Canine Arthritis - Osteoarthritis
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| Definition / Overview | Osteoarthritis | Other Types of Arthritis |
| Symptoms | Rheumatoid Arthritis | Risk Factors / Diagnosis |
Osteoarthritis in Dogs
By far the most prevalent form of canine arthritis is osteoarthritis, a progressive, debilitating disorder. It is characterized by the loss of cartilage that covers and protects the end of the bones in a movable (synovial) joint. The smooth cartilage has no nerves, so when it touches the cartilage of another bone, there is no pain. When the cartilage wears away, the bone is exposed. Bone does have nerves, so when the two bones in a joint touch (hence the term "bone on bone"), the result is pain and inflammation.
In plain terms, cartilage is the buffer between bones in a joint. The breakdown of cartilage can reduce the function of the joint and create pain or stiffness.
Chondroitin sulfate consists of repeating chains of molecules called mucopolysaccharides. Chondroitin sulfate is a major constituent of cartilage, providing structure, holding water and nutrients, and allowing other molecules to move through. This is an important property, as there is no blood supply to cartilage. In degenerative joint disease, there is a loss of chondroitin sulfate as the cartilage erodes.
Causes of Osteoarthritis in Dogs
Osteoarthritis can occur in any joint, but is most often found in the hip. The factors that can cause osteoarthritis or increase the potential for it to develop or worsen include the following:
- Developmental defects: abnormalities of growth that alter the shape or stability of a joint (e.g., hip dysplasia, luxation of the patella) or result in a defective joint surface (e.g., osteochondrosis) (These developmental defects are discussed further in the age factor section.)
- Obesity: overweight dogs are much more prone to osteoarthritis and are more likely to develop osteoarthritis in several joints because of the excessive load on their joints; they are also more difficult to treat than dogs in more ideal body condition
- Poor limb conformation: bow-legged, knock-kneed, or cow-hocked conformation causes uneven load across the joint surfaces, which can predispose the dog to osteoarthritis
- "Wear and tear": repeated load at or near the physical limits of the joint may, over time, lead to osteoarthritis in very active dogs (e.g., dogs working or competing in physically strenuous activities)
Regardless of the specific cause, the particular factor (or combination of factors) ultimately triggers a common sequence of events that result in the hallmarks of osteoarthritis. Depending on the cause, the process may begin with either (1) damage to, or degeneration of, the articular cartilage, or (2) inflammation of the joint capsule lining.
Effects of Inflammation
Inflammation begins at the cell level with production of pro-inflammatory substances by the outer membrane of a damaged cell. These substances begin a cascade of biochemical events in the surrounding tissue which cause the outward signs of inflammation: pain, heat, swelling, and redness (may not be obvious on hair-covered or pigmented areas). Following trauma or in the cases of joint defects and disease, inflammatory products are released into the joint fluid that can damage the articular cartilage. This causes the cartilage to lose thickness and elasticity, making it difficult for the joint to bear stress or weight load. Next, the chemical products produced from inflammation trigger the release of pro-inflammatory enzymes, leading to loss of function. All these materials then spill into the joint capsule, breaking the long chains of the molecules inside, causing the joint fluid to lose viscosity and cartilage to become more degraded. This then makes the joint even less capable of bearing stress and weight load, triggering more inflammation. Once this cycle of joint inflammation and cartilage degradation begins, it causes the secretion of even more inflammatory and catabolic enzymes. The result: increased pain to the joint and an ongoing cycle of joint deterioration within surrounding tissues.
Risk Factors for Young Dogs
Osteoarthritis mostly affects dogs in their middle years and old age. However, it can develop in young dogs in the following circumstances: (1) the joint has been badly damaged by injury or infection, or (2) the joint is misshapen or malformed as a result of a developmental defect. Developmental defects are abnormalities that occur during growth or development. The more common developmental problems that may predispose young dogs to osteoarthritis are:
- Osteochondrosis dissecans (OCD): defective development of the subchondral bone can result in flaps or loose fragments of cartilage or cartilage and bone (osteochondral fragments) in the joint.
- Elbow dysplasia: dysplasia, or abnormal growth, in the elbow joint includes such specific conditions as fragmented medial coronoid process, ununited anconeal process, and OCD (osteochondritis dissecans).
- Patellar luxation: displacement or dislocation of the patella (the equivalent of the human kneecap) is a fairly common problem in several small breeds of dog; it is usually accompanied by dysplasia of the groove in which the patella is located at the lower end of the femur (thigh bone).
While the developmental defect usually becomes apparent (typically as lameness) in the first 12 months of life, osteoarthritis may not become severe until later in life. Early diagnosis and appropriate treatment of the developmental defect can substantially decrease the risk for, and may even prevent, secondary osteoarthritis.
Osteoarthritis Diagnosis
Osteoarthritis can usually be diagnosed just from the symptoms and from what is known of your dog's age, breed, and medical history. Nevertheless, a veterinarian will probably perform a physical examination to evaluate your dog's general health and to rule out any other medical problems that could be causing the symptoms you report.
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