THE BIOMECHANICAL PROPERTIES OF LIGAMENTOUS RESTRAINTS OF THE FIRST METATARSAL

Abstract

The purpose of this study was to investigate the influence of ligamentous restraints on first metatarsal (MT1) movements in the context of hallux valgus (HV), the surgical correction of which relies on a sound understanding of factors leading to MT1 deformity. Hypermobility or instability of the first metatarsal at its tarsometatarsal joint (TMJ1) is associated with greater degrees of deformity and also greater risk of recurrence after surgery. Recent anatomical work has shown the importance of the plantar aponeurosis (PA), and the transverse ‘tie-bar’ system (TTB) of the metatarsophalangeal (MPJ) plantar plates and intervening deep transverse inter-metatarsal ligaments in the structure and function of the foot. These ligamentous systems are important in MPJ stability, but may also be important at TMJ1.

Ten normal cadaveric feet were dissected to expose the capsules and ligaments of the MPJs and TMTJs and the PA. They were then mounted in plaster of Paris leaving the MT1, MT2, and their articulations free. A loading fixture was constructed so that loads could be applied to MT1 in the transverse plane to produce moments in flexion, abduction or extension. The movements resulting from a load of 40N were measured relative to MT2 using an Isotrak II (Polhemus, US) magnetic measurement system. The tests were performed with the hallux mobile, fixed neutral and fixed dorsiflexed at the MPJ. After an initial test with all structures intact, the PA and the TTB were severed in random order and the test repeated. “Movement maps” were produced showing the range of motion available in different directions and with different ligamentous restraints.

Movement maps suggest that the TMJ1 behaves as a ball and socket joint with no preferred axis of motion. The contributions of the PA and TTB to stiffness in the sagittal plane are small (about 1° movement). However, the TTB provides significant control of the abduction of MT1. The control afforded by the TTB is particularly important since it can ensure that the PA acts to provide an adduction moment about the TMJ1. The integrity of these ligamentous structures is likely to be important in the success of corrective surgical procedures for HV, where disruption can allow up to 10° increase in MT1-2 angles.