We investigated whether the extension gap in total knee replacement (TKR) would be changed when the femoral component was inserted. The extension gap was measured with and without the femoral component in place in 80 patients with varus osteoarthritis undergoing posterior-stabilised TKR. The effect of a post-operative increase in the size of the femoral posterior condyles was also evaluated. The results showed that placement of the femoral component significantly reduced the medial and lateral

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... xtension gaps by means of 1.0 mm and 0.9 mm, respectively (p < 0.0001). The extension gap was reduced when a larger femoral component was selected relative to the thickness of the resected posterior condyle. When the post-operative posterior lateral condyle was larger than that pre-operatively, 17 of 41 knees (41%) showed a decrease in the extension gap of > 2.0 mm. When a specially made femoral trial component with a posterior condyle enlarged by 4 mm was tested, the medial and lateral extension gaps decreased further by means of 2.1 mm and 2.8 mm, respectively. If the thickness of the posterior condyle is expected to be larger than that pre-operatively, it should be recognised that the extension gap is likely to be altered. This should be taken into consideration when preparing the extension gap. Optimising the extension gap is one of the goals in total knee replacement (TKR) and the gap is often evaluated using laminar spreaders, spacer blocks or a tensing device without the femoral component in place. [1] [2] [3] [4] [5] [6] [7] However, the effect of the posterior condyle of the femoral component on the extension gap should also be taken into account. Prominent posterior condyles will tighten the posterior capsule, resulting in a smaller extension gap. Therefore, it is possible that in the absence of the femoral component the extension gap might be overestimated. In particular, when using the gap technique with a posterior cruciate ligament (PCL)-sacrificing TKR, a larger femoral component is sometimes selected to compensate for the enlarged flexion gap created by resecting the PCL. 8-10 This produces post-operative posterior condyles which are larger than the bone they replace, and can affect the extension gap after the femoral component is inserted. These relationships have not previously been evaluated. Another important consideration is the extent to which the extension gap is changed by the removal of posterior condylar osteophytes and the release of the posterior soft tissues. The effects of these ligamentous releases have been investigated mainly in normal cadaver knees, 2,5,11-14 and much less in osteo-arthritic knees. 6 The aims of this study were: (1) to investigate how much the extension gap would enlarge at each step of the surgical procedure; (2) to assess the effect on the extension gap of insertion of the femoral component; and (3) to examine the effect of a change in the thickness of the posterior condyles on the extension gap in varus osteo-arthritic knees undergoing TKR. We also evaluated how much the extension gap would be changed when the femoral component was increased by one size. Our hypothesis was that when a femoral component was implanted having posterior condyles larger than the bone it replaced, the extension gap would be tightened on its insertion.