Bone diseases range from extremely common conditions like osteoporosis, which affects roughly 1 in 6 people over age 50 worldwide, to rare genetic disorders that appear at birth. Some weaken bones silently over decades, others cause pain and deformity, and a few involve abnormal growths. Here’s a look at the major categories and what each one does to the skeleton.
Osteoporosis
Osteoporosis is by far the most widespread bone disease. It causes bones to lose density and become fragile, making fractures far more likely from minor falls or even everyday movements like bending over. Globally, about 28.5% of women and 5.1% of men over 50 have osteoporosis. The spine, hip, and wrist are the most vulnerable sites.
The condition develops quietly. Most people don’t know they have it until they break a bone. A bone density scan (called a DXA scan) measures how your bone mineral density compares to that of a healthy young adult. That comparison produces a number called a T-score. A score of negative 1 or higher is healthy. Between negative 1 and negative 2.5 indicates osteopenia, a milder form of bone loss that can progress to osteoporosis. A score at or below negative 2.5 means osteoporosis is present.
Risk factors include aging, menopause, low calcium and vitamin D intake, smoking, physical inactivity, and long-term use of certain medications like corticosteroids. Women are hit harder partly because estrogen, which helps maintain bone density, drops sharply after menopause.
Osteoarthritis and Bone Spurs
Osteoarthritis is primarily a joint disease, but it directly involves bone. What many people don’t realize is that damage to the bone just beneath the cartilage (called subchondral bone) can actually precede the cartilage breakdown that defines the condition. Repeated mechanical stress causes tiny fractures in this bone layer, triggering inflammation and thickening that disrupts the normal partnership between cartilage and bone.
As the disease progresses, cartilage erodes, exposing the underlying bone. The body responds by forming osteophytes, commonly known as bone spurs. These bony outgrowths develop at joint margins and appear to be the body’s attempt to stabilize a deteriorating joint and redistribute force. They don’t always cause symptoms on their own, but they can limit range of motion and contribute to pain. Osteoarthritis most commonly affects the knees, hips, hands, and spine.
Paget’s Disease of Bone
Paget’s disease causes bones to break down and rebuild in a disorganized way. In healthy bone, old tissue is steadily removed and replaced with new, well-structured tissue. In Paget’s disease, this cycle goes haywire in specific areas. The cells that break down bone become oversized and hyperactive, containing up to 100 nuclei compared to the normal 3 to 20. They also respond to growth signals at concentrations 10 to 100 times lower than what normal bone cells need.
The bone-building cells then rush to fill the gaps, but they deposit new bone chaotically instead of in the orderly layers that give healthy bone its strength. The result is bone that’s larger and misshapen but structurally weak and prone to fracture. The pelvis, spine, skull, and leg bones are most commonly affected. Many people with Paget’s disease have no symptoms and discover it through routine blood work or X-rays taken for another reason. When symptoms do appear, they typically include deep bone pain, enlarged or deformed bones, and joint stiffness near affected areas.
Rickets and Osteomalacia
These are essentially the same disease at different ages. Rickets affects children, osteomalacia affects adults, and both result from bones that fail to mineralize properly, usually because of severe vitamin D deficiency. Without enough vitamin D, the body can’t absorb calcium efficiently, and bones remain soft.
In children, the hallmark sign is bowed legs, where the legs curve outward leaving a visible gap between the knees. Children with rickets may also be small for their age, and their teeth may come through later than expected. In adults, osteomalacia causes widespread bone pain (particularly in the back, hips, pelvis, and legs), muscle weakness, difficulty climbing stairs or getting out of a chair, and a higher risk of fractures from minor incidents. Both conditions are treatable with vitamin D and calcium supplementation, though skeletal deformities that have already developed may need additional correction.
Osteomyelitis
Osteomyelitis is a bone infection, most often caused by Staphylococcus aureus bacteria. The infection can reach bone through the bloodstream, through an open wound or fracture, or from nearby infected tissue. In children, it tends to affect the long bones of the arms and legs. In adults, the spine and feet are common sites, particularly in people with diabetes, where poor blood flow to the extremities makes foot bones especially vulnerable.
Symptoms include deep, throbbing bone pain, fever, swelling, and warmth over the affected area. Treatment typically requires weeks of antibiotics. In children, 3 to 6 weeks of treatment is standard, with the risk of chronic infection rising significantly if effective therapy lasts less than 3 weeks. Adults with uncomplicated cases generally need up to 6 weeks of treatment. In some cases, surgery is necessary to remove dead bone tissue or drain infected areas. Diabetic foot osteomyelitis is particularly challenging because infections in this setting often involve multiple types of bacteria.
Osteogenesis Imperfecta
Osteogenesis imperfecta, often called brittle bone disease, is a genetic disorder that affects the body’s ability to produce type I collagen, the protein that gives bones their flexibility and strength. Mutations in two specific genes account for about 90% of all cases. There are several types, ranging from mild to fatal.
Type I is the mildest form. People with this type produce normal collagen, just not enough of it. They experience fractures during childhood and adolescence, often from minor incidents like falling while learning to walk, but fractures become less frequent in adulthood. They’re typically of normal or near-normal height, often have a blue or grey tint to the whites of their eyes, and about half develop hearing loss as adults.
Types III and IV fall in the middle of the severity spectrum and can cause short stature, spinal curvature, joint deformities, respiratory problems, and a tooth development disorder. Type II is the most severe form. Infants with this type may have an abnormally small, fragile rib cage and underdeveloped lungs, leading to life-threatening breathing problems that can be fatal shortly after birth. In the more severe types, the collagen itself is structurally abnormal rather than simply reduced in quantity.
Fibrous Dysplasia
Fibrous dysplasia is a rare condition in which normal bone is replaced by scar-like fibrous tissue. It can affect a single bone or multiple bones, and the most common sites are the skull, face, long bones in the arms and legs, and ribs. Symptoms include bone pain, deformities, recurring fractures, difficulty walking, and abnormal curvature of the spine.
When fibrous dysplasia occurs alongside hormone abnormalities and distinctive skin patches, it’s called McCune-Albright syndrome. The hormonal disruptions can cause early puberty in children, an overactive thyroid, excess growth hormone, and low blood phosphate levels that further weaken bones. The skin changes appear as patches of brown-colored skin with irregular, jagged edges. Depending on which bones are affected, people with McCune-Albright syndrome may also experience facial asymmetry, nasal congestion, vision loss, or hearing loss.
Primary Bone Cancer
Cancer that originates in bone (as opposed to cancer that spreads to bone from elsewhere) is relatively rare. The main types each tend to appear at different ages.
- Osteosarcoma forms in immature bone tissue, most often near the shoulder or knee. Risk is highest in children and adolescents ages 10 to 19, though it can occur in older adults. It tends to grow quickly and can spread to the lungs.
- Ewing sarcoma typically develops in the pelvis, legs, or ribs and also grows rapidly. It’s most common in children and adolescents under 19.
- Chondrosarcoma begins in cartilage, usually in the pelvis, upper leg, or shoulder. Unlike the first two, it occurs mainly in adults over 40, and risk increases with age. It generally grows more slowly.
- Chordoma is very rare and forms in the bones of the spine, most often at the base of the spine or base of the skull. It’s more common in older adults and affects men roughly twice as often as women.
Protecting Bone Health
Many bone diseases share overlapping risk factors: insufficient calcium and vitamin D, lack of weight-bearing exercise, smoking, and excessive alcohol use. The recommended daily calcium intake varies by age and sex. Children ages 9 to 18 need 1,300 mg per day during their peak bone-building years. Adults ages 19 to 50 need 1,000 mg. Women over 51 and everyone over 70 need 1,200 mg. Dairy products, leafy greens, fortified foods, and canned fish with bones are reliable dietary sources.
Vitamin D is equally important because it controls how well your body absorbs calcium. Sun exposure, fatty fish, fortified milk, and supplements are the primary sources. Weight-bearing exercise like walking, jogging, and strength training stimulates bone formation at any age and slows the rate of bone loss in older adults. For people already diagnosed with low bone density, medications that slow bone breakdown or stimulate new bone growth can significantly reduce fracture risk.