Posterolateral upslope of a low-conforming insert blocks the
This study determined differences in anterior–posterior movements of the femoral condyles, pivot locations, and internal tibial rotation in different arcs of flexion for two implants with different insert conformities in kinematically aligned TKA.

25 patients treated with a medial and lateral low-conforming, posterior cruciate ligament (PCL) retaining (LC CR) implant followed by a medial ball-in-socket and flat, lateral PCL sacrificing (B-in-S CS) implant in the contralateral knee underwent single-plane fluoroscopy during a deep knee bend.

Results:
--From 0º to 30º of flexion, a medial pivot occurred with the tibia rotating internally approximately 5º with both implants.

--From 30º to 90º, the pivot remained medial and internal rotation increased to 10º with the B-in-S CS implant. In contrast, neither femoral condyle moved more than 1 mm with the LC CR implant from 30º to 60º, but from 60º to 90º degrees, a lateral pivot occurred and internal rotation increased.

--Internal rotation of the tibia on the femur from 0° to maximum flexion occurred about a medial pivot similar to the native knee for the B-in-S CS implant and was 4.5° greater than that of the LC CR implant.

--There was no difference in the median patient-reported outcome scores between implant designs.

Tibial insert conformance, in particular, is a key factor in determining whether to pivot medially or laterally during a deep knee bend. The chock-block effect of the insert's posterolateral upslope, which impedes posterior movement of the lateral femoral condyle, is one explanation for the switch from a medial to lateral pivot between 30 and 60 degrees with the LC CR implant. Because the B-in-S CS implant's insert has no posterolateral upslope, the tibia pivots medially throughout flexion, just like the natural knee.

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