How often do we alter constitutional limb alignment, joint line obliquity, and Coronal Plane Alignment of the Knee (CPAK) phenotype when performing mechanically aligned TKA?

Aims While mechanical alignment (MA) is the traditional technique in total knee arthroplasty (TKA), its potential for altering constitutional alignment remains poorly understood. This study aimed to quantify unintentional changes to constitutional coronal alignment and joint line obliquity (JLO) resulting from MA. Methods A retrospective cohort study was undertaken of 700 primary MA TKAs (643 patients) performed between 2014 and 2017. Lateral distal femoral and medial proximal tibial angles were measured preand postoperatively to calculate the arithmetic hip -knee -ankle angle (aHKA), JLO, and Coronal Plane Alignment of the Knee (CPAK) phenotypes. The primary outcome was the magnitude and direction of aHKA, JLO, and CPAK alterations. Results The mean aHKA and JLO increased by 0.1 degrees (SD 3.4 degrees) and 5.8 degrees (SD 3.5 degrees), respectively, from preto postoperatively. The most common phenotypes shifted from 76.3% CPAK Types I, II, or III (apex distal JLO) preoperatively to 85.0% IV, V, or VI (apex horizontal JLO) postoperatively. The proportion of knees with apex proximal JLO increased from 0.7% preoperatively to 11.1% postoperatively. Among all MA TKAs, 60.0% (420 knees) were changed from their constitutional alignments into CPAK Type V, while 40.0% (280 knees) either remained in constitutional Type V (5.0%, 35 knees) or were unintentionally aligned into other CPAK types (35.0%; 245 knees). Conclusion Fixed MA targets in TKA lead to substantial changes from constitutional alignment, primarily a significant increase in JLO. These findings enhance our understanding of alignment alterations resulting from both unintended changes to knee phenotypes and surgical resection imprecision.