The role of the plantar fascia, flexor hallucis longus, and fibro-osseous tunnel in functional hallux rigidus.

BACKGROUND

Functional hallux rigidus is characterized by limited first metatarsophalangeal (MTP) dorsiflexion during weight bearing conditions, but normal motion otherwise. This phenomenon alters the biomechanics of the foot, potentially leading to first MTP pain and trauma to the MTP cartilage. The purpose of this study is to assess the role of the flexor hallucis longus (FHL), plantar fascia (PF), and fibro-osseous tunnel in functional hallux rigidus.

METHOD: Fourteen cadaveric lower limbs were analyzed. The Dananberg technique was used to dorsiflex the great toe and radiographs were taken after each of the following manipulations, measuring the dorsiflexion angle of the first MTP. The plantar fascia was transected with a Z-cut, shortened approximately 10 mm, then repaired. The fibro-osseous tunnel was released. The FHL was transected with a Z-cut then shortened approximately 7 mm.

RESULTS

Average passive dorsiflexion of the first MTP is 23.8° (95% CI 19.7-27.9) prior to manipulation. Shortening the plantar fascia and FHL did not decrease the MTP dorsiflexion significantly beyond baseline (p>0.05). Lengthening the plantar fascia increased the dorsiflexion by 6.7° (28.1% increase) and lengthening the FHL increased dorsiflexion by 7.0° (29.4% increase). The difference between the two methods was not significant (p>0.05). Releasing the fibro-osseous tunnel alone increased the dorsiflexion 3.4° (14.4% increase) and was not significantly different from PF or FHL lengthening.

CONCLUSION

Functional hallux rigidus causes limitation of first MTP dorsiflexion when the first metatarsal head is prevented from plantarflexing. There is no difference between the contribution of the FHL versus the plantar fascia to functional hallux rigidus. The true etiology may be multifactorial and involve other structures such as the flexor hallucis brevis, first metatarsal, and gastrocnemius/soleus/achilles complex. As such, no clear surgical intervention can be recommended to improve functional hallux rigidus except in cases with clear-cut causes of stenosis.