The role of the popliteus tendon in total knee arthroplasty: a cadaveric study

Abstract

Purpose

this study was conducted to investigate the influence of the popliteus tendon (PT) on the static stability of total knee arthroplasty (TKA).

Methods

twenty knees were used. In 10 right knees, a cruciate-retaining (CR) TKA trial prosthesis was implanted; in the other ten knees (left knees), the posterior cruciate ligament was cut and a posterior substitution (PS) TKA trial prosthesis was implanted. Lamina spreaders were set at 100 N of tension, one on the medial and one on the lateral articular space. Gaps were then measured with a caliper before and after PT sectioning.

Results

the correlation between femoral dimensions and popliteus insertion distance from articular surfaces was measured with the Pearson correlation index and considered significant.

In the CR-TKA group, medial and lateral gap measurements showed a significant increase after PT sectioning both in flexion and in extension. In the PS-TKA group, lateral gap measurements showed a significant increase after PT sectioning both in flexion and in extension, while the medial gap measurements increased significantly only in flexion.

Conclusions

PT sectioning destabilized both the lateral and the medial aspects of the knee. A greater effect was observed in the lateral compartment. The most statistically reliable effect was observed with the knee in flexion. In addition, we observed that preserving the PCL does not prevent lateral gap opening after PT sectioning.

Clinical relevance

PT should always be preserved when performing a TKA, because its resection can affect gap balancing, in flexion and in extension.

Type of study

controlled laboratory study.

Introduction

Soft-tissue balancing is a crucial step when performing a total knee arthroplasty (TKA). The current literature devotes little attention to the popliteus tendon (PT), whose function is still not completely understood. It is part of the posterolateral corner of the knee, a useful stabilizing structure in varus and external knee rotation, and it is commonly considered both a static and a dynamic constraint. It prevents anterior subluxation of the femur on the tibia during knee flexion (1), internal rotational laxity and varus deviation (2, 3). Functionally, the PT has been shown to contribute to stabilization of the knee in the range of movement (ROM) from 60 to 90 degrees of flexion (4). The tendon can be recognized on the posterolateral aspect of the knee; its insertion is located on the lateral facet of the lateral condyle, anteriorly and distally to the lateral collateral ligament (LCL). The integrity of the PT can be jeopardized during TKA bone cuts on account of its proximity to the articular space; this is particularly true in small patients in whom the insertion is close to the joint line (5). The purpose of the present study was to study the influence of the PT on the static stability of TKAs in cadaveric specimens. The hypothesis of the study was that the PT influences stability in the lateral part of the knee in both flexion and extension.

Methods

This study was conducted using 20 knees (10 whole cadavers): 4 males and 6 females. The following exclusion criteria were applied: previous surgeries, flexion contracture (> 5°) or deficit (< 110°), severe varus or valgus laxity [>grade 2 according to the Knee Society Score (KSS)], and severe malalignment (> 10° of varu/valgus deformity).

Age, gender, ROM, limb alignment, osteoarthritic changes and knee laxity were recorded preoperatively as baseline characteristics. ROM was measured using a manual goniometer. Fluoroscopy was used to assess limb alignment with the hip-knee-ankle angle method, and to identify osteoarthritic changes of the knee, which were classified using the Kellgren-Lawrence classification (6). With the knee in full extension and under maximum manual stress, knee stability was evaluated on the coronal plane and graded according to the KSS.

All the surgical procedures were performed by the senior author (RR) using Zimmer NexGen TKA instrumentation (Zimmer Inc., Warsaw, IN, USA). A median skin incision and a medial parapatellar capsular incision were performed. No soft-tissue balancing was necessary during any of the procedures. The PT insertion on the lateral condyle was clearly visualized by exposing the lateral condyle. Thereafter, the following measures were obtained: the distance between the PT insertion and the distal and posterior articular surfaces, respectively; the medial-lateral width of the distal femur, and the anterior-posterior depth of the lateral femoral condyle (Fig. 1). In 10 knees (right knees), a cruciate-retaining (CR) TKA trial prosthesis was implanted; in the other 10 knees (left knees), the posterior cruciate ligament (PCL) was cut and a posterior substitution (PS) TKA trial prosthesis was implanted. Alignment of bone cuts was obtained using intramedullary guides and resulted in a perpendicular cut of the tibia and a 5° valgus distal femoral cut. The femoral component was externally rotated by 3° using a posterior condylar reference guide. Flexion and extension gaps in both groups (CR and PS) were evaluated using two dynamometric laminar spreaders. Measurements were obtained first with the PT retained and then with the PT sectioned (Fig. 2).

Fig. 1.

Flexion gap with intact popliteus tendon.

Fig. 2.

Flexion gap after cutting the popliteus tendon.

Lamina spreaders were set at 100 N of tension, one on the medial and one on the lateral articular space. Gaps were measured with a digital caliper by three independent observers and the mean values were used for data analysis. The tibial tray was positioned using the posterolateral corner-locked technique and the femoral component was lateralized as much as possible. A 10-mm polyethylene trial prosthesis was used in all cases. Each knee achieved good stability and a full ROM.

Data analysis

All statistical analyses were performed using Microsoft Excel 2011 for Apple v. 14.2.2. Descriptive statistics of continuous variables were expressed in mean values and standard deviations (SDs). Pearson’s correlation coefficient was used to correlate the PT insertion position with the femoral dimensions. A paired Student’s t-test was performed to evaluate changes in flexion and extension gaps in both groups (CR and PS) after PT resection. A p value of <0.05 was considered statistically significant.

Results

The mean age of the specimens was 88 ± 4 years (range, 82–93 years). Alignment of the lower limb was neutral (0–2° of valgus) in six knees, 6° of valgus in eight knees, 8° of valgus in two knees, and 10° of valgus in four knees. Knee extension was complete in all the specimens; knee flexion ranged from 120–130° with no contractures.

Knee osteoarthritis was grade 0 in 10 knees, grade 1 in four knees, grade 2 in two knees, and grade 4 in four knees. The size of the trial femoral component was E in eight knees, F in eight knees, and D in the last four knees, with no side-to-side differences.

The average distance of the PT insertion from the distal articular surface was 14.2 ± 2.6 mm (range, 11.5 – 17.9 mm), while the distance from the posterior surface was 14.5 ± 5.6 mm (range, 10.3 – 21.0 mm). The average distal femur width was 74.8 ± 23.9 mm (range, 67.3 – 82.3 mm) and the average lateral condyle anterior-posterior depth was 65.3 ± 5.5 mm (range, 54.6 – 71.7 mm).

The thickness of the lateral femoral bone resections averaged 7.2 mm distally and 7.0 mm posteriorly. The distance of the PT insertion from the bone resection averaged 7 mm from the distal cut and 5.5 mm from posterior condylar cut. A significant correlation was found between femoral depth and the distance of the PT insertion from the articular surfaces (Tab. 1).

Table 1.

Analysis of correlations between the distance of the insertion of the popliteus tendon from the articular surface, both posteriorly and distally, and femoral dimensions.

Correlation coefficient (p value)	Femoral width (anterior-posterior)	Femoral depth (lateral)
Distal articular surface	0.77 (p=0.49)	0.91 (p<0.0001)
Posterior articular surface	0.82 (p=0.78)	0.82 (p<0.0001)

In the CR-TKA group, the measurements of the medial and lateral gaps increased significantly after PT section, both in flexion and in extension (Tab. 2). In the PS-TKA group, the lateral gap measurements showed a significant increase after PT section, both in flexion and in extension, while the medial gap measurements increased significantly only in flexion (Tab. 3).

Table 2.

Medial and lateral gap measurements before and after popliteus tendon section in flexion and extension for CR TKA.

Gap measurement (mm)		Intact tendon	Resected tendon	p value
Flexion	Lateral	21.7 ± 1.9	23.8 ± 1.8	0.003
	Medial	21.8 ± 1.2	22.6 ± 1.1	0.03
Extension	Lateral	21.3 ± 0.8	22.7 ± 2.1	0.016
	Medial	19.9 ± 0.9	21.6 ± 2.2	0.03

Table 3.

Medial and lateral gap measurements before and after popliteus tendon section in flexion and extension for CR TKA.

Gap measurement (mm)		Intact tendon	Resected tendon	p value
Flexion	Lateral	21.1 ± 2.3	23 ± 1.9	0.003
	Medial	20.1 ± 1.6	21.6 ± 0.9	0.005
Extension	Lateral	21.9 ± 2.3	23.6 ± 1.7	0.03
	Medial	23.3 ± 2.4	24.2 ± 2.8	0.06

Discussion

The PT is known to be an important knee stabilizer, on account of its position and its function in knee biomechanics. Numerous articles report isolated ruptures of the PT, particularly in athletes. Other authors have observed that a full or partial rupture of this structure in normal knees did not have significant consequences on knee stability (7–10).

However, since the biomechanics of a normal knee are different from those of a TKA, it is not immediately clear how a PT rupture can affect TKA stability. In this report, we demonstrated that PT sectioning destabilized both the lateral and medial aspects of the knee. A greater effect was observed in the lateral compartment both in extension and flexion. The most statistically reliable effect was observed with the knee in flexion. In addition, the authors proved that preserving the PCL does not prevent lateral gap opening after PT sectioning.

Flexion and extension gap imbalances after PT sectioning can make soft-tissue balancing more challenging. Data analysis revealed a greater influence of the PT in the lateral compartment, especially in flexion. Tendon sectioning caused a lateral gap increase at 90° of flexion of 2.1 mm in the CR group (p=0.003) and 1.64 mm in the PS group (p=0.003). The only non-significant increase in joint opening after PT sectioning was observed in the medial compartment of PS TKAs with the knee in extension (p= 0.06).

The resulting knee instability may lead the surgeon to increase the level of constraint of the implant.

Kesman et al. (11) reported no clinical difference in TKAs after PT rupture. However, no gaps were measured, which made it impossible to measure the influence of the PT in their study. On the basis of our study, we are not able to state with certainty how the PT influences the outcome of TKA. However, in the literature, significantly lower function was reported in patients in whom the PT was sectioned during TKA surgery (12).

In the presence of varus knee alignment it is crucial to avoid sectioning the PT, as this would produce asymmetrical spaces both in extension and flexion on the lateral aspect of the knee. Given the importance of the PT in stabilizing both flexion and extension gaps on the medial knee compartment, its sacrifice should also be avoided also in valgus alignment.

PCL preservation or sacrifice is a much debated topic in TKA surgery. It is well known that PCL retention reduces the flexion gap (13). Considering the PT as an articular retainer of the knee in flexion, this study was conducted to evaluate, albeit in a small cohort of cases, how sectioning this tendon affects the flexion gap in TKA, both with preservation and with sacrifice of the PCL. In order to enhance the validity of the evaluation, each cadaver underwent both a CR-TKA and a PS-TKA. No statistically significant difference was noted between the two groups. Therefore, the central pivot did not appear to influence lateral flexion gap opening in the presence of a concomitant PT lesion.

Tantavisut et al. (5) reported an increased risk of iatrogenic resection of the PT insertion in small knees. This is due to standard cutting blocks that are not suitable for small patients. In our small cohort of specimens we did not observe any iatrogenic transection of the tendon. The distance of the popliteus insertion from the articular space (14.2 and 14.5 mm from the distal and posterior femoral cuts, respectively) was found to be sufficient to avoid iatrogenic damage in this Caucasian population. We can therefore consider bone cut-PT insertion distances of about 5.5 mm posteriorly and 7 mm distally to constitute a “safe zone”. Moreover these distances were found to be directly and strongly correlated with the femoral medial-lateral and anterior-posterior dimensions.

In conclusion, we can say that the PT should always be preserved when performing a TKA. Its resection can affect gap balancing, in flexion and in extension. We observed significant differences in terms of stability, albeit in a small cohort.