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A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients Enrolled in a Randomized Controlled Trial
The Journal of Arthroplasty xxx (2015) xxx–xxx
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A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients Enrolled in a Randomized Controlled Trial Anders Troelsen, MD, PhD, DMSc a,b, Meridith E. Greene, PhD a, David C. Ayers, MD c, Charles R. Bragdon, PhD a, Henrik Malchau, MD, DMSc a a b c
Harris Orthopaedic Laboratory, Department of Orthopedics, Massachusetts General Hospital, Boston, Massachusetts Department of Orthopedic Surgery, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark Department of Orthopedics, University of Massachusetts Medical School, The Arthritis and Joint Replacement Center, Worchester, Massachusetts
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Article history: Received 15 April 2015 Accepted 25 June 2015 Available online xxxx Keywords: radiostereometric analysis (RSA) polyethylene wear novel method for wear measurement total hip replacement conventional polyethylene highly cross-linked polyethylene
a b s t r a c t Conventional radiostereometric analysis (RSA) for wear is not possible in patients with tantalum cups. We propose a novel method for wear analysis in tantalum cups. Wear was assessed by gold standard RSA and the novel method in total hip arthroplasty patients enrolled in a randomized controlled trial receiving either titanium or tantalum cups (n = 46). The novel method estimated the center of the head using a model based on identification of two proximal markers on the stem and knowledge of the stem/head configuration. The novel method was able to demonstrate a pattern of wear that was similar to the gold standard in titanium cups. The novel method offered accurate assessment and is a viable solution for assessment of wear in studies with tantalum cups. © 2015 Elsevier Inc. All rights reserved.
Wear of polyethylene (PE) liners in total hip arthroplasty (THR) can result in particle induced osteolysis [1–4]. This mechanism has been recognized as a frequent cause for aseptic implant loosening and late revisions in primary THR [5,6]. Advances in polymer engineering have aimed to reduce PE wear, and low wear has been reported for highly cross-linked PE [7–14]. Assessment of implant performance, including wear of the PE component, is very important and is achieved through clinical studies [15]. In these clinical studies radiographic images are used to assess, over time, the relative displacement between the femoral head and the acetabular cup. A change in this relative distance will indicate a movement of the femoral head relative to the cup which will indicate wear of the PE liner. The most common techniques that are available to analyze these radiographs for possible wear are computer-assisted edge-detection techniques and radiostereometric analysis (RSA) [16]. When comparing the edge detection techniques to RSA, RSA provides improved accuracy and better precision [17]. In RSA measurements, One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.06.050. Level of Evidence: Diagnostic Level 1. Reprint requests: Meridith E. Greene, PhD, Harris Orthopaedic Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ 1126, Boston, MA 02114-2696.
the acetabular cup segment is defined by edge detection of the acetabular shell and detection of the radiopaque markers inserted into the PE liner. When beads are not inserted into the PE liner, a markerless ‘backshell only’ method can be implemented utilizing only the edge detection of the acetabular shell; however in UmRSA, this results in fewer points defining the acetabular unit (5 points compared to up to 9 points). The femoral head segment is defined by edge detection of this component. The use of titanium acetabular cups allows visualization of the radiopaque PE liner markers and the contour of the femoral head. In recent years, the use of acetabular cups made of tantalum is increasingly more common. However, in tantalum acetabular cups the liner markers and the contours of the femoral head cannot be identified due to the radiopaque characteristics of tantalum. Therefore the cup segment can be defined by the acetabular shell only, and the femoral head segment cannot be defined. The same limitations apply to the computer-assisted edge-detection techniques. Thus, the use of tantalum cups prevents the assessment of PE wear, and currently there is no validated solution for clinical studies of in-vivo wear performance for this type of cup. The inability to measure wear performance for THR where tantalum cups are used, has important implications for in-vivo quality assessment when new PE liners are developed. The development of a technique to reliably assess PE wear in liners implanted with tantalum acetabular cups is therefore warranted. We propose a novel method in which specific knowledge about the size of the implanted femoral stem, the length and size of the implanted
http://dx.doi.org/10.1016/j.arth.2015.06.050 0883-5403/© 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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A. Troelsen et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
femoral head, and the position of two markers placed at the proximal end of the femoral stem are used to estimate the position of the femoral head center. Based on 5-year follow-up with RSA radiographs in a randomized controlled trial in which patients received either a titanium or a tantalum acetabular cup and then either a conventional PE or a highly cross-linked PE liner we aim to: (1) investigate if the novel method can calculate femoral head penetrations that are similar to those produced by gold standard RSA when assessed in titanium cups; (2) investigate the accuracy and variation of the novel method for assessment of femoral head penetration in titanium cups; and (3) investigate if assessment of femoral head penetration by the novel method demonstrates expected wear patterns in conventional and highly cross-linked PE liners when assessed in tantalum acetabular cups. We hypothesized that the novel method is able to convey accurate measurements of PE wear with expected increased variation of measures and that the novel method demonstrates expected PE wear patterns with the ability to show differences between conventional and highly cross-linked PE liners.
uniplanar calibration cage (cage 43, RSA Biomedical, Umeå, Sweden). The digitized RSA radiographic pairs were analyzed using UmRSA Digital Measure (Version 6, RSA Biomedical, Umeå, Sweden), including marker identification and edge detection, by one investigator. Software processing (UmRSA Analysis, Version 6, RSA Biomedical, Umeå, Sweden) of femoral head penetration was performed by one investigator. The absolute femoral head penetration at each follow-up was assessed using the postoperative RSA radiographic pair as reference. Femoral head penetration was assessed as the movement of the femoral head segment relative to the acetabular cup segment in the y-plane (proximal direction). For analysis by gold standard RSA the acetabular cup segment was defined by edge detection of the acetabular shell and detection of the radio-opaque markers in the PE liner, and the femoral head segment was defined by edge detection of this component. For analysis by the novel method, the cup segment was defined by the acetabular shell only, and the femoral head segment was defined by calculation of the head center (detailed later). Point correction was done to secure valid comparison of follow-up RSA radiographs to the postoperative film pair. Mean errors upon point correction of more than a predefined level of 0.250 mm led to exclusion from the analysis. All initially enrolled patients (n = 46) were eligible for inclusion in the present study. The following exclusion criteria were applied: Patients with no postoperative RSA radiographic assessment (n = 0), patients with no RSA radiographic assessment after the postoperative assessment (n = 1), or if valid comparison of follow-up RSA radiographs to the postoperative RSA film pair could not be done based on assessment by point correction as described (n = 0). Table 1 shows the follow-up availability.
Materials and Methods The eligible cohort included 46 THR patients (46 hips) enrolled between May 2005 and June 2008, in a randomized, controlled trial. The study was approved by an institutional review board and all patients gave informed consent prior to surgery. Patients (19 male/27 female) were less than 75 years of age (median age: 59 years, range: 42–73 years), had a BMI less than 40, and a diagnosis of osteoarthritis (n = 42) or avascular necrosis (n = 4). All acetabular cups implanted were uncemented, and patients were randomized to receive either a titanium (Trilogy, Zimmer, Warsaw, IN, USA) (n = 23) or a tantalum (Trabecular Metal, Zimmer) (n = 23) acetabular cup with either a conventional (Zimmer) or a highly crosslinked (Longevity, Zimmer) PE standard liner. Titanium cups were inserted with 11 conventional PE liners and 12 highly cross-linked PE liners. Tantalum cups were inserted with 11 conventional PE liners and 12 highly cross-linked PE liners. During surgery, radiopaque tantalum markers, 1 mm in diameter, were inserted along the nonarticulating edge of the PE liners. Similar markers were also implanted in the periacetabular bone and greater and lesser trochanter, however, these were not used in this study. The femoral component was an uncemented, tapered, proximally porous-coated, titanium stem (M/L Taper, Zimmer), with standard offset and a 28 mm head component. For purposes of the study the stems were manufactured with one radiopaque tantalum marker on each side of the prosthetic collar and one at the tip of the stem. The 2-year clinical outcomes were previously reported together with femoral head penetration assessments in the subgroup of patients receiving titanium cups [18]. RSA radiographic follow-ups were planned postoperatively, at 6 months, and annually from 1 to 5 years. At follow-up, sets of RSA radiographs were taken with patients standing and utilizing a standard
The Novel Method The novel “fictive head” method is based on identification of two radiopaque tantalum markers attached to the stem proximally and apriori knowledge of the femoral head/stem configuration of the inserted THR. A stem configuration library was created from a series of radiographs, for the same stem size, head diameter, neck length, e.g. all patients with stem size 14, 28 mm head diameter, and + 3.5 mm neck length. For all patients with the same stem configuration the library provided information as to the mean distance between the medial femoral neck marker and the head center as well as the mean angle created between the line connecting the medial and lateral proximal stem markers (“inter-marker” line) and the line connecting the medial marker and the femoral head center line (Table 2). This geometrical information was then used to place, in 3D space, a fictive femoral head center to be used by the RSA software (Fig. 1). Because wear occurs primarily in the y-direction [19], the z-axis position of the head was assumed to remain aligned with that of the acetabular cup and therefore, was assigned the z-axis position of the acetabular cup as calculated by the edge detection. The fictive head point (femoral head center) was then inserted into the RSA study database and kinematics were calculated
Table 1 Counts of Follow-Up (FU) Availability at Each Interval After Exclusions Were Applied. Decreases in Follow-Up Counts Were Due To Deaths, Loss to Follow-Up, Missed Appointments (MA), and Image Failure in the RSA Program. Titanium Cup/Highly Cross-Linked PE Liner (n = 11)
Titanium Cup/Conventional PE Liner (n = 11)
Tantalum Cup/Highly Cross-Linked PE Liner (n = 12)
Tantalum Cup/Conventional PE Liner (n = 11)
Follow-up Interval
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
6 Months 1 Year 2 Years 3 Years 4 Years 5 Years
10 11 9 8 9 10
0 0 1 0 0 0
1 0 1 2 1 0
10 11 10 5 9 8
0 0 1 1 0 0
1 0 0 4 0 1
11 12 10 8 9 7
0 0 2 0 0 0
1 0 0 2 1 3
10 10 7 9 8 9
0 0 0 0 0 0
1 1 4 2 3 2
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
A. Troelsen et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
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Table 2 Average and Standard Deviations of Distances from Medial Stem Bead to Center of Femoral Head and In-Plane Angles Between Medial-Head Line and Medial-Lateral Line. Stem Group
Average Distance from Medial Bead to Head Center (mm)
In-Plane Angle Between Medial-Head Line and Medial-Lateral Line (radians)
A B C D E F G H K M
34.4 37.7 34.6 37.6 34.4 37.5 34.4 37.7 34.6 37.6
1.10 1.13 1.13 1.16 1.16 1.22 1.22 1.26 1.23 1.30
± ± ± ± ± ± ± ± ± ±
0.1 0.06 0.02 0.02 0.1 0.04 0.07 0.02 0.02 0.1
± ± ± ± ± ± ± ± ± ±
0.001 0.0018 0.00097 0.00047 0.00052 0.0016 0.0039 0.001 0.0009 0.007
according the standards of the software. One author performed computer programming for placement of the fictive femoral head center.
Theory/Calculation Due to the relatively small sample sizes in some subgroups, we did not consider femoral head penetration data to be normally distributed. Accordingly, data are presented as median values with inter-quartile ranges, and comparisons are made using the two-sample Wilcoxon rank-sum (Mann–Whitney) test. However, in the largest subgroups data could be considered normally distributed. To facilitate interpretation of our data and to provide an instrument for sample size calculations of future studies, we also present penetration measures as means and standard deviations. Standard deviations are compared using a variance ratio test. Assessment of agreement between gold standard RSA and the novel method for assessment of femoral head penetration are made using a Bland–Altman approach with
Fig. 2. Femoral head penetration (in mm on y-axis) for conventional polyethylene liners in titanium cups assessed by gold standard RSA (blue) and the novel method (red) at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
presentation of data as difference of the means and 95% limits of agreement (LOA) [20]. The level of significance was set at P b 0.05. The cohort size (n = 46) was estimated based on considerations about the ability to detect statistically significant differences in linear femoral head penetration between two groups (conventional PE vs. highly cross-linked PE) [18]. For the purpose of the present study, we estimated the minimum required sample size for detection of statistically significant differences in femoral head penetration between conventional PE liners and highly cross-linked PE liners to be 4 patients in each group at 5 years (80% power and significance level of 0.05). Based on results of previous studies [8,13] we assumed 5 year femoral head penetration of 0.400 mm and 0.150 mm (SD: ±0.125) in conventional PE liners and highly cross-linked PE liners, respectively.
Source of Funding The randomized controlled trial was funded by Zimmer (Warsaw, IN, USA). However, for the development and investigation of the novel “fictive head” method there was no external funding source.
Fig. 1. The novel method utilizes information about the distance between the medial femoral neck marker and the head center (MC) as well as the angle (α) created between the line connecting the medial and lateral proximal stem markers (IM) and the line connecting the medial marker and the femoral head center line (MC).
Fig. 3. Femoral head penetration (in mm on y-axis) for highly cross-linked polyethylene liners in titanium cups assessed by gold standard RSA (blue) and the novel method (red) at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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A. Troelsen et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Table 3 Femoral Head Penetration for Conventional Polyethylene Liners in Titanium Cups Assessed by Gold Standard RSA and the Novel Method. Time Postoperatively (No. of Patients)
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) Gold Standard RSA
Novel Method
6 Months (n = 10)
0.078 (0.043–0.131) 0.085 (0.066) 0.177 (0.121–0.222) 0.169 (0.074) 0.238 (0.206–0.288) 0.241 (0.071) 0.330 (0.330–0.400) 0.328 (0.122) 0.340 (0.210–0.509) 0.358 (0.139) 0.379 (0.280–0.415) 0.366 (0.127)
0.032 (−0.022 to 0.068) 0.039 (0.089) 0.058 (0.011–0.117) 0.106 (0.146) 0.207 (0.082–0.248) 0.200 (0.135) 0.391 (0.243–0.394) 0.327 (0.138) 0.297 (0.274–0.610) 0.425 (0.210) 0.320 (0.250–0.446) 0.380 (0.199)
1 Year (n = 11) 2 Years (n = 10) 3 Years (n = 5) 4 Years (n = 9) 5 Years (n = 8)
Fig. 4. Bland–Altman plot showing the difference of means (solid red line) and 95% limits of agreement (dashed red lines) for paired assessments of femoral head penetration by gold standard RSA and the novel method.
Results Assessment of femoral head penetration in titanium acetabular cups by the novel “fictive head” method demonstrated wear patterns comparable to that of the gold standard RSA in both conventional (Fig. 2) and highly cross-linked (Fig. 3) PE liners. Cumulative femoral head penetration in titanium cups measured by each method showed increased wear over time in the conventional PE liners (Table 3), and although lower, both methods demonstrated increased penetration into the highly cross-linked PE liners as well (Table 4). Assessment of the standard deviations (SD) for paired assessments of femoral head penetration in titanium cups (n = 109) showed that measurement by the gold standard RSA (SD: ±0.137 mm) was significantly less than with the novel method (SD: ±0.183 mm) (P = 0.0006). We also evaluated the effect of defining the cup segment by edge detection of the acetabular shell alone (and not by both the shell and markers in the PE liner), and found an increase in the standard deviation from ± 0.137 mm to ± 0.163 mm (P = 0.07). Based on 109 paired assessments of femoral head penetration by the novel method and gold standard RSA in titanium cups, we evaluated the accuracy of the novel method. The difference between the means of the femoral head penetration values was 0.008 mm suggesting that little systematic error was introduced when penetration assessments were performed by the novel method. The 95% LOA were −0.218 mm to 0.234 mm, meaning that 95% of assessments performed by the
Table 4 Femoral Head Penetration for Highly Cross-Linked Polyethylene Liners in Titanium Cups Assessed by Gold Standard RSA and the Novel Method. Time Postoperatively (No. of Patients)
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) Gold Standard RSA
Novel Method
6 Months (n = 10)
0.060 (0.022–0.081) 0.053 (0.060) 0.069 (0.024–0.100) 0.057 (0.071) 0.105 (0.044–0.110) 0.094 (0.064) 0.039 (0.015–0.100) 0.053 (0.040) 0.090 (0.080–0.100) 0.091 (0.032) 0.082 (0.050–0.140) 0.091 (0.046)
0.024 (−0.035 to 0.100) 0.038 (0.084) 0.029 (−0.102 to 0.068) 0.010 (0.103) 0.067 (0.013–0.171) 0.072 (0.176) −0.020 (−0.040 to 0.198) 0.024 (0.181) 0.113 (0.018–0.186) 0.113 (0.120) 0.171 (0.128–0.212) 0.173 (0.102)
1 Year (n = 11) 2 Years (n = 9) 3 Years (n = 7) 4 Years (n = 9) 5 Years (n = 10)
novel method would fall within this interval of the corresponding gold standard RSA assessments of femoral head penetration (Fig. 4). Assessments of femoral head penetration by the novel method in tantalum cups showed expected patterns with less penetration in highly cross-linked liners compared with conventional liners at all followups (Fig. 5). After the 6-month follow-up, the difference in femoral head penetration between the two liner types was statistically significant (P = 0.003–0.04) at all follow-ups, except at the 3 year follow-up when tested by the non-parametric test (Table 5). Discussion Using the novel “fictive head” method we estimated femoral head penetration in titanium cups and demonstrated a pattern that was similar to that of gold standard RSA in both conventional and highly crosslinked PE liners. Little systematic error was introduced when penetration assessments were performed by the novel method, but standard deviations of assessed femoral head penetration were statistically different (P = 0.0006) from assessment by gold standard RSA. Assessments of femoral head penetration by the novel method in tantalum cups showed as expected less penetration in highly cross-linked liners compared with conventional liners at all follow-ups. Several factors should be acknowledged as contributors to increased variation in measurements by the novel “fictive head” method compared with gold standard RSA: The exact position of the proximal stem markers may vary between femoral stems. There may be differences in actual length of the head–neck segment between femoral components with the same configuration of stem and head sizes due to differences in femoral head impaction. These issues will affect the mathematical algorithms of the novel method. To our knowledge, there are no reports that document the influence of these potential variations on these types of measurements. Further, we found that accurate edge detection of the acetabular cup opening was more difficult in tantalum acetabular cups and that the radiopaque liner markers obstructed the edge detection of the opening. Therefore, when planning future studies utilizing the novel method we recommend that radiopaque markers are not inserted into the PE liner. Using the novel method, the acetabular cup segment is defined by the acetabular shell only and may result in greater variation of penetration compared to defining the segment by both the shell and PE liner markers [21,22]. In our study, we found that the standard deviation of femoral head penetration increased from ±0.137 mm to ±0.163 mm when defining the acetabular cup segment solely by edge detection of the acetabular shell. Utilizing the novel “fictive head” method is restricted by technical demands limiting the use of the method. First, radiopaque tantalum
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
A. Troelsen et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 5. Femoral head penetration (in mm on y-axis) for conventional (blue) and highly cross-linked (red) polyethylene liners in tantalum cups assessed by the novel method at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
markers must be attached to the femoral stems. However, as RSA based prospective follow-up of femoral head penetration is always done in well planned studies and in accordance with an IRB approved protocol, the supply of femoral stems with markers can be planned. Second, a library containing information on all combinations of stem sizes and femoral head–neck lengths and sizes is needed. However, this library has to contain information only about the stem configurations used in the study, and once obtained, the library can be used in subsequent studies. Femoral head penetration was reported as movement of the center of the femoral head relative to the acetabular cup segment along the x-axis, y-axis and z-axis. The need and usefulness of reporting wear along the z-axis remains debated [19,23]. Reporting penetration along the y-axis (proximal) or x-axis and y-axis (medial and proximal) is commonly accepted as the majority of wear occurs in frontal plane [16]. When utilizing the novel “fictive head” method, the calculated femoral head center is assigned to the same position along the z-axis
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as the acetabular cup center, and thus, any penetration in this plane is ignored. We found very little systematic error when applying the novel “fictive head” method, which supports our hypothesis that the novel method can convey accurate measures of femoral head penetration. As we also hypothesized, and have discussed above, we found statistically increased variation (standard deviations) of femoral head penetration measurements by the novel method compared with gold standard RSA. The femoral head penetration assessed by the novel method in conventional and highly cross-linked PE liners at 5 year follow-up showed results comparable to that observed by other authors [8,13,24]. Our hypothesis that the novel method demonstrates expected PE wear patterns with the ability to show differences between conventional and highly cross-linked PE liners was affirmed. Some overall methodological strengths and limitations should be acknowledged and discussed: RSA radiographs were recorded with patients bearing weight, which has been considered important in some reports, whereas other authors have reported no difference of femoral head penetration between supine and weight bearing assessment [25]; The study was performed on a patient cohort enrolled in a randomized controlled trial, thus limiting bias, but blinding of RSA observers was not possible due to the easily identifiable differences between titanium and tantalum cups; the a-priori sample size calculations that we performed suggested that our sample size was sufficient, however variation of assessments by the novel “fictive head” method turned out greater than expected in some subgroups, thus potentially compromising the ability to detect statistically significant differences. Using the overall standard deviation for the novel method of ±0.193 mm from this study a post-hoc analysis reveal that 10 patients in each group would be required to obtain statistical significance of the assumed difference at 5 year follow-up (80% power and significance level of 0.05). This has important implications for planning of studies utilizing the novel method. As we have shown the “fictive head” method results in higher standard deviations of femoral head penetration proving that the gold standard remains the best option for measurement of polyethylene wear. However, when radiodense tantalum cups are used clinically, the only option for measuring femoral head penetration is a method utilizing a fictive femoral head center.
Table 5 Femoral Head Penetration for Highly Cross-Linked and Conventional Polyethylene Liners in Tantalum Cups Assessed by the Novel Method. Time Postoperatively
6 Months
1 Year
2 Years
3 Years
4 Years
5 Years
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) No. of Observations Highly Cross-Linked PE Liner
Conventional PE Liner
P Value Wilcoxon Rank-Sum (Mann–Whitney) Test T-Test
0.028 (−0.085 to 0.219) 0.103 (0.277) N = 11 −0.001 (−0.073 to 0.131) 0.035 (0.157) N = 12 0.032 (−0.066 to 0.178) 0.054 (0.180) N = 10 0.155 (−0.045 to 0.229) 0.082 (0.187) N=8 0.155 (−0.110 to 0.290) 0.099 (0.286) N=7 0.078 (−0.062 to 0.290) 0.100 (0.251) N=8
0.105 (0.005–0.228) 0.127 (0.120) N = 10 0.124 (0.118–0.257) 0.170 (0.078) N = 10 0.269 (0.177–0.335) 0.251 (0.116) N=6 0.221 (0.165–0.480) 0.282 (0.182) N=9 0.381 (0.305–0.460) 0.395 (0.123) N=8 0.420 (0.346–0.430) 0.458 (0.156) N=9
0.18 0.80 0.02 0.02 0.04 0.03 0.18 0.04 0.03 0.02 0.02 0.003
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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Conclusions In conclusion, our results suggest that the novel "fictive head" method is accurate, however, as expected, the variation of femoral head penetration measures is greater than for gold standard RSA. Consequently using radio-transparent cups with polyethylene markers will give more reliable results. The novel method seems to be a viable solution for assessment of femoral head penetration in prospective studies on conventional and highly cross-linked PE liners, in which the implantation of tantalum cups would otherwise make wear assessment impossible. References 1. Amstutz HC, Campbell P, Kossovsky N, et al. Mechanism and clinical significance of wear debris-induced osteolysis. Clin Orthop Relat Res 1992;276:7. 2. Dumbleton JH, Manley MT, Edidin AA. A literature review of the association between wear rate and osteolysis in total hip arthroplasty. J Arthroplasty 2002;17(5):649. 3. Jacobs JJ, Roebuck KA, Archibeck M, et al. Osteolysis: basic science. Clin Orthop Relat Res 2001;393:71. 4. Maloney WJ, Galante JO, Anderson M, et al. Fixation, polyethylene wear, and pelvic osteolysis in primary total hip replacement. Clin Orthop Relat Res 1999;369:157. 5. Clohisy JC, Calvert G, Tull F, et al. Reasons for revision hip surgery: a retrospective review. Clin Orthop Relat Res 2004;429:188. 6. Harris WH. Conquest of a worldwide human disease: particle-induced periprosthetic osteolysis. Clin Orthop Relat Res 2004;429:39. 7. Bragdon CR, Kwon YM, Geller JA, et al. Minimum 6-year followup of highly crosslinked polyethylene in THA. Clin Orthop Relat Res 2007;465:122. 8. Digas G, Karrholm J, Thanner J, et al. 5-year experience of highly cross-linked polyethylene in cemented and uncemented sockets: two randomized studies using radiostereometric analysis. Acta Orthop 2007;78(6):746. 9. McCalden RW, MacDonald SJ, Rorabeck CH, et al. Wear rate of highly cross-linked polyethylene in total hip arthroplasty. A randomized controlled trial. J Bone Joint Surg Am 2009;91(4):773. 10. Muratoglu OK, Bragdon CR, O'Connor DO, et al. A novel method of cross-linking ultrahigh-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award. J Arthroplasty 2001; 16(2):149.
11. Mutimer J, Devane PA, Adams K, et al. Highly crosslinked polyethylene reduces wear in total hip arthroplasty at 5 years. Clin Orthop Relat Res 2010;468(12):3228. 12. Oral E, Malhi AS, Wannomae KK, et al. Highly cross-linked ultrahigh molecular weight polyethylene with improved fatigue resistance for total joint arthroplasty: recipient of the 2006 Hap Paul Award. J Arthroplasty 2008;23(7):1037. 13. Thomas GER, Simpson DJ, Mehmood S, et al. The seven-year wear of highly crosslinked polyethylene in total hip arthroplasty: a double-blind, randomized controlled trial using radiostereometric analysis. J Bone Joint Surg Am 2011;93(8):716. 14. Whittaker JP, Charron KD, McCalden RW, et al. Comparison of steady state femoral head penetration rates between two highly cross-linked polyethylenes in total hip arthroplasty. J Arthroplasty 2010;25(5):680. 15. Malchau H, Bragdon CR, Muratoglu OK. The stepwise introduction of innovation into orthopedic surgery: the next level of dilemmas. J Arthroplasty 2011;26(6):825. 16. McCalden RW, Naudie DD, Yuan X, et al. Radiographic methods for the assessment of polyethylene wear after total hip arthroplasty. J Bone Joint Surg Am 2005;87(10): 2323. 17. Bragdon CR, Malchau H, Yuan X, et al. Experimental assessment of precision and accuracy of radiostereometric analysis for the determination of polyethylene wear in a total hip replacement model. J Orthop Res 2002;20(4):688. 18. Ayers DC, Hays PL, Drew JM, et al. Two-year radiostereometric analysis evaluation of femoral head penetration in a challenging population of young total hip arthroplasty patients. J Arthroplasty 2009;24(6 Suppl):9. 19. Martell JM, Berkson E, Berger R, et al. Comparison of two and three-dimensional computerized polyethylene wear analysis after total hip arthroplasty. J Bone Joint Surg Am 2003;85-A(6):1111. 20. Bland JM, Altman DG. Applying the right statistics: analyses of measurement studies. Ultrasound Obstet Gynecol 2003;22(1):85. 21. Borlin N, Rohrl SM, Bragdon CR. RSA wear measurements with or without markers in total hip arthroplasty. J Biomech 2006;39(9):1641. 22. Bragdon CR, Greene ME, Freiberg AA, et al. Radiostereometric analysis comparison of wear of highly cross-linked polyethylene against 36- vs 28-mm femoral heads. J Arthroplasty 2007;22(6 Suppl. 2):125. 23. Hui AJ, McCalden RW, Martell JM, et al. Validation of two and three-dimensional radiographic techniques for measuring polyethylene wear after total hip arthroplasty. J Bone Joint Surg Am 2003;85-A(3):505. 24. Dorr LD, Wan Z, Shahrdar C, et al. Clinical performance of a Durasul highly crosslinked polyethylene acetabular liner for total hip arthroplasty at five years. J Bone Joint Surg Am 2005;87(8):1816. 25. Bragdon CR, Thanner J, Greene ME, et al. Standing versus supine radiographs in RSA evaluation of femoral head penetration. Clin Orthop Relat Res 2006;448:46.
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients Enrolled in a Randomized Controlled Trial Anders Troelsen, MD, PhD, DMSc a,b, Meridith E. Greene, PhD a, David C. Ayers, MD c, Charles R. Bragdon, PhD a, Henrik Malchau, MD, DMSc a a b c
Harris Orthopaedic Laboratory, Department of Orthopedics, Massachusetts General Hospital, Boston, Massachusetts Department of Orthopedic Surgery, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark Department of Orthopedics, University of Massachusetts Medical School, The Arthritis and Joint Replacement Center, Worchester, Massachusetts
a r t i c l e
i n f o
Article history: Received 15 April 2015 Accepted 25 June 2015 Available online xxxx Keywords: radiostereometric analysis (RSA) polyethylene wear novel method for wear measurement total hip replacement conventional polyethylene highly cross-linked polyethylene
a b s t r a c t Conventional radiostereometric analysis (RSA) for wear is not possible in patients with tantalum cups. We propose a novel method for wear analysis in tantalum cups. Wear was assessed by gold standard RSA and the novel method in total hip arthroplasty patients enrolled in a randomized controlled trial receiving either titanium or tantalum cups (n = 46). The novel method estimated the center of the head using a model based on identification of two proximal markers on the stem and knowledge of the stem/head configuration. The novel method was able to demonstrate a pattern of wear that was similar to the gold standard in titanium cups. The novel method offered accurate assessment and is a viable solution for assessment of wear in studies with tantalum cups. © 2015 Elsevier Inc. All rights reserved.
Wear of polyethylene (PE) liners in total hip arthroplasty (THR) can result in particle induced osteolysis [1–4]. This mechanism has been recognized as a frequent cause for aseptic implant loosening and late revisions in primary THR [5,6]. Advances in polymer engineering have aimed to reduce PE wear, and low wear has been reported for highly cross-linked PE [7–14]. Assessment of implant performance, including wear of the PE component, is very important and is achieved through clinical studies [15]. In these clinical studies radiographic images are used to assess, over time, the relative displacement between the femoral head and the acetabular cup. A change in this relative distance will indicate a movement of the femoral head relative to the cup which will indicate wear of the PE liner. The most common techniques that are available to analyze these radiographs for possible wear are computer-assisted edge-detection techniques and radiostereometric analysis (RSA) [16]. When comparing the edge detection techniques to RSA, RSA provides improved accuracy and better precision [17]. In RSA measurements, One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.06.050. Level of Evidence: Diagnostic Level 1. Reprint requests: Meridith E. Greene, PhD, Harris Orthopaedic Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ 1126, Boston, MA 02114-2696.
the acetabular cup segment is defined by edge detection of the acetabular shell and detection of the radiopaque markers inserted into the PE liner. When beads are not inserted into the PE liner, a markerless ‘backshell only’ method can be implemented utilizing only the edge detection of the acetabular shell; however in UmRSA, this results in fewer points defining the acetabular unit (5 points compared to up to 9 points). The femoral head segment is defined by edge detection of this component. The use of titanium acetabular cups allows visualization of the radiopaque PE liner markers and the contour of the femoral head. In recent years, the use of acetabular cups made of tantalum is increasingly more common. However, in tantalum acetabular cups the liner markers and the contours of the femoral head cannot be identified due to the radiopaque characteristics of tantalum. Therefore the cup segment can be defined by the acetabular shell only, and the femoral head segment cannot be defined. The same limitations apply to the computer-assisted edge-detection techniques. Thus, the use of tantalum cups prevents the assessment of PE wear, and currently there is no validated solution for clinical studies of in-vivo wear performance for this type of cup. The inability to measure wear performance for THR where tantalum cups are used, has important implications for in-vivo quality assessment when new PE liners are developed. The development of a technique to reliably assess PE wear in liners implanted with tantalum acetabular cups is therefore warranted. We propose a novel method in which specific knowledge about the size of the implanted femoral stem, the length and size of the implanted
http://dx.doi.org/10.1016/j.arth.2015.06.050 0883-5403/© 2015 Elsevier Inc. All rights reserved.
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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femoral head, and the position of two markers placed at the proximal end of the femoral stem are used to estimate the position of the femoral head center. Based on 5-year follow-up with RSA radiographs in a randomized controlled trial in which patients received either a titanium or a tantalum acetabular cup and then either a conventional PE or a highly cross-linked PE liner we aim to: (1) investigate if the novel method can calculate femoral head penetrations that are similar to those produced by gold standard RSA when assessed in titanium cups; (2) investigate the accuracy and variation of the novel method for assessment of femoral head penetration in titanium cups; and (3) investigate if assessment of femoral head penetration by the novel method demonstrates expected wear patterns in conventional and highly cross-linked PE liners when assessed in tantalum acetabular cups. We hypothesized that the novel method is able to convey accurate measurements of PE wear with expected increased variation of measures and that the novel method demonstrates expected PE wear patterns with the ability to show differences between conventional and highly cross-linked PE liners.
uniplanar calibration cage (cage 43, RSA Biomedical, Umeå, Sweden). The digitized RSA radiographic pairs were analyzed using UmRSA Digital Measure (Version 6, RSA Biomedical, Umeå, Sweden), including marker identification and edge detection, by one investigator. Software processing (UmRSA Analysis, Version 6, RSA Biomedical, Umeå, Sweden) of femoral head penetration was performed by one investigator. The absolute femoral head penetration at each follow-up was assessed using the postoperative RSA radiographic pair as reference. Femoral head penetration was assessed as the movement of the femoral head segment relative to the acetabular cup segment in the y-plane (proximal direction). For analysis by gold standard RSA the acetabular cup segment was defined by edge detection of the acetabular shell and detection of the radio-opaque markers in the PE liner, and the femoral head segment was defined by edge detection of this component. For analysis by the novel method, the cup segment was defined by the acetabular shell only, and the femoral head segment was defined by calculation of the head center (detailed later). Point correction was done to secure valid comparison of follow-up RSA radiographs to the postoperative film pair. Mean errors upon point correction of more than a predefined level of 0.250 mm led to exclusion from the analysis. All initially enrolled patients (n = 46) were eligible for inclusion in the present study. The following exclusion criteria were applied: Patients with no postoperative RSA radiographic assessment (n = 0), patients with no RSA radiographic assessment after the postoperative assessment (n = 1), or if valid comparison of follow-up RSA radiographs to the postoperative RSA film pair could not be done based on assessment by point correction as described (n = 0). Table 1 shows the follow-up availability.
Materials and Methods The eligible cohort included 46 THR patients (46 hips) enrolled between May 2005 and June 2008, in a randomized, controlled trial. The study was approved by an institutional review board and all patients gave informed consent prior to surgery. Patients (19 male/27 female) were less than 75 years of age (median age: 59 years, range: 42–73 years), had a BMI less than 40, and a diagnosis of osteoarthritis (n = 42) or avascular necrosis (n = 4). All acetabular cups implanted were uncemented, and patients were randomized to receive either a titanium (Trilogy, Zimmer, Warsaw, IN, USA) (n = 23) or a tantalum (Trabecular Metal, Zimmer) (n = 23) acetabular cup with either a conventional (Zimmer) or a highly crosslinked (Longevity, Zimmer) PE standard liner. Titanium cups were inserted with 11 conventional PE liners and 12 highly cross-linked PE liners. Tantalum cups were inserted with 11 conventional PE liners and 12 highly cross-linked PE liners. During surgery, radiopaque tantalum markers, 1 mm in diameter, were inserted along the nonarticulating edge of the PE liners. Similar markers were also implanted in the periacetabular bone and greater and lesser trochanter, however, these were not used in this study. The femoral component was an uncemented, tapered, proximally porous-coated, titanium stem (M/L Taper, Zimmer), with standard offset and a 28 mm head component. For purposes of the study the stems were manufactured with one radiopaque tantalum marker on each side of the prosthetic collar and one at the tip of the stem. The 2-year clinical outcomes were previously reported together with femoral head penetration assessments in the subgroup of patients receiving titanium cups [18]. RSA radiographic follow-ups were planned postoperatively, at 6 months, and annually from 1 to 5 years. At follow-up, sets of RSA radiographs were taken with patients standing and utilizing a standard
The Novel Method The novel “fictive head” method is based on identification of two radiopaque tantalum markers attached to the stem proximally and apriori knowledge of the femoral head/stem configuration of the inserted THR. A stem configuration library was created from a series of radiographs, for the same stem size, head diameter, neck length, e.g. all patients with stem size 14, 28 mm head diameter, and + 3.5 mm neck length. For all patients with the same stem configuration the library provided information as to the mean distance between the medial femoral neck marker and the head center as well as the mean angle created between the line connecting the medial and lateral proximal stem markers (“inter-marker” line) and the line connecting the medial marker and the femoral head center line (Table 2). This geometrical information was then used to place, in 3D space, a fictive femoral head center to be used by the RSA software (Fig. 1). Because wear occurs primarily in the y-direction [19], the z-axis position of the head was assumed to remain aligned with that of the acetabular cup and therefore, was assigned the z-axis position of the acetabular cup as calculated by the edge detection. The fictive head point (femoral head center) was then inserted into the RSA study database and kinematics were calculated
Table 1 Counts of Follow-Up (FU) Availability at Each Interval After Exclusions Were Applied. Decreases in Follow-Up Counts Were Due To Deaths, Loss to Follow-Up, Missed Appointments (MA), and Image Failure in the RSA Program. Titanium Cup/Highly Cross-Linked PE Liner (n = 11)
Titanium Cup/Conventional PE Liner (n = 11)
Tantalum Cup/Highly Cross-Linked PE Liner (n = 12)
Tantalum Cup/Conventional PE Liner (n = 11)
Follow-up Interval
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
Patients with FU
Death or Lost to FU
MA or Failure in RSA
6 Months 1 Year 2 Years 3 Years 4 Years 5 Years
10 11 9 8 9 10
0 0 1 0 0 0
1 0 1 2 1 0
10 11 10 5 9 8
0 0 1 1 0 0
1 0 0 4 0 1
11 12 10 8 9 7
0 0 2 0 0 0
1 0 0 2 1 3
10 10 7 9 8 9
0 0 0 0 0 0
1 1 4 2 3 2
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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Table 2 Average and Standard Deviations of Distances from Medial Stem Bead to Center of Femoral Head and In-Plane Angles Between Medial-Head Line and Medial-Lateral Line. Stem Group
Average Distance from Medial Bead to Head Center (mm)
In-Plane Angle Between Medial-Head Line and Medial-Lateral Line (radians)
A B C D E F G H K M
34.4 37.7 34.6 37.6 34.4 37.5 34.4 37.7 34.6 37.6
1.10 1.13 1.13 1.16 1.16 1.22 1.22 1.26 1.23 1.30
± ± ± ± ± ± ± ± ± ±
0.1 0.06 0.02 0.02 0.1 0.04 0.07 0.02 0.02 0.1
± ± ± ± ± ± ± ± ± ±
0.001 0.0018 0.00097 0.00047 0.00052 0.0016 0.0039 0.001 0.0009 0.007
according the standards of the software. One author performed computer programming for placement of the fictive femoral head center.
Theory/Calculation Due to the relatively small sample sizes in some subgroups, we did not consider femoral head penetration data to be normally distributed. Accordingly, data are presented as median values with inter-quartile ranges, and comparisons are made using the two-sample Wilcoxon rank-sum (Mann–Whitney) test. However, in the largest subgroups data could be considered normally distributed. To facilitate interpretation of our data and to provide an instrument for sample size calculations of future studies, we also present penetration measures as means and standard deviations. Standard deviations are compared using a variance ratio test. Assessment of agreement between gold standard RSA and the novel method for assessment of femoral head penetration are made using a Bland–Altman approach with
Fig. 2. Femoral head penetration (in mm on y-axis) for conventional polyethylene liners in titanium cups assessed by gold standard RSA (blue) and the novel method (red) at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
presentation of data as difference of the means and 95% limits of agreement (LOA) [20]. The level of significance was set at P b 0.05. The cohort size (n = 46) was estimated based on considerations about the ability to detect statistically significant differences in linear femoral head penetration between two groups (conventional PE vs. highly cross-linked PE) [18]. For the purpose of the present study, we estimated the minimum required sample size for detection of statistically significant differences in femoral head penetration between conventional PE liners and highly cross-linked PE liners to be 4 patients in each group at 5 years (80% power and significance level of 0.05). Based on results of previous studies [8,13] we assumed 5 year femoral head penetration of 0.400 mm and 0.150 mm (SD: ±0.125) in conventional PE liners and highly cross-linked PE liners, respectively.
Source of Funding The randomized controlled trial was funded by Zimmer (Warsaw, IN, USA). However, for the development and investigation of the novel “fictive head” method there was no external funding source.
Fig. 1. The novel method utilizes information about the distance between the medial femoral neck marker and the head center (MC) as well as the angle (α) created between the line connecting the medial and lateral proximal stem markers (IM) and the line connecting the medial marker and the femoral head center line (MC).
Fig. 3. Femoral head penetration (in mm on y-axis) for highly cross-linked polyethylene liners in titanium cups assessed by gold standard RSA (blue) and the novel method (red) at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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Table 3 Femoral Head Penetration for Conventional Polyethylene Liners in Titanium Cups Assessed by Gold Standard RSA and the Novel Method. Time Postoperatively (No. of Patients)
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) Gold Standard RSA
Novel Method
6 Months (n = 10)
0.078 (0.043–0.131) 0.085 (0.066) 0.177 (0.121–0.222) 0.169 (0.074) 0.238 (0.206–0.288) 0.241 (0.071) 0.330 (0.330–0.400) 0.328 (0.122) 0.340 (0.210–0.509) 0.358 (0.139) 0.379 (0.280–0.415) 0.366 (0.127)
0.032 (−0.022 to 0.068) 0.039 (0.089) 0.058 (0.011–0.117) 0.106 (0.146) 0.207 (0.082–0.248) 0.200 (0.135) 0.391 (0.243–0.394) 0.327 (0.138) 0.297 (0.274–0.610) 0.425 (0.210) 0.320 (0.250–0.446) 0.380 (0.199)
1 Year (n = 11) 2 Years (n = 10) 3 Years (n = 5) 4 Years (n = 9) 5 Years (n = 8)
Fig. 4. Bland–Altman plot showing the difference of means (solid red line) and 95% limits of agreement (dashed red lines) for paired assessments of femoral head penetration by gold standard RSA and the novel method.
Results Assessment of femoral head penetration in titanium acetabular cups by the novel “fictive head” method demonstrated wear patterns comparable to that of the gold standard RSA in both conventional (Fig. 2) and highly cross-linked (Fig. 3) PE liners. Cumulative femoral head penetration in titanium cups measured by each method showed increased wear over time in the conventional PE liners (Table 3), and although lower, both methods demonstrated increased penetration into the highly cross-linked PE liners as well (Table 4). Assessment of the standard deviations (SD) for paired assessments of femoral head penetration in titanium cups (n = 109) showed that measurement by the gold standard RSA (SD: ±0.137 mm) was significantly less than with the novel method (SD: ±0.183 mm) (P = 0.0006). We also evaluated the effect of defining the cup segment by edge detection of the acetabular shell alone (and not by both the shell and markers in the PE liner), and found an increase in the standard deviation from ± 0.137 mm to ± 0.163 mm (P = 0.07). Based on 109 paired assessments of femoral head penetration by the novel method and gold standard RSA in titanium cups, we evaluated the accuracy of the novel method. The difference between the means of the femoral head penetration values was 0.008 mm suggesting that little systematic error was introduced when penetration assessments were performed by the novel method. The 95% LOA were −0.218 mm to 0.234 mm, meaning that 95% of assessments performed by the
Table 4 Femoral Head Penetration for Highly Cross-Linked Polyethylene Liners in Titanium Cups Assessed by Gold Standard RSA and the Novel Method. Time Postoperatively (No. of Patients)
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) Gold Standard RSA
Novel Method
6 Months (n = 10)
0.060 (0.022–0.081) 0.053 (0.060) 0.069 (0.024–0.100) 0.057 (0.071) 0.105 (0.044–0.110) 0.094 (0.064) 0.039 (0.015–0.100) 0.053 (0.040) 0.090 (0.080–0.100) 0.091 (0.032) 0.082 (0.050–0.140) 0.091 (0.046)
0.024 (−0.035 to 0.100) 0.038 (0.084) 0.029 (−0.102 to 0.068) 0.010 (0.103) 0.067 (0.013–0.171) 0.072 (0.176) −0.020 (−0.040 to 0.198) 0.024 (0.181) 0.113 (0.018–0.186) 0.113 (0.120) 0.171 (0.128–0.212) 0.173 (0.102)
1 Year (n = 11) 2 Years (n = 9) 3 Years (n = 7) 4 Years (n = 9) 5 Years (n = 10)
novel method would fall within this interval of the corresponding gold standard RSA assessments of femoral head penetration (Fig. 4). Assessments of femoral head penetration by the novel method in tantalum cups showed expected patterns with less penetration in highly cross-linked liners compared with conventional liners at all followups (Fig. 5). After the 6-month follow-up, the difference in femoral head penetration between the two liner types was statistically significant (P = 0.003–0.04) at all follow-ups, except at the 3 year follow-up when tested by the non-parametric test (Table 5). Discussion Using the novel “fictive head” method we estimated femoral head penetration in titanium cups and demonstrated a pattern that was similar to that of gold standard RSA in both conventional and highly crosslinked PE liners. Little systematic error was introduced when penetration assessments were performed by the novel method, but standard deviations of assessed femoral head penetration were statistically different (P = 0.0006) from assessment by gold standard RSA. Assessments of femoral head penetration by the novel method in tantalum cups showed as expected less penetration in highly cross-linked liners compared with conventional liners at all follow-ups. Several factors should be acknowledged as contributors to increased variation in measurements by the novel “fictive head” method compared with gold standard RSA: The exact position of the proximal stem markers may vary between femoral stems. There may be differences in actual length of the head–neck segment between femoral components with the same configuration of stem and head sizes due to differences in femoral head impaction. These issues will affect the mathematical algorithms of the novel method. To our knowledge, there are no reports that document the influence of these potential variations on these types of measurements. Further, we found that accurate edge detection of the acetabular cup opening was more difficult in tantalum acetabular cups and that the radiopaque liner markers obstructed the edge detection of the opening. Therefore, when planning future studies utilizing the novel method we recommend that radiopaque markers are not inserted into the PE liner. Using the novel method, the acetabular cup segment is defined by the acetabular shell only and may result in greater variation of penetration compared to defining the segment by both the shell and PE liner markers [21,22]. In our study, we found that the standard deviation of femoral head penetration increased from ±0.137 mm to ±0.163 mm when defining the acetabular cup segment solely by edge detection of the acetabular shell. Utilizing the novel “fictive head” method is restricted by technical demands limiting the use of the method. First, radiopaque tantalum
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
A. Troelsen et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 5. Femoral head penetration (in mm on y-axis) for conventional (blue) and highly cross-linked (red) polyethylene liners in tantalum cups assessed by the novel method at each follow-up time point (in years on x-axis). Femoral head penetration is given as medians with interconnecting lines and bars representing inter-quartile ranges.
markers must be attached to the femoral stems. However, as RSA based prospective follow-up of femoral head penetration is always done in well planned studies and in accordance with an IRB approved protocol, the supply of femoral stems with markers can be planned. Second, a library containing information on all combinations of stem sizes and femoral head–neck lengths and sizes is needed. However, this library has to contain information only about the stem configurations used in the study, and once obtained, the library can be used in subsequent studies. Femoral head penetration was reported as movement of the center of the femoral head relative to the acetabular cup segment along the x-axis, y-axis and z-axis. The need and usefulness of reporting wear along the z-axis remains debated [19,23]. Reporting penetration along the y-axis (proximal) or x-axis and y-axis (medial and proximal) is commonly accepted as the majority of wear occurs in frontal plane [16]. When utilizing the novel “fictive head” method, the calculated femoral head center is assigned to the same position along the z-axis
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as the acetabular cup center, and thus, any penetration in this plane is ignored. We found very little systematic error when applying the novel “fictive head” method, which supports our hypothesis that the novel method can convey accurate measures of femoral head penetration. As we also hypothesized, and have discussed above, we found statistically increased variation (standard deviations) of femoral head penetration measurements by the novel method compared with gold standard RSA. The femoral head penetration assessed by the novel method in conventional and highly cross-linked PE liners at 5 year follow-up showed results comparable to that observed by other authors [8,13,24]. Our hypothesis that the novel method demonstrates expected PE wear patterns with the ability to show differences between conventional and highly cross-linked PE liners was affirmed. Some overall methodological strengths and limitations should be acknowledged and discussed: RSA radiographs were recorded with patients bearing weight, which has been considered important in some reports, whereas other authors have reported no difference of femoral head penetration between supine and weight bearing assessment [25]; The study was performed on a patient cohort enrolled in a randomized controlled trial, thus limiting bias, but blinding of RSA observers was not possible due to the easily identifiable differences between titanium and tantalum cups; the a-priori sample size calculations that we performed suggested that our sample size was sufficient, however variation of assessments by the novel “fictive head” method turned out greater than expected in some subgroups, thus potentially compromising the ability to detect statistically significant differences. Using the overall standard deviation for the novel method of ±0.193 mm from this study a post-hoc analysis reveal that 10 patients in each group would be required to obtain statistical significance of the assumed difference at 5 year follow-up (80% power and significance level of 0.05). This has important implications for planning of studies utilizing the novel method. As we have shown the “fictive head” method results in higher standard deviations of femoral head penetration proving that the gold standard remains the best option for measurement of polyethylene wear. However, when radiodense tantalum cups are used clinically, the only option for measuring femoral head penetration is a method utilizing a fictive femoral head center.
Table 5 Femoral Head Penetration for Highly Cross-Linked and Conventional Polyethylene Liners in Tantalum Cups Assessed by the Novel Method. Time Postoperatively
6 Months
1 Year
2 Years
3 Years
4 Years
5 Years
Penetration in mm Median (IQ-Range) Mean (Standard Deviation) No. of Observations Highly Cross-Linked PE Liner
Conventional PE Liner
P Value Wilcoxon Rank-Sum (Mann–Whitney) Test T-Test
0.028 (−0.085 to 0.219) 0.103 (0.277) N = 11 −0.001 (−0.073 to 0.131) 0.035 (0.157) N = 12 0.032 (−0.066 to 0.178) 0.054 (0.180) N = 10 0.155 (−0.045 to 0.229) 0.082 (0.187) N=8 0.155 (−0.110 to 0.290) 0.099 (0.286) N=7 0.078 (−0.062 to 0.290) 0.100 (0.251) N=8
0.105 (0.005–0.228) 0.127 (0.120) N = 10 0.124 (0.118–0.257) 0.170 (0.078) N = 10 0.269 (0.177–0.335) 0.251 (0.116) N=6 0.221 (0.165–0.480) 0.282 (0.182) N=9 0.381 (0.305–0.460) 0.395 (0.123) N=8 0.420 (0.346–0.430) 0.458 (0.156) N=9
0.18 0.80 0.02 0.02 0.04 0.03 0.18 0.04 0.03 0.02 0.02 0.003
Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050
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Conclusions In conclusion, our results suggest that the novel "fictive head" method is accurate, however, as expected, the variation of femoral head penetration measures is greater than for gold standard RSA. Consequently using radio-transparent cups with polyethylene markers will give more reliable results. The novel method seems to be a viable solution for assessment of femoral head penetration in prospective studies on conventional and highly cross-linked PE liners, in which the implantation of tantalum cups would otherwise make wear assessment impossible. References 1. Amstutz HC, Campbell P, Kossovsky N, et al. Mechanism and clinical significance of wear debris-induced osteolysis. Clin Orthop Relat Res 1992;276:7. 2. Dumbleton JH, Manley MT, Edidin AA. A literature review of the association between wear rate and osteolysis in total hip arthroplasty. J Arthroplasty 2002;17(5):649. 3. Jacobs JJ, Roebuck KA, Archibeck M, et al. Osteolysis: basic science. Clin Orthop Relat Res 2001;393:71. 4. Maloney WJ, Galante JO, Anderson M, et al. Fixation, polyethylene wear, and pelvic osteolysis in primary total hip replacement. Clin Orthop Relat Res 1999;369:157. 5. Clohisy JC, Calvert G, Tull F, et al. Reasons for revision hip surgery: a retrospective review. Clin Orthop Relat Res 2004;429:188. 6. Harris WH. Conquest of a worldwide human disease: particle-induced periprosthetic osteolysis. Clin Orthop Relat Res 2004;429:39. 7. Bragdon CR, Kwon YM, Geller JA, et al. Minimum 6-year followup of highly crosslinked polyethylene in THA. Clin Orthop Relat Res 2007;465:122. 8. Digas G, Karrholm J, Thanner J, et al. 5-year experience of highly cross-linked polyethylene in cemented and uncemented sockets: two randomized studies using radiostereometric analysis. Acta Orthop 2007;78(6):746. 9. McCalden RW, MacDonald SJ, Rorabeck CH, et al. Wear rate of highly cross-linked polyethylene in total hip arthroplasty. A randomized controlled trial. J Bone Joint Surg Am 2009;91(4):773. 10. Muratoglu OK, Bragdon CR, O'Connor DO, et al. A novel method of cross-linking ultrahigh-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award. J Arthroplasty 2001; 16(2):149.
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Please cite this article as: Troelsen A, et al, A Novel Method for Assessment of Polyethylene Liner Wear in Radiopaque Tantalum Acetabular Cups: Clinical Validation in Patients E..., J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.06.050