Canine-distal Antebrachial Fractures In Toy-breed Dogs

Vol. 19, No. 2 February 1997

Continuing Education Article

FOCAL POINT ★The causes of the high prevalence of antebrachial fractures and associated complications in toy breed dogs are unclear, but the treatment of choice is rigid stabilization with a bone plate in combination with cancellous bone autograft.

KEY FACTS ■ Fractures of the antebrachium can occur in toy breeds after apparently minimal trauma. ■ Treatment of distal antebrachial fractures in toy breeds with bone plate fixation has a low complication rate compared with other methods. ■ Delayed union or nonunion is particularly prevalent in toy breeds and may eventually necessitate amputation. ■ Bone is weakest in shear, followed by tension and compression; when a bending load is applied, the convex (tension) side will fail first, followed by the concave (compression) side.

Distal Antebrachial Fractures in Toy-Breed Dogs University of California

Peter Muir BVSc, MVetClinStud, PhD, MACVSc, MRCVS

F

ractures of the antebrachium account for approximately 17% of canine fractures.1 Motor vehicle trauma is a prevalent cause. In toy breeds, however, fractures of the antebrachium can occur after apparently minimal trauma, such as jumping or falling, and usually affect the distal region of the diaphysis.2 Treatment of distal antebrachial fracture in toy breeds with bone plate fixation has a low complication rate,3 whereas a high complication rate can be expected with other treatment methods, such as external coaptation or intramedullary pinning.3,4 More recently, external skeletal fixation has been used to successfully treat this type of fracture.5 Routine use of cancellous bone autograft for fracture treatment is also considered important.2 The prevalence of delayed union or nonunion in canine fractures is 3.4%.6 This complication occurs most commonly after fracture of the distal antebrachium and is particularly prevalent in toy breeds.6–8 Development of delayed union or nonunion after surgery is a potentially serious complication, because limb amputation may eventually be necessary.9 Although the prevalence of antebrachial fracture and complications after treatment in toy breeds has been recognized for many years,7,10 the causative biological mechanisms are poorly understood. Various hypotheses regarding the prevalence of delayed bone healing after distal antebrachial fracture in toy breeds have been suggested, including fracture instability,9 increased and persistent formation of cartilage within the fracture site,11 and decreased osteogenesis compared with larger dogs.4,8,11 Differences in vascular density at the metaphyseal–diaphyseal junction have also been implicated,12 but blood supply at the fracture site has not been measured directly. The hypothesis examined in this study of distal antebrachial fractures in toy breeds is that a small region of the distal part of the diaphysis fails mechanically after trauma. Fracture patterns, which have been identified radiographically, are evaluated in detail, with the long-term goal of better defining reasons why the antebrachium in toy breeds appears to be so vulnerable to fracture. Further-

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more, the results of treatment of distal antebrachial fractures have been compared to determine whether a particular treatment method was associated with improved outcome.

ment displacement, and regional location of the main fracture line were recorded. Using the mediolateral radiographic view, the distance between the distal end of the radius and the main fracture line was measured, and the result expressed as a percentage of the total length of the radius from distal (0%) to proximal (100%). Fractures were categorized as delayed union if the degree of fracture healing was considered abnormally reduced for the time elapsed between injury and presentation or treatment. Fractures with loss of bone adjacent to the fracture and a minimal healing response were categorized as atrophic nonunion.

MATERIALS AND METHODS The medical records of all dogs with antebrachial fracture presented to the University of California, Davis Veterinary Medical Teaching Hospital (UC Davis VMTH) from April 1987 to March 1996 were reviewed retrospectively using the UC Davis VMTH computerized data base for patient records. Of 127 dogs presented, 40 weighed less than 5 kg. Of these 40 dogs, medical records of 26 were complete and were examined for this study.

Treatment and Follow-up Clinical Findings Figure 1—Craniocaudal radiographic views of the right antebrachium. The treatment Signalment and (A) This transverse fracture of the distal radius and ulna, at 23% of the radial methods and use of body weight were length, occurred in a 2-month-old female miniature pinscher after it jumped cancellous bone aurecorded. Fracture from its owner’s arms. Transverse fractures typically result from bending over- tograft were deterinformation in- load. (B) This oblique fracture of the distal radius and ulna, at 24% of the ra- mined. Outcome cluded time from dial length, occurred when a 5-month-old miniature pinscher jumped off a was determined for coffee table. Oblique fractures typically result from compressive overload. fracture to presendogs reexamined at tation, fracture duUC Davis VMTH, ration, unilateral including healing of versus bilateral fracture, and open versus closed fracthe fracture and development of complications. ture. Prior treatment(s) and outcome were also recordRESULTS ed. Clinical Findings Radiography Breeds included toy poodles (n = 11), Pomeranians Mediolateral and craniocaudal radiographic views of (n = 5), Chihuahuas (n = 3), papillons (n = 2), miniathe antebrachium were examined. Fracture type, fragture pinschers (n = 2), Italian greyhound (n = 1), toy fox Figure 1A

Figure 1B

SIGNALMENT ■ RADIAL LENGTH ■ FRACTURE CLASSIFICATION ■ BREEDS

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Figure 2A

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Figure 2B

Figure 2C

Figure 2—Mediolateral radiographic views of the right antebrachium of an 18-month-old castrated male Yorkshire terrier

cross. (A) At presentation (8 weeks after fracture). Delayed union of the distal radius and ulna, at 26% of the radial length, developed after failure of the intramedullary pin and external coaptation technique. (B) Immediately after surgical revision. The fracture has been debrided and stabilized with a 2.0-mm dynamic compression plate. (C) Sixteen weeks after plating. The delayed union fracture has healed.

terrier (n = 1), and Yorkshire terrier (n = 1). Nine of the 26 dogs were younger than 1 year of age at the time the fracture was sustained. Sixteen dogs ranged in age from 1 to 4 years, and the age of one dog was unknown. Of 14 females, 6 were spayed; of 12 males, 6 were castrated. Body weight ranged from 1 to 4 kg (mean, 2.3 kg). Seventeen dogs were presented with acute fractures (<7 days from injury) without receiving definitive orthopedic treatment. Nine dogs had sustained fractures

1 to 3 months before presentation and had received prior treatment with intramedullary pinning, external coaptation, intramedullary pinning combined with external coaptation, or bone plating. The treatment history in one dog was unknown. Fracture etiologies included jumping or falling, often from a minimal height (n = 16), being stepped on (n = 3), motor vehicle trauma (n = 1), being attacked by another dog (n = 1), and being caught in a door (n = 1). In four dogs

PATIENT AGE ■ TREATMENT HISTORY ■ FRACTURE ETIOLOGY

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the etiology was unknown. In 6 of 26 dogs, fractures were bilateral. There were no grade II or grade III open fractures.

Radiography Of the 17 dogs presented with acute fractures, 14 had 16 transverse or short oblique fractures. All fractures involved the distal antebrachium (Figure 1), occurring between 15% and 37% of the radial length (mean ± SD, 25% ± 6%). An intraarticular fracture of the distal radius and a contralateral distal antebrachial fracture (22%) were sustained by one dog following motor vehicle trauma. A comminuted middiaphyseal antebrachial fracture (58%) was sustained by one dog after being stepped on. Bilateral comminuted mid-diaphyseal antebrachial fractures (54% and 70%) were sustained by one dog after being attacked by another dog. This dog also sustained a humeral fracture. Nine dogs presented with 11 chronic fractures, including 3 atrophic nonunion fractures, 3 delayed union fractures, and 5 healing malunions. Atrophic nonunions were associated with prior intramedullary pinning (n = 1), external coaptation (n = 1), and intramedullary pinning combined with external coaptation (n = 1). Delayed unions were associated with prior intramedullary pinning (n = 2) or intramedullary pinning combined with external coaptation (n = 1). Malunions were characterized by lateral deviation of the paw distal to the fracture and were associated with external coaptation (n = 3), bone plating (n = 1), and unknown treatment (n = 1). Remodeling of the fractures prevented precise determination of the original fracture pattern. Eight of 11 fractures affected the distal antebrachium (mean ± SD, 27% ± 13% of radial length; range, 16% to 55%). Treatment and Follow-up Bone plate fixation was used to stabilize 15 acute antebrachial fractures, and cancellous bone autograft was used to treat 4 of these fractures. Nine fractures healed without complication, and 5 fractures were lost to follow-up. Instability of the bone plate construct resulted in eventual fracture malunion in one dog, with lateral deviation of the paw distal to the fracture. External coaptation was used to treat four acute fractures in four dogs that were younger than 5 months of age. Three of these fractures healed, and one fracture was lost to follow-up. One dog with bilateral acute fractures was discharged to the referring veterinarian for treatment with external coaptation. Bone plate fixation was used to stabilize four chronic fractures, of which cancellous bone autograft

was used in three. Three of these fractures healed without complication (Figure 2). Instability of the bone-plate construct in one dog resulted in delayed malunion, with lateral deviation of the paw distal to the fracture, despite use of cancellous bone graft. Application of a type II external skeletal fixator, using small Kirschner wires and methylmethacrylate connecting bars, combined with cancellous bone autografting was used to treat three chronic fractures. Two fractures healed (Figure 3). One fracture was lost to follow-up at 2 months, at which time fracture union had not occurred. Three stable malunion fractures received no further treatment, and one fracture was lost to follow-up. Overall, bone–plate removal was performed for 8 of 19 fractures. Refracture of the antebrachium occurred in one dog after plate removal. This fracture healed after treatment by the referring veterinarian.

DISCUSSION Fracture of the distal antebrachium after apparently minimal trauma remains a prevalent clinical problem in toy breeds. Despite being recognized for many years as a clinical phenomenon, the etiology is still not completely understood. In this case series, the signalment of affected dogs was similar to that reported in other studies. 3,10,13 In dogs presented with acute fractures in which precise determination of fracture pattern was possible, the injury sustained by those with a history of minimal trauma was characteristically a transverse to oblique fracture of the distal region of the antebrachium. This fracture pattern is similar to Colles’ fracture,14 which occurs commonly in humans. In general, dogs with comminuted fractures located more proximally in the antebrachium appeared to sustain their fractures as a result of more severe injury. Bone is weakest in shear, followed by tension and compression.15,16 When a bending load is applied to bone, the convex side is loaded in tension and the concave side is loaded in compression. The convex (tension) side will fail initially, generating a crack. As the crack propagates, the neutral axis of the bone will be shifted toward the compressive side, thereby allowing propagation of the crack across the bone to create a transverse fracture.16 If a compressive load is applied to bone, an oblique fracture is typically produced, with bone failure occurring because of shear stresses within the bone. Such fractures typically occur in the metaphyseal regions of bones because cancellous bone is significantly weaker in compression than cortical bone.16 Comminuted fractures with a butterfly fragment are typically a result of a combined bending and compres-

FRACTURE TYPE ■ FRACTURE STABILIZATION ■ BENDING LOAD ■ HUMAN FRACTURES

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Figure 3A

Figure 3D

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Figure 3B

Figure 3E

Figure 3—The right antebrachium of a 3-year-old female toy poodle. (A) Transverse fracture of the distal radius and ulna, at 18% of the radial length, was sustained after jumping. (B) Mediolateral radiographic view. Before presentation, the fracture was stabilized using an intramedullary pin and external coaptation technique, with penetration of the antebrachiocarpal joint by the pin. (C) Mediolateral radiographic view on presentation 12 weeks after injury. Atrophic nonunion has developed with severe osteopenia of the antebrachium and carpus as a consequence of the initial treatment. (D) Craniocaudal radiographic view 4 weeks after revision surgery. Cancellous bone autograft has been placed around the fracture, and the fracture has been stabilized with a type II external skeletal fixator using Kirschner wires and acrylic connecting bars. Pins were placed in the proximal ulna and metacarpus as well as the antebrachium. Osteopenia imFigure 3C proved, and fracture dynamization was performed by removal of the proximal and distal pins. (E) Craniocaudal radiographic view of the right antebrachium 10 weeks after revision. The external fixator was removed because osteopenia further improved, and healing of the fracture has progressed.

sive failure load.16 Jumping or falling was the most common cause of distal antebrachial fracture in the dogs in this report, and such an event would be likely to create the compressive or bending failure loads that would be necessary to create the fracture patterns observed. Why toy breeds appear to have decreased antebrachial failure load compared with larger dogs remains obscure. Larger dogs typically sustain hyperextension injury to the carpus after jumping or falling.17 Recent ex vivo studies in dogs18 and humans19 have shown that geometric variables, such as cross-sectional cortical area and area moment of inertia, are at least as important as bone mineral density20 in determining bone failure load and risk of fracture. In this study, many of the antebrachial fractures that resulted from apparently minimal trauma occurred in a small region of the distal diaphysis, centered at the 25th percentile of radial length. The region from 30% to 40% of radial length appears to be mechanically weakest in humans, based on de-

FRACTURES IN LARGER DOGS ■ GEOMETRIC VARIABLES

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tailed evaluation of bone geometry.21 Fractures after minimal trauma are most common in the elderly, and age-related alterations in bone mineral density and bone geometry have been demonstrated in the distal radius.22 Bone mineral density declines with increasing age, and compensatory adaptive remodeling results in altered bone geometry that contributes to the preservation of bone strength.22 Distal antebrachial fractures in toy breeds typically occur in young growing or adolescent dogs.10 Whether age-related changes in antebrachial bone mineral density or bone geometry result in increased antebrachial failure load in toy breeds in later life has not been investigated. Further comparative study of the material and structural properties of the antebrachium of toy breeds versus larger dogs is needed to confirm whether this region of the antebrachium, which is so prone to fracture in toy breeds after apparently minimal trauma, is abnormally weak. Because the relative height of falling may tend to be greater for toy-breed dogs than for larger dogs, bone failure load may be exceeded more easily after jumping or falling. Successful fracture healing usually occurred after rigid stabilization of the antebrachium using a bone plating technique. The presence of chronic atrophic nonunion, delayed union, or malunion fracture at presentation was typically associated with prior treatment with intramedullary pinning and external coaptation. On the basis of this and other studies,3,13 rigid stabilization with a bone plate, combined with use of cancellous bone autograft, is the treatment of choice for all acute antebrachial fractures in toy breeds, although union of the fracture will usually occur after bone plate fixation alone. Intramedullary pin fixation and external coaptation of antebrachial fractures in toy breeds are contraindicated because these stabilization methods are associated with an unacceptably high complication rate,3,8,13 although delayed union or nonunion after external coaptation in young dogs is uncommon.10 The antebrachiocarpal joint is often damaged by intramedullary pin fixation of distal antebrachial fractures. Development of a complication such as atrophic nonunion is potentially serious and may ultimately necessitate limb amputation.9 Development of severe atrophic nonunion usually necessitates treatment with external skeletal fixation,5 because of loss of mineralized bone stock. Bone mass and bone architecture are controlled by adaptive bone modeling and remodeling.23 Mechanisms controlling modeling and remodeling of bone include functional load bearing, stimuli associated with bone growth in adolescence, and the influence of calciumregulating hormones.23 Bone remodeling is also influ-

enced by fatigue damage to bone.24 The apparently low failure load of the toy breed antebrachium, the higher risk of complicated fracture healing after inadequate stabilization, and the rapid decline in bone mineral density associated with external coaptation and functional disuse2–4,8,10,11 could hypothetically be influenced by any of the above-mentioned mechanisms controlling modeling and remodeling. Such hypotheses still require investigation.

About the Author When this article was submitted, Dr. Muir was affiliated with the Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, California. He is currently affiliated with the Department of Small Animal Medicine and Surgery, The Royal Veterinary College, University of London, London, UK. He is a Diplomate of the American College of Veterinary Surgeons and the European College of Veterinary Surgeons.

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16. Tencer AF, Johnson KD: Factors affecting the strength of bone and the biomechanics of bone fracture, in Biomechanics in Orthopedic Trauma. Philadelphia, JB Lippincott Co, 1994, pp 18–56. 17. Willer RL, Johnson KA, Turner TM, Piermattei DL: Partial carpal arthrodesis for third degree carpal sprains: A review of 45 carpi. Vet Surg 19:334–340, 1990. 18. Muir P, Markel MD: Geometric variables and bone mineral density as potential predictors for mechanical properties of the greyhound radius. Am J Vet Res 57:1094–1097, 1996. 19. Myers ER, Hecker AT, Rooks DS, et al: Geometric variables from DXA of the radius predict forearm fracture load in vitro. Calcif Tissue Int 52:199–204, 1993. 20. Horsman A, Currey JD: Estimation of mechanical properties

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