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the articulating bones, a synovial membrane that lines the joint capsule and secretes synovial fluid, and external supporting ligaments that reinforce the joint and limit movement to the desired range of motion. In addition, some joints have stabilizing ligaments inside the joint capsule (for example, the cruciate ligaments inside the knee). When a joint becomes inflamed, a number of problems can follow. Inflammatory chemicals damage articular cartilage, fluid builds up inside the space, and bones may alter their shape and density. The net result is that inflamed joints don't work well. They may lose some range of motion, they may become visibly and palpably swollen, but most of all, they hurt. When this happens at small joints like knuckles or wrists, this can obviously limit activity. But when it happens at the weight-bearing joints like knees and hips, joint inflammation can quickly become crippling, and may lead to a joint replacement surgery, as discussed in this column in the November/December 2010 issue ("Baby Boomers and Joint Replacement Therapy," page 96). Articular cartilage is at the heart of most joint dysfunction. At its best, cartilage is a smooth, rubbery, wet surface that slides along or pivots off a similar surface on the opposing bone. This allows for a lifetime of movement with a minimum of friction. Articular cartilage is invested with living cells called chondroblasts. These cells are able to create new connective tissue fibers and liquid matrix to preserve the health of articular cartilage, but they do not readily reproduce, so an injury to cartilage does not lead to an increase in the number and activity of chondroblasts. This means that when cartilage is damaged, its capacity for healing and repair is limited to what the existing chondroblasts can do. OSTEOARTHRITIS: USE IT UP, WEAR IT OUT Osteoarthritis is by far the most common type of arthritis. It is specifically related to wear and tear of the joint, which leads to cartilage breakdown and erosion of the contacting surfaces. Obviously, osteoarthritis is most debilitating at the weight-bearing joints (the knees and hips), but it can also occur at elbows, knuckles, shoulders, and elsewhere. Cartilage breakdown is only the beginning of osteoarthritis; inflammatory chemicals inhibit chondroblast activity, trigger the synovial membrane to swell, and ultimately stimulate osteocytes in the epiphyses of the affected bones to become more active. Consequently, the condyles of the bone become enlarged, osteophytes (bone spurs) may develop, and cyst-like cavities may develop under the cartilage of the affected bone. SPONDYLITIS: DEGENERATION OF THE SPINE A subtype of osteoarthritis occurs specifically at the spine. Called spondylitis, this degenerative condition can affect the facet joints in the same way that osteoarthritis affects other synovial joints, but it can also affect the cartilaginous joints between the vertebral bodies in the intervertebral discs. Remember Osteoarthritis is by far the most common type of arthritis. It is specifically related to wear and tear of the joint, that intervertebral discs are composed of a strong, dense outer ring of cartilage called the annulus fibrosus, and a softer gelatinous center called the nucleus pulposus. It can be a useful analogy to think of the vertebral bodies as articulating bones, the tough annulus fibrosus as a capsular ligament, and the gelatinous nucleus pulposus as the synovial fluid inside a joint. As the spine ages, especially if the stabilizing ligaments are lax or if the vertebrae are out of optimal alignment, shearing and compressive stresses can affect the health of the joint. The disc thins, and bone spurs may develop around the vertebral body or on the facet joints. Two major differences distinguish spondylitis from osteoarthritis found elsewhere. One is that while osteoarthritis away from the spine can be deeply achy or sharply painful depending on activity and other variables, uncomplicated spondylitis—especially in the neck—tends to be experienced as an essentially painless, but progressive, loss of range of motion. The other difference is that for which leads to cartilage breakdown and erosion of the contacting surfaces. Celebrate ABMP's 25th anniversary and you may win a refund on your membership. ABMP.com. 39