Osteogenesis Imperfecta
Collagen Defect and Fragile Bones
Sillence Classification
Critical Must-Knows
- Type I: Mildest. Normal stature. Blue sclerae.
- Type II: Lethal. Perinatal death.
- Type III: Severe deforming. Progressive.
- Type IV: Moderate. Variable sclerae.
- Bisphosphonates: Increase bone density, reduce fractures.
- Rodding: Bailey-Dubow or Fassier-Duval telescoping rods.
Examiner's Pearls
- "Sillence classification
- "Collagen I defect
- "Bisphosphonates reduce fractures
- "Telescoping rods for deformity
Distinguish from NAI
Osteogenesis Imperfecta vs Non-Accidental Injury (NAI)
- Multiple fractures in OI can mimic NAI.
- Look for: Blue sclerae, wormian bones, family history, dentinogenesis imperfecta.
- Metaphyseal corner fractures are NOT typical of OI (they are specific for NAI).
- OI does NOT exclude the possibility of concurrent abuse.
At a Glance Table
| Aspect | Details |
|---|---|
| Definition | Genetic bone fragility disorder due to Type I collagen defect |
| Genetics | COL1A1/COL1A2 mutations, autosomal dominant (most) |
| Classification | Sillence Types I-IV (expanded to VIII+) |
| Key Features | Fractures, blue sclerae, dentinogenesis imperfecta, hearing loss |
| Treatment | Bisphosphonates, telescoping rods, multidisciplinary care |
| Prognosis | Type I near-normal, Type II lethal, Types III/IV variable |
Sillence Classification
| Type | Severity | Sclerae | Stature |
|---|---|---|---|
| Mild | Blue | Normal | |
| Lethal | Blue | N/A (perinatal death) | |
| Severe | Blue/gray | Short | |
| Moderate | Normal/gray | Short |
OI Features
Memory Hook:BBDH - Brittle Bones, Blue sclerae, Dentinogenesis, Hearing loss.
Sillence Types
Memory Hook:I-II-III-IV: Mild, Lethal, Severe, Moderate.
Telescoping Rod Complications
Memory Hook:MFPIR - Migration, Failure, Peri-implant, Infection, Revision.
Overview/Epidemiology
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.
Anatomy and Pathomechanics
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.
Classification Systems
Sillence Classification
- Type I: Mild. Blue sclerae. Normal stature. Fewer than 20 fractures typically. Near-normal life.
- Type II: Lethal. Perinatal death. Severe bone fragility. Multiple intrauterine fractures.
- Type III: Severe deforming. Progressive. Blue/gray sclerae. Short stature. Often wheelchair-bound.
- Type IV: Moderate. Normal or gray sclerae. Variable short stature.
Clinical Assessment
History:
- Fracture history (number, age of first).
- Family history.
- Mobility and function.
Physical Exam:
- Sclerae: Blue (Types I, II, III) or normal/gray (Type IV).
- Teeth: Opalescent, weak (dentinogenesis imperfecta).
- Hearing: May be impaired.
- Stature: Normal (Type I) or short (Types III, IV).
- Limbs: Bowing, deformity.
- Skin: Thin, easy bruising.
Investigations
Genetic Testing:
- COL1A1/COL1A2 mutations: Confirmatory.
Imaging:
- X-rays: Osteopenia, wormian bones (skull), bowing, callus.
- DEXA: Low bone density.
Other:
- Audiometry: Hearing assessment.
- Dental: Dentinogenesis imperfecta.
Management Algorithm
Bisphosphonates
- Pamidronate (IV) or Zoledronate: Most common in children.
- Mechanism: Inhibit osteoclast activity → increase bone density.
- Benefits: Reduce fracture rate, improve vertebral shape, reduce pain.
- Timing: Start early in moderate-severe OI.
Surgical Techniques
Fassier-Duval Rodding
Indications: Progressive bowing, recurrent fractures in femur or tibia.
Technique: Multiple osteotomies (sofield procedure) to correct bowing. Telescoping rod inserted (two components that slide apart as child grows).
Post-op: Protected weight bearing, then full.
Complications: Rod migration, failure to telescope, peri-implant fractures.
Complications
| Complication | Context | Management |
|---|---|---|
| Recurrent Fractures | Disease-related | Bisphosphonates, rodding |
| Rod Migration | Telescoping rods | Revision |
| Peri-Implant Fracture | Weak bone | Careful technique |
| Basilar Invagination | Severe OI | Neurosurgical assessment |
| Hearing Loss | Otosclerosis | Audiology |
Postoperative Care
- Protected Weight Bearing: Then progress.
- Physiotherapy: Maintain strength.
- Continue Bisphosphonates: Per protocol.
Outcomes/Prognosis
- 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.
Evidence Base
- Original OI classification
- Types I-IV defined
- Based on clinical and inheritance pattern
- Pamidronate in OI
- Increased bone density
- Reduced fracture rate
- Fassier-Duval telescoping rod
- Reduces need for revision
- Continues to function as child grows
- Comprehensive OI review
- Expanded classification
- Multidisciplinary care
- Outcomes of telescoping rods in OI
- Reduced fracture frequency
- Migration and revision rates documented
Viva Scenarios
Practice these scenarios to excel in your viva examination
Recurrent Femur Fractures in OI
"5-year-old with Type III OI. Fourth femur fracture in 2 years. Progressive bowing. On bisphosphonates."
This child has **severe OI (Type III)** with recurrent fractures despite bisphosphonates. The progressive bowing indicates the need for **intramedullary rodding**. I would perform **Fassier-Duval telescoping rod** insertion. This involves multiple osteotomies to correct bowing and insertion of a rod that telescopes with growth. I would continue bisphosphonates post-operatively. Counsel family that more fractures and revisions are likely.
OI vs NAI
"Infant presents with multiple fractures. Parents claim OI. How do you differentiate from NAI?"
This is critical to differentiate. I would look for **clinical features of OI**: blue sclerae, family history, wormian bones on skull X-ray, dentinogenesis imperfecta. I would order **genetic testing** for COL1A1/COL1A2. However, I would ALSO complete a **full NAI workup** (skeletal survey, head imaging, fundoscopy) because OI does NOT exclude abuse. Key point: **metaphyseal corner fractures are NOT typical of OI** - they are specific for NAI. Posterior rib fractures are also more suspicious for NAI.
Bisphosphonate Therapy
"How do bisphosphonates work in OI?"
Bisphosphonates (e.g., **pamidronate, zoledronate**) are anti-resorptive agents. They inhibit **osteoclast activity**, reducing bone turnover. In OI, this leads to **increased bone mineral density** and **reduced fracture rate**. They also improve vertebral shape (reduce compression fractures) and reduce pain. They do not correct the collagen defect but improve bone quantity.
MCQ Practice Points
Classification MCQ
Q: Which OI type is lethal? A: Type II.
Genetics MCQ
Q: What collagen type is affected in OI? A: Type I collagen (COL1A1/COL1A2).
Treatment MCQ
Q: What is the mechanism of bisphosphonates? A: Inhibit osteoclasts → reduce bone resorption → increase bone density.
Rodding MCQ
Q: What is the advantage of Fassier-Duval rods? A: Telescoping - they grow with the child.
Sclerae MCQ
Q: Which OI type has normal sclerae? A: Type IV - moderate severity with normal or gray sclerae.
NAI Differentiation MCQ
Q: What fracture pattern is NOT typical of OI and suggests NAI? A: Metaphyseal corner fractures (bucket-handle) - these are specific for NAI.
Australian Context
- Bisphosphonates: PBS-listed for OI.
- Multidisciplinary Clinics: Metabolic bone disease clinics.
- Genetic Testing: Available clinically.
- Support: OI Foundation Australia.
OSTEOGENESIS IMPERFECTA
High-Yield Exam Summary
GENETICS
- •COL1A1/COL1A2
- •Type I collagen
- •Autosomal dominant
- •Wormian bones
CLASSIFICATION
- •I: Mild, blue sclerae
- •II: Lethal
- •III: Severe deforming
- •IV: Moderate
FEATURES
- •Fractures
- •Blue sclerae
- •Dentinogenesis
- •Hearing loss
TREATMENT
- •Bisphosphonates
- •Telescoping rodding
- •Protected mobilization
- •Multidisciplinary
OI vs NAI
- •Blue sclerae suggests OI
- •Wormian bones suggests OI
- •Metaphyseal corners = NAI
- •OI does NOT exclude abuse
RODDING PEARLS
- •Fassier-Duval = telescoping
- •Sofield osteotomies correct bowing
- •Complications: migration, failure
- •Continue bisphosphonates post-op
Differential Diagnosis
| Condition | Distinguishing Features |
|---|---|
| Non-Accidental Injury | Metaphyseal corner fractures, posterior rib fractures, inconsistent history |
| Rickets | Widened physes, cupping, bowing - but normal bone quality |
| Hypophosphatasia | Low alkaline phosphatase, premature tooth loss |
| Osteoporosis (other) | No blue sclerae, no dentinogenesis imperfecta |
| Ehlers-Danlos | Hypermobility predominant, not bone fragility |