Biomechanical Consequences of First Metatarsaophalangeal Joint Arthrodesis on Flexor Digitorum Longus Function: A Cadaveric Study

SLR - July 2019 - Woojung M. Lee

Scientific Literature Review

Reviewed By: Woojung M. Lee, DPM
Residency Program: Hunt Regional Medical Center – Greenville, TX

Podiatric Relevance: First metatarsophalangeal joint (MTPJ) arthrodesis is a common treatment modality for hallux rigidus. It has been reported to have union rate as high as 100 percent and significant improvement of patient satisfaction up to 85 percent. One of the most common problems encountered by patients after arthrodesis is the lesser toe metatarsalgia, defined as pain in either the heads of the metatarsal or the MTPJs. Several biomechanical studies have shown that the loss of FDL function causes significant change of forefoot loading from lesser toes to metatarsal heads. The goals of this study is to investigate the hypothesis that the 1st MTPJ arthrodesis leads to loss of FDL function and therefore causes abnormal forefoot loading and metatarsalgia.

Methods: Ten fresh frozen cadaveric specimens were used to measure lesser toe range of motion (ROM) and FDL excursion on simulated FDL contraction. Proximal tensile force measured 50N was exerted by a single investigator at FDL musculotendinous junction to simulate active plantar flexion of the lesser toes and the third toe MTPJ and proximal interphalangeal joint (PIPJ) flexion ROM were measured visually using goniometer with reference to metatarsals, with ankle in the stationary position at about 20 degrees of plantar flexion. On application of proximal tensile force at the FDL musculotendinous junction, the difference between the reference point and the surgical marker on the FDL was taken as the excursion of FDL. These measurements were taken in three difference scenarios: 1) before 1st MTPJ arthrodesis, 2) after 1st MTPJ arthrodesis, and 3) after the knot of Henry release.

Results: There was a significant decrease in the mean 3rd toe flexion ROM and FDL excursion post 1st MTPJ arthrodesis. Before 1st MTPJ arthrodesis, mean 3rd toe MTPJ ROM and PIPJ ROM were 20 degrees and 22 degrees with FDL excursion of 6mm, compared to post 1st MPTJ arthrodesis values of 9.5 degrees and 14 degrees with FDL excursion of 4.7 mm. However, there was improvement in these parameters after the knot of Henry release with mean 3rd toe MTPJ ROM and PIPJ ROM of 16.5 degrees and 19 degrees with FDL excursion of 5mm.

Conclusions: This study has shown reduced FDL function following 1st MTPJ arthrodesis. There was reported improvement after the knot of Henry release, which demonstrates that the knot of Henry is the major contributory cause of FDL function loss in 1st MTPJ arthrodesis. This finding is clinically important as it demonstrates the relevance of releasing the knot of Henry during 1st MTPJ arthrodesis to reduce forefoot loading, with subsequent reduced rate of metatarsalgia. In my opinion, one of the biggest limitation of this study is that goniometer was used for measurement of ROM which can be subjective and therefore less accurate. However, this study successfully suggested potential benefits of the knot of Henry release in 1st MTPJ arthrodesis to improve FDL function, thereby reducing the incidence of metatarsalgia and other associated complications of forefoot loading.