Dislocation is one of the common complications after total hip arthroplasty and the cause of dislocation must be understood prior to treatment. Patient-side risk factors include age over 70 years, female patients, ligamentous laxity, revision surgery, hip abduction, and patient education. Medical factors include the volume and experience of the operator’s annual hip replacement, surgical approach, whether eccentric distance and lower extremity length were restored, prosthesis placement, soft tissue tension, and impingement. Prosthesis-related factors include the design of the head and neck position, particularly the skirt design of the lengthened neck. Ideally an adjustable eccentric distance provides better restoration of soft tissue tension.
Anti-dislocation liner design provides better stability, and dislocation may also occur due to impact if the high side is poorly positioned. Late dislocations can be caused by polyethylene wear, soft tissue damage, weakened abductors, and infection. Understanding these causes of dislocation can help prevent dislocation after hip arthroplasty. Proper preoperative design, such as restoring a near-physiological eccentric distance, helps maintain proper soft tissue tone and allows the abductors to work better. The prosthesis must be correctly positioned to prevent impingement while maintaining stability. Most dislocations occur early and can be prevented by proper means.
Dislocation after total hip arthroplasty is a common cause of surgical failure. The percentage of revisions due to dislocation is 22% of all hip revisions in the US Medicare database, ranking first. A review of the various types of literature shows that the incidence of hip dislocation after initial replacement is 0.3-10%, and in hip revision it is even higher at 28%. Dislocation results in increased costs, the potential need for further surgical treatment for the patient, and inconvenience for the surgeon. Studying the causes of dislocation after hip arthroplasty helps the surgeon to understand the various risk factors associated with the patient, the prosthesis, and the surgery to avoid dislocation whenever possible.
Patient factors.
Most dislocations (60-70%) occur within six weeks of surgery, and about 1/3 of these will recur. Only about 1% of the first dislocations occur many years after surgery, and this can occur due to prosthesis wear, soft tissue injury, inter-rotor problems or adductor muscle tears, and infection. Patient-related risk factors include age greater than 70 years, multiple comorbidities, female patients, muscle ligament laxity, revision surgery, adductor muscle weakness, and inter-rotor problems. Patient awareness and acceptance of preoperative and postoperative education (e.g., rehabilitation) also play a role.
Prosthetic factors.
The design and selection of the prosthesis plays an important role in whether or not it is dislocated. The most common consideration is the diameter of the femoral head. In the authors’ opinion, the notion of using a large diameter ball head to reduce dislocation began to gain popularity with the use of gold-to-gold interfaces in recent years. With the various problems that have occurred with the use of gold-to-gold interfaces, physicians have begun to favor the use of a combination of large metal heads to polyethylene liners, but this actually goes against the idea of lower wear that was the original design of the gold-to-gold interface. Equally important is the design of the ball head diameter in relation to the lateral aspect of the femoral stem neck. The ratio of the femoral head to the neck length of the head determines the angle of movement of the femoral neck prior to impact with the socket cup during abduction.
Another important factor is the eccentric distance design, which is the horizontal distance from the center of rotation of the femoral head to the longitudinal axis of the femoral stem. Some surgeons choose to place the femoral stem in the inversion position to restore soft tissue tension, which may result in overlength of the affected limb in this case and may also introduce an insufficient eccentric distance. Several prosthetic designs now offer different eccentric distance options that can balance the soft tissues without increasing the length of the affected limb. Assembled neck collars offer a variety of eccentric distance, length, and anteversion options, but the extra interface also carries the potential risk of wear, rust, and fracture at the joints.
Polyethylene liners can be designed with a 10°-20° high edge, providing additional coverage to prevent dislocation. However, it is important to note that the extra high side may also cause dislocation due to impact and that the safe placement area for the high side is posteriorly superior.
The neck of the femoral shank may be designed to be longer for additional strength, and there is a “skirt” underneath the ball head after installation. The “skirt” may also lead to premature impingement and needs to be avoided whenever possible.
Surgical factors.
Surgical experience and the number of hip replacements per year correlate with the incidence of complications and dislocations. Preoperative design plays an important role in determining prosthesis selection, femoral neck osteotomy surface, and depth of prosthesis infusion. The choice of head and neck length, eccentric spacing, maintenance of soft tissue tension, lower extremity length, placement of the prosthesis, and removal of the bony masses that cause impingement are all important.
Another important factor is the patient’s position on the operating table, as the instruments used to position the cup angle are positioned with reference to a horizontal plane parallel to the floor. If the patient is positioned in a head-high to foot-low (reverse Trendelenburg) position, the cup will be positioned too vertically. The anterior or posterior position will affect the anterior tilt of the cup, and the size of the cup will also affect the angle.
The choice of approach also has an impact on stability. The postero-lateral approach, which is currently the most widely used, has a higher rate of dislocation compared to the lateral and anterolateral approaches. Reconstruction of the posterior soft tissues has been shown to have a significant effect on reducing dislocations; in one literature report, reconstruction of the posterior capsule reduced dislocations by 80%.
Improper placement of the prosthesis position is a common cause of dislocation. Proper placement of the femoral stalk requires not only the correct anterior tilt, but also attention to valgus/inversion. Excessive valgus will result in reduced eccentric distance, resulting in insufficient soft tissue tension and thus increasing the potential for impingement.
A cup that is too vertical can certainly lead to dislocation. However, a cup that is too horizontal may lead to anterior impingement when the patient is bent over, resulting in instability. The anterior tilt angle of the cup is more difficult to assess. The posterior lateral approach seems to require a greater anterior tilt angle compared to the lateral and anterolateral approaches.
Treatment of recurrent dislocations.
Dislocations are more effectively prevented than treated. Preoperative design using a template, proper prone position, soft tissue tension, proper placement of the prosthesis position, and proper intraoperative assessment of stability are all critical.
Before treating a recurrently dislocated hip, the cause of the dislocation needs to be known. Prosthesis position, eccentric distance and lower extremity length all need to be assessed. The anterior tilt of the socket cup can be assessed with a lateral radiograph, without the need for a lateral frog position. It is not advisable to change the correct position of the prosthesis on the other side during preoperative design in order to compensate for the lack of angulation of one side of the prosthesis. If needed, the patient will be informed that the affected limb may be longer than the healthy side in order to maintain stability.
Intraoperatively, the component may be replaced to increase soft tissue tension, or the ball head may be replaced with a larger diameter, including the option of a double- or triple-acting head prosthesis, or a polyethylene liner with a high rim. Improperly positioned and impacted prostheses must be addressed. A large thick augmentation nudge and a restrictive liner are the last modalities to be considered.
Conclusion.
Dislocation after hip arthroplasty is the result of a combination of patient, prosthesis, and surgical factors, and its incidence can be effectively reduced by careful preoperative design, good surgical technique, proper prosthesis selection and use, and good patient education.