Overview
A loose body is a fragment of bone, cartilage, or a combination of both that has broken free and is floating within the knee joint. Symptoms are usually intermittent because the loose body moves around within the knee. When it becomes trapped between the moving surfaces of the joint, it may suddenly cause catching, locking, or the feeling that something is stuck inside the knee. Once it moves out of the way, the symptoms often disappear just as quickly. I think of it like a small rock caught between the gears of a well-running machine. Everything works smoothly until the rock gets caught. Once it moves out of the way, everything functions normally again.
Causes
Degenerative Loose Bodies
The most common loose bodies result from arthritis. As the joint gradually wears out, small pieces of cartilage or cartilage with bone can break away from the joint surface and begin floating within the knee.
Traumatic Loose Bodies
Traumatic loose bodies occur after injuries such as an ACL tear, patellar dislocation, or other significant knee trauma. In these injuries, a fragment of cartilage or bone is literally knocked loose from the joint surface.
Less Common Causes
Conditions such as synovial chondromatosis can also produce loose bodies. In this disorder, the lining of the knee forms numerous small cartilage nodules that eventually break free into the joint.
Diagnosis
The hallmark symptom is intermittent catching or locking. The loose body only causes symptoms when it becomes trapped between the moving surfaces of the knee. Once it moves out of the way, the symptoms often disappear just as quickly. If the loose body contains bone, it is often visible on X-rays. Pure cartilage fragments usually require an MRI to identify them. One helpful clue is that a loose body may appear in different locations on different X-rays or MRIs, confirming that it is freely moving within the knee.
Treatment
If a loose body continues to cause recurrent catching, locking, or pain, arthroscopic removal is recommended. Fortunately, this is usually a straightforward arthroscopic procedure requiring only a short operative time. Patients are generally allowed immediate weight-bearing and progressively return to full activities over approximately 3–4 weeks. Interestingly, once a loose body becomes lodged in an area where it no longer moves, it may become completely asymptomatic and require no treatment. It is the mobile loose body that typically causes the symptoms.