Kinematic alignment preserves the mid-flexion trochlear line orientation in total knee arthroplasty: A prospective analysis from the FP-UCBM Knee Study Group

Purpose: Kinematic alignment (KA) in total knee arthroplasty (TKA) aims to restore the patient's native joint anatomy by resurfacing the distal and posterior femoral condyles. However, the trochlear anatomy is often overlooked, raising concerns about potential relative internal rotation of the femoral component. The aim of this study was to define the ?mid-flexion trochlear line’ (MTL) and assess its orientation relative to the posterior condylar line, hypothesising a parallelism between the two. Methods: A total of 158 knees (145 patients) undergoing KA TKA were prospectively analysed, after excluding post-traumatic osteoarthritis, cases with trochlear dysplasia, and femoral component flexion >5°. The anterior chamfer cut was conducted with a posterior referencing guide and the most prominent points of the medial and lateral trochlear facets—defining the MTL—were measured with a caliper. The MTL orientation relative to the posterior condylar line was calculated as the difference between the medial and lateral trochlear facets, with the two lines considered parallel for differences of 0 ± 1 mm. Two one-sided tests was implemented to assess equivalence between the two lines within a ±1 mm threshold. Correlations with coronal plane parameters (hip-knee-ankle angle [HKA], medial proximal tibial angle [MPTA] and lateral distal femoral angle [LDFA]) were assessed with Pearson's correlation coefficient. Statistical significance was set at p < 0.05. Results: The mean difference between the medial and lateral trochlear facets was 0.1 ± 1.40 mm, with 81.7% of cases falling within the 0 ± 1 mm range, indicating parallelism between the posterior condylar line and the mid-flexion trochlear line (p = 0.709). No significant correlations were observed between MTL orientation and HKA, MPTA or LDFA. Conclusions: Referencing the posterior condylar line accurately restores MTL orientation in the vast majority of patients, irrespective of coronal plane parameters. These findings support the biomechanical rationale of kinematic alignment, dispelling concerns about femoral component internal rotation. Level of Evidence: Level IV, prospective observational study.