Osteogenesis imperfecta (OI) is a genetic disorder characterized by bones that break easily, often from little or no apparent cause. There are at least four recognized forms of the disorder, representing extreme variation in severity from one individual to another.
For example, a person may have just a few, or as many as several hundred fractures in a lifetime. While the number of people affected with OI in the United States is unknown, the best estimate suggests a minimum of 20,000, and possibly as many as 50,000 are currently living with this condition.
OI is caused by a genetic defect that affects the body's production of collagen. Collagen is the major protein of the body's connective tissue and can be compared to the framework around which a building is constructed. In OI, a person has either less collagen than normal, or a poorer quality of collagen than normal, leading to weak bones that fracture easily.
It is often, though not always, possible to diagnose OI based solely on clinical features. Clinical geneticists can also perform biochemical (collagen) or molecular (DNA) tests that can help confirm a diagnosis of OI in some situations.
Most cases of OI are caused by a dominant genetic defect. Some children with OI inherit the disorder from a parent. Other children are born with OI even though there is no family history of the disorder. In these children, the genetic defect occurred as a spontaneous mutation.