Updated September 03, 2013.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
Pain is the primary symptom associated with knee osteoarthritis. Joint stiffness and limited range of motion of the knee are also signs that point to osteoarthritis or one of the other types of arthritis. Some people experience popping, locking, or giving out of the knee, too.
Ligament Tears
A knee that gives out, typically referred to as knee instability, is often associated with injury to one or more of the ligaments that stabilize the knee.
There are four major ligaments to the knee. There are two collateral ligaments, an anterior cruciate ligament, and the posterior cruciate ligament. The collateral ligaments are located on either side of the knee and limit sideways bending. The medial collateral ligament (MCL) is on the inner side of your knee and the lateral collateral ligament (LCL) is on the outer side of your knee. The anterior cruciate ligament (ACL) connects the top of the tibia near the front (anterior) to the femur at the center of the knee and limits rotation and forward motion of the tibia. The posterior cruciate ligament (PCL) connects to the top of the tibia near the rear (posterior) to the femur and limits backward motion of the tibia. Any ligament tear can contribute to knee instability.
Meniscal Tears
A torn meniscus also can contribute to the sensation of a knee giving out, especially if you squat down or go down stairs. There are two wedge-shaped pieces of cartilage, the menisci (each individually is called a meniscus), located between the femoral and tibial components of the knee.
Each meniscus cushions the knee joint and also plays a role in stabilizing the knee joint. Injury to one or both menisci increases instability.
Patellar Instability
Another cause of increased knee instability may involve the kneecap (patella) moving out of the groove where it sits on the femur. This type of instability is specifically known as patellar instability. Normally, the kneecap (patella) glides through in the patellofemoral groove, a track in the front of the femur (thigh bone) at the knee. The patella is attached to ligaments and tendons that serve to stabilize it. The patella lies within the quadriceps tendon, which anchors the quadriceps muscle to the upper tibia (shin bone). In addition, patellofemoral ligaments act as secondary ligament stabilizers from the sides of the patella. A direct blow to the kneecap, usually from an accident or sports injury, can dislocate it from the groove. Also, an unnatural twisting motion of the knee can have the same result. Patellar dislocation increases instability.
Worn and Uneven Cartilage in the Knee
The aforementioned causes of knee instability were primarily related to joint injury. But osteoarthritis of the knee is also associated with instability. This may be due to several problems seen in osteoarthritis, such as loose cartilage bodies inside the joint, degenerative meniscus, or ligament tears, pain, and muscular weakness.
Loose bodies are small fragments of calcified cartilage that can become caught between the bones of the knee, in the joint space, causing sudden pain and possibly causing the knee to give way. Aside from causing the joint to give way, loose bodies in the knee can also cause locking or may interfere with straightening of your leg.
Even if the loose bodies have been detected on imaging studies (x-rays, MRI, or CT scan), the instability they may cause is somewhat unpredictable, meaning you don't know when you will feel your knee give out. It happens suddenly and without warning.
Sources:
Reports of Joint Instability in Knee Osteoarthritis: Its Prevalence and Relationship to Physical Function. Fitzgerald GK et al. Arthritis Crae & Resaerch. Vol. 51, No. 6, December 15, 2004, pp 941–946
http://onlinelibrary.wiley.com/doi/10.1002/art.20825/pdf
Anterior cruciate ligament injuries of the knee in athletes. Duodecim. 2010;126(3):289-95. Kallio.T.
http://www.ncbi.nlm.nih.gov/pubmed/20402040
Handout on Health: Osteoarthritis. NIAMS. July 2010.
http://www.niams.nih.gov/health_info/Osteoarthritis/default.asp
Knee Symptoms. Giving Way (Instability). Jointhealing.com. Accessed 05/13/2012.
http://www.jointhealing.com/pages/knee/knee_sx_instab.html
Lower Extremity Considerations: Knee. Osteoarthritis. Diagnosis and Medical/Surgical Management. Fourth Edition. Chapter 21B. P.395-414. Moskowitz et al. Published by Lippincott.
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