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Tribological and Metal Ion Issues   |    
Tribology and Wear of Metal-on-Metal Hip Prostheses: Influence of Cup Angle and Head Position
Sophie Williams, BEng, PhD1; Ian Leslie, MEng, PhD1; Graham Isaac, BSc, MSc, PhD1; Zhongmin Jin, BEng, PhD, CEng, MIMechE1; Eileen Ingham, BSc, PhD1; John Fisher, BSc, PhD, DEng, CEng, FIMechE, FIPEM1
1 Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail address for J. Fisher: J.Fisher@leeds.ac.uk
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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 DePuy International. 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 (DePuy International, Tissue Regenix, and BITECIC). Also, a commercial entity (DePuy International) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2008 Aug 01;90(Supplement 3):111-117. doi: 10.2106/JBJS.H.00485
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Abstract

Background: Clinical studies have indicated that the angular position of the acetabular cup may influence wear in metal-on-metal total hip bearings. A high cup angle in comparison to the anatomical position may lead to the head being constrained by the superior lateral surface and rim of the cup, thus potentially changing the location of the contact zone between the head and the cup. The aim of this study was to test the hypothesis that both a steep cup angle and a lateralized position of the head can increase head contact on the superior rim of the cup, with the consequence of increased wear.

Methods: Hip-joint simulator studies of metal-on-metal bearings were undertaken with cup angles of 45° and 55°. The femoral head was either aligned to the center of the cup or placed in a position of microlateralization. Wear was measured gravimetrically over 5 million cycles.

Results: A steep cup angle of 55° showed significantly higher long-term steady-state wear than a standard cup angle of 45° (p < 0.01). The difference was fivefold. Microlateralization of the head resulted in a fivefold increase in steady-state wear compared with a centralized head. The combination of a steep cup angle and a microlateralized head increased the steady-state wear rate by tenfold compared with a standard cup angle with a centralized head.

Conclusions: These studies support the hypothesis that both an increased cup angle and a lateral head position increase wear in metal-on-metal hip prostheses.

Clinical Relevance: These experimental studies support the recent clinical observations of increased wear with increased cup angle. Furthermore, the experimental studies indicate that the position and alignment of the femoral head may also be an important determinant of wear in vivo.

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    References

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    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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    John Fisher, BSc, PhD
    Posted on March 11, 2009
    Dr. Fisher and colleagues respond to Mr. Jain
    Institute of Medical & Biological Engineering, University of Leeds, United Kingdom

    Thank you for the interest and attention that you have paid to our paper. We are delighted to offer the following responses to the specific questions you have raised about our study:

    Regarding your comments about construct validity, previous studies under standard conditions have demonstrated that even n=3 is sufficient to observe statistical differences. Sample sizes of between n=3 and n=5 have been used in simulator studies over the last ten years (1,2). However as seen in this study, when simulating more extreme conditions particularly with microseparation and rim loading, this type of instability can produce a greater variation in the data set.

    Regarding your comments about controlling for micro lateralization, this method has been previously published (3,4 for ceramic-on-ceramic bearings) and validated against clinical retrievals in terms of volumetric wear, wear scar geometry, wear mechanism and debris distribution. Full details of the simulator method can be found in these source references. There is growing recognition that microseparation and superior rim loading, which may be produced by mal-positioned components, soft tissue laxity and offset deficiency has a significant effect on clinical wear rates. We postulate that in surface replacements,lack of flexibility in restoration of the offset, makes this surgical intervention particularly susceptible to elevated rim wear especially with combination with steel cup angles.

    Regarding your comment that "The paper also fails to explain why a different mounting of the head was tested with a different number of cycles in study 3." - Studies 1 and 2 tested 28mm metal-on-metal total hip replacements and study 3 a larger diameter (39mm) surface replacement metal on metal bearing. In the surface replacement bearing there is clearly no femoral stem or spigot on which to mount the head in a surface replacement, so a simulated femoral neck and head were created for purposes of wear simulation testing. Clearly, the configuration of the head mounting may have an effect on the biomechanics and wear during microseparation. This is just one of several variables that we are currently investigating under microseparation and rim loading conditions in further studies, (which include a wider range of head sizes, wider range of cup angles, cup version angle, as well as design and metallurgy variables and comparison with ceramic-on-ceramic bearings). This multi-variable study, which is supported by the Furlong Charitable Research Foundation, will involve over 200 million cycles of simulator testing and will last until 2012. The results of the effect of individual and combined variables will be published as the study progresses. We believe these are important findings which may influence the future clinical use of surface replacements and which have not been previously reported, and therefore need to be published as they are generated.

    You state that "No confidence intervals were given for the main results." - Confidence limits (95%) are shown in Figures 3, 4 and 5 and in Table 1.

    Regarding your comment about internal validity of the study, measurements to assess wear involve quantitative (gravimetric assessment of wear) and therefore observer bias is avoided. It is not normal to blind experiments in laboratory studies with quantitative data. All simulator studies are carried out under strict quality assurance conditions in an ISO accredited laboratory. Over the last decade over one billion cycles of hip simulator studies have been under taken under strictly controlled standardized conditions and published in peer reviewed papers and abstracts.

    You state that "Study groups 2 & 3 are not matched and therefore cannot be compared." - See above, because other variables as well as head size change between study groups 2 and 3, such as method of mounting of head, this could also have contributed to the difference in the wear results. Of course, clinically, these are relevant differences between a large head surface replacement and a smaller head total joint. This is a “first in the world“ simulation study of effect of microseparation on surface replacement and further investigation of a wider range of variables is currently on going and will be published shortly. Recent results published during the last year confirm that the microseparation as well as cup angle is a critical determinant of wear in surface replacement metal-on-metal hips (5).

    We fully recognize that the study fails to include the effects of version angle which may affect the contact mechanics; this factor will be investigated in the present ongoing work.

    References

    1. Barbour PS, Stone MH, Fisher J. A hip joint simulator study using simplified loading and motion cycles generating physiological wear paths and rates. Proc Inst Mech Eng [H]. 1999;213;455-67.

    2. Firkins PJ, Tipper JL, Ingham E, Stone MH, Farrar R, Fisher J. Influence of simulator kinematics on the wear of metal-on-metal hip prostheses. Proc Inst Mech Eng [H]. 2001;215;119-21.

    3. Nevelos J, Ingham E, Doyle C, Streicher R, Nevelos A, Walter W, Fisher J. Microseparation of the centers of alumina-alumina artificial hip joints during simulator testing produces clinically relevant wear rates and patterns. J Arthroplasty. 2000;15;793-95.

    4. Stewart T, Tipper J, Streicher R, Ingham E, Fisher J. Long-term wear of HIPed alumina on alumina bearings for THR under microseparation conditions. J Mater Sci Mater Med. 2001;12;1053-56.

    5. Fisher J. Tribology of hard on hard bearings under adverse conditions. In: Proceedings of the Institution of Mechanical Engineers, Conference on Bearing Surfaces in Hip Replacement; 2008; London.

    Rohit Jain
    Posted on February 28, 2009
    Tribology and Wear of Metal-on-Metal Hip Prostheses
    Wrightington Hospital, United Kingdom

    To the Editor:

    I read the paper by Williams et al. (1) with great interest. Although the limitations of the study are well recognized and discussed by the authors,I would like to draw attention to the following points:

    Construct Validity: The size of the sample in each study has not been justified. There are no power calculations available. The authors fail to explain how micro lateralization was controlled through 2 million cycles of wear. The paper also fails to explain why a different mounting of the head was tested with a different number of cycles in study 3. No confidence intervals were given for the main results.

    Internal Validity: There is no blinding attempted to address the observer bias. Study groups 2 & 3 are not matched and therefore cannot be compared.

    The authors recognize that the study fails to include the effects of version angle which clinically may affect the radiographic appearance of inclination & contact mechanics.

    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.

    Reference

    1. Sophie Williams, Ian Leslie, Graham Isaac, Zhongmin Jin, Eileen Ingham, and John Fisher. Tribology and Wear of Metal-on-Metal Hip Prostheses: Influence of Cup Angle and Head Position. J Bone Joint Surg Am 2008; 90: 111-117.

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