[Osteoarthritis of rare etiology].

Although osteoarthritis is characterized by a uniform pattern of clinical and radiological manifestations, it is a syndrome that can be produced by a variety of causative factors. Rare causes of osteoarthritis can be categorized as follows: 1) systemic metabolic disorders due to known biochemical and/or genetic abnormalities, such as hemochromatosis, ochronosis, Wilson's disease, Ehlers-Danlos syndrome (and probably the "idiopathic" joint hypermobility syndrome), sickle cell anemia, and thalassemia; 2) endocrine disorders, such as acromegaly, whose joint manifestations are now well-known, and hypothyroidism; 3) Paget's disease of bone, osteopetrosis (which induces changes in bone elasticity), and other systemic bone diseases; 4) dysplasias, which form a vast group including familial polyepiphyseal dysplasia, spondyloepiphyseal dysplasia congenita (especially its milder forms), Stickler's syndrome, osteo-onychodysplasia, Kniest's dysplasia, trichorhinopharyngeal syndrome, and a group of diseases that affect the epiphyses; 5) endemic forms of osteoarthritis, e.g., Mselini disease, Kashin-Beck disease, and Malnad disease, which are unknown in western Europe but have been reported to affect thousands of individuals in endemic areas. All these disorders are usually responsible for premature osteoarthritis, whose presentation sometimes bears the imprint of the causative abnormality but can be identical to that of common osteoarthritis. The effects of toxic substances (Kashin-Beck disease) or genetically-determined collagen II abnormalities (epiphyseal dysplasias) may explain the occurrence of these rare forms of premature osteoarthritis.