A STUDY OF THE GROSS, MICROSCOPIC, ULTRASTRUCTURAL, AND COMPARATIVE ANATOMY AND DEVELOPMENT OF THE ARTICULAR SURFACES OF THE HUMAN SACROILIAC JOINT
Cassidy, David John
In this study, the gross, microscopic, ultrastructural, and comparative anatomy and development of the sacroiliac joint are presented. Sacroiliac joints were obtained from 108 human, 43 bovine, 24 equine, 21 porcine, and 21 canine autopsies, spanning a full range of ages. The specimens were photographed and examined under the light microscope. The human specimens were also examined under the transmission and scanning electron microscopes. The sacroiliac joint angle and the relative lengths of the caudal and cephalad limbs of the joint were compared between species. On gross inspection, the sacral surface has the smooth, shiny, creamy-white appearance of normal articular cartilage. The iliac surface has the rough, dull, bluish appearance of a thin layer of fibrocartilage. Histological and ultrastructural examination confirms the presence of hyaline cartilage on the sacral side and fibrocartilage on the iliac side in all species. The equine joint has a greater angle and a relatively shorter caudal limb than the other species. The unique juxtaposition of hyaline and fibrocartilage in the sacroiliac joint is explained by the developmental anatomy of the joint. The sacral articular cartilage develops directly from the primary hyaline cartilage anlage. Enchondral ossification progresses from the primary centre of ossification toward the articular surface. At maturity, a hyaline cartilage cap is left as the articular cartilage. On the iliac side, ossification of the cartilage anlage precedes cavitation of the joint space. Therefore, the iliac articular cartilage is derived from other progenitor cells and not directly from the primary cartilage anlage. As a result, it contains undifferentiated spindle cells and secondary cartilage that matures into a thin layer of fibrocartilage. The same situation is observed in the young animal specimens. Premature degenerative changes develop in the iliac cartilage in all species. In the human, ultrastructural signs of degenerative change can be observed in specimens from the second decade of life onward. Similar changes are observed on the sacral side by the third decade of life. A similar chronology of events occurs in the animal specimens. The iliac cartilage is affected at a very young age, followed soon after by the sacral cartilage.