The Journal of Bone and Joint Surgery 79:1539-44 (1997)
© 1997 The Journal of Bone and Joint Surgery, Inc.
Three-Dimensional Analysis of Multiple Wear Vectors in Retrieved Acetabular Cups*
MOTOI YAMAGUCHI, M.D., PH.D. ,
THOMAS W. BAUER, M.D., PH.D. and
YASUSHI HASHIMOTO, M.D., PH.D. , CLEVELAND, OHIO
Investigation performed at the Departments of Anatomic Pathology and Orthopaedic Surgery, The Cleveland Clinic Foundation, Cleveland.
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Abstract
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The wear of polyethylene acetabular components is an important issue in total hip arthroplasty. The amount of wear has been measured in many studies, but few have addressed other mechanical aspects of wear in vivo. We used the shadowgraph method to measure the wear vectors in 104 retrieved acetabular cups that had been made by a single manufacturer, and we identified more than one wear vector in thirty-one cups (30 per cent). We hypothesized that the most likely explanation of multiple wear vectors was loosening of the acetabular implant with a change in the orientation of the implant in the pelvis. To test this hypothesis, we estimated the extent of motion of the cup in situ on the basis of differences in angles measured from serial radiographs of sixteen hips. We then used linear transformation of the three-dimensional vectors to compare the wear directions measured in the retrieved implants with the wear directions predicted from the radiographs. The change in wear direction predicted on the basis of in vivo motion of the cup never corresponded to the actual difference between wear vectors in the retrieved implants. Our results suggest that multiple wear vectors may be commonly found in retrieved implants, but loosening of the acetabular cup does not account for the multiple vectors. Additional observations suggest that impingement between the edge of the acetabular cup and the femoral component may be associated with multiple wear vectors. These results have implications for the investigation of the general mechanisms of wear in vivo and suggest that clinical or wear-testing scenarios that assume a single direction of wear may underestimate the over-all amount of volumetric wear.
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Introduction
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Wear of a polyethylene acetabular component may be an important factor in loosening of the implant and development of osteolysis after total hip arthroplasty5,12,17,19. In many studies, the extent of wear has been shown to be associated with clinical and other implant-related variables1,7,11,13,16, but few authors have addressed the direction of the wear vector3,6,9,10,19. Since Charnley et al. first reported that wear advanced in a cylindrical pathway and in a single direction4, volumetric wear usually has been calculated from measured maximum linear wear10,11. However, several investigators recently have noted that some retrieved acetabular cups showed more than one wear vector9,15. In those reports, the authors presumed that multiple wear vectors were due to the migration of the acetabular cup. The purposes of the present study were to describe a group of retrieved implants with multiple wear vectors, to evaluate serial radiographs to determine whether migration of the implant might be responsible for multiple wear vectors, and to examine other factors that influence multiple wear vectors. We identified a group of retrieved acetabular cups with multiple wear vectors and tested the null hypothesis that the direction of the wear vector relative to the pelvis does not change but a change in the position of the cup in vivo causes a second wear vector to develop in the implant.
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Materials and Methods
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Measurement of Wear in the Retrieved Implants
We reviewed the implant registry at our institution and identified 104 retrieved acetabular components that had been made by a single manufacturer (Osteonics, Allendale, New Jersey). The diameter of the articular surface of the retrieved polyethylene liner was twenty-two millimeters in one component, twenty-six millimeters in thirty-four, twenty-eight millimeters in thirty-seven, and thirty-two millimeters in thirty-two. The rim was extended 10 degrees in ninety cups, 20 degrees in eight, and 0 degrees (neutral) in six. Casts of the articular surface were prepared. Profiles were made of both the face and the side orientations with use of a profile projector (model PH-350H; Mitutoyo, Tokyo, Japan). The three-dimensional location of the center of the original articular surface and that of the worn surface could be measured by superimposing the projected arcs onto a standardized grid with templates of concentric rings. On the basis of the coordinates for these two points, we determined the extent of linear wear and the direction of wear relative to the implant with use of a shadowgraph technique, which has been described previously18 (Fig. 1). The wear vectors of each cup were represented in three dimensions, and if the cup showed multiple wear vectors the angle between the vectors was calculated and defined as  1 (Fig. 2). Retrieved polyethylene liners also were inspected grossly for evidence of an indentation on the peripheral rim indicating prosthetic impingement. The potential influences of the age and gender of the patient, the duration of implantation, and the presence or absence of impingement on the existence of multiple wear vectors also were tested. We calculated the mean extent and annual rate of linear wear, on the basis of the maximum linear wear in each cup.
Radiographic Measurement
Adequate serial anteroposterior radiographs of the pelvis, made early after the primary arthroplasty and before the revision arthroplasty, were used to determine the three-dimensional orientation of the acetabular component by combining measurements of the inclination of the cup and the angles of anteversion and rotation. The angle of inclination of the cup was measured directly from anteroposterior radiographs, and the angle of anteversion of the cup was calculated as described by Hassan et al. The extended-rim polyethylene liners used in the present series have a wire marker at 5 degrees to the left of the apex of the rim. On the basis of the position of this wire marker, we calculated the rotational orientation of the polyethylene liner relative to the acetabulum (the rotation angle of the cup) (Fig. 3). Most of the cups with a rim that was not extended did not have a wire marker for determining the angle of rotation of the cup, and they were excluded from the radiographic analysis. If the extended rim of a prominently anteverted cup was positioned extremely posterior or anterior, the wire marker could not be seen on the anteroposterior radiographs of the pelvis because of overlap with the femoral head or the metal backing. Those cups also were excluded from the radiographic measurement. On the basis of estimated errors of measurement, we defined a cup as stable when there was a difference of less than 5 degrees between all angles.
Examination of the Hypothesis
On the basis of the change in orientation of the cup in the pelvis as measured from serial radiographs, the expected wear vector was calculated with use of linear transformation of the vectors. The angle between the primary wear vector ( ) and the expected wear vector (ex) was determined and defined as 2 (Fig. 4). We compared 1 and 2 to examine our hypothesis.
As it was unclear which vector represented primary wear, multiple vectors observed in a retrieved implant were paired with the corresponding expected vectors and calculated respectively—that is, in a retrieved implant with two vectors (1 and 2), if 1 was presumed to be the primary wear vector, the expected vector (1ex) and the angle (2) between 1 and 1ex could be calculated. Conversely, if 2 was regarded as the primary wear vector, the expected vector 2ex and the angle (2') could be calculated. These two angles (2 and 2') were not necessarily equal. As the primary wear vector could not be determined, we calculated two or three 2 angles for each cup and compared the 2 angles with the 1 angle.
Statistical Analysis
The Mann-Whitney U test was used for statistical analysis of continuous variables (age of the patient, duration of implantation, and extent of linear wear), and the chi-square test was used for categorical variables (movement of the cup, impingement, and diameter of the articular surface) between cups with multiple and single wear vectors. A value of p < 0.05 was regarded as significant.
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Results
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Adequate clinical information was available for ninety of the 104 retrieved cups. The duration of implantation of the ninety cups ranged from three weeks to eighty-six months (mean, 39.4 months). The age of the patients at the time of the revision operation ranged from twenty-seven to eighty years (mean, 52.7 years). Thirty-six (40 per cent) of the ninety patients were men. The extent of linear wear was measured in all retrieved cups with use of the shadowgraph technique, but the annual rate of wear could be calculated for only the ninety cups for which the duration of implantation was known. The mean extent of linear wear was 0.85 millimeter (range, 0.03 to 4.56 millimeters). The mean rate of linear wear was 0.33 millimeter per year (range, 0.02 to 2.98 millimeters per year). Measurement with the shadowgraph technique demonstrated more than one wear vector in thirty-one (30 per cent) of the 104 cups. Four of the thirty-one cups had three wear vectors, and the remaining twenty-seven cups had two.
Adequate serial radiographs were available for sixteen of the thirty-one cups that had multiple wear vectors and for thirty-two of the seventy-three cups that had a single wear vector. These radiographs were used to determine the orientation of the acetabular component after the primary arthroplasty and before the revision arthroplasty. We found no substantial change in the orientation of nine of the sixteen cups that had multiple wear vectors. The mean changes (and standard deviations) in the inclination of the cup and the angles of anteversion and rotation during the period that the implant was in situ were only 4.1 ± 10.2 degrees (range, -1.9 to 38.5 degrees), 0.8 ± 5.7 degrees (range, -10.5 to 14.9 degrees), and 2.1 ± 10.1 degrees (range, -15.0 to 24.0 degrees), respectively (Tables I and III). Eighteen (56 per cent) of the thirty-two cups that had a single wear vector were regarded as stable, and the mean changes in inclination, anteversion, and rotation were 4.7 ± 14.6 degrees (range, -12.0 to 68.0 degrees), 1.3 ± 8.8 degrees (range, -21.0 to 30.8 degrees), and -0.3 ± 4.9 degrees (range, -17.8 to 13.9 degrees) (Table III). With the numbers available, we found no significant difference in the change in the angles between the two groups.
For the cups that had at least two vectors, the mean angle between the measured wear vectors (1) was 27.8 ± 22.4 degrees (range, 4.5 to 72.5 degrees). When the retrieved cup had three different wear vectors (Cases 9 and 14), there were three pairs of angles between vectors. The anticipated changes in the wear direction between the measured vector and the expected vector as determined on the basis of migration of the cup (2) also were examined (Table II).
If our hypothesis were true, then we would expect that 1 would be equal to 2. However, the changes in wear direction that were anticipated on the basis of the radiographs (2) were smaller than the actual changes measured in the retrieved implants (1). Also, the direction of the vector that was expected on the basis of migration of the cup did not correspond to the actual direction of the vectors measured in any of the cups. We would expect not only 1 to equal 2 but also  1ex to equal 2 and ß1ex to equal ß2, or 2ex to equal 1 and ß2ex to equal ß1 (Table II). These results suggest that movement of the acetabular cup does not by itself account for the multiple wear vectors in retrieved implants.
We also analyzed the association between other clinical factors and multiple wear vectors. With the numbers available, we could detect no significant association between multiple wear vectors and the age (p = 0.59) or the gender (p = 0.56) of the patient. The retrieved polyethylene liners used in the present study had articulated with femoral heads of several sizes. Multiple wear vectors were found in sixteen (47 per cent) of the thirty-four cups that had articulated with a twenty-six-millimeter-diameter head, in seven (19 per cent) of the thirty-seven cups associated with a twenty-eight-millimeter-diameter head, and in eight (25 per cent) of the thirty-two cups associated with a thirty-two-millimeter-diameter head. The only available cup that had articulated with a twenty-two-millimeter-diameter head had a single wear vector. We found a significant association between cups that had articulated with a small-diameter femoral head and multiple wear vectors (p = 0.03). Forty-one (39 per cent) of the 104 cups had a peripheral indentation, indicating prosthetic impingement. Twenty (65 per cent) of the thirty-one cups with multiple wear vectors and twenty-one (29 per cent) of the seventy-three cups with a single wear vector had evidence of impingement. A significant association was found between impingement and multiple wear vectors (p = 0.0006).
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Discussion
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Although the measurement of polyethylene wear in acetabular components has been attempted in many studies3,4,7,9-11,15,18,19, the mechanisms of wear advancement in vivo are not completely understood. In most investigations3,4,7,10,11,15, volumetric wear of polyethylene has been calculated from radiographs or retrieved implants with the assumption that the femoral head migrates into the polyethylene in a single linear vector. If the path of migration is not linear or if there are multiple directions of wear, such calculations of volumetric wear are inaccurate. A more accurate depiction of migration of the femoral head is necessary to understand the mechanisms and the magnitude of polyethylene wear. The shadowgraph analysis of casts of retrieved implants provides information concerning both the magnitude and the direction of wear. With use of this method, we identified multiple wear vectors in a relatively high proportion (30 per cent) of retrieved implants. Izquierdo-Avino et al. noted that fifteen (28 per cent) of fifty-four retrieved acetabular components showed wear that was not unidirectional. To clarify the mechanisms of this phenomenon, we hypothesized that the most likely explanation of multiple wear vectors was loosening of the acetabular implant with a change in its orientation in the pelvis. Our analysis of the radiographs, however, showed that the vectors expected on the basis of movement of the implant were different than the measured vectors and that the anticipated changes in wear direction were smaller than the actual measured differences between wear vectors in most cups. Therefore, multiple wear vectors could not be explained by movement of the cup during the process of loosening. We recognize that our radiographic measurements may include some errors because the orientation of the acetabular component was determined from anteroposterior radiographs of the pelvis7,14, but in most of the cups the difference between 1 and 2 greatly exceeded the estimated error of measurement.
In addition to motion of the implant, several other factors could cause the multiple wear vectors seen in retrieved components. These include altered motion of the femoral head because of impingement on the rim, loosening and movement of the femoral component, a change in the gait cycle due to pain or muscle weakness, and progressive changes in biomechanical conditions such as contact stress, sliding distance, and friction coefficient. Our results suggested that, among these factors, peripheral impingement on the rim leading to abnormal motion of the femoral head may be associated with multiple wear vectors. We found a strong association between impingement and multiple wear vectors (p = 0.0006), but it should be noted that eleven (35 per cent) of thirty-one implants with multiple wear vectors had no evidence of impingement and that twenty-one (29 per cent) of seventy-three implants with only a single recognizable vector of wear had evidence of impingement. These observations may lack sensitivity, however, because we could identify impingement only on the basis of peripheral indentation of the rim. It is possible that there was osseous impingement on some of the eleven cups with multiple wear vectors but no gross evidence of such impingement was visible on the cup2. Despite the significant association found between impingement and multiple wear vectors in our study, it seems that impingement is neither a necessary nor a sufficient condition for the development of multiple wear vectors.
Our results demonstrated a significant association between the size of the femoral head and multiple wear vectors (p = 0.03). However, a previous study suggested that a small ratio between the diameters of the femoral head and neck increases the chance of impingement2. As we also found a significant association between the size of the femoral head and impingement (p = 0.0006), the relationship between multiple wear vectors and the size of the femoral head may be related to impingement.
The maximum linear wear in the cups with multiple wear vectors was greater than that in the cups with a single wear vector (p = 0.002), and the cups with multiple wear vectors tended to have been in situ longer than those with a single wear vector (p = 0.007). One explanation for this result might be that it is difficult to detect a second wear arc on the casts of retrieved cups that were in situ for a short duration, suggesting a limitation in the sensitivity of wear measurement with use of the shadowgraph method.
The rate of wear observed in the present study was higher than that seen in other studies involving radiographic measurement of well functioning implants7,11. This was true in part because all of the implants in our study were retrieved at the time of revision arthroplasty. In addition, some implants were sent to our laboratory from other institutions for analysis of wear.
It is unclear whether the multiple wear vectors demonstrated in our study illustrate the consequence of clinical failure or a more universal pattern of wear, but the multiple wear vectors cannot be explained by motion of the implant alone. Multiple wear vectors may have important implications for the investigation of the mechanisms of wear advancement, and our findings suggest that the amount of volumetric wear may have been underestimated in previous studies10,11 in which it was assumed that there was a single vector of wear. Additional studies are needed to evaluate mechanisms other than motion of the implant that may be responsible for multiple wear vectors after total hip arthroplasty.
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Footnotes
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*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
Department of Anatomic Pathology and Orthopaedic Surgery, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195.
Department of Orthopaedic Surgery, Kobe University School of Medicine, 7-5-2 Kusunoki-cho, Chuo-ku, Kobe, Japan.
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References
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-
Bartel, D. L.; Bicknell, V. L; and Wright, T. M.: The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J. Bone and Joint Surg., 68-A: 1041-1051, Sept. 1986.[Abstract/Free Full Text]
-
Chandler, D. R.; Glousman, R.; Hull, D.; McGuire, P. J.; Kim, I.; Clarke, I.; and Sarmiento, A.: Prosthetic hip range of motion and impingement. The effects of head and neck geometry. Clin. Orthop., 166: 284-291, 1982.
-
Charnley, J., and Halley, D. K.: Rate of wear in total hip replacement. Clin. Orthop., 112: 170-179, 1975.
-
Charnley, J.; Kamangar, A.; and Longfield, M. D.: The optimum size of prosthetic heads in relation to the wear of plastic sockets in total replacement of the hip. Med. and Biol. Eng., 7: 31-39, 1969.
-
Cooper, R. A.; McAllister, C. M.; Borden, L. S.; and Bauer, T. W.: Polyethylene debris-induced osteolysis and loosening in uncemented total hip arthroplasty. A cause of late failure. J. Arthroplasty, 7: 285-290, 1992.[Medline]
-
Dowling, J. M.; Atkinson, J. R.; Dowson, D.; and Charnley, J.: The characteristics of acetabular cups worn in the human body. J. Bone and Joint Surg., 60-B(3): 375-382, 1978.
-
Griffith, M. J.; Seidenstein, M. K.; Williams, D.; and Charnley, J.: Socket wear in Charnley low friction arthroplasty of the hip. Clin. Orthop., 137: 37-47, 1978.
-
Hassan, D. M.; Johnston, G. H.; Dust, W. N.; Watson, L. G.; and Cassidy, D.: Radiographic calculation of anteversion in acetabular prostheses. J. Arthroplasty, 10: 369-372, 1995.[Medline]
-
Izquierdo-Avino, R. J.; Siney, P. D.; and Wroblewski, B.M.: Polyethylene wear in the Charnley offset bore acetabular cup. A radiological analysis. J. Bone and Joint Surg., 78-B(1): 82-84, 1996.
-
Kabo, J. M.; Gebhard, J. S.; Loren, G.; and Amstutz, H. C.: In vivo wear of polyethylene acetabular components. J. Bone and Joint Surg., 75-B(2): 254-258, 1993.
-
Livermore, J.; Ilstrup, D.; and Morrey, B.: Effect of femoral head size on wear of the polyethylene acetabular component. J. Bone and Joint Surg., 72-A: 518-528, April 1990.[Abstract/Free Full Text]
-
Maloney, W. J., and Smith, R. L.: Periprosthetic osteolysis in total hip arthroplasty: the role of particulate wear debris. J. Bone and Joint Surg., 77-A: 1448-1461, Sept. 1995.[Free Full Text]
-
Morrey, B. F., and Ilstrup, D.: Size of the femoral head and acetabular revision in total hip-replacement arthroplasty. J. Bone and Joint Surg., 71-A: 50-55, Jan. 1989.[Abstract/Free Full Text]
-
Murray, D. W.: The definition and measurement of acetabular orientation. J. Bone and Joint Surg., 75-B(2): 228-232, 1993.
-
Ohlin, A., and Selvik, G.: Socket wear assessment. A comparison of three different radiographic methods. J. Arthroplasty, 8: 427-431, 1993.[Medline]
-
Ritter, M. A.; Stringer, E. A.; Littrell, D.A.; and Williams, J. G.: Correlation of prosthetic femoral head size and/or design with longevity of total hip arthroplasty. Clin. Orthop., 176: 252-257, 1983.
-
Schmalzried, T. P.; Kwong, L. M.; Jasty, M.; Sedlacek, R. C.; Haire, T. C.; O'Connor, D. O.; Bragdon, C. R.; Kabo, J. M.; Malcolm, A. J.; and Harris, W. H.: The mechanism of loosening of cemented acetabular components in total hip arthroplasty. Analysis of specimens retrieved at autopsy. Clin. Orthop., 274: 60-78, 1992.
-
Sychterz, C. J.; Moon, K. H.; Hashimoto, Y.; Terefenko, K. M.; Engh, C. A. Jr.; and Bauer, T. W.: Wear of polyethylene cups in total hip arthroplasty. A study of specimens retrieved post mortem. J. Bone and Joint Surg., 78-A: 1193-1200, Aug. 1996.[Abstract/Free Full Text]
-
Wroblewski, B. M.: Direction and rate of socket wear in Charnley low-friction arthroplasty. J. Bone and Joint Surg., 67-B(5): 757-761, 1985.
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Top
Abstract
Introduction
=(180/
)cos-1((Lxr)/(Rxr'))(degrees) 