Total hip arthroplasty (THA) is a successful, cost-effective operation for improving pain and function. In 2010, approximately 332,000 THAs were performed in the United States. Despite the long-term success of this procedure, numerous complications can lead to decreased patient satisfaction, decreased function, and, potentially, revision surgery.
The most common indications for revision include infection, osteolysis, and dislocation. Medicare claims data revealed a 3.9 percent overall dislocation rate, which places a large economic burden on the healthcare system. Given the frequency of instability and the increasing demand for THA, an understanding of how to prevent and treat this complication is critical.
Risk factors for dislocation can be grouped into two broad categories—patient-related or surgery-related (Table 1). In addition to the patient factors shown, preoperative diagnoses of femoral neck fracture, osteonecrosis, or rheumatoid arthritis also increase the odds of dislocation.
Although the posterior approach is associated with higher dislocation rates than the anterolateral, direct lateral, or direct anterior approaches, repair of the external rotators and posterior capsule minimizes the risk of instability. An abduction pillow or knee immobilizer is commonly used during the perioperative period to prevent early dislocation; however, the efficacy of this approach is not well supported in literature. Although anterior approaches may have lower dislocation rates, other complications may counteract the benefit of additional stability.
Multiple “safe zones” have been described for acetabular component positioning for optimal stability after THA. The most frequently cited safe zone recommends a cup position of 40° ± 10° of inclination and 15° ± 10° of anteversion. Based on a retrospective registry study of 1823 THA and hip resurfacing acetabular components that used slightly narrower acceptable ranges (30° to 45° of inclination and 5° to 25° of anteversion), only 63 percent of the cups were placed in the proper abduction range and 79 percent were properly anteverted. Only 50 percent of the cups fell in both target ranges. Therefore, whether the classic acetabular positioning is ideal for every patient remains controversial.
Workup and evaluation
The evaluation of a patient with postoperative instability requires a thorough history and physical exam. The history should include the number and timing of dislocations, as well as the position or activity that caused the dislocation. Outside medical records and operative reports should be obtained if the examiner did not perform the index operation.
A complete physical exam should be performed with special focus being placed on gait, abductor strength, soft-tissue contractures, and leg-length inequality. Appropriate radiographs should be obtained as well, including anteroposterior (AP) pelvis and AP and cross-table lateral views of the hip. Advanced imaging such as computed tomography may be indicated to further evaluate component position. Screening laboratory studies, including erythrocyte sedimentation rate and C-reactive protein concentration, are also recommended to rule out infection.
Dislocations can be classified as early (less than 6 months) or late (more than 5 years), based on the timing of the event from the index surgery. First-time dislocations that occur between 6 months and 5 years after surgery are termed intermediate.
Another classification system divides recurrent instability into the following six etiologies:
- acetabular component malposition
- femoral component malposition
- abductor deficiency
- late wear
Treatment of periprosthetic dislocation usually begins with a closed reduction attempt under sedation, either in the emergency department or operating room setting. In an early dislocation, closed reduction with activity modifications and possible abduction bracing has been shown to be effective in most cases. Late dislocations have a higher rate of recurrence, however. Recurrent dislocations are commonly treated surgically.
Surgical options include the following:
- modular component exchange
- revision of femoral and/or acetabular components
- bipolar or tripolar arthroplasty
- larger femoral head
- constrained liner
- trochanteric advancement
- soft-tissue augmentation
Isolated head and liner exchanges are indicated for late dislocations due to polyethylene wear, as long as the components are well-fixed and well-positioned. Lateralized liners can be used to increase offset and improve soft-tissue tension. Although elevated rim liners can provide a block to dislocation, they also can potentially lead to component impingement and increased wear.
Increasing the size of the femoral head is an option to increase the head-neck ratio in cases of impingement. Revision of the acetabular component is indicated for malposition or loosening. The use of constrained liners is viewed as a salvage option; indications include well-fixed, well-positioned components with either abductor deficiency or no definitive reason for dislocation. When the cause of instability has not been addressed, poor outcomes have been reported with constrained liners.
Recently, unconstrained modular dual-mobility and tripolar articulations have gained popularity. Modular dual-mobility components are available from several manufacturers and allow motion between the femoral head and the liner as well as between the liner and the cup. Biomechanically, these constructs allow greater range of motion before impingement and have been shown to have low dislocation rates in both primary and revision settings. Despite these favorable results, intraprosthetic dislocations have been reported in up to 2.4 percent of patients in whom contemporary implants were used.
Tripolar constructs, consisting of a bipolar femoral head with a large acetabular component and liner, have also been used. Studies demonstrate a 93 percent early success rate in revisions employing these constructs to treat recurrent instability. Long-term studies are still needed to evaluate polyethylene wear rates and implant survivorship in both dual-mobility and tripolar designs.
Periprosthetic hip dislocations are a leading complication after THA. Although many patients can be treated successfully with closed reduction, instability remains a common indication for revision THA. Controversy remains with respect to optimal implant position and the role of dual mobility versus constrained constructs in THA. Revision surgery for prosthetic hip dislocation should address the root cause of instability. Similarly, anterior approaches to the hip have gained popularity but the long-term effect on dislocation or other complications remains controversial.
One or more of the authors report potential conflicts of interest; for more information, visit the AAOS disclosure database at www.aaos.org/disclosure
Mark W. Dwyer, MD, is a fellow in the department of orthopaedic surgery at Case Western Reserve University School of Medicine and University Hospitals Case Medical Center; the late Victor M. Goldberg, MD, was a professor in the department of orthopaedic surgery at Case Western Reserve University School of Medicine and a member of the AAOS Now editorial board; Glenn D. Wera, MD, is an assistant professor in the department of orthopaedic surgery at Case Western Reserve University School of Medicine, University Hospitals Case Medical Center, and Cleveland Veterans Affairs.
- After THA, periprosthetic dislocation due to instability is a common complication.
- Reasons for instability are multifactorial and may be a combination of patient- and surgery-related factors.
- Successful revision surgery must address the underlying causes of instability.
- Research is required on the long-term effects of newer surgical approaches and implant types, particularly with regard to complications.
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