Slipped femoral epiphysis is a hip disorder that occurs in adolescents and is characterized by the displacement of the femoral epiphysis through the growth plate. In fact, the epiphysis is displaced upward and outward, while the epiphysis is still held in the acetabulum by the round ligament. The vast majority of cases are idiopathic slipped epiphysis, i.e., the cause is unknown; in a few cases, it is associated with endocrine disorders, renal bone dysplasia, and previous radiotherapy. 1. Etiology It is believed that idiopathic slipped femoral epiphysis may be related to physical and biochemical factors, the combination of which leads to gradual lesions of the fragile growth plate. The main physical factor is obesity, which increases the posterior tilt of the femur and increases the shear stress on the epiphyseal plate. Obesity is present in most children with slipped femoral heads. The average anterior femoral tilt is 10.6° in normal weight adolescents and 0.40° in obese adolescents. It has also been shown that the average acetabular C-E angle in affected children is 37°, compared to 33° in controls, which may predispose the greater the coverage of the femoral epiphysis, the more shear stress is generated through the epiphyseal plate, and the more likely it is to develop. Biochemical factors mainly refer to endocrine-related. Slipped femoral epiphysis is a disease of adolescence, a time when many hormonal changes occur, and hypothyroidism, hypogonadism, and receiving growth hormone therapy are closely related to the onset of the disease. The growth plate is sensitive to growth hormone during puberty, showing increased physiological activity of the epiphyseal plate and accelerated longitudinal growth, accompanied by widening of the epiphyseal plate and decreased strength of the epiphyseal plate. Estrogen decreases epiphyseal plate width and increases epiphyseal plate strength, and testosterone decreases epiphyseal plate strength, which seems to explain the prevalence in males and the rare onset in girls after menarche. According to histological and electron microscopic observations, the epiphyseal plate in children with slipped femoral epiphysis is deficient and abnormal in collagen and proteoglycan architecture, with abnormalities in both the hypertrophic and proliferative layers. In the thickened hypertrophic zone chondrocytes were clustered and disorganized, and collagen fibrils were lacking; the alterations in the proliferative zone were proteoglycan and glycoprotein concentrations. It is not clear whether these abnormalities are the cause of the slipped femoral epiphysis or the result of the lesion. 2 , diagnosis Slipped femoral epiphysis has been clinically classified into four types: pre-slip, acute, chronic and acute attack of chronic slippage. Pre-slip is often painful and limp, and most of the physical examinations have positive findings, such as limited internal rotation. x-ray shows osteoporosis of the affected proximal femur, and there may also be widening and irregularity of the epiphyseal plate. Acute slippage is sudden trans-epiphyseal plate displacement, 10%-15% of them are acute slippage, with symptoms lasting less than 3 weeks, external rotation deformity, and significant activity limitation, most of them have mild prodromal symptoms for 1~3 months. Chronic slipped femoral head is the most common, accounting for 85% of cases. It manifests as pain in the groin and knee, and the symptoms can last for months or even years, with limited internal rotation of the hip joint. The acute onset of chronic slippage is a sudden increase in the degree of slippage based on chronicity, as well as acute clinical manifestations. Most of the clinical cases are classified into two categories: stable and unstable, based on clinical manifestations and radiological findings. The clinical observation of the child’s ability to walk is differentiated by the use of crutches or not; the ultrasound examination pays attention to the presence or absence of hip joint exudation. If the child is able to walk, there are no signs of hip oozing and the epiphysis has plastic changes, the child is considered to have a stable slipped femoral head; conversely, the child is unstable and in acute exacerbation. This classification is a better predictor of ischemic necrosis, with a high chance of complications in unstable slippage. Slipped femoral epiphysis appears on X-ray as a downward and posterior slippage of the proximal femoral epiphysis relative to the diaphysis. On anteroposterior radiographs, the posterior cortical lip of the epiphysis slips posteriorly and overlaps with the epiphysis to present a double density shadow, which is called the Steel’s sign of the epiphysis. A line drawn along the anterior and superior femoral neck, which normally passes through the epiphysis, is called the Klein line. If the epiphysis is flush with or below this line, it suggests a possible slipped epiphysis of the femoral head. Bone scans and MR examinations can make an early diagnosis of ischemic necrosis and chondrolysis. Advanced ultrasound techniques can show exudation and plasticity of the femoral neck in the hip. a CT scan can provide a three-dimensional image showing forward displacement of the femoral neck and a partially posterior deformity of the proximal femur. The severity of slipped femoral epiphysis is commonly evaluated in two ways. One, according to the amount of epiphyseal displacement in the epiphysis, a displacement of less than 1/3 of the width of the femoral neck is considered mild slippage; between 1/3 and 1/2 is considered moderate, and greater than 1/2 is considered severe. Second, measuring the frog lateral epiphysis-stem angle (proposed by Southwick), a slipped angle <30° is considered mild, 30°~50° is considered moderate, and >50° is considered severe. 3. Treatment The principle of treatment for slipped femoral epiphysis is to prevent and avoid the continued development of slippage while reducing the occurrence of ischemic necrosis and chondrolysis complications. For stable slipped femoral head epiphysis, treatment includes ① hip herringbone cast braking. (ii) Single or multiple stitches or screws for in situ fixation. (iii) iliac or allograft bone graft with open epiphyseal fixation. ④corrective osteotomy via the epiphyseal plate with multiple pins for internal fixation. ⑤Compensatory osteotomy with the femoral neck as the base and multi-pin fixation. ⑥Internal fixation of intertrochanteric osteotomy. In recent years, the success rate of single screw nailing treatment is higher due to the familiarity with the three-dimensional anatomy of the slipped femoral epiphysis and the extensive use of intraoperative imaging techniques, which have significantly reduced intraoperative injuries and made the positioning more accurate. This is the most common method for the treatment of slipped femoral epiphysis. As for the treatment of unstable slipped femoral epiphysis, it is basically similar to that of the stable one, but the following aspects are more controversial: ① whether to reset immediately or to delay reset; ② whether to traction before surgery. In the author’s experience, the incidence of ischemic necrosis is high in unstable slipped femoral epiphysis with severe displacement, and early repositioning and fixation may be more prudent. For less severely displaced unstable slipped femoral head epiphysis, theoretically, the chance of ischemic necrosis is small, so delayed fixation may be safer.