Transfixation pin casts are generally used for stabilization of comminuted fractures at locations where it is difficult to apply internal fixation for fracture repair, although it is not uncommon for them to be used in conjunction with internal fixation. Typically, a transfixation pin cast consists of pins that pass through the bone proximal to a fracture and then are incorporated within a cast so that most of the animal's weight is supported by the cast, thereby partially unloading the fracture. In fact, the main purpose of a transfixation pin cast is to allow placement of a load on the bone distal to the pins appropriate for fracture healing but not too excessive so as to cause fracture overloading and bone deformation (strain).1
Transfixation pin casts have been used to treat fractures of large animals for > 50 years,2 albeit not without complications including pin loosening, secondary infection, and necrosis at the bone-pin interface.3,4 The bone-pin interface is typically a site of excessive motion that ultimately results in pin loosening and signs of pain.5–7 Consequently, it is often necessary to remove or replace pins, which in turn necessitates replacement of the cast. Many alternatives to decrease complications at the bone-pin interface have been investigated, such as use of pins of varying sizes, smooth versus positive-profile pins, self-tapping versus nontapping positive-profile pins, hydroxyapatite-coated positive-profile pins, and diaphyseal versus metaphyseal pin placement.8–11 However, the optimal fixation strategy for decreasing bone-pin interface complications, minimizing motion, and unloading the fracture site remains undefined.
Traditional transfixation pin casts involving an MC3 or third metatarsal bone consist of two 6.3-mm positive-profile pins placed proximal to the fracture. The pins are placed at 30° relative to each another in the transverse plane. That configuration is preferred over parallel pin placement because it provides better resistance to torsional loading as evidenced by the fact that, in vitro, MC3s stabilized with parallel pins fractured at lower torques than MC3s stabilized with divergent pins.12 Recently, it was suggested that a transfixation pin–cast construct consisting of four 4.8-mm smooth pins spaced 2 cm apart with some degree of divergence would be a more effective construct than two 6.3-mm positive-profile pins spaced 4 to 5 cm apart.a That alternative design arose from the supposition that 6.3-mm pin designs are too rigid and inhibit normal biomechanical stimuli needed to achieve appropriate healing.13 The use of pins with a smaller diameter allows for distribution of some load across the fracture site to facilitate fracture healing while still protecting the fracture from excessive strain.14 In 1 study,15 the amount of strain at the dorsal aspect of the proximal phalanx did not differ significantly between a transfixation pin–cast construct with two 6.3-mm centrally threaded positive-profile pins and a transfixation pin–cast construct with four 4.8-mm smooth Steinman pins, which suggested that loading at the fracture site was similar between those 2 constructs. However, that supposition has yet to be confirmed. The objective of the study reported here was to compare the strain at the bone-pin and cast-pin interfaces among 3 transfixation pin-cast constructs applied to the forelimbs of horses.
Supported by the Townsend Equine Health Research Fund at the Western College of Veterinary Medicine, University of Saskatchewan.
The authors declare that there were no conflicts of interest.
The authors thank Brennan Pokoyoway and Dr. David Wilson for technical assistance.
Third metacarpal bone
Bramlage LR, Rood and Riddle Equine Hospital, Lexington, Ky: Personal communication, 2017.
CEA-06-125UW-120, Vishay Micro-Measurements, Raleigh, NC.
M-Coat A, Vishay Micro-Measurements, Raleigh, NC.
Delta-Lite Plus, BSN Medical, Hamburg, Germany.
Model RRH-10010, Enerpac, Milwaukee, Wis.
LDI-119-200-A020A, Omega, Norwalk, Conn.
Model 1220-AF, 250 kN capacity, Interface, Scottsdale, Ariz.
SPSS, version 18.0, IBM Corp, Chicago, Ill.
ABAQUS, 3DS, Waltham, Mass.
1. McClure SR, Watkins JP, Bronson DG, et al. In vitro comparison of the standard short limb cast and three configurations of short limb transfixation casts in equine forelimbs. Am J Vet Res 1994;55:1331–1334.
3. McClure SR, Honnas CM, Walkins JP. Managing equine fractures with external skeletal fixation. Compend Contin Educ Pract Vet 1995;17:1054–1063.
5. Morisset S, McClure SR, Hillberry BM, et al. In vitro comparison of the use of two large-animal, centrally threaded, positive-profile transfixation pin designs in the equine third metacarpal bone. Am J Vet Res 2000;61:1298–1303.
6. Nunamaker DM, Richardson DW, Butterweck DM, et al. A new external skeletal fixation device that allows immediate full weightbearing application in the horse. Vet Surg 1986;15:345–355.
8. Zacharias JR, Lescun TB, Moore GE, et al. Comparison of insertion characteristics of two types of hydroxyapatite-coated and uncoated positive profile transfixation pins in the third metacarpal bone of horses. Am J Vet Res 2007;68:1160–1166.
9. McClure SR, Hillberry BM, Fisher KE. In vitro comparison of metaphyseal and diaphyseal placement of centrally threaded, positive-profile transfixation pins in the equine third metacarpal bone. Am J Vet Res 2000;61:1304–1308.
10. Bubeck KA, García-Lopez JM, Jenei TM, et al. In vitro comparison of two centrally threaded, positive-profile transfixation pin designs for use in third metacarpal bones in horses. Am J Vet Res 2010;71:976–981.
11. Aron DN, Toombs JP, Hollingsworth SC. Primary treatment of severe fractures by external skeletal fixation: threaded pins compared with smooth pins. J Am Anim Hosp Assoc 1986;22:659–670.
12. McClure SR, Watkins JP, Ashman RB. In vitro comparison of the effect of parallel and divergent transfixation pins on breaking strength of equine third metacarpal bones. Am J Vet Res 1994;55:1327–1330.
13. Smith GK. Biomechanics pertinent to fracture etiology, reduction, and fixation. In: Newton CD, David M, eds. Textbook of small animal orthopaedics. Philadelphia: Lippincott Williams and Wilkins, 1985;195–230.
14. Perren SM. Physical and biological aspects of fracture healing with special reference to internal fixation. Clin Orthop Relat Res 1979;175–196.
15. Williams JM, Elce YA, Litsky AS. Comparison of 2 equine transfixation pin casts and the effects of pin removal. Vet Surg 2014;43:430–436.
16. Cordey J, Gautier E. Strain gauges used in the mechanical testing of bones. Part II: “in vitro” and “in vivo” technique. Injury 1999;30(suppl 1):A14–A20.
17. Brianza S, Brighenti V, Boure L, et al. In vitro mechanical evaluation of a novel pin-sleeve system for external fixation of distal limb fractures in horses: a proof of concept study. Vet Surg 2010;39:601–608.
18. Nash RA, Nunamaker DM, Boston R. Evaluation of a tapered-sleeve transcortical pin to reduce stress at the bone-pin interface in metacarpal bones obtained from horses. Am J Vet Res 2001;62:955–960.
19. Nutt JN, Southwood LL, Elce YA, et al. In vitro comparison of a novel external fixator and traditional full-limb transfixation pin cast in horses. Vet Surg 2010;39:594–600.
20. Elce YA, Southwood LL, Nutt JN, et al. Ex vivo comparison of a novel tapered-sleeve and traditional full-limb transfixation pin cast for distal radial fracture stabilization in the horse. Vet Comp Orthop Traumatol 2006;19:93–97.
22. Rauber AA. Elastizitt und festigkeit der Knochen: anatomisch-physiologische studie. Leipzig, Germany: Verlag von Wilhelm Engelman, 1876.
24. Wytch R, Mitchell CB, Wardlaw D, et al. Mechanical assessment of polyurethane impregnated fibreglass bandages for splinting. Prosthet Orthot Int 1987;11:128–134.
25. Clary EM, Roe SC. Enhancing external skeletal fixation pin performance: consideration of the pin-bone interface. Vet Comp Orthop Traumatol 1995;8:1–8.
26. Lescun TB, McClure SR, Ward MP, et al. Evaluation of transfixation casting for treatment of third metacarpal, third metatarsal, and phalangeal fractures in horses: 37 cases (1994–2004). J Am Vet Med Assoc 2007;230:1340–1349.
27. Palmer RH, Hulse DA, Hyman WA, et al. Principles of bone healing and biomechanics of external skeletal fixation. Vet Clin North Am Small Anim Pract 1992;22:45–68.
28. Chamay A, Tschantz P. Mechanical influences in bone remodeling. Experimental research on Wolff's law. J Biomech 1972;5:173–180.
30. Perren SM. Evolution of the internal fixation of long bone fractures. The scientific basis of biological internal fixation: choosing a new balance between stability and biology. J Bone Joint Surg Br 2002;84:1093–1110.
31. Joyce J, Baxter GM, Sarrafian TL, et al. Use of transfixation pin casts to treat adult horses with comminuted phalangeal fractures: 20 cases (1993–2003). J Am Vet Med Assoc 2006;229:725–730.
33. Ashman RB, Rho JY, Turner CH. Anatomical variation of orthotropic elastic moduli of the proximal human tibia. J Biomech 1989;22:895–900.