Dozens of diseases can damage your bones and joints, ranging from extremely common conditions like arthritis and osteoporosis to rare genetic disorders like brittle bone disease. Some wear down cartilage over decades, others weaken bone from the inside, and a few attack the skeleton through infection or cancer. Understanding what each condition actually does to your bones helps you recognize early warning signs and know what to expect.
Osteoarthritis
Osteoarthritis is the single most common skeletal disease worldwide. In 2019, roughly 528 million people were living with it, a 113% increase since 1990. About 73% of those affected are older than 55, and 60% are women. The knee is the most frequently affected joint, with 365 million cases globally, followed by the hip and hand.
The disease works by wearing away the cartilage that caps the ends of bones inside a joint. Cartilage normally acts as a smooth, shock-absorbing surface that lets bones glide past each other. As it breaks down, bones begin rubbing together, causing pain, stiffness, and swelling that typically worsens over years. Unlike some other skeletal diseases, osteoarthritis isn’t driven by the immune system. It’s largely a mechanical process accelerated by aging, excess body weight, joint injuries, and repetitive stress. About 344 million people with osteoarthritis have moderate or severe symptoms that benefit from rehabilitation like physical therapy, exercise programs, and weight management.
Rheumatoid Arthritis
Rheumatoid arthritis looks similar to osteoarthritis on the surface, with painful, swollen joints, but the underlying cause is completely different. It’s an autoimmune disease: your immune system mistakenly attacks the lining of the joints. That lining, called the synovium, becomes inflamed and thickened, eventually eroding the cartilage and bone within the joint.
Rheumatoid arthritis tends to affect joints symmetrically (both hands, both wrists) and often starts between ages 30 and 60. Because it’s a systemic immune disorder, it can also affect organs beyond the skeleton, including the lungs, heart, and eyes. Early treatment focuses on controlling the immune response to slow joint destruction, which makes recognizing it early particularly important. Morning stiffness lasting more than 30 minutes, swelling in the small joints of the hands and feet, and fatigue are hallmark early signs.
Osteoporosis
Your bones are constantly being broken down and rebuilt by specialized cells. Osteoclasts dissolve old bone, and osteoblasts lay down new bone to replace it. In osteoporosis, osteoblasts can’t fully refill the gaps left by osteoclasts, so each remodeling cycle results in a net loss of bone. Over time, bones become porous, fragile, and prone to fractures, especially in the spine, hip, and wrist.
Osteoporosis is often called a “silent disease” because bone loss happens without symptoms until a fracture occurs. A bone density scan measures how much mineral is packed into your bones and reports it as 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. A score of negative 2.5 or lower points to osteoporosis. Women after menopause are at highest risk because the drop in estrogen speeds up bone breakdown, but men develop it too, particularly after age 70.
Rickets and Osteomalacia
When bones don’t get enough calcium and phosphorus to mineralize properly, the result is soft, weak bone. In children, this condition is called rickets. In adults, it’s called osteomalacia. Both are most commonly caused by vitamin D deficiency, since vitamin D is essential for absorbing calcium from food.
Rickets shows up most dramatically in areas where bone is growing fastest, which is why it peaks between 4 and 12 months of age. The signs can be striking: bowing of the legs once a child starts standing, a protruding breastbone (sometimes called pigeon chest), a softened skull, delayed closure of the fontanelle, and beading along the rib cage where cartilage meets bone. Tooth development suffers too, with delayed eruption and weak enamel. In adults, osteomalacia produces bone pain, a waddling gait, muscle weakness, and fatigue. It develops more subtly than rickets but increases fracture risk significantly.
Paget’s Disease of Bone
Paget’s disease disrupts normal bone remodeling in a different way than osteoporosis. Instead of slow, steady bone loss, affected areas cycle through rapid breakdown and equally rapid (but disorganized) rebuilding. The replacement bone is larger, softer, and structurally weaker than normal bone. It most often affects the pelvis, skull, spine, and legs.
Many people with Paget’s disease have no symptoms at all and discover it incidentally through blood tests or X-rays. When symptoms do appear, they include deep bone pain, bones that feel warm to the touch, and gradual deformity of affected bones over years. Doctors track disease activity through blood markers that reflect the rate of bone turnover, with alkaline phosphatase being the most commonly used. Paget’s disease is relatively rare before age 50 and becomes more common with age.
Scoliosis and Spinal Curvature Disorders
The spine naturally curves forward and backward, but when it curves sideways, that’s scoliosis. A sideways curve measuring more than 10 degrees on an X-ray meets the diagnostic threshold. Curves are typically S-shaped or C-shaped, and most cases develop during the growth spurt just before puberty.
The majority of scoliosis cases are idiopathic, meaning there’s no identifiable cause. Mild curves may never need treatment beyond monitoring. Moderate curves in a growing child may require bracing to prevent progression. Severe curves, generally above 40 to 50 degrees, can affect breathing and may require surgical correction. Two related conditions affect the spine’s front-to-back curves: kyphosis (excessive rounding of the upper back, sometimes called a hunchback) and lordosis (excessive inward curvature of the lower back). All three conditions can range from cosmetic concerns to sources of significant pain and limited mobility.
Osteomyelitis
Osteomyelitis is a bone infection, usually bacterial, that can cause serious damage if not treated promptly. Bacteria reach bone through three main routes: traveling through the bloodstream from an infection elsewhere in the body, spreading directly from an infected wound or surgical site, or extending from infected surrounding tissue into the bone.
In children, blood-borne infections tend to settle in the long bones of the arms and legs. In adults, the spine is a common target, and new or worsening back or neck pain paired with fever should raise suspicion. People with diabetes face a particular risk: foot ulcers can deepen until they reach bone. If a blunt probe can touch bone through an open ulcer, osteomyelitis is highly likely. Treatment typically requires weeks of targeted antibiotics, and in some cases, surgery to remove dead bone tissue. Delayed treatment can lead to chronic infection that’s far harder to resolve.
Bone Cancer
Primary bone cancer, where cancer originates in the bone itself, is rare. Osteosarcoma is the most common form, and it predominantly affects adolescents and young adults. It usually develops in areas of rapid bone growth near the ends of long bones: the lower thighbone, upper shinbone, and upper arm bone. Pain and swelling near a joint that worsens over time, sometimes initially mistaken for a sports injury, are typical first symptoms.
Far more common than primary bone cancer is metastatic bone disease, where cancer from another organ (breast, prostate, lung, kidney, or thyroid) spreads to the skeleton. Metastatic bone disease behaves very differently from primary bone cancer at the cellular level, and treatment strategies reflect that difference. Osteosarcoma itself can also metastasize, most commonly to the lungs and occasionally to other bones.
Osteogenesis Imperfecta
Osteogenesis imperfecta, commonly known as brittle bone disease, is a genetic disorder that weakens bones by disrupting the production of type I collagen, the main structural protein in bone. Mutations in two specific collagen genes account for about 90% of all cases. There are at least 19 recognized forms, but four are most clinically relevant.
Type I is the mildest: the body produces normal collagen but not enough of it, leading to bones that fracture more easily than average but without major deformity. Type II is the most severe and is often fatal around birth. Types III and IV fall between these extremes, with Type III causing progressive bone deformity and Type IV producing moderate fragility. Beyond fractures, people with osteogenesis imperfecta may have blue-tinged whites of the eyes, short stature, loose joints, hearing loss, and dental problems. Severity varies enormously, even among people with the same type.
Protecting Bone Health
Regardless of which skeletal conditions you’re most at risk for, the nutritional foundations are the same. Adults between 19 and 50 need 1,000 mg of calcium daily. Women over 50 and men over 70 need 1,200 mg. For vitamin D, adults up to age 70 need 600 IU per day, and those over 70 need 800 IU. Children and teens between 9 and 18 have the highest calcium needs at 1,300 mg per day, reflecting the rapid bone building that happens during growth.
Weight-bearing exercise, including walking, running, dancing, and resistance training, stimulates bone formation throughout life. The bone mass you build in your teens and twenties acts as your reserve for later decades, which is why childhood and adolescent nutrition and activity matter so much. Smoking, excessive alcohol, and prolonged inactivity all accelerate bone loss. For anyone over 50 or with risk factors for osteoporosis, a bone density scan provides a concrete baseline and helps guide whether preventive treatment makes sense.