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  • Author or Editor: Rhonda L. Aper x
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Abstract

Objective—To test the effects of bone diameter and eccentric loading on fatigue life of 2.7-mm-diameter cortical bone screws used for locking a 6-mm-diameter interlocking nail.

Sample Population—Eighteen 2.7-mm-diameter cortical bone screws.

Procedure—A simulated bone model with aluminum tubing and a 6-mm-diameter interlocking nail was used to load screws in cyclic 3-point bending. Group 1 included 6 screws that were centrally loaded within 19-mm-diameter aluminum tubing. Group 2 included 6 screws that were centrally loaded within 31.8-mmdiameter aluminum tubing. Group 3 included 6 screws that were eccentrically loaded (5.5 mm from center) within 31.8-mm-diameter aluminum tubing. The number of cycles until screw failure and the mode of failure were recorded.

Results—An increase in the diameter of the aluminum tubing from 19 to 31.8 mm resulted in a significant decrease in the number of cycles to failure (mean ± SD, 761,215 ± 239,853 to 16,941 ± 2,829 cycles, respectively). Within 31.8-mm tubing, the number of cycles of failure of eccentrically loaded screws (43,068 ± 14,073 cycles) was significantly greater than that of centrally loaded screws (16,941 ± 2,829 cycles).

Conclusions and Clinical Relevance—Within a bone, locking screws are subjected to different loading conditions depending on location (diaphyseal vs metaphyseal). The fatigue life of a locking screw centrally loaded in the metaphyseal region of bone may be shorter than in the diaphysis. Eccentric loading of the locking screw in the metaphysis may help to improve its fatigue life. (Am J Vet Res 2003;64:569–573)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To investigate the effects of disk fenestration and ventral slot formation on vertebral motion unit (VMU) range of motion (ROM) and determine the effects of fenestration and ventral slot width on VMU ROM.

Sample Population—C5-C6 VMUs from 10 skeletally mature canine cadavers.

Procedures—Specimens were assigned to 2 groups (5 specimens/group). Surgery was performed in which width of a fenestration and a ventral slot was 33% (group 1) or 50% (group 2) the width of the vertebral body. Flexion-extension, lateral bending, and axial torsion ROMs were measured during loading before surgery, after fenestration, and after ventral slot formation. Range of motion was compared within groups to determine effects of surgical procedure on stability and between groups to determine effects of width of fenestration and ventral slot on stability.

Results—For both groups, fenestration resulted in a significant increase in ROM during flexion-extension, compared with results for intact specimens. Ventral slot formation resulted in a significant increase in ROM during flexion-extension and lateral bending, compared with results for intact specimens. Ventral slot formation resulted in a significant increase in ROM only during flexion-extension, compared with results for fenestrated specimens. There were no significant differences in ROM of the intact, fenestrated, and ventral slot specimens between groups.

Conclusions and Clinical Relevance—Analysis of these results suggests that fenestration and ventral slot procedures each affect the biomechanics of the C5-C6 VMU. Width of a fenestration or ventral slot up to 50% of the width of C5-C6 may be clinically acceptable.

Full access
in American Journal of Veterinary Research