The distal metatarsal articular angle in hallux valgus deformities. Comparisons of radiographic and weightbearing CT scan measurements with variations in hindfoot position

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When evaluating hallux valgus (HV) deformity with anteroposterior (AP) foot radiographs, the distal metatarsal articular angle (DMAA) has been the subject of frequent debate. Although a straightforward indicator of structural alignment of the distal first metatarsal articular surface, inter- and intraobserver measurements can vary widely. Alterations in the radiographic appearance of bony "structure" with positional changes of the foot/foot bones in HV deformities in no small part contributes to these inaccuracies. The aim of this study was to determine the effect of hindfoot position on the DMAA. Four different radiologic images were obtained for 15 subjects with HV: three AP foot radiographs (standard weightbearing, foot supinated, foot pronated) and one weightbearing CT (WBCT) scan. For each image, five investigators measured the DMAA in order to assess reliability. Mean DMAA values measured from the images indicated that the angle was highest with the pronated foot (15.3 (95% CI, 10.3-20.3) degrees) and lowest when measured from the CT image (11.6 (95% CI, 7.3-16.0) degrees). For all image types, the intraclass correlation coefficient was greater than 0.9 and statistically significant (P<0.0005). Hindfoot positions affected radiographic DMAA/mean DMAA values, with values highest with feet in pronated attitudes. Unlike radiographic projections, the WBCT appears less likely to overestimate DMAA and is not subject to variations in foot positioning. In the preoperative evaluation of HV deformities, this points to the potential use of WBCT as a reference standard. For markedly severe HV deformities that may present with AP radiographs with marked first metatarsal head "roundness," the use of WBCT is the intuitive choice. The results indicate excellent reliability in measurements of DMAA between all five investigators. As such, the measurement method used to determine DMAA in this study was dependable and reproducible. Level II.