Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) is a genetic bone fragility disorder.

• Genetics: Most are autosomal dominant. COL1A1 or COL1A2 mutations (Type I collagen).
• Incidence: 1 in 10,000-20,000.
• Pathophysiology: Defective Type I collagen leads to weak bone matrix.

Collagen Abnormalities

• Type I collagen is the main organic component of bone.
• Defective collagen leads to poor bone quality despite normal mineral.

Why Fractures Occur

• Bones are structurally weak.
• Minimal trauma causes fractures.
• Bones may bow and deform due to repeated microfractures.

The topic states the defect is in COL1A1/COL1A2 and that "defective collagen leads to poor bone quality" without explaining the genotype-phenotype link that examiners probe - why a Type I patient is mild and a Type II/III patient is devastated.

Normal type I collagen is a triple helix of two pro-alpha-1 (COL1A1) and one pro-alpha-2 (COL1A2) chains, with a glycine at every third residue (Gly-X-Y) that lets the helix pack tightly.

• Quantitative defect (→ mild, Type I). A null/loss-of-function COL1A1 allele produces about half the normal amount of structurally NORMAL collagen (haploinsufficiency). Less-but-normal collagen gives the mildest phenotype - the classic Type I.
• Qualitative / structural defect (→ severe, Types II-IV). A glycine substitution (or other structural mutation) produces an abnormal chain that is still incorporated into the triple helix, where it disrupts folding of the whole molecule - a dominant-negative ("protein suicide") effect. Abnormal collagen is worse than too little collagen, which is why structural mutations cause the lethal (II) and severely deforming (III) forms. Substitutions nearer the carboxy-terminus tend to be more severe.
• Recessive OI. A minority are autosomal recessive, caused by defects in genes handling collagen post-translational modification and folding - CRTAP, P3H1 (LEPRE1) and PPIB (the prolyl-3- hydroxylation complex) and chaperones - producing severe phenotypes without a primary collagen-gene mutation; these are over-represented in consanguineous populations.

This is why the field is moving from the purely clinical Sillence types toward a gene-based classification. (The biochemistry of collagen synthesis itself is developed in our Proteoglycans & Collagen topic.)

View larger

This topic describes rodding, osteotomies and scoliosis surgery without addressing the OI-specific perioperative and anaesthetic risks that make any operation hazardous - high-yield because these patients return to theatre repeatedly.

• Airway & cervical spine. Fragile, brittle teeth (dentinogenesis imperfecta) are easily damaged at laryngoscopy; a relatively large head and short neck, cervical-spine fragility and possible basilar invagination mean the neck must be handled gently (avoid forceful extension/manipulation) - anticipate a potentially difficult airway.
• Positioning is dangerous. An anaesthetised OI patient can fracture from handling alone - careful padding and gentle positioning, avoid a standard automated blood-pressure cuff (repeated inflation can fracture the humerus - use the lowest cuff/least frequent cycling or an arterial line), avoid forceful limb manipulation and tourniquets where possible, and consider that suxamethonium fasciculations may provoke fractures/hyperthermia.
• Intra-operative hyperthermia. OI carries a recognised tendency to intra-operative hyperthermia and a hypermetabolic response (a metabolic phenomenon distinct from classic malignant hyperthermia, though the two can be confused) - monitor temperature, avoid over-warming, and treat hyperthermia actively.
• Bleeding. A mild platelet dysfunction and capillary/vessel fragility give an increased bleeding tendency - anticipate blood loss in osteotomy/spinal surgery.
• Regional anaesthesia & continuing therapy. Spinal deformity, short stature and fragile bone make neuraxial/regional technique harder (not absolutely contraindicated); continue bisphosphonate therapy perioperatively (it need not be postponed around rodding - see the Azzam evidence), and plan within the multidisciplinary team. (General anaesthetic and regional technique are covered in our General Anaesthesia in Orthopaedics and Regional Anaesthesia topics; the craniocervical problem in our Basilar Invagination topic.)

• Protected Weight Bearing: Then progress.
• Physiotherapy: Maintain strength.
• Continue Bisphosphonates: Per protocol.

• Type I: Near-normal lifespan and function.
• Type II: Lethal.
• Type III: Wheelchair-dependent. Significant disability.
• Type IV: Variable. Many ambulatory.
• Bisphosphonates + Surgery: Improved outcomes in modern era.

Global epidemiology

• Birth prevalence approximately 1 in 15,000-20,000; Type I (mild) is the most common form worldwide.
• Autosomal dominant COL1A1/COL1A2 mutations account for the majority; recessive (e.g. CRTAP, P3H1/LEPRE1) and other rare forms are over-represented in consanguineous populations.

Side-by-side practice across major settings

Registry & cohort evidence

• No dedicated implant registry equivalent to arthroplasty registries exists for OI; the evidence base is single-centre series (e.g. Shriners network) and natural-history cohorts.
• Key practical signal from cohorts: high lifetime revision burden for telescopic rods (around half of patients), driven mainly by growth.

High- vs limited-resource practice variation

• High-resource settings: early genetic confirmation, cyclic IV bisphosphonates from infancy, telescopic rodding, DXA surveillance, allied-health teams.
• Limited-resource settings: diagnosis is largely clinical/radiographic; bisphosphonate access and telescopic implants may be constrained, with greater reliance on bracing, non-telescopic rods (Rush rods/K-wires) and conservative fracture care.

• Do bisphosphonates actually reduce fractures? They reliably raise BMD, but the Cochrane review found fracture reduction is not conclusively proven and clinical-function benefits are inconsistent. BMD gain is a surrogate, not a guaranteed clinical endpoint.
• Optimal agent, dose and duration. Pamidronate vs zoledronate, IV vs oral, and how long to continue (risk of over-suppression, atypical fractures, delayed osteotomy healing) remain unresolved.
• Bisphosphonates and surgery. Whether to pause therapy around osteotomy/rodding is debated; some series (Azzam et al) continued therapy without postponement and reported acceptable union rates.
• Telescopic vs non-telescopic rods. Telescopic rods reduce re-operation for growth but carry their own failure modes (failure to telescope, migration); high overall revision rates persist regardless of implant.
• Emerging anabolics. Anti-sclerostin antibodies and other Wnt-pathway/anabolic agents show biochemical and BMD benefit in early trials, but fracture-reduction efficacy and paediatric safety are not yet established.
• OI and child protection. A genetic OI diagnosis does not exclude concurrent non-accidental injury; the threshold for safeguarding work-up remains a clinical judgement.