Mobility of the first tarsometatarsal joint in relation to hallux valgus deformity: anatomical and biomechanical aspects.

Hypermobility of the first tarsometatarsal (TMT 1) joint is suggested to be an important factor in the cause and progression of hallux valgus deformity. Hypermobility of the TMT 1 joint is tested clinically in the sagittal plane, but an important deformation also exists in the transversal plane: metatarsus primus varus. This in vitro study was undertaken to investigate the relation between the mobility of the TMT 1 joint in these two planes and to investigate the correlation of the mobility with morphological variables. A second aim was to study the possible stabilizing effect of the tibialis anterior muscle, flexor hallucis longus muscle, and peroneus longus muscle on the TMT 1 joint. Nine embalmed human specimens were tested under standardized conditions. A 30-N force was applied to the head of the first metatarsal (MT 1) to pull in either the dorsal or medial direction. To simulate muscle force, 21 N was applied to the three tendons: all seven possible combinations of muscle action were tested in each plane of motion. Angular displacements were measured using 2-dimensional LED video registration. TMT 1 mobility is a relevant factor in MT 1 mobility in the sagittal and transversal planes, the peroneus longus has a stabilizing effect on this joint, and the effect of the flexor hallucis longus on this joint is different in both planes. When considering a Lapidus procedure for surgically correcting a hallux valgus, the mobility of MT 1 in the transversal plane should also be assessed, but so far no objective clinical test in this plane has been described.