Axial rotation of the first metatarsal head in a normal population and hallux valgus patients.

HYPOTHESIS Does metatarsal pronation exist and, if so, what is its impact? INTRODUCTION Hallux valgus is a deformity associating angulation and a rotational component. The present study sought to investigate the nature and origin of the coronal plane displacement. MATERIALS AND METHODS A prospective single-center radiological and anatomic study was conducted on 100 feet operated on for hallux valgus. Baseline X-ray determined the preoperative position of the 1st metatarsal head in the coronal plane. The range of motion (ROM) of the cuneometatarsal joint in pronation-supination was measured peroperatively. An anatomic study investigated possible diaphyseal torsion. RESULTS Mean radiologic pronation in hallux valgus was 12.7° (range, 0°-40°). Cuneometatarsal rotational ROM was determined by adding peroperative ROM in pronation (mean, 9.3°; range, 0°-30°) and in supination (mean, 8.7°; range, 0°-20°). Intermetatarsal divergence showed no correlation with radiologic pronation or ROM in pronation. Radiologic pronation showed no correlation with peroperative ROM in pronation. Pronation of the metatarsal head was never observed without associated sesamoid pronation; the latter, however, was in some cases observed without the former. Twenty randomly selected metatarsal cadaver specimens from the anatomy laboratory of the University of Nice (France) showed diaphyseal torsion in 80% of cases, with the metatarsal head in neutral position or in supination with respect to the base. DISCUSSION In hallux valgus, 1st ray pronation appears to be systematic, in contrast to the typical supination found in the general population. Metatarsal rotation is always associated with sesamoid rotation, whereas the converse is not the case: displacement of the sesamoids appears to displace the metatarsal head via the metatarsosesamoid ligaments. This "drive-belt" effect, however, varies in its mechanical properties and the transmission is imperfect and likely subject to progressive ligament stretching, so that head rotation does not exactly follow and may even become independent of the sesamoid displacement. Radiologic and clinical rotation thus do not match any longer. The anatomic study showed that, while diaphyseal torsion cannot be ruled out, the metatarsal pronation mainly derives from cuneometatarsal joint rotational instability, the evolution of which does not parallel lateral instability, no correlation being found between degree of varus and rotational instability. CONCLUSION The present study found metatarsal pronation to be associated with hallux valgus, making a preoperative AP view useful; the underlying mechanism was generally cuneometatarsal instability. Although difficult to specify exactly without correlation between radiological and clinical data, any such pronation raises the question of whether replacing the metatarsal head on its sesamoid supports is sufficient to achieve stability in all planes, or whether on the contrary derotation should be associated to metatarsal valgization osteotomy to restore horizontal support. LEVEL OF EVIDENCE Level IV.