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Early Failure of a Cross-Linked Polyethylene Acetabular LinerA Case Report
K. David Moore, MD1; Preston R. Beck, MS2; Donald W. Petersen, PhD2; John M. Cuckler, MD3; Jack E. Lemons, PhD2; Alan W. Eberhardt, PhD4
1 Division of Orthopaedic Surgery, University of Alabama at Birmingham, FOT 920, 510 20th Street South, Birmingham, AL 35294
2 Department of Prosthodontics and Dental Biomaterials, University of Alabama at Birmingham, SDB 616 (P.R.B. and D.W.P.) and SDB 615 (J.E.L.), 1919 7th Avenue South, Birmingham, AL 35294
3 Division of Orthopaedic Surgery, University of Alabama at Birmingham, 1709 Somerset Circle, Mountain Brook, AL 35213
4 Department of Biomedical Engineering, University of Alabama at Birmingham, Hoehn 370, 1075 13th Street South, Birmingham, AL 35294. E-mail address: aeberhar@uab.edu
View Disclosures and Other Information
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the National Institutes of Health (National Institute of Biomedical Imaging and Bioengineering, Bioengineering Research Partnership 1 RO1 EB001715-01A2) and the Division of Orthopaedic Surgery and the Department of Biomedical Engineering, University of Alabama at Birmingham. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Biomet, Inc., Warsaw, Indiana). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the University of Alabama at Birmingham, Birmingham, Alabama

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2008 Nov 01;90(11):2499-2504. doi: 10.2106/JBJS.G.01304
5 Recommendations (Recommend) | 3 Comments | Saved by 3 Users Save Case

Extract

The use of highly cross-linked ultra-high molecular weight polyethylene in total hip arthroplasty has become a popular alternative to the use of conventional polyethylene. Evaluations with use of hip simulator tests1-4 and clinical evaluations of total hip replacements5-10 have independently shown that polyethylene materials processed with gamma irradiation followed by melting or annealing have extremely low wear rates. However, it also has been demonstrated that such cross-linking processes may reduce the fracture toughness and resistance to fatigue crack propagation of polyethylene11-16. Bradford et al.14 found that retrieved cross-linked polyethylene acetabular liners exhibited surface cracking that had not been predicted by in vitro hip simulator studies. Tower et al.17 observed fatigue failure at the superior aspect of the rim of four retrieved highly cross-linked acetabular bearings. They concluded that the factors contributing to these failures included thin polyethylene at the cup rim, a relatively vertical cup alignment, and compromised material properties of the cross-linked polyethylene in comparison with conventional polyethylene. In the current case report, we describe the failure of a highly cross-linked polyethylene acetabular liner less than three years after implantation. We believe that the mechanisms of failure were fracture of the cross-linked polyethylene where it was thin along the locking groove and abnormal loading of the cup because of improper seating of the liner in the shell at the time of the original procedure. This study was conducted with institutional review board approval and in compliance with HIPAA (Health Insurance Portability and Accountability Act) requirements at our institution.
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    References

    Accreditation Statement
    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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    Alan W. Eberhardt, PhD
    Posted on December 19, 2008
    Dr. Eberhardt and colleagues respond to Dr. McGrory
    University of Alabama at Birmingham

    We thank the correspondents for their letter, and agree completely with the observation that many designs of acetabular inserts that use the “first generation” highly cross linked polyethylene are not optimal with regard to the mechanical characteristics and limitations of the material.

    However, in addition to the mechanical failure of the liner reported in our case report [1], we would like to indicate that the immediate post- operative X-ray showed substantial asymmetry of the femoral head within the acetabular component, far more than can be explained by the design of this particular implant (Figure 1a). The degree of asymmetry can only be explained by incomplete seating of the acetabular polyethylene.

    We believe this case is another example of the risks of limited incision, limited exposure surgery, and that the failure of the implant was precipitated by incomplete seating of the component inferiorly, where the greatest asymmetry was evident. We believe this resulted in progressive varus position of the liner against the acetabular shell, resulting in high stress concentration in the superior portion of the liner, and leading to crack initiation and subsequent fracture of the implant.

    Surgeons need to balance the potential advantages of a shorter incision against the necessity of careful insertion and assembly of modular implants, in addition to understanding the limitations of the materials and potential sources of complications.

    Reference

    1. K. David Moore, Preston R. Beck, Donald W. Petersen, John M. Cuckler, Jack E. Lemons, and Alan W. Eberhardt Early Failure of a Cross- Linked Polyethylene Acetabular Liner. A Case Report J Bone Joint Surg Am 2008; 90: 2499-2504.

    Brian J. McGrory, MD
    Posted on December 02, 2008
    Offset Crosslinked Liners: How Safe?
    Maine Medical Center

    To The Editor:

    I wish to comment on one point not adequately addressed in the recently published case report by Moore,et al.(1)

    The authors hypothesize that the polyethylene liner was initially not seated because of the eccentric center of rotation noted on the original post-operative radiograph (figure 1-A), and certain markings on the disassociated components noted at retrieval. I would like to point out that in this type of cup (36 mm ID, Trilogy, Zimmer) the cross-linked polyethylene liner is offset, with the polar thickness 3.5 mm greater than the thickness at the equator. It is therefore possible, if not probable, that the liner in this case was actually seated at the time of surgery.

    This pattern of increased polar thickness is common in many large internal diameter polyethylene components currently on the market, and in my view makes no sense. Yamaguchi, et al (2) have demonstrated that the wear vector in total hip replacements is 8.1 degrees lateral and 4.1 degrees posterior - directly toward the thinnest polyethylene in this acetabular polyethylene design. The increased polar thickness makes the polyethylene appear safer, but in fact makes it more dangerous.

    As the authors of the case report eloquently point out, crosslinked polyethylene components are at risk of fracture. I would add that in this design of polyethylene insert, namely one that has the center of rotation lateralized, and the thinnest portion of the plastic at the locking ring trough near the unsupported lateral polyethylene, early failure is of great concern.

    The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.

    References:

    1. K. David Moore, Preston R. Beck, Donald W. Petersen, John M. Cuckler, Jack E. Lemons, and Alan W. Eberhardt Early Failure of a Cross-Linked Polyethylene Acetabular Liner. A Case Report J Bone Joint Surg Am 2008; 90: 2499-2504

    2. Yamaguchi M, Hashimoto Y, Akisue T, Bauer TW. Polyethlene wear vector in vivo: a three-dimensional analysis using retrieved acetabular components and radiographs. J Orthop Res. 1999;17(5):695-702.

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