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 deformit y. Hypermobility of the TMT 1 joint is tested clinically in the sagittal pl ane, but an important deformation also exists in the transversal plane: met atarsus 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 t o 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 i n either the dorsal or medial direction. To simulate muscle force, 21 N was applied to the three tendons: all seven possible combinations of muscle ac tion were tested in each plane of motion. Angular displacements were measur ed using 2-dimensional LED video registration. TMT 1 mobility is a relevant factor in MT 1 mobility in the sagittal and tr ansversal planes, the peroneus longus has a stabilizing effect on this join t, and the effect of the flexor hallucis longus on this joint is different in both planes. When considering a Lapidus procedure for surgically correct ing a hallux valgus, the mobility of MT 1 in the transversal plane should a lso be assessed, but so far no objective clinical test in this plane has be en described.