Mutation affects a single protein in fibroblast growth factor receptor-3 (FGFR–3) gene, changing glycine to arginine at position 380, leading to growth retardation of the proliferative zone of the growth plate.

The abnormality seen in the bone of patients with achondroplasia is failure of endochondral ossification. Intramembranous and periosteal ossifications are undisturbed.

Loss of normal chondrocyte proliferation.

Because the width of the long bones is a product of intramembranous periosteal ossification, the bones are of normal diameter.

The growth plates with the most growth (proximal humerus/distal femur) are most affected, resulting in rhizomelic (proximal more than distal) short stature.

Trunk height tends to be normal, but arm span and standing height are diminished.

They also have frontal bossing, button nose, trident hands, thoracolumbar kyphosis, lumbar stenosis with lordosis and short pedicles, posterior radial head dislocation, “champagne glass” pelvic outlet and genu varum.

Radiology: The metaphyses are widened and flared, but the epiphyses are uninvolved. The growth plates are U-shaped or V-shaped. The pelvis characteristically appears broad and flat, with squared iliac wings. ^. The foramen magnum is smaller than normal. There is posterior scalloping of the vertebral bodies and shortened pedicles.

Prenatal diagnosis of achondroplasia: Ultrasonography does reveal decreased femoral length for gestational age.

Nonsurgical treatment is usual for the thoracolumbar kyphosis present early on. Avoidance of unsupported sitting may help prevent it.

Genu varum is treated with osteotomies if symptomatic.

Foramen magnum/upper cervical stenosis may require urgent decompression if cord compression is present; this area does grow bigger in later life.

The main issue in adult life is lumbar stenosis requiring decompression and/or fusion. A more lateral decompression is often necessary.

Limb lengthening may be carried out. Adolescence usually is the time when lengthening is undertaken.

Administration of growth hormone continues on an investigational basis.

Autosomal dominant form due to a cytosine–adenine substitution in FGFR-3 in chromosome 4; but milder form of achondroplasia.

Rhizomelic shortening of extremities, apparent at 2–3 years.

Less distinct facial features than achondroplasia.

Diagnosis is rarely apparent before 2 years.

Lumbar canal stenosis, genu varum and short stature are seen.

Treatment is similar to achondroplasia.

Short-limbed and short-trunk dwarfism apparent from birth. Other common findings include cleft palate and hitchhiker thumbs.

Autosomal recessive

Mutation in sulphate transport protein that primarily affects cartilage matrix

Cleft palate is present in 60 %.

Cauliflower ears are present in 80 % and develop after birth from cystic swellings in the ear cartilage. Compressive wrapping is used for cystic swelling of the ears.

Cervical kyphosis and thoracolumbar scoliosis are often present.

Joint contractures (hip flexion, genu valgum with dislocated patellae) and rigid clubfoot or skewfoot are often present.

Surgery is indicated for progressive spinal deformity or cord compromise; note that cervical kyphosis often resolves spontaneously.

Surgery is also indicated for progressive, symptomatic lower extremity deformity; recurrence is common.

Autosomal dominant disorder due to mutation in type 2 collagen (COL2A1)

Short trunk and short limbs

Features include midface hypoplasia, myopia, retinal detachment and deafness.

A distinctive feature of Kniest dysplasia is hundreds of small holes in the bone cartilage, making it appear like Swiss cheese on an x-ray. The holes weaken the cartilage, which serves as connective tissue throughout the body and causes joint stiffness and swelling.

Dumbbell-shaped long bones with broad metaphysic and irregular dysplastic epiphysis

Other features; odontoid hypoplasia, kyphosis, mild scoliosis, joint contractures and limb malalignment, hip dysplasia

Degenerative arthritis as early as second decade of life

Every child’s condition is different, so treatment is determined on a case-by-case basis. Treatment may include nonsurgical options such as bracing and physical therapy or surgical options such as spinal fusion or implanting growing rods to stabilise the child’s spine as he continues to grow.

Proportionate dwarfism with multiple epiphyses involved but no spinal involvement (limbs shorter than trunk)

Often diagnosed in mid childhood

Autosomal dominant.

Genes identified causing this phenotype include COMP; COL9A2, which encodes a chain for type IX collagen (a link protein for type II collagen); and recently a similar gene, COL9A3.

Multiple abnormal and irregular epiphyses.

Shortened metacarpals and metatarsals are present.

Valgus knees with a double-layer patella are found.

May have mild to severe involvement of epiphyses; long-term prognosis ranges from mild joint problems to end-stage OA with severe joint contractures at a young age.

No spinal involvement.

For any bilateral Legg–Calvé–Perthes patients, rule out MED.

Progressive genu valgum can be managed by staple hemiepiphysiodesis or osteotomy.

Painful, stiff joints are managed with therapy and nonsteroidal anti-inflammatory drugs; end-stage OA with joint arthroplasty.

Most common autosomal dominant form is SED congenital, which is apparent from birth and caused by mutations in COL2A1, which encodes type II collagen found in articular cartilage and vitreous humor of the eyes. The proliferative zone of the growth plates is affected with inability of epiphyseal centres to ossify.

Rarer, X-linked recessive form is SED tarda, which is milder, with later onset from age 8 to 10 years and is thought to involve the SEDL gene.

Hypoplasia of odontoid and atlantoaxial instability is common in both forms.

Platyspondyly and delayed epiphyseal ossification are present in both, as is premature OA.

SED congenita: Coxa vara, genu valgum, planovalgus feet, retinal detachment, myopia and hearing loss also present.

SED tarda: No lower limb bowing, but dislocated hips are sometimes seen.

Cervical instability should be stabilised.

Progressive, symptomatic lower extremity deformity should be corrected with osteotomies, being careful to assess the whole limb for deformities in joints above and below and recognising that early OA and joint arthroplasty are still likely.

MPS is a type of lysosomal storage disorder where a particular enzyme deficiency leads to accumulation of products in the viscera, joints and brain. This group is characterised by excretion of mucopolysaccharide in urine (Tables 1, subtype II and 2).

Mucopolysaccharidoses are lysosomal storage diseases that result in the intracellular accumulation of mucopolysaccharides in multiple organs.

All are autosomal recessive except Hunter syndrome type II, which is X-linked recessive.

Urine test, using a toluidine blue spot test to see which mucopolysaccharide breakdown products are present

Testing enzyme activity in skin fibroblast culture

Chorionic villous sampling

All patients are short statured; additional features vary but often include corneal clouding, enlarged skull, bullet-shaped phalanges, mental retardation, visceromegaly, cervical instability, genu valgum and developmental dysplasia of the hip (DDH) that is later in onset.

Vertebra: Platyspondyly (ovoid hypoplastic flat vertebral body usually pointing anteriorly)

Skull: Abnormal J-shaped sella, thick calvarium

Chest: Wide oar-shaped ribs, wide clavicles, plump scapula

Long bones: Foreshortening of long bones with underdevelopment of epiphysis with submetaphyseal overconstriction

Short tubular bones: Bullet-shaped metacarpals

Pelvis: Wine glass appearance with shallow acetabulum

Hip: Coxa valga, fragmented ossification centre of femoral head

Knee: Genu valgum

Bone structure: Osteopenic with coarsely laced trabeculae

Hurler syndrome is now treated with a bone marrow transplant in the first year of life; intelligence is normal in some affected individuals, but short stature and orthopaedic deformities are always present.