(see also: Pseudoachondroplasia)
- autosomal dominant disorder;
- arises due to a point mutation for the gene encoding a fibroblast growth factor (Fibroblast growth factor receptor 3)
- over 80% of all persons with this disorder are born to parents who are not achondroplastic, & such parents rarely have second achondroplastic child;
- it is presumed in these instances that disorder is consequence of mutation and may be related to a mutation in a fibroblast growth factor gene;
- growth of endochondral bones is abnormal in achondroplasia and causes disproportionate and rhizomelic shortening of limbs;
- since intramembranous ossification is unaffected head is relatively large and the forehead is prominent;
- long bones are wide with flaring of the metaphyses;
- characteristics in infancy:
- rhizomelic short limbs and normal trunk height,
- prominent frontal bossing & depressed nasal bridge;
- midface hypoplasia, and relative mandibular prominence;
- maxillary hypoplasia may lead to dental crowding and malocclusion;
- recurrent otitis media will lead to higher incidence of conductive hearing loss in adults;
Development & Motor Milestones
- patients often have a delay in reaching milestones;
- many children do not begin to walk independently until 2-3 years;
- hypotonic ligamentous laxity, relatively large head, & disproportionate trunk and limbs may contribute to problem;
- obesity is common;
- intelligence is normal;
- early speech may be difficult because of tongue thrust, but this usually resolves by school age;
- careful preoperative assessment and postoperative monitoring of pulmonary function are necessary;
- significant respiratory problems develop in 10% of affected individuals because of an abnormal thoracic cage configuration, midfacial hypoplasia, upper airway obstruction, or spinal cord compression at the foramen magnum;
- 3% of affected individuals have hydrocephalus; detection is difficult because head size may run above 97th percentile;
- elbow extension is often diminished & posterolateral radial head dislocation may occur;
- hands are short and broad;
- trident hands (increased space between third and fourth fingers);
- fingers are three-pronged with a space between the third and fourth digits, producing characteristic trident hand, a feature that is lost late in childhood;
- pelvis is broad and short, w/ relatively wide iliac wings & horizontal acetabular margins.
- flexion contractures of the hip, and genu varum;
- genu varum and varus ankle deformities are found;
- unlike dysplasias that involve the epiphysis, DJD is rare, and prophylactic surgical correction of deformities is probably not indicated;
- bracing is not beneficial because orthosis often opens the joint medially, secondary to ligamentous laxity;
- fibular epiphysiodesis and/or tibial osteotomies may be required for severe genum varum;
- other problems include dental malocclusion, communicating hydrocephalus, arthritis, and resistant rickets.
- The effect of treatment on growth and deformity in hypophosphatemic vitamin D-resistant rickets.
- Orthopaedic aspects of achondroplasia.
- Mutations in the transmembrane domain of FGFR3 cause the most common genetic form of dwarfism, achondroplasia.
- Disorders of the Pediatric and Adolescent Spine.
- Spinal Manifestations of Skeletal Dysplasias.