Meniscus Tears
The meniscus is a thick, durable fibrocartilage that sits between the femur and tibia. There is one meniscus on the inside (medial) and one on the outside (lateral) of the knee. I think of the meniscus as both a shock absorber and a cushion that helps the rounded femur fit more smoothly against the relatively flat tibia while distributing forces across the knee. Meniscus tears may occur after an injury in younger patients or gradually as part of the aging and arthritic process.
Arthritis
Arthritis is one of the most common causes of knee pain. It occurs when the smooth articular cartilage covering the ends of the bones gradually wears away. As the cartilage deteriorates, the joint becomes painful, stiff, swollen, and progressively more difficult to move. Osteoarthritis is by far the most common type, although other forms, such as inflammatory arthritis and post-traumatic arthritis, may also occur.
Ligament Injuries
The ligaments are the primary stabilizing structures of the knee. They function much like strong restraints that prevent the knee from moving beyond its normal limits. Although there are four major ligaments, the MCL and ACL are the ones most commonly injured. Some ligament injuries can be treated successfully without surgery, while others require reconstruction to restore knee stability.
Patellar Instability
Patellar instability occurs when the kneecap partially or completely slips out of its normal groove, almost always toward the outside of the knee. This commonly occurs after a twisting injury but may also happen in patients whose kneecaps are naturally loose. When the patella dislocates, it often produces a large, painful swelling and may damage the articular cartilage of the kneecap or femur as it slips out of its normal groove.
Loose Bodies
Loose bodies are small fragments of bone or cartilage within the knee joint. They may develop as a result of arthritis, articular cartilage breakdown, or traumatic injury. These fragments can become trapped between the bones or other structures of the knee, producing catching, locking, or the sensation that something is moving around inside the joint.