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Restoration of Posterior Condylar offset in TKA: The Effect of Posterior-referencing Jig Design

February 19, 2016

Contributors: Ryan Coyle, MD; Marcel A Bas, MD; Matthew Stewart Hepinstall, MD; Matthew Stewart Hepinstall, MD

Whereas the native tibiofemoral joint line is typically oblique to the mechanical axis and the native posterior condylar axis is typically internally rotated the transepicondylar axis (TEA), mechanically-aligned total knee arthroplasty (TKA) alters these relationships. In mechanically-aligned TKA, femoral component rotation parallel to the transepicondylar axis optimizes patellofemoral and tibiofemoral mechanics. Collateral ligament isometry in flexion and extension depends on posterior condylar offset and the level of the joint line. Intramedullary distal femoral cutting jigs commonly set the distal femoral joint line in relationship to the most distal femoral condyle, typically the medial condyle. Posterior referencing jigs reference the posterior condylar axis, allowing variable degrees of external rotation. When rotation is greater than 0 degrees, it is impossible to anatomically restore the posterior offset of both femoral condyles. Some jigs dictate external rotation around a central point, resulting in small changes in the offset of both condyles. Other jigs reference one femoral condyle; with increasing external rotation, these jigs can markedly change the offset of the condyle not referenced. Referencing the lateral femoral condyle in flexion and the medial femoral condyle in extension may create mismatched gaps after bone resection. Using 32 identical sawbones, we performed distal and posterior femoral resections using cutting guides from 8 widely available TKA systems. Distal femoral valgus resection was set at 5 degrees for all specimens, whereas rotation was set at 3 degrees for 2 sawbones and 5 degrees for 2 sawbones with each system. We measured the thickness of all bone resections, and compared those values to known implant thickness. We report the resulting differences in thickness between the distal and posterior femoral resections, a marker for potential imbalance of the flexion and extension gaps. Our data offers insight for arthroplasty surgeons into the bony resections taken by widely-used TKA instrumentation systems. Our surgical video demonstrates how adjustments of anteroposterior position can be made to balance flexion and extension resections, even when jigs reference the medial condyle in extension and the lateral condyle in flexion.

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