External coaptation of distal radius and ulna fractures

Fractures of the radius and ulna comprise about 1 in 6 of the fractures seen in small animal practice (1,2). Management of these fractures can involve external coaptation or surgical stabilization. Repairs involving plate and external fixator have been the most common and most successful surgeries, especially when the surgeon is faced with fractures in the distal one-third of the radius, ulna, or both. With the availability of a wide variety of plate and screw sizes, as well as increasingly sophisticated external fixator systems, there is no longer a justification for placing an intramedullary pin in one of these fractures. Pins do not provide sufficient stability to justify the invasive surgery required to place them, and they produce an unacceptable degree of damage to the radiocarpal joint when placed proximad from the carpal joint. When faced with nondisplaced fractures or clients with limited budgets, external coaptation is frequently considered and can be effective when properly applied. The limitations of external coaptation, as applied to fractures of the radius and ulna, must be recognized by the practitioner and clearly outlined to the client. The most common fracture seen in the antebrachium is a transverse or short oblique fracture of the distal third of the radius and ulna. These fractures are most often seen in small breed dogs as injuries resulting from falls, and they have long been recognized as having a frequency of delayed union or nonunion as high as 80% (2,3) when treated with external coaptation. There are 3 main reasons for this:

1. High interfragmentary strain

Interfragmentary strain refers to the stresses between fracture fragments as a result of motion at the fracture site. The 2 main factors that influence interfragmentary strain are the rigidity of the technique used to stabilize the fracture and the surface area of the fracture fragments. The strain is defined as the relative motion over the surface area at the fracture site. In this way, comminuted fractures can tolerate relatively greater motion, since the strain is applied over a larger surface area of fracture fragments. If interfragmentary strain exceeds a critical level, the blood supply for healing at the fracture site is subjected to repeated trauma and cannot become established. Moreover, instability prevents the production of stable tissues at the fracture site. Fibrous tissue is relatively resistant to high strain forces, while cartilage and bone cannot form under conditions of high strain (4,5). As long as high strain forces exist at a fracture site (stability is inadequate), fibrous tissue will remain and stabilizing bony callus will be unable to form. If such conditions continue for long enough, a fibrous nonunion is the result. Since most fractures of the distal third of the radius and ulna in small dogs are transverse or short oblique, interfragmentary strain forces tend to be high and rigid fixation is essential to achieve successful outcomes.

2. Challenges in achieving stability with external coaptation

The first principle of treating fractures with external coaptation has always been to immobilize the joint above and the joint below the fracture. With fractures in the distal third of the radius or ulna, this can be difficult to achieve. First, most practitioners like to leave digits exposed to allow the detection of swelling under a cast or bandage, and to encourage early ambulation on the limb. While these are both desirable goals, exposed digits that bear weight invariably contribute to some degree of motion at a fracture site in the distal third of the radius or ulna. Second, immobilization of the elbow may be poor to non existent when a splint or padded bandage is used. Elbow immobilization is especially challenging when it is difficult to extend the cast much above the elbow, such as in chondrodystrophoid breeds. Further, it has been hypothesized that the contraction of flexor and extensor muscles of the antebrachium that occurs after the fracture makes stabilization of the fragments difficult and contributes to motion at the fracture site when external coaptation is used (4). In such an environment, oblique fracture lines are especially susceptible to shearing forces, which sabotages attempts to reduce the fragments and keep them in reduction.

3. Variations in blood supply

The biggest single reason why small breed dogs have problems with healing of fractures in the distal third of the radius or ulna is decreased density of the intraosseous blood supply in this region in small compared with larger dogs (3). Extraosseous blood supply is also limited due to minimal overlying soft tissues (6).

Does this mean that radius and ulna fractures cannot be successfully managed by external coaptation? Certainly not, but owners need to be apprised of the risks. In a survey of 66 radius and ulna fractures in our own practice, we noted the following: 46 cases were managed surgically and 20 were managed by external coaptation; 30 surgical cases and 11 nonsurgical cases were small dogs (62% of the total); 12 surgical and 9 nonsurgical cases were large dogs; 4 surgical cases were cats; 39 of 46 surgical cases and 12 of 20 nonsurgical fracture cases were in the distal third of the radius and ulna. Of the 20 nonsurgical cases, 1 was managed with a cast, while the other 19 were managed with mason metasplints. The small dogs in the nonsurgical group ranged in age and weight from 6 mo to 10 y and from 1.5 to 7 kg. The large dogs ranged in age from 8 mo to 5 y and from 20 to 40 kg. Eight of the 20 dogs treated with external coaptation experienced some form of complication. Seven of the 8 were bandage-related problems, such as pressure sores, chewed or soiled bandages, slipped or twisted bandages, or swelling of the paw. One 9-month-old bichon frise developed a delayed union, which, eventually, was judged to be healed at 20 wk. Of the 20 fractures treated by external coaptation, 6 were lost to follow-up with respect to declaring them healed. All of the remaining 14 cases healed between 2 and 20 wk.

How can this information be applied to clinical cases? First, the concentration of strain forces in transverse or oblique fractures of the distal third of the radius and ulna and the challenges these forces present in achieving successful results have to be recognized, particularly in small breeds with decreased vascular density at the fracture site. That means that all such fractures in small dogs are ideally treated with plate and screw repair. Cancellous bone grafting from the ipsilateral proximal humerus is a wise addition to the surgical treatment plan. If surgery is declined in such a case or if the veterinarian is presented with a young, larger patient, external coaptation can result in a successful outcome. The owner must be warned of the potential problems ranging from concerns with bandage or cast management to delayed or nonunion. To maximize the degree of stabilization that can be obtained, the practitioner must consider using a cast instead of splints or bandages, while realizing that what is gained in terms of stability is lost from the perspective of patient comfort and ease of bandage removal, replacement, or modification. Consideration must also be given to extending the cast well above the elbow and covering the digits to maximize stability. The advantages gained must be considered against the disadvantages of not having a portion of the paw exposed for monitoring purposes. In our practice, this means that when we are presented with a displaced transverse or oblique fracture of the distal third of the radius and ulna in nearly any patient, plate repair is recommended as the best option. If the patient is larger and young, if the fracture is minimally displaced, or if surgery has been declined, external coaptation can be considered. In a puppy with a non displaced fracture, this will likely take the form of a mason metasplint with the digits exposed and an effort being made to extend the splint and bandage material to immobilize the elbow as much as possible. If we are forced to use external coaptation in a toy breed dog, a fiberglass cast, extended well above the elbow and to the tips of the digits, with all appropriate disclaimers to the owner, would be the choice. Comminuted and/or open fractures of the radius make up a very small percentage of our cases, and they are usually best handled with an external fixator, placed without opening the fracture site (7).CVJ