Background and purpose — Cup wear in total hip arthroplasty (THA) can be affected by different manufacturing processes of the polyethylene (PE). We report the long-term wear pattern differences, as well as early creep behavior, between conventional PE and highly cross-linked PE (HXLPE) liners, as measured with radiostereometry (RSA) up to 10 years. We also compare migration and clinical outcome of 2 similar uncemented cups with different backside surface roughness. Patients and methods — We included 45 patients with primary osteoarthritis. 23 received a conventional liner and 22 an HXLPE liner in a similar uncemented cup, but with a slightly rougher surface. The patients were followed up with RSA and hip-specific outcome questionnaire (HOOS) at 3 months, 1, 2, 5, and 10 years. Results — During the first 3 months both liners showed expected deformation with mean proximal head penetration of 0.39 mm (conventional PE) and 0.21 mm (HXLPE). Between 3 months and 10 years there was a difference in annual wear with 0.12 mm/year for the conventional liner and 0.02 mm/year for the HXLPE liner. The cup with rougher surface had less initial migration but both types had stabilized after 3 months. The HOOS scores improved after surgery and remained high for both groups throughout the study period. Interpretation — Up to 10 years the HXLPE has consistent lower annual wear, possibly contributing to longer survival of the THA, compared with conventional PE. All patients reported good results regardless of liner type.
Osteolysis, attributed to polyethylene wear debris, is one of the main causes of aseptic loosening in THA (Jacobs et al. [
The CSF cup with standard conventional PE liner (JRI Orthopaedics Ltd, London, UK) has been on the market since 1991 showing satisfactory results (Datir and Angus [
This is a single-center prospective cohort study conducted at Skåne University Hospital of 50 patients who had surgery performed between April 2007 and June 2008. Mean age was 63 years (50–75), 25 were men, all had primary hip OA, Charnley class A or B (Table 1), and had been included in a published randomized controlled trial comparing 2 versions of the Furlong stem (Weber et al. [
Table 1. Patient characteristics
CSF CSF Plus Total n = 23 n = 22 n = 45 Mean age (range) 64 (50–74) 62 (53–75) 63 (50–75) Male/female sex 14/9 11/11 25/20 Mean BMI (SD) 27 (4.1) 29 (6.0) 28 (5.1)
1 SD = standard deviation.
Of the 50 patients, the first 25 were allocated to have a CSF cup with conventional PE liner and the following 25 patients a CSF Plus cup with HXLPE liner. The reason for this consecutive allocation on the cup side is that the CSF Plus cup was not available to us when the study was initiated but was obtainable later in the study period. Although all patients met the inclusion criteria and were suitable for an uncemented stem, 5 were considered unsuitable for an uncemented cup (women ≥ 70 years old with radiographical doubt as to bone quality in the acetabulum). Thus, 2 patients in the CSF group, and 3 in the CSF Plus group were excluded from the cup part of the study (Figure 1, Table 1).
PHOTO (COLOR): Figure 1. Consort flow chart.
In the cup migration analysis part of the study, due to the absence of adequate visible markers in the acetabulum affecting RSA cup migration measurements but not wear, 5 patients were excluded: 1 from the CSF group, 4 from the CSF Plus group. Liner analysis was conducted in all 35 patients.
Surgery was performed by 2 experienced hip surgeons (GF and MS) using a posterolateral incision. The patients were blocked randomized to have either the Furlong HAC or the Furlong Active stem (Weber et al. [
The liner in the CSF cup is made from Ticona grade GUR 1050 resin, which is ram extruded and sterilized with 2.5 Mrads. The HXLPE liner in the CSF Plus cup is made from the same material followed by irradiation with 7.5 Mrads of gamma sterilization to produce the cross-linking in the polyethylene. The liners are free from calcium stearate, a compound that has been associated with fusion defects and increased oxidation (McKellop et al. [
The CSF Plus cup metal shell, compared with its precursor CSF, has a thicker and rougher layer of titanium coating. The complete coating includes the same outer layer of hydroxyapatite, Supravit, 100–170 µ thick for both cups. This makes the total thickness for CSF Plus 365 µ with a roughness of 60–100 RZ, compared with a total thickness of 200 µ and a roughness of 30–50 RZ for the CSF cup shell.
RSA examinations were performed according to the guidelines for standardization for radiostereometry (Valstar et al. [
During the follow-up period, all patients had a double examination calculating the precision value (Table 2).
Table 2. Precision values of the cup wear and migration analysis
Liner wear Cup Cup Axis translation (mm) translation (mm) rotation (°) Transverse (X) – 0.14 0.76 Longitudinal (Y) 0.08 0.09 0.72 Sagittal (Z) – 0.38 0.22 3D 0.23 – –
2 The value given represents the smallest migration considered as statistically significant and is based on mean + 2 SD of the error obtained. This corresponds to the 95% confidence limit.
An upper limit for the condition number (CN) is normally set at 150 (Valstar et al. [
Point motion of the femoral head in relation to the cup segment was used for wear analysis. The cup segment was defined as cup opening and back shell as definitive points of the cup combined with the markers from the liner periphery (Börlin et al. [
Cup inclination was measured for all patients on the first postoperative radiographs as this can affect the wear of the liner (Tian et al. [
The Self-administered Hip Disability and Osteoarthritis Outcome Score (HOOS) (Nilsdotter et al. [
Power analysis was performed based on previously published RSA data on stems and cups. Assuming that the true difference of head penetration at 2 years is 0.1 mm with a common standard deviation (SD) of 0.1, 21 patients in each group would yield a power of 90% to find a statistically significant difference between the groups, using alpha = 0.05. To cover possible dropouts, 25 patients were included in each group.
Continuous variables are presented using mean and SD or range, and categorical variables are presented using counts and percentages. A significance level of 0.05 was used for all statistical tests and 95% confidence intervals (CI). Comparison between CSF and CSF Plus at single time-points were performed using two-sample t-tests. Linear regression was used to evaluate the effect of cup slope on wear.
Wear over time was analyzed using a piecewise linear mixed-effect model with a knot (breaking point) at 3 months after surgery where a clear pattern change from creep to deformation has been shown in an earlier study (Bergvinsson et al. [
The HOOS data was analyzed using Mann–Whitney U-test for comparison between groups.
The study was approved by the Ethics Committee of Lund University, Sweden (Dnr 2007/33). All patients gave informed written consent to participate in the study including follow-ups. The study was carried out according to the Helsinki Declaration of 1975, as revised in 2000. Data is available on reasonable request.
JRI Orthopaedics Ltd have financially supported part of the RSA examinations but had no influence on how this study was conducted or how the results were interpreted. The authors have no conflict of interest.
Both groups showed head penetration into the liner occurring during the first 3 months, known as initial polyethylene deformation or creep. The mean Y-penetration at 3 months was 0.39 (CI 0.21–0.60) mm for the conventional PE group and 0.21 (CI 0.10–0.32) mm for the HXLPE group. After this there is a clear change in the wear pattern, indicating change from the initial deformation phase followed by beginning of the wear phase. Based on this observation, a mixed–model analysis was performed with a knot at the 3-month follow-up moment. Between 3 months and 10 years the mean femoral head penetration in the 2 groups showed different patterns. The head penetration in the conventional PE group continued (p < 0.001, mixed models) whilst the HXLPE group experienced minimal penetration (p = 0.3, mixed models); at 10 years the total Y-translation was 1.56 (CI 1.21–1.91) mm and 0.40 (CI 0.20–0.60) mm, respectively. This results in a yearly wear rate of 0.12 mm for conventional PE and 0.02 mm for HXLPE after the initial creep period (Table 3 and Figure 2).
PHOTO (COLOR): Figure 2. Y-translation of the femoral head for conventional PE (CSF) and HXLPE (CSF Plus) with 95% CI bars.
Table 3. Wear measured with RSA as translation of femoral head. Values are mean (mm) and (95% confidence intervals)
Months Conventional PE HXLPE Y-axis translation 3 0.39 (0.22–0.57) 0.21 (0.09; 0.32) 0.07 12 0.51 (0.32–0.70) 0.21 (0.04–0.38) 24 0.60 (0.42–0.80) 0.24 (0.09–0.38) 60 0.92 (0.68–1.16) 0.27 (0.10–0.45) < 0.01 120 1.56 (1.21–1.92) 0.40 (0.20–0.60) 3D translation 3 0.62 (0.38–0.87) 0.40 (0.26–0.55) 0.09 12 0.71 (0.47–0.96) 0.50 (0.32–0.67) 24 0.81 (019–1.04) 0.50 (0.33–0.67) 60 1.12 (0.86–1.37) 0.54 (0.38–0.70) < 0.01 120 1.69 (1.30–2.08) 0.56 (0.31–0.81)
3 Mixed models analysis between 0 and 3 months and 3 months to 10 years, respectively
The values for the total penetration (3D vector) were similar' at 3 months, the wear was 0.62 (CI 0.37–0.87) mm for the conventional PE group and 0.40 (CI 0.26–0.54) mm for the HXLPE group. The total wear, at 10 years, for the conventional PE group was 1.69 (CI 1.30–2.08) mm and 0.56 (CI 0.31–0.81) mm for the HXLPE group. Thus, for the conventional PE the yearly wear rate is 0.11 mm/year compared with 0.02 mm/year for the HXLPE group (Table 3 and Figure 3).
PHOTO (COLOR): Figure 3. 3D-translation of the femoral head for conventional PE (CSF) and HXLPE (CSF Plus) with 95% CI bars.
The CSF cup in the conventional group migrated cranially (Y-translation) 0.28 mm (CI 0.13–0.43) during the first 3 months and then seemed to have stabilized with a migration of 0.34 mm at 10 years. The CSF Plus cups in the HXLPE group had a Y-translation from 0.09 (CI 0.01–0.17) mm at 3 months and –0.04 (CI –0.30 to 0.22) mm at 10 years. After initial settling-in, measured up to 3 months, there was, up to 10 years, generally very little translation and rotation of the cups in both groups (Figures 4 and 5 and Table 4).
PHOTO (COLOR): Figure 4. Y-translation of the CSF and CSF Plus cups with 95% CI bars.
PHOTO (COLOR): Figure 5. Z-rotation of the CSF and CSF Plus cups with 95% CI bars. Plus (+) rotation indicates decreased and minus (–) rotation increased inclination.
Table 4. Cup migration. Values are mean (mm/°) and (95% confidence intervals)
Months CSF CSF Plus X–axis translation, medial (+) or lateral (–) 3 0.37 (0.16 to 0.59) 0.17 (–0.06 to 0.40) 12 0.35 (0.12 to 0.57) 0.17 (–0.05 to 0.40) 24 0.37 (0.14 to 0.59) 0.25 (–0.01 to 0.50) 60 0.31 (0.04 to 0.58) 0.31 (0.03 to 0.60) 120 0.31 (0.02 to 0.60) 0.30 (0.04 to 0.56) Y–axis translation, proximal (+) or distal (–) 3 0.28 (0.13 to 0.43) 0.09 (0.01 to 0.16) 12 0.34 (0.17 to 0.50) 0.03 (–0.15 to 0.20) 24 032 (0.16 to 0.49) 0.04 (–0.15 to 0.22) 60 0.33 (0.16 to 0.49) –0.01 (–0.23 to 0.21) 120 0.34 (0.13 to 0.54) –0.04 (–0.31 to 0.22) Z–axis translation, anterior (+) or posterior (–) 3 0.03 (–0.15 to 0.21) 0.19 (–0.08 to 0.46) 12 –0.01 (–0.20 to 0.17) 0.35 (–0.03 to 0.74) 24 0.10 (–0.09 to 0.30) 0.38 (–0.04 to 0.80) 60 0.11 (–0.09 to 0.32) 0.13 (–0.27 to 0.53) 120 0.04 (–0.21 to 0.28) 0.23 (–0.27 to 0.73) X–axis rotation, anterior (+) or posterior (–) tilt 3 0.26 (–0.20 to 0.71) 0.10 (–0.13 to 0.33) 12 0.40 (–0.01 to 0.80) 0.21 (–0.24 to 0.67) 24 0.28 (–0.14 to 0.71) 0.08 (–0.43 to 0.58) 60 0.25 (–0.26 to 0.77) 0.17 (–0.37 to 0.72) 120 0.20 (–0.34 to 0.74) 0.23 (–0.60 to 1.06) Y–axis rotation, internal (+) or external (–) rotation 3 –0.15 (–0.60 to 0.29) 0.05 (–0.33 to 0.44) 12 –0.11 (–0.49 to 0.27) 0.00 (–0.42 to 0.42) 24 –0.16 (–0.57 to 0.25) –0.12 (–0.57 to 0.33) 60 –0.13 (–0.51 to 0.25) –0.29 (–0.77 to 0.20) 120 –0.03 (–0.56 to 0.51) –0.27 (–0.76 to 0.22) Z–axis rotation, decreased (+) or increased (–) inclination 3 –0.02 (–0.49 to 0.46) –0.16 (–0.46 to 0.14) 12 0.00 (–0.52 to 0.53) –0.23 (–0.44 to –0.02) 24 0.07 (–0.44 to 0.58) –0.12 (–0.37 to 0.12) 60 0.06 (–0.46 to 0.58) –0.06 (–0.32 to 0.19) 120 0.01 (–0.61 to 0.62) 0.06 (–0.21 to 0.32)
The mean cup inclination, as measured on the postoperative radiographs, was 43° (CI 41–46) for the conventional PE group and 44° (CI 42–46) for the HXLPE group.
The HOOS was similar for both groups preoperatively and at 6, 12, 24, 60, and 120 months. All patients had improved HOOS scores compared with preoperatively and the improvement remained up to 10 years (Figure 6).
PHOTO (COLOR): Figure 6. HOOS questionnaire outcome. HOOS outcome measures: Pain; Symptoms including stiffness and range of motion; Activity limitations – daily living (ADL); Sport and recreation function (Sport/Rec.); and Hip-related quality of life (QoL). A score of 0 indicates poor function/high number of symptoms, a score of 100 indicates excellent function/low number of symptoms.
1 cup was revised due to late hematogenic infection. 10 years after THA, none of the remaining cups had any clinical or radiological signs of loosening requiring revision.
This study was conducted in order to investigate the long-term difference between conventional PE and HXLPE. Our results confirms that the superiority of the HXLPE continues up to 10 years. The curves indicate that this pattern will continue and, so far, we cannot see any disadvantages with the change from conventional PE to HXLPE. Furthermore, we conclude that the deformation process of the PE liner can be divided into 2 phases: the initial deformation phase, known as creep (Sychterz et al. [
There are several ways of producing the HXLPE: by different radiation intensity, annealing or remelting, and sterilizing techniques, resulting in variations in characteristics of the liners. Although the superiority of each technique is debatable, studies to date indicate no increased risk in use of HXLPE compared with their precursors. To our knowledge, this is the 1st study presenting wear data on this specific manufacturer's HXLPE.
Our secondary aim was to investigate possible difference in migration behavior as well as time required until osseointegration for the CSF and the CSF Plus cup occurred. The newer design, with a rougher surface, showed less migration during initial bedding-in. However, both seemed to have osseointegrated within 3 months, and none of them showed further signs of migration and/or associated loosening throughout the 10-year follow-up. This is considered a good migration pattern for acetabular cups and indicates minimal risk for aseptic loosening with revision risk in the long term (Pijls et al. [
The mean pain score in HOOS was 92 (100 being no pain) for both groups 1 year after surgery and 88 after 10 years. Hence, patients experienced their hips still performing well after 10 years.
A limitation of the study is that this is not a randomized study for the cup part, but only for the stem part of the study. Instead, the patients were operated on consecutively with the 1st half of the patients receiving the CSF cup and conventional PE liner and the other half receiving CSF Plus cups and HXLPE liner. The reason for this was that the CSF Plus cups were not released when the study started. It should be noted that there was the same proportion of the different stems in each group. Another potential limitation is that we are comparing the 2 different kinds of polyethylene in 2 slightly different cup shells. It might be speculated that the HXLPE liner is somewhat affected by the slightly rougher surface of the CSF Plus shell compared with the conventional PE of the CSF shell. However, we find this unlikely as the migration behavior of the cups was very similar, except for a slightly less early migration of the CSF Plus cup. A further potential limitation is that due to loss to follow-up after 10 years, the remaining 20 and 15 patients in each respective group do not meet our initial criteria for power in the study. However, the differences in wear already at 5 years are far greater than the values used for power calculations, leading us to believe that these results have sufficient power.
In conclusion, our results indicate that this HXLPE has the wear characteristics expected from a modern HXLPE, with markedly less wear compared with the older conventional PE. Both the older cup, with conventional PE, and the newer cup with its slightly rougher surface and an HXLPE liner indicate very good stability up to 10 years.
HB: study conduction, data analysis, writing of the manuscript. GF and MS: study design and conduct, performing surgery, data analysis, and critical revision of the manuscript. VZ: critical revision of the manuscript.
The authors would like to thank Håkan Leijon at the RSA laboratory, Skåne University Hospital, Lund University for computerizing and analyzing the RSA pictures; Helene Jacobsson at Clinical Studies Sweden—Forum South, Skåne University Hospital, Lund is thanked for statistical guidance.
Acta thanks Lennard Koster and Matthew Teeter for help with peer review of this study.
By Halldor Bergvinsson; Vasilis Zampelis; Martin Sundberg and Gunnar Flivik
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