Purpose: Coronal mal-alignment contributes to total knee arthroplasty (TKA) failure. The surgical aim is to place the implant at right angles to the load-bearing axis, restoring the femoral and tibial mechanical axes (MA) to neutral. Mal-alignment of greater than 3° is associated with a poor outcome and reduced longevity. Pre-operative decisions for alignment correction are often made using anatomical axis (AA) measurements taken from standing short knee radiographs. The aim of this study was to determine how well the AA predicts the MA in subjects with mild and severe varus and valgus deformities. Several different methods of calculating the AA were also compared for their ability to predict the MA.

Method: The database of full-length lower extremity radiographs from the Multicenter Osteoarthritis (MOST) Study was used to select images for this study. All of the subjects in the MOST either had knee OA or were at high risk for developing knee OA. 120 full-length digital radiographs were assigned, with 30 in each of four alignment groups (0° to 4.9°, and ≥ 5° of varus and 0.1° to 4.9°, and ≥ 5° of valgus). The MA and 5 measures of the AA (using progressively shorter shaft lengths) were obtained from each radiograph using Horizons Analysis Software, OAISYS Inc. The offsets between the MA and the different versions of the AA were calculated (with 95% confidence intervals) for the complete sample of 120 limbs and for each alignment group. Pearson correlations were also calculated (α = 0.05).

Results: The average offset between the MA and the AA for the entire dataset was 5.0°. In varus limbs the shortened shaft AA measurements increased the offset from 5.1° to 7.0°. The opposite occurred with valgus limbs (from 5.0° to 3.7°). The CI for the offsets increased from less than 3° for the full-length AA measurements to over 8.3° for the shortest AAs. While correlations between MA and AA for the whole dataset were high (0.88 to 1.00), correlations for individual groups were much lower, especially for the shortest AA (0.41 to 0.66).

Conclusion: Using short knee radiographs to estimate the MA has important limitations. The offsets obtained using the shorter AAs vary depending on type and degree of mal-alignment, and do not provide reliable predictions for the MA. Full-length films are needed to consistently define the alignment in order to ensure the best outcome from TKA.

This study determined the inter-reader and intra-reader reliability of lower limb frontal plane alignment measures obtained from digital radiographs using a computer software program.

Measurements of lower limb frontal plane alignment were obtained from over 3000 full limb digital radiographs of both limbs of persons ‘at risk’ for developing knee osteoarthritis (OA), as part of the Multicenter Osteoarthritis Study (MOST). Three trained clinicians used a computer software program (Horizon Image Viewer, version 1.5, OAISYS Medical Inc.) to locate bone landmarks on the femur and tibia from which standard measures of alignment (e.g. the Hip-Knee-Ankle (HKA) angle) and bone lengths could be computed.

To assess the reliability of these alignment measurements, one hundred randomly assigned digital radiographs, representing two hundred limbs, were selected from the complete data set for a repeated analysis carried out two or more weeks after completion of the first measurements. Random effects two-way analysis of variance (ANOVA) models were applied to estimate the interclass and intraclass correlation coefficients (ICC), which correspondingly evaluated inter-reader and intra-reader reliability for each of the angles and bone lengths.

High reliability measures were obtained for the HKA angle (inter-reader reliability: ICC=0.995 (95% CI, 0.994–1); intra-reader reliability: ICC= 0.998 (95% CI, 0.998–1)). Reliability for additional angles between the femur and tibia ranged from 0.839 to 0.993 (inter-reader reliability) and 0.908 to 0.998 (intra-reader reliability). High reliability measures were also obtained for bone lengths (inter-reader reliability: ICC from 0.993 to 0.995; intra-reader reliability: ICC from 0.994 to 0.995).

Each of the lower limb alignment and bone length measurements were highly reliable. The outcome supports the use of computer software programs and software tools for analysis of lower limb frontal plane alignment.