Achondroplasia

· most common form of short-limb dwarfism

epidemiology

· 1 in 25 000

· transmitted as autosomal dominant

· single gene responsible

· 80% due to mutation

· because few achondroplastics have children

· more common with paternal age > 36 yrs

· homozygous form fatal

· occurs in 25% of children if both parents achondroplastic

pathology

· defect in enchondral bone formation

· periosteal and membranous bone formation normal

· defect is alteration in normal chondrocyte maturation, hypertrophy and degeneration

· proliferative zone - abnormal clustering of chondrocytes

· hypertrophic zone - narrow and irregular with cells of differing sizes

clinical

Limbs

· disproportionately short

· proximal segment particularly affected (rhizomelic)

Upper limb

· shoulders appear broad

· normal clavicular development

· loss of full extension of elbow

· due to posterolateral dislocation of radial head

· fingers short and stubby with trident appearance

· persistent space between middle and ring fingers

· fingertips only reach to hips

· difficulty with hygiene and dressing

Lower limb

· hip flexion contractures

· external rotation of limb

· genu recurvatum prior to walking age

· develop genu varum after walking

· due to tibial bowing

· waddling gait

· circumduction of legs while running

Facies

· enlarged head with frontal bossing

· nasal bridge depressed

· hypoplasia of midface

Trunk

· normal in length

· prior to walking

· thoracic lordosis

· thoracolumbar kyphosis

· after walking, develop

· lumbar lordosis

· forward pelvic tilt

· hip flexion contractures

· gives rise to

· sway back

· prominent abdomen and buttocks

Development

· speech normal

· motor milestones delayed 3-6 mths

· sitting height normal

· standing height below 3rd percentile

radiology

Skull

· enlarged calvarium

· midface hypoplasia

· frontal prominence

· foramen magnum small (best seen on CT)

Lumbar spine

· progressive narrowing of intepedicular distance

· short pedicles

· posterior scalloping of vertebral bodies

· thoracolumbar kyphosis prior to walking

· lumbar lordosis after walking

Pelvis

· square iliac wings (elephant ears)

· short narrow sciatic notch

· horizontal acetabular roof

· champagne-glass pelvic cavity

· horizontal sacrum

Long bones

· short and thick

· metaphyseal flaring

· overgrown trochanter

· inverted-V shaped distal femoral physis

· tibial bowing

· fibula longer than tibia

· marked humeral shortening

· radial head dislocation

· broad short proximal and middle phalanges of hand

Medical complications

General

· obesity

Midface hypoplasia

· dental crowding and malocclusion

· recurrent otitis media

· speech problems

Short base of skull

· cervicomedullary compression

Enlarged cranium

· hydrocephalus

· extension injuries of neck

Orthopaedic complications

Spinal stenosis

Pathology

· due to

· short thick pedicles

· interpedicular narrowing

· thickened laminae and inferior facets

· precipitated by

· disc herniation

· spondylolysis

· lumbar lordosis

· anterior vertebral body wedging

Clinical

· low back pain

· usually in 20’s

· radiculopathy

· neurogenic claudication

· cauda equina syndrome

Treatment

· weight reduction

· decompression laminectomy

Thoracolumbar kyphosis

Pathology

· may be due to

· enlarged head

· hypotonia

· ligamentous laxity

· hip contractures

· may be anterior wedging of apical vertebra (T12 or L1)

Clinical

· universal prior to walking age

· resolves in 90% as ambulation begins

· in those where it persists

· low back pain

· radiculopathy

· claudication

· paraparesis

Treatment

· observation until walking commences

· if persists, thoracolumbar brace

· consider surgery if

· > age 5

· kyphosis > 40^o

· wedging of apical vertebra

· surgery in form of

· anterior decompression and strut grafting

· posterior fusion (?noninstrumented)

Genu varum

Pathology

· may be due to

· relative overgrowth of fibula

· obesity

· ligamentous laxity

Clinical

· lower limb pain and cosmetic deformity

· do not often develop osteoarthritis of knee

Treatment

· bracing not effective

· surgery in form of

· proximal tibial osteotomy

· proximal and distal fibular epiphyseodesis

Short stature

· limb lengthening performed more often

· 4 segment procedures

· increase of up to 64%

· significant complications