Rotational Deformity of the First Ray Precludes Accurate Distal Metatarsal Articular Angle Measurement in Hallux Valgus.

The routine workup of hallux valgus includes measurements of the distal metatarsal articular angle (DMAA). However, this angle may not be a reliable measurement when rotational deformities occur. Eight matched cadaveric feet were evaluated radiographically using fluoroscopy. The intermetatarsal angle (IMA), hallux valgus angle, and DMAA were evaluated as a baseline. Then, we assessed whether axial rotation of 15°, 30°, 45°, and 60° affected the accuracy of the IMA and the DMAA significantly. Statistical evaluation was performed using Student's t tests and analysis of variance. The IMA remained stable for each anterior-posterior image despite rotation of the distal segment, with a mean difference of less than 2.5°. However, the DMAA was not constant, having different angles of rotation through the osteotomy site. We were able to demonstrate that measurement of the DMAA varies significantly with rotation of the distal first metatarsal. Using the DMAA to assess first metatarsophalangeal joint congruency should be done cautiously because it may estimate unreliably and inaccurately the 3D deformity often encountered in pathologic hallux valgus.