In this prospective, randomised study, we have compared the wear rate of cemented, acetabular polyethylene cups articulating with either a 22 mm or a 32 mm ...
Comparison of polyethylene wear with femoral heads of 22 mm and 32 mm A PROSPECTIVE, RANDOMISED STUDY S. Eggli, S. z’Brun, C. Gerber, R. Ganz From the University of Berne, Switzerland
n this prospective, randomised study, we have compared the wear rate of cemented, acetabular polyethylene cups articulating with either a 22 mm or a 32 mm cobalt-chromium head. We evaluated 89 patients who had a total of 484 radiographs. The mean follow-up period was 71.4 months (SD 29.1). All the radiographs were digitised and electronically measured. The linear wear rate was significantly higher during the first two years and decreased after this period to a constant value. We suggest that this is partly due to a ‘run-in’ process caused by irregularities between surfaces of the cup and head and an initial plastic deformation of the polyethylene. The mean volumetric 3 wear was 120.3 mm /year for the 32 mm head, which 3 was significantly higher than the 41.5 mm /year for the 22 mm heads. The mean linear wear rate was not significantly different. We were, however, unable to find radiological signs of osteolysis in the patients who had higher wear rates.
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J Bone Joint Surg [Br] 2002;84-B:447-51. Received 15 May 2000; Accepted after revision 24 May 2001
In 1995 Harris stated that “periprosthetic osteolysis is the 1 main problem in contemporary total hip replacement”. Wear particles initiate a foreign-body reaction and are a 1-4 major factor in producing osteolysis. Recent research has therefore aimed at reducing the rate of wear of polyethyl5-8 ene to a minimum. Besides other mechanical factors, several studies have suggested that the size of the femoral 9-12 head significantly influences wear.
S. Eggli, MD S. z’Brun, MD R. Ganz, MD Department of Orthopaedic Surgery, University of Berne, Inselspital, CH3010 Berne, Switzerland. C. Gerber, MD Department of Orthopaedic Surgery, University of Zürich, Balgrist, CH8008 Zürich, Switzerland. Correspondence should be sent to Dr S. Eggli. ©2002 British Editorial Society of Bone and Joint Surgery 0301-620X/02/311344 $2.00 VOL. 84-B, NO. 3, APRIL 2002
We have therefore compared in a prospective, randomised trial the in vivo polyethylene wear rate of femoral heads of 22 mm with those of 32 mm and its effect on the development of osteolysis.
Patients and Methods Between 1986 and 1987 we performed 126 primary total hip replacements using the same cemented SLS-86 stem (Müller straight-stem; implant material, hot-forged Ti6Al7Nb; head material, CoCrMo; Sulzer Orthopedics, Baar, Switzerland) combined with a cemented polyethylene cup by the same manufacturer (ultra-high molecular-weight polyethylene RCH-1000 Chirulen). The selection of the size of the femoral head was based on a random assignment of 22 mm and 32 mm heads determined by a randomnumber table. Thirty-seven patients were excluded either because of previous hip surgery (21), hip infection (1), or moderate or severe developmental dysplasia of the hip (15), leaving 89 patients in the study. There were 37 women and 52 men with a mean age of 66.5 years (SD 8.7) and a mean weight of 64.5 kg (SD 4.3) for the women and 76.1kg (SD 7.2) for the men. There was no statistically significant difference between the groups. The preoperative diagnosis of the affected hip was primary coxarthrosis in 73, avascular necrosis of the femoral head in ten and mild developmental dysplasia in six. All patients were operated on in the supine position using a transgluteal approach. In 49 implants the size of the femoral head was 22 mm and in 40 it was 32 mm. Both components were cemented with polymethylmethacrylate cement (PMMA; Palacos, Sulzer Orthopedics). The outer diameter of the cup ranged between 50 and 58 mm. The minimal thickness of polyethylene was 10 mm. The mean thickness of polyethylene for the 32 mm heads was 16.2 mm (SD 2.8) and for the 22 mm heads it was 13 mm (SD 1.4) which was statistically significant (p = 0.03). The stem size, cup size, and leg length were determined before 13 operation. All the hips were assessed radiologically by preoperative and postoperative anteroposterior (AP) and lateral radiographs. Further radiological evaluation was performed at three, six and 12 months after operation. After this period the patients were reviewed annually. A single observer made all the measurements. 447
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Fig. 1 Diagram showing the digital measurement of the horizontal and vertical distances between the centre of the femoral head and the centre of the cup.
The amount of wear was evaluated on digitised radiographs using software developed in collaboration with the 14 Maurice E. Müller Foundation, Berne, Switzerland. The area of the radiograph containing the acetabular and femoral components was scanned with a resolution of 2000 dots per inch (dPI) (Fig. 1). The head was then magnified tenfold and the contour defined with three points by the investigator. This information was used to calculate the centre of the femoral head. Using the known diameter of the femoral head (accuracy given by the manufacturer, SD 0.001 mm; Sulzer Orthopaedics) the image was calibrated. The centre of the cup was determined from the centre of a line crossing the two apices of an ellipse formed by the 10,11 metal wire in the border of the cup. With this information, the software calculated the centres of the femoral head and the cup and the vertical and horizontal distances of the
two centres in a co-ordinate system defined by a horizontal line through the teardrop and its perpendicular axis (Fig. 1). The measurement of each radiograph was repeated four times to calculate a mean value as well as the standard deviation and standard error. The direct linear distance between the two centres was calculated based on Pythago2 2 ras’ theorem (⌬d = 公(⌬x +⌬y ) where x is the horizontal distance and y the vertical distance between the two centres in the co-ordinate system described above. Volumetric wear was then calculated by the cylindrical formula 2 (⌬V = r ⌬d) where r is the known diameter of the femoral head. The accuracy of the described technique of digital measurement had an SD of 0.01 mm, determined by digitising in vitro ten radiographs of a 52 mm acetabular cup 14 together with a 28 mm prosthetic head. The radiological evaluation was based on the criteria 15 defined by Johnston et al. Osteolysis on the femoral side 16 was rated according to Gruen, McNeice and Amstutz and on the acetabular side according to DeLee and 17 Charnley. Statistical analysis. The mean rates of wear were compared statistically with the size of the head using the unpaired Student t-test with a significance level of p t 0.05. Other parameters tested for a significance correlation with the wear rate of the acetabular polyethylene were age, gender (Student’s t-test), the size of the cup and the rate of osteolysis. One-factor ANOVA analysis was used to compare ungrouped numerical data and the level of significance was p < 0.05.
Results The mean clinical and radiological follow-up was 71.4 months (SD 29.1). A total of 484 radiographs was measured digitally (5.4 follow-ups per patient). Table I shows the cumulated linear polyethylene wear and the 95% confidence intervals (CI) for the 22 and 32 mm heads. Table II
Table I. Mean cumulative linear polyethylene wear (mm) with 95% CIs for both sizes of head 22 mm head
32 mm head
Cumulated linear wear
95% CI
Cumulated linear wear
95% CI
3 mths
0.45
0.37 to 0.53
0.53
0.40 to 0.66
6 mths
0.88
0.77 to 0.99
0.92
0.75 to 1.09
Follow-up period
12 mths
1.20
1.03 to 1.37
1.31
1.18 to 1.44
2 yrs
1.38
1.27 to 1.49
1.48
1.33 to 1.63
3 yrs
1.49
1.36 to 1.62
1.64
1.49 to 1.79
4 yrs
1.59
1.51 to 1.67
1.80
1.38 to 2.22
5 yrs
1.70
1.43 to 1.97
1.94
1.79 to 2.09
6 yrs
1.80
1.38 to 2.22
2.09
1.84 to 2.34
7 yrs
1.91
1.44 to 2.39
2.22
1.90 to 2.54
8 yrs
2.04
1.51 to 2.57
2.39
2.22 to 2.56
9 yrs
2.16
1.86 to 2.46
2.55
2.19 to 2.91 THE JOURNAL OF BONE AND JOINT SURGERY
COMPARISON OF POLYETHYLENE WEAR WITH FEMORAL HEADS OF 22 MM AND 32 MM
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Table II. The mean annual linear wear rate (mm/year) and the mean calculated volumetric wear (mm /year) with 95% CI for both sizes of head 22 mm head
32 mm head
Linear wear
95% CI
Volumetric wear
3 mths
1.8
1.50 to 2.10
669.6
556.5 to 782.7
2.12
1.59 to 2.65
1668.4
1249.8 to 2087.1*
6 mths 12 mths
1.72 0.64
1.50 to 1.94 0.56 to 0.72
639.8 119.0
556.9 to 722.8 101.9 to 136.2
1.56 0.78
1.27 to 1.85 0.70 to 0.86
1227.7 613.9
999.5 to 1456.0* 551.7 to 676.0*
2 yrs 3 yrs
0.18 0.11
0.17 to 0.20 0.10 to 0.12
67.0 40.9
61.4 to 72.5 37.2 to 44.6
0.17 0.16
0.15 to 0.19 0.15 to 0.18
133.8 125.9
120.4 to 147.2* 114.1 to 137.7*
4 yrs 5 yrs
0.10 0.11
0.10 to 0.11 0.90 to 0.13
37.2 40.9
35.3 to 39.1 34.647.2
0.16 0.14
0.12 to 0.20 0.13 to 0.15
125.9 110.2
96.8 to 155.0* 101.5 to 118.8*
6 yrs 7 yrs
0.10 0.11
0.08 to 0.12 0.08 to 0.14
37.2 40.9
28.6 to 45.8 30.9 to 51.0
0.15 0.13
0.130.17 0.11 to 0.15
118.1 102.3
103.9 to 132.2* 87.4 to 117.3*
8 yrs 9 yrs
0.13 0.12
0.10 to 0.16 0.10 to 0.14
48.4 44.6
35.7 to 61.0 38.3 to 51.0
0.17 0.16
0.16 to 0.18 0.14 to 0.18
133.8 125.9
124.4 to 143.2* 107.8 to 144.0*
Follow-up
95% CI
Linear wear
95% CI
Volumetric wear
95% CI
* p < 0.05
gives the annual linear wear rate and the calculated volumetric wear rate for both sizes of head. During the first 12 months the wear rate was more than ten times higher than that after this period. After the second year it reached a constant value of 0.11 (SEM 0.017) mm/year for the 22 mm head and 0.15 (SEM 0.014) mm/year for the 32 mm head. 3 This gave a mean volumetric wear rate of 41.5 mm /year 3 (SEM 4.1) for the 22 mm head and 120.3 mm /year (SEM 10.8) for the 32 mm head (Figs 2 and 3). The difference in wear rate between the first and second years was highly significant for both sizes of head (Student’s t-test, 22 mm, p = 0.007; 32 mm, p = 0.004). Between the second and third years the difference was significant for the 22 mm head but not for the 32 mm head (Student’s t-test, 22 mm, p = 0.03; 32 mm, p = 0.2). After the third year there was no statistically significant difference in the annual wear rate. The linear wear rates of the 32 mm head were slightly higher than those of the 22 mm heads, but because of the high standard deviation of the measurements the differences were not significant (Table II). By contrast, the calculated volumetric wear rates showed significantly higher values with the 32 mm heads than with the 22 mm heads (Table II). Neither age (p = 0.095), gender (p = 0.09) nor the size of the cup (p = 0.122) showed a correlation with the amount of wear. Osteolysis was present in 17% of the hips mainly around the proximal stem with no difference between the 22 mm and the 32 mm heads (chi-squared test, p = 0.192). No stem showed signs of loosening during this period. In addition, the mean amount of osteolysis on the acetabular side was 8% which did not correlate with the amount of polyethylene wear (chi-squared test, p = 0.32).
Discussion In the past there has been controversy in the literature with 18,19 regard to the measurement of wear. Beckenbaugh and 20 Ilstrup suggested that three-dimensional wear cannot be calculated on two-dimensional radiographs. Wear is VOL. 84-B, NO. 3, APRIL 2002
orientated in the direction of the highest loading, that is, in 21,22 the cranial and medial directions in the hip. Wear in the sagittal plane is relatively small and does not significantly 18,19 23 Clarke et al found that on influence the wear results. conventional radiographs wear cannot be determined because the outer contour of the cup is not easily seen. Like 24 Afifi and Jacob we used the metal wire in the rim of the cup as a landmark to measure the distance between the cup and the centre of the femoral head (Fig. 1). Ohlin and 19 Selvick compared the acetabular wear, measured as the radiological distance between the centre of the femoral head and that of the ellipse formed by the metal wire in the cup, with direct measurement in retrieved cups and found it to differ by less than 5%. In order to decrease the error of measurement we digitised the radiographs. In this way we were able to calibrate all the images to obtain the same enlargement factor, which normally varies between 1.16 13 and 1.22. A second advantage of the digitisation was that, compared with the direct measurement on the hard copy of a radiograph, distances could be enlarged to define the measured points more accurately. With a resolution of 200 dPI the discrimination of two measured points was 0.036 mm. In our study, the mean error of measurement was 7.7%, but because we were not concerned with the absolute amount of wear but with the relationship between the wear rates of the two sizes of head this error could be neglected. With both sizes of head the linear wear rate was significantly increased during the first two years, and 25 declined to a constant rate subsequently. Dowling et al suggested an initial higher wear rate because of a ‘run-in’ caused by irregularities of the surfaces of the head and cup. 26 27 Rose et al and Zichner and Willert found in their investigations in vitro that a major part of the measured wear rates was initially caused by plastic deformation of the polyethylene. To our knowledge we are the first to demonstrate a reversed exponential wear behaviour of the polyethylene cup in vivo in relation to time (Figs 2 and 3). The values of linear wear rate are in agreement with other
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Fig. 2
Fig. 3
Comparison of the linear wear rates (mm/year) of the 22 mm and the 32 mm heads.
Comparison of the volumetric wear rates (mm /year) of the 22 mm and the 32 mm heads.
measurements in vitro and in vivo, which range between 0.11 and 0.15 for the 22 mm head and 0.1 and 0.214 for the 10,11,28 The differences are caused by varia32 mm head. tions in the techniques of measurement, the types of prosthesis used and the quality of the polyethylene, and do not allow comparison of the results. Nevertheless, the findings are in agreement in that 22 mm heads have less linear wear than 32 mm heads. Statistically, we could not find a significant difference in the linear wear between the two sizes of head because of the high deviation of the measured values, but a comparison of the volumetric wear showed that it was approximately three times higher with the 32 mm head than with the 22 mm head, which was significant. Although the different wear rates during the first two years cannot be compared because most of the wear may represent plastic deformation of the polyethylene, the values after this time are consistent and clearly indicate that the 32 mm head produces significantly more wear. We 10 agree with Kesteris et al that these measurements may not be taken as absolute values but are valid when comparing the wear rate between the two sizes of head. Possible causes for the increased wear rate in 32 mm heads are an 10-12,14 and in 22 mm heads betincreased sliding distance, 10,12-14,29 ter stress distribution and increased polyethylene 11,14 thickness. We now know that the primary reason for aseptic loosening is osteolysis caused by a foreign-body reaction initiated 2,4,30-32 by polyethylene wear. Several long-term studies have shown that there is a close correlation between the 33 amount of wear debris and osteolysis. Sochart found that the 25-year survivorship exceeded 90% for arthroplasties with a wear rate of less than 0.1 mm per year, but that the 20-year survivorship of acetabular components with a wear rate greater than 0.2 mm per year was below 30%. Also 34 Maloney et al showed that there was a significant correlation between the amount of wear and loosening of either 29 the acetabular or femoral components. Murray showed in
his survivorship analysis that a prosthesis with a 32 mm head had a shorter survival than that with a 22 mm articulation. In our study we could not find a significant correlation between the wear rate and the amount of osteolysis, but we consider that the mean follow-up time of 71.4 months was too short to allow conclusions to be made. Our findings have shown that 32 mm heads produce significantly higher polyethylene acetabular wear in vivo than 22 mm heads. The measured wear distance shows a reversed exponential course but we cannot comment on how much of this distance is produced by real wear and how much by plastic deformation. We could not find a significant correlation between the wear rate and the amount of osteolysis after a mean follow-up period of 71 months.
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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.
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