Orthopedic surgery in small mammals (Proceedings)

Fractures in small mammals generally involve the femur, tibia, and occasionally the humerus. These fractures are usually the result of being dropped or getting the leg caught in a wire cage floor. The patient usually presents acutely with minimal metabolic changes. The patient is treated for metabolic compromise or shock as needed. If the patient is very stressed, it should be placed in a warm (75-80o F), dark, quiet location and allowed to recover from shock prior to addressing the fracture.

Most fractures of the femur and humerus in small mammals are not open making the routine use of therapeutic antibiotics inappropriate. Perioperative antibiotic therapy is recommended with intravenous administration perioperative and every 90 min. during the surgery. Fractures of the distal extremity are more often open because there is little soft tissue coverage. The area surrounding the wound is clipped, cleaned, and dressed until the patient can safely be anesthetized for definitive treatment. If the patient is stable, under anesthesia any necrotic soft tissue and bone is debrided. The bone is replaced under the skin, the limb bandaged, and broad-spectrum antibiotic therapy initiated pending culture results. Cultures are obtained at the time of surgery. Care must be taken when selecting an antibiotic for use in rabbits and rodents.

Analgesia is especially important in small mammals because many are prey species and do not tolerate stress well. Buprenorphine (0.05-0.5 mg/kg SQ q 6-12 hr; 0.01-0.04 mg/kg in ferrets), butorphanol (0.1-0.4 mg/kg SQ q 4 hr) are useful for providing analgesia.

Preoperative laboratory evaluation should include a hematocrit, total serum solids, glucose, and urea evaluation. If the hematocrit is less than 20% surgery should be delayed or a blood transfusion should be considered. An increased urea may be an indication of dehydration or renal disease. The hematocrit and total solids are useful in determining which. Normal serum glucose in small mammals is 75-150 gm/dl. Hypoglycemic patients benefit from the addition of dextrose to their fluid regimen. Insulinoma should be considered in ferrets that are hypoglycemic.

Fracture management

The principles of fracture management in small mammals are similar to those established for larger mammals and include rigid stabilization and anatomic alignment with minimal disturbance of callus formation and soft tissue dissection. The inherent problems in working with rabbit and rodent bones are the small size and the thin, brittle cortices. Any apparatus used must be well tolerated by the patient and it is best to provide early return to function to prevent fracture disease. Practical considerations in fracture management include the cost of the materials, ease of application, availability of equipment, and the surgeon's level of expertise with various fixation devices.

Compression, rotation, bending, and shear forces are exerted on the fracture and must be neutralized to promote fracture healing. Comminuted fractures are susceptible to compression, rotation, bending, and shear forces. Any fracture fixation should neutralize the inherent forces acting on the fracture to prevent motion at the fracture site. The more forces that must be neutralized by the fixation, the higher the incidence of complications and failure.

External coaptation

External coaptation involves the use of splints, slings, and other bandages. External coaptation is difficult to apply to femoral and humeral fractures. Grade 1 open fractures (minimal soft tissue injury, little comminution) are amenable to management with external coaptation; however, more severe fractures are best managed surgically allowing better management of soft tissue injury and monitoring for the development of osteomyelitis.

A variety of splints and bandages may be used successfully in small mammals. External coaptation is most appropriate for patients that are too small for internal fixation, if there is minimal fracture displacement, if there are factors that make anesthesia and surgery especially risky, or if the fracture is highly comminuted making primary repair impractical. It is inexpensive and simple requiring little time and a short anesthesia with little risk of infection. External coaptation should immobilize the joints proximal and distal to the fracture. All forms of external coaptation are checked weekly for any signs of vascular compromise, odor, discomfort, soiling, slippage, or other problems that may require splint replacement.

Soft, conforming cast padding and conforming gauze work well for padding. The coaptation may be reinforced with wood applicator sticks, tongue depressors, aluminum rods, lightweight cast material or other substances that will add bending stability. Veterinary Thermoplastic (VTP, IMEX, Inc., Longview, TX), Orthoplast (Johnson & Johnson Products, Inc., New Brunswick, NJ) and Hexcelite (Kirschner Medical, Timonium, MD) are firm at room temperature but when heated in hot water become malleable and can be molded to conform closely to the shape of the limb.

Schroeder-Thomas (ST) splints may be used to treat fractures distal to the elbow and stifle. They may be used as a sole means of fracture stabilization or in conjunction with intramedullary pin. The ST splint is a traction splint modified by adjusting the configuration to conform to the shape of the limb and the location of the fracture. The configuration of the splint varies with the location of the fracture. It should not be applied to maintain all joints in hyperextension and traction. This will predispose the limb to severe fracture disease such as quadriceps contracture. It should be applied with the limb in a functional position with tension applied to separate the joints at each end of the fractured bone. Proper application of the ST splint requires tension on the joints proximal and distal to the fracture in the correct direction to allow the fracture fragments to be separated and aligned.

A tubular traction splint has been used with success to treat fractures of the femur and tibia. This involves the use of a plastic tube such as a syringe case that is of a diameter appropriate to the size of the patient's limb. The tube should be padded at its proximal end, which will be tightly pushed into the inguinal region. Tape stirrups are applied to the distal extremity and secured to the foot. Padding is added to the limb to prevent movement within the tube. The tape is then pulled through the end of the tube and the limb is placed in hyperextension and traction by wedging the tube into the inguinal area. The tape is secured to the outside of the tube to maintain the leg in traction. The disadvantages of this type of splint are that the limb is maintained in hyperextension predisposing to fracture disease and the splinted leg is not in a functional position.

It can be challenging to maintain coaptation, especially in rodents. They tend to chew on devices applied to their body. Administration of analgesics is often helpful. Some animals may need to be tranquilized for a few days until they adapt to having the device in place. Most rodents do not tolerate Elizabethan collars well. An alternative is a yoke device which prevents the animal from turning around but is better tolerated.

Internal fixation

Internal fixation should provide rigid immobilization, anatomic alignment, and early return to function. However, internal fixation requires general anesthesia, some degree of surgical expertise, and implants and instrumentation.

Intramedullary (IM) pins

IM pins are relatively inexpensive, provide axial alignment and bending stability, and require minimal tissue exposure for insertion. They are best inserted normograde in most cases. Sixty to 70% of the medullary canal is filled with the pin. They are not stable against rotation and shear forces. Stack pinning, adding cerclage or hemicerclage wires, or the addition of external skeletal fixation (ESF) may be used to counter shear and rotation forces. Cerclage wires should not be used as the sole means of fracture stabilization as they are not stable against bending forces.

External skeletal fixation

These devices provide good anatomic alignment and stability. When properly applied, they stabilize fractures against rotation, bending, and shear forces. ESF does not interfere with joint function and is easily removed once the fracture has healed. Fixators are lightweight and allow early return to function. Open, comminuted and metaphyseal fractures are well suited for stabilization with ESF as they bridge the fracture and allow management of soft tissue injury. Type I and type II fixators are applicable to fractures in small mammals. Staged removal can be used to gradually increase the load on the bone in a process called dynamization. Fixator may catch on objects in the cage or the patient may chew of wrappings designed to protect the device.

Biphasic ESF devices use various size Krischner wires, Steinmann pins, or hypodermic needles as fixation pins but the connecting bar and clamps are replaced by acrylic polymer or other stiff material. The fixation pins should be <20% of the diameter of the bone. Non-sterile hoof repair PMM (Technovit, Jorgensen Labs, Loveland, CO), VTP, Hexcelite, epoxy resin, and dental acrylics may be used.

Bone plates

Plate fixation in rabbits and rodents is difficult because of the small size of the bone and the thin cortices, which are susceptible to iatrogenic fracture during screw implantation. Bone plates provide the advantages of rigid internal fixation and anatomic alignment without interfering with joint function, allowing early return to function. Bone plates are most applicable to femur and humerus fractures in rabbits and ferrets. Bone plate application, however, is technically difficult to perform and requires specialized training. The equipment is expensive, the surgical exposure and tissue dissection is extensive, and the surgery time is prolonged. The ASIF 30 cm 50 hole veterinary cuttable plate (Synthes 243.99 or 242.99, Paoli, PA) is well suited for the small bones of rodents and lagomorphs. These plates can be used with both 1.5 mm or 2.0 mm screws (243.99 plate) and 2.0 mm or 2.7 mm screws (242.99 plate). They are small, light weight, and strong. The holes are closely placed and the plate may be cut to an appropriate length.

Postoperative care

The hospital stay should be kept to a minimum as rodent and lagomorphs do better in their home environment. Fluid and nutritional support, and analgesia are provided until the patient is eating and drinking voluntarily. Vegetable baby food or Critical Care for Herbivores (Oxbow Pet Products, Murdock, NE) can be syringe fed to meet the nutritional needs of the patient.

Activity restriction can also be challenging because they are free within the cage. Any exercise wheels, climbing platforms, or tubing should be removed during the convalescence.

Follow up radiographs are made at 4 weeks postoperatively and weekly thereafter until there is radiographic evidence of fracture union. Implants should only be removed after this has been documented.

Other orthopedic problems

Dislocations are more difficult to manage in small mammals because of anatomic differences and their tendency toward explosive movements. The anconeal process of rabbits and ferrets is not well developed making it difficult to maintain closed reduction of elbow luxations in these species. It appears that if the elbow luxation can be reduced, splinting the leg in flexion provides better results than extension as is recommended in dogs and cats. Often a surgical reduction is indicated. Even with an open reduction it can be difficult to maintain the reduction. A transarticular pin (through the proximal ulna into the distal humerus) is useful in maintaining reduction. The pin is removed in 2-3 weeks. Hip luxations are also easily reduced but difficult to maintain in reduction. In many cases, femoral head and neck excision is recommended to prevent degenerative joint disease. Patella luxations occur in rabbits and, if the patient is symptomatic, surgery may be beneficial.

Amputations are indicated for severe osteomyelitis, malignant tumors, and severe fractures that are highly comminuted and have severe soft tissue damage. They are performed at the same location as in dogs and cats and are well tolerated by small mammals. Because the bone is brittle, a hobby saw works well for cutting the bone if needed (mid-femoral).

Degenerative joint disease occurs in small mammals with some degree of frequency for the same reasons and mechanisms as in larger animals. Treatment involves the use of weight control, chondroprotectants, and nonsteroidal anti-inflammatory medications.

Vitamin C deficiency in guinea pigs affects the skeletal system. Deficiency interferes with collagen synthesis. Radiographically, the costochondral junctions and epiphyses appear enlarged. There may be soft tissue swelling around the joint due to hemarthrosis.