Chapter 7 Diseases of the Musculoskeletal System
The musculoskeletal system is responsible for movement and shape in all animals. Animals must be able to move, find food, seek shelter, and escape predators to survive. Without a rigid frame (the skeleton), flexible articulations (joints), and a system of pulleys (muscles, tendons, and ligaments), animals would be little more than lumps of tissue. The integration of these systems provides movement, one of the basic characteristics of life.
Injuries that involve the musculoskeletal system are painful, and analgesics should be used to increase the comfort of the patient. A pain-free patient will not only be more comfortable while healing, but will be able to function normally again more rapidly. Any disease or malfunction of this system compromises the animal’s ability to maintain homeostasis with its environment.
At least three-fourths of long-bone fractures occur as a result of motor vehicle accidents. Other causes include indirect violence, bone disease, or repeated stress. These fractures may be classified as open (bone exposed through the skin) or closed (bone not exposed through the skin), simple or comminuted (splintered or fragmented), and stable or unstable (Fig. 7-1). The type of fracture and its location determine the best method of repair.
Figure 7-1 Common traumatic fractures.
(From Christenson DE: Veterinary medical terminology, Philadelphia, 1997, WB Saunders, by permission.)
The job of the veterinary technician is to quickly assess the patient, especially in the case of motor vehicle accidents. After treatment for shock, hemorrhage, and soft-tissue trauma, the possibility of fractures should be addressed. Technicians should always be aware that fractures might exist. They should take care when moving the animal, protect any areas of suspected fractures with support bandages (such as a Robert Jones) if possible, and be careful not to make the injury worse by restraint methods or handling when obtaining radiographs.
Figure 7-2 A, The biomechanical advantage of intramedullary pins is that they are equally resistant to bending loads applied from any direction because they are round. B and C, Biomechanical disadvantages of intramedullary pins include poor resistance to rotational or axial (compressive) loads and lack of fixation (interlocking) with bone.
(From Fossum TW: Small animal surgery, ed 3, St Louis, 2007, Mosby, by permission.)
The anterior and posterior cruciate ligaments are intraarticular structures that help stabilize the stifle joint. Rupture of the cranial cruciate ligament is possibly the most common injury to the stifle of the dog and is a major cause of DJD in the stifle joint (Fig. 7-4). The ligament may rupture completely, resulting in gross instability of the joint, or it may tear, producing minor instability. Both injuries result in degenerative changes within the joint within a few weeks.
(From Olmstead ML: Small animal orthopedics, St Louis, 1995, Mosby, by permission.)
Cruciate ligament injuries are usually seen in middle-age, obese, inactive animals that suddenly hyperextend their stifle joint while exercising. Rupture may also occur in animals engaged in athletic endeavors (such as racing or jumping), resulting in a traumatic injury to the ligament. There may be an occult degenerative process present in the former group of animals that predisposes the ligament to atraumatic rupture. In both groups, rupture of the opposite cruciate ligament often occurs within a year after injury to the first ligament. Approximately 50% of dogs with ligament rupture also demonstrate meniscal injury.
Treatment of this type of injury involves removal of the damaged tissue and stabilization of the joint. Many repair techniques have been reported; the choice of technique is usually based on the size of the dog, the activity level required by the animal, and the skill of the surgeon.
Medial luxations of toy, miniature, and large breeds occur early in life and are not related to trauma. They are often called congenital because they are usually the result of anatomic deformities. Approximately 75% to 80% of patellar luxations are medial displacements. Anatomic derangements that predispose an animal to medial luxations include medial bowing of the distal third of the femur, shallow trochlear sulcus and a poorly developed medial ridge, medial torsion of the tibial tubercle, or medial bowing of the proximal tibia. Over time these derangements put added stress on the cranial cruciate ligaments, predisposing 15% to 20% of these ligaments to rupture.