There are a host of disorders that fall within the category of skeletal dysplasias. These dysplasias are usually categorized by comparing the relative shortening of the proximal versus the distal long bones (e.g. femur versus tibia or humerus versus radius). The various types of skeletal dysplasias include:

• rhizomelia - shortening of the proximal limb (humerus, femur) relative to the distal limb
• mesomelia - shortening of the middle limb (radius-ulna, tibia-fibula)
• acromelia - shortening of the distal limb (hands, feet) relative to the proximal limb
• micromelia - shortening of both the proximal and distal limbs

Achondroplasia, the most common of the skeletal dysplasias, is a rhizomelic growth deformity that results from abnormal bone formation at the growth plate. Achondroplasia is an autosomal dominant disorder with a lethal homozygous form and a heterozygous form that demonstrates the multiple clinical manifestations of the disease.

Radiographic imaging findings include:

• rhizomelic limb shortening
• craniofacial disproportion with large calvarium, diminuitive skull base, small foramen magnum (may compress brainstem) and small jugular foramina
• shortened spinal pedicles distally with a decreased interpedicular distance (on AP films) in the inferior lumbar spine compared with the superior lumbar spine
• spinal stenosis (secondary to above)
• rounded iliac wings with decreased acetabular angles providing a "tombstone" appearance