Osteogenesis imperfect ms ortho 15 marks

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Osteogenesis Imperfecta (OI) - MS Ortho 15 Marks

Definition

Osteogenesis imperfecta (OI), also called "brittle bone disease", is the most common inherited disorder of connective tissue. It is a phenotypically diverse group of disorders caused by mutations that impair the synthesis or structure of type I collagen, principally affecting bone and all other tissues rich in type I collagen (joints, eyes, ears, skin, teeth).

Etiopathogenesis / Molecular Basis

Genetic defect: Mutations in COL1A1 and COL1A2 genes encoding the alpha-1 and alpha-2 chains of type I procollagen.

Type I collagen is a heterotrimer: two alpha-1(I) chains + one alpha-2(I) chain
Assembly into a triple helix requires glycine at every third position (-Gly-X-Y- repeat)
A missense mutation substituting one glycine residue disrupts triple-helix formation, preventing proper post-translational modification → dominant negative effect (severe phenotype)
Loss-of-function mutations eliminating one allele produce less collagen but the remaining collagen is normal → haploinsufficiency (milder phenotype)

Histologic findings (Miller's Review of Orthopaedics):

Increased diameters of Haversian canals and osteocyte lacunae
Increased cell numbers
Replicated cement lines
Result: thin cortices seen on radiographs

Net effect: Synthesis of too little bone (Robbins), resulting in extreme skeletal fragility.

Inheritance:

Most forms: Autosomal dominant (COL1A1/COL1A2 mutations - heterozygous)
Some recessive forms: mutations in collagen-processing enzymes (CRTAP, LEPRE1, PPIB)

Classification - Sillence Classification

Originally Sillence (1979) described 4 types; now at least 8-9 types are recognized. Best considered a clinical continuum.

Type	Inheritance	Sclerae	Key Features
I (IA, IB)	AD	Blue	Mildest form; presents at preschool age (tarda); hearing loss in 50%; IA = no dentinogenesis imperfecta (DI); IB = DI present; normal/near-normal stature
II	AD (new mutation) / AR	Blue	Lethal in utero or perinatally; multiple fractures at birth; "beaded ribs" (accordion/concertina femur); absent calvaria; pulmonary hypertension
III	AR (rare AD)	Blue (lighten with age)	Fractures at birth; most severe survivable form; progressively short stature; long bone deformities; DI and hearing loss common
IV (IVA, IVB)	AD	Normal (gray)	Moderate severity; bowing of long bones, vertebral fractures; IVA = no DI; IVB = DI; hearing loss variable; normal sclerae
V	AD	Normal	Similar to IV; hypertrophic callus after fracture; ossification of interosseous membrane between radius-ulna and tibia-fibula; no DI
VI	?	Normal	Moderate; excess osteoid in bone; no DI; no Wormian bones
VII	AR	Blue	Similar to II/III; rhizomelic limb shortening + coxa vara; mutation in CRTAP
VIII	AR	-	Similar to II/III; mutation in LEPRE1
IX	AR	-	Similar to II/III; mutation in PPIB

Sources: Goldman-Cecil Medicine; Campbell's Operative Orthopaedics 15th Ed; Miller's Review of Orthopaedics 9th Ed

Clinical Features (Tetrad of OI)

1. Bone Fragility (Primary Feature)

Fractures from trivial trauma; may occur in utero (Type II, III)
Fractures occur less frequently with advancing age (usually cease at puberty)
Healing is normal, but bone does NOT remodel normally
Olecranon apophyseal avulsion fractures are characteristically seen

2. Blue Sclerae

Seen in Types I and II (and early Type III)
Caused by decreased collagen in sclerae → underlying choroid becomes visible through translucent sclerae, imparting blue color (Robbins)

3. Dentinogenesis Imperfecta (DI)

Small, misshapen, blue-yellow opalescent teeth that wear easily
Present in types IB, IVB (absent in IA, IVA)

4. Hearing Loss

Sensorineural deficit AND conductive loss (abnormalities of middle ear ossicles)
~50% in Type I

Other features:

Short stature (most types)
Ligamentous laxity
Scoliosis - common, progressive; can be severe
Wormian bones (multiple sutural bones in skull)
Basilar invagination - common in severe types (Types III, IV)
Compression fractures of vertebrae - "codfish vertebrae"
Bowing of long bones - anterolateral bow / proximal varus deformity of femur; anterior/anteromedial bow of tibia
Bowing results from multiple transverse fractures + muscle contraction across weakened diaphysis (Miller's Review)

Radiological Features

Osteopenia throughout skeleton
Multiple fractures at various stages of healing
Wormian bones (mosaic pattern in skull sutures)
Codfish vertebrae (biconcave compression deformities)
Gracile (pencil-thin) diaphyses with thin cortices
Accordion/concertina femora in Type II (multiple in utero fractures)
Beaded ribs (Type II)
Bowing deformities of long bones
Type V: calcification of interosseous membrane (radius-ulna); hypertrophic callus
Normal bone scan (fractures heal)
DXA: reduced bone mineral density

Differential Diagnosis

Non-accidental injury (child abuse) - critical distinction in infants with multiple fractures
Rickets
Osteoporosis (in adults)
Hypophosphatasia
Idiopathic juvenile osteoporosis
Osteopetrosis

Key distinction from child abuse: In OI, Wormian bones, blue sclerae, DI, and family history help differentiate; genetic testing confirms.

Treatment

A. Non-Operative Management

1. Bisphosphonates (Mainstay of medical treatment)

Pamidronate (IV) - most widely used in children
Zoledronic acid, alendronate, risedronate
Mechanism: inhibit osteoclastic bone resorption → increase bone density → reduce fracture incidence
Miller's Review: "Bisphosphonates reduce the incidence of fractures"
Given in cycles; radiographs show dense "zebra lines" in metaphyses
Complications: atypical subtrochanteric fractures with long-term use; osteonecrosis of jaw (rare)

2. Physical therapy and rehabilitation

Hydrotherapy (pool-based PT) - minimizes fracture risk while building muscle strength
Bracing of extremities to prevent deformity and minimize fractures

3. Growth hormone - recombinant GH increases bone density and growth velocity in some types

4. Genetic counseling

B. Operative Management

Indications for surgery:

Progressive long bone deformity
Recurrent fractures at same site
Inability to ambulate due to deformity
Spinal deformity (scoliosis >40-50°)

Sofield-Millar Procedure (Multiple Osteotomies + Intramedullary Rodding)

Classic operation for OI - remains the cornerstone of surgical treatment

Principle: Multiple osteotomies to straighten bowed long bone, followed by intramedullary fixation to maintain alignment and provide internal splinting

Types of IM rods:

Rod	Description	Advantage	Disadvantage
Rush/Williams rod (non-elongating)	Stainless steel, fixed length	Simpler insertion	Must be exchanged as child grows
Bailey-Dubow rod (telescopic)	Older telescoping nail	Elongates with growth	High complication rate (~50%), particularly T-piece pullout
Fassier-Duval (FD) nail (telescopic)	Modern, screw-in design - threaded into epiphysis and anchors in proximal/distal apophyses	Self-expanding; single incision insertion; can be inserted percutaneously	Proximal migration if not positioned correctly

Fassier-Duval nail is currently preferred for femur and tibia in children - single surgical portal, no need for separate epiphyseal incision, better containment within medullary canal.

Femoral rodding: Most common; indicated for lower extremity deformity preventing ambulation

Tibial rodding: For anterolateral tibial bowing + recurrent fractures

Humeral rodding: Indicated for severe bowing/recurrent fractures; functional outcomes good

Spinal Surgery

Scoliosis management:

Bracing is generally ineffective in OI (soft rib cage doesn't provide adequate purchase)
Surgical indications: curves >40-50°, progressive deformity
Pedicle screw instrumentation + fusion (cement augmentation used for poor bone quality)
Challenges: osteoporotic bone, poor screw purchase, risk of neurological injury

Basilar invagination:

Can cause brainstem compression → cranial nerve deficits, long tract signs, sleep apnea
Treatment: posterior fossa decompression ± occipito-cervical fusion

Fracture Management

Most fractures managed conservatively (lightweight splints, casts)
Avoid prolonged immobilization (worsens osteoporosis)
Lightweight fiberglass casts preferred
Operative fixation with IM rods for displaced fractures or those failing conservative treatment
Protect uninvolved limbs during casting (fragile bones)

Anesthetic Considerations

Risk of hyperthermia (OI is in differential for malignant hyperthermia - though not same mechanism)
Fragile bones: extreme care during positioning, intubation
Teeth and jaw fragility: careful laryngoscopy
Risk of cervical spine instability in severe forms

Prognosis

Type I: Near-normal lifespan; ambulatory; fracture frequency decreases post-puberty
Type II: Lethal (stillbirth or neonatal death from respiratory failure)
Type III: Severe disability; may require wheelchair; life expectancy reduced
Type IV: Variable; most ambulatory with treatment
Types V-IX: variable

Key Exam Points (High-Yield)

Gene defect: COL1A1 / COL1A2 (chromosome 17 and 7 respectively)
Most severe survivable type: Type III
Lethal type: Type II
Blue sclerae: Types I and II (choroid visible through thin sclera)
Hypertrophic callus + IOM ossification: Type V (pathognomonic)
Rhizomelia + coxa vara: Type VII (CRTAP mutation)
Sofield-Millar procedure = multiple osteotomies + IM rodding
Best current IM nail: Fassier-Duval (telescopic, self-elongating)
Medical Rx: Bisphosphonates (pamidronate) - reduce fracture incidence
Hearing loss in 50% of Type I cases - mixed conductive + sensorineural
Wormian bones on skull X-ray
Codfish vertebrae on spinal X-ray
DI absent in IA and IVA; present in IB and IVB
Fractures decrease after puberty due to sex hormones increasing bone density
Distinguish from child abuse: OI has blue sclerae, family history, Wormian bones, DI

Sources: Campbell's Operative Orthopaedics 15th Ed 2026; Miller's Review of Orthopaedics 9th Ed; Goldman-Cecil Medicine (International Edition); Robbins & Kumar Basic Pathology

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