Aligning Success: Insights From 85,000 CT Scans on Trochlear Groove Positioning With Kinematic Alignment.

Abstract

Patellar maltracking remains a notable cause of postoperative pain and dissatisfaction after total knee arthroplasty (TKA), often from femoral component internal rotation and trochlear groove (TG) malpositioning. Although the Coronal Plane Alignment of the Knee (CPAK) classification evaluates coronal alignment, it does not account for three-dimensional anatomy. As newer alignment philosophies like kinematic alignment (KA) gain traction, risk of patellar maltracking may increase, especially when using implants designed for mechanical alignment. This study evaluates TG orientation in axial and coronal planes after virtual KA-based placement of a standard femoral component in 85,604 patients undergoing primary TKA. We analyzed 85,604 CT scans of patients with arthritis. For each, we calculated CPAK classification, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), hip-knee-ankle (HKA) angle, and posterior condylar to transepicondylar axis (PCA to TEA). Using kinematic alignment principles, we virtually implanted a femoral component with a 6° lateral trochlear groove. The average quadriceps line of force (QLF), defined as 3.2° lateral to the femoral mechanical axis (FMA), was considered ideal for tracking. TG orientation was then measured relative to the QLF and the standard 6° reference point. Mean values were as follows: LDFA 87.1°, MPTA 85.7°, HKA 176.7°, and PCA to TEA 3.2°. In the coronal plane, 41% had a TG medial to the QLF, and 9% of cases had a TG medial to the native MA. CPAK types 2 (74%), 3 (100%), and 6 (58%) showed the highest internal rotation and medial TG orientation. To achieve KA, approximately 30% of patients required >3° of internal rotation and valgus tilt and approximately 2% required >5°. A large proportion of patients demonstrated TG malpositioning and excessive internal rotation when placing MA-designed implants in KA. This highlights the need for three-dimensional classification systems and alignment-specific designs or alignment guardrails to prevent implant malpositioning.