The knee joint is often damaged by arthritis. Arthritis of the knee is painful and gets worse with time. Though there are many different forms of arthritis, three kinds are the common culprits of knee pain:
- Osteoarthritis (also sometimes called degenerative arthritis) is the most common form of arthritis that affects the knee. Obesity, diabetes and frequent physical battering of the joint itself (think: baseball catcher's knees) are common causes, although we don't always know why osteoarthritis occurs. Osteoarthritis usually catches up to us in middle age or beyond -- among 40-year-olds, about two out of five show signs of osteoarthritis, but only half experience symptoms at this stage. Osteoarthritis can be very painful. Surviving tissue may be inflamed, bone can grind against bone when a joint is moved, and all of the aggravating stimulation to the end of the bone can cause it to form a bone spur, a newly formed bony protrusion, near the joint, leading to further discomfort.
- Rheumatoid arthritis occurs when the body's immune system attacks healthy tissue in reaction to bacteria, toxins and parasites. It can take just a year for some malfunctioning white blood cells around your knee joint to cause permanent damage to your joint [source: Abbott Laboratories]. This usually affects both knees simultaneously. Over time, rheumatoid arthritis changes the way you walk and move, and hobbling around leads to further bone and tissue damage. Rheumatoid arthritis can show up at any point in life.
- Post-traumatic arthritis occurs as a result of external damage to the knee. It may take months or years for signs of arthritis to appear after an injury.
As knees become arthritic, they stiffen, swell and grow weaker. When cartilage wears away, it can create too much space between the bones, and it can alter the tracking of the patella. Likewise, if bone spurs (outgrowths caused by bone stimulation) develop, there will be too little space between the joints.
As arthritis advances, the pain happens not only when your knee is in motion, but even when you're resting. The knee may start "going out" or "giving way," or, alternately, it may stiffen up and refuse to bend.