The knee is one of the most complex joints in the body. It bends and straightens many times a day, which is essential for activities like climbing the stairs or sitting in a chair. It also supports the weight of the whole leg, so it needs to be strong and stable. There are several muscles, ligaments, tendons, and bones that make up the knee, and problems with any of them can cause pain when bending.
The most common symptom is knee pain in front of the knee, below and to the side of the knee cap. It is often accompanied by a grinding or grating sound when the knee is bent. This is due to compression and pressure on the different structures in the knee. Other symptoms include swelling and a slight bulge behind the knee called a baker’s cyst, and a feeling that the knee is giving way or locking when bending. This is due to the tearing of the cartilage lining in the knee joint.
Other causes include damage to the meniscal cartilage, which is found between the femur and tibia, or a patellar subluxation or dislocation, which can occur when a sudden twist or change in direction occurs in sports such as football or tennis. This can lead to sharp and severe pain.
Arthritis is another common cause of pain in the knee when bending. This is usually caused by wear and tear in the knee joint, which can happen as we age. The most common type of arthritis in the knee is osteoarthritis, which can be associated with a dull or aching pain in the front of the knee. It can also be aggravated by descending stairs or long periods of kneeling. Other types of arthritis such as rheumatoid arthritis can have more severe and debilitating symptoms and require medical treatment.