Osteogenesis imperfecta (OI) literally means imperfectly formed bones. All of them lead to variable degrees of micromelic (short-limbed) dwarfism.
The primary pathology in OI is a disturbance in the synthesis of type I collagen, which is the predominant protein of the extracellular matrix of most tissues.
Type I is the most common and mildest type of OI. The collagen structure is normal, but there is less collagen than normal.
Type II is the most severe form of OI leading to still birth or death shortly after birth. The collagen is improperly formed.
Type III OI also has improperly formed collagen and often severe bone deformities. The infant is often born with fractures and blue sclera.
Type IV OI is moderately severe, with improperly formed collagen. Bones fracture easily, but the sclera is normal. Bone deformities are mild to moderate.
Radiographic features:
The radiologic sine qua non of OI is generalized osteoporosis of both the axial and appendicular skeleton.
Milder forms of OI result in thin, overtubulated (gracile) bones with thin cortices, and relatively few fractures. The short tubular bones are also affected, though they are less frequently fractured.
More severe forms of OI, such as in types II and III, feature thickened, shortened long bones with multiple fractures; these forms are often complicated by hyperplastic callus formation. The callus is most often found around the femur and often large, appearing as a dense, irregular mass arising from the cortex of bone. This callus is associated with thickened periosteum. Its presence causes other differential diagnostic considerations including: osteosarcoma, myositis ossificans, chronic osteomyelitis, and osteochondroma.
Radiographs of the skull may reveal normal skull development in milder forms of disease. With increasing disease severity, the skull demonstrates poor mineralization and multiple wormian, or intrasutural, bones .
The chest may be small. Multiple rib fractures are often found; these can cause the ribs to become broad and deformed.
Spinal abnormalities in all subtypes include platyspondyly, and scoliosis.
Popcorn calcifications occur commonly in type III in the metaphyseal-epiphyseal region of long bones, most commonly at the knee and ankle. This results from repeated microfractures at the growth plate. Soft craniofacial bones with a large, thin calvarium, causes triangular facies.
A feature more commonly associated with type IV than other types is basilar invagination (impression) with or without brainstem compression