Osteogenesis imperfecta (OI) is an inherited disorder that affects the connective tissues of the body. The clinical expression of the genetic defect is extremely variable. However, the hallmark of OI is bones that break easily. Other associated signs vary between individuals and even within families. Classically these signs include bone deformities due to recurrent fractures (Figure 1), the whites of eyes may appear blue (Figure 2), dentition may be poor (Figure 3) and hearing loss may occur later in life.
- The underlying cause of OI is genetic. The majority of cases are autosomal dominant, meaning only one parent need have the gene for a child to be affected. Also, both male and female offspring can have OI. Sometimes the defect occurs as a spontaneous new mutation with neither parent being a carrier.
- The gene defect results in the production of abnormal collagen, which is a major structural protein in bone and other connective tissues (such as in ligaments, heart valves and the skin).
- From an orthopaedic point of view, the end result is osteoporosis (reduced bone mass) and bone fragility.
The diagnosis of OI is essentially clinical, relying on the history, clinical examination and certain typical x-ray appearances (Figure 4).
- There is no single lab test that can make the diagnosis.
- Prenatally, ultrasound can be used early to detect severe forms of OI. Chorionic villus sampling can be used to detect abnormal proteins, and amniocentesis can be used to detect abnormal genes. However, these two techniques are not widely available and are not 100% accurate. These two procedures also carry risks to the pregnancy.
- A DEXA scan can confirm osteoporosis in a child. It is useful in adding information to support the clinical diagnosis of OI in mild cases. However, the scan must be interpreted with caution and in context appropriate to the age of the child.
- Other lab tests such as fibroblast culture and genetic testing for OI are not routinely available in South Africa. Even these tests are not 100% reliable and can miss the diagnosis of OI in some instances.
- Other conditions which cause fractures, deformity or short stature (e.g. rickets) are generally easy to diagnose clinically and with blood tests.
- Other causes of osteoporosis in children must be considered. Many of these are extremely rare.
- The biggest challenge is differentiating non-accidental injury (child abuse), which is widespread, from mild forms of OI; this can have major medico-legal implications.
There are many aspects to consider in the overall management of OI.
- Orthopaedic surgery and physiotherapy:
- Treat fractures
- Minimise fractures
- Restore / preserve long bone alignment
- Maximise mobility and function
- Prevent immobility induced bone loss
- A deformed long bone is straightened by making multiple cuts in the bone and realigning the bone on a straight metal rod (Figure 5).
- Medical treatment – Bisphosphonate therapy:
- Leads to a reduction in normal bone resorption; by this means improved bone mass and strength is indirectly achieved
- Improves bone pain, reduces fracture rate
- Used in severe forms of OI.
- Children with mild OI should not participate in contact sports.
- Consideration should be given for the more severely affected child to be educated at a school for the physically disabled.
- Provision for occupational therapy, hearing aids, wheelchairs and walking aids where necessary.
In terms of the Sillence classification there are classically four types of OI, with other less common expressions still being identified:
- Type 1: This form is mild and many patients stop fracturing at the end of growth. The patient may live a relatively normal life. Typically these patients also have blue sclerae and suffer from hearing loss later in life.
- Type 2: This form is severe. The child dies soon after birth as the ribs are so weak they cannot support breathing.
- Type 3: This form is severely deforming and is a common form in South Africa. Scoliosis (spine deformity) and chest wall deformity may also be severe. The sclerae may be blue.
- Type 4: This is an intermediate form in terms of severity. Commonly the child’s dentition is poor.
In terms of the Shapiro classification:
- Crumpled femurs in utero: 94% mortality at birth
- Intra uterine fractures, but normal bone contours: 8% mortality at birth; 59% wheelchair bound; 33% ambulatory
- Fractures before walking age: 33% wheelchair bound; 67% ambulatory
- Fractures after walking age: 100% ambulatory.