Stress protection afforded by a cast on plate fixation of the distal forelimb in the horse in vitro.

This research article focuses on a study where the impact of using a cast on plate fixation in the distal forelimb of horses was analyzed. The researchers found that the use of a cast improved the fatigue life of the implant, reducing the deformation observed.

Experimental Set-Up

• This study included six forelimb specimens taken from three adult horses. The fetlock joint in each of these specimens was fused through the application of a dorsal plate and a screw placed in the metacarpus leading to each proximal sesamoid bone.
• Five of these specimens had a single rosette strain gage instrumented on the central dorsal surface of the plate. However, the sixth specimen had four longitudinally oriented single-axis strain gages on the plate.

Procedure and Findings

• The specimens were subjected to axial compression up to 4,000 N in three different conditions: within a cast (test 1), within a cast that included a heel block (test 2), and without a cast (test 3).
• All specimens in all tests displayed a vertical orientation of strain under loads less than 1,000 N.
• However, the uncast specimens experienced a significant difference in strain orientation at loads greater than 1,000N. The strain angle became horizontal for these uncast specimens when the load was more than 3,000 N, indicating a change from tension to compression on the dorsal surface of the implant.
• In contrast, the cast specimens did not show a significant change in the principal strain angle even under loads greater than 1,000 N.
• The cast specimens (both with and without a heel block) endured less surface deformation than the uncast specimens.

Conclusions

• The cast was found to change the direction and magnitude of bending at the fixation point, leading to a decrease in the deformation of the implant. This outcome implies an improvement in the fatigue life of the implant when used within a cast, providing more effective stress protection.