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Algorithm

The algorithm for recurrent instability is used to determine the cause of failure and manner of treatment.

CRP , C-reactive protein level; ESR , erythrocyte sedimentation rate; THA , total hip arthroplasty; WBC , white blood cell count.

Introduction

Dislocation is second only to aseptic loosening as the most common cause of failure of total hip arthroplasty (THA). The incidence of dislocation is approximately 2% (Scottish and Kaiser Registries) after primary THA and between 5% and 20% after revision THA. The causes of instability after THA are often multifactorial and include component malposition, impingement, inappropriate soft tissue tensioning, and noncompliance with hip precautions ( Figs. 68.2 and 68.3 ). In some patients, the exact cause of dislocation is not clear. The key to optimizing the outcome of revision THA for recurrent dislocation is preoperative determination of the cause of dislocation.

A and B, The patient’s acetabular component was horizontal (i.e., in less abduction) with neutral anteversion, which caused anterior impingement.

A and B, The patient had an overly anteverted and abducted acetabular component. She suffered from anterior instability during extension and external rotation of the extremity. C and D, Postoperative radiographs show repositioning of the acetabular component.

When presented with an unstable THA, a thorough history, physical examination, and radiographic evaluation should be performed. The history should include the mechanism of dislocation (i.e., activity and position of the limb at the time of dislocation) and descriptions of previous episodes of subluxation or dislocation. The patient’s history of infection should be obtained. Both lower extremities should be examined for gait, range of motion, strength (particularly of the abductor musculature), neurovascular status, leg lengths, position of previous incisions, and leg position at rest.

Radiographic examination should include an anteroposterior view of the pelvis centered over the pubis and anteroposterior and cross-table lateral views of the affected hip. Component alignment and rotation can be delineated from radiographs. If component alignment remains unclear, computed tomography (CT) should be considered. Full serology, including a complete blood cell (CBC) count with a differential count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) level, should be performed. ESR and CRP values are used to diagnose indolent infection, which may be the cause of instability. If any value is out of the normal range, preoperative hip aspiration with or without an intraoperative frozen section biopsy should be done to rule out infection as a cause of dislocation.

Indications and Contraindications

The indications for revision THA are straightforward because dislocation is typically a traumatic event. First-time dislocations warrant an attempt at closed reduction with placement of a hip abduction brace. The brace should be worn full time for 4 to 6 weeks. For a posterior dislocation, the hip abduction brace should limit hip flexion to 40 degrees, with 10 degrees of abduction and no internal rotation. For an anterior dislocation, the limitations include maintenance of hip flexion to avoid hip extension and external rotation.

The position of the components should be thoroughly evaluated. If there is malpositioning, particularly of the acetabular component, revision THA should be considered because recurrent dislocations are likely. If component positioning is acceptable, a conservative course of hip abduction bracing should be considered.

After a second dislocation, closed reduction and bracing may be attempted again, but the patient should be counseled that revision surgery may be necessary if another dislocation occurs. Typically, revision THA is performed when a third dislocation takes place.

Contraindications for revision THA are few but include active periprosthetic infection, which requires treatment with a two-stage procedure. Any form of patient morbidity may be a contraindication to further surgical intervention.