Computed tomography scanogram compared to long leg radiograph for determining axial knee alignment

Background and purpose - Supine computed tomography scanogram (CTS) is a commonly used alternative to weight bearing long leg plain radiograph (LLR) in measuring knee alignment. No published studies have validated its use in the native knee and the post-unicompartmental replacement knee (UKR). We quantified the difference in measurements obtained from CTS and LLR for knee alignment. Patients and methods - Supine CT scanograms and weight bearing long leg plain anteroposterior radiographs were obtained for 40 knees (in 25 patients), 17 of which were native, and 23 of which were post-UKR. The mechanical and anatomical axes of the tibio-femoral joint were measured. Bland-Altman plots were used to calculate the 1.96 standard deviation limits of agreement between CTS and LLR. Intraclass correlation was used to assess intra-rater and inter-rater reliability (where values > 0.81 indicate very good reliability). Results - CTS and LLR were equally reliable in measurement of the mechanical and anatomical axes of the tibio-femoral joint (intraclass correlation coefficient (ICC) > 0.9 for all parameters). Statistically significant and clinically relevant differences were found between CTS and LLR in measurement of the mechanical axis (limits of agreement: UKR -3.2 degrees to 6.3 degrees; native -3.2 degrees to 5.6 degrees) and the anatomical axis (limits of agreement: UKR -3.7 degrees to 8.7 degrees; native -2.0 degrees to 8.8 degrees). Interpretation - Although it is a reliable tool, CTS is not necessarily an accurate one for measurement of knee alignment when compared to LLR. We recommend that CTS should not be used as a substitute for LLR in measurement of the mechanical or anatomical axes of the knee.