An insert with less than spherical medial conformity causes a loss of passive internal rotation after calipered kinematically aligned TKA

Abstract

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.

Adjusting Insert Thickness and Tibial Slope Do Not Correct Internal Tibial Rotation Loss Caused by PCL Resection: In Vitro Study of a Medial Constraint TKA Implanted with Unrestricted Calipered Kinematic Alignment

“…The present study showed that PCL retention with a medial ball-in-socket with a flat lateral insert restored passive internal tibial rotation comparable to values reported for the native knee even with excision of the ACL, which is an unexpected finding not previously reported for the TKA. 40 In summary, the methods of using a thicker insert and reducing the tibial component's posterior slope were unsuccessful in restoring the loss of internal tibial rotation from PCL resection.…”

“…Because the internal tibial rotation with a spherical medial ball-in-socket insert with PCL retention was comparable to values reported for the native knee, and there was no anterior lift-off, the PCL and flexion space were not over-tensioned. 40 In the extended native knee, the normal screw home movement assists in capturing the patella through tensioning of the patellar ligament and the lateral and medial retinacula when it is not yet confined in the trochlear groove. 41 In the native knee throughout flexion, the tibia progressively internally rotates, which decreases the Q-angle and optimizes the retinacular ligaments' tension that guides patellofemoral tracking.…”

A TKA Insert with A Lateral Flat Articular Surface Maximizes External and Internal Tibial Orientations without Anterior Lift-Off Relative to Low- and Ultracongruent Surfaces

“…In TKA, determining how the level of lateral insert congruency determines external and internal tibial orientation requires an insert with medial ball-in-socket congruency that mimics the morphology of the native knee and an intraoperative tool to measure it. A medial congruency less conforming than a ball-in-socket causes a loss of tibial rotation [ 3 , 13 ]. A medially inscribed radial dial converts the trial insert into a goniometer that can measure external tibial orientation at extension, which indicates the extent of the screw-home movement and the internal tibial orientation in 90° flexion relative to a sagittal line on the femoral trial component ( Figure 1 ) [ 13 ].…”

“…A medial congruency less conforming than a ball-in-socket causes a loss of tibial rotation [ 3 , 13 ]. A medially inscribed radial dial converts the trial insert into a goniometer that can measure external tibial orientation at extension, which indicates the extent of the screw-home movement and the internal tibial orientation in 90° flexion relative to a sagittal line on the femoral trial component ( Figure 1 ) [ 13 ].…”

More passive internal tibial rotation with posterior cruciate ligament retention than with excision in a medial pivot TKA implanted with unrestricted caliper verified kinematic alignment

“…The difference in orientation between flexion angles computes internal–external (I–E) rotation over a motion arc (Fig. 1 ) [ 20 , 23 ]. The occurrence of anterior lift-off or 'booking' of the trial insert (or baseplate) indicates an over-tight flexion space (Fig.…”