The Role of the Plantar Fascia, Flexor Hallucis Longus, and Fiberosseous Tunnel in Functional Hallux Rigidus

Abstract

Category: Midfoot/Forefoot Introduction/Purpose: Functional hallux rigidus is characterized by limited first metatarsophalangeal (MTP) dorsiflexion during weight bearing conditions, but normal motion when the metatarsal is allowed to plantarflex. This phenomenon alters the mechanics of the foot, potentially leading to lateral column overloading and dynamic cavovarus. Furthermore, it is suspected that the abnormal forces across the first MTP alters its biomechanics, leading to cartilage destruction and eventual structural hallux rigidus. The purpose of this study is to assess the role of the flexor hallucis longus (FHL) and plantar fascia in functional hallux rigidus and to determine the best surgical treatment. Methods: Six cadaveric lower limbs disarticulated at the hip were analyzed. The feet were positioned manually in the same manner that would be used in a clinical exam of patients. The examiner's thumb was used to stabilize the first metatarsal head and prevent plantarflexion to simulate the closed-chain kinetics of weight bearing. The same hand was used to hold the ankle neutrally aligned to the leg. The other hand was used to maximally dorsiflex the great toe. X-ray was used to obtain images of the feet. The plantar fascia was transected with a Z cut, shortened approximately 5 mm, then repaired. The fiberosseous tunnel was released. The FHL was transected with a Z cut then shortened approximately 5 mm. Great toe dorsiflexion was assessed and X-rays taken after each of the manipulations. Range of motion of the first MTP was measured on Xrays and analyzed using Student's T-test. Results: Average passive dorsiflexion of the first MTP is 29.8° (range 17-46°) prior to manipulation. Shortening the plantar fascia and FHL did not decrease the MTP dorsiflexion significantly beyond baseline (P = 0.08, 0.09 respectively). Transecting the plantar fascia increased the dorsiflexion by 14° (standard deviation (SD) 9.5) and transecting the FHL increased dorsiflexion by 13.7° (SD 4.0). Dorsiflexion increased significantly beyond baseline with both these methods (P = 0.02 and P < 0.001 respectively), but the difference between the two methods was not significant (P = 0.93). Releasing the fiberosseous tunnel alone increased the dorsiflexion 8.8° (SD 5.7). Increased dorsiflexion beyond baseline was significant (P = 0.03). The increase in dorsiflexion with fiberosseous tunnel release was not significantly different than transecting the plantar fascia or the FHL (P = 0.40 and 0.42 respectively). Conclusion: By definition, functional hallux rigidus is diagnosed when first MTP dorsiflexion cannot reach 60°. Its prevalence may be more widespread than previously identified. Both the FHL and plantar fascia are implicated in functional hallux rigidus, as transection of either improves the dorsiflexion of the great toe. However, accessing the plantar fascia comes with significant risks and complete transection of the FHL is not feasible. As an alternative, releasing the fiberosseous tunnel is a simple, minimally morbid procedure that can also significantly improve functional hallux rigidus to a similar degree.