An in vitro biomechanical comparison of the insertion variables and pullout mechanical properties of ao 6.5-mm standard cancellous and 7.3-mm self-tapping, cannulated bone screws in foal femoral bone.

This research paper compares the screw insertion properties and pullout mechanical characteristics of AO 6.5-mm cancellous and 7.3-mm cannulated bone screws in foal femoral bone using in vitro methods. The study found that both types of screws exhibit similar strengths and weaknesses in different areas of the bone but have more holding power at the mid-diaphyseal (middle-shaft) location. There is no obvious advantage in using screws larger than 6.5 mm in diameter, but the 7.3-mm cannulated screws could be more suitable in cases where more precision is required or when bone threads for the 6.5-mm cancellous screw have been damaged.

Methods

• The researchers used seven pairs of femora from young (1-7 months old) foals for their in vitro mechanical study.
• Both the 6.5-mm cancellous and 7.3-mm cannulated screws were inserted in predetermined proximal and distal metaphyseal, and mid-diaphyseal locations in the bones.
• Measurements were then taken for the insertion torque, force, time to drill, tap (for the 6.5-mm cancellous screw), guiding wire insertion (for the 7.3-mm cannulated screw), and the time for the screw insertion.
• Screw pullout properties such as load until deformation and failure, displacement, energy, and stiffness, were evaluated using mechanical tests.
• A repeated measure ANOVA was used to assess the effects of the screw type and their location in the bone on the obtained insertion and pullout values. Wherever necessary, post hoc contrasts were used for pairwise comparisons. The set significance level for all comparisons was P < .05.

Results

• The 7.3-mm cannulated screws showed significantly greater insertion torques than the tap required for the 6.5-mm cancellous screws, but lower than the 6.5-mm cancellous screws themselves.
• There were no significant differences observed in the total times it took to insert either types of screws.
• Both types of screws showed similar pullout properties for each location of insertion in the femora.
• The holding power, or ability to withstand pullout forces, for both types of screws was significantly higher in the mid-diaphysis location as compared to either metaphyseal location.
• Failures during pullout tests occurred due to shearing of the bone at the bone-screw interface for all specimens under study.

Conclusions

• Both the 6.5-mm cancellous and 7.3-mm cannulated screws showed variations in their insertion properties but had similar pullout characteristics in the mid-diaphysis, proximal, and distal metaphysis of the foal femurs.
• Both types of screws displayed greater holding power when inserted at the mid-diaphyseal location as compared to the metaphyseal locations in the bones.
• The overall similarity in the holding power of the two types of screws suggested that merely increasing the screw diameter beyond 6.5 mm may not lead to an increase in holding power in a foal’s femur.
• However, the 7.3-mm cannulated screw could potentially be used for more precise treatments or in instances where the bone threads for the 6.5-mm cancellous screw have been stripped.