[Development and functional structure of the epiphyseal plate]

The longitudinal growth of long bones occurs in the epiphyseal plates at their ends. After a center of ossification has developed in the epiphysis, a cartilaginous plate, characterized by a typical zonal arrangement is formed. The cartilaginous plate proliferates using cells from a reservoir at the epiphyseal (i.e. articular) side of the plate. The cells are orientated to longitudinal rows and become thickened towards the metaphysis. The extracellular matrix between the hypertrophic chondrocytes becomes calcified and is subsequently replaced in an ossification process by lamellar bone. Cell proliferation in the epiphyseal plate is regulated by a number of different factors (e.g. IGF-I and BMP-7). The extracellular matrix is rich in glycosaminoglycans, proteoglycans and type II collagen, whereas especially at the edges type I and type VI collagen can be observed, too. The three dimensional orientation of the epiphyseal plate is determined by local mechanical influences. The adjacent bony endplates exhibit a higher central mineralization pattern with lower density values at their edges. The region of failure predominantly concerns the proximal zone of the longitudinal rows. The mechanical properties of the epiphyseal plate are influenced by endocrine as well as by metabolic factors. Any case of an epiphysiolysis is caused by a misproportion between acting shear forces and cartilaginous strength.