Paget’s disease of bone is a chronic condition where the body’s normal process of breaking down and rebuilding bone goes haywire, producing bones that are larger, weaker, and misshapen. It’s the second most common metabolic bone disorder after osteoporosis, affecting 1 to 2% of adults over age 55. Many people with Paget’s disease have no symptoms at all and only discover it through a blood test or X-ray done for another reason.
How Bone Remodeling Goes Wrong
Your skeleton is constantly renewing itself. Specialized cells called osteoclasts break down old bone, and other cells called osteoblasts lay down fresh bone in its place. In Paget’s disease, the osteoclasts become abnormal. They grow larger than usual, multiply faster, and contain far more nuclei than healthy bone-resorbing cells. These overactive osteoclasts chew through bone at an accelerated rate.
The body tries to keep up by producing new bone just as fast, but the replacement bone is disorganized. Instead of the neat, layered structure of normal bone, pagetic bone forms in a chaotic mosaic pattern of woven tissue. The result is bone that looks thicker and denser on an X-ray but is actually structurally weaker and more prone to fracture, bowing, and deformity. This abnormal remodeling stays highly localized, often affecting just one or a few bones rather than the entire skeleton.
Where It Typically Shows Up
Paget’s disease can affect any bone, but it has strong preferences. The most commonly involved sites are the pelvis, spine, skull, and the long bones of the legs (the femur and tibia). In many cases, only a single bone is affected. The disease doesn’t spread from one bone to another the way cancer does. Whichever bones are involved at the time of diagnosis are generally the only ones that will ever be affected.
Symptoms and Warning Signs
Bone pain is the most common symptom, and it’s often mistaken for arthritis or general aging. The pain tends to be deep, aching, and persistent, sometimes worse at night. Because the disease frequently involves bones near joints, it can damage nearby cartilage and lead to genuine arthritis over time.
Visible changes depend on which bones are involved:
- Skull: gradual increase in head size, headaches, and hearing loss if the thickening bone compresses the nerves near the ear
- Spine: curvature of the back, and tingling or numbness in the arms and legs when enlarged vertebrae press on spinal nerves
- Leg bones: bowing of the limb, an altered gait, and increased fracture risk
- Pelvis: hip pain, especially when the disease involves the pelvis or upper thighbone
Many people, however, never develop any of these symptoms. Their disease is discovered incidentally when a routine blood test shows an unexplained spike in alkaline phosphatase, an enzyme released during bone formation.
What Causes It
The exact cause remains unclear, but Paget’s disease appears to result from a combination of genetic susceptibility and environmental triggers. About 30% of people with a family history of the disease carry a mutation in a gene called SQSTM1, which plays a role in the signaling pathways that control osteoclast formation. Around 10% of people with no family history also carry this mutation. However, having the mutation alone isn’t enough to cause the disease. Animal studies have confirmed that mice engineered with the same genetic change don’t develop Paget’s on their own, pointing to additional unknown factors.
Geographically, the disease is most common in people of British descent. Prevalence is highest in northwest England and in countries with significant British ancestry, such as Australia and New Zealand. Interestingly, both the prevalence and the severity of Paget’s disease have been declining over recent decades, which suggests environmental factors play a meaningful role, though researchers haven’t pinpointed exactly which ones.
How It’s Diagnosed
Diagnosis typically involves three tools. A blood test measuring alkaline phosphatase is the simplest screening method. Elevated levels suggest increased bone turnover, though the same enzyme can rise with liver disease, so further testing is needed to confirm the source. In cases where only a single bone is affected, alkaline phosphatase can sometimes remain normal, so doctors may order more specific bone turnover markers.
X-rays reveal the characteristic structural changes of Paget’s disease: areas of thickened, disorganized bone that look distinctly different from healthy tissue. A bone scan, which uses a small amount of radioactive tracer injected into the bloodstream, can map the full extent of the disease across the skeleton. The tracer collects wherever bone-forming activity is elevated, lighting up affected areas. This is particularly useful for identifying bones that don’t yet show symptoms.
Occasionally, the changes on imaging can resemble other conditions, particularly certain bone cancers that produce dense, bony growths. When imaging and blood tests aren’t enough to tell the difference, a bone biopsy provides a definitive answer. Under a microscope, pagetic bone has a distinctive mosaic pattern of woven bone and abnormally large osteoclasts that look nothing like tumor cells.
Treatment Options
The primary treatment is a class of drugs called bisphosphonates, which work by slowing down the overactive osteoclasts. For most patients with active disease who are at risk of complications, treatment with a bisphosphonate is recommended. The most effective option is an intravenous infusion that takes about 15 minutes and can produce disease remission lasting more than six years in the majority of patients. This long duration means many people need only a single treatment session and periodic monitoring afterward.
Oral bisphosphonates are an alternative for people who can’t receive the intravenous form. Treatment decisions weigh the potential benefits against risks. For someone with active disease in a weight-bearing bone or near a joint, early treatment helps prevent fractures, deformity, and premature arthritis. For someone with a single, stable area of disease in a non-critical location, monitoring without immediate treatment can be reasonable.
During treatment, adequate calcium (1,000 to 1,500 mg per day) and vitamin D (400 IU per day) intake is important. Bisphosphonates suppress bone turnover so effectively that calcium levels can temporarily drop if dietary intake is insufficient.
Potential Complications
Left untreated, Paget’s disease can lead to several serious problems. Fractures through weakened pagetic bone are one of the more common complications, particularly in the legs. Arthritis in joints adjacent to affected bones develops frequently because the misshapen bone alters how forces are distributed across the joint surface.
Hearing loss occurs when the disease involves the skull, as thickened bone can compress the auditory nerves. Other neurological symptoms, including numbness, weakness, or pain radiating down the limbs, happen when enlarged vertebrae narrow the spinal canal.
In rare cases, usually well under 1% of patients, pagetic bone can undergo malignant transformation into a type of bone cancer called osteosarcoma. This is a serious but uncommon complication, and a sudden increase in pain or swelling in a previously stable area of disease should prompt immediate evaluation. Heart complications can also develop in people with very extensive disease, because the increased blood flow demanded by highly active pagetic bone forces the heart to work harder.
Living With Paget’s Disease
For most people, Paget’s disease is a manageable condition. Many never need treatment at all. Those who do typically respond well to a single course of bisphosphonate therapy and then return for periodic blood tests to check alkaline phosphatase levels and confirm the disease remains in remission. If levels start creeping up again years later, retreatment is straightforward.
Staying physically active helps maintain joint flexibility and bone strength, though activities should be adapted to avoid excessive stress on affected bones. Weight management matters too, since extra body weight increases the load on bones and joints that may already be compromised. Regular follow-up ensures that complications like hearing changes, nerve compression, or joint deterioration are caught early enough to address effectively.