Arthrodesis of the equine proximal interphalangeal joint: A biomechanical comparison of three 4.5-mm and two 5.5-mm cortical screws

Objective-To compare the biomechanical characteristics and mode of failure of 2 parallel-screw techniques for proximal interphalangeal joint arthrodes is in horses. Study Design-Randomized block design, blocking for horse (1-5), method of s crew fixation (three 4.5-mm vs two 5.5-mm), side (left limb vs right limb), and end (front limb vs hind limb). Constructs were loaded to failure in 3- point bending in a dorsal-to-palmar (plantar) direction. Sample Population-Twenty limbs (10 limb pairs) from 5 equine cadavers. Methods-A combined aiming device was used to facilitate consistent screw pl acement. Three parallel 4.5-mm cortical screws were placed in lag fashion i n 1 limb of a pair, and 2 parallel 5.5-mm cortical screws were placed in la g fashion in the contralateral limb. Arthrodesis constructs were tested in 3-point bending in a dorsal-to-palmar (plantar) direction using a materials -testing machine. Loading rate was 19 mm/s. Maximal bending moment at. fail ure and composite stiffness were obtained from bending moment-angular defor mation curves. Data were analyzed using ANOVA and chi (2) analysis. Results-There were no significant differences in bending moment (P > .05, p ower = 0.8 @ delta = 19%) or composite stiffness (P > .05, power = 0.8 @ de lta = 19%) between the 2 fixation techniques. Higher maximal bending moment was found in front limbs than hind limbs, and Front limbs with two 5.5-mm screws than hind limbs with two 5.5-mm screws. In all cases, constructs com pletely failed. A greater number of 4.5-mm cortical screws failed than 5.5- mm cortical screws. Conclusions-In pastern arthrodesis constructs loaded in 3-point bending, en d (front limb vs hind limb) affected maximal bending moment at failure of c onstructs. There was no significant effect of horse, treatment, or side on maximal bending moment or stiffness. Two 5.5-mm cortical screws should prov ide a surgically simpler pastern arthrodesis than three 4.5-mm cortical scr ews while maintaining similar biomechanical characteristics. Clinical Relevance-Three 4.5-mm screws or two 5.5-mm screws will provide si milar biomechanical characteristics in bending when performing equine paste rn arthrodesis. (C) Copyright 2001 by The American College of Veterinary Su rgeons.