Assessment of stiffness and strength of 4 different implants available for equine fracture treatment:: A study on a 20° oblique long-bone fracture model using a bone substitute

Objective-To compare the mechanical properties of 4 stabilization methods for equine long-bone fractures: dynamic compression plate (DCP), limited contact-DCPlate (LC-DCP), locking compression plate (LCP), and the clamp-rod internal fixator (CRIF-formerly VetFix). Study Design-In vitro mechanical study. Sample Population-Bone substitute material (24 tubes) was cut at 20 degrees to the long axis of the tube to simulate an oblique mid-shaft fracture. Methods-Tubes were divided into 4 groups (n = 6) and double plated in an orthogonal configuration, with I screw of I implant being inserted in lag fashion through the "fracture". Thus, the groups were: (1) 2 DCP implants (4.5, broad, 10 holes); (2) 2 LC-DCP implants (5.5, broad, 10 holes); (3) 2 LCP implants (4.5/5.0, broad, 10 holes) and 4 head locking screws/plate; and (4) 2 CRIF (4.5/5.0) and 10 clamps in alternating position left and right of the rod. All constructs were tested in 4-point bending with a quasi-static load until failure. The implant with the interfragmentary screw was always positioned on the tension side of the construct. Force, displacement, and angular displacement at the "fracture" line were determined. Construct stiffness under low and high loads, yield strength, ultimate strength, and maximum angular displacement were determined. Results-None of the implants failed; the strength of the bone substitute was the limiting factor. At low loads, no differences in stiffness were found among groups, but LCP constructs were stiffer than other constructs under high loads (P=.004). Ultimate strength was lowest in the LCP group (P=.01), whereas yield strength was highest for LCP constructs (409 Nm, P=.004). CRIF had the lowest yield strength (117 Nm, P=.004); no differences in yield strength (250 Nm) were found between DCP and LC-DCP constructs. Differences were found for maximum angular displacement at the "fracture" line, between groups: LPC < DCP < LC-DCP < CRIF (P < .037). Conclusions-DCP, LC-DCP, and LCP constructs provided sufficient biomechanical stability to withstand single-cycle loads that might be experienced postoperatively. LCP constructs showed the best performance because of the highest yield strength, above which irreversible deformation occurred. Inadequate biomechanical properties, excessive motion, and shape of the device create concern about the use of CRIF in these large sizes. Clinical Relevance-CRIF does not meet the demands for equine long-bone fracture treatment. With respect to biomechanical properties, DCP, LC-DCP, and LCP constructs did not show critical differences so other factors may direct clinical selection of these implants. We prefer the LCP implants because of the high yield strength, high stiffness under high-load application, and the least movement at the fracture line. (c) Copyright 2005 by The American College of Veterinary Surgeons.