An Insert Goniometer Identifies One Insert Thickness That Provides Maximum Tibial Rotation During Kinematically Aligned TKA Using a Medial Parapatellar Approach.

Abstract

Introduction: A surgeon performing kinematically aligned total knee arthroplasty (KA TKA) through a mid-vastus approach can use an insert goniometer to select the optimal insert thickness. The insert goniometer provides the value of soft-tissue-driven tibial internal rotation during passive knee flexion. This study tested whether the insert goniometer can identify one insert thickness that maximizes tibial rotation while restoring full knee extension during KA TKA performed through a medial parapatellar approach.

Materials and methods: One surgeon trialed goniometer inserts from a 10 to 14mm thickness with a medial ball-in-socket and flat lateral articular conformity in 33 patients treated with unrestricted caliper-verified KA, PCL retention, and a medial parapatellar approach. The medial insert had markings that measured tibial orientation relative to a sagittal line on the femoral trial component at full extension and 90° flexion (+ external/ - internal). The difference of tibial external orientation in extension and tibial internal orientation at 90° flexion computed tibial internal rotation.

Results: Of the 33 TKAs, a 10mm (N =13) or 14mm thick (N =2) insert maximized tibial internal rotation. In these TKAs, a 1mm thinner or thicker liner was not trialed, respectively. The remaining liner thicknesses maximizing tibial rotation were 11mm in 12 TKAs, 12mm in four TKAs, and 13mm in two TKAs. Mean ± SD maximal tibial rotation was 17 ± 4.1°, which was 4° more than with a 1mm thinner (13 ± 5°) and 1mm thicker (13 ± 5.3°) insert (p<0.001).

Conclusions: Surgeons performing KA TKA through a medial parapatellar approach can utilize the insert goniometer to select the insert thickness that maximizes tibial rotation. Maximizing tibial rotation with a medial ball-in-socket and lateral flat insert conformity restores physiologic knee kinematics and dynamically stabilizes the patella.