Restoration of the trochlear peaks is unnecessary with a kinematic alignment-optimized femoral component as under-stuffing results in equivalent or better patient-reported outcome scores.

Abstract

Purpose: Kinematic alignment (KA) total knee arthroplasty (TKA) aligns the femoral component to restore the pre-arthritic posterior joint line, potentially altering the heights of the medial and lateral trochlear peaks. It remains unclear whether the femoral component should be adjusted to correct deviations in peak height. This study assessed whether >2 mm of under- or over-stuffing in peak height negatively impacted patient-reported outcome (PRO) scores compared to restoration within ±2 mm.

Methods: The study included 115 KA TKAs performed with a KA-optimized femoral component featuring a trochlea with a lateral ridge opening that creates a 20° valgus trochlear groove and a flattened medial ridge, and PROs at a mean of 22 (12-28) months. The surgeon measured the height of the trochlear peaks on the anterior femoral resection.

Results: Peak under-stuffing >2 mm occurred medially in 66% and laterally in 43%. Over-stuffing >2 mm was too infrequent for statistical analysis. Compared to restoration within ±2 mm, medial under-stuffing resulted in a non-equivalent 6-point higher Forgotten Joint Score (FJS) (p = 0.1087) and equivalent but 9- and 3-point higher Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS JR) (p < 0.0001) and Oxford Knee Score (OKS) (p = 0.0020). Lateral under-stuffing yielded equivalent but 6-, 12- and 3-point higher FJS (p = 0.0484), KOOS JR (p < 0.0001) and OKS (p < 0.0001).

Conclusion: The KA-optimized femoral component features a patient-specific trochlea that addresses anterior arthritic trochlear variations, which are reported to range from -24° varus to 30° valgus. One possible explanation for why >2 mm of under-stuffing leads to superior PROs is that reducing the peak height compensates for over-stuffing above the native trochlea caused by the prosthesis's proximal overreach, with a reported average of 17 mm.

Level of evidence: Level III.