Osteogenesis imperfecta (OI) is a genetic condition that weakens bones throughout the body, making them fracture easily, sometimes from little or no trauma at all. But its effects reach well beyond the skeleton. Because the underlying problem involves the protein that gives structure to bones, skin, teeth, and other connective tissues, OI can affect hearing, lung function, the heart, joints, and dental health. It occurs in roughly 1 in 10,000 to 20,000 people worldwide, and its severity ranges from mild (occasional fractures) to fatal at or before birth.
The Underlying Problem: Faulty Structural Protein
OI is caused by mutations in genes responsible for producing type I collagen, the main structural protein in bone, skin, tendons, and the whites of the eyes. In most cases, the body either makes too little collagen or makes collagen that is structurally abnormal. Think of collagen as the rebar inside concrete: without it, the material crumbles under stress. When collagen is defective, bones lack the flexible framework they need to resist force, and every tissue that relies on collagen is compromised to some degree.
Most cases are inherited from a parent who carries the abnormal gene, though new mutations can appear in families with no history of the condition. Some rarer forms don’t affect the collagen itself but instead disrupt the bone-forming cells that build and maintain the skeleton.
How OI Severity Varies by Type
OI is classified into several types. The most useful way to think about them is on a spectrum from mild to severe.
Type I is the mildest and most common form. People with Type I typically fracture bones from mild to moderate impacts, with most fractures happening before puberty. They usually reach near-normal height, though loose joints and muscle weakness are common. A blue, purple, or gray tint to the whites of the eyes is a hallmark sign.
Type II is the most severe form. Bones are extremely fragile, and mortality is essentially 100%, either before or shortly after birth. X-rays often reveal fractures that occurred in the womb.
Types III and IV fall between these extremes. Type III causes moderate to severe bone deformity, noticeably short stature, a barrel-shaped rib cage, a triangular face, shortened upper arm and thigh bones, and poor muscle development in the arms and legs. Type IV is similar but generally less severe. In Type III, excess mortality is very high in childhood and remains significantly elevated through the mid-30s.
Type V and rarer forms are generally moderate in severity. Type V has a distinctive feature: the body produces unusually large calluses at fracture sites, and a membrane between the two forearm bones can calcify, restricting arm movement.
Bones: Fractures, Bowing, and Deformity
The hallmark of OI is bones that break repeatedly, often from forces that would never injure a healthy skeleton. Fractures are most common in the shafts of long bones (the femur, tibia, humerus) and in the spine. Children are typically recognized when they present with multiple, repeated, or unexplained fractures, especially when no significant trauma occurred.
Beyond fractures, the long bones can gradually bow and thin. In severe forms, toddlers may develop a characteristic “shepherd’s crook” deformity of the thighbone or a “saber shin” curvature of the lower leg. These deformities result from a cycle of fracture, healing, and re-fracture on the curved side of the bone. The spine is also vulnerable: progressive scoliosis (side-to-side curvature) and kyphosis (forward rounding) are common and worsen over time if untreated.
In the skull, the bones may contain an unusually high number of small, irregular bone fragments called wormian bones, giving a mosaic-like appearance on X-ray. More severely affected individuals have more of these fragments. Skull shape changes are possible but rare.
Teeth and Dental Health
About half of people with OI also have dentinogenesis imperfecta, a condition where the inner layer of the teeth (dentin) forms abnormally soft. The teeth appear discolored, typically blue-gray or yellow-brown, and look somewhat translucent. Both baby teeth and permanent teeth are affected. Because the dentin is weak, teeth wear down rapidly, chip, break, and are more prone to decay and early loss. This isn’t just cosmetic; it can significantly affect chewing and nutrition, especially in children.
Hearing Loss Over Time
Hearing impairment is one of the less well-known effects of OI, but it’s common. Studies estimate that roughly one-third of people with OI develop some degree of hearing loss, with some research placing the figure as high as 58% in those with mild OI.
The pattern changes with age. Hearing loss is rare before age 20. Between ages 20 and 30, about 7% show hearing problems, typically conductive loss (meaning the tiny bones of the middle ear, which also depend on healthy collagen, don’t transmit sound efficiently). By ages 40 to 50, the rate climbs to 56%, and by age 50 and beyond, 65% have measurable hearing loss. In older adults, the type shifts from conductive to sensorineural or mixed, meaning the inner ear or auditory nerve becomes involved as well. Because it develops gradually, regular hearing checks starting in early adulthood are important for catching it early.
Lungs and Heart
In moderate to severe OI, the rib cage and spine don’t develop normally. Rib fractures, a barrel-shaped chest, scoliosis, collapsed vertebrae, and short stature all reduce the space available for the lungs to expand. The result is restrictive lung disease: the lungs themselves may be healthy, but they physically cannot inflate fully. People with Type III OI show significantly reduced lung capacity compared to those with milder forms, along with altered breathing muscle coordination and inefficient ventilation patterns.
The heart is affected too. Collagen is a structural component of heart valves and blood vessel walls. People with OI may develop widening of the aortic root (the base of the body’s main artery), thickening of the heart’s walls, and valve problems that allow blood to leak backward. Over time, the combination of lung restriction and cardiac strain can lead to pulmonary hypertension, where pressure in the blood vessels serving the lungs rises, forcing the right side of the heart to work harder. In severe cases, this progresses to right-sided heart failure.
Joints, Muscles, and Mobility
Loose, overly flexible joints are a near-universal feature of OI. The same faulty collagen that weakens bones also weakens the ligaments that hold joints together. This hypermobility increases the risk of joint dislocations and sprains. It also contributes to flat feet and an unstable gait, particularly in children.
Muscle weakness compounds the problem. Poor muscle development, especially in the arms and legs, is common in more severe types. Weak muscles provide less support for already fragile bones, creating a cycle: fractures lead to immobilization, immobilization causes further muscle loss, and weaker muscles offer less protection against the next fracture.
How OI Is Managed
There is no cure for OI, but treatment focuses on strengthening bones, preventing fractures, and managing complications as they arise.
Bone-Strengthening Medication
Bisphosphonates are the most widely used medications. They work by slowing the cells that break down bone, allowing the skeleton to retain more mineral density over time. Clinical trials show that oral bisphosphonates can significantly increase spine bone density within 12 months. Some trials also found reductions in fracture rates, though results have been mixed across studies. Intravenous and oral forms are both used, and the choice depends on the individual’s age, severity, and tolerance.
Surgery for Severe Deformity
For people with moderate to severe OI who develop disabling deformities or recurrent fractures in the same bone, surgery to straighten and internally reinforce long bones is a well-established option. The procedure involves cutting the bone in multiple places to realign it, then threading a metal rod down the center of the bone’s hollow canal to act as an internal splint. In children, expanding rods (such as the Fassier-Duval rod) are commonly used because they telescope outward as the child grows, reducing the need for repeat surgeries. Expanding rods are used in roughly 70% of femur procedures. Non-expanding rods are more common in the shinbone.
Physical Therapy and Daily Life
Carefully designed exercise programs help maintain muscle strength and joint stability without putting bones at excessive risk. Low-impact activities like swimming and water therapy are popular choices. Physical therapy also helps children reach motor milestones they might otherwise miss due to fractures and immobilization. For many people with mild OI, fracture frequency drops significantly after puberty, and daily life becomes considerably easier in adulthood.
Life Expectancy Across Types
For people with Type I OI, life expectancy is near normal. The condition causes inconvenience and pain, but it is rarely life-threatening. Types IV and related moderate forms carry modestly elevated mortality, with a statistically significant increase at younger ages that becomes less pronounced by middle age. Type III carries substantially higher mortality in childhood and young adulthood, primarily driven by respiratory complications from chest and spine deformities. Type II is fatal at or before birth in virtually all cases.
Respiratory failure, not fractures, is the leading cause of death in severe OI. This is why monitoring lung function and treating spinal curvature aggressively are priorities in more severe types.