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This study aimed to explore the changes in lower limb axial alignment and knee joint function after arthroscopic partial resection of the discoid lateral meniscus.
Methods
Preoperative and postoperative full-length weight-bearing radiographs of the lower limb were obtained from a total of 161 patients with lateral menisci tears from September 2018 to September 2020 who underwent partial meniscal resection. The patients were divided into discoid meniscus group (DMG) and ordinary meniscus group (OMG). The measured mechanical axis deviation (MAD), proximal tibia angle (PTA), and distal femoral angle (DFA) in the axial alignment of the lower limb were determined before and after surgery. Knee joint function on the affected side was determined using the International Knee Documentation Committee (IKDC) subjective scale and the Lysholm knee scoring scale before surgery and at one, six, 12, and 24 months after surgery.
Results
For MAD and DFA, there were statistically significant differences between the preoperative and postoperative findings within each group (P < 0.01); the MAD and DFA were smaller after surgery. The difference in preoperative and postoperative PTA of the DMG was statistically significant (P < 0.01), meaning that the PTA becomes larger after surgery. The differences in preoperative and postoperative PTA of the OMG, preoperative PTA between the two groups, and postoperative PTA between the two groups were not statistically significant (P > 0.05). Intragroup comparisons of the IKDC subjective scale and the Lysholm knee scoring scale before and after surgery revealed significant differences (P < 0.05).
Conclusion
Arthroscopic partial resection of the discoid lateral meniscus is a safe and effective surgical method that can significantly improve knee joint function. Although the axial arrangement of the lower limbs will be slightly changed in the early stage, it will still be within the normal range after surgery in patients with normal lower limb axial alignment. For patients with varus or valgus before surgery, this procedure should be applied with caution.
The discoid lateral meniscus (DLM) is the most common congenital variant of the meniscus, which was first observed in a cadaver specimen by Young in 1889 [
]. The disorder of the circular collagen fiber system in the DLM matrix makes the meniscus two times more prone to tearing up when compared with the normal lateral meniscus (20.4 vs. 9.9%) [
]. Some authors have reported that magnetic resonance imaging or arthroscopic evaluation reveals high rates (79–97%) of bilateral DLM in patients presenting with symptomatic unilateral DLM [
Diagnostic performance of clinical examination and selective magnetic resonance imaging in the evaluation of intraarticular knee disorders in children and adolescents.
]. Although several studies have reported the impact on DLM, they were evaluated using the functional score (Lysholm score and hospital for special surgery knee score) and visual analog scale [
Arthroscopic partial meniscectomy with repair of the peripheral tear for symptomatic discoid lateral meniscus in children: results of minimum 2 years of follow-up.
]). In addition, these studies rarely mentioned the change in force line before and after arthroscopic partial meniscal resection. In this regard, this study aimed to explore the changes in lower limb axial alignment using a full-length X-ray of the lower limbs after treatment of the DLM by arthroscopic partial meniscus. Moreover, Allen et al. used a full-length X-ray of the lower limbs and found that surgical removal of a total meniscus caused changes in lower limb axial alignment [
]. Based on this finding, we hypothesized that the axial alignment of the lower limb might be affected after arthroscopic partial resection of DLM.
2. Materials and methods
2.1 General information
This was a retrospective cohort study conducted at a tertiary level-A teaching hospital in Chongqing China and the institutional review board approved the study protocol. The research study was registered in the Chinese Clinical Trial Registry (registration ID: ChiCTR2100049003). To be included in the study, patients must have undergone (1) arthroscopy, (2) intraoperative diagnosis of simple lateral meniscus tear, with partial resection as treatment, (3) both preoperative and three to five days postoperative standing anteroposterior radiography, and (4) unilateral lower limb surgery only. Converesely, the exclusion criteria were patients who had ligament injury combined with synovitis, cartilage injury, or other intra-articular diseases during arthroscopy, postoperative joint infections, and lower limbs with varus or valgus deformity before surgery. We reviewed the medical records obtained from September 2018 to September 2020. We selected a total of 161 patients who had undergone arthroscopic partial meniscus resection to treat lateral meniscus tears. The patients were divided into the discoid meniscus group (DMG) and ordinary meniscus group (OMG). The study was reported in line with the criteria for strengthening the reporting of cohort studies in surgery (STROCSS) [
All patients signed a written informed consent before undergoing the arthroscopic surgery. All procedures were performed under general anesthesia and in the supine position. A standardized diagnostic arthroscopy was performed to confirm the diagnosis and classify the tear size. A partial tear is defined as a tear that accumulates only in the non-vascularized area of the meniscus. If the meniscus is torn in both the non-vascularized and vascularized area, it is a complete tear. For partial tears, partial meniscal resection was performed. A torn DLM was treated by trimming the meniscus into a semilunar shape and the width of the retained meniscus was about 5 mm (Figure 1). All operations were performed by a single surgeon and the rehabilitation program was supervised by a single therapist.
Figure 1Partial meniscus resection for treatment of simple lateral meniscus tear.
All measurements were performed using an image measurement tool (Digimizer, Bruges, Belgium). We determined three positions on the full length of the lower limbs. First, we determined the center of the femoral head (A) (Figure 2(a)) and found that all femoral heads were morphologically normal. The other two positions were the center of the knee joint (B) and the center of the ankle joint (C) (Figure 2(b) and (c)), where B is the midpoint of the medial and lateral femoral condyles, and C is the midpoint of the ankle fossa.
Figure 2(a) Center of the femoral head. (b) Center of the knee joint. (c) Center of the ankle joint (C).
The mechanical axis of the lower limb is the distance from the center of the femoral head to the center of the ankle joint and this axis typically intersects or is near the center of the knee joint in normal cases. If the knee joint center is lateral to the mechanical axis, then the knee is in a varus inclination and when it is medial to the axis, the knee is in a valgus inclination [
]. Mechanical axis deviation (MAD) was defined as the distance from the knee joint center to the mechanical axis (If the knee joint center is outside the mechanical axis, the value is positive, otherwise it is negative) (Figure 3(a)).
Figure 3The mechanical axis deviation (MAD) (a), distal femur angle (DFA) and proximal tibia angle (PTA) of the lower limb (b).
We defined the lateral angle between the femoral mechanical axis and the line of the femur condyle as distal femoral angle (DFA). We also defined the lateral angle between the tibial mechanical axis and the tibial plateau as proximal tibial angle (PTA). Under normal anatomy, the line of the femoral condyle is parallel to the tibial plateau (Figure 3(b)). Paley found that the DFA of normal people is 88° (85–90°), while the PTA is 93° (90–95°). If the angle is too large or too small, it indicates that the knee joint is either varus or valgus [
For all patients, MAD, DFA, and PTA were measured before and after surgery. Anatomic landmarks were marked manually by two independent readers. Subsequently, the aforementioned angles and distances were measured using the same software (Digimizer, Bruges, Belgium). Each reader measured the same parameter twice and the average value was used for further calculation. The knee joint functions on the affected side were scored using the International Knee Documentation Committee (IKDC) subjective scale and the Lysholm knee scoring scale before surgery and one, six, 12, and 24 months after surgery.
2.7 Statistical analysis
All analyses were performed using SPSS 17.0 software (SPSS, IBM, Chicago, IL, USA). Measurements with a normal distribution were expressed as mean ± standard deviation Continuous variables between two groups were compared by independent Student’s t-test. Preoperative and postoperative continuous data were analyzed by paired Student’s t-tests. Intragroup comparisons were performed by a nonparametric test. The level of significance was set at P < 0.05. To evaluate the intra- and inter-observer agreement of the measurements, the intra-class correlation coefficients (ICCs) of each parameter were calculated using the two-way mixed-effects model. ICCs ≥ 0.7 were considered sufficient for reliability. The sample size was calculated based on these data from Zhang’s results, which detected a mean MAD change of 6.05 mm before and after surgery [
]. With these assumptions, we estimated that at least a total of 116 patients would provide 90% power to detect a significant difference (at 5% type I error, 10% type II error, P = 0.05, two-sided). To compensate for any non-evaluable patient, the authors planned to enroll 10% more patients.
3. Results
3.1 General characteristics
There were 98 patients in the OMG and 63 patients in the DMG. All patients received arthroscopic partial meniscal resection and underwent serial clinical and radiological follow up at regular intervals. Their data were retrospectively collected for this study. The detailed characteristics of the subjects are presented in Table 1. Patients in the two groups were similar in terms of age, sex, height, weight, and body mass index.
Table 1General characteristics of patients.
OMG
DMG
P
No.
98
63
–
Age
48.22 ± 5.69
46.87 ± 4.90
0.123
Sex (F/M)
64/34
44/19
0.550
Height
161.52 ± 2.91
160.86 ± 2.80
0.154
Weight
59.29 ± 3.96
58.67 ± 3.22
0.293
BMI
22.72 ± 1.28
22.67 ± 1.15
0.812
BMI, body mass index; DMG, discoid meniscus group; OMG, ordinary meniscus group.
There was no significant difference between the OMG and DMG in terms of MAD, whether preoperative or postoperative (P > 0.05); however, there were statistically significant differences between preoperative and postoperative MAD within each group (P < 0.01) (Table 2). In the OMG, the average preoperative and postoperative MAD was 5.26 ± 2.57 mm and 4.44 ± 2.71 mm, respectively. In the DMG, the average preoperative and postoperative MAD was 5.15 ± 3.12 mm and 4.04 ± 3.26 mm, respectively.
Table 2Mechanical axis deviation (MAD) preoperative and postoperative.
There was no significant difference in preoperative DFA between the OMG and DMG (P > 0.05); however, there was a statistically significant difference in DFA between the two groups after surgery. In addition, there were statistically significant differences in DFA between each group before and after surgery (P < 0.01). The OMG average preoperative and postoperative DFA was 87.44 ± 1.30° and 86.95 ± 2.35°, respectively, whereas the DMG preoperative and postoperative DFA average was 87.31 ± 1.42° and 86.08 ± 1.20°, respectively (Table 3).
Table 3Distal femoral angle (DFA) preoperative and postoperative.
The preoperative and postoperative PTA of the DMG was statistically significant (P < 0.01), while the preoperative and postoperative PTA of the OMG, preoperative PTA between the two groups, and postoperative PTA between the two groups were not statistically different (P > 0.05). The average PTA of the OMG before and after surgery was 92.55 ± 1.43° and 92.57 ± 1.30°, respectively. The average PTA of the DMG before and after surgery was 92.58 ± 1.34° and 92.84 ± 1.41°, respectively (Table 4).
Table 4Proximal tibia angle (PTA) preoperative and postoperative.
With respect to MAD, DFA, and PTA, the ICCs were greater than 0.7, thus suggesting high intra- and inter-observer reliabilities for these parameters (Table 5).
Table 5Intra- and inter-observer agreement for lower limb axial alignment parameters.
Using the IKDC subjective scale or the Lysholm knee scoring scale, there were significant differences in the different time periods in each group (P < 0.01). Considerable improvement was observed after surgery in each group; however, there was no significant difference in the IKDC subjective scale and the Lysholm knee scoring scale between the groups at the same stage (P > 0.05) (Table 6).
This study demonstrates that regardless of the shape of the meniscus, partial meniscus resection will affect lower limb axial alignment. In this study, almost all knee joints have valgus changes after partial meniscus resection; for the ordinary meniscus tear, the valgus changes are mainly on the femoral side, while the angle on the tibial side does not change significantly. However, for patients with the discoid meniscus, the femur side and tibia side all have valgus after surgery. Therefore, this change is more obvious after partial meniscus resection for discoid meniscus. However, all valgus changes were small and all patients’ lower limb axial alignments were still within the normal range after surgery. These results will provide orthopedists with data on changes in lower limb axial alignment after partial meniscus resection. To our knowledge, this was the first study measuring the femoral angle and tibial angle separately, such that lower limb axial alignment changes after partial meniscus could be studied in detail. In addition, this was also the first study using professional measurement software to measure the center of the circle, which can reduce the measurement error. In terms of knee function recovery, partial meniscus resection yielded good results in both groups; moreover, satisfactory results can be achieved within six months after surgery (Figure 4).
Figure 4Alteration in the axial alignment of the lower limb and knee joint function in all groups before and after operation.
Previous studies opine that the meniscus plays an important role in the load transmission, shock absorption, and maintenance of knee stability between the knee cartilage and shock absorption [
]. Changes in the contact between the cartilage and meniscus during partial meniscus resection will result in changes in the force of the meniscus, which, in turn, result in changes in lower limb axial alignment. Wilson et al. believe that most of the load (84–85%) of the knee is borne by the meniscus. The contact area was reduced by 40–62% after the meniscus was removed [
]. Based on the findings from this present study, this change mainly affects the angle of the femur side. The partial meniscus resection of the discoid meniscus had more area removed, resulting in more obvious changes in the lower limb force line after surgery. At the same time, we found that patients with DLM also had changes in the tibial angle after surgery and the mechanism underlying this change remains unknown. This may be due to the removal of more areas of the discoid meniscus, which results in a greater bias in the femoral condyle.
In this study, we did not include cases of knee varus or valgus, considering that previous studies believed that the knee varus or valgus would accelerate the progression of osteoarthritis. Sharma et al. [
] studied a total of 2958 knees (1752 participants) and found that the varus and valgus alignment each increased the risk of progression in the biomechanically stressed compartment. Wang et al. [
] observed that the varus angle was notably reduced after meniscectomy of the DLM in adolescents and thus the original varus deformity was somewhat rectified. Our study also obtained similar results; however, we screened the cases without deformity of the knee joint before surgery. Although there was a small amount of knee joint valgus after the surgery, lower limb axial alignment was still within the normal range. In other words, the changes of the knee valgus angle of the patients after partial meniscus resection were not sufficient to correct the knee varus deformity. We did not include patients with medial and lateral deformities because we could not determine whether the pain of the patient’s knee joint was due to pure meniscus tear. Moreover, it may be difficult to achieve the desired results with only partial meniscus resection. Prakash et al. [
] conducted a study on a total of 674 patients undergoing high tibial osteotomy and concluded that the high tibial osteotomy could result in accelerated lateral compartment osteoarthritis in patients with the complete discoid meniscus. Therefore, patients with knee varus or valgus who have a meniscus tear should be treated with caution.
It has been previously reported that a total meniscectomy was associated with more severe knee joint degeneration [
]. In our study, the functional knee joint scores increased over time, regardless of whether the patients have ordinary meniscus or discoid meniscus. All patients achieved satisfactory results in the knee joint function six months after the surgery. Furthermore, two years after the surgery, the knee joint function still showed a slow upward trend and, in the same time period, there was no statistical difference in function between the two groups (P > 0.05).
This study has several limitations. First, the sample size included is small and the results may be biased. Second, we only measured lower limb axial alignment in the early stages after the surgery and did not follow up with the patients for a long duration to ascertain whether lower limb axial alignment changed after the surgery. Based on the retrospective nature of this study, we cannot determine the specific cause of preoperative pain in patients with varus or valgus lateral meniscus tear; thus, we did not include it in the study. In the future, there is a need to further design randomized controlled trials to observe the effect of partial meniscus resection under different lower limb axial alignments.
5n conclusion, arthroscopic partial resection of DLM is a safe and effective surgical method that can significantly improve knee joint function. Although the axial arrangement of the lower limbs may slightly change in the early stages, for patients with normal lower limb axial alignment, it will still be within the normal range after surgery. For patients with varus or valgus before surgery, this procedure should be applied with caution.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author contributions
Y.H. wrote the paper. Y.F. and Y.Z. provided the cases. Y.H. and H.C. provided the medical images and interpretation of the data. W.B. and Y.H. reviewed and edited the manuscript. All authors read and approved the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
Young R.B.
The External Semilunar Cartilage as a Complete Disc.
Diagnostic performance of clinical examination and selective magnetic resonance imaging in the evaluation of intraarticular knee disorders in children and adolescents.
Arthroscopic partial meniscectomy with repair of the peripheral tear for symptomatic discoid lateral meniscus in children: results of minimum 2 years of follow-up.
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