Relevance of asymmetric extension gap on initial post-operative and weight-bearing x-rays after total knee replacement

Abstract

Introduction Asymmetric gap between the femoral and tibial component after primary total knee replacement (TKR) is a known cause of post-operative pain. This study investigates whether significant asymmetric extension gaps seen on initial post-operative non-weight-bearing (NWB) radiographs persist on follow-up weight-bearing (WB) radiographs. Method 203 consecutive patients who underwent primary TKR, and had adequate radiographic follow-up during a four-year period, were included. The distance between the tibial tray and femoral condyle on the medial and lateral sides was measured on initial NWB and follow-up WB radiographs, to identify significant asymmetric extension gaps. Previous studies showed medial extension gaps of ≥1.5mm may be associated with increased pain post-operatively. Therefore, we defined a significant gap as ≥1.5mm. The results were examined to determine whether initial post-operative gaps remain appreciable on follow-up x-rays. Results Of the 203 radiographs reviewed, 29 (14.3%) patients had an extension gap mismatch of ≥1.5mm on initial post-operative imaging. 13 (44.8%) patients had a medial opening gap whilst 16 (55.2%) had a lateral opening. Only 4 (13.8%) of these patients showed no significant change in the asymmetrical extension gap (remained ≥1.5mm) on WB x-rays. In the remaining patients, the gap was no longer appreciable. Conclusion The study showed that if patients present with post-TKR pain, which could be related to a medial opening extension gap, clinicians should use initial NWB x-rays to investigate as the gap mismatch may not be detectable on follow-up WB radiographs.