Crutch Weightbearing on Comminuted Humeral Shaft Fractures: A Biomechanical Comparison of Large versus Small Fragment Fixation for Humeral Shaft Fractures

Abstract

This study evaluated the failure properties of length unstable humerii secured with small or large fragment plates. Two nonlocking plate constructs were examined, a nine-hole 4.5-mm limited contact dynamic compression plate (large fragment group) and a 12-hole 3.5-mm limited contact dynamic compression plate (small fragment group), both on composite humerii with a 1-cm defect to simulate comminution (n = 12 for each group). Each plate construct had similar working lengths and number of fixation points. Mechanical testing was first randomized for stiffness measurements in axial and torsional loads. All constructs were then tested in cyclic axial loads to failure. For axial testing, the large fragment group had a mean stiffness of 1020 ± 195 N/mm compared with 268 ± 67 N/mm in the small fragment group (P < 0.0001). For torsional testing, the large fragment group had a mean stiffness of 1.5 ± 0.05 Nm/degree compared with 0.9 ± 0.04 Nm/degree in the small fragment group (P < 0.0001). Plastic deformation in the large fragment and small fragment groups were 0.09 ± 0.07 mm and 0.20 ± 0.24 mm, respectively (P = 0.1) assessed during cyclic testing up to 300 N. The postcyclic yield force in the large fragment group was 227 ± 30 N and in the small fragment group was 153 ± 5 N (P < 0.0001). The ultimate load in the large fragment and small fragment groups were 800 ± 87 N and 307 ± 15 N, respectively. The results corroborate anticipated plate mechanical behavior with plate stiffness increasing as both plate width and thickness increase. The calculated yield force data suggest that both small and large fragment constructs would experience plastic deformation during bilateral crutch ambulation in a patient weighing 50 kg or more. The large fragment construct is not expected to catastrophically fail when subjected to loads in a patient 90 kg or less. The small fragment construct is predicted to catastrophically fail in patients weighing 70 kg or more.