AGE-RELATED-CHANGES IN THE ARTICULAR-CARTILAGE OF HUMAN SACROILIAC JOINT

Iliac and sacral articular cartilage of 25 human sacroiliac joints (1- 93 years) are examined by light microscopy and immunohistochemistry in order to gain further insight into the nature and progress of degener ative changes appearing during aging. These changes can already be see n in younger adults as compared to cartilage degeneration known in oth er diarthrodial joints. Structural differences between sacral and ilia c cartilage can already be observed in the infant: the sacral auricula r facet is covered with a hyaline articular cartilage, reaching 4 mm i n thickness in the adult and staining intensely blue with alcian blue at pH1. Iliac cartilage of the newborn is composed of a dense fibrilla r network of thick collagen bundles, crossing each other at approximat ely right angles. A faint staining with alcian blue suggests a low con tent of acidic glycosaminoglycans. In the adult, iliac cartilage becom es hyaline and its maximal thickness reaches 1-2 mm. Both articular fa cets exhibit morphological changes during aging that are more pronounc ed in the iliac cartilage and resemble osteoarthritic degeneration; th e staining pattern of the extracellular matrix becomes inhomogenous, c hondrocytes are arranged in clusters and the articular surface develop s superficial irregularities and fissures. Sometimes fibrous tissue fi lls up these defects. Nevertheless, large areas of iliac cartilage rem ain hyaline in nature. Sacral articular cartilage often remains largel y unaltered until old age. The sacral subchondral bone plate is usuall y thin and shows spongiosa trabeculae inserted at right angles, sugges ting a perpendicular load on the articular facet. Iliac subchondral sp ongiosa shows no definite alignment and joins the thickened subchondra l bone plate in an oblique direction. The iliac cartilage therefore se ems to be stressed predominantly by shearing forces, arising from the changing monopodal support of the pelvis during locomotion. The subcho ndral bone plate on both the iliac and sacral auricular facet is penet rated by blood vessels that come into close contact with the overlying articular cartilage. These vessels may contribute to the high inciden ce of rheumatoid and inflammatory diseases in the human sacroiliac joi nt. Immunolabelling with an antibody against type Il collagen reveals a diminished immunoreactivity in the upper half of adult sacral cartil age and only a faint and irregular labelling in the iliac cartilage. T ype I collagen can be detected in a superficial layer on the sacral ar ticular surface and around chondrocyte clusters in iliac cartilage, as in dedifferentiating chondrocytes during the development of osteoarth ritis.