The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 90, Issue Supplement 3

Tribological and Metal Ion Issues | August 01, 2008

Design Considerations and Life Prediction of Metal-on-Metal Bearings: The Effect of Clearance

Michael A. Tuke, HNC, ME¹; Gareth Scott, FRCS²; Anne Roques, PhD¹; Xiao Q. Hu, PhD¹; Andrew Taylor, PhD¹

¹ Finsbury Development Limited, 13 Mole Business Park, Randalls Road, Leatherhead KT22 0BA, Surrey, United Kingdom. E-mail address for A. Roques: anne.roques@finsbury.org
² Orthopaedic Department, Royal London Hospital, Whitechapel Road, Whitechapel E1 1BB, London, United Kingdom

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. 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 (Finsbury). 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.

The Journal of Bone and Joint Surgery, Inc.

J Bone Joint Surg Am, 2008 Aug 01;90(Supplement 3):134-141. doi: 10.2106/JBJS.H.00610

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Abstract

Background: Clinical observations suggest that metal-on-metal arthroplasties that have been implanted for more than twenty years do fail. It is proposed that there are not two, but three distinct phases of wear life for any metal-on-metal implant system: bedding-in, steady state, and end point. In this study, we asked two questions: can we explain late failure due to wear, and will there be a late failure mechanism due to a change in the frictional torque?

Methods: In order to characterize wear failure, an analysis was made of five retrieved metal-on-metal couples that were mapped with use of a roundness machine. A geometrical model was developed on the basis of these observations, and wear at the end point was calculated. The literature on first-generation metal-on-metal implants retrieved for aseptic loosening was reviewed to assess the agreement with the retrieval findings as well as the wear model.

Results: A wear patch of an appreciable and constant size could be measured in all five retrieved couples. The end point of revision was observed to occur when the wear progression reached a contact area corresponding to approximately 75% of the projected diameter of the ball. The wear volume was calculated from the geometry. The available literature describing the wear characteristics of retrieved bearings after successful clinical use showed good agreement with the calculated wear model.

Conclusions: During the implant life of long-term successful metal-on-metal devices, a wear patch develops, as evident from retrieved failed devices. Failure often occurs through loosening, and the observed wear patch is similar in size for devices measured by us and for those described in the literature. We hypothesized that failure by loosening occurs through the accumulation of wear, which eventually leads to high friction within the bearing and increased torsional forces across the joint and its fixation.

Clinical Relevance: This study will lead to a better understanding of long-term failure of first-generation metal-on-metal devices, which may lead to improvements in design and increased survival for more modern devices.

Figures in this Article

Introduction

The low rates of wear and excellent survivorship reported for some historical metal-on-metal total hip prostheses, such as the Ring and the McKee-Farrar prosthesis^1-3, stimulated the reintroduction of metal-on-metal bearings in the early 1990s for hip resurfacing and large-diameter modular-head devices. The success of the early implants depended on a number of factors, including the presence of an ideal clearance (diametral clearance of approximately 120 to 200 µm) between the articulating components¹, which has been utilized in many modern devices.

Retrieval analysis⁴, simulator studies^5-16, and mathematical models^17,18 are widely available to demonstrate two phases of wear representing the life cycle of the device. There is an initial fast-wear phase, termed running-in, followed by a phase of wear at a lower rate, termed the steady state. However, a failure, often due to component loosening, after twenty-five to thirty years of successful life¹, demonstrates that there may be a third phase—termed the end point—in the life of a metal-on-metal device. This ultimate failure is currently neither predicted by lubrication theories^10,19,20 nor simulated experimentally. We hypothesized that an eventual loosening failure mode can be explained by progressive wear and increased friction.

A likely consequence of increased friction at the bearing surface is an increase in torsional stresses at the implant-bone interface, which will be added to in vivo forces (such as torque forces in retroversion when walking, climbing stairs, and getting up out of a chair). The increased friction in the bearing may therefore result in seizing of the bearing. In addition, the increased friction particle load may lead to osteolysis with eventual loosening of the prosthesis. Therefore, consideration must be given to the conflict of achieving acceptable running-in rates of wear while maximizing the time that leads to too-high friction, which decreases prosthetic longevity.

To test this failure model, we studied the geometrical characteristics of long-term clinically successful metal-on-metal devices that failed after many years of use. These characteristics were derived from measurements made on retrieved devices and an analysis of the literature on wear of historical metal-on-metal devices. A simple geometrical model of the observations was developed. This allowed for wear-volume calculations, and the model was further validated against data from the literature.

Materials and Methods

Wear patches were measured on five retrieved metal-on-metal bearing couples that had been in successful use for more than seven years (range, seven to thirty-four years). The measurements were performed with a Series 211 RA 300/400 Round test machine (Mitutoyo, Kawasaki, Japan). This apparatus is analogous to a surface profilometer that can trace the surface profile of nonlinear shapes to a resolution of 0.01 µm and a magnification power of up to 50,000 times. The wear measurement method is illustrated in Figure 1.

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Fig. 1: Polar roundness measurement setup for a head (left) and a cup (right).

The wear patch was characterized on the basis of the maximum depth of wear, measured as the maximum linear deviation of the worn surface from the original surface (derived from the unworn part of the surface). Typical wear maps for a head and acetabular component are shown in Figure 2. By taking a minimum of five traces longitudinally around the spherical shape, the location of the wear patch could be marked on the surface. The wear patch radius and the initial component radius were used to express the size of the wear patch as an angle (the semi-angle from the apex of the sphere, also called the contact angle), as illustrated in Figure 3. These measurements were supplemented by the linear wear rate^1,21-23 and by contact patch measurement data²⁴ as reported in the literature.

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Fig. 2: Typical wear patches (left) and wear maps for head (top) and acetabular component (bottom). The example shown is not included in this study.

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Fig. 3: Illustration of the mathematical model of wear. RH = head radius, RC = cup radius, Rwear patch = wear patch radius.

The literature on historical metal-on-metal implant retrieval analysis was reviewed. The wear results of fifty-seven clinically retrieved implants of various first-generation metal-on-metal bearings all having exceeded nine years of survival before revision due to aseptic loosening were reviewed with particular attention to the wear-patch characteristics. The review of the literature was performed with use of a PubMed search engine and the following key words: hip AND Ring AND prosthesis, hip AND Metasul AND prosthesis, hip AND McKee. For the simulator studies, the search was performed using the PubMed search engine with the following key words: hip AND simulator AND metal.

The findings from the retrievals and the literature review were used to develop a mathematical model to obtain the total wear volume generated before failure occurred (see Appendix). Based on the results of measuring the retrieved components, the assumption made was that the worn volume is generated by the head wearing into the cup while the cup wears into the head. This generates a spherical cap of worn material as illustrated in Figure 4.

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Fig. 4: Illustration of the growth of the wear patch during metal-on-metal articulating surface wear. Left, end of bedding-in phase. Right, end of steady-state phase.

The calculation of the wear volume at failure for a given implant is based on the knowledge of the original radius of the head, the original radial clearance between components, and the contact angle at failure. The head size and clearance are design parameters; the contact angle at failure is obtained from retrieval measurements, as previously described (see Appendix).

Since linear wear and initial radial clearance are related, it is possible to calculate the congruency angle at failure. This was done for various metal-on-metal bearing couples that had been implanted for not less than nine years and for which published clinical linear wear results were available^1,13,25.

Results

For the retrieved coupled components, the corresponding cup and head wear patches were worn into a matching pair of new, almost spherical surfaces. The position of the wear patch center ranged from 10° to 15° from the vertical²⁶. For the head, the radius of the wear patch was slightly increased compared with the initial radius, and, for the acetabular component, the radius was slightly decreased (Fig. 3). The measurements are summarized in Table I. These show that the worn contact surface patch with abrasive changes reached almost 75% of the articulating diameter at the time of failure, equating to a mean semi- angle of 51° (range, 47° to 58°) (Figs. 2 and 3). These measurements are also similar to what has been reported in the literature (Table II).

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TABLE I Measurement Results for the Five Retrieved First-Generation Metal-on-Metal Prostheses as Measured by the Authors

	Device Type	Age at Time of Surgery (yr)	Years of Survival	Indication for Revision	Bore Radius (mm)	Diametral Clearance (mm)	Total Linear Wear (µm) (Head + Cup)	Contact Angle (deg)	Calculated Linear Penetration (µm)	Calculated Total Wear Volume (mm³)
1	Ring	50	20	Cup loosening	17.35	0.2	51	47	93	28
2	McKee-Farrar	Not known	20	Cup loosening	17.40	0.25	84	53	82	31
3	Müller	54	11	Cup loosening	18.40	0.35	146	58	155	77
4	McKee-Farrar	61	7	Cup and stem loosening	17.40	0.31	98	52	97	35
5	Ring	Not known	34	Stem fracture	20.60	0.2	30	47	93	40

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TABLE I Measurement Results for the Five Retrieved First-Generation Metal-on-Metal Prostheses as Measured by the Authors

	Device Type	Age at Time of Surgery (yr)	Years of Survival	Indication for Revision	Bore Radius (mm)	Diametral Clearance (mm)	Total Linear Wear (µm) (Head + Cup)	Contact Angle (deg)	Calculated Linear Penetration (µm)	Calculated Total Wear Volume (mm³)
1	Ring	50	20	Cup loosening	17.35	0.2	51	47	93	28
2	McKee-Farrar	Not known	20	Cup loosening	17.40	0.25	84	53	82	31
3	Müller	54	11	Cup loosening	18.40	0.35	146	58	155	77
4	McKee-Farrar	61	7	Cup and stem loosening	17.40	0.31	98	52	97	35
5	Ring	Not known	34	Stem fracture	20.60	0.2	30	47	93	40

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TABLE II Congruency Angle Data Obtained from the Literature and Comparison with the Present Results

Reference	Stem Type	Head Diameter (mm)	Years of Follow-up	Average Linear Wear (µm)	Number of Bearing Couples	Congruency Angle (deg)
Schmidt et al.¹³	Müller	42	9 to 25	69 (head + cup)	10	53 (calculated)
Semlitsch et al.²⁵	Müller	40	10 to 20	56 (head + cup)	8	50 (calculated)
McKellop et al.¹	McKee-Farrar	34.9 to 41.3	20 to 24	67 (head + cup)	5	53 (calculated)
Howie et al.²⁴				13.2 (heads);
				16.2 (cups)
	McKee-Farrar (Vinertia)	Not specified	11 to 22		6	52.5 (measured)
	McKee-Farrar (Howmedica)	Not specified	9 to 25		11	60.1 (measured)
	McKee-Farrar (Downs)	Not specified	10 to 24		3	51.0 (measured)
	McKee-Farrar (Zimmer)	Not specified	12 to 18		4	67.5 (measured)
Present study	McKee-Farrar/Müller/Ring	34 to 41	7 to 35	23 (heads); 27 (cups)	55	48 (heads, measured); 54 (cups, measured)

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TABLE II Congruency Angle Data Obtained from the Literature and Comparison with the Present Results

Reference	Stem Type	Head Diameter (mm)	Years of Follow-up	Average Linear Wear (µm)	Number of Bearing Couples	Congruency Angle (deg)
Schmidt et al.¹³	Müller	42	9 to 25	69 (head + cup)	10	53 (calculated)
Semlitsch et al.²⁵	Müller	40	10 to 20	56 (head + cup)	8	50 (calculated)
McKellop et al.¹	McKee-Farrar	34.9 to 41.3	20 to 24	67 (head + cup)	5	53 (calculated)
Howie et al.²⁴				13.2 (heads);
				16.2 (cups)
	McKee-Farrar (Vinertia)	Not specified	11 to 22		6	52.5 (measured)
	McKee-Farrar (Howmedica)	Not specified	9 to 25		11	60.1 (measured)
	McKee-Farrar (Downs)	Not specified	10 to 24		3	51.0 (measured)
	McKee-Farrar (Zimmer)	Not specified	12 to 18		4	67.5 (measured)
Present study	McKee-Farrar/Müller/Ring	34 to 41	7 to 35	23 (heads); 27 (cups)	55	48 (heads, measured); 54 (cups, measured)

When the head radius, diametral clearance, and contact angle measured on the five retrieved couples were input into equations (1) and (2) (see Appendix), the linear penetration and total wear volume could be calculated. Good agreement was observed between the calculated linear penetration and the measured linear wear.

The critical contact angle at failure that was derived from the retrieved components from the literature review was approximately 50°, regardless of head size (Table II). This is in good agreement with the values physically measured by us.

The literature on wear rates obtained both from simulators and retrieval analysis studies^{1,5-16,21-23,27,28} is shown in a table in the Appendix. The linear wear rates from retrieval studies reported in that table are in good agreement with those calculated in the present study (range, 0.8 to 14 µm per year). The volumetric wear rate ranged from 1.1 to 7 mm³ per year, in good agreement with reported clinical wear rates. It is important to note that the steady-state rate of wear appears independent of the starting clearance.

Discussion

The evidence gathered from retrieval observations and simulator studies suggests that, for any given pair of components of whatever clearance, metallurgy, or activity level of the patient, the formation of a penetrating congruent wear patch will occur as a result of a period of bedding-in wear plus the duration of the steady-state wear. An understanding of what is likely to happen afterward lies in observations of successful metal-on-metal historical devices (failed due to aseptic loosening after many years of use). Although variations are likely due to factors such as patient sensitivity, component alignment, loading, infection, or patient death, the eventual failure appears to be related to the size of the wear patch, which corresponds to a proportion of the articulating diameter. The wear life of metal-on-metal bearings can therefore be divided into three phases: running-in, steady state, and end point.

The wear mechanism is explained by hip-contact mechanics and lubrication theories. As they are loaded, spherical surfaces with any diametral clearance have a minimal but real contact area at implantation due to elastic deformation of the metal^29,30. As the components are in relative motion, wear occurs at a high rate and the area of contact between the surfaces increases. The two components are slowly shaped by wear to form a contact patch. Within the contact patch, the clearance between the two components is reduced toward zero. This promotes a geometry that is more favorable to fluid-film lubrication. When optimal fluid-film lubrication conditions prevail, the wear during the steady-state phase occurs at a slower rate; the contact patch continues to develop, but at a much slower rate. Thus, the effective clearance within the contact patch and within the surrounding entraining geometry is slowly consumed by wear. Our evidence suggests that once the wear patch has reached a critical size (a semi-angle of approximately 50°), an increasing frictional torque results. An increased frictional torque was also reported within metal-on-metal bearings designed with a low clearance^27,31. A small clearance between the two spherical parts is likely to reduce or prevent the ingress of lubricant and the egress of wear particles by modifying the entraining geometry. This will in turn be detrimental to the friction conditions and wear. Research by Farrar and Schmidt²⁷ on a 28-mm metal-on-metal system demonstrated the anticipated result of reduction in wear rate as the clearance became smaller, until at low clearances, a reverse in the trend was observed. After approximately 2 million cycles with a diametral clearance of 14 µm, the wear rate was over five times greater than that observed with a clearance of 74 µm. A further reduction in clearance, simulating equatorial bearing, resulted in total seizure at about 20,000 cycles following the highest wear rates observed.

A comparison of the wear results from simulator and retrieval studies is difficult. Simulator results depend on the machine setup and retrievals on the manner in which the components were implanted and subsequently used. Retrieval of components only occurs at their end point, so that any transitional changes from bedding-in to steady state are unknown. However, ex vivo observations^{5-7,9-13,16,22,32} and more recent ion level monitoring studies³³ do indicate that a steady state of low wear follows the initial bedding-in process.

Data from wear simulators suggest that the steady-state wear phase begins approximately after the first postoperative year. These studies show similar steady-state wear rates regardless of the initial clearance. As the current simulator-study results only consider the first few postoperative years (assuming 2 × 10⁶ cycles per year³⁴), they are insufficient to predict the long-term outcome of increasing size of the wear patch. The fact that wear is continuing throughout the bearing life is evident from the long-term follow-up results of metal ion studies, which show similar levels of continued release followed by a return to background ion levels when the implant is replaced by a metal-on-polyethylene bearing³⁵. This also indicates that corrosion of the metal alone or slow release from early wear debris accumulated in the body does not completely account for the observed release of ions. It is also noted that the ion levels reported for well-functioning Ring devices were similar to those observed for modern devices after two or more years of successful use³⁶.

The third phase of metal-on-metal wear (end point) is the least well understood. We propose that during the steady-state phase, as wear occurs, the effective clearance between the components decreases slowly, both within the wear area and within the surrounding geometry, as the clearance is slowly consumed by wear³⁷. We believe that there is a rise in friction when the wear patch reaches a critical size and that this results in component-loosening. It is recognized that component-loosening may be caused or influenced by other factors. Specially, particulate debris may initiate osteolysis^38,39, which might also compromise fixation.

It is interesting to note that the assumption of component wear and the implications described above are topics that have been neglected in currently available lubrication studies, in which the components are modeled as unworn, perfect geometries with a small contact area due to high stress deformation^10,29. Our model included contact patch dimensions that were based on retrieval observations. The linear wear penetration can be calculated from knowledge of the head dimensions and component clearance. The calculated linear wear is in good agreement with clinically reported values. The wear volume can be calculated from the same assumptions. If the wear volume corresponding to a critical contact patch is known and if the wear rate is known, the implant life can be calculated. The metal-on-metal systems that displayed long-term survival (greater than twenty years) had low total wear volumes of approximately 60 mm³ and starting diametral clearances generally in excess of 120 µm (typically 200 to 300 µm)^1,13. With use of McKee component data from McKellop et al.¹ and wear rates from the literature (see Appendix), and assuming a contact angle of 50° at the time of failure, the predicted life for the implant was in good agreement with the reported clinical results.

Although our model is based on observations of first-generation metal-on-metal total hip replacements with a relatively small diameter (approximately 41 mm), there is no evidence to suggest that the same principles will not apply to smaller or larger-diameter metal-on-metal bearings. Indeed, recent measurements of ion levels in the blood of patients suggest that the wear rate of large modern resurfacing devices after one to five years of use during the steady state is similar in magnitude to that of current well-functioning historical devices (Ring or McKee) or to Metasul devices (Zimmer, Winterthur, Switzerland) with a similar length of follow-up⁴⁰.

The manufactured starting clearance is related to each of the three phases in the life of a metal-on-metal implant, but in different ways. With a small starting clearance (50 to 120-µm), the bedding-in wear volume is lower than it would be with a higher starting clearance (120 to 300-µm) before the steady-state wear phase is reached when partial fluid-film lubrication can be sustained. Thus, higher "historical" starting clearances result in a higher bedding-in wear volume prior to achieving the same steady-state equilibrium phase as can be achieved with a lower starting clearance. However, the bedding-in volume represents a small proportion of the anticipated life of the implant (less than 8%), regardless of the initial starting clearance, as demonstrated by simulator studies^5,10,15. If all component and surgical factors and patient activity levels are assumed to be normal, the present study indicates that the length of time that the steady-state phase lasts depends only on the amount of starting clearance. This is because the steady-state wear rate is not related to the starting clearance and the observed implants all failed with similar end-point wear patch sizes. Thus, with a smaller starting clearance, the number of cycles that occur in the steady state before the clearance between the components is consumed to a critical point will be less than with a higher starting clearance. Clinical reported wear rates in the steady-state phase vary by a large factor (1.1 to 7 mm³ per year) and may be influenced by metallurgy, acetabular component inclination angle, and activity levels but do not appear to be influenced by starting clearance (see Appendix). In summary, if the overall objective for the patient is longevity for the purpose of prolonging the time until the third phase or end point is reached, then there is a need to preserve traditional higher starting clearances, which result in higher initial bedding-in wear volume but provide a longer steady-state wear phase.

Conclusions

Retrieved metal-on-metal implants that are of a clinically successful design but that undergo long-term failure demonstrate a typical wear patch that is geometrically close to being a matched pair with its counterpart. The patch area corresponds to approximately 75% of the head diameter at the time of aseptic loosening. The wear patch position is in agreement with a loading direction of 10° to 15° to the vertical, as reported for metal-on-polyethylene devices²⁵. The life of successful first-generation metal-on-metal implants is a three-stage process, with an initial bedding-in phase followed by a steady-state wear rate and with the eventual occurrence of end-point failure.

A mathematical model was developed that could represent the observed wear patch and predict the end point of a given metal-on-metal design. This model was validated with use of clinical data obtained from the literature for first-generation metal-on-metal implants.

It is proposed that the overall life of a metal-on-metal bearing is directly related to its starting clearance, with higher clearances providing a higher likelihood of long life if all other failure modes are avoided.

Appendix

Details of the mathematical model and a table presenting wear-rate data from the literature are available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). Image Not Available

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Topics

steady state ; drug clearance ; metals

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Michael A. Tuke, Gareth Scott, Anne Roques, Xiao Q. Hu, Andrew Taylor; Design Considerations and Life Prediction of Metal-on-Metal Bearings: The Effect of Clearance. The Journal of Bone & Joint Surgery. 2008 Aug;90(Supplement_3):134-141.

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