STRESS PROTECTION AFFORDED BY A CAST ON PLATE FIXATION OF THE DISTAL FORELIMB IN THE HORSE IN-VITRO

Six forelimb specimens from three adult horses had the fetlock joint f used by application of a dorsal plate and by a screw placed in lag fas hion through the metacarpus to each proximal sesamoid bone. Five speci mens were instrumented on the central dorsal surface of the plate with a single rosette strain gage, and the plate of the sixth specimen was instrumented with four longitudinally oriented single-axis strain gag es. The specimens were loaded axially in compression to 4,000 N in a c ast (test 1), in a cast with a heel block (test 2), and uncast (test 3 ). The principal angle of strain in all specimens, in all tests, close ly approximated the vertical axis at loads < 1,000 N. The principal an gle in uncast specimens was significantly different at loads > 1,000 N than the cast specimens (P < .05). At loads > 3,000 N, the principal angle in test 3 closely approximated the horizontal axis, indicating a change from tension to compression on the dorsal surface of the plate , whereas the principal angle of the cast specimens was unchanged. Spe cimens in a cast (tests I and 2) suffered less surface deformation tha n did uncast specimens (test 3). Therefore, the cast changed the direc tion and extent of bending at the point of fixation, and thereby decre ased the deformation of the plate. This effect would lead to greater f atigue life of the implant in the cast specimens compared with the unc ast specimens. (C) Copyright 1995 by The American College of Veterinar y Surgeons