The growth plate of the tibial tuberosity does not develop until several months after birth and is structurally different than most growth plates that are loaded primarily in compression. Histologically, it is particularly different from the juxtaposed proximal tibial growth plate. The physis of the tibial tuberosity is composed primarily of fibrocartilage and fibrous tissue, with bone being added to the anterior portion of the tibial metaphysis by membranous bone formation. Initially very little of the growth plate is comprised of columnated cells, but by the time of maturation of the tuberosity, with the exception being the most distal region, the columnar portion has extended distally and is found under most of the tuberosity. These structural features would be an adaptation to the strong tensile forces exerted in this region. That human "traction" apophyses may be histologically different from "compression" epiphyses seems not to have been demonstrated previously. Osgood-Schlatter's disease would appear to be an inability of the developing secondary ossification center to withstand tensile forces, resulting in avulsion of segments of the ossification center, and eventual formation of extra bone(s) between the fragments.