Osteogenesis Imperfecta
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Direct answer
Osteogenesis imperfecta is a heritable disorder of type I collagen that makes bones fragile and prone to fracture. Most cases are caused by mutations in COL1A1 or COL1A2 and follow an autosomal dominant pattern, although recessive forms exist. The Sillence classification into types I to IV guides prognosis, and blue sclerae and dentinogenesis imperfecta are its classic extraskeletal features.
What you must remember
- Defect: mutations in COL1A1 or COL1A2 reduce or distort type I collagen, the main organic matrix of bone, dentine and sclera.
- Sillence type I: mild, autosomal dominant, with blue sclerae and normal stature; the fracture rate falls after puberty.
- Sillence type II: perinatal lethal, with multiple fractures and rib fractures at birth.
- Sillence type III: progressive deformity and short stature; the most severe survivable form.
- Sillence type IV: moderately severe, with fractures and deformity less marked than in type III and a usually normal sclera.
- Dentinogenesis imperfecta: opalescent, brittle teeth that wear rapidly, seen in a substantial proportion of affected children.
- Treatment: intravenous bisphosphonates reduce fracture frequency and bone pain, while telescoping intramedullary rods correct deformity in long bones.
- Hearing: conductive hearing loss from stapes fixation can appear in adolescence and adulthood, especially in type I.
From a repeated fracture to a rodded limb
A 6-year-old boy has a femoral fracture after a minor fall, with blue sclerae and a mother who had several fractures in childhood. The reasoning moves from recognition to protection, and only then to correction.
- Recognise the pattern: fractures after trivial trauma, blue sclerae, wormian skull bones and a family history point towards osteogenesis imperfecta.
- Exclude abuse: a child with repeated fractures must be assessed for non-accidental injury, because the two conditions can mimic each other and both must be considered.
- Confirm and classify: collagen or genetic testing, where available, is combined with the clinical features to assign the Sillence type and guide prognosis.
- Start bisphosphonates: intravenous pamidronate or zoledronate reduces fracture frequency and bone pain, with monitoring for hypocalcaemia and an acute reaction after infusions.
- Rod the deforming long bones: telescoping intramedullary rods are placed in femora and tibiae with recurrent fractures or progressive bowing, and they lengthen as the child grows.
- Protect and rehabilitate: physiotherapy, safe mobility aids and a review of hearing, teeth and vision support function alongside the medical and surgical steps.
Where the exam tries to catch you
Examiners test whether you can separate OI from abuse, and understand what each treatment achieves.
- Trap: labelling every fracture in a child as non-accidental injury. Correct: OI must be considered first, since blue sclerae, wormian bones and a family history point away from abuse, but abuse must still be excluded when suspicion arises.
- Trap: expecting bisphosphonates to heal the bone fully. Correct: they reduce fracture frequency and pain but do not correct the collagen defect, so rodding is used for the deformity.
- Trap: defining type I by blue sclerae alone. Correct: blue sclerae occur in type I and in some type III, so the classification uses fracture pattern, stature and sclera together.
Frequently asked questions
What is the Sillence classification of osteogenesis imperfecta?
The Sillence classification divides osteogenesis imperfecta into types I to IV according to severity and clinical features. Type I is mild with blue sclerae, type II is perinatal lethal, type III is severe and deforming, and type IV is moderately severe.
Why are blue sclerae important in osteogenesis imperfecta?
Blue sclerae arise because the thin, collagen-deficient sclera lets the underlying choroid show through. The sign is most prominent in type I but is not diagnostic alone, since it can also appear in other types.
Which drug reduces fracture frequency in osteogenesis imperfecta?
Intravenous bisphosphonates such as pamidronate or zoledronate are standard for reducing fracture frequency and bone pain in moderate to severe disease. Patients are monitored for hypocalcaemia and vitamin D status during treatment.
When is intramedullary rodding used in osteogenesis imperfecta?
Telescoping intramedullary rods are placed in long bones with repeated fractures or progressive bowing, and they lengthen with the growing bone. Rodding improves mobility and reduces deformity, and it is often combined with bisphosphonate therapy.
Why must non-accidental injury be considered in children with fractures?
Repeated fractures can arise from abuse, and osteogenesis imperfecta can mimic it, so both must be considered in every case. A careful history, skeletal survey, and examination of the sclerae, teeth and family history help distinguish them.