Joint Space
The gap between bones in a joint, visible on X-rays. Narrowing of joint space indicates cartilage loss and is a key diagnostic sign of osteoarthritis.
Extended Definition
Joint space isn’t actually empty—it contains cartilage covering bone ends and synovial fluid. On X-rays, cartilage is invisible, so the “space” represents cartilage thickness.
Normal vs. Narrowed
Normal Joint Space:
- Adequate cartilage cushion
- Even spacing throughout joint
- Bones well-separated
Narrowed Joint Space:
- Cartilage has worn thin
- May be uneven (worse in one compartment)
- Indicates OA progression
Measurement
Knee:
- Normal: 4-5mm
- Mild OA: 3-4mm
- Moderate OA: 2-3mm
- Severe OA: less than 2mm
Clinical Significance
Joint space narrowing helps:
- Confirm OA diagnosis
- Grade OA severity
- Track progression over time
- Guide treatment decisions
Treatment Connection
HA injections may help slow joint space narrowing by improving lubrication and potentially protecting remaining cartilage. However, they cannot regenerate lost cartilage or widen the joint space.
Important Note
Joint space on X-ray doesn’t always predict symptoms. Some people with significant narrowing have minimal pain, while others with mild narrowing have substantial symptoms.
Related Terms
More Anatomy Terms
View allAcetabulum
The cup-shaped socket of the pelvis that holds the head of the femur (thigh bone) in the hip joint.
Articular Cartilage
The smooth, white tissue covering the ends of bones in joints. Its breakdown is the primary feature of osteoarthritis.
Bursae
Small, fluid-filled sacs that act as cushions between bones, tendons, and muscles near joints, reducing friction during movement. Singular form is 'bursa.'
Bursa
Small, fluid-filled sacs that cushion bones, tendons, and muscles near joints. Bursitis (inflammation of a bursa) can cause joint pain similar to arthritis.
Cartilage
A smooth, rubbery connective tissue that covers and protects the ends of bones where they meet to form joints, allowing them to glide smoothly during movement.