In Vivo surface wear mechanisms of femoral components of cemented total hip arthroplasties

The Journal of Arthroplasty Vol. 19 No. 1 2004

In Vivo Surface Wear Mechanisms of Femoral Components of Cemented Total Hip Arthroplasties The Influence of Wear Mechanism on Clinical Outcome J. R. Howell, FRCS,* L. A. Blunt, PhD,† C. Doyle, PhD,‡ R. M. Hooper, PhD,‡ A.J.C. Lee, PhD,‡ and R.S.M. Ling, FRCS‡

Abstract: The appearance and mechanism of femoral stem wear was studied in 172 retrieved femoral components, of which 74 stems had been stable in vivo. Macroscopic, microscopic, and nano-level scales of examination were used. Loss of stem surface in response to micromotion (wear) was found to affect 93% of stems. However, changes were frequently difficult to see with the naked eye, and in 19% of cases they would have been missed completely without the use of light microscopy. The surface finish of the prosthesis determined the mechanism of stem wear. Matte surfaces showed typical abrasive processes that also damage the cement, releasing particulate debris from the cement and metal surfaces. This may destabilize the stem within the cement. Polished stems showed a typical fretting appearance with retention of debris on the stem surface and without significant damage to the cement. These differences in wear mechanism between matte and polished stems have significant effects on stem function. Key words: cemented hip arthroplasty, femoral stem, wear, micromotion, debris. © 2004 Elsevier Inc. All rights reserved.

inner aspect of the cement in cemented prostheses, or between the stem and bone in uncemented prostheses, may also be an important source of debris. Both radiosteriometric analysis (RSA) and postmortem studies have shown that micromotion will always occur between stem and cement [4 – 6]. Consequently, it is highly likely that wear will also occur with 2 concomitant effects: first, the production of particulate debris and, second, damage to the cement [7] sometimes leading to enlargement of the internal dimensions of the cement mantle. Several authors have described the occurrence of wear on the surface of explanted femoral stems [8 – 16]. However, given its potential importance, such wear damage and its sequelae have received relatively little attention as a source of stem failure from loosening, possibly because the changes are often impossible to see with the naked eye. Thus, there is a lack of widespread recognition that the process of

Although controversies exist [1] concerning the causes of loosening of the components of artificial hip joints, many researchers now believe that biological reactions to particulate debris play an important role [2,3]. Particles produced from the articulation are most commonly blamed. However, wear processes occurring between the femoral component and the From *Orthopaedics and Trauma, Princess Elizabeth Orthopaedic Centre, Royal Devon and Exeter Hospital, Exeter, the †Centre for Precision Technology, School of Engineering and Computer Science, University of Huddersfield, Huddersfield, and the ‡School of Engineering & Computer Science, University of Exeter, Exeter, United Kingdom. Submitted June 8, 2002; accepted May 27, 2003. Benefits or funds were received in partial or total support of the research material from Stryker Howmedica, Osteonics, Stryker House, Hambridge Road, Newbury, Berkshire, UK. Reprint requests: J. R. Howell, FRCS, 3 The Old School, New Street, Chagford, Devon. TQ13 8BB, United Kingdom. © 2004 Elsevier Inc. All rights reserved. 0883-5403/04/1901-0015$30.00/0 doi:10.1016/S0883-5403(03)00278-X

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Stem Wear and Its Clinical Significance • Howell et al. Table 1. Design Details for Stems Studied by Light Microscopy Stem Design Exeter Charnley CPT JRI hemiarthroplasty Exeter Charnley McKee Muller Stanmore JRI Howse Lubinus SPII J&J Profile Protasul 10 SS Monoblock Duolock Charnley-Muller Furlong J&J Ultima Ring Aufranc-Turner Minneapolis McKee Zimmer Monoblock MS 30

To our knowledge, this is the first time that evidence of wear has been sought on such stable implants.

Surface Finish

Number

Polished Polished Polished Polished Matte Matte Matte

41 4 1 1 38 30 14 8 7 6 4 3 2 2 2 2 1 1 1 1 1 1 1

Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte Matte

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wear and debris production at the stem cement interface may be as common and influential as wear at the articulation. This study originated with certain hypotheses: i) Wear processes are evident at the stem cement interface where the morphology of damage can be used to determine the mechanisms of the wear. ii) Although wear may be evident on all cemented stems to a greater or lesser degree, the mechanism (which is the key determinant of clinical effect) is influenced by stem geometry and surface finish. If wear is to be proven, then surface changes relating to the loss of stem material should be shown and quantified. Comparison of regions of zero and high stem– cement interface strain provide an in-built control. Because it is impossible to see damage in all cases with the naked eye, more sensitive techniques to facilitate a systematic study are required. We therefore examined a wide range of retrieved femoral stems of different geometry, surface finish, and alloy. A methodology of ascending degrees of resolution was established using macroscopic (resolution millimeters), microscopic (resolution microns) and nanoscale (resolution nanometers) measurements. A significant proportion of the stems that we studied were clinically stable and successful.

Materials and Methods At the Princess Elizabeth Orthopaedic Centre in Exeter, Hip Unit surgeons routinely retain implants removed at repeat surgery. Between 1983 and 1998, 172 stems representing 23 different designs were collected at repeat surgery after implantation times varying between 4 and 300 months. Details of the stems that were studied are shown in Table 1. All stems were cemented; 125 had matte surfaces and 47 were polished. The reasons for revision surgery are outlined in Table 2. Of the 172 stems in this study, 74 were stated by the operating surgeon to be “absolutely fixed” at the time of revision, based on the presence or absence of visible stem movement in response to vigorous attempts to move the stem through manual force applied to the head. In this group of 74 stems, only 5 were revised for reasons attributable to the femoral component; 3 were revised for localized femoral lysis, and 2 for a combination of cup loosening and femoral lysis. The remainder of the stems in this well-fixed group were either removed for infection or were exchanged to facilitate cup revision for indications such as cup loosening or recurrent dislocation. Three methods of investigation were used to establish the surface topography and any evidence of changes to the topography of the stems. First, stems were assessed by plain visual examination and light microscopy using a scoring system devised for this purpose. The second method involved the use of quantitative 3-dimensional (3-D) interference profilometry [17,18]. Finally, selected stems were assessed using scanning electron microscopy.

Table 2. Reasons for Revision of Stems Included in the Study Reason for Revision Loose acetabular component Both components loose Loose femoral component Sepsis Localized lysis of femur Recurrent dislocation Not known Pain from other causes Femoral component fracture Localised lysis of acetabulum Failed impaction grafting Massive subsidence Acetabular component wear

Cases (n) 58 51 13 12 10 9 7 6 2 1 1 1 1

90 The Journal of Arthroplasty Vol. 19 No. 1 January 2004 Table 3. Scoring System for Polishing and Pitting Changes and Debris Deposition Score Polishing changes 0 1

Grade

Nil Slight

2

Moderate

3

Marked

4

Very Marked

Pitting Changes 0 1

Nil Slight

2

Moderate

3

Marked

4

Very Marked

Debris deposition 0 1 2 3 4 5 6

Description

No surface changes seen Slight loss of surface matt finish or change of surface hue Not obviously polished Microscopy: Isolated asperities are rounded Areas of polished surface without reflectivity Microscopy: Most asperities are rounded Areas of polished and reflective surface Microscopy: Asperities are flattened Polished and reflective surface completely cover zone Smaller areas of very highly polished and reflective surface Microscopy: Asperities are markedly flattened over entire zone No surface changes seen Slight loss of surface polish Light microscopy shows patchy shallow pits Pitting visible with naked eye Light microscopy shows widespread uniform shallow pits Severe damage visible with naked eye Microscopy shows deep pits with definite banks to the edge Will often vary in depth with “islands” within the crater May be a grain to the wear or heaping of metal within crater Severe damage covering entire zone or widespread damage of exceptional depth Microscopy shows changes as for 3 but covering entire zone No surface changes seen Light covering of brown deposit within area of surface wear Light covering of black deposit within area of surface wear Heavy covering of brown deposit within area of surface wear Heavy covering of black deposit within area of surface wear Heavy covering of brown deposit within area of surface wear and with deposit piled around edge of wear Heavy covering of black deposit within area of surface wear and with deposit piled around edge of wear

Light Microscope Study In this study, 26 zones on all 172 stems were examined using unassisted eye examination and light microscope examination. The zones were developed from those described by Gruen, McNeice, and Amstutz [19] and Johnston and Fitzgerald et al. [20]. Three principal changes were seen on the surfaces of explanted stems. These were polishing, pitting, and debris collection on the stem surface. These are considered to be indicators of different wear mechanisms. Thus a scoring system was devised for the assessment of these changes. This system is outlined in Table 3. Each stem surface zone was scored for polishing, pitting, and the presence of debris. Regions of the stems were compared for wear changes by summating scores for each type of wear change from each stem zone and then comparing these scores using the Mann-Whitney test. Stems were classified by the length of time that they had been in situ, with periods of 60 months used to divide the groups of

stems. These groups and the wear scores for wellfixed and loose prostheses were compared using the Mann-Whitney test. The influence of several variables on the wear scores was investigated by logistic regression analysis and by multiple regression analysis using SAS (SAS Institute, Cary, NC). Patient factors that were investigated included age, gender, and weight, and stem-related factors included the alloy and the presence or absence of a collar. 3-D Interference Microscopy Study Interference profilometry is a powerful tool for the measurement of surface topography, providing quantitative information at a nanometer scale together with surface mapping and parametric analysis. Generally, measurement systems for 3-D surface topography may be characterized by the physical principle used in the surface-sensor interaction [21]. Profilometric 3-D instruments may be either contacting or noncontacting. The former use a stylus to trace the contours of a surface and are

Stem Wear and Its Clinical Significance • Howell et al. Table 4. Stems Studied with Interference Microscopy Stem Design Exeter Matte Exeter Polished Charnley Matte Charnley Polished Stanmore Lubinus SPII Protasul 10 CPT Howse Zimmer Monoblock J&J Profile JRI hemiarthroplasty

Number

Unworn Surface Roughness (Sa)

8 6 7 3 5 3 2 1 1 1 1 1

1.03 ␮m–1.83 ␮m 0.019 ␮m–0.0386 ␮m 0.661 ␮m–1.21 ␮m 0.016 ␮m–0.026 ␮m 0.606 ␮m–0.832 ␮m 0.389 ␮m–0.933 ␮m 1.02 ␮m–1.47 ␮m 0.096 ␮m 1.03 ␮m 2.59 ␮m 2.19 ␮m 0.096 ␮m

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microscopy. Four stems had matte surfaces and 4 were polished. Before insertion of each stem segment into the SEM, the area of wear was marked and the orientation of the stem segment within the microscope was recorded. By systematically changing the degree of tilt of the specimen, the topography could be seen either at a true magnification (90° to the beam) or with enhanced directionality of surface features because of the shadowing effect realized by a glancing electron beam (specimen tilted 20° to the beam).

Results limited by the geometry of the stylus, which may be unable to register narrow or deep grooves within surface topography [18]. The WYKO NT 2000 (WYKO NT 2000 Veeco, Tucson, AZ) interferometer is a noncontacting optical instrument that uses the interference patterns of light reflected by a surface to measure surface topography. The accuracy of this method is limited only by the wavelength of the light used for assessment and so this method represents a significantly more accurate technique than contacting methods [18]. For logistical reasons, it was not possible to perform interference profilometry on all 172 femoral components. The authors chose to study the 4 stem designs (Exeter, Lubinus, Charnley and Stanmore) that were proven to be most successful by a systematic review of outcomes and cost-effectiveness [22] and which had good long-term outcomes on several national arthroplasty registries [23–25]. In addition, the CPT design was also investigated because it was an alternative polished stem to the Exeter prosthesis. Twenty-nine matte and 11 polished stems were measured, and the details are shown in Table 4. Assessment of stem surface topography produced 2 types of data. First, images of the surface were collected for each score of wear. Second, 5 surface parameters were used to quantify the degree of wear exhibited on different areas of the stem. The parameters chosen to quantify wear are shown in Table 5, along with a simple explanation of each.

Light Microscope Study Morphology and Incidence of Wear. The macroscopic topographic changes seen on matte and polished stems were very different. The data show that matte-surfaced stems had undergone a polishing process, affecting 118 of 125 matte stems (94%). In contrast, polished stems exhibited a pitting process, which affected 44 of 47 (94%) polished stems. Of the 118 matte-surfaced stems that exhibited polishing, 20 stems exhibited pitting within the polished areas. The retention of debris on the stem surfaces differed between matte and polished stems. Thirtyeight of 44 (86%) polished stems that showed pit-

Table 5. Three-Dimensional Surface Parameters Used to Assess Wear Parameter Average surface roughness Root mean surface roughness Peak surface roughness

Abbreviation

Explanation

Sa

Mean height of surface topography A more “stable” expression of mean surface roughness Height between tallest peak and deepest valley Indicates the direction of surface irregularities; zero for randomly rough surfaces, positive where peaks predominate, and negative where pits predominate Ratio of real surface area to the projected area; tends to value of 1 for perfectly flat surfaces

Sq St

Surface skewness

Ssk

Surface area index

SAI

Scanning Electron Microscopy A further subset of 8 stems (from the series of 40 studied using interference microscopy) were sectioned and examined using scanning electron microscopy (SEM) (Hitachi S3200 N, Tokyo, Japan). This group of stems included examples of all the designs that had been studied with interference

92 The Journal of Arthroplasty Vol. 19 No. 1 January 2004 Table 6. Wear Morphology and Distribution on Matte and Polished Stems Matte Stems 92% (122/132) of stems exhibited polishing wear 3% of stems showed retention of debris on surface Anterior Surface Lateral scores ⬎ Medial scores* Posterior Surface Medial scores ⬎ Lateral scores* Collar Under surface High scores*

Polished Stems 94% (44/47) of stems exhibited pitting wear 86% of stems showed retention of debris on surface Anterior Surface Lateral scores ⬎ Medial scores* Posterior Surface Medial scores ⬎ Lateral scores*

*P ⬍ .001

ting changes also showed retention of debris associated with the surface damage. This contrasted with the matte stems, in which only 6 of 98 (6%) matte stems that exhibited purely polishing changes retained debris in the region of the wear. These cases showed only light staining of the stem surface. This result would indicate that in the vast majority of matte stems, the material removed during wear had been released into the stem– cement interface. Thirty of the 165 stems that exhibited wear had grade 1 changes only. This means that in 18% of cases, the wear was impossible to detect with the unassisted eye and would have been entirely missed without microscopic techniques. These cases included 21 matte stems and 9 polished stems. Furthermore, anything less than meticulous eye examination, in appropriate light, would have missed many more. Of the 74 stems for which the surgical notes stated that the stem was firmly fixed at revision, 46 were matte-finished stems and 28 were polished. Forty-one of 46 (89%) matte stems exhibited surface changes, as did 27 of 28 (96%) polished stems. This confirms that local interfacial motion occurred in most cases. Distribution of Wear. The distribution of wear on the stem surfaces is summarized in Table 6. On the anterior stem surface, significantly higher scores were recorded over the lateral half than the medial half (P⬍.001) for both matte and polished stems. On the posterior surface, the medial half had significantly higher scores than the lateral half (P⬍.001). Again, this result was seen in both polished and matte-finished stems. The distribution of

wear was identical for stainless steel, cobalt chrome, and titanium stems. The inferior surfaces of proximal collars or flanges exhibited marked wear changes, and such stems also showed statistically significantly increased wear on the lateral aspects of the distal thirds, by comparison with the same areas on collarless stems. This presumably arose as a consequence of the stem “toggling” (the calcar pivot effect [19]) on the collar or flange with a windscreen-wiper effect on its distal end within the cement. Magnitude of Wear. Logistic and multiple regression analyses failed to show any relationship between patient gender, age, or weight and the stem wear scores. In addition, neither the stem alloy nor the presence or absence of a collar influenced the magnitude of the wear score. However, despite the presence of wear on wellfixed stems, when the wear scores of well-fixed and loose stems were compared using the Mann Whitney test, the loose stems were found to be significantly more worn (P⬍.001). In addition, stems that had been in situ for between 0 and 60 months were significantly less worn (P⬍.01) than stems that had been revised after 240 and 300 months. Stems that had been revised between 60 and 120 months after primary surgery also showed significantly less wear (P⬍.01) than the stems that had been in situ for between 240 and 300 months, but no other significant differences were found between groups of stems of different service duration. The presence or absence of cement opacifier within the cement had no statistically significant effect on the wear scores, although the number of cases with radiolucent cement was small. 3-D Interferometry Study Forty stems were measured using a total of 145 scans, each one producing a set of 5 surface parameters and 3 images of the stem surface: a contour plot, a 3-D plot, and a bearing curve. On each stem, baseline measurements were taken of unworn areas. The results are shown in Table 4. Reporting the full results of the surface statistics produced by the WYKO NT 2000 would require a table with over 1000 cells. Although these data can be made available to any interested reader, they are not included here for reasons of brevity. Figs. 1 and 2 show typical results for stable matte and polished stems, respectively. Fig. 1 shows the results obtained from a matte Vaquasheen Charnley stem that had been revised for loosening of the acetabular component and endosteal lysis of the femur. The

Stem Wear and Its Clinical Significance • Howell et al.

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The contour plots and 3-D plots show that the asperity tips have been removed during the polishing process leading to a progressively flatter surface, leaving only the deepest valleys between the asperities. This is confirmed by the bearing curves, which show a loss of the left-hand tail on the graph, indicating a loss of the points lying above the average surface and therefore a loss of the tallest asperities. As the surface becomes polished, the remaining valleys dominate the bearing curve, which produces a long and deep right-hand tail to the curve. The surface statistics confirm the visual appearances. As areas of increased wear are examined, the 2 statistics concerned with average roughness (Sa and Sq) decrease. Concurrently, the total roughness (St) also decreases as the surface polishes. The surface skewness (Ssk) results show that the polishing process removed the asperity tips, leaving the valleys, and thus produces a progressively negative value of Ssk. The results for the surface area index (SAI) for areas of increasing wear approach a value of 1 as the surfaces become more highly polished. Fig. 2 was taken from the edge of a pit present on the surface of a polished cobalt-chrome CPT stem. The wear changes are typical of those seen in the polished stems examined with this technique and are characterized by pitting below the level of the original stem surface, sometimes with overlying debris. Scanning Electron Microscopy Four matte and 4 polished stems were examined using SEM. Fig. 3 shows grazing incidence views of the surface changes seen in these matte stems. Fig. 3A shows the unworn surface of a matt stem with prominent asperities separated by deep valleys. Fig.

Fig. 1. Three-dimensional interferometry results for a matte Charnley stem. (A) An unworn area of stem. (B) An area of slight polishing wear. (C) An area of marked polishing.

results are from areas of increasing degrees of polishing, starting with those from an apparently unworn area in Fig. 1A, through to an area of highly polished wear in Fig. 1C.

Fig. 2. Three-dimensional interferometry image taken from the edge a pit on a cobalt-chrome CPT stem.

94 The Journal of Arthroplasty Vol. 19 No. 1 January 2004

Fig. 4. Scanning electron microscope image showing a grain of alumina uncovered by polishing wear.

Fig. 3. Grazing incidence SEM images showing increasing degrees of polishing wear of matte-surfaced stems. (A) An unworn area of matte stem. (B) An area of slight polishing wear. (C) An area of marked polishing wear.

3B shows an area of slight polishing characterized by loss of the asperity tips at a constant height, consistent with abrasive wear. The appearances shown were present in the worn areas of all three designs of stem that were examined. When areas of marked polishing wear were examined, they showed more extensive changes, producing an almost flat surface with only the deepest valleys remaining, as shown in Fig. 3C. The white spots seen on the stem surface in Fig. 3C were analysed with energy dispersive x-ray analysis (EDX) and found to be grains of alumina, shown in high magnification in Fig. 4. During stem manufacture, these grains are used in the blasting of the stem surface to achieve the matte finish. Some of these grains become embedded within the stem and then are exposed in vivo by the abrasive wear process, and possibly released into the stem– cement interface. Again, this was a consistent finding in all 3 designs of matte stems examined. Fig. 5 shows a highly worn area of a Stanmore stem removed for loosening of both components. It shows a comet-tail appearance on one side of all surface depressions. This appearance is typical of the SEM findings produced by erosion of a surface caused by high-pressure fluid containing hard particles, so called “slurry” wear or erosion [7,26]. The orientation of these comet-tails was straight up the stem from caudal to cephalad. Fig. 6 shows an area of wear on a polished Exeter stem and it shows a pattern of elliptical pitting, arranged in a unidirectional manner. The orientation of these ellipses was checked and found to be lying at approximately 45° to the long axis of these stems. The appearances of this elliptical pattern are

Stem Wear and Its Clinical Significance • Howell et al.

Fig. 5. Scanning electron microscope appearances typical of “slurry” wear.

consistent with those produced by fretting of a metal surface because of micromovement [27].

Discussion The Relationship Between Stem Surface Finish and Wear Morphology and Mechanism Several authors have reported wear of femoral components [8 –11,13–16,28] and several terms have been used to describe these changes, including “rub marks,” “burnishing,” “abrasions,” and “polishing.” Additionally, some authors have reported the occurrence of corrosive changes [10,29], most often in the context of cemented titanium implants. In 1998, Crowninshield and Jennings et al. [30] reported the results of pin on disk experiments in which they found that cement abrasion depended on pin surface roughness, but they did not consider wear of the metal component. To our knowledge, no previous reports of the interdependency of prosthesis surface finish and the morphology of surface changes seen on the stem have been published. The results of this study show that the morphology of surface changes, and thus the wear mechanism, differed fundamentally between matt and polished stems. Matt stems exhibited polishing changes, and in general, debris was not found on the surfaces. In contrast, stems manufactured with a polished surface exhibited pitting below the level of the original surface, and the majority had debris retained in the region of the pits. The morphology of the wear of matte and polished stems described here suggests that both polishing and pitting wear

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have arisen because of stem micromotion within the cement mantle. The results from interference microscopy show that matte stems wear through the loss of surface asperities, starting with the tallest peaks and wearing down to a flat surface, leaving only the deepest surface depressions. This was confirmed by our SEM findings. This suggests an abrasive process between the cement and the stem as a consequence of micromotion at the interface between the 2 surfaces. Use of the term “burnishing” [9,11] in connection with the wear changes seen on matte-surfaced stems is incorrect. “Burnishing” is, in fact, a machining process in which the surface asperities are smeared over the general surface using a hardened steel roller. The wear seen on matte-surfaced stems is abrasive or truncation wear, and this wear has been confirmed by detailed surface metrology and computer-modelling techniques [31]. Abrasive wear leads to liberation of metallic particles from the stem surface. In addition, “mirror image” wear of the cement occurs and thus large volumes of cement and metal debris are produced that then has a third body effect, amplifying the wear process. When the polished stems were analyzed using interference microscopy, researchers found that the deep pits exhibited a ductile appearance typical of fretting wear [32]. SEM confirmed these findings and showed that these pits were oriented in a unidirectional manner, which is another characteristic feature of fretting [32] caused by stem micromotion. Despite the different wear morphology found on matte and polished stems, the light microscopy results show that a similar and clear pattern of wear

Fig. 6. Fretting wear of a polished Exeter stem.

96 The Journal of Arthroplasty Vol. 19 No. 1 January 2004 distribution is present in stems of both surface finishes, and that in both cases wear particularly affects the anterolateral and posteromedial aspects of the stem. This distribution of wear was unaffected by the stem alloy. It also agrees with previous reports of the distribution of wear changes [11,15], suggesting that a common causal factor is present. Indeed, these wear changes occurred in regions of the stem that are subjected to the highest contact forces against the cement under the influence of torsional forces applied through the femoral head [33]. This suggests that the common causal factor is torsional micromotion in response to these forces. The Surface Finish That Separates Fretting and Abrasive Wear Mechanisms The unworn areas of the polished stems that we studied with interferometry varied in surface roughness (Sa) from 0.016 to 0.0386 ␮m. The worn areas of these stems all exhibited evidence of a fretting wear process. The unworn areas of the nonpolished stems that we studied with interferometry varied in surface roughness (Sa) from 0.389 to 2.59 ␮m. The worn areas of these stems all showed evidence of an abrasive type of wear. A number of stem designs have appreciably rougher surfaces than those that we studied, especially those with proximal “macrotexturing.” A “Vives” stem that had been removed for focal lysis was studied by Hale and Lee et al. [12], using a stylus-type profilometer. They found it to have a surface roughness in unworn areas of 3.69 ␮m. Marked areas of abrasive wear of that stem were noted in the same distribution as described in this study. Retrieved Centralign (Zimmer, Warsaw IN) and Versys (Zimmer) stems with unworn surface Ra measurements of 2.5 and 2.64 ␮m, respectively, were found to have sustained abrasive surface wear [34]. In the same study, a Precision (Howmedica, Rutherford, NJ) stem with an unworn surface Ra of 10.76 ␮m was also found (after experimental cyclic loading) to have sustained abrasive wear damage. The evidence thus suggests that any stem with a surface roughness of approximately 0.4 ␮m or greater will probably wear by an abrasive mechanism. The upper limit of stem surface roughness that makes a stem liable to undergo abrasive rather than fretting wear is uncertain and of evident importance, especially from the standpoint of concomitant damage to the cement mantle and consequent stem destabilization. The Ceraver Osteal titanium alloy stem has an anodized surface with a surface

roughness of 0.08 to 0.1 ␮m (L. Sedel, personal communication, 2001). Excellent results have been reported with this device [35,36], even with thin cement mantles. Therefore, presumably abrasive wear at the stem-cement interface did not occur. The crucial roughness is thus probably somewhere between 0.1 and 0.3 ␮m, and is likely to be related to the degree of mechanical interlock that is achieved between the stem surface and the cement at surgery. The Incidence of Stem Wear In this study, we found wear changes on the surfaces of 162 of 172 stems examined using light microscopy. This indicates that surface stem wear is much commoner than may have been thought in the past. Clinicians should remember that the group of stems reported here does not represent a consecutive series, but it does include a significant number of stems exchanged for reasons other than stem failure, and a significant number on which no abnormalities were visible to the naked eye. In the light of the recent experimental finding that stem cement “debonding” and instability can occur in the presence of normal plain radiographic findings [37] and the radiostereometric [4] finding that stem migration within the cement is the rule rather than the exception, our findings are not as surprising as they may seem. Even the figures mentioned previously may be an underestimation of the true incidence, because surface changes are often extremely difficult to see and this is illustrated by the results from interferometry. When this technique was used to examine the apparently unworn surface of a Lubinus SPII stem that had been revised for localized femoral endosteal bone lysis and loosening of the socket, the surface statistics indicated that areas of the stem that had been in contact with the cement differed from those that had not in all 5 parameters used to assess the surface morphology. This result confirms that changes to the stem surface may occur that are not detectable by visual inspection or even by conventional light microscopy. Based on naked eye examination and light microscopy, wear affected a total of 93% of stems in this study. If it had been possible to examine all the stems using interferometry, the surface statistics may well have revealed the presence of wear in areas that were normal on light microscopy, as it had with the Lubinus SPII that was studied. Thus, the vast majority, and possibly all of these stems had been subject to micromotion within the cement mantle.

Stem Wear and Its Clinical Significance • Howell et al.

Most importantly, we found evidence of wear on the surface of 74 of the stems that were stated to be “rigidly fixed” by the revising surgeon. Thus, it appears that wear may occur in the absence of evident loosening of the stem within its cement mantle. This contradicts the findings of Witt and Swann [13] and Buly and Huo et al. [38], who found evident wear changes only in loose prostheses. However, the stems in these studies were not examined using interferometry, which may detect more subtle changes on apparently normal surfaces. O’Connor and Burke et al. [6] studied 9 clinically successful and stable femoral components retrieved at autopsy. They measured the relative movement at the 2 interfaces of the implant– cement– bone composite and found movements of the order of 25 ␮m, particularly in response to torsional forces that are produced in activities such as stair climbing. This movement, occurring in stable stems, was most marked at the implant– cement interface. Our findings of stem wear in similar clinical cases are further evidence of such micromovement. Indeed, the results of the present study add to an increasing body of evidence [4,9,39 – 45] that suggests that debonding, either localized or generalized, is much more common than hitherto appreciated and may be difficult or impossible to identify on plain radiography [37]. Indeed, the RSA studies of Karrholm and Nivbrant et al. [4] have shown stem-cement axial migration to be detectable in all designs of stem that were studied, calling into question the whole notion of stem– cement interface bonding. The Initiation of Stem Wear The initiation of wear processes in the stem– cement interface is a crucial issue, but precisely how the wear process starts is not yet certain, though the authors continue to investigate the matter. This article reports the occurrence of localized wear, the distribution of which concurs with that found in several other reports [9,11,13]. We have found such wear even on the surfaces of stems that have been clinically stable for many years and that have been found to be “stable” at repeat surgery for indications other than stem failure. The presence of wear on such a stem means that cyclical micromovement of too small an amplitude to be detected by the surgeon at repeat surgery must have been occurring over at least parts of the stem– cement interface, possibly in association with bending or twisting of the stem. Furthermore, the material from the worn stem together with worn cement must have formed third bodies in the stem– cement

97

interface in the worn area and contributed to accelerated wear. The Clinical Significance of Stem Wear Crowninshield and Jennings et al. [30] suggested that, in the case of a debonded stem, the long-term survival of the implant may depend on the ability of the implant to minimize cement wear during micromotion. That a femoral prosthesis with a polished surface can achieve prolonged clinical success despite the appearance of debonding at the stem– cement interface is beyond doubt [46 –50]. This study has shown that when wear of a polished stem occurs, metal is lost below the level of the original surface. The result is that the stem– cement interface and cement surrounding the worn area remain undisturbed. This means that the resulting debris is trapped within the worn area, and it is not released into the interfaces and bearing surfaces of the prosthesis. This is in contrast to the wear of matte stems, in which neither acrylic nor metal debris are retained at the site of wear damage and considerable damage of cement occurs [44] from the associated wear damage to the corresponding areas of the inner aspect of the cement mantle. Such wear will therefore lead to the production of large amounts of metal, polymethylmethacrylate (PMMA), alumina, and radio-opacifier particulate debris as well as enlarging the internal dimensions of the cement mantle, thus providing a space for fluid flow and leading to increased stem– cement micromovement with cyclical loading. As well as further accelerating wear, the latter may have major hydrostatic consequences as the presence of slurry wear [7] shows. Slurry wear is caused by fluid containing hard particles. The particle velocities in association with slurry wear are commonly between 5 and 500 m/s [7] so that the hydrostatic implications of the presence of slurry wear are substantial. This is especially true because experimental evidence suggests that pressure changes and mechanical strain can modulate the output of cytokines from macrophages, both on their own and synergistically with particles [51–54]. Moreover, our SEM evidence of the liberation of particulate alumina from the worn surfaces of femoral stems is particularly significant. This is one of the hardest materials known to mankind and could be involved in slurry wear, as well as in producing catastrophic third body effects not only at the stem– cement interface but also within the articulation itself, leading to an increase in the production of polyethylene debris. It is no coincidence that Shardlow and Hailey et al. [15] reported a correlation between

98 The Journal of Arthroplasty Vol. 19 No. 1 January 2004 Table 7. Outcomes of Stems That Have Existed With Different Surface Finishes During Their Clinical Histories Ra (␮m)

Follow-up (y)

Loosening

EN58J

0.03

3–17

3.1%

316L

0.6

3–17

11.4%

REX734‘Orthinox’

0.01–0.03

8–10

0%

REX734‘Orthinox’

0.8–1.2

8–10

21.7%

REX734‘Orthinox’

0.01–0.03

8–10

0%

REX734‘Orthinox’

0.8–1.2

8–10

20%

316L CoCrMb CoCrMb CoCrMb CoCrMb

0.03 0.9–1.2 0.75 2.0 0.8

11.04 10.48 10.46 7.21 11.3

CoCrMb

2.1

8.2

2000

Iowa (gb†) ⫹ pre-coat T28

316L

0.03

17–21

11.1%

2000

TR28

CoCrMb

0.9–11.2

17–21

15.8%

1999

Charnley Kerboull Mk I Charnley Kerboull Mk II Charnley Kerboull Mk III CMK MkIII

316L

0.01–0.03

20

1.0%

316L

0.6–0.8

10

4%

316L

0.01–0.03

12

0%

316L

?

Author

Year

Stem

Dall and Learmonth et al. [56] Dall and Learmonth et al. [56] Crawford, Gie, and Ling [57]

1993

Crawford, Gie, and Ling [57]

1998

Howie, Middleton, and Costi [58]

1998

Howie, Middleton, and Costi [58]

1998

Collis and Mohler [59] Collis and Mohler [59] Collis and Mohler [59] Collis and Mohler [59] Sporer and Callaghan [60] Sporer and Callaghan [60] Meding, Wassif, and Ritter [61] Meding, Wassif, and Ritter [61] Kerboull [62]

1998 1998 1998 1998 1999

Charnley Flat back Subsequent Charnleys Polished Exeter Monoblock Matt Exeter Monoblock Polished Exeter Monoblock Matt Exeter Monoblock T28 TR28 Iowa (bb*) Iowa (gb†) Iowa (bb*)

Kerboull and Lefevre [63]

2000

Kerboull

2002

Kerboull

2002

1993 1998

1999

Alloy

Focal Lysis

Revision for ASL

0% 4.9% ‡ ‡ 6% 18%

12%–15%

*Bead blasted. †Grit blasted. ‡In the first 5 years, revision rate was 5 ⫻ higher for the grit-blasted and precoated stems than for the bead blasted stems.

stem wear and socket wear. Fluid flow has been shown to occur experimentally even along the apparently well-bonded stem– cement interface of rough surfaced stems [41]. This provides the probable explanation for the mode of dispersal of debris during the wear of matte stems. Such mechanisms are likely to be important in the natural history of focal femoral lysis [55]. Considering the marked differences between the possibility of polished and nonpolished surfaces damaging the inner aspect of the cement mantle as a consequence of abrasive wear, and thus, potentially destabilizing the stem, it would not be surprising to find that where stems of similar or identical geometry exist with different surface finishes, there is an association between surface finish and out-

come. Table 7 summarizes the available data on this matter [56 – 63, M. Kerboull, personal communication, 2002], and it is clear that, other things being equal, surface finish has a profound effect on clinical outcome, because none of the stems listed in the table exhibited an improved clinical performance with a rougher surface finish. Major questions remain to be clarified in connection with stem wear, including the amplitudes of micromovement occurring at the stem– cement interface in vivo, the precise way in which stem wear starts, and the crucial surface finish that separates fretting from abrasive wear. Nevertheless, it is a common phenomenon that not only increases the particulate load, both directly and indirectly, but may also lead directly to enlargement of the inside

Stem Wear and Its Clinical Significance

of the cement mantle and thus to stem instability within the cement mantle with all its sequelae. It may in fact be a far more important phenomenon in the genesis of aseptic stem loosening than primary mechanical failure of cement from fracture. We believe that the findings in this study fully support the 2 hypotheses on which the study was based.

Acknowledgments The authors would like to thank the surgeons at the Princess Elizabeth Orthopaedic Centre, Exeter, for providing most of the specimens on which this study has been based, and the staff of the Medical Graphics Department at the Royal Cornwall Hospital, Truro, for the figures.

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