An ex vivo biomechanical study of equine neonatal rib fracture repair by different fixation methods

Abstract

Introduction: Rib fractures result in significant morbidity and mortality of equine neonates. Various methods to repair these fractures have been described but biomechanical testing in immature equine neonatal bone has not been performed. The study objective was to compare strength, stiffness, and mode of failure for four different methods of equine neonatal rib fracture repair. Methods: Equine neonatal cadavers (< 2 weeks of age) without history of dystocia or thoracic trauma present were used for this study. Ribs three through six on both left and right side were randomly allocated to one of four repair groups: nylon cable tie (NCT), modified reconstruction plate (MRP), string of pearls plate (SOP), or rib clip (RC). Intact ribs were first fractured under three-point bending to induce a simple, transverse fracture. The fracture was then reduced and repaired according to the assigned repair group. Repaired ribs were then tested to failure under single-cycle, four-point bending. Strength (maximum force, Fmax, and force at failure, Ffail) and mode of failure were recorded. Stiffness (N/mm2) was calculated. Data were analyzed using linear mixed models Results: In intact ribs, no significant differences in Fmax (P=0.46), Ffail (P=0.35), or stiffness (P=0.90) were observed between groups. Among repair groups, Fmax, Ffail, and stiffness differed significantly (P<0.0001). NCT repairs had significantly lower Fmax and Ffail than MRP and SOP, while MRP and SOP had greater stiffness compared to NCT and RC. NCT and RC groups failed at the original fracture site, whereas MRP and SOP failed mostly due to screw pullout. Conclusion: Rigid fixation with MRP or SOP provides stronger fracture repair than NCT or RC when equine neonatal ribs are tested under single-cycle load to failure. Further investigation of both rigid and non-rigid repair methods with cyclic loading is warranted.