A biomechanical comparison of equine third metacarpal condylar bone fragment compression and screw pushout strength between headless tapered variable pitch and AO cortical bone screws.

The research explores a biomechanical comparison of bone fragment compression and screw pushout strength in horse bones, particularly the third metacarpal (MC3), using headless tapered variable pitch screws (6.5-mm Acutrak Plus, AP) and AO cortical bone screws (4.5-mm). The study indicates that the AP screws provide a comparable holding strength to AO screws but offer reduced interfragmentary compression for treating complete lateral condylar fractures in horses.

Objectives and Methodology

• The study aimed at comparing the bone fragment compression and mechanical pushout strength of AP and AO bone screws after stabilizing a simulated equine MC3 lateral condylar fracture.
• The researchers followed an in vitro paired study design of screw insertion variables, bone fragment compression, and screw pushout tests using a bone screw stabilized simulated lateral condylar fracture model.
• The study used six pairs of cadaveric equine MC3s. The metacarpi were appropriately positioned and centred on a biaxial load cell in a materials testing system to measure torque, compressive force, and time required for drilling, tapping, and screw insertion.
• Measured fragment compression was done using a pressure-sensing device placed between the simulated fracture fragments during the process of screw insertion for fragment stabilization. Eventually, the screws were pushed out of the stabilized bone fragments in a single cycle to failure.

Results

• The study’s results showed that drilling variables for both AO and AP were comparable. However, AP tap had significantly greater insertion torque and force.
• The mean settling screw insertion torque was much more substantial for AO screws. For fragment compression, AP screws generated 44% to 65% of the compressive pressure and force, respectively, created by AO screws.
• AP screws had higher pushout strength than AO screws. However, their degree of stiffness was similar.

Conclusions

• Based on the gathered data, the researchers concluded that the tapered 6.5-mm AP screw generated less interfragmentary compression pressure and force, but had the same degree of pushout stiffness.
• Evaluation of failure patterns suggested that AP screws had greater pushout strength than the AO screws for the fixation of a simulated complete lateral condylar fracture.
• Hence, for the repair of complete lateral condylar fractures in horses, the 6.5-mm tapered AP screw could offer a secure holding strength but would deliver less interfragmentary compression compared to 4.5-mm AO screws.