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

Dr. Ronald A. NashComparative Orthopedic Research Laboratory, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348.
Present address is 977 Stephens Hwy, Magnolia, AR 71753.

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David M. NunamakerComparative Orthopedic Research Laboratory, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348.

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Ray BostonComparative Orthopedic Research Laboratory, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA 19348.

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Abstract

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 (loaddeflection) and higher loads at yield point were detected for the TSP (stiffness for conventional 9.50- mm 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). ( Am J Vet Res 2001;62:955–960)

Abstract

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 (loaddeflection) and higher loads at yield point were detected for the TSP (stiffness for conventional 9.50- mm 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). ( Am J Vet Res 2001;62:955–960)