The radius is the third most frequently fractured bone in dogs,1,2 with 85% of radial fractures involving the distal third of the diaphysis.1,2 Plate osteosynthesis is one of the most common methods used to stabilize radial fractures in dogs and promotes rapid union as well as rapid return to function.3,4 Some studies,4,5 however, reveal that up to 54% of small-breed dogs with radial fractures treated with plate osteosynthesis develop complications. In addition, small-breed dogs have a greater risk of developing certain complications, such as delayed union and nonunion, than do large-breed dogs.6–8 Although the definitive cause of these complications has yet to be elucidated, biomechanical and vascular characteristics unique to small-breed dogs have been identified as possible risk factors.4,6–8 A microvascular study8 performed in cadaveric radii revealed that there is decreased microvascular density in the distal diaphyses of small-breed dogs, compared with large-breed dogs, which purportedly might impair bone healing in small-breed dogs. Other biological factors that may contribute to delayed union in small-breed dogs include poor soft tissue coverage of the distal portion of the antebrachium and extensive soft tissue dissection performed during direct fracture reduction.5,9 Biomechanical factors such as stiffness and interfragmentary strain of the bone-plate construct may also play a role in the development of complications in small-breed dogs because these variables influence fracture healing.10 Experimentally, clinically normal dog femurs plated with stiff plates become more osteoporotic than femurs plated with less-stiff plates.11 Interfragmentary strain is another important mechanical factor because it stimulates the formation of callus and accelerates healing, but excessive motion at the gap may not be tolerated and may be detrimental to callus formation.12
Radial fractures stabilized with plates in large-breed dogs are associated with an excellent prognosis and rarely develop complications, compared with dogs weighing < 5 kg.2,5 These positive results suggest that plating radial fractures in large-breed dogs13 provides an appropriate mechanical and biological environment for fracture healing. The rigidity of any fixation device needs to be sufficient to prevent excessive interfragmentary strain during the early postoperative convalescent period, yet flexible enough to promote load sharing with the bone as the callus mineralizes.14–16 Excessive plate stiffness may also cause stress shielding, which can lead to atrophy of existing bone, in accordance with Wolff's Law,11,17 and increase the risk of refracture in cases requiring implant removal.18 Excessive plate stiffness has been suggested as a cause of healing complications in radial fractures in small-breed dogs.4 Therefore, it is important to determine whether plating causes a similar change in stiffness in small- and large-breed dog radii.
The purpose of the study reported here was to compare in vitro axial compression, abaxial compression, and torsional stiffnesses of intact and plated radii from small- and large-breed dogs. We hypothesized that plated radii would be stiffer than the intact radii in small-breed and large-breed dogs and that plated radii from small-breed dogs would have a greater relative stiffness than plated radii from large-breed dogs. To test the hypothesis, data were normalized by expressing the stiffness of the plated radii as a percentage of the stiffness of the intact bones and the normalized stiffness of the bone-plate constructs was compared between large- and small-breeds dogs.
Dynamic compression plate
Limited-contact dynamic compression plate
Synthes USA, Paoli, Pa.
Bondo Corp, Atlanta, Ga.
MTS 858 Mini Bionix II, MTS Systems Corp, Eden Prairie, Minn.
Mini driver K100 with K148 blade (0.6 mm), 3M, Maplewood, Minn.
Microsoft Office Excel 2003, Microsoft Corp, Redmond, Wash.
PASW Statistics GradPack, version 17.0, SPSS Inc, Chicago, Ill.
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