Achondroplasia.

Achondroplasia is the commonest disproportionate dwarfism. It is inherited as an autosomal dominent. 85% of case incidence is from a spontaneous mutation. The main feature is a symmetrical proximal limb shortening, rhizomelia, the humeri and femora are particularly affected. Metaphyseal irregularity takes the form of cupping or a 'V' shaped metaphyseal notch.

The other features are: Smaller square iliac bones, horizontal acetabular roofs, delayed appearance of ossification, pelvic tilt with lumbar lordosis, narrow sacro-iliac notch, flattened verebral bodies, occasional short a-p diameter of vertebrae or occasionial bullet shape to anterior vertebral border, spinal stenosis, posterior scalloping of vertebrae, small skull base, compensating large vault with appearance of frontal bossing, small foramen magnum, short ribs with widened anterior ends, Trident hand, similar lengths of generally shorter fingers.

Neurological bladder and hydrocephalus may complicate the axial spinal stenoses.

Some of the appearance is predictable. A process that results in short bones of normal thickness in the hand and wrist will, with a shorter radius of curvature (length of finger and metacarpal), have to accommodate them in a smaller area unless the angle in which they are confined is increased. Thus the divergence of (shortened) index and little fingers is increased.

Some sort of metaphyseal abnormality has to result from a failure of bone lengthening, since this has to involve the function of the tissue beyond the margins of the epiphyseal cartilagenous plate. Similar deformities have to result from abnormal growth at other epiphyses or apophyses, which are separated from the core of the bone by cartilage. Regardless of the accuracy of this contention, it serves as an aid in remembering the features of this syndrome. Another example is the small skull base; being cartilage, it is more affected than the vault, which is membrane bone and accomodates the same brain size with pronounced bossing of the vault.

Hypochondroplasia

There exist less severe forms of achondroplasia of which the commonest is hypochondroplasia. Our example merely shows minor metaphyseal changes and mild spinal stenosis with posterior vertebral scalloping.