First metatarsal head shape in juvenile hallux abducto valgus.

A round first metatarsal head has been implicated as a predisposing factor in hallux abducto valgus. The shape of one hundred first metatarsal heads was analyzed on 50 weightbearing dorsoplantar x-rays of 10-year-old children with a hallux abductus angle in excess of 15 degrees and osteophytic thickening of the first metatarsophalangeal joint. The metatarsals were subjectively described as either round or square. A measurement technique was then devised to quantify in millimeters the difference between the two types. Using chi-square analysis, a very strong statistical association (p less than 0.001) was found between the subjective assessment and the objective measurement. A weak relationship was found between the metatarsal head shape and the degree of hallux abducto valgus (HAV) (r = -0.294). While the trend of this correlation is consistent with accepted wisdom, that is, the squarer the metatarsal head the less the hallux abducto valgus, the association is not strong enough to scientifically confirm it. This finding along with the presence of so many square metatarsal heads in children with advanced HAV indicates that assessment of metatarsal head shape has little place in the scientific assessment of first metatarsophalangeal joint pathology.