High-yield overview
Collagen Defect and Fragile Bones
COL1A1/COL1A2Genetics
I-IV (Sillence)Types
Classic SignBlue Sclerae
Reduce FracturesBisphosphonates
Sillence Classification
Type I
PatternMild. Blue sclerae. Normal stature.
TreatmentMinimal treatment needed
Type II
PatternLethal. Perinatal death.
TreatmentNot survivable
Type III
PatternSevere deforming. Progressive.
TreatmentBisphosphonates, rodding
Type IV
PatternModerate. Normal or gray sclerae.
TreatmentBisphosphonates ± rodding
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.
Clinical 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 |
Mnemonic
OI FOI Features
| B | Brittle Bones Recurrent fractures |
| B | Blue Sclerae Classic Type I/II/III |
| D | Dentinogenesis Imperfecta Opalescent teeth |
| H | Hearing Loss Otosclerosis |
| B | Brittle Bones Recurrent fractures | D | Dentinogenesis Imperfecta Opalescent teeth |
| B | Blue Sclerae Classic Type I/II/III | H | Hearing Loss Otosclerosis |
Hook:BBDH - Brittle Bones, Blue sclerae, Dentinogenesis, Hearing loss.
Mnemonic
IIIIIIIVSillence Types
| I | Mild Blue sclerae, normal stature |
| II | Lethal Perinatal death |
| III | Severe Deforming Progressive, wheelchair |
| IV | Moderate Normal sclerae |
| I | Mild Blue sclerae, normal stature | III | Severe Deforming Progressive, wheelchair |
| II | Lethal Perinatal death | IV | Moderate Normal sclerae |
Hook:I-II-III-IV: Mild, Lethal, Severe, Moderate.
Mnemonic
MFPIRTelescoping Rod Complications
| M | Migration Rod moves proximally or distally |
| F | Failure to Telescope Components do not slide |
| P | Peri-implant Fracture Fracture at rod tip |
| I | Infection Rare but serious |
| R | Revision Needed As child outgrows rod |
| M | Migration Rod moves proximally or distally | I | Infection Rare but serious |
| F | Failure to Telescope Components do not slide | R | Revision Needed As child outgrows rod |
| P | Peri-implant Fracture Fracture at rod tip |
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.
Pathophysiology, 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
Landmark
Sillence, Rimoin & Danks
Key Findings:
- Foundational genetic-clinical classification of OI into four types (I-IV)
- Distinguished phenotypes by severity, sclerae, inheritance and fracture pattern
- Established variable expressivity vs genetic heterogeneity framework
Clinical Implication: The Sillence types remain the clinical backbone for prognosis and counselling, now supplemented by molecular subtyping.
Source: Birth Defects Orig Artic Ser 1979
Level III
Glorieux et al (pamidronate)
Key Findings:
- Uncontrolled observational study, 30 children with severe OI on cyclic IV pamidronate
- Mean BMD rose 41.9% per year; z-score improved from -5.3 to -3.4
- Fracture incidence fell by 1.7 per year (p less than 0.001); healing and growth unaffected
Clinical Implication: Landmark study that established cyclic IV bisphosphonates as standard medical therapy in moderate-to-severe paediatric OI.
Source: N Engl J Med 1998
Review
Rauch & Glorieux (seminar)
Key Findings:
- Authoritative seminar; expanded the four classical types to seven distinct phenotypes
- Most cases due to COL1A1/COL1A2 mutations; some have no detectable collagen mutation
- Bisphosphonates are an adjunct, not a cure; gene therapy remains preclinical
Clinical Implication: Frames OI as a heterogeneous collagen-related disorder managed by combined medical, surgical and rehabilitative care.
Source: Lancet 2004
Level I
Dwan et al (Cochrane review)
Key Findings:
- Systematic review of 14 trials (819 participants), oral and IV bisphosphonates
- Consistently increase lumbar-spine BMD in children and adults
- Fracture reduction not conclusively proven; zoledronate vs pamidronate showed no clear superiority
Clinical Implication: Bisphosphonates reliably raise BMD, but fracture-prevention and optimal regimen/duration remain uncertain - counsel families accordingly.
Source: Cochrane Database Syst Rev 2016
Level IV
Azzam et al (Fassier-Duval rodding)
Key Findings:
- Single-surgeon series: 58 children, 179 lower-limb Fassier-Duval telescopic rods
- Revision required in 53% (mean 52 months), mostly growth-related; nonunion 14.5%
- Bisphosphonates continued perioperatively without postponement; mobility improved
Clinical Implication: Telescopic rodding with realignment osteotomy is effective and less invasive, but families must expect a high lifetime revision rate.
Source: J Pediatr Orthop 2018
Level II
Glorieux et al (BPS804 anti-sclerostin)
Key Findings:
- Randomised phase 2a trial, 14 adults with moderate OI, anti-sclerostin antibody (setrusumab/BPS804)
- Stimulated bone formation markers (P1NP +84%) and reduced resorption (CTX-1 -44%)
- Lumbar-spine aBMD increased 4% (p=0.038); generally well tolerated
Clinical Implication: Anti-sclerostin antibodies are an emerging anabolic approach for OI, now advancing through later-phase trials.
Source: J Bone Miner Res 2017
Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
CLINICAL SCENARIOStandard
Recurrent Femur Fractures in OI
CLINICAL PROMPT
"5-year-old with Type III OI. Fourth femur fracture in 2 years. Progressive bowing. On bisphosphonates."
PRACTICAL APPROACH
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.
KEY CLINICAL POINTS
Telescoping rods for recurrent fractures
Correct bowing with osteotomies
Continue bisphosphonates
COMMON PITFALLS
Using non-telescoping rods
Stopping bisphosphonates
FURTHER QUESTIONS
"What is the Sillence classification?"
CLINICAL SCENARIOStandard
OI vs NAI
CLINICAL PROMPT
"Infant presents with multiple fractures. Parents claim OI. How do you differentiate from NAI?"
PRACTICAL APPROACH
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.
KEY CLINICAL POINTS
Look for OI features
Genetic testing
OI does not exclude NAI
Metaphyseal corners are NAI
COMMON PITFALLS
Not investigating for NAI
Assuming OI excludes abuse
FURTHER QUESTIONS
"What fractures are specific for NAI?"
CLINICAL SCENARIOStandard
Bisphosphonate Therapy
CLINICAL PROMPT
"How do bisphosphonates work in OI?"
PRACTICAL APPROACH
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.
KEY CLINICAL POINTS
Inhibit osteoclasts
Increase bone density
Reduce fractures
COMMON PITFALLS
Thinking they fix the collagen
FURTHER QUESTIONS
"What are the side effects?"
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.
Guidelines, Registries & Global Practice
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
| Domain | Consensus / Society Position |
|---|---|
| Medical therapy | Cyclic IV bisphosphonates (pamidronate or zoledronate) are standard for moderate-severe OI across North American, European and international metabolic bone units; oral agents used in milder disease |
| Surgical platform | Telescopic intramedullary rodding (Fassier-Duval, Sheffield/Bailey-Dubow) with Sofield-type osteotomies is the global standard for deforming long-bone disease |
| Care model | International OI consensus and specialist centres advocate multidisciplinary clinics (orthopaedics, endocrinology/metabolic bone, genetics, rehab, dental, audiology) |
| Emerging therapy | Anti-sclerostin antibodies (e.g. setrusumab) and combination anabolic strategies are in later-phase trials, not yet routine |
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.
OSTEOGENESIS IMPERFECTA
Clinical 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 |
Controversies & Areas of Uncertainty
- 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.