What AVN actually does to the femoral head

The ball of your hip joint — the femoral head — sits at the top of the thigh bone and relies on a single dominant vessel, the medial circumflex femoral artery, to keep its bone cells alive. Most joints receive blood from several overlapping sources, which gives them a degree of redundancy. The femoral head does not enjoy that luxury. When this artery is blocked or disrupted, whether by trauma, prolonged steroid use, or another cause, the bone tissue inside the femoral head begins to die. That process is avascular necrosis (AVN), and because the bone can no longer repair itself from within, the damage quietly accumulates.

This is quite different from the more familiar picture of hip osteoarthritis, where cartilage wears down gradually over decades, typically in older adults. AVN most commonly strikes adults aged 30 to 65 — often people who had no prior joint problems at all — and it does not wait patiently. Dead bone loses its structural strength. Over months to years, the weight of the body pressing through the joint eventually overwhelms the compromised femoral head, and the surface begins to crack and collapse inward. At that point, the smooth ball that once glided through the hip socket becomes irregular and misshapen, causing secondary damage to the socket itself.

The collapse timeline makes early recognition particularly important. In symptomatic, untreated cases, some studies suggest up to 80–85% progress to structural failure within two years. That window — between the first sign of bone cell death and irreversible collapse — is where treatment choices still exist and where the difference between joint preservation and joint replacement may be decided.

Risk factors and who gets AVN

Several distinct histories raise the likelihood of AVN significantly, though the condition can occasionally appear with no obvious explanation at all.

Hip trauma — a fracture through the femoral neck or a dislocation of the hip joint — is the most common single cause. The mechanical injury can directly tear the vessels supplying the femoral head, cutting off blood flow immediately.

Long-term high-dose corticosteroids rank as the leading non-traumatic cause and are often prescribed for autoimmune conditions, inflammatory disorders, and organ transplant regimens. Steroids are thought to disrupt fat metabolism, promoting tiny fatty deposits that may block the small vessels inside the femoral head.

Chronic heavy alcohol use is the second leading non-traumatic cause, possibly through a similar mechanism of microvascular occlusion over time.

Less common causes include sickle cell disease (where misshapen blood cells can obstruct small vessels), inherited or acquired clotting disorders, and radiotherapy to the pelvis.

Idiopathic AVN — where no cause can be identified — accounts for a meaningful proportion of cases, which is a reminder that the absence of an obvious risk factor does not rule out the diagnosis.

One additional point worth noting: when a systemic cause such as corticosteroid use or sickle cell disease is present, AVN may develop in both hips simultaneously or sequentially, so the opposite hip warrants imaging even if it is currently symptom-free.

Whatever the underlying cause, it does not substantially alter the staging and treatment logic that guides management — the condition follows the same collapse pathway regardless of how the blood supply was first disrupted.

Symptoms and why early diagnosis is harder than it sounds

Pain is usually the first signal — a deep, aching discomfort centred in the groin, sometimes spreading down the front of the thigh to the knee. Early on it tends to arrive with weight-bearing: walking, climbing stairs, or rising from a chair. Over weeks and months it typically becomes more constant, eventually intruding on rest and waking people at night. Hip stiffness follows, making ordinary tasks — pulling on a sock, squatting, cutting toenails — noticeably harder. A worsening limp often develops without the person fully registering it until someone else points it out.

What makes early diagnosis genuinely difficult is that this pattern is almost identical to advancing hip arthritis, groin strain, or referred pain from the lumbar spine. Without the right imaging, clinicians and patients alike can spend months attributing symptoms to the wrong cause.

A normal X-ray does not rule out AVN. In the early stages, the femoral head looks intact on plain films even when significant bone cell death has already taken place. MRI is the critical tool here: it detects changes in the bone's internal signal — the earliest sign of compromised blood flow and dying tissue — long before any structural failure shows up on an X-ray. That matters because the treatment options available at the pre-collapse stage are far broader than those available once the bone has begun to cave in.

Anyone with known risk factors — steroid use, a history of hip trauma, heavy alcohol use — who develops persistent groin pain should ask specifically whether an MRI has been arranged, rather than accepting reassurance based on a normal X-ray alone.

Staging AVN: why the Ficat system matters to treatment

Think of the femoral head as a bridge. Stages I and II describe a structure that still looks sound from the outside but whose internal steel is weakening — the geometry is intact, the bone has not buckled, and joint-preserving procedures such as core decompression or bone grafting remain on the table. Stage III is when the first cracks become visible: a thin line just beneath the joint surface — the crescent sign — appears on imaging, marking the point where the bone is beginning to separate from the cartilage above it. This is the critical inflection. The window for preservation has not yet closed entirely, but it is closing fast. By Stage IV, structural collapse has occurred: the femoral head loses its rounded shape, the joint space narrows, and the acetabulum — the socket — begins to suffer secondary damage. At that point, procedures aimed at saving the native joint are no longer viable.

The Ficat and Arlet classification (Stages I–IV) is the system patients are most likely to encounter when AVN is discussed in clinic, and understanding its logic is practically useful. Staging is not a formality; it is what determines which interventions remain possible when two people sit down to discuss imaging results. A patient who arrives knowing that Stage III means impending collapse — and that Stage IV means replacement is the realistic path — is far better placed to have a meaningful conversation about what comes next.

From preservation to replacement: matching treatment to stage

At Stages I and II — before the femoral head geometry has failed — a small number of interventions aim to buy the bone time to recover or to unload the damaged segment. Core decompression, which involves drilling a channel into the femoral head to relieve pressure and encourage blood vessel in-growth, is the most widely performed; evidence is moderate, and outcomes depend heavily on lesion size, underlying cause, and how early the procedure is carried out. Bone grafting — packing that cavity with structural or vascularised graft — adds mechanical support and may improve success rates in selected patients. Osteotomy, which repositions the femoral head so that the diseased area bears less load, is a more technically demanding option that remains available only in specialist centres.

None of these procedures reverses bone cell death. They work by interrupting the pathway to collapse, not by undoing the damage already done. The window in which they can meaningfully intervene closes quickly — among symptomatic patients, 80–85% will reach structural collapse within two years without effective treatment.

Conservative management — crutches, anti-inflammatory medication, physiotherapy — can reduce pain during this period but cannot halt the underlying process. It is a bridge to a decision, not a destination.

Once collapse has occurred, the treatment arithmetic changes entirely. The bony architecture that joint-saving procedures depend on has failed, and there is nothing left to reconstruct around. Total hip arthroplasty at this point is not a reluctant last resort; it is the straightforward right answer. It removes pain reliably, restores walking mechanics, and returns most patients to activities that AVN had progressively taken from them. For appropriately selected patients, survivorship studies now place THA for AVN on a par with THA performed for osteoarthritis — durable, well-characterised outcomes that were not available to patients a generation ago, and that with modern implant design may extend well beyond 25 years.

Delay beyond collapse does not simply defer the operation. It allows hip muscle bulk and conditioning to deteriorate further, making the eventual recovery more demanding than it needed to be.

Hip replacement for AVN: what makes the approach matter for younger patients

Younger patients facing THA for AVN carry a different set of concerns from those presenting with age-related osteoarthritis. An implant placed at 45 needs to function for several decades; a person who was hiking or cycling before the diagnosis wants more than pain relief — they want to resume a physical life. These stakes sharpen two practical questions: how durable will the replacement be, and how quickly and safely can normal movement be re-established?

The surgical approach plays a more significant role in answering those questions than is sometimes appreciated. Conventional posterolateral techniques require dividing the short external rotator muscles and tendons that stabilise the joint from behind. When those structures are cut and subsequently repaired rather than kept intact, the recovering hip can be less stable in the early weeks, dislocation precautions may extend for months, and patients often feel uncertain about loading the joint with full confidence.

Muscle-sparing approaches that preserve the posterior tendons aim to address this directly. The SPAIRE technique — Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus — is built on this principle: leaving the piriformis and obturator internus intact preserves the native tension and proprioceptive feedback of the posterior soft-tissue envelope, which may support earlier stable weight-bearing and a lower dislocation rate in suitable patients. Professor Paul Lee developed SPAIRE at the Exeter Hip Unit and uses it as his standard approach for hip arthroplasty, with the technique's rationale centred on reducing surgical disruption to structures that matter for early, confident recovery.

Within this pathway, the surgical approach is reinforced by a structured biological recovery programme that, for selected patients, supports same-day or next-day discharge rather than extended hospitalisation — integrating implant choice, mobilisation planning, and recovery design from the outset. For a younger patient weighing up surgical options, the approach used and the recovery framework built around it are concrete questions worth raising with a specialist before a decision is made.

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