Advertisement

Passing return to sports tests after ACL reconstruction is associated with greater likelihood for return to sport but fail to identify second injury risk

  • Wouter Welling
    Correspondence
    Corresponding author at: University of Groningen, University Medical Center Groningen, Center for Human Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, the Netherlands.
    Affiliations
    University of Groningen, University Medical Center Groningen, Center for Human Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, the Netherlands

    Medisch Centrum Zuid, Sportlaan 2-1, 9728 PH Groningen, the Netherlands
    Search for articles by this author
  • Anne Benjaminse
    Affiliations
    University of Groningen, University Medical Center Groningen, Center for Human Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, the Netherlands

    School of Sport Studies, Hanze University Groningen, Zernikeplein 17, 9747 AS Groningen, the Netherlands
    Search for articles by this author
  • Koen Lemmink
    Affiliations
    University of Groningen, University Medical Center Groningen, Center for Human Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, the Netherlands
    Search for articles by this author
  • Alli Gokeler
    Affiliations
    University of Groningen, University Medical Center Groningen, Center for Human Movement Science, Antonius Deusinglaan 1, 9713 AV Groningen, the Netherlands

    Exercise Science and Neuroscience, Department Exercise & Health, Faculty of Science, Paderborn University, Paderborn, Germany
    Search for articles by this author
Open AccessPublished:April 01, 2020DOI:https://doi.org/10.1016/j.knee.2020.03.007

      Abstract

      Background

      A limited number of patients return to sport (RTS) after an anterior cruciate ligament reconstruction (ACLR) and patients who RTS have a relatively high risk for second ACL injury. The purpose of the current study was to compare the results of a test battery between patients who returned to the pre-injury level of sport (RTS group) and patients who did not (NO-RTS group).
      It was hypothesized that the RTS group showed better test results.

      Methods

      Sixty-four patients (age 27.8 ± 8.8 years) were included. The results of a multicomponent test battery (jump-landing task assessed with the Landing Error Scoring System (LESS), three hop tests, isokinetic strength test for quadriceps and hamstring) were compared between groups with a 2 × 2 ANOVA.

      Results

      The RTS group showed a significantly lower LESS score (p = 0.010), significantly higher absolute scores on hop tests with both legs (injured leg: single leg hop test p = 0.013, triple leg hop test p = 0.024, side hop test p = 0.021; non-injured leg: single leg hop test p = 0.011, triple leg hop test p = 0.023, side hop test p = 0.032) and significantly greater hamstring strength in the injured leg (p = 0.009 at 60°/s, p = 0.012 at 180°/s and p = 0.013 at 300°/s). No differences in test results were identified between patients who sustained a second ACL injury and patients who did not.

      Conclusion

      Patients after ACLR with better jump-landing patterns, hop performance and greater hamstring strength have greater likelihood for RTS. However, our findings show that RTS criteria fail to identify patients who are at risk for a second ACL injury.

      Keywords

      1. Introduction

      Return to pre-injury level of sport is often the goal for patients after a reconstruction of the anterior cruciate ligament (ACLR) [
      • Lynch A.D.
      • Logerstedt D.S.
      • Grindem H.
      • et al.
      Consensus criteria for defining “successful outcome” after ACL injury and reconstruction: a Delaware-Oslo ACL cohort investigation.
      ]. After primary ACLR, patient expectations to return to the pre-injury level of sport are high both preoperatively (94%) [
      • Feucht M.J.
      • Cotic M.
      • Saier T.
      • Minzlaff P.
      • Plath J.E.
      • Imhoff A.B.
      • et al.
      Patient expectations of primary and revision anterior cruciate ligament reconstruction.
      ] and postoperatively (88%) [
      • Webster K.E.
      • Feller J.A.
      Expectations for Return to Preinjury Sport Before and After Anterior Cruciate Ligament Reconstruction.
      ]. For professional athletes, return to sport (RTS) rates to pre-injury level range from 78% to 98% around two years after ACLR [
      • Busfield B.T.
      • Kharrazi F.D.
      • Starkey C.
      • Lombardo S.J.
      • Seegmiller J.
      Performance outcomes of anterior cruciate ligament reconstruction in the National Basketball Association.
      ,
      • Erickson B.J.
      • Harris J.D.
      • Cole B.J.
      • Frank R.M.
      • Fillingham Y.A.
      • Ellman M.B.
      • et al.
      Performance and return to sport after anterior cruciate ligament reconstruction in national hockey league players.
      ,
      • Harris J.D.
      • Erickson B.J.
      • Bach Jr., B.R.
      • Abrams G.D.
      • Cvetanovich G.L.
      • Forsythe B.
      • et al.
      Return-to-sport and performance after anterior cruciate ligament reconstruction in national basketball association players.
      ]. Unfortunately, for amateur athletes only 65% of the patients return to their pre-injury level of sport two years after ACLR [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Webster K.E.
      Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors.
      ]. The risk for second ACL injury for athletes is relatively high, with the highest rates of second ACL injury in young athletes (up to 23% for patients younger than 25 years) [
      • Wiggins A.J.
      • Grandhi R.K.
      • Schneider D.K.
      • Stanfield D.
      • Webster K.E.
      • Myer G.D.
      Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: a systematic review and meta-analysis.
      ]. Additionally, 74% of second ACL injuries occur within the first two years [
      • Grooms D.R.
      • Page S.J.
      • Nichols-Larsen D.S.
      • Chaudhari A.M.
      • White S.E.
      • Onate J.A.
      Neuroplasticity associated with anterior cruciate ligament reconstruction.
      ,
      • Webster K.E.
      • Feller J.A.
      Exploring the high reinjury rate in younger patients undergoing anterior cruciate ligament reconstruction.
      ].
      Several factors contribute for RTS after ACLR [
      • Novaretti J.V.
      • Franciozi C.E.
      • Forgas A.
      • Sasaki P.H.
      • Ingham S.J.M.
      • Abdalla R.J.
      Quadriceps strength deficit at 6 months after ACL reconstruction does not predict return to preinjury sports level.
      ]. Research show that patients with greater muscle strength and higher scores on functional tests are more likely to return to pre-injury level of sport [
      • Novaretti J.V.
      • Franciozi C.E.
      • Forgas A.
      • Sasaki P.H.
      • Ingham S.J.M.
      • Abdalla R.J.
      Quadriceps strength deficit at 6 months after ACL reconstruction does not predict return to preinjury sports level.
      ,
      • Ardern C.L.
      • Webster K.E.
      • Taylor N.F.
      • Feller J.A.
      Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery.
      ,
      • Lentz T.A.
      • Zeppieri Jr., G.
      • Tillman S.M.
      • Indelicato P.A.
      • Moser M.W.
      • George S.Z.
      • et al.
      Return to preinjury sports participation following anterior cruciate ligament reconstruction: contributions of demographic, knee impairment, and self-report measures.
      ]. Furthermore, psychological readiness for RTS predicts outcomes one year after ACLR [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Whitehead T.S.
      • Webster K.E.
      Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery.
      ]. Therefore, monitoring patient-reported outcome measures (PROMs) seems to be essential for RTS after ACLR [
      • Kvist J.
      • Ek A.
      • Sporrstedt K.
      • Good L.
      Fear of re-injury: a hindrance for returning to sports after anterior cruciate ligament reconstruction.
      ]. Consequently, multicomponent test batteries are recommended in the RTS readiness decision-making to determine if patients are capable for RTS after ACLR [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ,
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ].
      Currently, limited research has been done in the relations of passing RTS criteria, the ability to RTS and the potential risk for second ACL injury. These relations are interesting for clinicians regarding the usefulness of current RTS criteria. Unfortunately, recent evidence shows conflicting results for the relation between RTS criteria and potential risk for second ACL injury [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ,
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ,
      • Nawasreh Z.
      • Logerstedt D.
      • Cummer K.
      • Axe M.J.
      • Risberg M.A.
      • Snyder-Mackler L.
      Do patients failing return-to-activity criteria at 6 months after anterior cruciate ligament reconstruction continue demonstrating deficits at 2 years?.
      ,
      • Sousa P.L.
      • Krych A.J.
      • Cates R.A.
      • Levy B.A.
      • Stuart M.J.
      • Dahm D.L.
      Return to sport: does excellent 6-month strength and function following ACL reconstruction predict midterm outcomes?.
      ,
      • Losciale J.M.
      • Zdeb R.M.
      • Ledbetter L.
      • Reiman M.P.
      • Sell T.C.
      The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis.
      ,
      • Webster K.E.
      • Hewett T.E.
      What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
      ,
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ]. One recent systematic review concluded that passing RTS criteria did not result in a decreased risk for second ACL injury [
      • Losciale J.M.
      • Zdeb R.M.
      • Ledbetter L.
      • Reiman M.P.
      • Sell T.C.
      The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis.
      ]. It has been found however that passing RTS criteria reduced the risk of a second ACL injury in the ipsilateral leg by 60%, but increased the risk of a contralateral ACL injury by 235% [
      • Webster K.E.
      • Hewett T.E.
      What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
      ].
      The primary purpose of the current study was to compare the results of a test battery at the end of the rehabilitation between patients who returned to the pre-injury level of sport (RTS group) and patients who did not (NO-RTS group) within two years after ACLR. The secondary purpose was to compare the results of the test battery between patients who sustained a second ACL injury and patients who did not. It was hypothesized that the RTS group showed better test results compared to the NO-RTS group. Additionally, it was hypothesized that patients who did not sustain a second ACL injury showed better results compared to patients who sustained a second ACL injury.

      2. Methods

      For this study, 100 patients after ACLR were identified from an outpatient physical therapy database. All patients were involved in amateur team ball sports (Table 1) and had the ambition to return to the pre-injury level of sport. During their rehabilitation, patients followed the same rehabilitation program. The early phase of the rehabilitation protocol (the first six weeks after ACLR) focused on reducing inflammation and swelling, restoring full knee extension, gait training and neuromuscular training for quadriceps activity. After that, muscle strength and endurance training and more advanced neuromuscular training commences. At 12 weeks, hypertrophy strengthening was started. Additionally, running and jumping exercises were added to the rehabilitation protocol. During the period of 24–44 weeks after ACLR, plyometric activities, running/cutting drills and on field rehabilitation, including sport-specific agility drills were added [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ,
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ]. Inclusion criteria were as follows: 1) participating in competitive, pivoting sports for at least four hours every week, 2) age > 18 years old, 3) primary isolated ACL lesion and 4) arthroscopic ACLR with a hamstring tendon graft or a bone-patellar tendon with an anteromedial portal technique. Patients were excluded if there was 1) a presence of swelling and/or pain of the injured knee during a test moment, 2) no ambition to return to competitive sport or 3) a feeling of instability in the injured knee. Patients performed a multicomponent test battery at the end of the rehabilitation and were followed up to two years after ACLR. Of the patients identified in the database, 71% responded to the invitation to participate in the study, there was missing data of seven patients and therefore, 64 patients were included in the current study (Table 1, Figure 1). All patients signed an informed consent prior to data collection. The study protocol was approved by the Institutional Review Board of the University of Groningen.
      Table 1Descriptive statistics study population for the RTS group and the NO-RTS group.
      RTS, Mean ± SDNO-RTS, Mean ± SDp-Value
      n4618N.A.
      Age (years)25.5 ± 5.833.6 ± 12.20.001*
      Gender33(M), 13(F)11(M), 7(F)N.A.
      Weight (kg)74.7 ± 7.076.3 ± 13.40.492
      Length (cm)179.3 ± 7.0180.4 ± 7.20.577
      Type of graft (n)HT(29), PT(17)HT(16), PT(2)N.A.
      Time post surgery at test battery (months)10.1 ± 0.910.1 ± 1.00.898
      Time post surgery questionnaire (months)24.8 ± 9.725.4 ± 10.10.559
      Number of therapy sessions72.4 ± 17.170.9 ± 8.80.802
      SportSO(32), HA(6), BA(5), KO(2), VO(1)SO(15), HA(2), KO(1)N.A.
      * = significant difference, RTS = patients who returned to the pre-injury level of sport, NO-RTS = patients who did not, M = males, F = females, kg = kilogram, cm = centimeter, HT = hamstring tendon graft, PT = bone-patellar tendon graft, SO = soccer, HA = handball, BA = basketball, KO = korfball, VO = volleyball. N.A. = not applicable.
      Figure 1
      Figure 1Flow chart of the included patients in the current study. ACLR = anterior cruciate ligament reconstruction.

      2.1 Multicomponent test battery (T1)

      The test battery was performed at an average of 10.1 ± 1.0 months after ACLR and included the following tests: a jump-landing task assessed with the Landing Error Scoring System (LESS) [
      • Padua D.A.
      • Marshall S.W.
      • Boling M.C.
      • Thigpen C.A.
      • Garrett Jr., W.E.
      • Beutler A.I.
      The landing error scoring system (less) is a valid and reliable clinical assessment tool of jump landing biomechanics: the jump-ACL study.
      ], three hop tests (single leg hop (SLH) test, triple leg hop (TLH) test and side hop (SH) test), isokinetic strength test for quadriceps and hamstring at a velocity of 60°/s, 180°/s and 300°/s and two questionnaires: the International Knee Documentation Committee Subjective Knee Form (IKDC) to measure self-reported knee function [
      • Logerstedt D.
      • Di Stasi S.
      • Grindem H.
      • Lynch A.
      • Eitzen I.
      • Engebretsen L.
      • et al.
      Self-reported knee function can identify athletes who fail return-to-activity criteria up to 1 year after anterior cruciate ligament reconstruction: a Delaware-Oslo ACL cohort study.
      ] and the Anterior Cruciate Ligament–Return to Sport after Injury (ACL-RSI) [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Whitehead T.S.
      • Webster K.E.
      Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery.
      ] to measure psychological readiness for RTS. All tests used were highly reliable (LESS: intraclass correlation coefficient (ICC) = 0.91; SLH: ICC = 0.97; TLH: ICC = 0.80–0.92; SH: ICC = 0.84–0.96; isokinetic device: ICC = 0.91–0.99) [
      • Kockum B.
      • Heijne A.I.
      Hop performance and leg muscle power in athletes: reliability of a test battery.
      ,
      • Munro A.G.
      • Herrington L.C.
      Between-session reliability of four hop tests and the agility T-test.
      ,
      • Padua D.A.
      • Boling M.C.
      • Distefano L.J.
      • Onate J.A.
      • Beutler A.I.
      • Marshall S.W.
      Reliability of the landing error scoring system real time, a clinical assessment tool of jump-landing biomechanics.
      ,
      • Tiffreau V.
      • Ledoux I.
      • Eymard B.
      • Thevenon A.
      • Hogrel J.Y.
      Isokinetic muscle testing for weak patients suffering from neuromuscular disorders: a reliability study.
      ]. The criteria for passing the test battery were: LESS <5 [
      • Padua D.A.
      • Marshall S.W.
      • Boling M.C.
      • Thigpen C.A.
      • Garrett Jr., W.E.
      • Beutler A.I.
      The landing error scoring system (less) is a valid and reliable clinical assessment tool of jump landing biomechanics: the jump-ACL study.
      ], limb symmetry index (LSI) >90% for all three hop tests (SLH, TLH, SH) [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ], LSI >90% for isokinetic quadriceps and hamstring strength at 60°/s, 180°/s and 300°/s [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ], quadriceps strength normalized to body weight (BW) >3.0 Nm/kg for the injured leg at 60°/s [
      • Kuenze C.
      • Hertel J.
      • Saliba S.
      • Diduch D.R.
      • Weltman A.
      • Hart J.M.
      Clinical thresholds for quadriceps assessment after anterior cruciate ligament reconstruction.
      ], hamstring/quadriceps (H/Q) ratio >55% for females and >62.5% for males for the injured leg at 300°/s [
      • Kuenze C.
      • Hertel J.
      • Saliba S.
      • Diduch D.R.
      • Weltman A.
      • Hart J.M.
      Clinical thresholds for quadriceps assessment after anterior cruciate ligament reconstruction.
      ], ACL-RSI >56 points [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Whitehead T.S.
      • Webster K.E.
      Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery.
      ] and IKDC score within 15% of healthy, gender- and age-matched controls [
      • Logerstedt D.
      • Di Stasi S.
      • Grindem H.
      • Lynch A.
      • Eitzen I.
      • Engebretsen L.
      • et al.
      Self-reported knee function can identify athletes who fail return-to-activity criteria up to 1 year after anterior cruciate ligament reconstruction: a Delaware-Oslo ACL cohort study.
      ].

      2.2 Two years after ACLR (T2)

      In the current study, mean follow-up was 25.1 ± 9.9 months after ACLR. A web-based questionnaire was developed by three of the authors (WW, AB, AG) using Google Forms (Google LLC, Mountain View, CA, USA). Data collection took place between December 2017 and April 2018. For data collection, patients were contacted by e-mail and up to two reminders were sent to patients who did not respond after two weeks. Non-responders were contacted by telephone and were asked to participate in the current study. The web-based questionnaire contained the following three questions: “Did you return to the pre-injury level of sport? (yes/no)”, “If not, did you return to a lower level of your pre-injury sport or do you currently perform a different sport compared to your pre-injury sport? (open answer)” and “Did you sustain a second ACL injury? (yes/no)”. Furthermore, patients were asked to complete the IKDC and ACL-RSI questionnaires again, which were included within the web-based questionnaire. Figure 2 shows the overview of the study design. The results of the test battery at the end of the rehabilitation were compared between the RTS and the NO-RTS groups.
      Figure 2
      Figure 2Overview of the study design. ACLR = anterior cruciate ligament reconstruction, T1 = at 10.1 ± 1.0 months after ACLR, T2 = at 25.1 ± 9.9 months after ACLR, LESS = Landing Error Scoring System test, SLH = single leg hop test, TLH = triple leg hop test, SH = side hop test, IKDC = the International Knee Documentation Committee Subjective Knee Form, ACL-RSI = the Anterior Cruciate Ligament–Return to Sport after Injury Scale, IKDC1 = IKDC score at 10.1 ± 1.0 months after ACLR, ACL-RSI1 = ACL-RSI score at 10.1 ± 1.0 months after ACLR, IKDC2 = IKDC score at 25.1 ± 9.9 months after ACLR, ACL-RSI2 = ACL-RSI score at 25.1 ± 9.9 months after ACLR.

      2.3 Data reduction

      The LESS test was analyzed by playing frontal and sagittal videos frame by frame [
      • Padua D.A.
      • Marshall S.W.
      • Boling M.C.
      • Thigpen C.A.
      • Garrett Jr., W.E.
      • Beutler A.I.
      The landing error scoring system (less) is a valid and reliable clinical assessment tool of jump landing biomechanics: the jump-ACL study.
      ]. For hop tests and strength tests, LSI values were calculated by dividing the scores of the injured leg by the non-injured leg, × 100 [
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ]. Furthermore, absolute values of the quadriceps strength in the injured leg were normalized to BW for the isokinetic peak torque test at 60°/s [
      • Kuenze C.
      • Hertel J.
      • Saliba S.
      • Diduch D.R.
      • Weltman A.
      • Hart J.M.
      Clinical thresholds for quadriceps assessment after anterior cruciate ligament reconstruction.
      ] and H/Q ratios at 300°/s were calculated [
      • Hewett T.E.
      • Myer G.D.
      • Zazulak B.T.
      Hamstrings to quadriceps peak torque ratios diverge between sexes with increasing isokinetic angular velocity.
      ]. Besides the LSI values, all absolute data of the hop tests and strength tests were analyzed. IKDC and ACL-RSI scores were calculated for the two time points (end of rehabilitation and two years after ACLR). Furthermore, the data of the three additional questions was analyzed and groups were created based on the answers.

      2.4 Statistical analysis

      All data was normally distributed as analyzed with SPSS version 20 (IBM SPSS 244 Inc., Chicago, IL). A 2 × 2 ANOVA was conducted to compare the demographic data and the data of the test battery between the RTS group and the NO-RTS groups. In addition, paired sample t-tests were used to investigate the progress in IKDC and ACL-RSI score over time for each group. To analyze the secondary purpose, a 2 × 2 ANOVA was conducted to compare the demographic data and the data of the test battery between patients who sustained a second ACL injury and patients who did not.

      3. Results

      Two years after ACLR, 71.9% (n = 46) were in the RTS group and 28.1% (n = 18) were in the NO-RTS group (Figure 3). Patients in the RTS group were significantly younger (25.5 ± 5.8 years vs. 33.6 ± 12.2 years; p = 0.001) compared to patients in the NO-RTS two years after ACLR. For the NO-RTS group, 27.8% (n = 5) were active on a lower level in the same sport and 66.7% (n = 12) were active in a different sport (non-pivoting sports like running and cycling) two years after ACLR. In addition, one patient was not active in sport at all two years after ACLR.
      Figure 3
      Figure 3Overview of the percentages of patients returned to the returned to the pre-injury level of sport and patients who did not. RTS = patients who returned to the pre-injury level of sport, NO-RTS = patients who did not.
      Patients in the RTS group showed a significantly lower LESS score compared to the NO-RTS group (p = 0.010) (Table 2). Although LSI criteria for all hop tests were met (LSI >90%) for both groups, patients in RTS group had significantly higher absolute scores in both the injured leg and non-injured leg on the SLH, TLH test and the SH test compared to the NO-RTS group (injured leg: SLH p = 0.013, TLH p = 0.024, SH p = 0.021; non-injured leg: SLH p = 0.011, TLH p = 0.023, SH p = 0.032) (Table 2). No differences were found in quadriceps strength between patients in the RTS and NO-RTS group. Patients in the RTS group showed significantly greater hamstring strength in the injured leg at 60°/s (p = 0.009), at 180°/s (p = 0.012) and at 300°/s (p = 0.013) and significantly greater LSI values for hamstring strength at 60°/s (p = 0.001) and at 180°/s (p = 0.012) compared to patients in the NO-RTS group.
      Table 2Results of the test battery for the RTS group and the NO-RTS group.
      RTS (Mean ± SD)NO-RTS (Mean ± SD)p-Value
      Second ACL injury (n)6 (13.0%)4 (22.2%)N.A.
      Ipsilateral/contralateral (n)5/13/1N.A.
      Passing all RTS criteria (n)6 (13.0%)1 (5.6%)N.A.
      LESS3.1 ± 1.44.4 ± 2.50.010*
      SLH injured leg (cm)163.7 ± 27.4142.4 ± 35.60.013*
      SLH non-injured leg (cm)168.8 ± 25.6149.4 ± 29.50.011*
      LSI SLH (%)97.1 ± 8.495.0 ± 13.40.445
      TLH injured leg (cm)524.8 ± 84.4469.3 ± 91.50.024*
      TLH non-injured leg (cm)538.9 ± 81.2487.4 ± 75.20.023*
      LSI TLH (%)97.4 ± 6.496.1 ± 9.70.509
      SH injured leg (number)53.3 ± 13.143.8 ± 17.10.021*
      SH non-injured leg (number)54.4 ± 11.646.9 ± 14.00.032*
      LSI SH (%)98.0 ± 14.191.3 ± 17.10.116
      Quadriceps strength 60°/s injured leg (Nm)223.9 ± 51.2208.0 ± 42.60.246
      Quadriceps strength 60°/s non-injured leg (Nm)239.3 ± 53.6223.4 ± 45.00.272
      LSI quadriceps strength 60°/s (%)93.9 ± 9.293.5 ± 9.20.869
      Hamstring strength 60°/s injured leg (Nm)136.8 ± 31.9113.1 ± 31.40.009*
      Hamstring strength 60°/s non-injured leg (Nm)137.0 ± 31.7125.1 ± 36.60.203
      LSI hamstring strength 60°/s (%)100.1 ± 9.891.3 ± 8.70.001*
      Quadriceps strength 180°/s injured leg (Nm)158.2 ± 37.4146.7 ± 34.70.266
      Quadriceps strength 180°/s non-injured leg (Nm)167.4 ± 37.2155.2 ± 34.00.230
      LSI quadriceps strength 180°/s (%)94.7 ± 9.189.6 ± 24.80.186
      Hamstring strength 180°/s injured leg (Nm)107.9 ± 25.289.9 ± 24.80.012*
      Hamstring strength 180°/s non-injured leg (Nm)109.3 ± 23.898.4 ± 27.30.119
      LSI hamstring strength 180°/s (%)98.7 ± 9.791.7 ± 9.50.012*
      Quadriceps strength 300°/s injured leg (Nm)121.3 ± 30.2112.2 ± 30.60.286
      Quadriceps strength 300°/s non-injured leg (Nm)129.5 ± 30.5120.1 ± 30.10.271
      LSI quadriceps strength 300°/s (%)93.7 ± 0.8.793.7 ± 9.10.979
      Hamstring strength 300°/s injured leg (Nm)91.4 ± 21.376.3 ± 21.40.013*
      Hamstring strength 300°/s non-injured leg (Nm)91.4 ± 20.480.7 ± 22.20.071
      LSI hamstring strength 300°/s (%)100.1 ± 9.494.8 ± 10.40.057
      Quadriceps peak torque at 60°/s normalized to BW (Nm/kg)3.0 ± 0.62.8 ± 0.50.124
      H/Q ratio at 300°/s0.8 ± 0.20.7 ± 0.20.179
      IKDC182.3 ± 7.285.6 ± 8.40.387
      ACL-RSI171.5 ± 17.769.5 ± 20.10.697
      IKDC291.5 ± 7.689.6 ± 8.10.375
      ACL-RSI278.7 ± 19.762.6 ± 24.40.008*
      * = significant difference, RTS = patients who returned to the pre-injury level of sport, NO-RTS = patients who did not, ACL = anterior cruciate ligament, N.A. = not applicable, * = significant difference, Nm = newton meter, LSI = limb symmetry index, kg = kilogram, H/Q = hamstring/quadriceps, SLH = single leg hop test, TLH = triple leg hop test, SH = side hop test, cm = centimeter, LESS = Landing Error Scoring System test, IKDC = the International Knee Documentation Committee Subjective Knee Form, ACL-RSI = the Anterior Cruciate Ligament–Return to Sport after Injury Scale, IKDC1 = IKDC score at 10.1 ± 1.0 months after ACLR, ACL-RSI1 = ACL-RSI score at 10.1 ± 1.0 months after ACLR, IKDC2 = IKDC score at 25.1 ± 9.9 months after ACLR, ACL-RSI2 = ACL-RSI score at 25.1 ± 9.9 months after ACLR.
      Six patients in the RTS group sustained a second ACL injury compared to four patients in the NO-RTS (Table 2). Patients who sustained a second ACL injury were significantly younger (21.7 ± 4.7 years vs. 28.9 ± 9.0 years; p = 0.017) compared to patients who did not. Patients who sustained a second ACL injury did not differ in test results compared to patients who did not sustain a second ACL injury.
      The RTS group showed a significantly higher ACL-RSI score compared to the NO-RTS group two years after ACLR (p = 0.008) and the RTS group significantly increased their IKDC score over time (p < 0.001) (Table 2).

      4. Discussion

      The primary findings of the current study were that a lower LESS score, higher absolute scores on hop tests and greater hamstrings strength result in higher RTS rates to pre-injury level two years after ACLR. Passing or not passing the RTS criteria was not correlated with second ACL injury, indicating that RTS criteria used in this study fail in identifying patients who are at risk for a second ACL injury.
      In the current study, around 70% of the patients returned to the pre-injury level of sport two years after ACLR which is comparable to previous reported 65% RTS rates [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Webster K.E.
      Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors.
      ,
      • Ardern C.L.
      • Webster K.E.
      • Taylor N.F.
      • Feller J.A.
      Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery.
      ]. Patients in the RTS group were younger compared to patients in the NO-RTS group which indicates that younger patients have increased likelihood to return to pre-injury level of sport after ACLR. This is in line with earlier studies linking younger age with increased RTS rates [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Webster K.E.
      Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors.
      ,
      • Lentz T.A.
      • Zeppieri Jr., G.
      • George S.Z.
      • Tillman S.M.
      • Moser M.W.
      • Farmer K.W.
      • et al.
      Comparison of physical impairment, functional, and psychosocial measures based on fear of reinjury/lack of confidence and return-to-sport status after ACL reconstruction.
      ,
      • Brophy R.H.
      • Schmitz L.
      • Wright R.W.
      • Dunn W.R.
      • Parker R.D.
      • Andrish J.T.
      • et al.
      Return to play and future ACL injury risk after ACL reconstruction in soccer athletes from the Multicenter Orthopaedic Outcomes Network (MOON) group.
      ,
      • Feller J.
      • Webster K.E.
      Return to sport following anterior cruciate ligament reconstruction.
      ,
      • Webster K.E.
      • Feller J.A.
      Return to level I sports after anterior cruciate ligament reconstruction: evaluation of age, sex, and readiness to return criteria.
      ].
      Overall, around 15% of patients sustained a second ACL injury within two years after ACLR, which is similar to earlier reported studies [
      • Webster K.E.
      • Feller J.A.
      • Leigh W.B.
      • Richmond A.K.
      Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
      ,
      • Zwolski C.
      • Schmitt L.C.
      • Quatman-Yates C.
      • Thomas S.
      • Hewett T.E.
      • Paterno M.V.
      The influence of quadriceps strength asymmetry on patient-reported function at time of return to sport after anterior cruciate ligament reconstruction.
      ]. Furthermore, our results showed that patients who sustained a second ACL injury were younger compared to patients who did not. Similar findings were found in earlier studies, showing that younger age results in higher rates for second ACL injury [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Webster K.E.
      Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors.
      ,
      • Webster K.E.
      • Feller J.A.
      • Leigh W.B.
      • Richmond A.K.
      Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
      ,
      • Paterno M.V.
      • Huang B.
      • Thomas S.
      • Hewett T.E.
      • Schmitt L.C.
      Clinical factors that predict a second ACL injury after ACL reconstruction and return to sport: preliminary development of a clinical decision algorithm.
      ]. Although no information was collected about the patients' athletic exposures and/or level of exposures, these results are logical since patients in the RTS group were also younger compared to patients in the NO-RTS group. Therefore, these younger patients have increased risk for a second ACL injury since returning to a pivoting sport is more demanding compared to a non-pivoting sport [
      • Seil R.
      • Mouton C.
      • Lion A.
      • Nuhrenborger C.
      • Pape D.
      • Theisen D.
      There is no such thing like a single ACL injury: profiles of ACL-injured patients.
      ].

      4.1 Multicomponent test battery (T1)

      Only seven patients (10.9%) passed all RTS criteria which is comparable with previous research, showing that low rates of patients meeting RTS criteria after ACLR [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ,
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ,
      • Webster K.E.
      • Hewett T.E.
      What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
      ,
      • Toole A.R.
      • Ithurburn M.P.
      • Rauh M.J.
      • Hewett T.E.
      • Paterno M.V.
      • Schmitt L.C.
      Young athletes cleared for sports participation after anterior cruciate ligament reconstruction: how many actually meet recommended return-to-sport criterion cutoffs?.
      ,
      • Herbst E.
      • Hoser C.
      • Hildebrandt C.
      • et al.
      Functional assessments for decision-making regarding return to sports following ACL reconstruction. Part II: clinical application of a new test battery.
      ]. This raises the question regarding the usefulness of current RTS criteria, since the majority of the patients fail in passing RTS criteria. On the other hand, 85% of the patients who passed all RTS criteria, returned to the pre-injury level of sport. The problem with a large array of RTS criteria is that the overall pass rate for the test battery is dependent on the total number of tests and cut-off criteria [
      • Webster K.E.
      • Hewett T.E.
      What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
      ]. The more tests and criteria included, the more difficult it is for patients to pass all criteria. For example, the study of Grindem et al. [
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ] used seven criteria (compared to 14 criteria the current study), resulting in a 24% pass rate compared to 11% in the current study. For patients in the current study, passing all criteria of the test battery is almost utopian. The current study is part of an ongoing project which started in 2017 with the development of a RTS test battery for patients after ACLR [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ]. Since low percentages of patients passed RTS criteria at both six [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ] and nine months after ACLR [
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ], the current RTS criteria might be revised and potentially limited. For example, in the current study eight criteria were used related to muscle strength compared to only one criterion in the study of Grindem et al. [
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ], using only LSI >90% for quadriceps strength at 60°/sec besides LSI >90% for four hop tests and two questionnaires. If we use these criteria for the patient population in the current study, 54.7% (n = 35) of patients would have passed the RTS criteria. These findings indicate that the number of tests used in the current study might need revision. In addition, no differences were found in quadriceps strength between the RTS and the NO-RTS group. Furthermore, no differences in quadriceps strength were found between patients who sustained a second ACL injury and patients who did not. These findings are in conflict with earlier studies showing the importance of quadriceps strength in the reduction of a second ACL injury [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ,
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ]. For example, the study of Grindem et al. [
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ] found that patients who sustained a second ACL injury had a lower LSI value for quadriceps strength compared to patients who did not. Although the same isokinetic test was used for quadriceps strength, the percentage of included female patients was higher compared to the current study population (54% in [
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ] compared to 31% in the current study). This might explain the conflicting results. Furthermore, this study did not present absolute quadriceps strength values (only LSI values), which makes it hard to compare with the results in the current study [
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ].
      The results of the test battery indicate that better jump-landing patterns at the end of the rehabilitation result in higher RTS rates. Therefore, clinicians are encouraged to analyze movement quality including jump-landing patterns before RTS [
      • Bizzini M.
      • Hancock D.
      • Impellizzeri F.
      Suggestions from the field for return to sports participation following anterior cruciate ligament reconstruction: soccer.
      ]. Less optimal movement quality during functional movements could potentially increase the risk for second ACL injury [
      • Dingenen B.
      • Gokeler A.
      Optimization of the return-to-sport paradigm after anterior cruciate ligament reconstruction: a critical step back to move forward.
      ,
      • van Melick N.
      • van Cingel R.E.
      • Brooijmans F.
      • Neeter C.
      • van Tienen T.
      • Hullegie W.
      • et al.
      Evidence-based clinical practice update: practice guidelines for anterior cruciate ligament rehabilitation based on a systematic review and multidisciplinary consensus.
      ]. However, in the current study no differences in jump-landing patterns were found between patients who sustained a second ACL injury and patients who did not. This indicates that the drop vertical jump (assessed with the LESS) is a poor test to identify second ACL injury risk [
      • Krosshaug T.
      • Steffen K.
      • Kristianslund E.
      • et al.
      The vertical drop jump is a poor screening test for ACL injuries in female elite soccer and handball players: a prospective cohort study of 710 athletes [published correction appears in Am J Sports Med. 2017 Jul;45(9):NP28-NP29].
      ]. However, this is in conflict with earlier findings, showing that altered movements in hip and knee (for example more knee valgus displacement) during a dynamic landing task was correlated with a second ACL injury in young athletes [
      • Paterno M.V.
      • Schmitt L.C.
      • Ford K.R.
      • et al.
      Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport.
      ]. One potential reason for these conflicting results might be related to the 3D motion analysis system used in the study of Paterno and colleagues [
      • Paterno M.V.
      • Schmitt L.C.
      • Ford K.R.
      • et al.
      Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport.
      ]. Furthermore, the study population used in [
      • Paterno M.V.
      • Schmitt L.C.
      • Ford K.R.
      • et al.
      Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport.
      ] was significantly younger (16.4 ± 3.0 in vs. 27.8 ± 8.8 years in the current study). These differences might explain the conflicting results in the relation between altered jump-landing patterns and increased risk for second ACL injury.
      No differences were found in LSI values on all three hop tests (SLH test, TLH test and SH test) between groups. However, when comparing the absolute scores the RTS group jumped further in the SLH and in the TLH test and scored higher on the SH test with both legs compared to the NO-RTS group. The results of the current study are similar compared to previous research demonstrating greater absolute functional performance on single leg hop test in patients who returned to the pre-injury level of sport after ACLR [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ]. Although patients in Ithurburn et al. [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ] were younger (17.1 ± 2.4 years in Ithurburn et al. [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ] compared to 27.8 ± 8.8 years in the current study) and more female patients were included compared to the current study (75% female patients in Ithurburn et al. [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ] compared to 31% in the current study), both the study of Ithurburn et al. [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ] and the current study indicate that underlying, overall athleticism may contribute to RTS. Furthermore, these findings show that only using LSI values for the RTS decision making can mask bilateral deficits and overestimate performance [
      • Gokeler A.
      • Welling W.
      • Zaffagnini S.
      • Seil R.
      • Padua D.
      Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
      ,
      • Welling W.
      • Benjaminse A.
      • Seil R.
      • Lemmink K.
      • Zaffagnini S.
      • Gokeler A.
      Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
      ,
      • Gokeler A.
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Seil R.
      • Zaffagnini S.
      A critical analysis of limb symmetry indices of hop tests in athletes after anterior cruciate ligament reconstruction: a case control study.
      ,
      • Zwolski C.
      • Schmitt L.C.
      • Thomas S.
      • Hewett T.E.
      • Paterno M.V.
      The utility of limb symmetry indices in return-to-sport assessment in patients with bilateral anterior cruciate ligament reconstruction.
      ]. Therefore, the use of LSI values could give clinicians incomplete information about the patients' performance [
      • Gokeler A.
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Seil R.
      • Zaffagnini S.
      A critical analysis of limb symmetry indices of hop tests in athletes after anterior cruciate ligament reconstruction: a case control study.
      ]. In addition, leg dominance is suggested to affect LSI values [
      • Zwolski C.
      • Schmitt L.C.
      • Thomas S.
      • Hewett T.E.
      • Paterno M.V.
      The utility of limb symmetry indices in return-to-sport assessment in patients with bilateral anterior cruciate ligament reconstruction.
      ] since the dominant leg is often stronger that the non-dominant leg [
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Dingenen B.
      • Gokeler A.
      Progressive strength training restores quadriceps and hamstring muscle strength within 7 months after ACL reconstruction in amateur male soccer players.
      ]. Clinicians are encouraged to use absolute norm values instead of only LSI values to determine RTS readiness [
      • Gokeler A.
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Seil R.
      • Zaffagnini S.
      A critical analysis of limb symmetry indices of hop tests in athletes after anterior cruciate ligament reconstruction: a case control study.
      ,
      • Zwolski C.
      • Schmitt L.C.
      • Thomas S.
      • Hewett T.E.
      • Paterno M.V.
      The utility of limb symmetry indices in return-to-sport assessment in patients with bilateral anterior cruciate ligament reconstruction.
      ].
      The current study used a standardized rehabilitation program. Interestingly, a recent study [
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Dingenen B.
      • Gokeler A.
      Progressive strength training restores quadriceps and hamstring muscle strength within 7 months after ACL reconstruction in amateur male soccer players.
      ] found that using more progressive strength training within ACLR rehabilitation results in increased quadriceps strength normalized to BW in the injured leg compared to a standardized rehabilitation (3.2 ± 0.6 Nm/kg with a passing rate of 71.1% for >3.0 Nm/kg in [
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Dingenen B.
      • Gokeler A.
      Progressive strength training restores quadriceps and hamstring muscle strength within 7 months after ACL reconstruction in amateur male soccer players.
      ] compared to 2.9 ± 0.6 Nm/kg with a passing rate of only 46.9% in the current study). This raises the question if the quality of rehabilitation in the current study was sufficient enough for patients to RTS. Increasing the quality of the ACLR rehabilitation by implementing more progressive strength training results in higher passing rates for RTS strength criteria [
      • Welling W.
      • Benjaminse A.
      • Lemmink K.
      • Dingenen B.
      • Gokeler A.
      Progressive strength training restores quadriceps and hamstring muscle strength within 7 months after ACL reconstruction in amateur male soccer players.
      ] which potentially increase RTS rates and decrease the risk for second ACL injury. Future research should focus on longitudinal follow up studies after implementing progressive strength training within rehabilitation.
      Patients in the RTS group showed greater hamstring strength compared to patients in the NO-RTS. In addition, the RTS group showed more symmetrical hamstring strength. This is in line with earlier findings, showing more symmetrical hamstring strength in patients who successfully resumed pre-injury sports participation compared to patients who did not [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ]. On the other hand, no differences in hamstring strength were found between patients who sustained a second ACL injury and patients who did not. However, decreased hamstring strength is suggested to be a contributing mechanism to ACL injuries, especially in female athletes [
      • Myer G.D.
      • Ford K.R.
      • Barber Foss K.D.
      • Liu C.
      • Nick T.G.
      • Hewett T.E.
      The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes.
      ]. Instead of measuring absolute hamstring strength values and symmetry, the H/Q ratio might be a more relevant variable for investigating risk for second ACL injury [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ]. In the current study, H/Q ratios at 300°/s were used as RTS criteria. However, H/Q ratios at 60°/s might be more relevant since Kyritsis and colleagues found a 10 times greater risk for every 10% difference in H/Q ratio at 60°/s [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ]. Comparable values were found for H/Q ratios at 60°/s in the current study between patients who sustained a second ACL injury and patients who did not (53.8 ± 9.5 vs. 61.1 ± 12.5% in the current study compared to 53 ± 11 vs. 58 ± 10% in [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ]), indicating that a decreased H/Q ratio at 60°/s might be correlated with risk for second ACL injury. Many studies in the area of ACL injuries are focused on achieving quadriceps strength [
      • Ardern C.L.
      • Webster K.E.
      • Taylor N.F.
      • Feller J.A.
      Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery.
      ,
      • Novaretti J.V.
      • Franciozi C.E.
      • Forgas A.
      • Sasaki P.H.
      • Ingham S.J.M.
      • Abdalla R.J.
      Quadriceps strength deficit at 6 months after ACL reconstruction does not predict return to preinjury sports level.
      ,
      • Webster K.E.
      • Feller J.A.
      • Leigh W.B.
      • Richmond A.K.
      Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
      ,
      • Cristiani R.
      • Mikkelsen C.
      • Forssblad M.
      • Engstrom B.
      • Stalman A.
      Only one patient out of five achieves symmetrical knee function 6 months after anterior cruciate ligament reconstruction.
      ] but the results of the current study also highlight the importance of hamstring strength.
      Patients who sustained a second ACL injury did not differ in test battery results compared patients who did not. This is in line with the evidence showing a poor relation between passing RTS criteria and decreased risk for second ACL injury [
      • Kyritsis P.
      • Bahr R.
      • Landreau P.
      • Miladi R.
      • Witvrouw E.
      Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
      ,
      • Grindem H.
      • Snyder-Mackler L.
      • Moksnes H.
      • Engebretsen L.
      • Risberg M.A.
      Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
      ,
      • Nawasreh Z.
      • Logerstedt D.
      • Cummer K.
      • Axe M.J.
      • Risberg M.A.
      • Snyder-Mackler L.
      Do patients failing return-to-activity criteria at 6 months after anterior cruciate ligament reconstruction continue demonstrating deficits at 2 years?.
      ,
      • Sousa P.L.
      • Krych A.J.
      • Cates R.A.
      • Levy B.A.
      • Stuart M.J.
      • Dahm D.L.
      Return to sport: does excellent 6-month strength and function following ACL reconstruction predict midterm outcomes?.
      ,
      • Losciale J.M.
      • Zdeb R.M.
      • Ledbetter L.
      • Reiman M.P.
      • Sell T.C.
      The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis.
      ]. This raises the question on what the best clinical factors are to evaluate risk for second ACL injury [
      • Ithurburn M.P.
      • Longfellow M.A.
      • Thomas S.
      • Paterno M.V.
      • Schmitt L.C.
      Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
      ]. In addition, a recent systematic review questioned if current test batteries are valid instruments to determine the risk for second ACL injury [
      • Webster K.E.
      • Hewett T.E.
      What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
      ]. Our findings suggest that current test batteries can be used to determine the likelihood that patients resume sports at pre-injury level, but fail in identifying patients who are at risk for a second ACL injury. Furthermore, our findings show that younger patients have increased risk for a second ACL injury since younger patients are more likely to return to the pre-injury level of a pivoting sport compared to older patients who are more likely to do a less demanding non-pivoting sport [
      • Seil R.
      • Mouton C.
      • Lion A.
      • Nuhrenborger C.
      • Pape D.
      • Theisen D.
      There is no such thing like a single ACL injury: profiles of ACL-injured patients.
      ].

      4.2 PROMs two years after ACLR (T2)

      Psychological readiness for RTS is essential for RTS after ACLR since this is a predictor for returning to pre-injury level of sport in amateur athletes [
      • Ardern C.L.
      • Taylor N.F.
      • Feller J.A.
      • Whitehead T.S.
      • Webster K.E.
      Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery.
      ,
      • Kvist J.
      • Ek A.
      • Sporrstedt K.
      • Good L.
      Fear of re-injury: a hindrance for returning to sports after anterior cruciate ligament reconstruction.
      ]. The findings of the current study are in agreement with previous research showing that patients in the RTS group showed higher psychological readiness for RTS two years after ACLR compared to patients in the NO-RTS. It is suggested that clinicians can influence both the physical and psychological recovery of an athlete [
      • Webster K.E.
      • Feller J.A.
      • Leigh W.B.
      • Richmond A.K.
      Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
      ]. Therefore, psychological readiness for RTS should be monitored during rehabilitation, and if needed, targeted interventions should be used to increase psychological factors [
      • Bizzini M.
      • Hancock D.
      • Impellizzeri F.
      Suggestions from the field for return to sports participation following anterior cruciate ligament reconstruction: soccer.
      ]. Our results showed that only patients in the RTS group demonstrated a clinically relevant improvement of the IKDC score. More in detail, the absolute increase in IKDC score of patients in the RTS group was 9.2 which is similar to the minimal detectable change of 8.8 [
      • Grevnerts H.T.
      • Terwee C.B.
      • Kvist J.
      The measurement properties of the IKDC-subjective knee form.
      ]. Self-reported knee function and psychological readiness should be monitored after the rehabilitation since this can significantly influence RTS rates.

      5. Limitations

      A number of potential study limitations should be noted. There is a risk for response bias, since the response rate of the current study was 71%. Additionally, the questionnaire was conducted at an average of 25.1 months after ACLR, with a standard deviation of 9.9 months. This relatively high standard deviation could influence the results significantly. In the current study, the potential effects of different athlete exposures and/or level of exposures were not investigated. Also, defining groups (RTS vs. NO-RTS) was based on the results of a questionnaire and it was unknown if patients actually returned to the pre-injury level of sport. Future research should focus on measuring RTS, in terms of games played, scoring and career length in amateur athletes after ACLR [
      • Kester B.S.
      • Behery O.A.
      • Minhas S.V.
      • Hsu W.K.
      Athletic performance and career longevity following anterior cruciate ligament reconstruction in the National Basketball Association.
      ,
      • Arundale A.J.H.
      • Silvers-Granelli H.J.
      • Snyder-Mackler L.
      Career length and injury incidence after anterior cruciate ligament reconstruction in major league soccer players.
      ]. Furthermore, patients were tested at 10.1 ± 1.0 months after ACLR and it was unknown if and what kind of training patients did after rehabilitation. In other words, it is unknown how strength or hop performance developed over time. Measuring patients at one moment in time and comparing the test results with their performance two years later, might be methodological flawed. Therefore, future research should focus on repeated measurements over time in patients after ACLR. Also, LSI values were used in the current study which could result in incomplete information about the patients' performance. In the current study, it was unknown if all second ACL injuries were non-contact injuries. Last, future research should consider patients as complex biological systems, without direct relations between isolated factors (strength, hop performance, jump-landing patterns, PROMs) and the outcome (RTS) [
      • Bittencourt N.F.N.
      • Meeuwisse W.H.
      • Mendonca L.D.
      • Nettel-Aguirre A.
      • Ocarino J.M.
      • Fonseca S.T.
      Complex systems approach for sports injuries: moving from risk factor identification to injury pattern recognition-narrative review and new concept.
      ].

      6. Conclusion

      Better jump-landing patterns, improved hop performance and greater hamstring strength result in greater likelihood for RTS in patients after ACLR. These findings can help clinicians to identify patients who are more likely to RTS. Of caution, current RTS criteria fail in identifying patients who are at risk for a second ACL injury.

      Conflict of interest

      The authors have declared no conflict of interest.

      Ethical approval

      The study was approved by the Review Board at the University of Groningen.

      Funding

      None declared.

      Acknowledgements

      None.

      References

        • Lynch A.D.
        • Logerstedt D.S.
        • Grindem H.
        • et al.
        Consensus criteria for defining “successful outcome” after ACL injury and reconstruction: a Delaware-Oslo ACL cohort investigation.
        Br J Sports Med. 2013; https://doi.org/10.1136/bjsports-2013-092299
        • Feucht M.J.
        • Cotic M.
        • Saier T.
        • Minzlaff P.
        • Plath J.E.
        • Imhoff A.B.
        • et al.
        Patient expectations of primary and revision anterior cruciate ligament reconstruction.
        Knee Surg Sports Traumatol Arthrosc. 2016; 24: 201-207
        • Webster K.E.
        • Feller J.A.
        Expectations for Return to Preinjury Sport Before and After Anterior Cruciate Ligament Reconstruction.
        Am J Sports Med. 2019; 47: 578-583
        • Busfield B.T.
        • Kharrazi F.D.
        • Starkey C.
        • Lombardo S.J.
        • Seegmiller J.
        Performance outcomes of anterior cruciate ligament reconstruction in the National Basketball Association.
        Arthroscopy. 2009; 25: 825-830
        • Erickson B.J.
        • Harris J.D.
        • Cole B.J.
        • Frank R.M.
        • Fillingham Y.A.
        • Ellman M.B.
        • et al.
        Performance and return to sport after anterior cruciate ligament reconstruction in national hockey league players.
        Orthop J Sports Med. 2014; 5 ([2325967114548831])
        • Harris J.D.
        • Erickson B.J.
        • Bach Jr., B.R.
        • Abrams G.D.
        • Cvetanovich G.L.
        • Forsythe B.
        • et al.
        Return-to-sport and performance after anterior cruciate ligament reconstruction in national basketball association players.
        Sports Health. 2013; 5: 562-568
        • Ardern C.L.
        • Taylor N.F.
        • Feller J.A.
        • Webster K.E.
        Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors.
        Br J Sports Med. 2014; 48: 1543-1552
        • Wiggins A.J.
        • Grandhi R.K.
        • Schneider D.K.
        • Stanfield D.
        • Webster K.E.
        • Myer G.D.
        Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: a systematic review and meta-analysis.
        Am J Sports Med. 2016 Jul; 44: 1861-1876
        • Grooms D.R.
        • Page S.J.
        • Nichols-Larsen D.S.
        • Chaudhari A.M.
        • White S.E.
        • Onate J.A.
        Neuroplasticity associated with anterior cruciate ligament reconstruction.
        J Orthop Sports Phys Ther. 2017 Mar; 47: 180-189
        • Webster K.E.
        • Feller J.A.
        Exploring the high reinjury rate in younger patients undergoing anterior cruciate ligament reconstruction.
        Am J Sports Med. 2016 Nov; 44: 2827-2832
        • Novaretti J.V.
        • Franciozi C.E.
        • Forgas A.
        • Sasaki P.H.
        • Ingham S.J.M.
        • Abdalla R.J.
        Quadriceps strength deficit at 6 months after ACL reconstruction does not predict return to preinjury sports level.
        Sports Health. 2018; 10: 266-271
        • Ardern C.L.
        • Webster K.E.
        • Taylor N.F.
        • Feller J.A.
        Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery.
        Am J Sports Med. 2011; 39: 538-543
        • Lentz T.A.
        • Zeppieri Jr., G.
        • Tillman S.M.
        • Indelicato P.A.
        • Moser M.W.
        • George S.Z.
        • et al.
        Return to preinjury sports participation following anterior cruciate ligament reconstruction: contributions of demographic, knee impairment, and self-report measures.
        J Orthop Sports Phys Ther. 2012; 42: 893-901
        • Ardern C.L.
        • Taylor N.F.
        • Feller J.A.
        • Whitehead T.S.
        • Webster K.E.
        Psychological responses matter in returning to preinjury level of sport after anterior cruciate ligament reconstruction surgery.
        Am J Sports Med. 2013; 41: 1549-1558
        • Kvist J.
        • Ek A.
        • Sporrstedt K.
        • Good L.
        Fear of re-injury: a hindrance for returning to sports after anterior cruciate ligament reconstruction.
        Knee Surg Sports Traumatol Arthrosc. 2005; 13: 393-397
        • Gokeler A.
        • Welling W.
        • Zaffagnini S.
        • Seil R.
        • Padua D.
        Development of a test battery to enhance safe return to sports after anterior cruciate ligament reconstruction.
        Knee Surg Sports Traumatol Arthrosc. 2017; 25: 192-199
        • Welling W.
        • Benjaminse A.
        • Seil R.
        • Lemmink K.
        • Zaffagnini S.
        • Gokeler A.
        Low rates of patients meeting return to sport criteria 9 months after anterior cruciate ligament reconstruction: a prospective longitudinal study.
        Knee Surg Sports Traumatol Arthrosc. 2018; 24
        • Kyritsis P.
        • Bahr R.
        • Landreau P.
        • Miladi R.
        • Witvrouw E.
        Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture.
        Br J Sports Med. 2016 Aug; 50: 946-951
        • Grindem H.
        • Snyder-Mackler L.
        • Moksnes H.
        • Engebretsen L.
        • Risberg M.A.
        Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study.
        Br J Sports Med. 2016; 50: 804-808
        • Nawasreh Z.
        • Logerstedt D.
        • Cummer K.
        • Axe M.J.
        • Risberg M.A.
        • Snyder-Mackler L.
        Do patients failing return-to-activity criteria at 6 months after anterior cruciate ligament reconstruction continue demonstrating deficits at 2 years?.
        Am J Sports Med. 2017 Apr; 45: 1037-1048
        • Sousa P.L.
        • Krych A.J.
        • Cates R.A.
        • Levy B.A.
        • Stuart M.J.
        • Dahm D.L.
        Return to sport: does excellent 6-month strength and function following ACL reconstruction predict midterm outcomes?.
        Knee Surg Sports Traumatol Arthrosc. 2017 May; 25: 1356-1363
        • Losciale J.M.
        • Zdeb R.M.
        • Ledbetter L.
        • Reiman M.P.
        • Sell T.C.
        The association between passing return-to-sport criteria and second anterior cruciate ligament injury risk: a systematic review with meta-analysis.
        J Orthop Sports Phys Ther. 2019 Feb; 49: 43-54
        • Webster K.E.
        • Hewett T.E.
        What is the evidence for and validity of return-to-sport testing after anterior cruciate ligament reconstruction surgery? A systematic review and meta-analysis.
        Sports Med. 2019 Jun; 49: 917-929
        • Ithurburn M.P.
        • Longfellow M.A.
        • Thomas S.
        • Paterno M.V.
        • Schmitt L.C.
        Knee function, strength, and resumption of preinjury sports participation in young athletes following anterior cruciate ligament reconstruction.
        J Orthop Sports Phys Ther. 2019 Mar; 49: 145-153
        • Padua D.A.
        • Marshall S.W.
        • Boling M.C.
        • Thigpen C.A.
        • Garrett Jr., W.E.
        • Beutler A.I.
        The landing error scoring system (less) is a valid and reliable clinical assessment tool of jump landing biomechanics: the jump-ACL study.
        Am J Sports Med. 2009; 37: 1996-2002
        • Logerstedt D.
        • Di Stasi S.
        • Grindem H.
        • Lynch A.
        • Eitzen I.
        • Engebretsen L.
        • et al.
        Self-reported knee function can identify athletes who fail return-to-activity criteria up to 1 year after anterior cruciate ligament reconstruction: a Delaware-Oslo ACL cohort study.
        J Orthop Sports Phys Ther. 2014; 44: 914-923
        • Kockum B.
        • Heijne A.I.
        Hop performance and leg muscle power in athletes: reliability of a test battery.
        Phys Ther Sport. 2015; 16: 222-227
        • Munro A.G.
        • Herrington L.C.
        Between-session reliability of four hop tests and the agility T-test.
        J Strength Cond Res. 2011; 25: 1470-1477
        • Padua D.A.
        • Boling M.C.
        • Distefano L.J.
        • Onate J.A.
        • Beutler A.I.
        • Marshall S.W.
        Reliability of the landing error scoring system real time, a clinical assessment tool of jump-landing biomechanics.
        J Sport Rehabil. 2011; 20: 145-156
        • Tiffreau V.
        • Ledoux I.
        • Eymard B.
        • Thevenon A.
        • Hogrel J.Y.
        Isokinetic muscle testing for weak patients suffering from neuromuscular disorders: a reliability study.
        Neuromuscul Disord. 2007; 17: 524-531
        • Kuenze C.
        • Hertel J.
        • Saliba S.
        • Diduch D.R.
        • Weltman A.
        • Hart J.M.
        Clinical thresholds for quadriceps assessment after anterior cruciate ligament reconstruction.
        J Sport Rehab. 2015; 24: 36-46
        • Hewett T.E.
        • Myer G.D.
        • Zazulak B.T.
        Hamstrings to quadriceps peak torque ratios diverge between sexes with increasing isokinetic angular velocity.
        J Sci Med Sport. 2008; 11: 452-459
        • Lentz T.A.
        • Zeppieri Jr., G.
        • George S.Z.
        • Tillman S.M.
        • Moser M.W.
        • Farmer K.W.
        • et al.
        Comparison of physical impairment, functional, and psychosocial measures based on fear of reinjury/lack of confidence and return-to-sport status after ACL reconstruction.
        Am J Sports Med. 2015 Feb; 43: 345-353
        • Brophy R.H.
        • Schmitz L.
        • Wright R.W.
        • Dunn W.R.
        • Parker R.D.
        • Andrish J.T.
        • et al.
        Return to play and future ACL injury risk after ACL reconstruction in soccer athletes from the Multicenter Orthopaedic Outcomes Network (MOON) group.
        Am J Sports Med. 2012; 40: 2517-2522
        • Feller J.
        • Webster K.E.
        Return to sport following anterior cruciate ligament reconstruction.
        Int Orthop. 2013 Feb; 37: 285-290
        • Webster K.E.
        • Feller J.A.
        Return to level I sports after anterior cruciate ligament reconstruction: evaluation of age, sex, and readiness to return criteria.
        Orthop J Sports Med. 2018 Aug 2; 6 (2325967118788045)
        • Webster K.E.
        • Feller J.A.
        • Leigh W.B.
        • Richmond A.K.
        Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
        Am J Sports Med. 2014; 42: 641-647
        • Zwolski C.
        • Schmitt L.C.
        • Quatman-Yates C.
        • Thomas S.
        • Hewett T.E.
        • Paterno M.V.
        The influence of quadriceps strength asymmetry on patient-reported function at time of return to sport after anterior cruciate ligament reconstruction.
        Am J Sports Med. 2015; 43: 2242-2249
        • Paterno M.V.
        • Huang B.
        • Thomas S.
        • Hewett T.E.
        • Schmitt L.C.
        Clinical factors that predict a second ACL injury after ACL reconstruction and return to sport: preliminary development of a clinical decision algorithm.
        Orthop J Sports Med. 2017; 19 ([2325967117745279])
        • Seil R.
        • Mouton C.
        • Lion A.
        • Nuhrenborger C.
        • Pape D.
        • Theisen D.
        There is no such thing like a single ACL injury: profiles of ACL-injured patients.
        Orthop Traumatol Surg Res. 2016; 102: 105-110
        • Toole A.R.
        • Ithurburn M.P.
        • Rauh M.J.
        • Hewett T.E.
        • Paterno M.V.
        • Schmitt L.C.
        Young athletes cleared for sports participation after anterior cruciate ligament reconstruction: how many actually meet recommended return-to-sport criterion cutoffs?.
        J Orthop Sports Phys Ther. 2017 Nov; 47: 825-833
        • Herbst E.
        • Hoser C.
        • Hildebrandt C.
        • et al.
        Functional assessments for decision-making regarding return to sports following ACL reconstruction. Part II: clinical application of a new test battery.
        Knee Surg Sports Traumatol Arthrosc. 2015; 23: 1283-1291https://doi.org/10.1007/s00167-015-3546-3
        • Bizzini M.
        • Hancock D.
        • Impellizzeri F.
        Suggestions from the field for return to sports participation following anterior cruciate ligament reconstruction: soccer.
        J Orthop Sports Phys Ther. 2012; 42: 304-312
        • Dingenen B.
        • Gokeler A.
        Optimization of the return-to-sport paradigm after anterior cruciate ligament reconstruction: a critical step back to move forward.
        Sports Med. 2017; 47: 1487-1500
        • van Melick N.
        • van Cingel R.E.
        • Brooijmans F.
        • Neeter C.
        • van Tienen T.
        • Hullegie W.
        • et al.
        Evidence-based clinical practice update: practice guidelines for anterior cruciate ligament rehabilitation based on a systematic review and multidisciplinary consensus.
        Br J Sports Med. 2016; 50: 1506-1515
        • Krosshaug T.
        • Steffen K.
        • Kristianslund E.
        • et al.
        The vertical drop jump is a poor screening test for ACL injuries in female elite soccer and handball players: a prospective cohort study of 710 athletes [published correction appears in Am J Sports Med. 2017 Jul;45(9):NP28-NP29].
        Am J Sports Med. 2016; 44: 874-883https://doi.org/10.1177/0363546515625048
        • Paterno M.V.
        • Schmitt L.C.
        • Ford K.R.
        • et al.
        Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport.
        Am J Sports Med. 2010; 38: 1968-1978https://doi.org/10.1177/0363546510376053
        • Gokeler A.
        • Welling W.
        • Benjaminse A.
        • Lemmink K.
        • Seil R.
        • Zaffagnini S.
        A critical analysis of limb symmetry indices of hop tests in athletes after anterior cruciate ligament reconstruction: a case control study.
        Orthop Traumatol Surg Res. 2017; 103: 947-951
        • Zwolski C.
        • Schmitt L.C.
        • Thomas S.
        • Hewett T.E.
        • Paterno M.V.
        The utility of limb symmetry indices in return-to-sport assessment in patients with bilateral anterior cruciate ligament reconstruction.
        Am J Sports Med. 2016; 44: 2030-2038https://doi.org/10.1177/0363546516645084
        • Welling W.
        • Benjaminse A.
        • Lemmink K.
        • Dingenen B.
        • Gokeler A.
        Progressive strength training restores quadriceps and hamstring muscle strength within 7 months after ACL reconstruction in amateur male soccer players.
        Phys Ther Sport. 2019 Aug 9; 40: 10-18
        • Myer G.D.
        • Ford K.R.
        • Barber Foss K.D.
        • Liu C.
        • Nick T.G.
        • Hewett T.E.
        The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes.
        Clin J Sport Med. 2009 Jan; 19: 3-8
        • Novaretti J.V.
        • Franciozi C.E.
        • Forgas A.
        • Sasaki P.H.
        • Ingham S.J.M.
        • Abdalla R.J.
        Quadriceps strength deficit at 6 months after ACL reconstruction does not predict return to preinjury sports level.
        Sports Health. 2018; 10: 266-271
        • Webster K.E.
        • Feller J.A.
        • Leigh W.B.
        • Richmond A.K.
        Younger patients are at increased risk for graft rupture and contralateral injury after anterior cruciate ligament reconstruction.
        Am J Sports Med. 2014; 42: 641-647
        • Cristiani R.
        • Mikkelsen C.
        • Forssblad M.
        • Engstrom B.
        • Stalman A.
        Only one patient out of five achieves symmetrical knee function 6 months after anterior cruciate ligament reconstruction.
        Knee Surg Sports Traumatol Arthrosc. 2019; (Feb 18)https://doi.org/10.1007/s00167-019-05396-4
        • Grevnerts H.T.
        • Terwee C.B.
        • Kvist J.
        The measurement properties of the IKDC-subjective knee form.
        Knee Surg Sports Traumatol Arthrosc. 2015; 23: 3698-3706
        • Kester B.S.
        • Behery O.A.
        • Minhas S.V.
        • Hsu W.K.
        Athletic performance and career longevity following anterior cruciate ligament reconstruction in the National Basketball Association.
        Knee Surg Sports Traumatol Arthrosc. 2017; 25: 3031-3037
        • Arundale A.J.H.
        • Silvers-Granelli H.J.
        • Snyder-Mackler L.
        Career length and injury incidence after anterior cruciate ligament reconstruction in major league soccer players.
        Orthop J Sports Med. 2018; 6 (2325967117750825)
        • Bittencourt N.F.N.
        • Meeuwisse W.H.
        • Mendonca L.D.
        • Nettel-Aguirre A.
        • Ocarino J.M.
        • Fonseca S.T.
        Complex systems approach for sports injuries: moving from risk factor identification to injury pattern recognition-narrative review and new concept.
        Br J Sports Med. 2016; 50: 1309-1314