ONJ stands for osteonecrosis of the jaw, an area of jawbone that loses its blood supply and dies, leaving exposed bone that will not heal over. Most people who search for it have been prescribed a bone-strengthening medication and want to know how worried to be. The honest answer for the great majority is: considerably less than the internet suggests. This page covers what ONJ is, the actual numbers, who to see about it, and the version of osteonecrosis that does fall to physiotherapy, which is the one that affects the hip, knee and shoulder.
Long-term bone medication can also announce a problem as a dull ache in the leg, which PhysioHub's guide to thigh pain covers.
Bone is living tissue with its own blood supply. Osteonecrosis means an area of bone has lost that supply and died. In the jaw, the dead bone tends to become exposed through the gum and fails to heal, which is why the condition is usually noticed as an area of bare bone in the mouth that persists for weeks, often with pain, swelling or a bad taste.
Almost all cases are linked to medication, which is why the current term is MRONJ, medication-related osteonecrosis of the jaw. The drugs involved are anti-resorptives, which slow the cells that break bone down: bisphosphonates such as alendronic acid and zoledronic acid, and denosumab. Some anti-angiogenic cancer drugs are also implicated. The same property that makes these medicines effective against fracture, slowing bone turnover, also slows the jaw's ability to repair itself after an injury or an infection.
The jaw is affected rather than other bones because it is unusual: it is separated from a bacteria-rich mouth by a thin layer of gum, it undergoes constant remodelling around the teeth, and it is regularly injured by dental procedures. The strongest triggers are invasive dental treatment, particularly extractions, and dental infection.
The figure that matters is how different the risk is between the two groups who take these drugs.
For people taking anti-resorptives for osteoporosis, the reported incidence of MRONJ is in the region of 0.01%. For people receiving the much higher, more frequent doses used to manage cancer that has spread to bone, reported figures run up to 15%. That is a difference of roughly a thousandfold, and it is the single most important thing to establish about your own situation before worrying further.
UK dental practice reflects this. The Scottish Dental Clinical Effectiveness Programme guidance on oral health management of patients at risk of MRONJ, whose development process is NICE accredited, stratifies people into low and high risk groups and states plainly that the risk for patients treated with oral anti-resorptive drugs for osteoporosis is lower than for patients treated for cancer. Low-risk patients are advised to have routine dental care rather than anything restrictive.
Practical implications, in order of usefulness: have a dental check before starting an anti-resorptive if that is possible, keep on top of routine dental care while taking one, tell your dentist which medication you are on and for what reason, and report persistent mouth pain, loose teeth or exposed bone promptly. Stopping an osteoporosis medication because of ONJ anxiety carries its own risk, since the fractures these drugs prevent are common and consequential, and that is a decision for the prescriber rather than a precaution to take alone.
To be direct about scope: ONJ is a dental and oral surgical condition, and physiotherapy has no role in treating it. If you have symptoms in your mouth, the people you need are your dentist first, and an oral and maxillofacial surgery team if it is confirmed. Treatment centres on infection control, careful oral hygiene, conservative management of the exposed area and, in more advanced cases, surgery.
The reason a physiotherapy site covers it at all is that ONJ sits inside a wider story about bone health that overlaps heavily with musculoskeletal care. The people taking these medications are largely the same people managing osteoporosis, arthritis, fracture risk and falls, and the questions that follow an ONJ warning leaflet are usually about whether to keep taking the drug, whether exercise is still safe, and what else is happening to their skeleton.
The same process happens in other bones, where it is usually called avascular necrosis or osteonecrosis. Unlike the jaw version, this one is squarely musculoskeletal, and it is the reason "osteonecrosis" turns up in the context of hip pain.
The hip is the most commonly affected site, specifically the femoral head. Known associations include long-term or high-dose corticosteroid treatment, significant alcohol intake, sickle cell disease, previous hip fracture or dislocation, and decompression illness. A meaningful proportion of cases have no identified cause.
What it feels like: a deep groin ache that builds over weeks to months, worse on weight-bearing, often with night pain, and sometimes with a hip that starts catching or losing range. It is easily mistaken for early osteoarthritis, and it particularly warrants consideration when the person is younger than a typical arthritis presentation or has one of the risk factors above.
Why it matters to identify it: unlike osteoarthritis, this can progress to collapse of the bone surface, and the treatment options that preserve the joint depend on catching it before that happens. Plain X-rays are often normal early, and MRI is the test that finds it. Persistent, unexplained deep hip pain in someone who has taken long-term steroids is worth mentioning to a clinician specifically.
Physiotherapy has a genuine role here, largely around protecting the joint while a diagnosis is made, managing weight-bearing, maintaining strength and range around the hip, and rehabilitation if surgery follows. Our guides to hip replacement rehab and seeing a specialist for arthritis pain cover the routes on either side of that.
A common worry after an ONJ leaflet is whether exercise is now risky. For osteoporosis, the answer runs the other way: loading is part of the treatment.
Bone responds to mechanical load, and the exercise that maintains bone density is resistance training and weight-bearing impact rather than swimming or cycling. Alongside it, balance and strength work reduces falls, and falls are what turn low bone density into a fracture. UK consensus guidance on exercise for osteoporosis supports progressive resistance training and balance work for the large majority of people, including those with existing vertebral fracture, with modifications rather than avoidance.
The exceptions are worth knowing rather than fearing: repeated end-range spinal flexion under load is generally advised against where vertebral fractures have occurred, and anyone with a recent fracture needs a specific plan. Neither of those is a reason to stop exercising, and both are reasons to have the programme set up properly. Our article on rehab for seniors covers what strength and balance work looks like in practice.
The question that arrives with a bone-protection leaflet, almost always, is a version of: "should I be doing less?"
The answer that follows from the evidence is the opposite, and the reasoning is worth having. The medication is addressing bone density; it does nothing for muscle strength, balance or the confidence to move, and those are what decide whether a stumble on the stairs becomes a fracture. Someone on alendronic acid who has stopped walking and stopped lifting anything heavy has traded one risk factor for two. In the Uckfield clinic, the people who do best after a fracture-risk diagnosis are the ones whose programme was made harder over time rather than gentler, which is also what the falls-prevention evidence has consistently shown. Age UK East Sussex runs strength and balance and Otago programmes locally, and they are a good complement to individual work rather than a substitute for it where someone needs a specific plan.
If you have been started on a bone-protection medication and want a strength and balance programme built around your actual fracture risk, booking an assessment is where that starts.
What does ONJ stand for?
Osteonecrosis of the jaw. It is now more often called MRONJ, medication-related osteonecrosis of the jaw, because almost all cases are linked to medication.
How likely is ONJ on osteoporosis medication?
Reported incidence is around 0.01% for people taking anti-resorptives for osteoporosis, compared with figures up to 15% at the much higher doses used in cancer care.
Should I stop my bisphosphonate?
That is a decision for your prescriber. The fractures these drugs prevent are far more common than ONJ, so stopping on your own carries its own risk.
Can a physiotherapist treat ONJ?
No. It is managed by dental and oral and maxillofacial teams. Physiotherapy is relevant to the wider bone health picture and to osteonecrosis in the hip, knee and shoulder.
Is osteonecrosis of the hip the same condition?
It is the same process, bone losing its blood supply, in a different site. The causes, symptoms and treatment differ, and the hip version is a musculoskeletal problem rather than a dental one.
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