It is helpful to think of a joint not just as a mechanical location, but as an active organ system. Your pet’s joints contain living tissues which require nutrients, modify in response to demands, attempt repair of injury, and fall victim to infection or chronic inflammation. Understanding the normal joint helps understand how arthritis develops and guides treatment when things go wrong.
As in humans, your pet’s joints are surrounded by a capsule made of tough, fibrous connective tissue which attaches directly to bone. In synovial (pronounced sin-OH-vee-al) joints, which occur in the limbs and jaw, the joint capsule helps hold the joint together and seals in synovial fluid. Synovial fluid is a thick specialized substance providing critical lubrication, nutrition, and immune support to cartilage. Within the joint capsule, cartilage (a living tissue) covers the ends of bones, providing a smooth gliding surface and absorbing impact forces directed along the bone. Covering the inner layer of the joint capsule is the synovium (or synovial lining), a thin layer of living tissue responsible for producing synovial fluid. In areas of particular stress, the joint capsule becomes especially thick and strong, forming ligaments. The knee and hip joints contain additional ligaments inside the joint capsule which directly connect bone to bone. Additional support is provided by muscles attaching to bones via tendons that cross the joint.
This video provides an overview of normal and arthritic synovial joint features. Spinal joints can also develop arthritis, though their structure is somewhat different and intervertebral disc disease can also occur.
Causes of Arthritis in Dogs and Cats
Arthritis in dogs and cats can develop as a primary disease or secondary to other problems. It affects pets of all ages, becoming increasingly common with age. The term “arthritis” (from the Greek “arthron”, meaning joint and “-itis”, meaning inflammation) refers simply to inflammation of the joint, from any cause.
Bacterial infections cause septic arthritis and autoimmune diseases cause immune-mediated arthritis. However, the more common self-perpetuating form that can result spontaneously or from any sustained inflammation, including infection, autoimmunity, injury, or improperly formed joints (dysplasia) is called osteoarthritis (OA) or degenerative joint disease (DJD). Anything that causes synovial inflammation (injury, infection, autoimmunity) or abnormal forces on a joint (limping, joint “looseness” secondary to injury or malformation) causes the joint to remodel, and usually to break down. Common joint problems that lead to OA in dogs include medial patellar luxation (MPL), hip dysplasia, cranial cruciate ligament rupture (CCL), or orthopedic injury. There is some evidence that systemic and/or genetic factors impact the development of arthritis, though these are less well understood. Often, no inciting event is identified, and OA remains a chronic management issue for many aging pets.
Mechanisms of OA in Dogs and Cats
OA is characterized by inflammation of the synovium and swelling of the joint capsule, which itself is painful. Over time, cartilage breaks down, impairing its lubricant and cushioning functions. In response to altered forces and inflammation, bones on either side of the joint may remodel or even fracture. In an effort to stabilize the joint, scar tissue is produced which limits mobility.
Regardless of cause, OA progressively destroys cartilage and remodels bone for life, causing permanent pain and loss of mobility. The rate at which this occurs, however, is unique to each patient and can be slowed with early diagnosis and appropriate therapy. Prevention as well as early detection and treatment of joint disease or injury can prevent the initiation of OA. It is therefore important to seek and follow your veterinarian’s advice regarding any lameness as soon as it is noted.
Michigan State University Osteoarthritis Overview