Joint space narrowing

What the X-ray shows

Cartilage and meniscus are invisible on a plain X-ray, so they appear as a dark gap between the bones. When they wear or the meniscus is lost or extruded, the gap shrinks. Reports may say mild, moderate or severe narrowing, and name the compartment: medial (inner), lateral (outer) or patellofemoral (behind the kneecap).

When there is no gap left, it is often called bone on bone.

Why it matters

Joint space narrowing, along with osteophytes, is how osteoarthritis is graded on X-ray. Narrowing on one side often goes with a bow-legged or knock-kneed alignment, which loads that side more.

It is important that the X-ray is taken standing up. Lying down can make a worn knee look healthier than it is, so Dr. Gelber usually requests weight-bearing views.

What can be done

Treatment depends on symptoms more than the picture. Many people with narrowing manage well with exercise, weight management, pain relief and injections. If one compartment is worn and alignment is off, an osteotomy or partial knee replacement can be options. Severe narrowing in all compartments with disabling pain may lead to a total knee replacement.