Abstract
Introduction
Positioning of a femoral sizing guide has been cited as being a critical intraoperative step during measured-resection based TKA as it determines femoral component rotation. Consequently, modern femoral sizing guides permit surgeons to ‘dial in’ external rotation when placing the guide. Although this feature facilitates guide placement, its effect on posterior femoral condylar resection and flexion gap stability is unknown. This study examines the effect of rotation on posterior femoral condylar resection among different posterior-referencing TKA designs.
Methods
Left-sided posterior-referencing femoral sizing guides and cutting blocks from nine posterior-referencing femoral sizing guides belonging to six TKA manufacturers were collected. Each guide underwent high-resolution photography at a setting of zero, three and greater than three degrees of external rotation. The axis of rotation for each guide was then identified and its location from the posterior condylar axis was recorded (figure). Cutting blocks from each system were then photographed and the amount of posterior condylar resection from the medial and lateral condyles was calculated for each setting of external rotation (figure). The posterior resection was then compared to the standard distal resections for each system.
Results
Two sizing guides had axes of rotation that were eccentrically located and in proximity to the posterior condylar axis, six were centrally based and one was slightly eccentric. Axis of rotation location had substantial effects on posterior condylar resection. Guides with centrally-based axes tended to resect more medial posterior condyle and less lateral condyle as rotation increased. Guides with eccentric axes tended to resect either less lateral or more medial condyle only.
Discussion
This study is the first to investigate femoral rotation and posterior condylar resection, and the first to compare different sizing guide designs. Our results indicate that guides with centrally-based axes of rotation increase medial condylar resection as external rotation increases. This increased resection may unintentionally create a larger flexion gap in the case of a valgus knee.
