What an X-ray can and cannot show in a hip joint

When a hip starts causing persistent pain or stiffness, an X-ray of the pelvis is almost always the first investigation ordered — and for good reason. Recommended as the standard starting point by NHS guidance, the EBI/AOMRC (reviewed September 2024), and specialist orthopaedic bodies, a plain antero-posterior (AP) pelvis radiograph gives a rapid, reliable picture of what is happening inside the joint.

What it cannot do is show cartilage directly. Cartilage is soft tissue and does not absorb X-ray in the way bone does, so it appears as empty space on the image — the gap between the rounded femoral head and the cup-shaped acetabulum. When that gap shrinks, it signals that cartilage has worn away. Beyond joint space narrowing, the radiograph reveals several bony consequences of that loss: osteophytes (spurs of new bone growing at the joint margins), subchondral sclerosis (a whitening and hardening of the bone surface beneath where cartilage has thinned), and subchondral cysts (small fluid-filled pockets within the bone).

The standard view is taken with the patient standing. Under the load of body weight, any narrowing that is hidden when lying down becomes apparent — a clinical detail a supine MRI cannot replicate.

One important limitation: in early disease, cartilage damage can be well under way before any bony changes have had time to develop. A normal or near-normal X-ray does not rule out meaningful joint deterioration, which is why the imaging needs to be read alongside the patient's symptoms — a point the grading system explored in the next section makes concrete.

The Kellgren-Lawrence stages: what each grade means

The Kellgren-Lawrence (KL) system gives radiologists and surgeons a shared language for describing what a hip X-ray shows. Running from 0 to 4, the scale tracks the progression of visible joint damage — from no abnormality at all to end-stage wear.

Grade 0 — Normal

No radiographic features of osteoarthritis. The joint space is preserved, the bone surfaces appear smooth, and there are no osteophytes.

Grade 1 — Minimal / Doubtful

Tiny, possible osteophytes at the joint margins and questionable narrowing of the joint space. These changes may reflect very early wear or simply normal ageing — at this grade, a radiologist may describe findings as 'possible early OA' or 'within normal limits.'

Grade 2 — Mild

Definite osteophytes are now visible, confirming structural change. Joint space may still look near-normal or only slightly reduced. OA is likely, but meaningful cartilage remains present.

Grade 3 — Moderate

The joint space has narrowed clearly, with multiple osteophytes, early subchondral sclerosis, and sometimes a slight change in the shape of the femoral head. Function typically begins to be affected noticeably at this stage.

Grade 4 — Severe (bone-on-bone)

Joint space is severely reduced or entirely absent — the femoral head and acetabulum are in direct bony contact. Sclerosis, subchondral cysts, and joint deformity are common accompanying findings. Grade 4 is the imaging finding most strongly associated with suitability for hip replacement; surgical series from major specialist centres indicate that patients with confirmed bone-on-bone arthritis tend to achieve the best outcomes after the procedure.

The scale describes a continuum rather than a set of hard boundaries, and individual X-rays are always interpreted alongside the patient's full clinical story.

When your X-ray grade and your pain level do not match

Radiology grades are consistent; human pain experience is not. A Grade 2 X-ray and a Grade 4 X-ray do not predict, with any reliability, how much a particular person is suffering — and this surprises many patients when they first encounter the idea.

Some people with Grade 4 bone-on-bone changes continue to manage daily life with relative ease, having adapted over many years of gradual decline. Others with Grade 2 findings are barely able to dress themselves or sleep through the night. A NICE evidence review noted this discordance explicitly, particularly in earlier OA stages. Neither situation is unusual, and neither invalidates what the image shows.

What this means in practice:

• A 'bad-looking' X-ray does not automatically mean surgery is the next step.
• A 'mild-looking' X-ray does not mean the pain is imagined or unimportant.

Age, activity level, the rate at which wear has developed, pain adaptation, and other health conditions all influence how significantly a given level of joint damage affects daily life. The X-ray confirms structural disease; it cannot measure pain, functional loss, or quality-of-life impact — those are assessed through clinical interview and careful examination.

NICE guidance reflects this directly: osteoarthritis can be diagnosed on clinical grounds alone in patients aged 45 or over with activity-related joint pain and morning stiffness lasting fewer than 30 minutes. Imaging then confirms and stages what is already suspected from the patient's lived experience — it does not replace it.

When MRI adds something X-ray cannot

Plain X-ray answers most questions about established hip osteoarthritis quickly and reliably — but there are situations where it cannot provide what is clinically needed, and MRI steps in to fill those gaps.

The key advantage of MRI is sensitivity to changes that precede visible bony damage. Early cartilage thinning, bone marrow oedema, joint effusion, and soft-tissue pathology may all be present before a single feature appears on a radiograph. This makes MRI the modality of choice when a patient's symptoms seem disproportionate to what their X-ray shows, or when the diagnosis is not straightforward.

One of the most important differentials MRI helps resolve is avascular necrosis (AVN) of the femoral head — a condition in which the bone loses its blood supply and begins to collapse from within. AVN is frequently identifiable on MRI at a stage when plain X-rays appear entirely normal or only equivocal; missing it carries serious consequences for the joint, so early detection matters considerably.

However, UK clinical guidelines issued by the EBI and AOMRC (reviewed September 2024) are clear that MRI should not be requested routinely once a clinical assessment and X-ray have already confirmed typical OA features. In that scenario, MRI rarely adds useful diagnostic or treatment information, may extend waiting times, and can generate unnecessary patient anxiety. Ordering it is not protecting thoroughness — it is adding a test whose result is unlikely to change the management plan.

CT scanning occupies a narrower role still: it is most useful for detailed pre-operative bone anatomy mapping and implant templating in complex cases, rather than for routine diagnosis.

What actually moves the conversation toward hip replacement

Seeing a Grade 4 X-ray does not make hip replacement automatic. Seeing a Grade 2 does not rule it out. The decision rests on a combination of what the image shows, what daily life has become, and what conservative management has — or has not — achieved.

The clinical signals that most reliably shift the conversation toward surgical assessment are functional ones. Nocturnal pain that repeatedly interrupts sleep, start-up pain that lasts more than a few steps after sitting or lying, and the growing inability to perform basic tasks — pulling on socks, tying footwear, walking a meaningful distance without stopping — all carry more weight in the timing decision than a single radiographic grade.

Conservative management is not simply a box to be ticked before surgery becomes available. Analgesia, physiotherapy, weight management, and activity modification can provide meaningful relief at Grade 2 or 3, and in some cases delay the need for replacement by months or years. What matters, clinically, is whether these measures are still working — and documenting when they no longer are.

Before any surgical planning proceeds, one important differential must be excluded: pain radiating down the back of the thigh toward the knee can originate from the lumbar spine rather than the hip joint itself. Operating on a hip will not resolve spinal pain, which is why this distinction is resolved carefully before assessment moves forward.

Longevity is also part of the conversation, particularly for younger patients. Approximately 58% of total hip replacements are estimated to last 25 years — a figure that is neither alarming nor dismissive, but one that means surgeons weigh the severity of current functional impairment against the realistic expectation that revision surgery may eventually be needed. The goal is to operate at the right moment, not the earliest one the X-ray might suggest.

Why specialist assessment looks beyond the X-ray

A radiograph describes a joint's structure. It cannot describe how that joint behaves when someone rises from a chair, rotates their leg to dress, or walks the length of a supermarket aisle. Physical examination fills that gap — measuring actual range of motion, noting whether internal rotation is the first movement to go (as is typical in hip OA), assessing gait, and identifying whether hip movement or spinal loading reproduces the pain. Set alongside the KL grade and the documented response to conservative treatment, these findings produce something an imaging report cannot: a clinical picture with a functional timeline.

Senior surgeons experienced in hip replacement apply this integrative judgement in both directions — recommending surgery when the functional picture justifies it, and deferring when it does not. Knowing when to delay or avoid replacement is as much a part of specialist assessment as knowing when to proceed.

Professor Paul Lee, Consultant Orthopaedic Surgeon at the Royal London Hospital and 108 Harley Street, works specifically at this assessment stage — weighting imaging, examination, and functional history together to determine where a patient sits on the pathway. For those moving toward the replacement conversation, Professor Lee advocates the SPAIRE technique: a muscle-sparing posterolateral approach designed to preserve the posterior soft tissues that contribute to joint stability and early recovery in selected patients.

For anyone whose daily life is meaningfully limited by hip pain — particularly where the KL grade has progressed and conservative management is no longer holding — a specialist review can answer what the previous sections have deliberately left open: whether the clinical picture, not just the scan, now places the replacement conversation within reach.