Orthopedics

Tips & Techniques 

Conventional Versus Cross-Linked Polyethylene for Total Hip Arthroplasty

Michele F. Surace, MD; Luca Monestier, MD; Ettore Vulcano, MD; Steven F. Harwin, MD, FACS; Paolo Cherubino, MD

Abstract

The clinical and radiographic outcomes of 88 patients who underwent primary total hip arthroplasty with either conventional polyethylene or cross-linked polyethylene (XLPE) from the same manufacturer were compared. There were no significant differences between the 2 subpopulations regarding average age, gender, side affected, or prosthetic stem and cup size. The average follow-up was 104 months (range, 55 to 131 months). To the authors’ knowledge, this is the longest follow-up for this particular insert. Clinical and radiographic evaluations were performed at 1, 3, 6, and 12 months and then annually. Results showed that XLPE has a significantly greater wear reduction than that of standard polyethylene in primary total hip arthroplasty. At the longest available follow-up for these specific inserts, XLPE proved to be effective in reducing wear. [Orthopedics. 2015; 38(9):556–561.]

The authors are from the Department of Orthopaedic and Trauma Surgery (MFS, LM, EV, PC), Università dell’Insubria, Varese, Italy; and the Adult Reconstruction Service (SFH), Mount Sinai Beth Israel, New York, New York.

The authors have no relevant financial relationships to disclose.

Dr Harwin was not involved in the peer review of this manuscript.

Correspondence should be addressed to: Ettore Vulcano, MD, Department of Orthopaedic and Trauma Surgery, Università dell’Insubria, Viale Borri 57, Varese, Italy 21100 ( ettorevulcano@hotmail.com).

Received: May 27, 2014
Accepted: June 24, 2014

Abstract

The clinical and radiographic outcomes of 88 patients who underwent primary total hip arthroplasty with either conventional polyethylene or cross-linked polyethylene (XLPE) from the same manufacturer were compared. There were no significant differences between the 2 subpopulations regarding average age, gender, side affected, or prosthetic stem and cup size. The average follow-up was 104 months (range, 55 to 131 months). To the authors’ knowledge, this is the longest follow-up for this particular insert. Clinical and radiographic evaluations were performed at 1, 3, 6, and 12 months and then annually. Results showed that XLPE has a significantly greater wear reduction than that of standard polyethylene in primary total hip arthroplasty. At the longest available follow-up for these specific inserts, XLPE proved to be effective in reducing wear. [Orthopedics. 2015; 38(9):556–561.]

The authors are from the Department of Orthopaedic and Trauma Surgery (MFS, LM, EV, PC), Università dell’Insubria, Varese, Italy; and the Adult Reconstruction Service (SFH), Mount Sinai Beth Israel, New York, New York.

The authors have no relevant financial relationships to disclose.

Dr Harwin was not involved in the peer review of this manuscript.

Correspondence should be addressed to: Ettore Vulcano, MD, Department of Orthopaedic and Trauma Surgery, Università dell’Insubria, Viale Borri 57, Varese, Italy 21100 ( ettorevulcano@hotmail.com).

Received: May 27, 2014
Accepted: June 24, 2014

Although total hip arthroplasty is often extremely successful, polyethylene wear with the production of particulate debris is the main factor leading to periprosthetic osteolysis, aseptic loosening, and long-term failure of the implant.1–4 Cross-linked polyethylene (XLPE) was introduced to limit the wear and is produced by the formation of numerous bivalent bonds between the various molecular chains that form the conventional polymer structure.5 Several recent reports provide encouraging short-term results using XLPE.6–10 However, longer-term studies are needed to corroborate these early results.

The objective of this study was to evaluate the clinical and radiographic outcomes of 88 consecutive patients who underwent primary total hip arthroplasty for hip arthritis with either conventional polyethylene or XLPE in order to assess differences in functional outcomes, polyethylene wear, and the incidence of osteolytic lesions at an average follow-up of 8.4 years. To the best of the authors’ knowledge, this study presents the longest follow-up regarding XLPE.

Materials and Methods

Eighty-eight consecutive patients underwent total hip arthroplasty between January 1999 and December 2004. There were 61 women (69.32%) and 27 men (30.68%). The average age at the time of surgery was 68 years (range, 46 to 83 years). The indication for surgery was primary hip arthritis in 83 cases (94.32%), whereas 4 cases (4.54%) involved post-traumatic hip arthritis and 1 case (1.14%) involved avascular necrosis of the femoral head. The left hip joint was treated in 39 cases (44.32%) and the right hip joint in 49 cases (55.68%). All of the patients were operated on by the same surgeon (M.F.S.). The same types of implants were used in all patients: a Trilogy metal-backed cup (Zimmer, Warsaw, Indiana), a VerSys uncemented femoral stem (Zimmer), and a 28-mm VerSys Hip System femoral head (Zimmer).

The Trilogy standard polyethylene inserts (Zimmer) were implanted in 43 (48.9%) of the patients, while the Trilogy Longevity XLPE inserts (Zimmer) were used for 45 (51.1%) of the patients. The Trilogy standard polyethylene inserts were used until 2001, when the Longevity XLPE insert was introduced. There were no significant differences between the 2 subpopulations regarding average age, gender, side affected, or prosthetic stem and cup size. The average follow-up was 104 months (range, 55 to 131 months); clinical and radiographic evaluations were performed at 1, 3, 6, and 12 months and then annually.

Clinical outcomes were assessed with the Harris Hip Score (HHS) because it is easy to use and has widely accepted results.11 A total HHS between 90 and 100 points was considered excellent, between 80 and 89 points was considered good, between 70 and 79 points was considered fair, and less than 70 points was considered poor.

Radiographic assessment consisted of standard anteroposterior and lateral radiographs at all scheduled intervals. Linear wear of the polyethylene insert corresponding to the prosthetic head migration was assessed according to Livermore’s method, along 4 directions: superior, medial, superolateral, and inferomedial (Figure 1).12 Superolateral is the direction along which most migration of the head occurs and is the one used to describe linear wear. According to Geerdink et al,13 the measurements were made using Roman version V.170 freeware software (Cook and Poullain [2002–2005, Institute of Orthopaedics, Oswestry, United Kingdom]), which accepts all common image formats. The linear wear was then divided by the number of years over which it occurred, to obtain the yearly linear wear rate. To take into account the possible effect of higher than 5-mm offset variations on linear wear, as suggested by Little et al14 in 2009, the same software was employed to assess all patients’ pre- and postoperative radiographs.

Directions of linear penetration as described by Livermore et al.12 (Abbreviations: IM, inferomedial; M, medial; S, superior; SL, superolateral.)

Figure 1:

Directions of linear penetration as described by Livermore et al.12 (Abbreviations: IM, inferomedial; M, medial; S, superior; SL, superolateral.)

Periacetabular osteolysis was also evaluated in this study. The topographic classification of periprosthetic osteolytic areas, defined as areas of periprosthetic radiolucency that were not present on the radiographs of the immediate postoperative period, was described by DeLee and Charnley.3 They defined 3 distinct places in which the osteolytic area can be localized.

In the current study, a more recent evolution of the original topographic classification method, based on the angular identification of the osteolytic area (Figure 2), was employed.15 In this method, the angle corresponding to the superolateral end of the cup was considered to be 0°, while that corresponding to the inferomedial end was considered to be 180°. The most lateral point and the most medial point of the osteolytic area on the acetabular surface were taken as the “proximal limit” and the “distal limit” of the area, respectively. The Roman software finally calculated the extension of the area. Where several osteolytic areas were present, an angular definition of all areas and the sum of the osteolytic surfaces were provided.

Topographic identification of periacetabular osteolytic areas.

Figure 2:

Topographic identification of periacetabular osteolytic areas.

Finally, intraoperative and postoperative complications (ie, fractures, dislocations, venous thromboembolism, aseptic or septic loosening, infection, component cracks) were recorded for all patients.

The data were analyzed using SPSS version 11.0 software (IBM, Armonk, New York). Multivariate analysis was performed to detect possible effects between variables, factors, and cofactors. Then, the Student’s t test was performed on independent and paired samples for continuous variables, and nonparametric Wilcoxon tests were conducted for ordinal variables. The linear regression was also analyzed to evaluate the strength of correlations between the studied variables and factors. The significance level was set at P<.05.

Results

All of the tested variables showed no effect on the functional or radiographic outcomes, nor was there an effect on polyethylene wear.

The average HHS increased significantly, from 54.06 points preoperatively (range, 29.97 to 89 points) to 90.15 points at the last follow-up (range, 45.70 to 100 points) (P<.001). The results obtained at the most recent follow-up were excellent in 55 cases (62.51%), good in 21 cases (23.86%), fair in 7 cases (7.95%), and poor in 5 cases (5.68%). Three patients had a lower than preoperative HHS for causes not directly related to the procedure. One patient suffered a myocardial infarction and 2 patients developed cardiac failure.

In both of the groups, there was a significant increase in the HHS (P<.001). In patients with conventional polyethylene inserts, the mean score increased from 55.42 points (range, 29.97 to 89.00 points) to 90.67 points (range, 49.98 to 100.00 points) (P<.001). In patients with XLPE inserts, the mean score increased from 52.77 points (range, 30.50 to 77.85 points) to 89.65 points (range, 45.70 to 100.00 points) (P<.001). However, there was no statistically significant difference in the HHSs when comparing the 2 subpopulations.

Evaluation of the radiographs obtained using the Roman software indicated an average linear wear between the first postoperative follow-up and the 1-year postoperative follow-up of 0.085 mm (P<.001). Between the first postoperative follow-up and the 2-year postoperative follow-up, significant linear wear of the acetabular insert of a mean of 0.141 mm (P<.001) was observed. In the period between the first postoperative follow-up and the final radiographic follow-up, the mean significant linear wear of the polyethylene was 0.333 mm (P=.002). In the period between 1 year and 2 years postoperatively, the mean migration of the prosthetic head was 0.056 mm (P<.001). Significant mean linear wear of 0.192 mm (P<.001) was also recorded in the period extending from 2 years postoperatively to the final radiographic follow-up. The mean polyethylene wear rate of the entire population was 0.039 mm per year in the period between the first postoperative follow-up and the final follow-up, 0.032 mm per year between the 1-year follow-up and the final follow-up, and 0.029 mm per year between the 2-year follow-up and the final follow-up.

Wear of the acetabular inserts with conventional polyethylene and XLPE at various time intervals is detailed in Table 1. In patients with conventional polyethylene inserts, the mean wear at the first postoperative radiographic follow-up was 0.079 mm, vs 0.093 mm for those with XLPE. The difference was not significant. At 1 year postoperatively, the mean conventional polyethylene wear was 0.180 mm, vs 0.163 mm for XLPE. Hence, at 1 year postoperatively, the mean migration of the head was 0.017 mm greater for standard inserts compared with XLPE inserts. However, the difference between the 2 groups was not significant.

Comparison of Wear of the Acetabular Insert Between Subpopulations With Standard UHMWPE and XLPE Inserts

Table 1:

Comparison of Wear of the Acetabular Insert Between Subpopulations With Standard UHMWPE and XLPE Inserts

At 2 years postoperatively, the mean wear was 0.252 mm and 0.203 mm for standard and XLPE inserts, respectively. At this time interval, the mean wear of the standard polyethylene inserts was 0.049 mm, which was significantly higher than that of the XLPE inserts (P=.020).

The mean wear at final follow-up was 0.584 mm and 0.262 mm for standard and XLPE inserts, respectively. At the last radiographic follow-up, the mean wear of the standard inserts was 0.322 mm, which was significantly higher than that of the XLPE inserts (P<.001).

In the period between the 2-year postoperative follow-up and the final radiographic follow-up for patients with a conventional insert, the mean linear wear was 0.332 mm, with a rate of 0.044 mm per year. In the XLPE group, the femoral head penetrated by 0.059 mm, at a rate of 0.011 mm per year. Figure 3 shows a linear plot of wear over time of the standard ultra-high-molecular-weight polyethylene (UHMWPE) and XLPE groups. The average offset variation between the preoperative and postoperative radiographs was −1.4 mm, without a significant difference between the 2 groups. In all cases, the offset variation was less than 5 mm.

Linear plot of wear over time of the standard ultra-high-molecular-weight (blue line) and cross-linked polyethylene (red line) groups.

Figure 3:

Linear plot of wear over time of the standard ultra-high-molecular-weight (blue line) and cross-linked polyethylene (red line) groups.

Regarding osteolysis, at the most recent radiographic follow-up, 3 patients (3.40%) had a single osteolytic area, while 2 patients (2.70%) had 2 distinct areas. In 4 of these patients (80%), a standard polyethylene insert was implanted, while only 1 patient (20%) had an XLPE insert (Table 2). Correlation between the type of acetabular insert (standard vs XLPE) and the formation of periacetabular osteolytic areas showed no statistical significance. Similarly, correlation between the type of insert and the dimensions of the osteolytic phenomenon had no statistical significance.

Angular Values and Surface of Osteolytic Areas in 5 Patients

Table 2:

Angular Values and Surface of Osteolytic Areas in 5 Patients

Intra- and postoperative complications are detailed in Table 3.

Intra- and Postoperative Complications

Table 3:

Intra- and Postoperative Complications

Discussion

From the clinical point of view, the increase in the HHS from preoperatively to the most recent follow-up was statistically significant, but the difference between the 2 study groups was not. Therefore, in these populations, total hip arthroplasty was an effective treatment for arthritis and the type of polyethylene did not affect the clinical outcome. Regarding the 3 patients with a decreased HHS, the implant played no role. This suggests that this scoring system could be influenced by several factors, including patient comorbidities and the perception of pain, both of which can accentuate pain or affect normal daily activities (ie, climbing stairs or walking).

Ultra-high-molecular-weight polyethylene particles produced by wear are known to induce the phenomenon of periprosthetic osteolysis, which is the main cause of prosthetic implant failure.1,12 The production of polyethylene debris can be reduced by increasing the density of the cross-links in its structure. Some in vitro and in vivo studies have shown that acetabular inserts made from XLPE have higher resistance to long-term wear than do those made from traditional polyethylene.4,16

In one of their studies, Geerdink et al7 reported wear rates of 0.142 mm per year for traditional UHMWPE inserts and 0.088 mm per year for XLPE inserts. Digas et al17 and Manning et al18 showed 95% and 90% reductions, respectively, of wear in XLPE inserts compared with traditional polyethylene inserts. Heisel et al8 found a 70% reduction in the wear of cross-linked UHMWPE inserts compared with standard polyethylene inserts. McKellop et al19 reported that cross-linked UHMWPE showed 83% less wear than did traditional inserts, and Muratoglu et al20 reported a reduction of 85%.

In the current study, data were obtained from patients who underwent surgery with the same prosthetic components. Thus, biasing factors that could possibly affect wear were eliminated. The cup and the heads were the same for all of the patients and were part of the same prosthetic system, as were the 2 acetabular inserts compared in this study. Furthermore, the use of 28-mm femoral heads for all patients eliminated the effect described by Livermore et al,12 who demonstrated that polyethylene wear increased proportionally with the increase in the diameter of the femoral head and that debris production was greater with 32- vs 28-mm heads. Further, the effect of the different head/neck lengths on wear proved to be negligible, as the mean offset reduction of 1.4 mm resulted in a physiological offset restoration in all cases. In all patients, the offset variation was less than 5 mm, representing the cut-off point beyond which significantly higher wear occurs.14

Evaluation of the polyethylene wear is mainly based on radiographic assessment. Several studies have shown how the extent of wear of the material can be obtained by measuring the migration over time of the femoral head in the acetabular component.21–24 Other than wear, the migration of the femoral head is also caused by the process of plastic deformation of the material, or creep. To obtain accurate and consistent measurements of polyethylene wear, the analysis should not start before 12 to 24 months postoperatively (ie, after most of the “bedding-in” process has occurred).25 These data were confirmed by Sychterz et al26 and Olyslaegers et al,27 who also showed that polyethylene wear reaches a steady-state after 2 years postoperatively. Therefore, comparing femoral head penetration between the 2 types of polyethylene after the first 2 years postoperatively yields more realistic results. Indeed, because the bedding-in is negligible by then, the 2-year time frame allows data representing wear alone to be obtained.5–10

Based on the current study’s findings, it is clear that, over time, wear of the acetabular insert occurs in the superolateral direction. This confirms the results of previous studies.28 Moreover, if the average rates of polyethylene wear in the entire study population for the different postoperative periods are compared, an interesting finding is revealed. It appears that the penetration of the head into the polyethylene tends to decrease over time, reaching higher values in the first 2 years after total hip arthroplasty. The steady-state is reached at approximately 24 months postoperatively. This phenomenon can be attributed mainly to creep, as described by Jacobs et al.21

At the first radiographic postoperative follow-up, the presented results demonstrated a greater penetration of the head in the XLPE inserts than in the standard inserts. This was not statistically significant, but it may have been influenced by the bedding-in process. The same applies to the results obtained at the radiographic evaluation 1 year postoperatively, which may have been influenced by the polyethylene deformation process. Conversely, the results at the 2-year follow-up were significant. They showed that the wear of the XLPE was already less than that of the traditional polyethylene. The difference between the 2 materials was 0.049 mm for the first 2 years postoperatively, meaning that the XLPE inserts had 19.44% less wear than did the traditional polyethylene inserts. Therefore, the presence of the bedding-in phenomenon notwithstanding, there was a significant reduction in the wear of the XLPE acetabular insert already. Additionally, the presented results demonstrated that XLPE was also capable of reducing the bedding-in phenomenon. This confirms the findings reported by Digas et al.17

As presented in Table 1, XLPE demonstrated an 82.23% reduction in wear compared with standard polyethylene in the medium- to long-term, con-firming prior reports.7,9 However, it is not possible to conclude that the decreased production of XLPE wear particles, over standard polyethylene, correlated to a significant reduction of osteolysis. Despite the greater number of periacetabular osteolytic areas in patients with a traditional insert, the results of this study could not identify a statistically significant difference between the type of polyethylene insert and the number or size of periacetabular osteolytic areas. This could be related to the few patients presenting osteolytic lesions and the insufficient length of observation.

Finally, the intra- and postoperative complication rates reported in Table 3 are comparable to those reported in the literature.29

Conclusion

This study shows that XLPE is associated with a significant wear reduction over standard polyethylene in primary total hip arthroplasty. At the longest available follow-up for these specific inserts, XLPE proved to be effective in reducing wear.

References

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  21. Jacobs CA, Christensen CP, Greenwald AS, McKellop H. Clinical performance of highly cross-linked polyethylenes in total hip arthroplasty. J Bone Joint Surg Am. 2007; 89(12):2779–2786.
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  24. Sychterz CJ, Engh CA Jr, Shah N, Engh CA Sr, . Radiographic evaluation of penetration by the femoral head into the polyethylene liner over time. J Bone Joint Surg Am. 1997; 79(7):1040–1046.
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Comparison of Wear of the Acetabular Insert Between Subpopulations With Standard UHMWPE and XLPE Inserts

Time and InsertNo.Mean Follow-up, yWear, mmSDP

MeanMinimumMaximum
Postoperative
  Standard UHMWPE430.2−0.079−0.2900.2900.105.544
  XLPE450.2−0.093−0.3800.1900.111
At 1 year
  Standard UHMWPE431−0.180−0.3800.0300.089.419
  XLPE451−0.163−0.4350.0350.103
At 2 years
  Standard UHMWPE432−0.252−0.455−0.0450.089.020
  XLPE452−0.203−0.4750.0550.101
Follow-up
  Standard UHMWPE439.6−0.584−1.245−0.0800.278<.001
  XLPE457.2−0.262−0.5000.0700.119

Angular Values and Surface of Osteolytic Areas in 5 Patients

Patient No.UHMWPENo. of Osteolytic AreasProximal AngleDistal AngleTotal AnglePenetration, mmArea, mm2Total Area, mm2
1Standard227.8°51.2°23.4°17.2152.5177.6
67.9°78.7°10.8°9.425.1
2Standard135.4°44.0°8.6°5.720.420.4
3Standard142.5°53.8°11.3°3.78.78.7
4Standard262.1°75.4°13.3°8.647.6160.3
87.5°147.8°60.3°3.7112.7
5Cross-linked129.4°55.6°26.2°5.984.784.7

Intra- and Postoperative Complications

ComplicationNo. of Patients (%)Notes
Postoperative anemia requiring transfusions37 (38.9)20 patients received transfusion of 500 cc of red blood cells; 17 patients received transfusion of 1000 cc of red blood cells
Hospital complicationsa1 (1.1)1 case of venous thromboembolism
Early complicationsb (<6 mo)0Not available
Intermediate complicationsb (>6 mo and <5 y)0Not available
Late complicationsb (>5 y)2 (2.1)First patient: polyethylene liner was replaced by cross-linked polyethylene liner for cracking after 11 y. Second patient: revision of the acetabular and femoral components due to aseptic looseningafter 9 y.

10.3928/01477447-20150902-05

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