Resemblance of valgus malalignment of the distal tibia in 15-degree craniocaudal radiographs.

PURPOSE To describe and quantify a radiological phenomenon where the distal tibial plafond appears in valgus malalignment in intra-operative fluoroscopy owing to 15 degrees craniocaudal angulation of the X-ray beam. METHODS The lateral distal tibial angle (LDTA) of 14 male and 9 female skeletally mature patients was measured by a single reviewer using 2 types of anteroposterior radiographs, in which the X-ray beam was projected at 0 (orthogonal to the ankle) and then at 15 (in a craniocaudal direction) degrees. The LDTA was the angle between the long axis of the tibia and a line drawn across the most radiodense part of the tibial plafond. The paired t-test was used to compare the LDTA of the 2 measurements. RESULTS The mean LDTA on the 0-degree orthogonal radiographs was 89 (range, 87-92) degrees, whereas the mean LDTA on the 15-degree craniocaudal radiographs was 79 (range, 77-81) degrees. The mean difference was 10 degrees (range, 9-12; p<0.0001). In the 15-degree craniocaudal radiographs, the ankle joint appeared to have valgus malalignment. CONCLUSION During intramedullary nailing of the tibia, the knee is usually flexed and the image intensifier may not swing over far enough. This can result in well-aligned reduction being incorrectly viewed as having valgus malalignment or a varus-malaligned fracture being incorrectly viewed as reduced.