Validity and reliability of the first distal metatarsal articular angle

A laboratory study was undertaken to quantify the validity, the intra- and interobserver reliability, and the effect of first metatarsal orientation on the distal metatarsal articular angle (DMAA). First metatarsals from six cadavers were isolated by sharp dissection and attached to a custom-made triplanar protractor. This allowed controlled positioning of the first metatarsal in the three cardinal planes. Each specimen was moved through a stepwise sequence of first metatarsal orientation changes. Radiographs were taken at each position. The distal (metatarsophalangeal) articular surface of the first metatarsal was then circumscribed with metallic paint and the radiographic series was repeated. DMAA measurements were made on each radiograph multiple times by two measurers blinded to the parameters of variation. The difference between the standard (clinical) radiographic technique for estimating the DMAA and the metallic paint DMAA averaged -0.9 degrees (SD = 2.4 degrees). Longitudinal rotation (P < .0001) and varus deviation (P < .02 at 10 degrees) of the first metatarsal had a significant effect on the radiographic determination of the DMAA. Although intraobserver reliability for DMAA measurement was high, interobserver reliability for the clinical technique of measurement was poor (pooled intraclass correlation coefficient = 0.14).