Patient and Surgical Variability Influences Loading Across Metatarsal Osteotomy
Authors: Dharia Mehul A. , Bischoff Jeffrey E. , Woodburn Jim , Telfer Scott , Al-Munajjed Amir A. 
Published: October 2019

Category: Midfoot/Forefoot Introduction/Purpose: A metatarsal osteotomy is sometimes used to correct hallux valgus deformities in the foot, by placing an angular bone graft or spacer in the osteotomy wedge held together using an osteotomy plate. Information on loads through the osteotomy is essential to ensure that the device is adequately tested to perform safely. This study used a validated musculoskeletal computer model of the lower extremities [J Biomech Eng. 2015] with a detailed foot section [Carbes, ISB 2011, Brussels], for evaluating loads across the 1st metatarsal osteotomy during the stance phase of walking gait. This study also evaluates the effect of anatomic variability (osteotomy location relative to anatomical landmarks) and surgical variability (plane of the osteotomy cut) on the peak loading across the osteotomy. Methods: The foot section of the model included 26 bones and major soft tissue structures (Figure 1a), resulting in more than 75 forces acting on the 1st metatarsal. Four healthy patient specific models were analyzed (body weight: 774±73 N; foot length: 246±16 mm), for five osteotomy planes. The osteotomy plane perpendicular to the long axis of the 1st metatarsal (posterior- anterior) was considered a neutral-N osteotomy. Four additional planes were considered to capture surgical variability of 5 degrees in abduction-AB, adduction-AD, dorsiflexion-DF and plantarflexion-PF. Compressive (Fx) and shear (Fy, Fz) forces as well as three moments: varus/valgus (Mx), flexion/extension (My) and internal/external rotation (Mz) were predicted in body weight (BW) multiples. Anatomic variability (standard deviation in peak values between patients, averaged across all osteotomies) and surgical variability (Standard deviation in peak values between osteotomies, averaged across all patients) were computed for the peak force and moment components. Results: Similar trends for peak forces and moments were predicted for all patients. An average of 2.2xBW peak force was predicted along the long axis of the 1st metatarsal (Fx) at 50% gait position (Figure 1b, 1c). The predicted shear forces (Fy, Fz) were much lower than the axial compressive force (Fx). The average moment predictions were similar among all five osteotomy planes, with peak moments predicted in the internal/external direction (My) at 43% and 55% of gait positions. Thus, load variability due to patient anatomy and surgical cuts were studied for Fx and My (Figure 1c). Load variability between patients of 18% and 21% were predicted for the Fx and My, respectively. The highest surgical variability of 7% and 3% were predicted for Fx and My. Conclusion: A validated musculoskeletal computer model can be used to inform physiological loading across the 1st metatarsal osteotomy throughout gait, which can be used to evaluate the device safety. The model further helps to understand the influence of anatomical and surgical variability on the peak loading passing through the 1st metatarsal osteotomy, providing guidance in surgical planning. Although peak loading passing through the 1st metatarsal osteotomy is affected by both anatomical as well as surgical variability, it was influenced more by anatomical variability than surgical. This modeling approach can be used to evaluate loading for patients with numerous specific disorders.