Researchers 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.
120 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.
In 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).
The 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.