Let’s look at a case where the patient was a 72-year-old male with a history of previous left acetabular fracture and had a total hip replacement on the left side. Postoperatively, the patient experienced multiple hip dislocations. The dislocations occurred even after increasing the diameter of the femoral head during revision. A second revision with a restrictive acetabular liner resulted in a recurrence of dislocation. In the third revision, the liner was removed, a metal cement shell was inserted into the cup, and a larger femoral head was used to increase the head and neck ratio to the maximum, but the dislocation was still not spared. In this patient, a single factor was revised after each dislocation, indicating that the cause of the dislocation was multifactorial. The final revision included the position of the component (both cup and stem), maximization of the head and neck rate, restrictive acetabular liner, and anterior displacement of the greater trochanteric osteotomy. Since then, no further hip dislocations have occurred. So how to obtain a stable hip joint, there are many details that we need to pay attention to before, during and after surgery. First of all, we must fully understand the patient’s medical history and provide a detailed physical examination before surgery. Patients over 80 years of age are at a much higher risk of dislocation, which may be related to their decreased cognitive ability and muscle coordination. Other risk factors include alcohol abuse, neurological disorders (e.g., epilepsy, stroke, Parkinson’s disease), previous history of hip fracture, history of hip trauma, and history of hip revision. This is especially true for patients with preoperative abductor muscle relaxation and weakness, atrophy, etc., which seriously affect the patient’s standing and walking functions, resulting in complications such as postoperative claudication, weakness and even prosthetic dislocation and excessive wear. The risk of hip dislocation is significantly increased in patients with dementia and mental disorders. There is often nothing that can be done for these patients, and it is important to consider the appropriate endoprosthesis preoperatively and to inform the instability. Draw the plans before building the house. Again preoperative template measurements are used to assess the type of size of the prosthesis and the position of the components, to assess the patient’s bone quality, bone reserve, the presence of developmental deformities, bone redundancy and other possible deformities. Initially determine which method of fixation of the prosthesis will be used, cemented or non-cemented. If a non-cemented prosthesis is used, a decision is also made as to whether a proximal or distal loading prosthesis should be used in order to optimize the biomechanical environment of the hip. This also includes assessing the limb length discrepancy, accurately documenting the preoperative bilateral lower extremity length discrepancy, planning how to make both lower extremities isometric intraoperatively and assessing the level of femoral neck osteotomy. In addition we want to reconstruct the normal hip center. The optimal femoral eccentric distance is obtained. It is usually necessary to evaluate the hip with reference to the contralateral joint in order to obtain valuable preoperative information. In order to restore the normal biomechanical environment of the hip, it is crucial to choose the right prosthesis among the many available. Eighty percent of hip dislocations occur in the direction of the surgical approach. The risk rate of dislocation is higher in the posterior approach compared to the lateral approach. However, reconstructing the soft tissue with a posterior dissection, maintaining the integrity of the external rotator muscle groups and the joint capsule, and increasing the anterior tilt angle may reduce the risk of dislocation to as low as that of the lateral approach. In general, the mechanism of dislocation is an impingement of the joint at its maximum initial arc of motion. When the joint moves to its “jump distance”, a bar action is generated to trigger dislocation. The “jump distance” is half the diameter of the femoral head, so increasing the femoral head increases the stability of the joint. However, the large diameter femoral head can use a thinner polyethylene liner and increased volumetric wear, causing loosening of the prosthesis. Smaller diameters have greater linear wear and tend to cause dislocation and liner wear. The geometry of the femoral neck also affects impingement; the slimmer the femoral neck, the greater the initial arc of joint motion, so maximizing the head and neck rate will significantly increase the stability of the hip joint. The greatest variable in hip replacement surgery is the orientation of the component. The recommended safe orientation of the socket cup is 40+/-10 degrees of abduction and 15+/-10 degrees of anteversion, with an increased incidence of dislocation if this boundary value is exceeded. Too much anterior tilt is prone to anterior dislocation, too little anterior tilt is prone to posterior dislocation, too much abduction is prone to superior dislocation, and too little abduction is prone to anterior or posterior dislocation. The transverse acetabular ligament is a useful marker for the placement of the cup in order to minimize the patient’s postural factors on the placement of the prosthesis. By placing the cup parallel to the transverse acetabular ligament, it prevents excessive anterior tilting of the hip joint, which reduces the reactive forces through the hip joint and reduces the rate of polyethylene wear. Due to the anatomical characteristics of the hip joint itself, the abductor muscles must maintain a certain level of tension in order to maintain the stability of the hip joint during stance and gait. Insufficient eccentric distance may affect the stability of the prosthesis and result in micro-separation between the hip joint interfaces during walking. If the eccentric distance is too large, the shear force at the neck of the femoral prosthesis increases, the stress transfer of the prosthesis is uneven, the prosthesis moves slightly, and eventually leads to loosening, osteolysis, and fracture of the femoral prosthesis. The smaller the neck stem angle is, the larger the eccentric distance is, which can improve the force arm of the abductor muscle and increase the stability of the hip joint. However, it also increases the bending force arm of the prosthesis, which is subjected to greater tensile stress on the tension side and may result in fatigue fracture of the prosthesis. With improvements in materials and design, various manufacturers have introduced high eccentric distance prostheses that grow the femoral neck while internally shifting the femoral neck, strengthening the abductor muscle without affecting the length. The basic principle of acetabular reconstruction is the centration of the femoral head to reduce the gravity arm while the acetabulum is internally displaced to the base of the acetabular notch, which is the lateral edge of the teardrop. The internal displacement of the acetabular prosthesis increases the contact between the cup and the bone and the coverage of the cup on the femoral head, which facilitates prosthesis and joint stability. However, simple internal displacement of the acetabulum can cause relaxation and weakness of the hip abductor muscle, and the eccentric distance of the femur should be increased appropriately to maintain the tension of the abductor muscle. Due to excessive grinding of the subchondral bone, the peak stress value of the trabecular bone in cancellous bone increases, which is not conducive to the fixation and stability of the acetabular prosthesis. Therefore, try to retain 50% of the subchondral bone and increase the prosthetic coverage as much as possible. Of course even if the eccentric distance is correct, there are many other reasons that affect the abductor arm. The position of the prosthetic stem at the time of insertion can affect the cervical stem angle and thus the eccentric distance. If the stem is placed in a neutral position, the cervical stem angle is equal to the cervical stem angle of the prosthesis; if the stem is inserted with inversion, the cervical stem angle is reduced; if there is ectropion, the cervical stem angle is increased. The preoperative plan is very good, the correct eccentric distance, all requirements are met, but your femur is cut too much, the prosthesis is deep, resulting in a downward shift of the center of rotation, the abductor arm is shortened, and the muscle tone is insufficient. Therefore, a good preoperative plan also requires good intraoperative execution to ensure a total hip result. There are varying degrees of bone defects in hip revision. A successful revision should achieve close contact between the prosthesis and the bone surface, stable fixation of the prosthesis, minimize micro-movement between the prosthesis and the bone surface, and ensure long-term bone growth into the surface of the prosthesis. Immediately after the intraoperative prosthesis installation, check the mobility of the hip joint in all planes, flexion, internal retraction and internal rotation to evaluate the posterior stability of the hip joint, and posterior extension and external rotation to evaluate the anterior stability of the hip joint. If instability is evident, then the acetabulum should be examined for anterior tilt angle to the femur, as well as to assess soft tissue or bony impingement, the presence of thickening of the joint capsule and periarticular bone growth. The iliofemoral and pubofemoral ligaments add further stability to the hip anteriorly, with the sitiofemoral ligament strengthening the posterior joint capsule posteriorly. The iliofemoral ligament is tightened during hyperextension of the hip joint, thus preventing anterior dislocation of the hip joint due to hyperextension and increasing the anterior stability of the hip joint. This ligament should be released in a total hip replacement when the patient has a flexion and internal rotation deformity of the hip due to disease. The pubofemoral ligament is located anteriorly and inferiorly to the joint capsule and is stretched when the hip is hyperextended or abducted. When total hip replacement surgery is needed to correct the abduction deformity, the pubofemoral ligament should be released to restore the abduction function of the hip joint. The joint capsule and ligaments surrounding the hip joint are important factors in maintaining the stability of the hip joint and preventing dislocation due to extreme movement of the hip joint. Hip instability due to soft tissue factors is either due to a deficiency in abduction muscle strength or a reduction in the initial arc of motion of the hip due to soft tissue contracture, which makes the femoral head susceptible to barring out of dislocation. Avoid hyperextension, hyperextension and hyperextension and hyperextension of the hip on the operated side for 6 weeks. The technical goal of total hip replacement is to achieve initial stability and late stability. We have to follow the principle of press fit. The hemispherical acetabular cup does not have inherent stability, and increased stability relies on screws and tight press fit. The shape of the biologic femoral stem prosthesis has evolved from the original straight stem to the more tapered stem designed in the last decade to provide a tighter and stronger press fit mechanism. Later stability is dependent on bone ingrowth on the surface of the prosthesis, and for the prosthesis to achieve good bone ingrowth, adequate initial stability must first be obtained. Only then is it beneficial for bone tissue to grow into the coated micro-pores and provide later prosthetic stability. After the initial stability of the prosthesis is obtained, the different coatings on the surface of the prosthesis will have different effects on bone ingrowth, thus affecting the later stability of the prosthesis. Fully coated stems provide a greater range of strong fixation, while proximally coated stems can only be fixed in the proximal femoral region, where stresses are transmitted in a more physiological manner, mainly through the proximal end, avoiding stress masking and bone loss. Its surface treatment technology has changed from a simple rough surface + hydroxyapatite at the beginning to a three-dimensional porous surface/pearl surface design with better long-term stability. It is believed that by weighing the pros and cons and following the above principles, then we can obtain a stable hip joint.