Imaging in Osteogenesis Imperfecta 

Osteogenesis imperfecta (OI) is a common heritable disorder of collagen synthesis that results in weak bones that are easily fractured and are often deformed. Several distinct subtypes have been identified, all of which lead to micromelic (short-limbed) dwarfism of varying degree. Depending on severity, the bone fragility may lead to perinatal death or cause severe deformities that persist into adulthood.^{[1, 2, 3, 4, 5, 6]}

The variability of the modes of inheritance, family history, clinical features, and radiologic findings forms the basis for the current accepted classification system of osteogenesis imperfecta, which Sillence et al first proposed in 1979. Two radiographs depicting type I disease appear below.

Types of osteogenesis imperfecta

Bone fragility with multiple fractures and bony deformities are the common hallmark of all types.

In type I osteogenesis imperfecta, bone fragility is mild, and there are minimal bony deformities. Approximately 20% of patients have kyphoscoliosis.

Type II is the most severe form of osteogenesis imperfecta. The ribs are thin and beaded, the long bones are crumpled, there is limited cranial and/or facial bone ossification, and the limbs are short, curved, and angulated. Type II disease can be further subdivided into types IIA, IIB, and IIC on the basis of the radiographic features of the long bones and ribs (see Radiograph for details).

Type III osteogenesis imperfecta is the next most severe form after type II and is probably the form that is best known to radiologists and orthopedic surgeons. Its hallmark feature is severe bone fragility and osteopenia, which is progressively deforming. Multiple fractures and progressive deformity affect the long bones, skull, and spine and are often present at birth. Kyphoscoliosis is common. Children with this type of osteogenesis imperfecta tend to have severe dwarfism caused by spinal compression fractures, limb deformities, and disruption of growth plates.

Type IV osteogenesis imperfecta is distinguished from type I by the slightly increased, although still variable, severity of bone fragility. Mild to moderate bony deformity of the long bones and spine is present; the incidence of fracture is variable. Basilar impression of the skull, with consequent brainstem compression, is common (reported in 70% of patients).

A type V category has been added to include patients with osteoporosis or interosseous membrane ossification of the forearms and legs, as well as patients who are prone to the development of hypertrophic calluses.^{[7, 8, 9]}

See Special concerns, below, for a controversial, self-limiting variant of osteogenesis imperfecta, known as temporary brittle-bone disease.

Preferred examination

The preferred examination for the initial investigation of osteogenesis imperfecta is plain radiography. Indeed, most of the imaging characteristics of this disease are apparent on plain radiographs.

Prenatal ultrasonography plays a role in the diagnosis of osteogenesis imperfecta; this condition is one of the more common skeletal dysplasias detected with prenatal ultrasonography. Most cases are found incidentally on ultrasonographic examinations performed for other reasons; typical incidental findings include fractures, decreased calvarial ossification, or calvaria that are compressible with transducer pressure. Most cases of osteogenesis imperfecta that are recognized in this way are type II, and the patients have no family history of the disease.

Magnetic resonance imaging (MRI) plays an adjunct problem-solving role in assessing for associated complications, such as basilar invagination.^{[10, 11]}

Differential diagnosis and other problems to be considered

Because osteoporosis and multiple fractures are hallmark features of osteogenesis imperfecta, other disorders that cause multiple fractures or decreased bone mineralization may be considered in the differential diagnosis, such as child abuse, juvenile osteoporosis, steroid-induced osteoporosis, Menkes (kinky-hair) syndrome, hypophosphatasia, battered child syndrome (syndrome X) and temporary brittle-bone disease.

The multiplicity of fractures seen in osteogenesis imperfecta commonly raises a concern about child abuse; however, key imaging hallmarks help distinguish osteogenesis imperfecta from child abuse (ie, nonaccidental injury) (see Radiograph). Because the radiologist plays a central role in distinguishing between these 2 entities, he or she must have an understanding of this disease, its genetic variability, and its imaging appearance.^{[7, 12, 13]}

Special concerns

Some authors have suggested that there exists a self-limiting variant of osteogenesis imperfecta), known as temporary brittle-bone disease, which has been described as a fundamental transient defect in collagen formation that is associated with multiple fractures in infants younger than 6 months. The radiologic and clinical features of this variant are the same as those noted in cases of child abuse. Because there is little scientific evidence to support the existence of this self-limiting entity, controversy about how to deal with cases of possible child abuse exists in the medical and legal communities.^{[14, 15]}

In cases of suspected osteogenesis imperfecta, postnatal radiographs should include views of the long bones, skull, chest, pelvis, and thoracolumbar spine. The radiographic features are related to the type of osteogenesis imperfecta and the severity of disease. Some findings, however, may be seen in all subtypes.

General radiographic features of osteogenesis imperfecta

The radiologic sine qua non of osteogenesis imperfecta is generalized osteoporosis of both the axial and appendicular skeleton. Milder forms of this condition result in thin, overtubulated (gracile) bones with thin cortices and relatively few fractures (see the images below). The short tubular bones are also affected, though they are less frequently fractured. In addition, radiographs of the skull in milder forms of osteogenesis imperfecta may reveal normal skull development.

More severe forms of osteogenesis imperfecta, such as types II and III, feature thickened, shortened long bones with multiple fractures; these forms are often complicated by hyperplastic callus formation (see the following image). The callus is most often found around the femur and is often large, appearing as a dense, irregular mass arising from the cortex of bone. This callus is associated with thickened periosteum, and its presence causes other differential diagnostic considerations, including osteosarcoma, myositis ossificans, chronic osteomyelitis, and osteochondroma.

Moreover, with increasing disease severity, the skull demonstrates poor mineralization and multiple wormian, or intrasutural, bones (see the images below).

The chest may be small. Multiple rib fractures are often found; these can cause the ribs to become broad and deformed. In addition, spinal abnormalities in all subtypes of osteogenesis imperfecta include platyspondyly and scoliosis (see the section on Type III-specific radiographic features of osteogenesis imperfecta, below).

Recent advances in the treatment of osteogenesis imperfecta with bisphosphonates have resulted in specific imaging findings. Cyclical pamidronate treatment produces sclerotic growth recovery lines in the long bones (see the images below). The amount of bone growth between doses of pamidronate may be measured by the distance between these growth lines.

Some radiographic findings are more specific to certain subtypes of osteogenesis imperfecta than others.

Type II-specific radiographic features of osteogenesis imperfecta

Type II osteogenesis imperfecta is further categorized into 3 subtypes on the basis of radiologic features of the long bones and ribs. In types IIA and IIB, the long bones are short and broad because of undermodeling; the bones are also crumpled. In type IIC, the long bones are thinner (cylindrical) and longer than in the other subtypes, although they are still undermodeled.

The ribs in type IIA are short and broad with continuous beading. In type IIB, beading is absent or minimal and discontinuous. In type IIC, the ribs are thin and beaded.

Type III-specific radiographic features of osteogenesis imperfecta

Scoliosis of the thoracolumbar spine is specific to type III osteogenesis imperfecta: As many as 25% of patients with osteogenesis imperfecta have scoliosis, and the association is even higher in patients with type III disease (see the image below). Most affected patients have an S -shaped scoliosis.

Severe platyspondyly with vertebral compression fractures and "codfish vertebrae" are more common in this type of osteogenesis imperfecta than in other types (see the following image).

"Popcorn calcifications" also occur commonly 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 cause triangular facies.

Type IV-specific radiographic features of osteogenesis imperfecta

Radiographic findings of type IV osteogenesis imperfecta are similar to the general findings and findings specific to type I disease. However, one feature that is more commonly associated with type IV than other types is basilar invagination (impression), with or without brainstem compression. This may be detected on plain radiography of the skull or cervical spine. The McGregor line, as the straight line connecting the upper surface of the posterior edge of the hard palate to the most caudal point of the occipital curve, may be used to assess for this complication. Projection of the tip of the odontoid process above the McGregor line suggests the presence of basilar invagination (see the CT scan and MRI images below).

The presence of a large, thin cranium with platybasia and cranial settling may lead to the appearance of the Tam O'Shanter skull.

Degree of confidence

Upon the detection of hallmark bone findings of osteogenesis imperfecta on plain radiographs, the diagnosis may be made with a high degree of confidence; confirmation with other imaging modalities is not needed.

Currently, the major role of CT scanning is in adjunctive problem-solving. This modality may be used to further assess for basilar impression (see the image below) to evaluate the petrous bone for narrowing of the middle ear or otosclerosis, and to support bone mineral densitometry (BMD) (see Nuclear Medicine, below).

The following is an MRI through the cervical spine in the same patient as in the CT scan above.

The major role of MRI in osteogenesis imperfecta is in problem-solving. MRI is also used to image complications of this disease, such as basilar impression. Although cervical spinal radiography and CT scanning (see the first image below) may demonstrate this abnormality well, MRI has the advantage of detecting associated compression of the spinal cord (see the second image below, which is from the same patient as that of the CT scan in the first image).

Basilar impression is frequently associated with type IV osteogenesis imperfecta. In particular, in type IVB disease, the incidence of neurologic symptoms is increased. Other associated conditions that may be imaged better with MRI than with plain radiography include syringohydromyelia and communicating hydrocephalus, especially if these conditions develop after fontanelle closure.

Osteogenesis imperfecta is one of the most common skeletal dysplasias detected on prenatal ultrasonography. Most cases involve type II disease and are found incidentally.

First trimester of pregnancy

The diagnosis of osteogenesis imperfecta may be made reliably by week 17 of gestation. The diagnosis may be made by detecting morphologic abnormalities on ultrasonograms or by analyzing collagen synthesized by chorionic villus cells after ultrasonography-guided chorionic villus sampling.

Second trimester of pregnancy

Ultrasonographic findings of osteogenesis imperfecta during the second trimester scanning include decreased echoes from the calvarium with supervisualized (too easily seen) intracranial structures; bowing and angulation of the long bones, implying platic deformities and fractures; decreased length of the long bones; and multiple rib fractures.^[16]

Bone mineral densitometry (BMD) results may confirm the severity of osteoporosis in patients with osteogenesis imperfecta; it may also confirm the presence of demineralization in mild cases of type I or type IV disease.

Currently accepted BMD measurement techniques include the following: (1) cortical radial BMD measured by use of single-photon absorptiometry (SPA); (2) BMD of the lumbar spine (in children older than 1 y) and femoral neck (in children older than 6 y), in which BMD is obtained by use of dual-energy x-ray absorptiometry (DXA); and (3) lumbar spinal BMD measured by means of CT scanning in children older than 4 years.

Degree of confidence

There are only a few reported cases in which bone mineral densitometry measurements were made in young children with osteogenesis imperfecta; as such, the reliability of these measurements is unknown.