Osteogenesis imperfecta
OI and sometimes known as Brittle Bone Disease, or ‘Lobstein syndrome’.Osteogenesis imperfecta is disorder of congenital bone fragility caused by mutations in the genes that codify for type I procollagen. It is a common heritable disorder of collagen synthesis that results in weak bones that are easily fractured and are often deformed. It is also known as Brittle Bone Disease, or ‘Lobstein syndrome’. This condition affects an estimated 6 to 7 per 100,000 people worldwide. Several distinct subtypes have been identified. All of them 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. A wide array of clinical manifestations of the disease may be seen. These partly depend on the genetic subtype. Types I and IV are the most common forms of osteogenesis imperfecta, affecting 4 to 5 per 100,000 people.

The following 4 types of osteogenesis imperfecta have been reported. Type I - mild forms, type II - extremely severe, type III, severe type IV – undefined.

People with this disease are born with defective connective tissue, or without the ability to make it, usually because of a deficiency of Type-I collagen. This deficiency arises from an amino acid substitution of glycine to bulkier amino acids in the collagen triple helix structure. The larger amino acid side-chains create steric hindrance that creates a "bulge" in the collagen complex. As a result, the body may respond by hydrolyzing the improper collagen structure. If the body does not destroy the improper collagen, the relationship between the collagen fibrils and hydroxyapatite crystals to form bone is altered, causing brittleness. Another suggested disease mechanism is that the stress state within collagen fibrils is altered at the locations of mutations. These recent works suggest that osteogenesis imperfecta must be understood as a multi-scale phenomenon, which involves mechanisms at the genetic, nano-, micro- and macro-level of tissues.
In osteogenesis imperfecta, the modes of inheritance, family history, clinical features, and radiologic findings vary.Four distinct types are identified: type I, which is the dominantly inherited form with blue sclerae; type II, which is the perinatal lethal form; type III, which is the progressively deforming form with normal sclerae; and type IV, which is the dominantly inherited form with normal sclerae.
In general, type I is the mildest form of disease; type IV, type III, and type II, respectively, increase in severity.



As a genetic disorder, Ti is an autosomal dominant defect. Most people with OI receive it from a parent but it can be an individual (de novo or "sporadic") mutation. Osteogenesis imperfecta is relatively rare. In some cases, the parent has osteogenesis imperfecta and the condition has been genetically transmitted to the child. But, the child's symptoms and the degree of disability could be very different from that of the parent. In some children, neither parent has osteogenesis imperfecta. In these cases, the genetic defect is a spontaneous mutation.
The primary pathology in osteogenesis imperfecta is a disturbance in the synthesis of type I collagen, which is the predominant protein of the extracellular matrix of most tissues. In bone, this defect of extracellular matrix causes osteoporosis, which leads to an increase in the tendency to fracture. Besides bone, type I collagen is also a major constituent of dentin, sclerae, ligaments, blood vessels, and skin; therefore, individuals with OI may also have abnormalities of these structures.
The process of collagen molecule formation starts with the synthesis of procollagen. This precursor consists of a long triple-helix protein flanked by 2 propeptides at its 2 terminals. Procollagen is synthesized and then secreted into the extracellular compartment, where the amino- and carboxy-terminal propeptides are cleaved; thus, the functional collagen molecule is formed. These molecules then assemble into an ordered fibril. Mutations that interfere with expression of the collagen gene, formation of the triple helix (amino acid sequencing), or procollagen secretion affect the structure and function of collagen fibrils, resulting in a form of OI.
Electron microscopic studies of OI demonstrate a decrease in the diameter of the collagen fibril, relative to the collagen fibril of healthy persons, and smaller-than-normal apatite crystals.
A number of genetic defects cause the abnormal type I collagen synthesis that leads to OI. OI generally arises from mutations in 1 of 2 genes that encode for the synthesis and/or structure of type I collagen: the COL1A1 gene on chromosome 17, and the COL1A2 gene on chromosome 7. Mutations in these genes may cause abnormal collagen to be produced and may lead to a decrease in the production of normal collagen. The varying degree to which these 2 factors manifest themselves results in the different phenotypic expressions of OI. Milder forms of OI are caused primarily by a decrease in production of normal collagen, whereas more severe forms are caused primarily by the production of abnormal collagen. These abnormalities may be dominantly inherited, or they may be the result of sporadic mutation.
Common causes of nonorthopedic morbidity in type I and type IV OI are joint hypermobility, which causes chronic joint pain, hearing impairment, and brainstem compression.Children with type III OI often require orthopedic care because of their progressive deformities. Standing and walking are often impossible because of spinal compression fractures and scoliosis. Progressive thoracic deformities are associated with recurrent pneumonias that often limit the patient's lifespan.


Type I: The life expectancy of patients with all forms of OI other than type III is often assumed to be shortened. However, according to Paterson et al, the life expectancy of patients with OI type IA is the same as that of the general population. Type IA is a subtype of type I OI in which dentinogenesis imperfecta (tooth abnormalities) does not occur. Type IB is a rare form of type I OI in which dentinogenesis imperfecta does occur. In types IB and IV, mortality is modestly increased in comparison with that of the general population; there is no statistically significant difference in life expectancy. Type II: This form of OI is fatal in the perinatal period.

Type III: Only in type III OI is life expectancy affected. However, patients with type III OI who survive beyond the age of 10 years have a better outlook than other patients with OI.
Osteogenesis imperfecta does not seem to have a predilection for any particular race. No known sex predilection is reported for osteogenesis imperfect. The onset of fractures and deformities varies according to the type of osteogenesis imperfecta (OI) that is present.
For type I, the age of onset is variable. This form most commonly appears during the preschool years when the child is starting to stand. Onset after puberty is uncommon, although fractures may recur in adulthood after menopause or after periods of inactivity, such as after childbirth. Type II occurs in utero. In type III, abnormalities are present at birth (ie, abnormalities develop in utero) in more than 50% of patients. Fractures are frequent during the first 2 years of life.Type IV abnormalities are present at birth in approximately 30% of patients. The onset of this form is during infancy or the preschool years.
The clinical features of osteogenesis imperfecta (OI) depend on the type, but bone fragility with multiple fractures and bony deformities are the common hallmark of all types.
The major presenting signs and symptoms of OI include blue sclerae, hearing loss, tooth abnormalities (dentinogenesis imperfecta), joint laxity, and abnormal skin texture (smooth and thin skin). Other features that are common to multiple OI types include bleeding diathesis (easy bruising) and respiratory distress.
OI is classified into 4 distinct types: I-IV. Some cases of OI do not fit easily into any of the 4 types. 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.




The type 1 prototypical and most common form of OI is associated with the best prognosis. The mode of inheritance is autosomal dominant. The distinguishing clinical features of type I are blue sclerae, which occurs in patients of all ages, and presenile conductive hearing loss; in addition, most patients with type I OI have a family history of hearing loss. Bone fragility is mild, and there are minimal bony deformities. The stature of patients with type I OI is often normal or near normal. Ligamentous hyperlaxity, resulting in joint hypermobility or subluxation, is common. Approximately 20% of patients have kyphoscoliosis.
Dentinogenesis imperfecta is present in some families but not in others.12 Therefore, type I OI is subclassified to distinguish patients without dentinogenesis imperfecta (type IA, more common) from those with dentinogenesis imperfecta (type IB, rare). Some investigators have suggested that these 2 subgroups are biochemically distinct and that individuals with OI type IB, whose bodies make structurally abnormal collagen, are more similar to those with OI type IV than to those with other types of OI, including type IA.
Type II is the most severe form of OI. It is characterized by extreme bone fragility that almost invariably leads to intrauterine or early infant death. The cause of death is most often respiratory failure. The mode of inheritance is autosomal recessive. The sclerae are blue and occasionally dark blue or black. Clinically distinguishing features include intrauterine growth retardation, thin and beaded ribs, crumpled long bones, and limited cranial and/or facial bone ossification. Limbs are short, curved, and angulated.
Type II OI can be further subdivided into types IIA, IIB, and IIC on the basis of the radiographic features of the long bones and ribs. Patients with type IIA or IIC inevitably die in the perinatal period; rarely, patients with type IIB survive into early childhood.
Type III is the next most severe form of OI after type II. It is the most severe form in which survival extends beyond the perinatal period.
Its hallmark feature is severe bone fragility and osteopenia, which is progressively deforming. The mode of inheritance is thought to be autosomal recessive. Multiple fractures and progressive deformity affect the long bones, skull, and spine and are often present at birth. Postnatal growth failure is severe. Kyphoscoliosis is common. Sclerae are either normal from birth, or they progress from pale blue in infancy to a normal appearance by adolescence.
Type III OI is probably the form that is best known to radiologists and orthopedic surgeons. Children with type II OI tend to have severe dwarfism caused by spinal compression fractures, limb deformities, and disruption of growth plates.
Type IV OI is distinguished from type I OI by the slightly increased, though still variable, severity of bone fragility and by the presence of normal sclerae. The mode of inheritance is autosomal dominant. 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; it is reported in 70% of patients.


Hearing loss or a family history of hearing loss is noted in patients with this type of OI, as is dentinogenesis imperfecta. Type IV OI is also subclassified to distinguish patients without dentinogenesis imperfecta (type IVA) from those with it (type IVB). Compared with type I OI, hearing loss is less common in type IV, and dentinogenesis imperfecta (type IVB) is more common. Some authors have distinguished a self-limiting variant of OI, known as temporary brittle-bone disease. Its clinical features are identical with those found in cases of child abuse.
While there is no cure for osteogenesis imperfecta, there are opportunities to improve the child's quality of life. Treatment must be individualized and depends on the severity of the disease and the age of the patient. Care is provided by a team of health-care professionals, including several types of doctors, a physical therapist, a nurse-clinician and a social worker.
In most cases, treatment will be nonsurgical.
Medical bisphosphonates, given to the child either by mouth or intravenously, slow down bone resorption. In children with more-severe osteogenesis imperfecta, bisphosphonate treatment often decreases the number of fractures and bone pain. These medications must be administered by properly trained doctors and require close monitoring.
Casting, bracing, or splinting of fractures is necessary to immobilize the bone so that healing can occur. Movement and weight bearing are encouraged as soon as possible after fractures to increase mobility and decrease the risk of future fractures.
In surgical treatment, repeated fractures of the same bone, deformity, or fractures that do not heal properly are all indications that surgery may be necessary. Metal rods may be inserted in the long bones of the arms and legs. Some rods are a fixed length and must be replaced as the child grows. Other rods are designed like telescopes so they can expand along with the bone growth. However, other complications may occur with telescoping rods.
In many children with osteogenesis imperfecta, the number of times their bones fracture decreases significantly as they mature. However, osteogenesis imperfecta may become active again after menopause in women or after the age of 60 years in men. Scoliosis, or curvature of the spine, is a problem for many children with osteogenesis imperfecta. Bracing is the usual treatment for scoliosis, but it is often ineffective in children with osteogenesis imperfecta. Spinal fusion, in which the vertebrae are realigned and fused together, may be recommended to prevent excessive curvature.
At present there is no cure for OI. Treatment is aimed at increasing overall bone strength to prevent fracture and maintain mobility.
There have been many clinical trials performed with Fosamax (Alendronate), a drug used to treat women experiencing brittleness of bones due to osteoporosis. Higher levels of effectiveness apparently are to be seen in the pill form versus the IV form, but results seem inconclusive.

Bone infections are treated as and when they occur with the appropriate antibiotics and antiseptics.
Physiotherapy used to strengthen muscles and improve motility in a gentle manner, while minimizing the risk of fracture. This often involves hydrotherapy and the use of support cushions to improve posture. Individuals are encouraged to change positions regularly throughout the day in order to balance the muscles which are being used and the bones which are under pressure.
Children often develop a fear of trying new ways of moving due to movement being associated with pain. This can make physiotherapy difficult to administer to young children. With adaptive equipment such as crutches, wheelchairs, splints, grabbing arms, and/or modifications to the home many individuals with OI can obtain a significant degree of autonomy.
Spinal fusion can be performed to correct scoliosis, although the inherent bone fragility makes this operation more complex in OI patients. Surgery for basilar impressions can be carried out if pressure being exerted on the spinal cord and brain stem is causing neurological problems.
Because osteoporosis and multiple fractures are hallmark features of osteogenesis imperfecta (OI), other disorders that cause multiple fractures or decreased bone mineralization may be considered in the differential diagnosis. Such disorders including juvenile osteoporosis, steroid-induced osteoporosis, menkes (kinky-hair) syndrome, hypophosphatasia, battered child syndrome (syndrome X), temporary brittle-bone disease.

References:
http://emedicine.medscape.com/article/947588-overview

http://emedicine.medscape.com/article/411919-overview

http://ghr.nlm.nih.gov/condition=osteogenesisimperfecta

http://orthoinfo.aaos.org/topic.cfm?topic=A00051

http://hwmaint.jmg.bmj.com/cgi/content/abstract/16/2/101

N.B.: This article is excerpted from the book MUSCULOSKELETAL INJURIES for UNDERGRADUATES