Arthritis strictly means inflammation of the joint and there are many possible causes of this. When we talk about arthritis we are usually talking about a specific type of arthritis called degenerative joint disease (DJD) or osteoarthritis.
DJD is the commonest joint disease seen in many different breeds of dogs and cats and is often a major reason for euthanasia. DJD affects the movable synovial joints such as the elbow and hip joints. Synovial joints are formed where the ends of two bones meet. The ends of the bones are covered in very smooth ARTICULAR CARTILAGE. The SYNOVIAL MEMBRANE lines the inside of the joint capsule and produces SYNOVIAL FLUID which lubricates the joint. The tough JOINT CAPSULE covers the outside of the joint. Articular Cartilage is made up of :
- cartilage cells called chondrocytes
- the matrix ( material that fills the gaps between the cells). This is made of collagen fibres and a ground substance rich in substances called proteoglycans such as chondroitin, hyaluronic acid, keratin sulphates and glycosaminoglycans. Articular cartilage is translucent and glass-like to the naked eye.
WHAT HAPPENS IN DJD?
- The main biochemical change that occurs in cartilage when DJD is present is loss of proteoglycans. This is due to the proteoglycans in the cartilage matrix being broken down by enzymes. It is unclear whether these enzymes are produced by the chondrocytes within the cartilage or come from outside the joint. The major role of articular cartilage is to provide a smooth, friction free movement between the bone ends. Due to this proteoglycan loss the cartilage loses the ability to do this. Furthermore, the cartilage becomes eroded away because it loses its normal structure.
- Synovial fluid production reduces further reducing joint lubrication.
- New bone forms around the joint. These are called osteophytes.
- Synovial membrane is sometimes inflamed (called synovitis)
- Surrounding joint capsule becomes thickened (fibrosis)
- Bone ends underneath the cartilage becomes thickened (sclerosis)
Causes of DJD
1.Primary DJD is a disorder of ageing. Cartilage degeneration occurs but the cause is unknown. 2.Secondary DJD much more common in animals. Secondary DJD occurs as a result of a number of problems including: -abnormal loading of the joint due to the joint not developing properly eg hip dysplasia, elbow dysplasia -due to instability in the joints e.g cruciate ligament rupture -as a result of infections or immune mediated damage to the joint -due to joint fractures
Clinical Signs of DJD
Clinical signs vary depending on the severity of the DJD and the number of joints affected. Stiffness (especially after rest) and reduced activity are very common. These signs are often attributed to getting old and as a result treatment is often not given. Animals who are stiff and lame are in pain even though they rarely cry out or complain. Affected animals may be lame on one or a number of legs depending on how many joints are affected.
Examination by the veterinarian may reveal pain, crepitus (clicking or crunching), swollen joints. Xrays are the best way to diagnose DJD and there are a number of characteristic findings. As secondary DJD is most common it is important to look for underlying reasons for the DJD e.g hip dysplasia, elbow dysplasia etc. Xray changes and clinical signs sometimes do not always agree e.g changes may be mild on an Xray but the dog may be very lame and vice versa. Treatment is based on clinical signs rather than Xray changes.
DJD is usually progressive regardless of treatment. Identifying an underlying cause e.g FCP in the elbow may mean that surgery is required to deal with the initiating cause. Most cases of DJD are treated as follows:
- Weight control. The importance of this can not be over emphasised. Many dogs and cats are overweight and even a small amount of extra weight can make a major difference to the forces put on the joints e.g during the movement of going from sit to standing a force that is nine times your pets weight is loaded throughout each knee. Therefore, if your pet is 5kg overweight there is an additional 35kg on each knee just when standing up !!. Furthermore, obesity is now recognised as an inflammatory condition. Obese animals have higher levels of inflammatory products in their blood stream and these may aggravate the DJD.
- Exercise Put simply, too much exercise is as bad as not enough. Excessive exercise will cause pain and lameness in dogs with DJD. This may manifest itself as lameness during exercise or, more often, as stiffness after rest after exercise. Little and often gentle exercise is much better than long walks irregularly. On the other hand , too little exercise means that joint mobility and muscle tone is adversely affected. Swimming, especially controlled hydrotherapy, is an excellent exercise for dogs with DJD. Also, physiotherapy can be hugely beneficial
- Drugs a) NSAIDs. The cornerstone of DJD treatment is Non Steroidal Anti-Inflammatory Drugs (NSAIDS). They have both anti-inflammatory and analgesic (pain killing) effects. All NSAIDs have the potential to cause side effects in some animals. These include: – Gastro-intestinal side effects such as vomiting and diarrhoea can occur. More seriously ulceration of the gastro-intestinal tract can occur. This causes vomit with blood in it, black tar like faeces (malaena), and can be fatal if left untreated. – Exacerbation of degradation of the articular cartilage by increasing cartilage breakdown or by reducing glycosaminoglycan synthesis. Thus, the drug that is most commonly used for treating DJD has been shown to speed up the progress and deterioration of the disease. – Kidney toxicity. Nsaids can be damaging to kidneys especially where there is existing kidney disease or when there is risk of low blood pressure such as dehydration, heart disease, general anaesthetic – Affect blood clotting – Liver toxicity especially if there is pre-existing liver disease. Occasionally bizarre liver reactions occur. Therefore, as with all drugs they must be used carefully and under strict veterinary supervision. Despite these potential side effects thousands of dogs and cats are safely treated with NSAIDS. b) Steroids. Steroids will help to reduce joint inflammation but have many side effects making them generally unsuitable for treating DJD c) Surgical treatment to correct underlying problems. Also joint replacements such as hip replacements. d) Polysulphated glycosaminoglycans (PSGAG). This is a compound that is used to try and reduce cartilage degeneration in the the joint i.e it slows down the progress of DJD. PSGAG is administered via an injection. The proposed way that it works is by inhibiting the enzymes that breakdown the cartilage and stimulating the production of lubricant in the joint. There have been a number of laboratory studies showing the effect of PSGAG on reducing the activity of the degrading enzymes. There is nota huge amount of clinical trial data to prove the efficacy of PSGAGs but clinical experience suggests that there is a benefit in certain individuals. Dogs that have cancer should not be treated with PSGAGS because it can cause bleeding in such dogs. e) Nutraceuticals include chondroitin, glucosamine, and magnesium ascorbate. The theory behind using these products is that they provide precursors for the repair of damaged articular cartilage or at least slowdown the degeneration of the cartilage. There is anecdotal evidence they work but no high quality clinical trial data. They have very few reported side effects but they may not have any effect at all! Also, because they are not highly regulated the purity and composition of some of these products cannot be verified. f) Fatty acids may reduce inflammation in DJD by reducing the production of a substance called prostaglandins. Long chain omega 3 polyunsaturated fatty acids such as eicosapentaenoic acid seem to be effective. g) Antioxidants such as vitamin C and vitamin E may also have beneficial effects in DJD. Again clinical data in dogs is scarce but there are possible benefits demonstrated in mice experimentally.