Hip dysplasia is one of the most common causes of secondary osteoarthritis of the hip joint. Young patients who fail to receive appropriate treatment will eventually progress to severe hip osteoarthritis, requiring artificial joint replacement. Hip osteotomy can avoid or delay the occurrence and development of osteoarthritis, and there are various methods of periacetabular osteotomy, each with its own advantages and disadvantages. In this paper, we investigated the key points of surgical technique and medium-term efficacy in the treatment of early and mid-stage osteoarthritis secondary to hip dysplasia by performing a rotational osteotomy of the acetabulum through the lateral “U” approach of the Ollier hip. Data and methods: From May 2000 to May 2006, 12 patients (14 hips) with osteoarthritis secondary to early hip dysplasia underwent acetabular rotational osteotomy, including 2 on the right, 8 on the left, and 2 on both sides. All patients were female, and the mean age at the time of surgery was 28.9 years (13-46 years). All patients were closely followed up with a mean follow-up of 6.0 years (3.1 to 9.1 years). Patients were placed in the lateral position, and an Ollier lateral “U” approach was used, starting anteriorly at the anterior superior iliac spine, arcing downward posteriorly to 2 cm distal to the base of the greater trochanter of the femur, and then arcing upward posteriorly to end at the posterior superior iliac spine. The gluteus maximus muscle fibers were bluntly separated, and the external rotators such as the pear-shaped muscle and the internal occlusal muscle were cut at the end point, leaving the femoral square muscle intact to preserve the blood supply of the medial rotator femoral artery. The greater trochanter was osteotomized with a bone knife, preserving a thickness of 1.0-1.5 cm and pulling proximally to reveal the lateral aspect of the iliac wing, the anterior inferior iliac spine and the posterior inferior portion of the acetabulum. The periacetabular osteotomy line was marked with an electric knife, with the proximal end approximately 1.5 cm from the acetabulum and the posterior edge approximately 1.0 cm from the sciatic notch. a straight osteotome was first inserted vertically along the osteotomy line approximately 1.0 cm, and the osteotome was osteotomized with a curved osteotome under fluoroscopic guidance. the thickness of the osteotome should be >10 mm to prevent necrosis of the osteotome. The osteotomy block was rotated and displaced laterally and anteriorly and then fixed with long cortical bone screws without bone grafting. The greater trochanter is repositioned and fixed with wire or cortical bone screws. The patient did not need external fixation after surgery, and functional exercises for quadriceps muscle strength and hip range of motion were performed, with partial weight-bearing of the affected limb on the ground with the help of two abductors 2 weeks after surgery, and full weight-bearing of the abductor was abandoned after 2 months. Orthopantomographs of the pelvis were taken preoperatively, postoperatively and at follow-up (6 weeks, 12 weeks, 24 weeks and every year after surgery), and the CE angle (α), acetabular apex angle (β), head displacement index (B÷1/2A) and minimum joint space width were measured. The Harris score was used to determine the hip function. Paired t-test was performed on the radiographic measurements using statistical software (SPSS10.0). RESULTS: The patients’ pain symptoms were significantly improved at the final follow-up of this group of cases, with a Harris score of 72 (50-95) preoperatively and 91 (72-100) postoperatively (P<0.001). The mean operative time was about 2.4 hours (2.0-3.1 hours), and the mean intraoperative blood loss was about 650 ml (400-1200 ml). the CE angle was 0.9° preoperatively and 27° postoperatively (P<0.001); the acetabular apex angle was 29° preoperatively and 5° postoperatively; the head displacement index was 0.68 preoperatively and 0.65 postoperatively; the minimum joint space width was 2.0 mm preoperatively and 2.2 mm postoperatively. In all cases, the osteotomy block and the osteotomy of the greater trochanter healed well. There were no complications such as infection, nerve and vascular injury. In no case did the osteoarthritis secondary to hip dysplasia progress to the point of arthroplasty. Discussion: All patients with hip dysplasia progress to secondary osteoarthritis without effective treatment. periacetabular osteotomy for hip dysplasia in young patients can reduce the contact stress on the femoral head and delay or prevent further exacerbation of secondary osteoarthritis of the hip. chiari osteotomy can provide good bone coverage of the femoral head but not the hyaline cartilage of the articular surface; salter [11] osteotomy provides only a smaller improvement in coverage of the femoral head in pediatric patients and cannot be used in adult patients; the triple pelvic osteotomy [3] allows correction of acetabular coverage problems while changing the symmetry of the pelvis. The periacetabular osteotomy avoids the disadvantages of the above osteotomies and provides adequate lateral coverage of the femoral head. The acetabular rotational osteotomy, one of the peri-acetabular osteotomies, was first reported by Ninomiya and Tagawa in 1984, providing effective acetabular cartilage coverage of the femoral head with satisfactory mid-term and long-term results at clinical follow-up. Although the Ganz periacetabular osteotomy can correct the anterior tilt of the acetabulum and internally displaced the femoral head, the procedure is complex and requires a higher level of skill and a longer learning curve. In particular, it is difficult to visualize the sciatic branch and often requires extensive experience, and there is a high risk of injury to the internal iliac vessels. None of the acetabular rotational osteotomies require bone grafting at the osteotomy site, and they are firmly fixed with good postoperative bone healing. Our patients underwent proper functional recovery after surgery without complications such as screw fracture or bone displacement. The surgical approach is crucial for performing the rotational acetabular osteotomy. Currently, orthopedic surgeons in Europe and the United States prefer the medial approach, mainly to avoid compromising the abductor muscles of the hip joint and to reduce the occurrence of limited heterotopic ossification. The lateral approach has been accepted by orthopaedic surgeons in most Asian countries, but some experience is required to expose the acetabulum. the Ollier lateral "U" approach via the greater trochanter osteotomy adequately exposes the lateral aspect of the pelvis and the acetabular rim, simplifying the surgical technique and allowing easy osteotomy under direct vision. ko et al [15] concluded that the Ollier lateral "U" approach suggested that the Ollier lateral "U" approach is easier to perform acetabular osteotomy than other surgical approaches. However, a potential complication of the transfemoral greater trochanteric osteotomy approach is the possibility of nonunion and necrosis of the osteotomy block, which may reduce the abduction strength of the hip joint. In this group of cases, the osteotomy and repositioning were meticulously performed with wire and pressurized cortical bone screw fixation, and there was no bone necrosis and bone healing in all cases. The severity of hip dysplasia can be determined by measuring the CE angle. stulberg and harris measured the CE angle on radiographs of 60 healthy adults, with a mean of 37° in men and 35° in women. murphy measured the CE angle in 46 hips over 65 years of age without hip osteoarthritis, with a mean of 34°. The mean postoperative CE angle in this group of cases was 27°, which is close to normal, and it is important to emphasize that increasing the CE angle by excessive bone outgrowth often causes impingement with the greater trochanter of the femur and shifts the acetabular pear fossa to a weight-bearing position. The purpose of the acetabular rotational osteotomy is to provide adequate hyaline cartilage coverage of the femoral head, increase the weight-bearing surface of the acetabulum, and reduce its contact stress with the femoral head, so the degree of improvement in acetabular coverage can affect the long-term outcome after surgery. The causes of short-term failure of acetabular rotational osteotomy are related to the preoperative condition of the articular cartilage and postoperative chondrolysis, with the preoperative severity of articular cartilage degeneration affecting the postoperative outcome to a greater extent in older patients than in younger patients; chondrolysis is mainly related to incorrect surgical technique, with failure to preserve sufficient thickness of the acetabular osteotomy block causing its necrosis or resorption, resulting in chondrolysis and narrowing of the joint space on radiographs[19] . . The postoperative anastomosis of the acetabulum to the femoral head and the deformation of the femoral head determine to a certain extent the rate of progression of osteoarthritis and affect the long-term outcome after surgery. Ko et al. reported a group of 38 hips treated with acetabular rotational osteotomy through the lateral "U" approach of Ollier, with a mean follow-up of 5.5 years (2-10 years) and a satisfaction rate of 84%. Hsieh et al. reported that after a mean follow-up of 4.2 years (2-5 years) for 46 hips with periacetabular osteotomy via the greater trochanteric approach, 41 hips (89%) had progression-free osteoarthritis. ), with an excellent rate of 73% (52 hips). Most of the cases in this group were early to mid-stage hip dysplasia, although the number of cases was small and the outcome was satisfactory after a mean follow-up of 6.0 years (3.1 to 9.1 years). In conclusion, Ollier's lateral "U" approach with rotational acetabular osteotomy revealed the acetabular rim adequately; the treatment of early to mid-stage hip dysplasia can relieve pain and slow down the progression of osteoarthritis, and the results were satisfactory at mid-term follow-up.