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Tribological and Metal Ion Issues:
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 [Abstract] [Full text] [PDF]
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[Read Letter to the Editor] Dr. Fisher and colleagues respond to Mr. Jain
John Fisher, BSc, PhD, Sophie Williams, BEng, PhD; Ian Leslie, MEng, PhD; Graham Isaac, BSc, MSc; Zhongmin Jin, BEng, PhD; Eileen Ingham, BSc, PhD   (19 March 2009)
[Read Letter to the Editor] Tribology and Wear of Metal-on-Metal Hip Prostheses
Rohit Jain   (19 March 2009)

Dr. Fisher and colleagues respond to Mr. Jain 19 March 2009
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John Fisher, BSc, PhD,
Professor of Mechanical Engineering
Institute of Medical & Biological Engineering, University of Leeds, United Kingdom,
Sophie Williams, BEng, PhD; Ian Leslie, MEng, PhD; Graham Isaac, BSc, MSc; Zhongmin Jin, BEng, PhD; Eileen Ingham, BSc, PhD

Send letter to journal:
Re: Dr. Fisher and colleagues respond to Mr. Jain

j.fisher{at}leeds.ac.uk John Fisher, BSc, PhD, et al.

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.
Tribology and Wear of Metal-on-Metal Hip Prostheses 19 March 2009
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Rohit Jain,
Speciality Registrar, Trauma & Orthopaedics
Wrightington Hospital, United Kingdom

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Re: Tribology and Wear of Metal-on-Metal Hip Prostheses

rohitjain{at}hotmail.co.uk Rohit Jain

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.