# Achondroplasia > Achondroplasia for NEET-PG Orthopaedics: FGFR3 gain-of-function, rhizomelic short stature, trident hand, foramen magnum and spinal stenosis complications. - Canonical URL: https://prepelephant.com/topics/neet-pg/orthopaedics/achondroplasia - Exam / course: NEET-PG · Subject: Orthopaedics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-14 - Last updated: 2026-10-14 - Reference basis: Maheshwari's Essential Orthopaedics; Apley's System of Orthopaedics and Fractures - How to cite: "Achondroplasia", PrepElephant, https://prepelephant.com/topics/neet-pg/orthopaedics/achondroplasia ## Direct answer Achondroplasia is the commonest skeletal dysplasia, caused by a gain-of-function mutation in the FGFR3 gene that impairs endochondral ossification of the long bones. It produces rhizomelic short stature, a trident hand, frontal bossing and a narrow spinal canal. Most cases arise from new mutations, and the commonest variant is G380R. ## What you must remember - **Gene:** FGFR3 gain-of-function overactivates the receptor and slows chondrocyte proliferation in the growth plate. - **Inheritance:** autosomal dominant; most cases are sporadic, and advancing paternal age is a recognised factor. - **Rhizomelia:** shortening of the proximal limb segments, humerus and femur, more than the forearm and leg, is the key clinical sign. - **Hands:** a trident hand, with wide separation of the middle and ring fingers, and short, splayed digits. - **Spine:** short pedicles and a narrow lumbar canal predispose to stenosis, and foramen magnum narrowing threatens the cervicomedullary junction. - **Homozygous form:** homozygous achondroplasia is usually fatal in the perinatal period because of severe thoracic compromise. - **Growth and mind:** height is well below average on achondroplasia-specific charts, while intelligence is normal. - **Surgery:** limb lengthening is an option in selected patients, but spinal and foramen magnum problems take priority in childhood. ## From a newborn with short limbs to a surveillance plan A newborn has short proximal limbs, a large head and a trident hand on examination. The sequence moves from recognising the pattern to confirming it and then protecting the spinal canal over the years. 1. **Recognise the pattern:** rhizomelic shortening, frontal bossing and a relatively normal trunk form the typical clinical picture at birth. 2. **Confirm radiographically:** a skeletal survey shows short tubular bones with metaphyseal flaring, a champagne-glass pelvis and a narrow interpedicular distance that narrows caudally. 3. **Confirm genetically:** FGFR3 testing confirms the diagnosis where the picture is unclear, and most affected children carry the G380R change. 4. **Screen the foramen magnum and spine:** serial imaging and a sleep assessment look for foramen magnum stenosis, apnoea, hydrocephalus and lumbar stenosis that may need decompression. 5. **Manage the deformity:** genu varum, thoracolumbar kyphosis in infancy, which usually improves, and limb lengthening in selected cases are addressed in turn. 6. **Monitor throughout:** growth, neurodevelopment, weight and ear health are reviewed regularly, using achondroplasia-specific growth charts. ## Where the exam tries to catch you Examiners test whether you can separate achondroplasia from its look-alikes and recognise its serious complications early. - **Trap:** assuming the spinal problems wait until adulthood. **Correct:** foramen magnum stenosis and cervicomedullary compression matter in infancy, so early neurological and imaging screening is needed. - **Trap:** mixing achondroplasia with osteogenesis imperfecta because both cause short stature. **Correct:** achondroplasia shows rhizomelic shortening and a trident hand without fracture tendency, whereas OI shows fractures and often blue sclerae. - **Trap:** believing a normal family history excludes the diagnosis. **Correct:** most cases are new mutations, so a normal family history does not rule achondroplasia out. ## Frequently asked questions ### What causes achondroplasia at the molecular level? A gain-of-function mutation in FGFR3, most often G380R, overactivates the receptor and suppresses growth-plate chondrocyte proliferation. The result is short long bones with a relatively normal trunk. ### Why is the trident hand a feature of achondroplasia? The trident hand reflects wide separation of the middle and ring fingers, giving a bedside clue to the diagnosis. Similar hand patterns occur in some other skeletal dysplasias, so radiographic and genetic review is still needed. ### What spinal complication is most important in achondroplasia? Foramen magnum stenosis and lumbar spinal stenosis are the major spinal complications of achondroplasia. Foramen magnum narrowing can cause cervicomedullary compression and apnoea, while lumbar stenosis causes neurogenic claudication later in life. ### Is achondroplasia inherited or does it arise as a new mutation? Most cases of achondroplasia arise from new mutations, while the condition follows an autosomal dominant pattern when inherited. An affected parent passes the mutation on in half of pregnancies. ### Why is homozygous achondroplasia usually fatal? Homozygous achondroplasia is typically fatal in the perinatal period because of severe thoracic compromise and respiratory failure. Heterozygous individuals, who have the usual form of the condition, survive with normal intelligence.