Does measurement of the anatomic axis consistently predict hip-knee-ankle angle (HKA) for knee alignment studies in osteoarthritis? Analysis of long limb radiographs from the multicenter osteoarthritis (MOST) study

SummaryObjectiveResearchers commonly use the femoral shaft–tibial shaft angle (FS–TS) from knee radiographs to estimate the hip-knee-ankle angle (HKA) in studies examining risk factors for knee osteoarthritis (OA) incidence and progression. The objective of this study was to determine the relationship between HKA and FS–TS, depending on the method of calculating FS–TS and the direction and degree of knee deformity.Methods120 full-length digital radiographs were assigned, with 30 in each of four alignment groups (0.0°–4.9°, and ≥5.0° of varus and valgus), from a large cohort of persons with and at risk of knee OA. HKA and five measures of FS–TS (using progressively shorter shaft lengths) were obtained using Horizons Analysis Software, Orthopaedic Alignment & Imaging Systems Inc. (OAISYS). The offsets between HKA and the different versions of FS–TS were calculated, with 95% confidence intervals (CIs). Pearson correlations were calculated.ResultsIn varus limbs use of a shorter shaft length increased the offset between HKA and FS–TS from 5.1° to 7.0°. The opposite occurred with valgus limbs (from 5.0° to 3.7°). Correlations between HKA and FS–TS for the whole sample of 120 individuals were excellent (r range 1.00–0.88). However, correlations for individual alignment groups were low to moderate, especially for the shortest-shaft FS–TS (r range 0.41–0.66).ConclusionsThe offsets obtained using the shorter FS–TS measurements vary depending on direction and degree of knee deformity, and therefore may not provide reliable predictions for HKA We recommend that full-length radiographs be used whenever an accurate estimation of HKA is required, although broad categories of alignment can be estimated with FS–TS.