Abstract

Lapidus arthrodesis is used in the treatment of hallux valgus, first ray instability and midfoot arthritis. Despite being commonly performed, few studies have addressed the regional biomechanical implications of this procedure. Our objective was to analyse the stress concentrations caused by two commonly performed Lapidus arthrodesis on surrounding bone and soft tissue structures of the foot. A finite element model was used to simulate the normal intact foot and scenarios of tissues deficiencies that are often associated when a Lapidus arthrodesis is performed. Our model includes all the foot bones, cartilage and major tendons and ligaments that support the foot arch. Both tensile stress and compressive forces were measured in the midfoot bones, joints and tibialis posterior tendon. Results showed that the classical Lapidus arthrodesis is associated with an increase of about 76% in the compressive stress generated around the first and second cuneiform joint, while the isolated metatarsocuneiform arthrodesis showed a non-significant increase in stress in this region. The Lapidus procedures slightly offload tensile stresses in the tibialis posterior tendon but were not alone able to compensate for the lack of a calcaneonavicular (spring) ligament failure despite increasing the rigidity of the arch. We concluded that the Lapidus arthrodesis does have regional implications on soft tissues and bone that are difficult to define. Whilst helping to decrease stresses in the tibialis posterior tendon stress and allows correcting both the first ray deformity and stability, it cannot compensate the proximal talonavicular laxity when trying to restore the arch integrity.

Keywords: Biomechanics, Flatfoot, Finite element modelling, Hallux valgus, Lapidus