SLR - June 2020 - Julio D. Perez-Mustelier
Reference: Geng X, Shi J, Chen W, et al. Impact of First Metatarsal Shortening On Forefoot Loading Pattern: A Finite Element Model Study. BMC Musculoskelet Disord. 2019;20(1):625. Published 2019 Dec 27. doi:10.1186/s12891-019-2973-6Scientific Literature Review
Reviewed By: Julio D. Perez-Mustelier, DPM
Residency Program: Bethesda Hospital East – Boynton Beach, FL
Podiatric Relevance: Surgical management of first metatarsal deformities forms the foundation of many podiatric practices. This study was designed to further expand on how much shortening of the first metatarsal can be safely accommodated for, post hallux valgus reconstruction. In 2003, Maestro et al. found that the length of the first metatarsal should be within 2 mm of the second metatarsal. This 2 millimeters variance has become widely accepted among foot and ankle surgeons, however in a 2016 study Ahn et al. found no significant correlation between first metatarsal shortening and postoperative transfer metatarsalgia.
Methods: Using advanced computer software and imaging, the authors performed a 3D finite element analysis and investigated the relationship between first metatarsal shortening and corresponding plantar loading patterns. A single volunteer subject’s foot kinematics were measured. Using a customized foot plate, CT scans were also performed with the subject’s ankle and metatarsophalangeal joints held in positions designed to mimic push-off phase. 3D reconstruction and simulation of the osseous and ligamentous structures were performed. The reconstructed foot model, including the osseous and soft tissue structures were input in ABAQUS. A standardized metatarsal shortening procedure was performed, and all confounding factors were eliminated. The metatarsal was then shortened from 2 to 8 millimeters, in increments of 2 millimeters. After the osteotomy, the distal part of the metatarsal segment was moved proximally and parallel to both the longitudinal axis of the second metatarsal and the mentioned reference plane.
Results: The model-calculated results were similar to the measured values, in terms of the peak plantar pressures. Thus validating model’s ability to calculate the peak plantar pressure as well as the pressure distribution. With increased shortening the plantar pressure of the first ray decreased rapidly, with concomitant increase in the lesser metatarsals. The authors assumed all the alignment and other structural deformities have been corrected, so the confounding factors due to different deformities that may potentially influence model results are excluded
Conclusions: Geng et al. found that post osteotomy patients had substantially increased risk of metatarsalgia when plantar loading ratio of the central rays reached 55 percent during push-off phase of gait. By using the forefoot loading ratio of 55 percent as a threshold value, the author calculated a maximum value for the first metatarsal shortening to reduce transfer metatarsalgia. The loading ratio of the central rays increased up to 54.8 percent when the first metatarsal was shortened by 6 millimeters, approaching the 55 percent nearing threshold based on 2017 study. Ultimately the authors found that when the first metatarsal is shortened less than 6 millimeters, the increased loading ratio of the central rays does not approach the threshold for transfer metatarsalgia.