gms | German Medical Science

Objectives: An ongoing debate in total knee arthroplasty (TKA) is the level of conformity a medial stabilized (MS) implant needs to restore native (i.e., healthy) knee kinematics without over-tensioning the flexion space when the surgeon chooses to retain the posterior cruciate ligament (PCL). Whether an insert with a medial ball-in-socket and flat lateral surface conformity like the native knee or a less than spherical medial conformity restores higher and closer to native internal tibial rotation without anterior lift-off, an over-tension indicator, when implanted with calipered kinematic alignment (KA), is unknown.

Methods: Two surgeons treated 21 patients with calipered KA and a PCL retaining MS implant. Validated verification checks that restore native tibial compartment forces in passive flexion without release of healthy ligaments were used to select the optimal insert thickness. A goniometer etched onto trial inserts with the ball-in-socket and the less than spherical medial conformity measured the tibial orientation relative to the femoral component at extension and 90 and 120 degrees of flexion (Figure 1 [Fig. 1]).

The surgeon recorded the incidence of anterior lift-off of the insert.

Results: The insert with the medial ball-in-socket and flat lateral surface restored more internal tibial rotation than the one with less than spherical medial conformity, with mean values of 19 vs. 17 degrees from extension to 90 degrees (p < 0.01), and 23 vs. 20 degrees to 120 degrees of flexion (p < 0.002), respectively. There was no anterior lift-off of the insert at 90 and 120 degrees of flexion.

Conclusions: An MS insert with a medial ball-in-socket and flat lateral surface that matches the native knee's conformity as described by Freeman and Pinskerova restored the highest internal tibial rotation with values comparable to those reported for the native knee without over-tensioning the PCL and flexion space when implanted with calipered KA.