A NEW ALGORITHM FOR IDENTIFYING THE JOINT LINE IN PRIMARY AND REVISION TOTAL KNEE ARTHROPLASTY USING INTER-EPICONDYLAR DISTANCE

Abstract

Restoration of the position of the prosthetic joint line to the same level as the natural joint line, is a challenging problem in primary and revision knee arthroplasty and there is no reliable method for achieving this objective. We hypothesise that there is a constant ratio between the inter-epicondylar distance and the distance from this interepicondylar line to the joint line.

We analysed one hundred Computerised Tomography (CT) scans of the knee in the non arthritic population to study this relationship. The inter-epicondylar distance and the perpendicular distance from this inter-epicondylar line to the joint line was measured using both the clinical and surgical epicondylar axes for each knee as described in previous literature.

The results showed that using the clinical epicondylar axis the inter-epicondylar distance was 3 times the perpendicular distance from the inter-epicondylar line to the joint line (the median and mean ratio 3.0, Standard Deviation ±0.21). Using the surgical epicondylar axis the inter-epicondylar distance was 3.3 times the perpendicular distance from the inter-epicondylar line to the joint line (the median and mean ratio 3.3, SD ±0.25).

Landmarks such as inferior pole of patella or fibular head have been used to estimate the joint line position, but these methods have been shown to be unreliable. Our method will give an accurate estimate of the position of the joint line from the clinical epicondylar axis distance. This distance is easily calculated when using Computer Navigation for the surgery in both the primary and revision setting and the modern software programmes for Computer Assisted TKR should be modified accordingly. We conclude that the position of the joint line from the inter-epicondylar line is one-third of the inter-epicondylar distance which is valuable especially when there is significant bone loss at the tibio-femoral articulation.