Clinical Manifestations. At birth, weight is normal but body length is reduced. The configuration of the head and face is normal. The elbows do not extend fully. The excessive length of the distal fibulas in relation to the short tibias results in ankle deformity, and unilateral bowleg or knock-knee may develop in childhood. The hands and feet are short and pudgy; the foreshortened nails are normal in width and grow normally. Ligamentous laxity of the fingers and toes permits extraordinary hypermobility in the joints. Atlantoaxial instability and odontoid hypoplasia are common. A prominent sternum and mild flaring of the lower ribs with Harrison’s grooves are also typical. In many patients, a distinctive feature is the sparse, fine, light-colored hair, which grows slowly and breaks easily. Cross-sectional microscopic examination reveals a reduced, somewhat elliptic hair shaft of small diameter that frequently lacks a pigment core. Body hair is similarly affected. However, in some patients, the hair is nearly normal.

About 10% of patients with the McKusick-type metaphyseal chondrodysplasia manifest intestinal malabsorption and Hirschsprung disease. They may be unusually susceptible to chickenpox. Neutropenia, persistent lymphopenia, normal serum immunoglobulins, and diminished delayed skin hypersensitivity may also be present. Adult height ranges from 41 to 57 inches.

Radiographic Findings. Radiographic abnormalities do not become evident until the patient is 9 to 12 months old. Although changes are seen primarily in the limbs and ribs and around the knees (where they are most severe), subtle changes occur in other bones such as the vertebrae and pelvic bones. The metaphyses are widened and irregular with sclerosis and cystic alterations. Other findings include cupping of the ribs and ankle deformity.

Histologic Findings. Microscopic examination of the metaphysis shows normal ossification but a hypoplastic cartilage. Chondrocytes are decreased in number, and columnization is disorganized. The cartilaginous cores on which bone mineral can deposit appear to be inadequate.