Abstract
Introduction: The goals of TKR are restoration of the mechanical axis, joint line and Q –angle. Reproduction of the offset of the extensor mechanism during arthroplasty is less well understood. The lever arm of the extensor mechanism is primarily affected by femoral component position, patella tracking and overall patellar thickness. Changing this lever arm alters quadriceps muscle and patellofemoral joint reaction force. Some TKR designs purposefully aim to increase this offset in order to reduce PFJ contact pressures. Overstuffing the PFJ will however adversely affect outcome.
The aims of this study were to measure and compare the pre and postoperative quadriceps lever arm and its effect on function in a consecutive series of patients undergoing TKR.
Methods: Fifty consecutive patients who had an LCS TKR without patellar resurfacing by a single surgeon were reviewed. Patient demographics were recorded. We measured the pre and post-operative quadriceps lever arm using comparable lateral radiographs and digital imaging software. Functional outcome in these patients was determined using the American Knee Society Score pre and 6-months post-operatively. Patients were specifically questioned regarding the presence or absence of anterior knee pain
Results: Quadriceps lever arm was on average 6 mm greater post-operatively than pre-op but this difference was not significant [t-test]. The functional outcome in these patients was independent of any change in quadriceps lever arm.
Conclusions Reproduction of normal biomechanics is essential in order to optimise outcome from TKR.
Using the LCS system, there is a small but insignificant increase in the quadriceps moment arm. We have not found that this has any bearing on functional outcome in these patients.
The abstracts were prepared by Emer Agnew, Secretary to the IOA. Correspondence should be addressed to him at Irish Orthopaedic Association Secretariat, c/o Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
