A geometric ratio to predict the flexion gap in total knee arthroplasty

Measured resection is a common technique for obtaining symmetric flexion and extension gaps in posterior-stabilized (PS) total knee arthroplasty (TKA). A known limitation of measured resection, however, is its reliance on osseous landmarks to guide bone resection and component alignment while ignoring the geometry of the surrounding soft tissues such as the medial collateral ligament (MCL), a possible reason for knee instability. To address this clinical concern, we introduce a new geometric proportion, the MCL ratio, which incorporates features of condylar geometry and MCL anterior fibers. The goal of this study was to determine whether the MCL ratio can predict the flexion gaps and to determine whether a range of MCL ratio corresponds to balanced gaps. Six computational knee models each implanted with PS TKA were utilized. Medial and lateral gaps were measured in response to varus and valgus loads at extension and flexion. The MCL ratio was related to the measured gaps for each knee. We found that the MCL ratio was associated with the flexion gaps and had a stronger association with the medial gap (β = -7.2 ± 3.05, P < .001) than with the lateral gap (β = 3.9 ± 7.26, P = .04). In addition, an MCL ratio ranging between 1.1 and 1.25 corresponded to balanced flexion gaps in the six knee models. Future studies will focus on defining MCL ratio targets after accounting for variations in ligament properties in TKA patients. Our results suggest that the MCL ratio could help guide femoral bone resections in measured resection TKA, but further clinical validation is required.