The Knee
Volume 18, Issue 1 , Pages 11-14, January 2011

Effect of voluntary soft tissue tension and articular conformity after total knee arthroplasty on in vivo anteroposterior displacement☆☆☆

  • Y. Ishii

      Affiliations

    • Ishii Orthopaedic and Rehabilitation Clinic, Japan
    • Corresponding Author InformationCorresponding author. Ishii Orthopaedic and Rehabilitation Clinic, 1089 Shimo-Oshi, Gyoda, Saitama 361-0037, Japan. Tel.: +81 485 55 3519; fax: +81 485 55 3520.
    • ,
  • H. Noguchi

      Affiliations

    • Ishii Orthopaedic and Rehabilitation Clinic, Japan
    • ,
  • M. Takeda

      Affiliations

    • Ishii Orthopaedic and Rehabilitation Clinic, Japan
    • ,
  • H. Kiga

      Affiliations

    • Ishii Orthopaedic and Rehabilitation Clinic, Japan
    • ,
  • S.I. Toyabe

      Affiliations

    • Division of Information Science and Biostatistics, Niigata University Graduate School of Medical and Dental Sciences, Japan

Received 27 August 2009; received in revised form 1 December 2009; accepted 17 December 2009. published online 21 January 2010.

Article Outline

Abstract 

The in vivo relationship between the degree of voluntary soft tissue tension and articular conformity after total knee arthroplasty (TKA) and anteroposterior (AP) displacement was simultaneously investigated by analyzing LCS prostheses (posterior cruciate ligament-sacrificing rotating platform design) in 20 knees from 20 patients. AP displacement was measured using the KT-2000 arthrometer, at 30° and 75° flexion, while patients were conscious and under anesthesia; 30° flexion was regarded as high conformity and 75° as low conformity. Mean displacements at 30° and 75° were 5.1mm and 7.0mm, respectively, in conscious patients, and 6.7mm and 7.7mm, respectively, in patients under anesthesia. AP displacement was significantly associated with soft tissue tension (p=0.026) and conformity (p=0.001). No interaction was observed between the two variables (p=0.193). Surgeons should recognize that AP displacement is greater in anesthetized patients than in conscious patients, regardless of the degree of conformity, and that higher conformity shows less displacement, regardless of the degree of soft tissue tension. These results may help surgeons to determine the intra-operative AP displacement required for proper postoperative displacement in the current prosthetic design.

Keywords: Soft tissue tension, Conformity, Anteroposterior displacement, KT-2000 arthrometer, PCL-sacrificed total knee arthroplasty

 

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1. Introduction 

Proper anteroposterior (AP) joint displacement is an important indicator of good clinical outcome following total knee arthroplasty (TKA) [1], [2], [3], [4], [5], [6], [7], [8], [9]. Factors affecting the degree of AP displacement can be divided into two categories: soft tissue factors, such as the retention of ligaments, muscle tension, and capsules, and hard tissue factors, such as the component geometry, movement of tibial inserts, and the position of the components. Recently, the constraints of implant design [10], muscle activity, and ligamentous tension [11] were reported to affect kinematics in patients with successful TKAs, using fluoroscopy.

Although anesthesia would tend to exclude the contributions of voluntary soft tissue tension, its effect on knee-joint displacement has not been studied in TKA, although it has been studied in the repair of the anterior cruciate ligament [12], [13], [14], [15]. Additionally, although only a few studies refer to the effects of geometry on in vivo AP laxity [3] or stability [16], they have examined only the effect of soft tissue condition in conscious individuals. Thus, we examined the in vivo relationship between the degree of voluntary soft tissue tension and articular conformity after TKA and AP joint displacement simultaneously and clarified the interaction between the two variables. Our hypothesis was that the less soft tissue tension and the lower articular conformity after TKA revealed the larger in vivo AP joint displacement.

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2. Materials and methods 

We analyzed LCS prostheses (posterior cruciate ligament [PCL]-sacrificing, rotating platform design) in 20 knees of 20 patients. The current prosthetic design is constrained in the AP axis and unconstrained in the rotational axis. In the current system, there is full contact between the femoral component and the tibial insert from 0° to 30°, and the geometry of the prostheses involves a progressive posterior decrease in the radius of curvature of the femoral condyle, and a decrease in the constraint with flexion between the tibial and femoral components (Fig. 1) [17]. Thus, 30° is regarded as high conformity, and 75° as low conformity (Fig. 2).

[From Pappas MJ. Engineering design of the LCS replacement. In Hamelynck KJ, Stiehl JB, (eds) LCS mobile bearing knee arthroplasty. 25years of worldwide experience. Springer Berlin, Springer-Verlag, 2002, p. 48].

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  • Fig. 2 

    Sagittal plane view of the LCS rotating platform TKA demonstrating femoral–tibial congruence at 30° (left, full contact) and 75° (right, partial contact) of flexion.

All of the TKA procedures were judged to be clinically successful (Hospital for Special Surgery [HSS] scores >90) [18], with no ligamentous instability or pain at the time of measurement. All surgeries were performed by a single surgeon (Y.I.), using a standardized technique, including the necessary soft tissue release for proper balance. Proper intra-operative AP stability was confirmed manually, although it was not quantified intra-operatively. In all knees, the femoral components were fixed without cement and the tibial components were fixed with cement. No revision knee replacement or conversion from high tibial osteotomy was included in the study. The clinical characteristics of the patients are summarized in Table 1 [19].

Table 1. Patient characteristics.
ParameterPCLS
Knees/patients20/20
Gender: male/female5/15
Mean age (range) in years73 (66–82)
Mean flexion±SD111°±13°
HSS score±SD92±2
Mean sagittal alignment*
Femur4.8
Tibia82

*Radiographic analysis was performed using the Knee Society Radiographic Assessment [19]. PCLS, posterior cruciate ligament-sacrificing prosthesis; HSS, Hospital for Special Surgery.

Informed consent, including a description of the protocol and potential arthrometer-related complications, was obtained from all patients. We also received institutional review board approval.

AP displacement was measured using a KT-2000 arthrometer (MED metric, San Diego, CA), following a standard protocol, at 30° and 75° flexion. Both angles were confirmed with a goniometer during each testing of the arthroplasty. An anterior force of 133N and a posterior force of 89N were applied while all patients were under general anesthesia, and then after they had regained consciousness. The measurement under anesthesia was performed during TKA surgery on the contralateral side (Fig. 3). The average interval between initial TKA and the final measurement was 19months (range, 6–66months). We performed measurements in conscious patients within the first week following surgery on the contralateral side of the knee. Using the KT-2000, all patients were observed to relax their quadriceps. The same observer performed all tests, to eliminate inter-observer variation. Three measurements were made and subjected to statistical analysis; intra-subjective errors were <1mm. AP joint stability was analyzed by two-way analysis of variance (ANOVA) for repeated measures (triplicate experiments).

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  • Fig. 3 

    Anteroposterior displacement was measured using a KT-2000 arthrometer at 75° flexion. An anterior force of 133N and a posterior force of 89N were applied while the patients were under anesthesia. The measurement under anesthesia was performed just before TKA surgery on the contralateral (right) side.

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3. Results 

The mean AP displacements at 30° and 75° in conscious patients were 5.1mm (range, 1.9–9.3mm) and 7.0mm (range, 2.7–11.5mm), respectively, compared to 6.7mm (range, 1.7–12.4mm) and 7.7mm (range, 3.1–14.0mm), respectively, in anesthetized patients (Table 2, Fig. 4). AP joint displacement was significantly associated with soft tissue tension (p=0.026) and constraint (p=0.001). No interaction was observed between the two variables (p=0.193; Table 3).

Table 2. Mean anteroposterior displacement measurements at 30° and 75° of flexion, performed while conscious and under anesthesia.
Total displacement (mean±SD)30°75°
Conscious5.1±2.1mm7.0±2.0mm
Under anesthesia6.7±2.7mm7.7±3.4mm
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  • Fig. 4 

    Three measurements of mean total displacement (TD) were made while conscious (A, 30° of knee flexion; B, 75° of knee flexion) and while under anesthesia (C, 30° of knee flexion; D, 75° of knee flexion).

Table 3. Statistical analysis (two-way analysis of variance for repeated measures) for experiments performed in triplicate.
VariablesFSignificant (p value)
Soft tissue tension5.80.026
Conformity of joint motion15.60.001
Interaction1.80.193

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4. Discussion 

Relationships between AP knee AP displacement after TKA and clinical results have been examined in vitro [20], theoretically [7], [16], and in vivo [1], [2], [3], [4], [5], [6], [7], [8], [9]. Additionally, the effect of AP displacement has been reported mainly to detect ligament injuries [12], [13], [14], [15]. Recently, AP joint displacement was identified as an important indicator of good clinical outcome following TKA [3]. Approximately 5–10mm is believed to be the preferred value for TKA in many studies using various arthrometers [1], [2], [3], [4], [5], [6], [7], [8], [9], [16]. In a comparative study of three prosthetic designs, Warren et al. [7] concluded that anteroposterior laxity in excess of 5mm in the prosthetic knee was desirable for unimpaired joint function. They measured 6.5mm, 5.5mm, and 5.1mm for the Kinemax, Insall–Burstein, and Oxford Meniscal knees, respectively, although the measurements were made using a different arthrometer from that used in the present study. A study of displacement after TKA using a KT-1000 or KT-2000 system revealed between 3 and 10mm. In the Oxford Meniscal knee, White et al. [8] reported anterior displacement averaging 5.4mm in the anterior cruciate ligament normal group and 7.3mm in the ligament-deficient group, at 25° of knee flexion. Matsuda et al. [4] reported on 19 knees repaired using the Miller/Galante I prosthesis via TKA, with a minimum follow-up of 87months; the mean values at 30° and 75° of knee flexion were 10.1mm and 8.0mm, respectively, with PCL-retaining prostheses. Ishii et al. [2] reported on 77 knees repaired with the Genesis I prosthesis in TKA, with a minimum follow-up of 5years; the mean values at 30° and 75° of knee flexion were 5.8mm and 4.8mm, respectively, with PCL-retaining prostheses, and 5.3mm and 3.4mm, respectively, with PCL-substituting prostheses. For the current design, low-contact stress TKA, Matsuda and Ishii [5] found that the mean displacement values in the sagittal plane were 10.5mm and 9.3mm at 30° and 90° of flexion, respectively, with a PCL-retaining prosthesis, and 9.8mm and 9.7mm, respectively, with a PCL-sacrificing prosthesis, using a Telos arthrometer (Telos, Medizinisch-Technische GmbH, Griesheim, Germany). Recently, Jones et al. [3] concluded that the optimal sagittal laxity in a cruciate-retaining TKA was between 5 and 10mm, with 5.4–9.9years of follow-up.

However, because most of these studies [1], [2], [3], [4], [5], [6], [7], [8], [9], [16] evaluated AP joint displacement in various constraints and only in conscious patients, the effects of voluntary soft tissue contraction may not have been eliminated. The current study is the first one that evaluated the in vivo relationship between the degree of soft tissue tension and articular conformity after TKA and AP displacement simultaneously and clarified the interaction between two variables.

The effect of anesthesia, which would tend to exclude the contributions of voluntary soft tissue tension, on knee AP joint displacement has primarily been studied as it applies to the repair of the anterior cruciate ligament [12], [13], [14], [15]. These previous studies determined the cut-off point for ACL injury [14] and the diagnostic accuracy of the KT-1000 arthrometer [15].

In this study, AP in vivo joint displacement was significantly associated with soft tissue tension (p=0.026), with no interaction between articular conformities (p=0.193); thus, AP displacement under anesthesia showed a significantly larger range than in conscious patients, regardless of the different conformities. If the displacement values under anesthesia could be regarded as the same condition as those in an intra-operative one (that is, to exclude the contribution of voluntary soft tissue tension), the results of this study indicate that intra-operative displacement is greater than displacement in conscious patients. Thus, surgeons should be aware that intra-operative AP displacement is larger than the displacement defined as optimal in the awake condition, regardless of the degree of conformity.

Moreover, the current study showed that AP joint displacement was significantly associated with conformity (p=0.001), with no interaction with soft tissue tension (p=0.193). Surgeons should take into account that higher (or lower) conformity has less (or larger) displacement, regardless of the degree of soft tissue tension. We should not, therefore, determine the proper AP displacement consistently for every PCL-sacrificed TKA, but rather according to the degree of conformity both under anesthesia and while conscious, although this may not hold true for cruciate-retaining TKA designs. Additionally, surgeons should pay careful attention to changes in the conformity of the femoral and tibial couple with flexion, especially in the current prostheses versus those of femoral component design having a single radius.

This study has some limitations. First, the muscle tension might be also affected by knee flexion angle as well as with or without anesthesia. The quadriceps might be tighter with the knee flexed and hamstrings tighter with the knee extended. Further studies are needed on how this may have affected the differences observed at the two flexion positions. Second, the interval between measurements of AP displacement varied between 6 and 66months, which may have affected our results. However, Mizu-uchi et al. [6] reported that AP displacement in joints repaired using the PCL-retaining design without clinical complaints did not change significantly over a 5-year period. Additionally, we did not refer to the effect of differences in soft tissue structures, such as in PCL-retaining or -substituting designs, on AP displacement, because this study was intended to analyze only PCL-sacrificed designs. We believe that AP displacement after TKA may also be controlled by the geometry of the prosthesis, soft tissue structures, and their tension, as previously reported by Lafortune et al. [21] in relation to knee kinematics. Moreover, displacement was only measured when there was no axial load, due to the characteristics of the arthrometer that we used. Assessment of displacement under load-bearing conditions may provide a better understanding of the factors influencing clinical performance during activity.

In conclusion, this in vivo study demonstrated that AP displacement after TKA was controlled by the degree of conformity and soft tissue tension, without any apparent interaction between these factors. Surgeons should recognize that intra-operative displacement is greater than in conscious patients, regardless of the degree of conformity, and that higher conformity has less displacement, regardless of the degree of soft tissue tension. These results may help surgeons to determine the intra-operative AP displacement required for proper postoperative displacement in the current prosthetic design.

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☆ No benefits or funds were received in support of the study.

☆☆ Informed consent was obtained from all patients, with institutional review board approval.

PII: S0968-0160(09)00252-X

doi:10.1016/j.knee.2009.12.006

The Knee
Volume 18, Issue 1 , Pages 11-14, January 2011

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