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 fused by application of a dorsal plate and by a screw placed in lag fashion through the metacarpus to each proximal sesamoid bone. Five specimens 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 gages. The specimens were loaded axially in compression to 4,000 N in a cast (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, closely approximated the vertical axis at loads < 1,000 N. The principal angle 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. Specimens in a cast (tests 1 and 2) suffered less surface deformation than did uncast specimens (test 3). Therefore, the cast changed the direction and extent of bending at the point of fixation, and thereby decreased the deformation of the plate. This effect would lead to greater fatigue life of the implant in the cast specimens compared with the uncast specimens.
Parente
E J
EJ
Department of Clinical Studies, New Bolton Center, University of Pennsylvania, Kennett Square.
Nunamaker
D M
DM
D000818:Animals; D002370:Casts, Surgical / Q000662:veterinary*; D005593:Fracture Fixation, Internal / Q000662:veterinary*; D050723:Fractures, Bone / Q000503:physiopathology / Q000601:surgery / Q000662:veterinary*; D006736:Horses; D013314:Stress, Mechanical; D014033:Toe Joint / Q000293:injuries*; D016474:Weight-Bearing / Q000502:physiology