Slipped capital femoral epiphysis. The mechanical function of the periosteum: new aspects and theory including bilaterality.

This particular ailment has many designations in the literature but none is quite adequate. Here we use the terminology slipped capital femoral epiphysis (SCFE). The anatomy of the proximal femur in all mammals reflects their growth and function. The main forces acting on the head are perpendicular to the growth plate. The thick tear-proof perichondrium-periosteum on the femoral neck is like a stocking attached to the epiphysis and the trochanter region. Growth in length causes a strong tensile stress in the periosteum, pressing the epiphysis against the metaphysis and thus stabilizing the vulnerable growth plate-the periosteum theory. Several factors may diminish the stability of the growth plate. SCFE begins with fissures, which coalesce to a fracture in the growth plate, invisible on a radiograph. As the slip progresses, an increasing angulation between the epiphysis and the remainder of the femur occurs. Weight and muscular forces displace the epiphysis posteriorly in a flexed hip. A rift in the ventral half of the periosteal stocking occurs at the border to the perichondrium and, after that, a longitudinal rift in the periosteum at the anterior midline of the femoral neck. This rift becomes broader as the epiphysis slips posteriorly, withdrawing the ruptured periosteum. Displacement of the epiphysis is due to a rotational slip and tilt, made possible by a compression fracture in the posterior part of the metaphysis. Parts of the periosteum function as reins steering the slip direction and counteracting the displacement. SCFE may be regarded as a pseudoarthrosis in the growing cartilage of the plate. The periosteum theory extended to a pseudoarthrosis theory has been supported by findings at surgery and on true lateral radiographs of usual and unusual cases of SCFE presented in this opus. On a true lateral view, the displacement can be measured as the slipping angle (SA) based on anatomical and geometrical considerations. SA values from 95 normal hips and from 22 contralateral asymptomatic hips from SCFE patients are presented in a histogram and bar graph. Statistically, SCFE is always bilateral, but in about 1/3 of the asymptomatic, contralateral hips, the physis ossifies and closes with SA below 13 degrees, and surgery is not necessary. It is most important that the position of the femur on the X-ray table is exactly defined in two dimensions: 1) the angle between the femoral shaft and the tabletop (angle of elevation), 2) the degree of rotation of the femur around its axis. A precisely defined positioning of the femur is a prerequisite for an exact reproducible measurement of the SA on a true lateral view and is also valuable for the evaluation of radiographic "signs". An aid, the Youth Hip Triangle (YHT), has been designed to facilitate positioning of the femur and measurement of SA. YHT is recommended for routine use in every X-ray facility. The method is quick, cost effective and makes it possible to diagnose SCFE in the contralateral hip before clinical signs or symptoms have occurred.