Pre-operative analysis of lower limb coronal alignment - A comparison of supine MRI versus standing full-length alignment radiographs

BackgroundIn this study we compare the results of pre-operative standing full-length alignment (SFLA) radiographs with supine MRI assessment of the lower limb alignment prior to MRI based patient specific total knee arthroplasty (TKA).MethodsImaging was performed in 45 knees (45 patients). Assessment of SFLA radiographs was performed by three independent assessors. Inter-observer correlation was high and so the mean values were calculated. This data was then compared to MRI alignment data used to create the patient specific cutting jigs.ResultsThe range of alignment on SFLA radiographs ranged from + 25° to − 13° versus + 20° to − 11° with MRI. The mean difference between techniques was 2° (range 0-8°, SD ± 3°). Supine MRI under-estimated the degree of deformity in 31/45 (69%) cases. In 25/45 (56%) cases the supine MRI result was within ± 2° of the value on SFLA radiographs, 31/45 (69%) were within ± 3° and 38/45 (84%) within ± 5°. There was no correlation between the degree of varus/valgus deformity and the magnitude of the difference between imaging modalities (Spearman's r2 = 0.02, p = 0.41).ConclusionsThe findings from this study would indicate that supine MRI underestimates the degree of deformity at the knee joint, a conclusion which may be important for pre-operative planning or follow-up of corrective osteotomy or TKA.