Evaluation of a tapered-sleeve transcortical pin to reduce stress at the bone-pin interface in metacarpal bones obtained from horses.

OBJECTIVE

To evaluate stiffness and bone-pin interface stress for a transcortical tapered-sleeve pin (TSP) that incorporates bilateral tapered sleeves over a transcortical pin.

SAMPLE POPULATION

14 third metacarpal bones (MCIII) collected from adult horses of various breeds.

PROCEDURE

Each MCIII was cut in half to provide 2 test specimens. Pins (conventional and TSP) of 3 diameters (6.35, 7.94, and 9.50 mm) were inserted in specimens (3 specimens for each diameter and each type of pin). The test fixture simulated a typical sidebar-span skeletal fixation device for horses. Single cycle load-deflection tests were performed. Cyclic fatigue tests of TSP were performed to evaluate fatigue characteristics and stress conditions at the bone-pin interface. Maximum stress and strain were calculated, and results were compared with existing data on fatigue characteristics of bone.

RESULTS

Significant increases in stiffness (load-deflection) and higher loads at yield point were detected for the TSP (stiffness for conventional 9.50mm pins, 4,500 N/mm; stiffness for TSP, 19,988 N/mm). Results of cyclic tests revealed a close correlation with existing data on fatigue characteristics.

CONCLUSIONS AND CLINICAL RELEVANCE

The TSP described here is stiffer than conventional transcortical pins, and stress across the bone-pin interface is more evenly distributed. Use of this TSP should minimize major problems encountered during external fixation associated with the transcortical pin and bone-pin interface (ie, bone necrosis, infection of the pin track, pin loosening, and bone failure).