Total Hip Arthroplasty Frequently Alters Coronal Plane Alignment of the Knee Classification: Implications for Knee Arthroplasty Planning.

Background: Total hip arthroplasty (THA) can alter lower-limb alignment. Although the coronal plane alignment of the knee (CPAK) classification describes constitutional alignment and is associated with outcomes after total knee arthroplasty, whether CPAK classification is preserved following THA remains unclear. We investigated changes in CPAK classification after THA and factors associated with CPAK transition and postoperative alignment. Methods: This retrospective study evaluated 76 knees undergoing THA. Preoperative and 1-year postoperative standing long-leg radiographs were analyzed. Lateral distal femoral angle, medial proximal tibial angle (MPTA), arithmetic hip-knee-ankle angle (aHKA), and joint line obliquity (JLO) were measured, and CPAK type was determined preoperatively and postoperatively. Multivariable logistic, multinomial logistic, and linear regression analyses were performed. Results: CPAK classification changed in 43 knees (56.6%) after THA. Lateral distal femoral angle increased from 86.7° to 88.2° and JLO from 173.6° to 175.6°, whereas aHKA decreased from 0.15° to -0.81°; MPTA showed no significant change. Valgus knees shifted toward varus CPAK types (21 knees), whereas varus knees shifted toward valgus types (9 knees). The most common transition was type 2 to type 1 (6 knees). Preoperative aHKA and JLO independently predicted postoperative changes. In 54 unilateral cases, greater leg length difference correlated negatively with ΔMPTA and ΔaHKA. Conclusions: More than half of knees demonstrated a CPAK change after THA. These transitions were driven by femoral alignment-related changes rather than tibial parameters. It suggests that surgical sequencing of THA and total knee arthroplasty should be carefully considered in patients with concomitant hip and knee osteoarthritis.