A typical case of recurrent dislocation of the hip joint. This case is a good illustration of the challenges associated with treating recurrent instability after hip arthroplasty. A 72-year-old man with a previous history of left acetabular fracture had a total hip replacement on his left side. Postoperatively, the patient experienced multiple hip dislocations (Figure 3). Even after increasing the head diameter and using a large femoral head during revision, dislocation still occurred. A second revision with a restrictive acetabular liner still resulted in another dislocation. In this patient, a single factor was revised after each dislocation, indicating that the cause of the dislocation was multifactorial (Figure 10), and the final revision included the position of the components (including the cup and stem), maximization of the head and neck rate, restrictive acetabular lining, and anterior displacement of the greater trochanteric osteotomy to improve soft tissue tension. Since then, no further hip dislocations have occurred, and these principles form the cornerstone of joint dislocation prevention. Mechanism of dislocation In general, the mechanism of dislocation is produced by impingement of the joint at the arc of maximum initial motion. When the joint moves to its maximum distance of motion or “jump distance”, a bar action is generated that triggers dislocation. The “jump distance” is half the diameter of the femoral head, so increasing the femoral head increases the stability of the joint. The geometry of the femoral neck also affects impingement; the longer the femoral neck, the greater the initial arc of motion of the joint, so maximizing the head-neck ratio will significantly increase the stability of the hip joint. When we have a large femoral head, the large femoral head will increase the volumetric O loss of the joint which must be taken into account (hard¬-soft friction surfaces, i.e. metal to HPPE), although volumetric wear can be reduced when a highly cross-linked HPPE is used. The use of liquid lubricants between the hard-hard surfaces can also reduce wear, so the use of large femoral heads is not always detrimental. Preoperative planning The patient’s medical history should be fully understood and a detailed physical examination should be performed before surgery. The aim of treatment is to restore the biomechanical environment of the joint, reestablish hip stability and maintain the soft tissue balance of the hip. Preoperative templating is used to assess the size and type of the prosthesis and the position of the components in order to optimize the biomechanical environment of the hip, including assessment of limb length discrepancies, hip center, femoral eccentric distance, and femoral neck cut. Evaluation with reference to the contralateral joint usually yields valuable preoperative information. In order to restore the normal biomechanical environment of the hip, it is critical to select the appropriate prosthesis among the many available in the clump. Instability is usually multifactorial and generally falls into the following five major categories: Patient factors Patient-side factors contributing to joint instability are mainly derived from the assessment of medical history. Patients older than 80 years of age report a greatly increased risk of dislocation, up to 15%, which may be related to decreased cognitive ability and muscle coordination. Other risk factors thought to be significantly associated with dislocation 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, such as a patient’s history of hip replacement after failure of internal fixation for a hip fracture. Patient compliance is also an obvious risk factor. Patients with dementia and mental disorders are at significantly increased risk of hip dislocation, and there is often nothing that can be done for these patients; appropriate endoprosthesis must be considered preoperatively and instability must be communicated. Surgeon factors Data from joint registries and insurance companies indicate that hip replacements performed by highly qualified and experienced surgeons, working in specialized medical centers, have a lower rate of dislocation. 80% of hip dislocations occur in the direction of the surgical approach. The rate of risk of dislocation was higher with the posterior approach compared to the lateral approach. However, suturing the incised tissue through the bone and maintaining the integrity of the short external rotators and the joint capsule reduces the risk of dislocation to as low as in the posterolateral approach [2]. Likewise, a transfemoral approach will increase instability of the hip if the greater trochanter does not heal. The choice of the posterolateral approach is particularly important when the hip is to be repaired at hip extension, in the presence of anterior hip dislocation and in the presence of postoperative Trendelenburg claudication. A precise repair of the soft tissues is essential. Implant design As already mentioned, a slender femoral neck and a large femoral head increase the initial curvature of the joint and can significantly increase the stability of the hip joint. A long femoral neck increases the vertical height, the length of the leg and the eccentric distance. However, long femoral necks tend to have collars, which increase the incidence of impingement and should therefore be used with caution. The use of a collar prosthetic stem in combination with an acetabular restrictive liner is not recommended. In general, a high eccentricity femoral component rather than a collared component is still the safer choice. The design of the acetabular component has an important influence on the initial arc of motion and stability of the joint, especially the design of the liner and the high-sided socket. High-sided pads reduce the initial arc of motion of the joint and cannot be routinely used to compensate for poor prosthetic position. The proper liner should be obtained by changing the angle of the socket, not at the expense of the initial arc of motion of the joint. A restrictive acetabular liner “grips” the femoral head and therefore increases the force required to bar out the femoral head (Figure 14). Restrictive pads are available in a variety of designs, and successful mid-term applications have been reported for both initial and revision surgery. The restrictive liner reduces the initial arc of motion of the joint and, therefore, has a greater potential for impingement when combined with poor prosthetic position. Restrictive liners are not a substitute for malpositioned prostheses and can lead not only to increased stress on the articular surface but also to failure of the liner itself and loosening of the acetabular prosthesis. Other positions are mainly used in patients with some soft tissue hypertonicity, but also in patients with neurological disorders or mechanical instability. Orientation of the prosthesis The greatest variable in hip replacement surgery is the orientation of the component; Lewnineck et al. recommended a safe orientation of the socket cup of 40 +/- 10 degrees of abduction and 15 +/- 10 degrees of anteversion, with an increased incidence of dislocation of 1.5-6.1% if this cut-off value is exceeded. Many factors influence the placement of the socket cup, including the patient’s position on the operating table, and maintaining the position of the pelvis has an important impact on the placement of the prosthesis position. The transverse acetabular ligament is a useful marker for placement of the cup in order to minimize the effect of patient position on the placement of the prosthesis [6]. The acetabular cup is parallel to the transverse acetabular ligament to prevent excessive anterior tilting of the cup. stem is parallel to the cup, at which point the cup covers the femoral head in all directions equally. The angle of internal rotation of the femur is measured. In the ideal position where the components are parallel, the femur should be internally rotated by 45 degrees, and the joint anteversion angle of the cup and stem is 45 degrees. Soft tissue factors Soft tissue factors cause hip instability, either due to deficiencies in abductor muscle strength or because of soft tissue contracture resulting in a reduction in the initial arc of hip motion and a tendency for the femur to be barred out of dislocation. The risk of dislocation is 2.8% when the stalk is in the inversion of the stalk and the risk of dislocation is 2.8% when the fracture heals and the risk of dislocation increases to 17.6% when the large trochanter is displaced and does not heal. increased to 17.6%. Intraoperative stability assessment The attempted repositioning of the joint and the assessment of limb length and eccentricity are critical steps prior to placement of the prosthesis. The method used aside, accurate preoperative assessment and patient positioning are key to measuring limb length. The shuck test allows assessment of eccentric distance and jump distance. During hip flexion and extension movements, the index and middle fingers are placed on the neck of the shank and stress is applied to the shank to eccentrate the head away from the ball and socket; excessive mobility suggests inadequate soft tissue tension and may require an increase in eccentric distance. The initial mobility and stability of the hip joint must also be evaluated. First, keep the hip and knee straight and judge the stability of the front of the joint by external rotation of the limb; in addition, flex the hip joint to 90 degrees and internally rotate the lower limb to evaluate the posterior stability of the hip joint. If the hip joint is only dislocated at this point, it indicates good stability and the minimum angle recommended for dislocation should be 45 degrees. If instability is evident, then the acetabular and femoral anteversion angles should be examined, as well as soft tissue or bony impingements should also be evaluated, such as the presence of a thickened joint capsule and periarticular bone growth. Conclusion The goal of hip replacement is to provide the patient with a pain-free, stable joint. Dislocation of the joint in the primary and revision joints is a common postoperative complication, and the cause is usually multifactorial. Patients at high risk for dislocation should be carefully evaluated preoperatively so that surgical measures can be taken to stabilize the joint to the greatest extent possible. x-ray templates help the surgeon to select the appropriate prosthesis to restore the biomechanical environment around the hip joint. Proper placement of the prosthetic component is a critical step in preventing joint dislocation. It is also extremely important to assess the stability of the joint intraoperatively and, if it is not as stable as intended, to check that any tissue that could cause impingement has been removed or that the prosthetic component is correctly positioned.