Doctors call the outside of the knee the lateral side of the knee. There are several common causes for pain in this area that vary by age and activity level. Once the source of the lateral knee pain is identified, it usually can be eliminated or decreased.
Anatomy
The knee is composed of four bones: patella, femur, tibia and fibula. The two largest bones, the femur in the thigh and the tibia or shinbone, make up the weight-bearing surface of the knee. The medial and lateral meniscuses are between the tibia and femur. The patella, kneecap, is on the front of the knee. The fibula, a thin bone on the lateral side of the leg, connects to the tibia on the outside of the knee. The peroneal nerve passes behind the fibula. The cruciate ligaments inside the knee and the collateral ligaments outside the joint connect the bones. Muscles include the biceps femoris and the iliotibial band insert on the lateral side of the knee.
Injury
Lateral knee pain can arise from an injury. Lateral ligament injuries usually occur with ACL or PCL injuries, which involve twisting of the knee and sometimes hyperextension. Isolated LCL injury is uncommon. Motor vehicle accidents and sports are the usual causes of these injuries. Patients may notice instability with activity with the knee in extension.
Twisting injuries that are often sport related can result in lateral meniscus tears. There can be disruption of the tibia fibular joint, causing outside knee pain. These injuries have tenderness over the injured area.
In younger patients, knee fractures result from falls from heights or significant injuries. In older patients with poor quality bone, falls can cause tibial plateau fractures fibula head fractures or patella fractures. The elderly patient may not feel that the fall was enough to cause a fracture and may wait several days to see a doctor.
Adults
Lateral collateral knee pain can begin gradually without an injury. A frequent cause of pain on the outside of the knee is arthritis, which is the wearing down of cartilage on the surface of the bone. Extra bone forms, creating spurs. Pieces of cartilage and bone may break off and be loose in the joint. The American Academy of Orthopaedic Surgeons states that the most common form of arthritis is osteoarthritis. After an injury, post-traumatic arthritis can develop. Rheumatoid and psoriatic arthritis are two of the many types of inflammatory arthritic diseases that affect the knee.
The cartilage surface of the patella can wear unevenly. Pain from the patella femoral area on the front of the knee can be sensed as lateral knee pain. Inside the knee, the meniscuses lose water with aging. After age 40, degenerative tears of the lateral meniscus occur, causing pain on the outside of the knee. Young adults can develop painful meniscal cysts.
Active adults may develop irritation of the Iliotibial band or biceps tendonopathy. These conditions cause pain on the outside of the knee. They are associated with increased activity or a change in training routines.
Children
The structure of children's bone is different so they have different reasons for lateral knee pain. At the knee, the femur, tibia, and fibula develop from growth plates. The causes of outside knee pain in children include metabolic diseases causing abnormal growth plate development, fractures through the growth plate, and avulsion fractures, where a ligament or muscle insertion pulls a piece of bone away. On the outside of the knee, children may develop pain due to an incompletely developed meniscus, which is called Discoid meniscus. It is rare for children under 10 to have meniscus tears.
In heavy children presenting to a physician with knee pain, the hips will be examined to rule out slipped capital femoral epiphysis. This is a fracture through the growth plate on top of the femur.
Gradual onset of knee pain is unusual in children. When a child has six weeks of knee swelling, juvenile arthritis is a likely cause.
Sudden Unexplained Swelling
Patients usually seek medical attention when their knee becomes painful and swollen. Swelling typically involves the entire knee, but pain can be experienced more on the lateral side. Redness, fever and swelling may indicate infection. Gout and pseudogout are forms of arthritis that cause sudden painful swelling of the knees. In areas with a deer population, a late presentation of Lyme disease can be a cause of painful knee swelling.
Bursitis usually involves the anterior knee. There can be painful inflammation of the lateral bursa associated with tendonitis.
Rare Causes
Lateral knee pain can be caused by peroneal nerve compression or nerve abnormalities. Pain from other areas such as the back or hip can travel down nerves and be felt on the outside of the knee. This is called referred pain.
New bone tumors are an unlikely cause of knee pain. There were less than 2,400 bone sarcomas diagnosed in the USA in 2007. Cancer can metastasize to the knee.
References
- "American Academy of Orthopaedic Surgeons"; Comprehensive Orthopaedic Review; Lieberman, J, ed.; 2009
- Mayo Clinic: Knee Pain: Symptoms
- Clinical Sports Medicine: Lateral Posterior and Medial Knee Pain; Bunker and Kahn; 2006
