Osteogenesis imperfecta (brittle bone disease)

Causes and biological basis

Most commonly OI results from mutations in the genes that encode type I collagen, especially COL1A1 and COL1A2, which are inherited most often in an autosomal dominant pattern. Other, less common forms are caused by mutations in genes involved in collagen processing and bone mineralization and may be inherited recessively. The abnormal collagen impairs the bone matrix, making bones more brittle and prone to deformity under normal mechanical loads.

Key clinical features

• Frequent fractures with minimal trauma; long bones, ribs and vertebrae are often affected.
• Blue, gray or purple‑tinged sclerae (the whites of the eyes) in many affected individuals owing to thin connective tissue over the underlying choroid.
• Dentinogenesis imperfecta—defective tooth formation producing discolored, fragile teeth.
• Hearing loss that may develop in childhood or adulthood due to ossicular chain abnormalities or inner ear involvement.
• Short stature, scoliosis and bone deformities in moderate to severe forms.

Classification, diagnosis and course

Historically OI was divided into four main types (Sillence types I–IV) based on clinical severity, but later research identified many additional molecularly defined forms. Type I is typically the mildest; type II is often perinatally lethal; type III produces severe deformity with fractures beginning in infancy; type IV is of intermediate severity. Diagnosis combines clinical signs, radiographic evidence of bone fragility, family history and increasingly, genetic testing to identify causative mutations.

Management and prognosis

There is no cure for OI, but modern care focuses on reducing fracture risk, improving mobility and quality of life, and treating complications. Approaches include:

• Medical therapies such as bisphosphonates to increase bone density in many patients (effects and indications vary by age and type).
• Orthopedic interventions including intramedullary rodding to support long bones, corrective surgery for deformities, and fracture management.
• Rehabilitation, physiotherapy, adaptive equipment and pain management to maintain function.
• Dental care, hearing assessment and supportive measures for extra‑skeletal features.
• Genetic counseling and prenatal or preimplantation testing offered to families when a causative mutation is known.

Historical notes and research

Descriptions of unusually fragile bones date back to the 19th century, and classification has evolved with genetic discoveries. Research continues into targeted molecular therapies, improved anabolic drugs for bone, and gene‑based approaches, but these remain experimental. Outcomes depend strongly on the type and severity of OI; many people with mild forms lead active lives with appropriate care, while severe forms require lifelong multidisciplinary management.

For further information, see clinical summaries and patient resources: Genetics overview, clinical management guidance, patient and family support, and research updates.