Why imaging is ordered before any replacement discussion

Most people arrive at this article having already been sent for a scan — or having received a report they are not quite sure how to interpret. That experience is worth starting with, because it shapes how confusing imaging can feel before anyone has explained the sequence.

A plain weight-bearing X-ray of the hip is almost always the first investigation ordered when groin pain or hip stiffness is being assessed. It is quick, widely available, and gives a clear picture of bony architecture: how much space remains between the femoral head and the acetabular socket, whether bone spurs have formed, and whether the joint surfaces have begun to change shape. X-rays pass through soft tissue and register bone, which makes them ideal for this structural overview.

MRI uses magnetic fields rather than radiation, and images soft tissue in far greater detail — cartilage, tendons, the joint lining, and bone marrow changes that X-ray cannot detect. It is typically added when the X-ray does not explain the severity of symptoms, or when a condition affecting blood supply to the femoral head is suspected.

Neither scan is interpreted in isolation. The sections below explain what each one actually reveals, and why those findings are only part of what a surgeon weighs before any replacement discussion begins.

What a hip X-ray can and cannot show

Taken standing up — so the joint bears the body's full weight — a hip X-ray captures bone in sharp detail. The single most important measurement is the distance between the femoral head and the acetabular socket: this joint space is the X-ray surrogate for cartilage. As cartilage wears, that gap narrows. In advanced osteoarthritis it closes entirely, leaving bone pressed directly against bone.

Alongside joint space, the radiologist or surgeon looks for three further changes. Osteophytes — bone spurs growing at the femoral head and acetabular rim — are the skeleton's attempt to spread load across a failing surface. Subchondral sclerosis, seen as increased whiteness beneath the joint, reflects bone thickening in response to stress. Bone cysts or remodelling of joint contours indicate that structural change has been under way for some time. Together, these findings can be graded using the Kellgren-Lawrence scale, which classifies X-ray severity from no change through to advanced joint destruction — a useful shorthand for tracking how a joint changes over serial reviews.

What X-ray cannot show matters just as much. Cartilage itself is invisible on plain film, as is the joint lining, surrounding tendons, and soft-tissue inflammation. Early osteoarthritis, where cartilage is damaged but the bony landmarks have not yet shifted, may produce a near-normal-looking image. Early avascular necrosis — where the bone inside the femoral head is already dying — can similarly pass entirely undetected until collapse is close. These are not faults in the technology; they are the precise reasons MRI was developed as a complementary investigation.

What MRI adds — and when it matters most

Cross-sectional MRI images the hip in slices, revealing structures that bone-focused X-ray cannot reach: cartilage thickness and surface integrity, synovial inflammation, the labral ring around the acetabular socket, and the periarticular muscles that stabilise the joint. None of these appear on plain film.

The clearest illustration of MRI changing a clinical pathway is avascular necrosis of the femoral head. When blood supply through the medial circumflex femoral artery is disrupted — by trauma, prolonged corticosteroid use, or sometimes without an identifiable cause — bone tissue inside the femoral head begins to die silently. An early MRI shows characteristic bone marrow oedema and ischaemic signal changes long before the femoral head surface buckles. Detecting AVN at this stage may open management options that collapse forecloses entirely.

Surgeons classify AVN severity using staging frameworks such as Ficat-Arlet and the ARCO system, both of which rely primarily on MRI findings to place patients on a spectrum from early marrow change through to irreversible joint destruction. These tools help determine how much time, if any, remains before femoral head collapse makes hip replacement the only viable path.

Beyond staging known AVN, MRI is also valuable whenever a patient's symptoms seem disproportionate to a mildly abnormal X-ray — when restricted movement and groin pain exceed what the plain-film picture can explain. It can equally identify conditions that mimic osteoarthritis, such as early inflammatory arthritis or occult stress fractures, ensuring the clinical picture is complete before any replacement discussion begins.

Why your scan grade is not the whole story

Scan grades and lived experience frequently diverge — and this is one of the best-established findings in osteoarthritis research. Some patients whose X-rays show near bone-on-bone contact manage reasonable distances, sleep through the night, and report modest pain. Others with moderate joint space narrowing are unable to put on a shoe without help, lie awake in discomfort, and have long since stopped walking to the shops. Neither presentation is unusual, and neither means the scan is inaccurate.

The mismatch exists partly because of individual variation in how pain is processed, and partly because of what plain X-ray actually measures: structural bony change, not the nerve activity or soft-tissue inflammation that often drives daily symptoms.

For a surgeon, this means the clinical examination carries equal weight to the radiograph. Reduced internal rotation of the hip, groin pain reproduced on loading, and a restricted arc of movement can tell a more immediate story than the Kellgren-Lawrence grade alone. Quality-of-life factors — disrupted sleep, inability to walk a meaningful distance without stopping, loss of independence with basic tasks — are assessed alongside the imaging, not as an afterthought.

A patient who arrives with a report marked 'severe' is not automatically booked for replacement. Equally, someone whose scan is described as 'moderate' may find, on clinical review, that their functional loss and pain burden make specialist assessment entirely appropriate. Imaging answers the structural question; examination and quality-of-life history answer the human one.

How a specialist reads imaging as part of the decision

Reading a scan in clinic is not a pattern-matching exercise. A surgeon brings the radiograph into conversation with the patient's age, activity demands, symptom history, and the treatments already tried — physiotherapy, analgesics, or injection — and what each of those did or did not achieve. The same degree of joint space narrowing can carry very different clinical weight depending on everything surrounding it.

Once imaging confirms the diagnosis and replacement looks appropriate, the scans take on a second role: surgical planning. Pre-operative templating uses standardised radiographs to estimate implant size and geometry, plan restoration of leg length, and assess acetabular offset. For a muscle-sparing approach such as the SPAIRE technique, where preserving the posterior tendon envelope is central to the operation, this anatomical preparation before theatre shapes what is possible on the day — imprecise templating translates directly into imprecise surgical execution.

Professor Paul Lee's integrative approach at the Royal London Hospital and 108 Harley Street reflects this philosophy. His training in the SPAIRE technique at the Exeter Hip Unit is rooted in a surgical tradition where radiographic rigour and technical precision go together; the scan informs not just the diagnosis but the specific anatomy the surgeon will encounter. Imaging also extends beyond the procedure itself: radiographic review of implant position and bone integration is part of how outcomes are monitored over time, not a one-off step. For patients at the decision stage, that perspective matters — it means the surgeon reading a pre-operative X-ray is thinking not only about today's findings but about the decades of function they are being asked to support.

When imaging findings point toward replacement assessment

No single scan finding carries a referral letter inside it. What moves a conversation toward replacement assessment is convergence: joint space narrowing severe enough to suggest bone-on-bone contact, combined with pain that conservative treatment has not controlled, and symptoms that genuinely limit sleep, walking distance, or daily independence. When those three strands — structural, symptomatic, and functional — point in the same direction, specialist assessment becomes a reasonable next step rather than a premature one.

For avascular necrosis, the timeline can be more pressing. MRI evidence of femoral head collapse, or bone changes indicating collapse is imminent, often accelerates the pathway — not because surgery is automatic, but because the structural window for good implant positioning narrows quickly once the femoral head loses its spherical shape.

If any of this maps onto your own situation — scans showing significant joint deterioration, pain that disrupts sleep or limits you to short walks, and conservative measures that have plateaued — then asking for a specialist opinion is not a commitment to an operation. It is a way of establishing what the imaging actually means for this hip specifically: whether the structural picture and the functional picture have converged enough to make replacement worth discussing, or whether other steps remain.

1. [1] Osteoarthritis – Wikipedia. https://en.wikipedia.org/?curid=504841 https://en.wikipedia.org/?curid=504841
2. [2] Avascular Necrosis – Wikipedia. https://en.wikipedia.org/?curid=1498676 https://en.wikipedia.org/?curid=1498676