Knee giving way occurs when the joint that connects the upper and lower leg becomes weak and unstable, causing the joint to fail and the leg to collapse. Knee giving way can cause someone to stumble or fall. Knee giving way is a common sign of joint instability or weakness, and may be caused by conditions such as osteoarthritis or rheumatoid arthritis, or some type of injury to the knee joint. Knee giving way can also be caused by an injury to the ligaments and muscles that support the knee. Knee giving way that is caused by osteoarthritis may also be accompanied by swelling, deformity, heat, pain, and loss of flexibility in the knee joint.