Alignment philosophy in total knee arthroplasty has more impact on trochlear anatomy restoration as the lateral distal femoral angle decreases

PURPOSE: Most implants currently used in total knee arthroplasty (TKA) were designed based on mechanical alignment (MA) principles. Modern alignment philosophies alter the orientation of these classic designs by following each individual's knee phenotype, which may have important implications on trochlear restoration. This study aims to compare the influence of different alignment strategies on trochlear anatomy recreation within predefined lateral distal femoral angle (LDFA) groups. METHODS: A total of 147 robotic-assisted, functionally aligned (FA) TKAs, performed with a digital ligament tensioner, were reviewed. Simulated femoral component positions were generated according to different alignment techniques; kinematic alignment (KA), restricted KA (rKA), restricted inverse KA (riKA), gap balancing (GB) and MA. Changes in trochlear angle (∆TA), mediolateral sulcus offset and trochlear over/understuffing were calculated relative to native. Alignment philosophies were compared across following LDFA groups: <84°(n = 22), 84°-87° (n = 57), 87°-90° (n = 50) and >90° (n = 18). Paired Wilcoxon-rank-sum tests were performed with Bonferroni corrections. RESULTS: In the lower LDFA groups more significant differences in ∆TA were identified between strategies with FA, KA and rKA showing lower values than riKA, GB and MA (p < 0.01). The LDFA < 84° group had the greatest range in mean ∆TA (2.8° ± 3.4 to 9° ± 3.2). At 10° flexion, MA and GB led to a significantly more lateral sulcus position in each LDFA group compared to the more native sulcus offset obtained with KA, rKA and FA (p < 0.05). All alignment strategies understuffed the trochlea in each LDFA group (3 mm ± 1.2 to 5.3 mm ± 1.5), with KA and rKA creating the least understuffing (p ≥ 0.05). CONCLUSION: KA, rKA and FA more closely recreate trochlear anatomy than riKA, MA and GB. The difference between strategies, especially in ∆TA, becomes more pronounced as the LDFA decreases. LEVEL OF EVIDENCE: Level IV.