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.
Authors: Johnson Nicole L, Galuppo Larry D, Stover Susan M, Taylor Ken T
Journal: Veterinary surgery : VS
Summary
# Editorial Summary This 2004 biomechanical study directly compared two commonly used screw types for foal femoral fracture fixation: the AO 6.5-mm standard cancellous screw versus the 7.3-mm self-tapping cannulated alternative, using paired femoral samples from seven immature foals. Researchers measured insertion variables (torque, force, and time) across three anatomical locations (proximal metaphysis, distal metaphysis, and mid-diaphysis) and subsequently performed pullout testing to evaluate mechanical holding properties including yield load, failure load, stiffness, and energy absorption. The 7.3-mm cannulated screws required significantly greater insertion torque than the 6.5-mm tapping step but less than the final 6.5-mm cancellous screw insertion, with no difference in overall insertion time; critically, both screw types demonstrated equivalent pullout properties at all three locations, though holding power was substantially superior in the mid-diaphysis compared to either metaphyseal region. For practitioners selecting fixation methods in foal femoral fractures, these findings suggest that the 7.3-mm cannulated option offers no mechanical advantage in terms of bone-holding capacity, making it most valuable when improved surgical accuracy via guide-wire placement is prioritised or when 6.5-mm threads have been compromised—otherwise the simpler 6.5-mm cancellous screw remains the biomechanically equivalent choice.
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Practical Takeaways
- •Choose 6.5-mm cancellous screws as standard for foal femoral fracture repair, as they provide equivalent holding power to larger diameter screws at lower cost
- •Consider 7.3-mm cannulated screws when improved surgical accuracy is needed or if 6.5-mm screw threads have been compromised in bone
- •Position screws in the mid-diaphysis of the femur when possible to maximize holding power compared to metaphyseal locations
Key Findings
- •6.5-mm cancellous and 7.3-mm cannulated screws showed similar pullout properties across all femoral locations tested
- •Mid-diaphyseal screw placement provided significantly greater holding power compared to metaphyseal locations in both screw types
- •7.3-mm cannulated screws required greater insertion torque than 6.5-mm taps but less than 6.5-mm cancellous screws
- •Increasing screw diameter beyond 6.5 mm does not provide increased holding power in immature foal femoral bone