Inter-observer reliability of measurements performed on digital long-leg standing radiographs and assessment of validity compared to 3D CT-scan.

BACKGROUND: Long-leg radiographs (LLR) are often used in orthopaedics to assess limb alignment in patients undergoing total knee arthroplasty (TKA). However, there are still concerns about the adequacy of measurements performed on LLR. We assessed the reliability and validity of measurements on LLR using three-dimensional computed tomography (3D CT)-scan as a gold standard.

METHODS: Six different surgeons measured the mechanical axis and position of the femoral and tibial components individually on 24 LLR. Intraclass correlation coefficients (ICC) were calculated to obtain reliability and Bland-Altman plots were constructed to assess agreement between measurements on LLR and measurements on 3D CT-scan.

RESULTS: ICC agreement for the six observer measurements on LLR was 0.70 for the femoral component and 0.80 for the tibial component. The mean difference between measurements performed on LLR and 3D CT-scan was 0.3° for the femoral component and -1.1° for the tibial component. Variation of the difference between LLR and 3D CT-scan for the femoral component was 1.1° and 0.9° for the tibial component. 95% of the differences between measurements performed on LLR and 3D CT-scan were between -1.9 and 2.4° (femoral component) and between -2.9 and 0.7 (tibial component).

CONCLUSION: Measurements on LLR show moderate to good reliability and, when compared to 3D CT-scan, show good validity.

CLINICAL TRIAL REGISTRATION NUMBER: institutional review board Atrium-Orbis-Zuyd, number: 11-T-15.

LEVEL OF EVIDENCE: Prospective cohort study, Level II.