Influence of soft tissue composition and arm diameter on fracture strain in simulated humeral shaft fractures undergoing functional bracing.

Abstract

Functional bracing is a common non-operative treatment for humeral shaft fractures. The effects of patient-specific soft tissue characteristics on fracture site biomechanics during bracing are poorly understood and may alter healing. This study leveraged finite element analysis (FEA) to characterize the impact of arm diameter and muscle-adipose composition on fracture site strain during bracing. In conjunction with other factors, researchers and clinicians may apply these findings toward optimizing outcomes in patients with humeral shaft fractures. Nine humerus FEA models were constructed with concentric cylindrical tubes representing fractured diaphyseal bone, muscle, adipose, and a plastic brace. Models had varying arm diameters (small, medium, and large, based on institutional data) and muscle-to-adipose tissue ratios (25%/75%, 50%/50%, 75%/25%). To simulate bracing and physiological bending movements, a uniform radial pressure (5.33 kPa) was applied to the brace, and a lateral force (40 N) was applied to the distal humerus with the proximal end fixed. Fracture site strain values were computed for each arm configuration. FEA findings were validated with biomechanical testing of a cadaveric arm that was braced and subjected to the same bending forces. For a specific arm size, an increase in adiposity, as indicated by a lower muscle-to-adipose ratio, resulted in increased Perren strain values at the simulated fracture site. Furthermore, at a given ratio of muscle-to-adipose, an increase in arm size corresponded to a decreased level of strain experienced at the fracture site. Cadaveric biomechanical testing yielded comparable strain values to FEA models of similar arm composition. These findings suggest that smaller diameter arms and increased adipose levels may increase fracture instability during functional brace treatment of humeral shaft fractures. Further, these findings may inform patient selection for functional bracing versus surgery for humeral shaft fractures or guide modifications to functional brace design.