Biomechanical comparison of the bone-screw-fastener to conventional cortical buttress screw in a simulated ex vivo model of equine midbody proximal sesamoid bone fracture repair.

Overview

• This study compares the biomechanical performance of a new bone-screw-fastener (BSF) to traditional cortical screws (CS) for repairing simulated fractures in equine proximal sesamoid bones (PSB).
• It evaluates implant failure, displacement, and mechanical strength in different screw configurations using ex vivo horse cadaver limbs.

Study Purpose and Importance

• Proximal sesamoid bone fractures are common injuries in horses that can challenge surgical repair due to biomechanical stresses.
• The research aims to determine if the newer BSF provides improved or comparable biomechanical fixation compared to standard cortical screws (CS).
• Understanding which implant better resists mechanical failure is critical for improving surgical outcomes and horse recovery.

Materials and Methods

• Subjects: 14 paired cadaver forelimbs from horses were used to simulate fractures.
• Fracture Model: Medial mid-body osteotomies (artificial fractures) were created on each proximal sesamoid bone.
• Surgical Repair Groups:
  • Group 1: Single 3.5 mm bone-screw-fastener (BSF)
  • Group 2: Single 4.5 mm cortical screw (CS)
  • Group 3: Two 3.5 mm BSFs
  • Group 4: Two 3.5 mm cortical screws (CS)
• Number of samples: n = 7 per group.
• Testing: Each repaired bone underwent a single cycle to failure test assessing yield load and gap displacement between fracture segments.
• Statistical Analysis: Comparisons used Wilcoxon-matched pairs or Mann-Whitney tests; significance set at p < .05.

Key Findings

• Yield loads (force at which the implant starts to fail) were significantly higher when using two screws versus one screw, irrespective of screw type, suggesting that double screw fixation is biomechanically superior.
• No significant difference in biomechanical properties was detected when comparing single BSF to single 4.5 mm CS repairs, indicating comparable fixation strength.
• Gap displacement measurements showed greater movement on the abaxial side (away from the midline) than on the axial side for all groups at the point of construct failure, indicating lateral forces play a critical role in fixation stability.
• Overall, the BSF performed similarly to the traditional cortical screws in stabilizing the simulated fractures.

Interpretation and Clinical Implications

• Single BSF screws can provide fixation strength comparable to the commonly used 4.5 mm cortical screws, suggesting they are a viable alternative for PSB fracture repair.
• Using two screws—regardless of type—significantly improves mechanical stability compared to single screw repairs, recommending a preference for double screw fixation in surgery.
• The observation of increased gap displacement abaxially implies that lateral forces may contribute to implant failure or fracture instability; therefore, surgical techniques or implant designs that better counteract these forces could improve outcomes.
• Further research into mitigating abaxial forces and validating in vivo performance would be beneficial.

Limitations and Future Directions

• The study was conducted ex vivo using cadaver limbs, which may not fully replicate live tissue conditions such as healing response and dynamic loading during movement.
• The sample size was relatively small (7 repairs per group), which may limit the generalizability of findings.
• Future studies should investigate long-term clinical outcomes, implant performance in live animals, and techniques to address abaxial displacement forces during and after surgery.