Introduction In total knee arthroplasty (TKA), 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) is unknown. Whether an insert with a medial ball-in-socket conformity and lateral flat surface 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 and materials 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 rotation relative to the femoral component at extension and 90° and 120° flexion. The surgeon recorded the incidence of anterior lift-off of the insert. Results The insert with the medial ball-in-socket and lateral flat surface restored more internal tibial rotation than the one with less than spherical medial conformity, with mean values of 19° vs. 17° from extension to 90° flexion (p < 0.01), and 23° vs. 20°–120° flexion (p < 0.002), respectively. There was no anterior lift-off of the insert at 90° and 120° flexion. Conclusion An MS insert with a medial ball-in-socket and lateral flat surface that matches the native knee’s spherical conformity restores native tibial internal rotation when implanted with calipered KA and PCL retention without over-tensioning the flexion space.

Manipulation under anesthesia (MUA) for stiffness within 6 to 12 weeks after mechanically aligned total knee arthroplasty (TKA) generally yields better outcome scores than an MUA performed later. However, the timing of MUA after unrestricted, caliper-verified, kinematically aligned (KA) TKA remains uncertain. A retrospective review identified 82 of 3558 (2.3%) KA TKA patients treated with an MUA between 2010 and 2017. Thirty patients treated with an MUA within 3 months of the TKA (i.e., early) and 24 in the late group (i.e., >3 months) returned a questionnaire after a mean of 6 years and 5 years, respectively. Mean outcome scores for the early vs. late group were 78 vs. 62 for the Forgotten Joint Score (FJS) (p = 0.023) and 42 vs. 39 for the Oxford Knee Score (OKS) (p = 0.037). Subjectively, the early vs. late group responses indicated that 83% vs. 67% walked without a limp, 73% vs. 54% had normal extension, and 43% vs. 25% had normal flexion. An MUA within 3 months after unrestricted KA TKA provided excellent FJS and OKS at final follow-up relative to a late MUA. A late MUA performed after 3 months is worth consideration because of the good FJS and OKS scores, albeit with a risk of a persistent limp and limitation in knee extension and flexion.

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