Orthopaedic Surgery - Achondroplasia

Basics

Achondroplasia is the most common skeletal dysplasia and is characterized by disproportionate short stature caused primarily by impaired endochondral bone growth.

Adult height is usually less than approximately 4.5 ft, with the greatest shortening affecting the proximal portions of the limbs, particularly the humerus and femur. This pattern of proximal limb shortening is known as rhizomelia.

Typical craniofacial features include frontal bossing and midface hypoplasia. Although degenerative arthritis is uncommon, spinal stenosis is one of the most important and potentially serious complications.

The disorder primarily affects the skeletal and neurologic systems. Characteristic skeletal features include ligamentous laxity and impaired longitudinal growth of the long bones. Neurologic complications may result from narrowing of the foramen magnum in infancy or lumbar spinal stenosis later in life.

Most characteristic features are already apparent at birth. In heterozygous individuals, life expectancy is usually near normal.

Epidemiology

Achondroplasia affects males and females approximately equally.

The frequency of spinal stenosis and degenerative disc disease increases with age. Neurologic manifestations may begin during childhood, although they become considerably more common in later adulthood.

Approximately 10% of affected individuals may demonstrate neurologic signs by around 10 years of age, while a much larger proportion may develop findings such as neurogenic claudication, hyperreflexia, or other manifestations of spinal stenosis by the sixth decade.

Prevalence

Achondroplasia occurs in approximately 1 in 15,000 individuals.

It is found across different populations and ethnic groups and represents the most frequently encountered form of disproportionate short-limb dwarfism.

Risk Factors

Advanced paternal age has been associated with an increased risk of new mutations causing achondroplasia, with parental age greater than approximately 33 years identified as a contributing factor in some studies.

Having an affected parent also increases the likelihood that a child will inherit the disorder because achondroplasia follows an autosomal dominant inheritance pattern.

Genetics

Achondroplasia is inherited as an autosomal dominant condition.

However, approximately 80% of affected individuals are born to parents of average stature and develop the disorder as a result of a new spontaneous mutation.

When one parent has achondroplasia, each child has a 50% chance of inheriting the affected gene.

When both parents have achondroplasia, there is a possibility of inheriting two abnormal copies of the gene. The homozygous form is usually lethal in early infancy.

Pathophysiology

The primary abnormality occurs within the growth plates, where there is reduced cellular organization and diminished chondrocyte activity.

This causes defective endochondral ossification, resulting in impaired longitudinal bone growth.

The overall width of the growth plate may remain relatively normal, but microscopic examination demonstrates disruption of the usual orderly arrangement of cartilage cells into longitudinal columns.

Other tissues and bone formed by intramembranous ossification remain largely normal.

Because circumferential growth of the shafts of long bones depends primarily on membranous bone formation, the bones may maintain relatively normal thickness despite marked reduction in length.

Characteristic skeletal changes can be recognized radiographically as early as approximately 3 months of gestation.

Etiology

Achondroplasia results from an abnormality involving the fibroblast growth factor receptor 3 (FGFR3) protein.

The pathologic process begins during fetal development. Excessive FGFR3 signaling inhibits normal chondrocyte proliferation and maturation at the epiphyseal growth plates.

As a result, cartilage growth becomes slowed and disorganized, leading to reduced longitudinal growth of the long bones.

Associated Conditions

Common associated problems include spinal stenosis, possible hydrocephalus, and a tendency toward overweight or obesity.

Neurologic complications may arise from compression at the foramen magnum during infancy or narrowing of the spinal canal later in life.

Diagnosis

Signs and Symptoms

The characteristic physical appearance is usually evident at birth.

Affected individuals have disproportionately short stature, a relatively long trunk, and pronounced proximal shortening of the limbs.

The thighs and upper arms are affected more severely than the distal portions of the extremities.

A large head with a prominent forehead and parietal or frontal bossing is common, together with midfacial hypoplasia.

Lower Limb Findings

The lower extremities may appear relatively straight in infancy, but genu varum frequently develops as the child grows.

Ligamentous laxity may contribute to progressive angular deformity of the knees.

Internal tibial torsion may accompany the varus deformity.

Children may also demonstrate varus-valgus instability and knee hyperextension.

Upper Limb and Hand Findings

The upper limbs demonstrate marked rhizomelic shortening, particularly involving the humerus.

The fingers are typically short, broad, and relatively thick.

A characteristic separation between the middle and ring fingers produces the classic “trident hand”, in which the fingers appear divided into three groups.

Elbow extension may be limited, and some patients develop flexion contractures or radial head abnormalities. These findings usually cause relatively minor functional impairment.

Spine and Posture

An exaggerated lumbar lordosis is common and is frequently associated with anterior pelvic tilt.

A thoracolumbar kyphosis may develop during infancy, particularly around the time the child begins sitting.

In many children, this kyphosis improves after independent walking develops and is subsequently replaced by increased lumbar lordosis.

A waddling gait is common because of altered limb proportions, hip mechanics, and lumbar posture.

Adult Height

Adult height typically ranges from approximately 42 to 56 inches.

Despite marked short stature, most individuals are capable of independent mobility and participation in routine activities of daily living.

Neurologic Manifestations

Spinal canal narrowing can produce several neurologic complications.

Lumbar stenosis may develop from narrowing of the spinal canal, disc prolapse, osteophyte formation, and characteristic vertebral abnormalities.

Compression of the spinal cord or nerve roots may lead to neurogenic claudication, weakness, sensory changes, and, in severe cases, bladder dysfunction.

Cervical or thoracic stenosis may produce upper motor neuron findings such as hyperreflexia.

History

A detailed developmental history should be obtained.

Children with achondroplasia often reach gross motor milestones later than children of average stature because of short limbs, relative hypotonia, and altered body proportions.

However, developmental expectations should be compared with achondroplasia-specific developmental standards, rather than with conventional pediatric milestones.

Cognitive development is generally normal.

In some infants, narrowing at the foramen magnum may improve as growth progresses, although persistent or severe compression requires careful monitoring.

Physical Examination

Head and Face

Typical findings include frontal bossing, a relatively large head, and midface hypoplasia.

Spine

The spine should be assessed for exaggerated lumbar lordosis, thoracolumbar kyphosis, scoliosis, and evidence of neurologic compression.

Thoracolumbar kyphosis commonly becomes apparent at approximately sitting age.

Extremities

The limbs are shortened, particularly proximally at the humerus and femur.

Muscles may appear prominent because of the relative shortness of the underlying bones.

Elbow flexion contracture may be present, sometimes associated with radial head dislocation.

The hands often demonstrate the characteristic trident configuration.

Genu varum is common, and varus-valgus laxity or hyperextension may be observed during childhood.

Neurologic Examination

Adults should be assessed carefully for evidence of lumbar spinal stenosis, including weakness in the ankles or feet, sensory abnormalities, changes in gait, and bladder dysfunction.

Hyperreflexia may suggest cervical or thoracic spinal cord compression.

Serial neurologic examination is important throughout life because neurologic complications can develop gradually.

Laboratory Tests

There are no characteristic routine laboratory abnormalities associated with achondroplasia.

Diagnosis is generally based on characteristic clinical and radiographic findings.

Genetic testing is not routinely required when the phenotype is typical, although molecular testing may be useful in diagnostically uncertain cases.

Imaging

Skull Radiographs

Characteristic cranial findings include a shortened skull base and a relatively enlarged cranium.

Frontal and occipital prominence may be evident.

The foramen magnum is characteristically small, contributing to the risk of cervicomedullary compression during infancy.

Lumbar Spine Radiographs

Typical findings include progressive narrowing of the interpedicular distance in the lower lumbar spine, spinal canal stenosis, and posterior scalloping of the vertebral bodies.

These abnormalities contribute to the high incidence of lumbar stenosis in adulthood.

Thoracolumbar Spine

Thoracolumbar kyphosis is common in infancy.

In many children, the kyphosis resolves after independent walking begins and is subsequently replaced by exaggerated lumbar lordosis.

Persistent or progressive kyphosis requires close observation because severe deformity can increase the risk of neurologic compromise.

Pelvic Radiographs

The pelvis is characteristically broad with short, wide, and relatively square iliac wings.

The greater sciatic notches are small and deep.

The superior acetabular margins tend to be relatively horizontal.

These pelvic abnormalities contribute to the characteristic body proportions and lower-limb alignment.

MRI

MRI of the brain, craniocervical junction, and upper cervical spinal cord should be considered when an infant or child demonstrates developmental delay exceeding expected achondroplasia-specific norms or when signs suggest neurologic compression.

MRI can identify brainstem compression, upper cervical cord abnormalities, hydrocephalus, and stenosis of the foramen magnum.

MRI is also useful when symptoms of lumbar or cervical spinal stenosis develop later in life.

Diagnostic Procedures

Sleep studies may be considered in infants with developmental delay or symptoms suggestive of sleep-disordered breathing.

Sleep abnormalities may indicate compression around the foramen magnum or cervicomedullary junction.

Routine genetic testing is generally unnecessary in individuals with classic clinical and radiographic findings.

Pathological Findings

There are no specific gross pathological findings beyond the characteristic growth plate abnormalities associated with defective endochondral ossification.

Other tissues are generally structurally normal.

Differential Diagnosis

Pseudoachondroplasia

Pseudoachondroplasia can also produce disproportionate short stature but typically has a normal facial appearance.

Radiographs demonstrate irregular epiphyses, particularly around the hips and knees.

Hypochondroplasia

Hypochondroplasia generally produces a milder phenotype.

Adult height is commonly greater than approximately 54 inches, craniofacial abnormalities are less pronounced, and significant spinal stenosis is less characteristic.

The typical progressive caudal narrowing of the interpedicular distance seen in achondroplasia may also be absent or less pronounced.

Treatment

General Measures

There is no treatment that completely corrects the underlying growth plate abnormality responsible for achondroplasia.

Management therefore focuses on prevention and treatment of complications, maintenance of function, and correction of specific musculoskeletal deformities when necessary.

Osteotomy may occasionally be required for significant angular deformities of the limbs.

Management of Back Pain

Adults with low back pain are initially managed with appropriate conservative measures, including activity modification, analgesia, physical therapy, and management of contributing mechanical factors.

Persistent neurologic symptoms require evaluation for spinal stenosis.

Neurologic Complications

Neurologic problems such as hydrocephalus, spinal cord compression, nerve root compression, severe spinal stenosis, or paraplegia may require surgical treatment.

Close neurologic surveillance is therefore an important part of long-term care.

Foramen Magnum Stenosis

Severe narrowing of the foramen magnum during infancy can compress the brainstem and upper cervical spinal cord.

When clinically significant compression is present, foramen magnum decompression may be required.

The procedure should be performed by an experienced neurosurgical team, ideally following multidisciplinary evaluation.

Thoracolumbar Kyphosis

Thoracolumbar kyphosis should be monitored throughout infancy and childhood.

If significant kyphosis persists beyond approximately 2–3 years of age, bracing may be considered.

Persistent severe deformity may eventually require surgical correction.

Lumbar Spinal Stenosis

Severe lumbar spinal stenosis may require decompressive surgery.

A laminectomy may be performed when progressive neurologic symptoms, significant claudication, weakness, or functional impairment are present.

Fusion may also be required in skeletally immature patients or when severe kyphosis or instability is present.

Activity and Lifestyle

Routine restriction from sporting activity is usually unnecessary unless specific neurologic or orthopedic complications are present.

Environmental modifications can improve independence.

Custom-designed chairs, appropriately positioned household equipment, and automobile hand controls can be particularly useful.

Developmental Monitoring

Children with achondroplasia should not be assessed exclusively according to developmental milestones established for children of average stature.

Gross motor milestones are often delayed because of short limbs, joint laxity, hypotonia, and altered body proportions.

Cognitive and language development are usually normal and tend to occur at expected ages.

Physical Therapy

Physical therapy can assist children in adapting to their environment and developing functional mobility.

Therapists may also monitor motor development, posture, strength, joint motion, spinal alignment, and lower-limb deformities.

Therapy should focus on optimizing function without forcing developmental activities that may increase spinal deformity.

Surgical Management

Tibial Valgus-Derotation Osteotomy

A tibial valgus-derotation osteotomy may be indicated for persistent genu varum associated with internal tibial torsion.

The procedure corrects both angular and rotational deformity of the tibia.

Fibular shortening has not been clearly demonstrated to provide consistent benefit in achondroplasia.

Spinal Fusion

Spinal fusion may be indicated for severe thoracolumbar kyphosis exceeding approximately 40° when the deformity does not respond adequately to bracing.

Surgery is most commonly considered in persistent severe deformity by approximately 8–10 years of age.

Surgeons must recognize that pedicle anatomy and vertebral morphology in achondroplasia differ substantially from those of the average spine, making instrumentation technically challenging.

Laminectomy

When decompression is required for spinal stenosis, it often needs to be extensive.

In many cases, decompression involves most or all of the lumbar spine and may extend into the lower thoracic region.

Severe kyphosis may need to be corrected simultaneously, particularly in growing children or when deformity contributes to neural compression.

One important operative complication is dural tear, which occurs relatively frequently in decompressive surgery in the achondroplastic spine.

Limb Lengthening

Limb-lengthening procedures can produce substantial increases in overall height.

Lengthening may also improve lumbar lordosis and potentially reduce some manifestations of spinal stenosis by stretching tight hamstrings and changing lower-limb mechanics.

However, achieving proportionate stature usually requires staged bilateral lengthening of the femora, tibiae, and often the humeri, making treatment prolonged and demanding.

The overall treatment program may continue for several years.

Humeral Lengthening

Humeral lengthening using monolateral external fixation can provide significant increases in upper-limb length and may improve functional reach.

Potential complications include radial nerve palsy, infection, joint stiffness, and problems with regenerate bone formation.

Excessive Limb Lengthening

Lengthening of the lower extremity beyond approximately 50% of the original bone length is associated with a higher risk of complications.

These may include impaired subsequent bone growth, reduced range of motion in adjacent joints, delayed regenerate bone formation, prolonged treatment, and delayed return to full weight-bearing.

Careful patient selection and staged planning are therefore essential.

Foramen Magnum Decompression

Foramen magnum decompression should be considered when clinically significant neurologic compression persists or progresses.

Because of the anatomical complexity and neurologic risk, the operation should be undertaken by an experienced neurosurgeon after multidisciplinary discussion.

Follow-Up

Patients with achondroplasia require long-term orthopedic and neurologic follow-up because complications can develop at different stages of life.

Particular attention should be paid to spinal alignment, lower-limb deformities, neurologic function, gait, strength, and bladder control.

Prognosis

The overall prognosis for individuals with heterozygous achondroplasia is generally good.

Approximately two-thirds of children with thoracolumbar kyphosis experience spontaneous improvement after about one year of independent walking.

There is generally no major visceral organ involvement.

The rare homozygous form is usually lethal within the first weeks or months of life.

Heterozygous individuals usually have a near-normal life span and normal intelligence.

Most individuals are able to live independently and participate fully in daily life, although adaptations may be required because of short stature.

Mortality Considerations

Compared with the general population, individuals with achondroplasia have an increased risk of mortality from neurologic and cardiovascular complications.

Reported data suggest an approximately eight-fold higher risk of neurologic-related mortality and an approximately three-fold higher risk of mortality from cardiac causes.

Careful monitoring of neurologic symptoms, sleep-disordered breathing, obesity, and cardiovascular risk factors is therefore important.

Complications

Important complications include spinal stenosis, nerve root compression, and knee pain associated with genu varum and abnormal lower-limb mechanics.

Other possible complications include foramen magnum stenosis, hydrocephalus, thoracolumbar kyphosis, lumbar lordosis, degenerative spinal disease, obesity, and sleep-disordered breathing.

Patient Monitoring

Because individuals with achondroplasia have a high lifetime risk of musculoskeletal and neurologic complications, regular clinical assessment is recommended.

Periodic examination should include evaluation of limb strength, gait, reflexes, spinal alignment, lower-limb deformity, and functional capacity.

Bladder function should also be reviewed because changes in urinary control may be an early indicator of significant spinal cord or nerve root compression.