Knee pain can occur from several anatomical structures that can be injured or inflamed. Understanding the anatomy, listening to the history of the problem from the patient, performing a comprehensive examination of the knee structures and function, and reviewing proper imaging of the knee (x-rays, MRIs) will help lead to the correct diagnosis. In some cases, blood examination is a key part of the evaluation of some inflammatory conditions. Certain conditions require drawing fluid from the knee (a joint aspiration) to help provide the correct diagnosis, such as in the case of infections or gout.
Tendonitis occurs at the end of a muscle where it connects to a bone when it becomes acutely inflamed. This causes pain in the area of the tendon insertion when the muscle is being used. In the knee, there are many locations where muscles are connecting from the quadriceps muscles, the hamstring muscles, and the calf muscles that can cause knee pain when walking, running, rising and sitting, and stair climbing.
Tendinosis occurs when the tendon has been chronically inflamed and the fibers of the tendon develop micro-tears and swelling in them.
Tendon rupture is when a tendon tears acutely. The rupture can be complete or partial. In the knee, the quadriceps tendon is one of the more common tendon ruptures. In a complete tear, this causes an inability to extend the knee or walk without the leg giving out and requires surgical repair. There is also associated bleeding in the joint that can cause swelling and pain. Partial tears can be managed sometimes without surgery using braces to protect the tendon and can be augmented to heal using biologic injections.
Bursas are anatomic cushions at tendon insertions that can become inflamed and cause knee pain at their locations.
Ligaments connect bones to bones in the knee and act as static stabilizers that can be injured, stretched, or torn. The four main ligaments in the knee are the ACL, PCL, MCL, LCL. When these ligaments are injured, the stability of the knee is compromised and can cause pain and swelling.
- ACL tears are often repaired by replacing the ligament with another ligament or similar tissue from the patient or a donor patient.
- Complete PCL tears in young active patients are often repaired.
- MCL tears are commonly incomplete tears and can heal with bracing and protection without surgery. Some complete tears require surgical repair.
Medial and Lateral Meniscus cartilage injuries can cause pain when they are torn in small or large areas. These cartilaginous shock absorbers are critical in the transfer of load across the knee and protection of the surface cartilage on the ends of bones. A torn meniscus is a common meniscus cartilage injury. These tears can include both partial meniscus tears and full meniscus tears.
Articular Surface Cartilage Damage
Articular surface cartilage damage from trauma, arthritis, infection, and osteonecrosis can cause pain, swelling, loss of motion, and deformity of the knee.
Arthritis is the general term for joint inflammation. Arthritis usually causes pain, swelling, stiffness, loss of motion and reduction in activity level when it occurs in the knee. Treatment depends on the type of arthritis each person presents with. This can be the result of many disease processes in the knee, which include:
- Osteoarthritis (degenerative cartilage wear-down condition)
- Rheumatoid arthritis (inflammatory cartilage destructive disease)
- Psoriatic arthritis (inflammatory cartilage destructive disease)
- Gout (crystalline deposit disease from uric acid levels that are elevated)
- Infections (bacterial, viral, fungal, Lyme Disease)
Osteonecrosis means “bone death”. The bone cells die due to blood flow reduction, lack of oxygen to the cells, and the bone begins to cause pain. This can occur in any bone, but it is commonly seen in the knee and hip joints. It is most often associated with steroid use, alcohol use, sickle cell disease, and it can occur spontaneously. It is commonly seen on the end of the femur and more often on the inner side (medial side).
As osteonecrosis advances and the underlying bone dies, the surface coating on the knee joint called the cartilage can eventually collapse down into the underlying bone causing more pain and disability requiring joint replacement.
Synovitis is a condition inside the knee joint when the lining tissues become inflamed. The inner lining of the knee joint is called the synovium. A variety of conditions can activate this lining to become more active in producing synovial fluid, which is the lubricating fluid of the knee joint. With chronic activation, this lining can become swollen and thick and chronically produce too much joint fluid, creating knee pain and loss of motion from the fluid pressure.