Background:​ A recent clinical study suggested that restrictive tenosynovitis of the flexor hallucis longus (FHL) may play an important causative role in hallux rigidus. The goals of this research were to assess normal function of the FHL and the effect of restricted FHL gliding on the loading of the hallux metatarsophalangeal joint (MTPJ). We hypothesized that proximal displacement of the FHL would increase the forces carried by the FHL tendon, causing increased loading of the first metatarsal and MTPJ.

Materials and Methods:​ Dynamic simulations of the stance phase of walking were created by loading 5 non-embalmed, non-paired cadaver lower extremities (2 male, 3 female;​ mean age, 69;​ range, 56 to 75) in the Robotic Dynamic Activity Simulator (RDAS). The RDAS recreates lower leg motion and muscle activity based on kinematic and electromyographic data of healthy subjects. FHL tendon forces and excursions were measured for the entire stance phase of gait. Bone strains in the mid-shaft of the first metatarsal were recorded and used to derive the in situ forces and moments imposed on the metatarsal and the hallux MTPJ.

Results:​ Under force-feedback control, FHL excursion averaged 6.57 (+/-3.13) mm during gait. When the FHL was held 2-, 4-, and 6-mm proximal to the mid-point of normal excursion to simulate progressive stenosis, the forces in the FHL tendon, first metatarsal and first MTP joint were progressively, and significantly, increased (p < 0.05).

Conclusion:​ These findings support the hypothesis that progressive fibrosis at the FHL myotendinous junction can cause increased loading of the hallux MTPJ, thereby leading to hallux rigidus.