JAMES L. CARMALT, MA, VetMB, MVetSc, Diplomate ABVP & ACVS 1 , LANA DELANEY, DVM 1 , and DAVID G. WILSON, DVM, Diplomate ACVS 1 1 Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, SK, Canada Veterinary Surgery January 2010 39 1 91 - 94 Objective—To compare the biomechanical cyclic fatigue properties of 2 and 3 parallel transarticular 5.5 mm cortical screws used in arthrodesis of the proximal interphalangeal (PIP) joint. Study Design—Randomized block design, for horse, fixation method (2 versus three 5.5 mm cortical screws), side (right, left) and end (front, hind) in cadaveric equine limbs. Sample Population—Cadaveric adult equine fore- and hindlimbs (n=5 pairs each). Methods—Two parallel 5.5 mm cortical screws were inserted in lag fashion, transarticularly through the PIP joint in 1 limb of a pair, and in the contralateral limb, three 5.5 mm cortical screws were inserted in similar fashion. Constructs were then tested in 3-point bending in a dorsal-to-palmar (plantar) direction using a materials testing machine using a cyclic load of −500 to −3500 N at a rate of 6 Hz. Results—There was no significant difference in displacement at failure, force at failure or number of cycles between limbs. Forelimb constructs tended to fail at a greater mean displacement than hindlimb constructs but this difference was not significant (P=.06). There was no statistical difference in any tested biomechanical variable between left- and right-sided limbs. Conclusions—There was no significant difference in the number of cycles to failure for the 2 methods tested. Clinical Relevance—Two 5.5 mm cortical screws inserted in parallel for PIP joint arthrodesis is surgically simpler, results in comparable biomechanical performance in the current model and should perform as well as three 5.5 mm screws under cyclic fatigue conditions.