osteogenesis imperfecta

definition

· heterogenous group of inheritable diseases characterised by an abnormality of type 1 collagen

· manifested by bone fragility, deformity, and propensity to fracture

epidemiology

· incidence 33 per million

· type 1 - 70%

· type II - 2%

· type III - 20%

· type IV - 5%

pathogenesis

· errors in synthesis of collagen

· bone contains predominantly type III and IV (cf. type I)

· defects in cross-linking

· type I has quantitative defect in collagen

· insufficient synthesis

· type II - IV have qualitative defect in collagen

· abnormal synthesis

· reasons for fractures

· osteopenia

· reduced external volume of bone

· reduced tensile srength of lamellar bone

pathology

· bone usually woven

· little lamellar bone

· thin cortices

· poorly developed Haversian systems

· trabeculae markedly attenuated

· increased numbers of osteocytes

· wide osteoid seams

· collagen fibres in soft tissues have thinner, looser, more random arrangement

clinical features

Skeletal

Long bones

· characteristically fragile

· osteoporotic

· thin cortices and attenuated trabeculae

· narrow diaphyses

· fractures common

· fracture healing rapid with hyperplastic callus

· callus is plastic and deformable

· deformities common

· multiple fractures

· deformation of callus

Pelvis

· trefoil shape

· protrusio acetabulae

Trunk

· short stature

· scoliosis

· vertebral fractures with flat or concave vertebrae

Joints

· ligamentous laxity

· joint dislocation

Head

· defective dentinogeneisis

· soft brownish translucent teeth

· middle ear deafness

· blue sclerae and tympanic mebranes

· misshaped skull

· wide intertemporal diameter

· triangular face

· wormian bones

classification

· after Sillence (1981)

TYPE	GENETIC	CLINICAL FEATURES
I *	AD	MILD, blue sclerae, hearing loss
II	AR	LETHAL, dark blue sclerae
III	AR	SEVERE, normal sclerae, normal hearing
IV *	AD	MOD. SEVERE, normal sclerae, normal hearing

* A - without dentinogenesis impertecta

B - with dentinogenesis imperfecta

Type I

· commonest

· majority develop fractures by age 2

· fractures heal and deformities not severe

· blue sclerae

· teeth may be affected

· fractures less frequent in adolescence

· mild kyphoscoliosis

· only moderate short stature

· may develop impaired hearing

Type II

· severe lethal disorder

· if survive birth have multiple fractures

· crumpled or concertina appearance

· beaded ribs

Type III

· severe deforming type

· classic form

· by age of 6, numerous fractures and severe deformities

· severe kyphoscoliosis

· marked joint laxity

· delayed closure of fontanelles

· enlarged skull and triangular face

· sclerae blue but become normal

· usually do not survive into adulthood

Type IV

· moderately severe

· similar to type I

· sclerae pale blue but become normal

orthopaedic complications

Long bone fracture and deformity

Fractures

· difficult to treat

· bone soft and defomed

· ligamentous laxity

· frequency of fractures

· fractures heal well with abundant callus

· callus is identical to bone (weak)

· deformities or shortening common

· best treated closed

· if treated open, intramedullary fixation best

Deformity

· when deformities progress sufficiently (esp. in lower limb) ambulation difficult

· deformity can be treated with multiple osteotomies, realignment and intramedullary rod fixation

Scoliosis

· difficult to treat

· curves progressive

· bracing ineffective

· internal fixation hampered by poor bone

· early fusion with segmental instrumentation is useful