The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 94, Issue 9

Scientific Articles | May 02, 2012

Spontaneous Dissociation of Offset, Face-Changing Polyethylene Liners from the Acetabular Shell: A Report of Four Cases

Chancellor F. Gray, MD¹; Ryan E. Moore, MD, PhD¹; Gwo-Chin Lee, MD¹

¹ Department of Orthopaedic Surgery, Hospital of the University of Pennsylvania, 2 Silverstein Building, 3400 Spruce Street, Philadelphia, PA 19104. E-mail address for G. Lee: gwo-chin.lee@uphs.upenn.edu

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Investigation performed at Penn Presbyterian Medical Center, Philadelphia, Pennsylvania

J Bone Joint Surg Am, 2012 May 02;94(9):841-845. doi: 10.2106/JBJS.K.00506

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Abstract

Background:

Dissociation of the polyethylene liner from the acetabular shell is an uncommon event. Offset, face-changing polyethylene liners theoretically increase femoral head coverage, allow for larger heads with smaller cup diameters, and offer improved stability without risk of impingement. However, we present four cases of liner dissociations from the acetabular shell that necessitated revision hip surgery.

Methods:

Four patients with spontaneous dissociations of offset, face-changing polyethylene acetabular liners underwent revision hip arthroplasty between January 2007 and June 2010. All patients were women with an average age of fifty-three years. All cases involved the combination of a Pinnacle acetabular component with an offset, face-changing polyethylene liner. We reviewed the presenting signs and symptoms associated with liner dissociations and analyzed the radiographs for the acetabular component position.

Results:

All cases of liner dissociation occurred without direct trauma and were associated with squeaking sounds reported by the patient. The average cup abduction angle was 55° (range, 51° to 60°) and the average anteversion was 21.5° (range, 17° to 24°). Examination of the dissociated liners at the time of revision surgery revealed plastic deformation of the polyethylene liner rim posterosuperiorly at the eleven o’clock position on the right hip and the one o’clock position on the left hip.

Conclusions:

The combination of a vertically positioned acetabular component and an offset, face-changing liner resulted in impingement and dissociation of the liner from the shell in four patients. Proper acetabular component positioning, rather than the routine use of uniquely designed polyethylene liners, is critical to maximizing implant stability in total hip arthroplasty.

Level of Evidence:

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion

Liner dissociation from the acetabular shell in total hip arthroplasty is an uncommon mode of failure. This complication has been described with the Harris-Galante Porous I and II acetabular components (Zimmer, Warsaw, Indiana) and has been attributed to the failure of the locking mechanism, which functions to secure the polyethylene liner to the metal acetabular shell^1-5. A weak locking mechanism in the setting of impingement of the femoral component and liner micromotion can result in liner deformation, rim wear, and dissociation and failure of these implants^1-5. Liner dissociations have also been described with alternative bearings such as the Metasul metal-on-metal liners (Zimmer, Warsaw, Indiana)^6-8 and the Trident tripolar constrained liner (Stryker, Mahwah, New Jersey)⁹. These cases have also been attributed to impingement of the femoral component on the liner.

Offset polyethylene liners are available in a variety of offsets and various degrees of face-change, for use in both primary and revision total hip arthroplasty¹⁰. The theoretical advantages of offset, face-changing liners include increased femoral head coverage and increased polyethylene thickness at smaller cup diameters, allowing for the use of larger femoral heads. In addition, the surgeon can use the face-change to make fine adjustments to the position of the liner within the shell to improve version and decrease impingement, thus improving hip stability. Prior to the introduction of these liners, the only way a surgeon could gain stability once the acetabular shell had been implanted was to either revise the position of the cup or use an elevated-rim or hooded liner to improve femoral head coverage. Although the use of elevated-rim polyethylene liners can increase hip stability^11,12 and decrease hip dislocation¹³, the use of elevated-rim liners can cause impingement¹⁴ and thus lead to increased stresses at the cup locking mechanism.

Offset, face-changing liners differ from elevated-rim or hooded liners. First, although portions of the polyethylene liner overhang the metal acetabular shell in both hooded and offset, face-changing liners, the addition of offset theoretically increases hip motion prior to impingement and stability by lateralizing the center of rotation of the hip below the equator of the cup. Second, because the apex of the bore is positioned eccentric to the center of the acetabular shell, the internal version of the cup is modified. The face-change theoretically allows the surgeon to make fine adjustments (of up to 10°) to the ultimate effective cup version without changing the version or abduction angle of the implanted metal shell.

We present four cases of spontaneous dissociations of offset, face-changing polyethylene liners and review the clinical presentations, radiographic analysis, and intraoperative findings at the time of revision surgery for each case. We describe impingement as the primary mode of failure and caution against relying on offset, face-changing liners to address failures in surgical technique.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion

Patients and Surgical Procedure

Four female patients with dissociation of an offset, face-changing polyethylene liner underwent revision total hip arthroplasty between January 2007 and June 2010. The average patient age was fifty-three years (range, thirty-three to seventy-one years) and the average time to revision following the index procedure was 13.1 months (range, three to thirty-six months). All patients had had primary total hip arthroplasty with a Pinnacle acetabular component (DePuy, Warsaw, Indiana) and a +4-mm offset, 10° face-changing liner, which is designed to lock into the shell by means of a variable interference fit with an inclined taper mechanism; six tabs within the shell and polyethylene provide rotational stability. The liner itself has not been shown to have a higher than expected failure rate¹⁵.

Three hip arthroplasties were performed through a mini-posterolateral approach to the hip and one was performed through an anterolateral approach. At the time of the index procedure, the goals for acetabular inclination and anteversion were 45° and 20°, respectively. The liners were placed with the apex of the 10° face-change oriented posterosuperiorly (eleven and one o’clock for right and left hips, respectively) in the hips that were treated through the posterolateral approach. In the right hip that was treated through the anterolateral approach, the location was placed anterosuperiorly (one o’clock) on the basis of surgeon preference at the time of the index procedure. This position was chosen on the basis of the surgical approach, as a posterosuperior buildup has been previously reported to increase stable hip motion in patients in whom the arthroplasty has been performed through a posterior approach¹². By orienting the apex of the face-change posteriorly, however, the primary effect was on anteversion rather than abduction or lateral coverage. The effect of the liner in the anterior approach was to effectively neutralize the version of the cup.

Prior to closure, all hips were assessed for hip range of motion, stability, and impingement. For arthroplasties performed through a posterior approach, hip stability (i.e., no instability or impingement) was determined at 90° of flexion, adduction, and a minimum 50° of internal rotation, whereas for arthroplasties performed through an anterolateral approach, stability was determined at extension and full external rotation of the hip. Despite this examination, a complete assessment of the two positions at risk for impingement, namely (1) extension and external rotation and (2) flexion, adduction, and internal rotation, was not performed for all patients.

All four patients consented to the presentation of their cases. The patient medical records, intraoperative findings, and pre-revision and post-revision radiographs were reviewed. Acetabular component orientation (abduction and anteversion) was measured from an anteroposterior radiograph of the pelvis with use of methods previously described by Wan et al.¹⁶. Changes in patient limb length and offset from the index procedure were also measured with use of techniques previously described by Della Valle et al.¹⁷ and Knight and Atwater¹⁸. The average duration of follow-up in the present study was twenty-seven months (range, sixteen to fifty-four months).

Case Presentations

The first patient (Case 1) was a fifty-seven-year-old woman who had undergone left total hip arthroplasty through a mini-posterolateral approach for the treatment of osteoarthritis of the hip. At 10.5 months postoperatively she noted painless squeaking of the hip during normal walking without history of trauma. Radiographs revealed that the polyethylene liner was dissociated from the acetabular component. Cup angles were measured as 54° of abduction and 24° of anteversion. At the time of revision surgery, the liner was plastically deformed posterosuperiorly at the one o’clock position in the left hip. A 50-mm acetabular shell was revised to a 54-mm acetabular shell at 33° of abduction and 25° of anteversion. The femoral component was judged to be in 10° of anteversion. A +4-mm offset, 10° face-changing polyethylene liner was inserted into the metal shell. At the time of the latest follow-up at eighteen months postoperatively, the patient was doing well.

The second patient (Case 2) was a seventy-one-year-old woman who had undergone right total hip arthroplasty through a mini-posterolateral approach for the treatment of osteoarthritis of the hip. At three months postoperatively she reported a sudden onset of hip pain and squeaking. She reported that she had not had any preceding trauma and was able to walk without difficulty. Radiographs revealed that the liner was dissociated from the cup. Cup angles were measured as 51° of abduction and 22° of anteversion. At the time of revision surgery the liner was deformed posterosuperiorly at the eleven o’clock position in the right hip. A 52-mm shell was revised to a 54-mm shell at 39° of abduction and 24° of anteversion. The femoral component was in approximately 15° of anteversion. A +4-mm offset, 10° face-changing polyethylene liner was inserted into the cup. At fifty-four months postoperatively, the patient was doing well.

The third patient (Case 3) was a fifty-two-year-old woman who had undergone left total hip arthroplasty through a mini-posterolateral approach for the treatment of osteoarthritis of the hip. At thirty-six months postoperatively she reported the sudden onset of hip pain and squeaking during normal walking. Radiographs revealed an eccentric position of the femoral head, which suggested liner dissociation. Cup angles were measured as 60° of abduction and 23° of anteversion. At the time of revision surgery, the liner had plastic deformation in the posterosuperior aspect at approximately the one o’clock position in the left hip. A 52-mm acetabular shell was revised to a 56-mm acetabular shell. The femoral component was anteverted at 10° and was not revised. A +4-mm offset, 10° face-changing polyethylene liner was inserted into the shell. A 32-mm ceramic ball head was revised to a 36-mm metal ball head. At the time of the latest follow-up at sixteen months postoperatively, the patient reported no pain.

The fourth patient (Case 4) was a thirty-three-year-old woman who had undergone a right total hip arthroplasty through an anterolateral approach for the treatment of osteonecrosis of the femoral head. This patient had a substantial lengthening (34 mm) of the involved limb at the time of the index procedure in order to equalize leg length discrepancy due to extensive collapse of the femoral head preoperatively. At three months postoperatively, the patient reported a popping sensation following physical therapy and reported a sense of instability without frank dislocation. She did not report any trauma. The patient was able to walk without pain or limp but reported the onset of squeaking in the hip. Radiographs revealed that the polyethylene liner was dissociated from the cup. Cup angles were measured as 55° of abduction and 17° of anteversion. At the time of revision surgery, the liner was deformed anterosuperiorly at the one o’clock position in the right hip. A 50-mm shell was revised to a 52-mm shell. The femoral component was anteverted at 10° and was not revised. A neutral liner was inserted into the cup, and a 32-mm ceramic ball head was exchanged for a 32-mm metal ball head. At the time of the latest follow-up at twenty months postoperatively, the patient was doing well.

Source of Funding

No external funding sources were used in performing this study.

Results

Introduction | Materials and Methods | Results | Discussion

Clinical and Intraoperative Findings

The sudden onset of squeaking in the hip region was the only consistent symptom experienced by all patients. Two patients reported vague, nonlocalized hip pain, and two patients had no pain. No patient had a noticeable limp at presentation. All patients were able to bear full weight. Intraoperative findings were similar in all patients and included plastic deformation of the polyethylene liner at the posterosuperior position of the three hips that were treated through a posterior approach (that is, at the eleven o’clock position for the left hips and at the one o’clock position for the right hip) and at the anterosuperior (one o’clock) position in the right hip that was treated through an anterolateral approach (Fig. 1). Additionally, there were signs of metal transfer to the ceramic head and signs of metallosis in the hip joint. None of the acetabular components were overtly damaged or loose, and there was no obvious damage to the locking mechanism. All four patients underwent revision of the acetabular component to address the dissociated polyethylene liner. Three patients had revision to another offset, face-changing liner, primarily because of surgeon concern for stability, despite excellent cup position and intraoperative stability. One patient had revision to a neutral liner.

Fig. 1

Intraoperative photograph showing the retrieved offset, face-changing polyethylene liner from the left hip, revealing plastic deformation at the apex position of the face-change, at approximately the one o’clock position. The twelve o’clock position on the liner is denoted by the long white arrow, and the six o’clock position is represented by the shorter white arrow. The deformed liner edge is denoted by the black arrows. Blood is seen embedded within the walls of the cup.

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Radiographic Findings

Radiographic findings were similar in all four patients. All radiographs demonstrated eccentric placement of the femoral head within the metal shell with displacement in the superolateral direction (Fig. 2). The average cup abduction angle was 55° (range, 51° to 60°). The average anteversion of the cup was 21.5° (range, 17° to 24°).

Fig. 2

Anteroposterior radiograph showing the left hip of the third patient (Case 3) prior to the time of revision surgery, revealing superolateral displacement of the femoral head within the acetabular shell, implying liner dissociation.

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Discussion

Introduction | Materials and Methods | Results | Discussion

Dissociation of the polyethylene liner from the acetabular shell is an uncommon failure mode for total hip arthroplasty. Historically, these complications have resulted from the combination of poor locking mechanisms and impingement of the femoral component against the acetabular cup. In recent years, offset polyethylene liners with a face-change have been available for use in primary total hip arthroplasty. Despite advantages in femoral head size compatibility, head coverage, and theoretically improved distance to impingement compared with that of a hooded liner, these liners can promote impingement leading to dissociation if the liners are inserted in excessive abduction. We reported a series of four patients with spontaneous dissociations of offset, face-changing liners and cautioned against the routine use of these components to maximize hip joint stability as a substitute for obtaining proper cup orientation in primary total hip replacement.

The use of offset, face-changing liners has some potential disadvantages. The eccentric loading pattern increases torsional forces at the liner-shell-bone interfaces, which is also worsened by increasing the abduction angle. Giori reported that an optimally aligned acetabular offset liner increases torsion at the shell-bone interface by 3.9 Nm in normal walking, which increases to 6.3 Nm if an extra 20° of abduction occurs¹⁹. All patients in our series had excessive abduction of the acetabular component. Furthermore, an increased rate of acetabular component loosening has been reported with the use of offset liners. Archibeck et al. found an aseptic loosening rate of 4.2% in their extended offset acetabular group of patients who had undergone primary total hip arthroplasty, compared with 0.12% in a standard offset group, a thirty-five-fold increase²⁰.

It is incumbent on the surgeon to examine the reconstructed hip in the operating room for evidence of impingement. With the lower limb of the patient in maximum extension and external rotation, the surgeon should use the index finger to feel if the lesser trochanter impinges against the ischium; the greater trochanter, against the pelvis; and the metal femoral neck, against the polyethylene or metal rim. Retrieval studies have shown that the most common site of impingement of the metal neck against the polyethylene is the ten o’clock position in a right hip, which occurs during the extension of the hip during normal gait¹⁴. This finding was similar to the position of impingement identified in our series. Likewise, the hip should be examined at 90° of flexion and maximum internal rotation (usually 45° to 50°). In this position the surgeon index finger should again be able to pass between the greater trochanter and pelvis and the metal femoral neck and polyethylene or metal rim of the cup. Clearance should be maintained when the hip is further adducted. Both of these at-risk positions should be examined intraoperatively, despite the surgical approach used.

If this avoidance of impingement is accomplished with the hip reconstruction and the hip is stable throughout this impingement testing, there is no need for any uniquely designed polyethylene. Dislocation is avoided best by correct component positioning, which means knowledge of the femoral anteversion so that the acetabular anteversion is mated to it, and by inclination of the acetabular component at ≤45°. Osseous impingement is avoided by correct offset and hip length.

In conclusion, the combination of a vertically positioned acetabular component and an offset, face-changing liner resulted in impingement and dissociation of the liner from the shell in four patients. Proper acetabular component positioning, rather than the routine use of uniquely designed polyethylene liners, is critical to maximizing implant stability in total hip arthroplasty.

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Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, no author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

Topics

hip joint ; hip replacement arthroplasty ; polyethylene ; dissociation ; abduction

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Chancellor F. Gray, Ryan E. Moore, Gwo-Chin Lee; Spontaneous Dissociation of Offset, Face-Changing Polyethylene Liners from the Acetabular ShellA Report of Four Cases. The Journal of Bone & Joint Surgery. 2012 May;94(9):841-845.

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