Casts and splints are available in most primary care veterinary practices and can be used successfully to treat selected fractures in dogs and cats. Unfortunately, when used improperly or in contraindicated scenarios, these fixation methods frequently cause severe complications including fracture disease, quadriceps contracture, mal-union, delayed or non-union, and/or infection; any of which can lead to limb amputation or euthanasia. Unexpected complications can negate their apparent affordability and, thus, lead to client dissatisfaction. Therefore, the key to successful treatment using these fixation modalities is a thorough knowledge of their indications, proper application and limitations.
Splints & Casts for Fracture Fixation
Coaptation includes both splinting and casting techniques. Neither splints nor casts provide rigid fixation, but cylindrical casts provide greater fracture zone immobilization than splints. Specifically, one must consider the ability of any fixation method to resist the disruptive forces acting upon the fracture to be treated. Disruptive forces to consider include bending, rotation, axial compression (axial collapse), and tension. We will consider each of these disruptive forces individually.
Control of Bending with Casts
All long bone fractures are subjected to bending due, in part, to the irregular shape of bones and the inherent eccentric loading at the joint surfaces and sites of muscular attachment. In order to control bending, casts must bridge a joint above and below the fracture. Since the patient’s body wall prevents effective cast bridging of the hip and shoulder, casts cannot be used effectively for fractures above the knee or elbow. In theory, the most rigid cast stabilization would be imparted by direct application of the cast to the bone; this, of course, cannot be performed because of the surrounding soft tissues. Nonetheless, using this mechanical principle, one can readily appreciate that anything that increases the distance between the bone and the cast (muscle mass, soft tissue swelling, excessive cast padding, improper cast molding, etc.) reduces its ability to control disruptive bending forces. Cast padding is necessary to protect from soft tissue injury over bony prominences, but it is important to use no more cast padding than is necessary for this purpose.
Control of Rotation with Casts
All long bone fractures are subjected to some rotational forces (properly referred to as “rotational moments”). These rotational moments are due, in part, to eccentric insertions of muscle-tendon units (MTUs). If you imagine application of a cast to a cylindrical pipe, you recognize that it would have no ability to resist rotation. Fortunately, the limbs of our patients have some bony prominences and joint angulation with which the cast can interact; these prominences and contours give casts their limited ability to resist rotational forces. The pelvic limb has more inherent prominences (tuber calcis) and angulation (tarsus) than the thoracic limb. It is, therefore, advisable to be certain to contour thoracic limb casts to the region of the olecranon as well as to place a small amount of flexion in the elbow and carpal joints.
Control of Axial Compression (Axial Collapse) with Casts
Gravitational force acting upon the body is countered by a ground reaction force. These opposed forces create axial collapse of long bones when there is complete fracture. Casts do not sufficiently interact with each bony segment to prevent axial collapse. Therefore, cast fixation is dependent upon bony architecture to resist axial collapse. The 3 main ways that bony structure can prevent axial collapse are: (1) incomplete fracture (also called a “greenstick” fracture), (2) fracture of only 1 bone in a 2-bone system (ex. an intact fibula adjacent to a tibia fracture), and (3) appropriate reduction of a transverse fracture. As a rule of thumb, 50% reduction of transverse fractures in the worst of two orthogonal radiographic views is acceptable in order to resist axial collapse.
Control of Tension with Casts
Pure tensile forces originate from the insertion of MTUs on traction apophyses. These traction apophyses ((humerus - greater tubercle, ulna - olecranon, femur - greater trochanter, tibia - tibial tubercle, tarsus - tuber calcis) are nothing more than levers and the muscles are motors that make the skeleton move through space. Casts have zero ability to resist these pure tensile forces. Thus, casts are unable to effectively treat fractures of traction apophyses. In most instances, either a figure of 8 tension band or a tension band plate will be required for adequate treatment of these fractures.
Other Relevant Factors for Splints & Casts
Articular fractures require rigid stabilization and perfect anatomic reduction; therefore, coaptation is not recommended for treatment of articular fractures (including most fractures of carpal and tarsal bones). Coaptation is prone to cause skin irritation and soft tissue wounds when used long-term or in instances of poor patient/pet owner compliance. Similarly, external coaptation is prone to cause joint stiffness/immobility when used long-term because it restricts normal limb use. I am cautious with the use of coaptation in puppies during early skeletal development (<4–5 months of age) because abnormal limb use in the cast can contribute to skeletal developmental abnormalities such as patellar luxation, hip dysplasia, etc. When used in very young puppies, I strive to place the limb in a functional position (i.e., pelvic limb has normal knee and tarsal angles rather than full extension) and I remove the cast at the earliest sign of functional radiographic healing (as early as 2–3 weeks in many instances). Alternatively, even though the fracture will heal with coaptation, I often opt for a surgical fixation that will permit immediate restoration of normal limb use (minimally invasive techniques) in these developing animals. Coaptation is best avoided in the treatment of distal fractures of the radius/ulna, especially in toy breed dogs, as it commonly results in nonunion; surgical fixation with a bone plate or ESF is favored for most of these fractures. Malunion in valgus and/or external rotation often occurs with coaptation because direction of the direction of the traction force applied upon the tape stirrups during cast application; if the team is aware of this tendency, these mal-alignments can usually be prevented. Though coaptation often seems to be the most economical means of fracture treatment, repeated recheck exams, cast changes with sedation, and complications often minimize this advantage except when applied to the most predictable of scenarios.
Splints & Casts Summary
Effective use of coaptation is restricted to a relatively narrow spectrum of long bone fractures encountered in veterinary small animal practice. Coaptation is best avoided in the following scenarios:
- Fractures above the elbow or knee
- Fractures of traction apophyses (such as tibial tuberosity, tuber calcis, olecranon, etc.)
- Articular fractures including most carpal and tarsal bone fractures
- Fractures other than reducible/transverse fractures, greenstick fractures, or fractures with an intact adjacent bone
- Cases at risk of poor patient/pet owner compliance and/or slow healing