Clinical Manifestations. The condition is usually evident at birth. Although the average birth length is 161/2 inches, adult height varies widely depending in part on the degree of contractures and kyphoscoliosis. The characteristic facies is round with midfacial flatness, a depressed and wide nasal bridge, protruding eyes in shallow orbits, and a broad mouth. Myopia occurs in 50% of patients and may become severe; retinal detachment is also common. About 50% of patients have a cleft palate without harelip. Recurrent otitis media and hearing loss, both conductive and neurosensory, are frequent.

At birth, the limbs are short in relation to the trunk but the proportions change and the trunk becomes comparatively shortened and kyphotic by early childhood. The knee and elbow joints are particularly prominent and enlarged, with limited range of motion; widespread flexion contractures develop. The fingers are relatively long and have bulbous and knobby joints. Stiffness of the metacarpophalangeal and interphalangeal joints prevents the patient from making a complete fist. Precocious osteoarthritis develops and may become incapacitating by late childhood. Lumbar lordosis is pronounced by early childhood, and kyphoscoliosis is common. Adult height ranges from 41 to 57 inches.

Radiographic Findings. Generalized platyspondyly with anterior wedging of the vertebral bodies in the lower thoracic and upper lumbar spine is a major feature. In infancy, coronal clefting may be seen in the lumbar vertebrae. The ilia are broad with hypoplastic basilar portions. Ossification of the femoral head may not be apparent until age 3 or even later. The short femoral necks are extremely broad, and in the newborn period the femurs are dumbbell shaped. The epiphyses at the knees are relatively large, and a peculiar flocculent calcification develops in the metaphyses of the long bones. The hands are affected by osteoporosis, large carpal centers, and bulbously enlarged interphalangeal joints with narrowed joint spaces.

Histologic Findings. The histopathology in Kniest dysplasia is unique. The resting cartilage contains large chondrocytes in a loosely woven matrix with numerous empty spaces (like Swiss cheese). In contrast, the growth plate is hypercellular. Electron microscopy reveals these cartilage cells to be filled with dilated cisterns of the rough endoplasmic reticulum.

Differential Diagnosis. Radiographs help to distinguish Kniest dysplasia from similar disorders; in the neonatal period, the ribs are essentially normal and there is moderately elongated platyspondyly. Metatropic dysplasia is characterized by wafer-like vertebral bodies and very short ribs. In spondyloepi-physeal dysplasia congenita (see Plate 4-15), ossification centers are not present in the neonatal period and the femurs are not dumbbell shaped.