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A 75-year-old woman presents with recurrent dislocation of her right THA
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A 75-year-old woman presented with a complaint of recurrent dislocations of her right total hip arthroplasty. She underwent an uncomplicated right total hip arthroplasty 4 months prior to the current presentation through a posterolateral approach with a posterior soft-tissue repair. Postoperatively, she was instructed to follow posterior hip precautions for 6 weeks, but suffered her first posterior dislocation at 5 weeks, while reaching forward from a seated position on a 12-inch-tall stool. A successful closed reduction was performed at that time. Radiographs demonstrated components in acceptable alignment. Therefore, she was managed nonoperatively with an abduction brace for 6 weeks, restricting hip adduction and flexion. After discontinuing the brace, posterior hip precautions were continued for another 6 weeks. At the conclusion of this period, she suffered a second posterior dislocation, while leaning forward to put on a left sock. Again, a successful closed reduction was performed in an emergency room.
Her past medical history was significant for hypothyroidism, coronary artery disease, and bradycardia. Her surgical history was significant for coronary angioplasty, pacemaker placement and the right total hip arthroplasty (THA). Her medications consisted of anti-hypertensives, aspirin and clopidogrel. She had allergies to penicillin and sulfa medications. She was not a smoker, and consumed one alcoholic drink every other day, on average.
On physical examination prior to the second dislocation, she perceived her leg-lengths as equal. She had a clinically obvious dextroscoliosis affecting her thoracic spine with a compensatory lumbar curve, balancing her in the coronal and sagittal planes. She was able to ambulate without a limp. She had a well-healed incision over the posterolateral aspect of the right hip. She did not have a flexion contracture, and right hip flexion was 95°. At 90° of right hip flexion, internal rotation was 10° and external rotation was 45°. With the hip in extension, there were 10° of adduction and 30° of abduction. She exhibited signs of generalized hypermobility. The neurovascular examination of the right lower extremity was normal. In particular, ipsilateral abductor power was adequate, and she could perform a straight-leg raise.
Figure 1. Supine anteroposterior plain radiograph of the pelvis demonstrates equal leg lengths, restoration of right femoral offset and acetabular inclination of 37° (a). Crosstable lateral radiograph demonstrates 25° of acetabular anteversion (b). Qualitatively, the femoral component appears appropriately anteverted.
Images: McLawhorn AS and colleagues
Routine plain radiographs demonstrated a concentrically reduced right THA (Figure 1A). Leg lengths were equal, as judged by the perpendicular distances from the inter-teardrop line and the superomedial aspects of the lesser trochanters. Acetabular inclination measured 37° on the anteroposterior (AP) supine pelvis X-ray, and the crosstable lateral X-ray demonstrated 25° of acetabular anteversion, using the Woo and Morrey method of measurement. Thus, the acetabular component appeared to be within the “safe zone” for reduced risk of dislocation, which Lewinnek and colleagues defined as 40° ± 10° of inclination and 15° ± 10° of anteversion. The femoral component appeared anteverted on the crosstable view (Figure 1B).
What is your management?
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Revision of her acetabular component and femoral head
Postoperative dislocation remains a common complication after primary THA, affecting 2% to 4% of patients with modern implants, and it is one of the most common reasons for THA revision. Nearly 80% of dislocations occur within the first 3 months postoperatively. The etiology of dislocation can be multifactorial, with patient age, gender, body mass index and comorbid conditions all reportedly contributing to the risk of postoperative THA instability. Surgical approach, restoration of leg length and femoral offset, femoral head size, and component orientation are also important surgical factors that can impact the incidence of dislocation.
Lewinnek’s “safe zone” (40° ± 10° of inclination; 15° ± 10° of anteversion) is the traditional target for acetabular component orientation in order to minimize the risk of postoperative dislocation. However, a recent paper measured cup position in 147 patients who experienced dislocation and found that there is no “safe zone” within which the risk of dislocation is low. It is logical that one “safe zone” for acetabular component position does not fit all patients, since the position of the cup is dependent on the orientation of a patient’s pelvis during daily activities. Sagittal pelvic tilt, defined as the angle between the coronal plane and the anterior pelvic plane (APP) delineated by the two anterior-superior iliac spines and the pubic tubercle, has particular influence on the functional position of the acetabular component. Pelvic tilt is different among patients, and for each patient pelvic tilt changes depending on whether the patient is supine, standing or sitting. In most patients, the change in pelvic tilt from supine to standing is less than 10º. However, the change in the functional position of the pelvis sitting to standing can be significant. Pelvic tilt from sitting to standing may be an important variable in patients who dislocate posteriorly when rising from a chair. Furthermore, changes in pelvic tilt can occur after THA in approximately 15% of patients.
Sagittal pelvic tilt changes the functional inclination and anteversion of an acetabular component. Functional inclination is the angle between the pelvic longitudinal axis and the acetabular axis when this is projected onto the coronal plane, and functional anteversion is the angle between the acetabular axis and the coronal plane. The coronal plane reference is different in supine and standing AP radiographs. Alterations in sagittal pelvic tilt in supine and standing will affect measurement of acetabular anteversion and inclination on plain films. Functional anteversion will change approximately 4° for every 5° change in pelvic tilt. Given the inability to measure sagittal tilt on an AP radiograph, static 2D imaging alone likely provides an insufficient assessment of the malfunctioning THA, particularly in patients with a significant amount of sagittal pelvic tilt and/or significant spinopelvic motion.
Ideally, acetabular orientation should be individualized, taking into account femoral component anteversion in order to achieve maximum range of motion prior to impingement and minimal bearing surface wear. Femoral component position is just as important during functional positions and the concept of combined anteversion (acetabular version plus femoral anteversion) is a biomechanically sound principle to follow in order to avoid impingement. Impingement can be alleviated through one or more of the following options: acetabular reorientation, femoral reorientation, modular exchange of components, removing sources of bony impingement and increasing the articulation diameter. A kinematic analysis of a patient with recurrent THA instability is a useful preoperative planning tool to evaluate the potential impact of each of these options.
Figure 2. An anteroposterior and lateral EOS radiograph shows marked dextroscoliosis of the thoracic spine, and a secondary curvature of the lumbar spine with its convex to the left (a). Lateral pelvis radiograph shows measurement of 20° of posterior pelvic tilt in the standing position (b).
Images: McLawhorn AS and colleagues
Singular episodes of early postoperative THA instability are often treated successfully with reinforcement of hip precautions and bracing, if the components are properly aligned and hip mechanics have been restored. The patient who develops recurrent instability presents a challenging problem to the reconstructive surgeon. An algorithmic approach to the work-up of these patients is useful, beginning with an assessment of component positioning and abductor function. As in our case, in which the implants are not grossly malaligned and the abductors are not dysfunctional, sources of impingement should be identified and the treatment targeted at ameliorating the motion conflict predicating dislocation. However, accurate preoperative determinations of functional implant position and impingement are complicated, and they are dependent upon defining the spatial orientation of the pelvis and femur for an individual patient throughout the usual activities of daily living and the provocative position causing instability.
Work-up of our patient
A comprehensive imaging assessment using multiple modalities was performed to develop a preoperative plan for revision surgery. Standing biplanar lower extremity radiographs (EOS Imaging; Paris) (Figure 2) and a CT scan were obtained. A 3D computer model of the CT was reconstructed and used to measure pelvic tilt in the supine position. The CT model was then superimposed over the EOS image to measure pelvic tilt and cup orientation in the standing position (Figure 3).
Figure 3. This 3-D-CT reconstruction demonstrates the orientation of the patient’s pelvis and right total hip arthroplasty lying supine on the CT table (a). The 3-D-CT model was superimposed over the standing EOS image to show the functional position of the patient during standing (b). There was an increase in posterior pelvic tilt from 0° supine to 20° standing. Posterior pelvic tilt is evident qualitatively from the oblong appearance of the obturator foramina and the sacrococcygeal joint inferior to the pubic symphysis. The functional anteversion of the acetabular component in the AP view increased to 43° compared to 20° in the supine CT model in Figure 3a.
Images: Lipman JD and Esposito CI
We found a considerable increase in posterior pelvic tilt from the supine to standing position that was visible with the changes in the appearance of the obturator foramina and the sacrococcygeal-symphyseal distance; sagittal pelvic tilt was 20° posterior when standing. The resulting functional inclination and functional anteversion of the acetabular component were 32º and 43º when the patient was standing. Since sitting imaging was not obtained, a dynamic computer simulation was used to model collision of the proximal femur and acetabulum in deep flexion on a low stool. From a seated position, with an additional 52º of anterior pelvic tilt, there was implant-implant impingement (Figure 4A). Furthermore, modeling suggested that increasing cup inclination by approximately 10° would mitigate implant-implant impingement (Figure 4B). In this case, the patient had severe spine deformity and a significant change in pelvic tilt from supine to standing, which underscores the potential impact of the spine on pelvic mobility.
Figure 4. Dynamic 3-D computer simulations showed that there was superolateral implant-implant impingement in deep hip flexion when the patient was sitting (a). Computer modeling suggested that increasing acetabular inclination by 10° would prevent implant-implant impingement in the seated position (b).
Images: Lipman JD and Esposito CI
Based on our assessment, a decision was made to revise the acetabular component. Infection was ruled out preoperatively with serological testing and a right hip aspiration for cell count and culture.
Final Management
Two weeks after the second dislocation, the patient underwent revision of her acetabular component and femoral head (Figure 5). An imageless navigation system (AchieveCAS; Smith & Nephew Inc., Memphis, Tenn.) assisted acetabular component positioning. This system referenced the APP. A larger acetabular component was placed in order to accommodate a 36- mm femoral head, and supplemental screw fixation was used. Intraoperative navigation confirmed that an additional 10° of acetabular inclination was added to the position of the new component. Acetabular anteversion was decreased by 5° as a contingency, in case a constrained liner might be required in the future. Postoperatively, hip precautions were followed for 6 weeks.
The patient was ambulating without an assistive device at her latest follow-up. At nearly 1 year postoperatively from her revision, she has not had another dislocation. On exam, right hip flexion was 100°. At 90° of right hip flexion, internal rotation was 10° and external rotation was 45°. With the hip in extension, there were 10° of adduction and 30° of abduction.
Figure 5. Postoperative anteroposterior pelvis radiograph demonstrates approximately 45° of acetabular inclination. Articulation diameter was increased by revising the acetabular component and implanting a larger diameter femoral head (a). Acetabular anteversion measured approximately 20° on the postoperative crosstable radiograph(b).
Images: McLawhorn AS and colleagues
Recurrent THA dislocation is troubling for the patient and surgeon alike. Work-up requires an algorithmic approach, focusing on assessment of patient risk factors, implant alignment, abductor function, and sources of impingement. Although traditional, static, 2D radiographic views are the standard for initial assessment of THA component alignment, advanced, dynamic, 3D imaging may be necessary for a comprehensive assessment of the persistently unstable THA without gross implant malalignment or abductor dysfunction. If computer simulation is not available, plain radiographs of the patient in different functional positions can elucidate relative acetabular component orientations during normal activities, and axial imaging accurately determines absolute component version in relation to fixed reference planes, such as the APP. If the surgeon has experience with computer-assisted surgical techniques, they can be especially useful for precisely executing the preoperative plan in the setting of revision surgery. Failure rates after revision remain high, and contingencies for persistent instability should be considered during preoperative planning.
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For more information:
Alexander S. McLawhorn, MD, MBA; Christina I. Esposito, PhD; Joseph D. Lipman, MS; Seth A. Jerabek, MD; and Douglas E. Padgett, MD, can be reached at The Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021; McLawhorn’s email: mclawhorna@hss.edu; Esposito’s email: espositoc@hss.edu; Lipman’s email: lipmanj@hss.edu; Jerabek’s email: jerabeks@hss.edu; Padgett’s email: padgettd@hss.edu.
Disclosures: McLawhorn has no relevant financial disclosures; Esposito has no relevant financial disclosures; Lipman is an unpaid consultant for Exactech Inc. and Extremity Medical LLC, and receives royalties from Mathys Ltd. and Ortho Development Corp.; Jerabek is a paid consultant for Stryker Corp. and Mako Surgical Corp.; and Padgett is a consultant for Mako Surgical Corp., Stryker Corp., and Medical Compression Systems LLC, and he is a board member for the Hip Society, the Hospital for Special Surgery and the Journal of Arthroplasty.