The failure mode of a mechanically loaded equine medial femoral condyle analog with a void and the impact of lag and neutral screw placement.
Authors: Moreno Caitlin R, Santschi Elizabeth M, Younkin Jarrod T, Larson Robert L, Litsky Alan S
Journal: Veterinary surgery : VS
Summary
Subchondral bone defects of the medial femoral condyle (MFC) present a challenging orthopaedic problem in equine practice, yet the biomechanical behaviour of different fixation approaches remains poorly characterised. Moreno and colleagues used composite material disks engineered to replicate yearling MFC bone properties to investigate how simulated defects fail under loading and whether screw placement technique influences structural integrity. All defective specimens failed by crack propagation from the superior aspect of the void or screw exit hole; crucially, both lag and neutral screw placement substantially improved resistance to cracking compared with unfilled voids, with neutral screws producing the smallest displacement at physiological loads (0.11 mm versus 0.32 mm for void-only controls). Under cyclic loading, neutral screw fixation proved markedly superior, with cracks developing in only 3 of 6 specimens compared with all void-only controls and all lag-screw specimens, suggesting that neutral placement distributes compressive forces more effectively across the defect site. These findings challenge conventional orthopedic dogma and suggest that neutral screw positioning may offer a simpler, equally effective alternative to lag techniques for managing MFC lucencies in clinical practice—a potentially significant consideration for surgical decision-making and implant selection in high-value horses.
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Practical Takeaways
- •Neutral screw placement may be as effective or superior to lag screw placement for treating MFC subchondral voids, potentially simplifying surgical technique while maintaining biomechanical stability
- •Screw fixation substantially reduces displacement and crack propagation in simulated MFC defects under weight-bearing loads, supporting surgical intervention for subchondral lucencies
- •This in vitro model demonstrates failure modes and screw performance but does not account for biological healing, cartilage coverage, or clinical outcomes—clinical validation is needed before changing surgical protocols
Key Findings
- •MFC voids failed by superior cracking under both monotonic and cyclic compression, with cracks observed in 6/6 void-only specimens under monotonic loading
- •Neutral screw placement reduced displacement at peak load to 0.11 mm compared to 0.32 mm in void-only specimens, outperforming lag screw placement (0.24 mm)
- •Cyclic loading revealed superior crack resistance with screw fixation: only 3/6 void/lag screw specimens cracked versus 6/6 void-only specimens
- •Neutral screw placement prevented cracking in 1/6 specimens during cyclic testing, suggesting potential as an alternative fixation method for subchondral lesions