Abstract
Introduction
A small medial extension gap (EG) needs posterior soft tissue release to avoid undesirable additional resection of the distal femur in total knee arthroplasty (TKA). However, the effect of this procedure on the EG is not always sufficient because the EG is influenced not only by the posterior soft tissue but also by the medial collateral ligament (MCL). We hypothesize that contracture of the posterior capsule prevents full elongation of the MCL in extension and we investigated the efficacy of posteromedial vertical capsulotomy (PMVC) on the medial EG which separate MCL from the posterior capsule (Fig. 1).
Materials and Methods
The PMVC was performed on 128 knees in which the medial extension gap was considered too small. The EG was initially created with a standard femoral distal cut and tibial cut. To estimate the gaps more precisely before flexion gap (FG) adjustment at the final step of the surgery, we performed a 4 mm precut of the posterior femoral condyle and measured the gaps with the patella reduced after setting a precut trial component that had a usual distal part and 4 mm thick posterior part of the femoral component. This situation was the same as after setting the usual femoral trial component by using the measured resection technique with preservation of the posterior cruciate ligament (PCL) (Fig. 2). The semimembranosus tendon was not released in any cases.
Results
After the precut trial was set to the femur, the average EG and FG were 5.6 ± 2.0 mm and 10.0 ± 2.0 mm, respectively (mean ±SD). After performing the PMVC, the average increase of the EG and FG were 2.3 ± 1.4 mm and 0.1 ± 0.3 mm, respectively. The EG increase was significantly larger than the FG increase (p < 0.001). Twenty eight knees showed a 1 mm or less increase in the EG; however, 100 knees (78 %) had a 2 mm or greater increase in the EG with little increase in the FG. Initial gap difference (FG – EG) showed a positive corelation with EG increase after PMVC (R = 0.51, p < 0.001) (Fig. 3).
Conclusions
To make adequate EG and FG, it is important to understand which soft tissue management is effective to increase the FG or the EG. To increase the FG only, PCL resection is useful. However, the effective methodology of widening the EG without changing the FG is unknown. The EG of the varus knee is influenced by several factors such as tightness of the MCL, the posterior capsule, the semimembranosus tendon and protrusion of the posterior femoral component. In this study, a precut trial component was used to take into account the effect of posterior protrusion of the femoral component and the semimembranosus tendon was not released and we achieved a selective EG increase without changing the FG by the PMVC which allowed the MCL and the posterior capsule to act freely from each other.
