Abstract
Distal femur resection for correction of flexion contractures in total knee arthroplasty (TKA) can lead to joint line elevation, abnormal knee kinematics and patellofemoral problems. The aim of this retrospective study was to establish the contribution of soft tissue releases and bony cuts in the change in maximum knee extension in TKA.
Data were available for 209 navigated TKAs performed by a single surgeon using a medial approach. All patients had the same cemented implant, either CR or PS, which both required a minimum thickness of 10 mm for the tibial and 9mm for the femoral component. Intra-operatively pre- and post-implant extension angles and the size of bone resection were collected using a commercial navigation system. The thickness of polyethylene insert and the extent of soft tissue release performed (no release, moderate and extensive release) were collected from the patient record. A univariate linear regression model was used to predict change in maximum extension from pre- to post-implant.
The mean bone resection was 19mm (15 to 28 mm) (Figure 1).79% of polyethylene inserts were 10mm thick (10 to 16 mm). 71% of knees had no soft tissue release. The mean increase in extension was 5° (11° decrease to 23° increase) (Figure 1). The analysis showed that bone cuts (p<0.001), soft tissue release (p=0.001) and insert thickness (p=0.010) were all significant terms in the model (r2adj=0.170). This model predicted that carrying out a TKA with 19mm bone cuts, 10mm insert and no soft tissue release would give 4.2° increase in extension. It predicted that a moderate release would give a 2.8° increase in extension compared to no release, with an extensive release giving 3.9° increase over no release. For each mm increase in bone cuts the model predicted a 0.8° increase in extension and for each mm increase in insert size a decrease extension by 1.1°.
Preoperative FFC contracture is a frequent condition in TKA that the surgeon has to address either by resecting more bone or by extending soft tissue release to increase the extension gap to fit the knee implant. This analysis of 209 navigated knee arthroplasty showed that both options are suitable to increase the extension gap. The modelling results show that in general to increase maximum extension by the same as an extensive soft tissue release that bone cuts would have to be increased by 4–5mm. However this model only accounted for 17% of the variation in change in extension pre- to post-implant so is poor at predicting outcomes for specific patients. The large variation in actual FFC correction indicates that this relies on factors other than bone cuts and soft tissue releases as quantified in this study.
