Do Tibial and Femoral Bowing Explain the Natural History of Valgus (Coronal Plane Alignment of the Knee Types 3, 6, and 9) Knees?

Abstract

Coronal plane alignment of the knee (CPAK) characterizes knee phenotypes using arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). Medial tibial and lateral femoral bowing (fBOW) increase valgus aHKA and apex-proximal JLO, revealing the natural history of valgus knees. This study correlated bowing with aHKA and JLO and asked if arithmetic correction of long bone bowing shifts outliers toward CPAK₂. We retrospectively studied 192 knees between April 2024 and September 2025. Tibial bowing (tBOW) and fBOW angles were measured between proximal and distal metadiaphyseal axes. Radiographic parameters (lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), aHKA, and JLO) were calculated, and CPAK classifications determined. Pearson correlation assessed tBOW and alignment parameters. A one-way analyses of variance with Tukey post hoc test compared bowing across alignment groups. Corrected MPTA (MPTA without tBOW) and LDFA (LDFA without fBOW) were calculated, CPAK classifications recalculated, and reclassification patterns analyzed. Tibial bowing correlated with both MPTA (r = 0.56, P < 0.001) and aHKA (r = 0.47, P < 0.001). Femoral bowing correlated with LDFA (r = -0.60, P < 0.001) and JLO (r = -0.50, P < 0.001). Valgus knees had significantly greater mean tBOW (2.64°) compared to neutral (0.71°) and varus knees (-0.14°) (P < 0.001). Correction for tBOW reclassified 71.3% of all valgus knees (60.3% of CPAK₃ cases, 100% of CPAK_6, and 100% of CPAK₉ cases) toward more neutral alignment classifications. Tibial and fBOW alter CPAK classification, with correction returning many valgus knees to neutral, apex-distal alignment. Small degrees of bowing do not necessitate an osteotomy but should be considered by knee resurfacing surgeons who are guided by collateral ligament positions. With bowing, the normal apex-distal target joint line is displaced in part by the amount of the bow.