A biomechanical comparison of headless tapered variable pitch and AO cortical bone screws for fixation of a simulated lateral condylar fracture in equine third metacarpal bones.

The research was conducted to compare the effectiveness of two types of screws (AO and Acutrak Plus (AP)) used in fixing fractures in horse’s metacarpal bones. The factors tested included the process of inserting screws and their mechanical strength and stiffness. The results indicated that the type of screw used had similar effects on the procedure and fracture stability.

Research Objectives

• The main focus of the study was to compare the biomechanics of 4.5-mm AO screws and 6.5-mm Acutrak Plus (AP) screws in fixing equine metacarpal bone fractures. The objective was to find out which type of screw offers optimal handling and performance.
• The secondary objective was to assess the shear strength and stiffness of a simulated lateral condylar fracture of the equine third metacarpal bone (MC3) stabilized with either an AO or AP screw.

Methodology

• The researchers performed various tests on pairs of cadaveric equine MC3s in a controlled lab setting.
• The metacarpi (part of the horse’s leg where the fracture is simulated) were centered on a biaxial load cell in a materials-testing system. This setup was used to measure torque, compressive force, and time taken for drilling, tapping, and screw insertion, thereby assessing the efficiency of these procedures.
• A simulated lateral condylar fracture was made and fixed using either of the two screw types (AO or AP) and tested in shear until failure. They used standardized requirements to ensure reliability.
• Statistical tools like a paired t test were used to assess differences between screws, with the significance level set at P < .05.

Findings

• The research findings showed that both AO and AP screws demonstrated comparable parameters when it comes to insertion procedure and mechanical shear testing. This means that both types of screws performed similarly in these respects.
• From these results, the 6.5-mm tapered AP screw showed equivalent mechanical stability when inserted in equine third metacarpal condyles, thereby being as effective as the 4.5-mm AO screw in fixing a simulated lateral condylar fracture.

Conclusion

• Given the similar mechanical behavior, the 6.5-mm tapered AP screw provides an attractive alternative for the repair of incomplete lateral condylar fractures in horses.
• The researchers also highlight the advantages of the AP screw’s headless design, which can potentially reduce irritation in soft tissues after surgery, and its ability to achieve interfragmentary compression when it is inserted in a hole drilled perpendicular to the fracture plane.