Anatomy

Skeletal anomalies associated with achondroplasia reflect retarded endocardial bone formation. Therefore, the long bones are short but wide because appositional bone growth is unaffected. The skull is not dependent on endocardial bone; therefore, it is generally large. The spinal column is of relatively normal length but becomes kyphotic as a result of vertebral anomalies and body habitus.

The achondroplastic foramen magnum is small at birth. This markedly diminished growth results not only from abnormal endochondral bone growth but also from abnormal placement and premature fusion of the synchondroses.

In people with achondroplastic dwarfism, stenosis of the spinal canal is secondary to abnormalities of endochondral ossification with premature synostosis of the ossification centers of the vertebral body and the posterior arch. This results in thickening of the laminae, shortening of the pedicles, and a reduction in the height of the vertebral bodies. Additional factors, such as prolapsed intervertebral disks, osteophytes, and progressive thoracolumbar kyphosis, contribute to the narrowing of the spinal canal