Joint Space Narrowing in the Hip

Where the gap is. On an X-ray, bone shows up white, while cartilage does not show at all. The dark band between the ball at the top of the thigh bone (femoral head) and the socket in the pelvis (acetabulum) is the joint space. When that band looks thinner than expected, the report calls it joint space narrowing. It is read as a sign that the cartilage lining the ball and the socket has thinned, one of the changes of hip osteoarthritis. The same phrase on a knee X-ray has its own guide: joint space narrowing in the knee.

How wide is normal. In 223 people with no history of hip or lower back pain, the gap at the top of the joint averaged about 4 mm on X-rays taken lying down, with a range of 2 to 7 mm. It was narrower in women than in men, and the authors concluded that normal values vary widely (Lequesne et al., 2004). Studies have used a gap of 2 mm or less at its narrowest point as their X-ray definition of hip osteoarthritis (Jacobsen et al., 2004; Terjesen and Gunderson, 2012), and some have used 2.5 mm.

The words that come with it. Reports say where the gap is thinnest, how much has been lost and what the bone around it looks like. These are the terms you are most likely to see.

Kellgren-Lawrence grades. Some reports give an overall grade from 0 to 4: normal, doubtful, mild, moderate and severe. For the hip, grade 1 (doubtful) means possible narrowing and possible bony spurs. Grade 2 (mild) means definite narrowing, definite spurs and slightly denser bone. Grade 3 (moderate) means marked narrowing with some denser bone, cysts and a change in the shape of the ball and socket. Grade 4 (severe) means gross loss of the joint space with large spurs and a marked change in shape (Terjesen and Gunderson, 2012). The Framingham study counted grade 2 or higher as osteoarthritis on an X-ray (Kim et al., 2015). The grade names describe the whole X-ray, so “mild osteoarthritis” and “mild narrowing” are not the same statement.

Tönnis grades. Hip surgeons commonly use a second scale, from 0 to 3. Grade 0 means no signs of osteoarthritis. Grade 1 means denser bone at the joint with minimal narrowing and spur formation. Grade 2 means small cysts with moderate narrowing. Grade 3 means large cysts, a joint space that has closed and a ball that has lost its shape (Shapira et al., 2020).

The same hip can be worded differently. The severity words are not standardised, and readers do not always agree. When a radiologist and an orthopaedic surgeon graded the same 61 hips, their Kellgren-Lawrence grades agreed only moderately, while measuring the gap at its narrowest point agreed much better (Terjesen and Gunderson, 2012). Five hip surgeons grading the same 40 X-rays on the Tönnis scale reached only fair agreement (Shapira et al., 2020). A change in wording from one report to the next is not proof that the hip has changed.

It is not a measure of pain. Across groups of people, X-ray findings and hip pain match loosely. In the Framingham study of 946 people aged 50 and over, about one in five hips with osteoarthritis on the X-ray was painful on most days, and fewer than one in six of the hips that were painful on most days showed osteoarthritis on the X-ray. A second study of 4,366 people found the same pattern (Kim et al., 2015).

Of the X-ray findings, narrowing is the one most tied to pain. In a Danish survey of the pelvic X-rays of 3,807 adults, a gap of 2 mm or less at its narrowest was the finding most strongly linked with hip pain in people aged 60 and over: the odds of reporting pain were about three times higher. The authors regarded bony spurs, cysts and denser bone as relatively inconsequential by comparison (Jacobsen et al., 2004).

A clear X-ray does not rule the joint out. In the Framingham and Osteoarthritis Initiative studies, most of the older people whose pain or examination pointed to hip osteoarthritis, with groin or front-of-hip pain or pain when the hip was turned inward, did not have it on their X-ray (Kim et al., 2015). An X-ray cannot show cartilage directly, and it shows nothing of the labrum, the tendons on the outside of the hip or the lower back, each of which can produce pain in the same region.

It does not narrow at the same pace in everyone. Among 463 people with hip osteoarthritis followed for two years, the gap at its narrowest point went from 2.2 mm to 1.7 mm on average, and about one in three lost more than half a millimetre (Dougados et al., 1997). The changes in the joint’s structure progress over time, but symptoms go up and down a lot, and many people keep excellent function with the right management.

Where the pattern and the grade do matter. A review of 57 papers found strong evidence that a higher Kellgren-Lawrence grade, a ball that has shifted upward or up and outward, and denser bone under the joint surface each went with a greater chance of going on to a hip replacement. Body mass index did not, and for most other factors the evidence was limited or conflicting (Teirlinck et al., 2019). That is a pattern across groups, not a forecast for one hip. Referral for a hip replacement still rests on symptoms and how much they affect your life, not on the X-ray grade (NICE, 2022).

It is not expected to reverse. No standard treatment has been shown to restore the joint space on X-ray. Pain and function can still improve without the X-ray changing.

The main guidelines name the same core treatments for everyone with hip osteoarthritis: information about the condition and a structured exercise program (NICE, 2022; Bannuru et al., 2019). The trials show what those can and cannot be expected to do.

• Exercise. Across 18 trials of 1,368 people, exercise probably eased pain and improved function slightly compared with no treatment, usual care or brief education, by amounts the reviewers judged too small to count as an important average benefit. Compared with a placebo or attention-only treatment, it may make little or no difference to pain and may improve function slightly, and that evidence is less certain (Hall et al., 2026). These are averages; the size and timing of improvement vary from person to person.
• Over the long term. In a trial of 109 people with painful hip osteoarthritis on X-ray, 22 of those given exercise therapy plus education had a hip replacement within six years, compared with 31 of those given education alone, and the median time to surgery was 5.4 years against 3.5 (Svege et al., 2015). One trial of this size is not a promise that exercise will prevent or delay an operation.
• With severe changes. In a trial of 109 people aged 50 or older with severe hip osteoarthritis and an indication for surgery, a hip replacement improved pain and function far more at six months than a resistance training program did: 15.9 points against 4.5 on a 48-point hip questionnaire. By six months, 12 of the 56 people assigned to training had gone on to have the operation (Frydendal et al., 2024). For severe osteoarthritis that already qualifies for surgery, the operation gave the larger gain.
• Weight. NICE recommends support with weight loss, where relevant, for people with osteoarthritis (NICE, 2022). The OARSI guideline lists weight management among the core treatments for the knee but not for the hip (Bannuru et al., 2019).
• Hands-on treatment. NICE says manual therapy can be considered for hip osteoarthritis only alongside exercise (NICE, 2022).
• Injections. The guidelines differ. NICE says a steroid injection can give short-term relief (NICE, 2022), while the OARSI guideline did not recommend steroid injections for the hip (Bannuru et al., 2019). In a later UK trial of 199 adults with hip osteoarthritis and at least moderate pain, one ultrasound-guided injection of steroid and local anaesthetic added to advice and education eased pain more over six months than advice and education alone, but no more than an injection of local anaesthetic alone (Paskins et al., 2022). Injections are arranged through your doctor.
• Not recommended. Hyaluronic acid injections (NICE, 2022; Bannuru et al., 2019) and opioid painkillers (Bannuru et al., 2019).
• A surgical opinion. Worth asking for when symptoms substantially affect your quality of life and non-surgical care has not been enough or is not suitable (NICE, 2022). Your family doctor can arrange the referral.

• The narrowing is described as concentric or uniform rather than at the top of the joint, or you have morning stiffness lasting well over half an hour or several swollen joints. Narrowing that is even all the way round is the expected X-ray pattern of an inflammatory arthritis (Abrahams et al., 1988). See your family doctor.
• It describes collapse, flattening or a fracture line in the ball of the hip, or the pain came on or worsened quickly without a clear reason. Ask a doctor to review it promptly, and the same day if the pain changed quickly. A fracture or loss of blood supply to the bone (avascular necrosis) needs to be ruled out.
• The report says the hip has changed quickly since the last X-ray. Ask your doctor for a review before starting or changing rehab.
• A newly hot, swollen or suddenly very painful hip needs medical assessment today, even without a fever. With fever, chills or feeling generally unwell, go to emergency now. This can be a joint infection.
• The report says there is no narrowing, or very little, but the symptoms continue. A clear X-ray does not rule out osteoarthritis of the hip (Kim et al., 2015), and the other sources of pain in this region do not show on it.
• It also mentions cam or pincer morphology, or dysplasia. Cam and pincer describe the shape of the bones, and the same shapes are common in people who have no pain; the hip impingement page and the hip labral tear guide cover them. Dysplasia, a shallow socket, is a different problem with a different plan, and worth raising with your doctor.

I start with how the hip feels and behaves: where the pain sits, how long the hip is stiff in the morning, and what you can no longer do that you want to do. Pain from the joint itself is often felt in the groin or the front of the thigh and can travel to the knee. Pain over the bony point on the outside of the hip more often points to the gluteal tendons than to the joint, and the gluteal tendinopathy page covers that.

On examination I watch you walk, measure how far the hip bends and turns, test the strength of the muscles at the side and back of the hip, and check standing on one leg and getting up from a chair. Losing the ability to turn the hip inward (internal rotation) is one of the more useful examination findings in hip osteoarthritis, so I measure it on both sides. I screen the lower back, because it can send pain to the same area. I record a few measures I can repeat, such as a 30-second sit-to-stand and a timed walk, so you can see whether the plan is working. I cannot order imaging; when imaging would change the plan, I flag it to your family doctor or specialist and refer you.

The plan is usually built around strengthening, particularly the muscles that move the leg out to the side and back (abductors and extensors), with walking or cycling in amounts the hip tolerates. Joint mobilization and soft tissue work can sit alongside the exercise where they make it easier. Flares tend to follow a sudden jump in what you ask of the hip, so the load is built up gradually and a plan for flare-ups is part of it. Many people see meaningful improvements in pain and function within 6 to 12 weeks of consistent effort, but the real benefits come from building these habits into your life for good. If pain and loss of function remain unacceptable despite appropriate non-surgical care, a hip replacement assessment is a sensible next step, arranged through your family doctor.