Unexpected Wear of a Uniquely Designed Moderately Cross-Linked Polyethylene in Total Hip Arthroplasty.

BACKGROUND: A uniquely designed, non-heat-treated moderately cross-linked acetabular polyethylene liner used in total hip arthroplasty (THA) demonstrated excessive wear during routine follow-up, prompting an evaluation of the linear wear rate. METHODS: All THAs were performed by the senior author. The study group included 38 THAs using the uniquely designed polyethylene in question, compared to a control group of 21 THAs using another moderately cross-linked polyethylene with good 10-year outcomes. Two-dimensional linear head penetration wear measurements were obtained using the Martell Hip Analysis Suite, and retrieval analysis was performed on two liners. RESULTS: The study group had a significantly higher average penetration rate of 0.089 mm/y than the control group average rate of 0.047 mm/y (P = .04). Forty-five percent of the study group had a wear rate above the osteolysis threshold (0.1 mm/y), compared to 24% in the control group. Macroscopic analysis of two retrieved liners validated the radiographic findings. CONCLUSION: The data suggest unexpectedly higher wear rates for a moderately cross-linked polyethylene design, with nearly half of the study group at risk for osteolysis. Further registry or database analyses of this particular moderately cross-linked polyethylene are warranted.

Are Radiographic and Direct Measures of Acetabular Polyethylene Wear Comparable?

BACKGROUND: All polyethylene acetabular liners wear over time, and numerous methods for calculating linear wear rates exist. The objective of this study was to compare 2-dimensional wear rates between direct, micrometer measurements and the computerized, edge-detection method using Hip Analysis Suite (HAS) 8.0.4.3. METHODS: Two groups of retrieved acetabular liners from Harris-Galante Prosthesis I and Harris-Galante Prosthesis II implants in situ for more than 10 years were evaluated. Group 1 (n = 18) contained liners with both early postoperative (<6 months) and prerevision radiographs taken within 1 month of explantation. Group 2 (n = 55) included liners with only prerevision X-rays (ie, 1 radiograph for wear assessment). Average and maximum direct linear wear was calculated from thicknesses measured at 6 consistent, well-separated locations (3 in the worn and 3 in the unworn regions) using a calibrated, digital micrometer. HAS 8.0.4.3 was used to calculate 2-dimensional wear from anteroposterior pelvic radiographs. RESULTS: Aggregate wear rates calculated by HAS were higher than those calculated by the average of direct measurements for group 1 (P = .020) and group 2 (P < .001). However, comparing the maximum direct micrometer measurements to HAS showed no difference for either group 1 (P = .351) or group 2 (P = .451). Linear regression analysis showed a strong correlation between HAS and both average and maximum direct wear measures for both groups, though the coefficient for the direct maximum measurement comparisons were closer to one, indicating a better one-to-one correspondence between HAS and direct maximum wear. CONCLUSION: To our knowledge, this is the first study to compare and validate 2-dimensional wear rates in polyethylene acetabular liners between direct measurements from retrieved components and a radiographic computer-assisted technique (as opposed to comparison against a phantom component). Wear rates determined by direct measurements from retrievals were consistent with computer-assisted 2-dimensional methods when comparing maximum wear measurements. In addition, a single prerevision radiograph appears to be sufficient to assess 2-dimensional in vivo wear.

Dual-Energy X-Ray Absorptiometry (DEXA) Evaluation of the Bone Remodeling Effects of a Low-Modulus Composite Hip Stem After 2 Decades of Follow-Up.

Background: The Epoch FullCoat Hip Stem (Zimmer) was an isoelastic composite femoral stem developed to address stem stiffness concerns. Purpose: We sought to evaluate the long-term bone mineral density (BMD) of a cohort of patients who underwent total hip arthroplasty (THA) using the Epoch isoelastic stem and having more than 2-decade follow-up. Methods: We conducted a retrospective chart review of all patients who were study subjects at our institution in a multicenter prospective trial for the Food and Drug Administration of the Epoch implant in the mid-1990s. Through this, we identified 16 patients who had dual-energy X-ray absorptiometry (DEXA) scans, with which we could determine BMD preoperatively and at 3 points postoperatively. Of these, 5 agreed to participate in the study (the others were deceased, unable or declined to participate, or were lost to follow-up) with mean follow-up of 22 years. These participants underwent clinical and radiographic evaluation consisting of a Harris hip score, anteroposterior (AP) pelvis and AP and lateral hip X-rays, and DEXA evaluation of both hips. BMD in the 7 Gruen zones at last follow-up was compared with immediate postoperative and 2-year follow-up. Results: At last follow-up, all stems were well-fixed with signs of extensive osteointegration. In proximal Gruen zones 1 and 7, patients underwent a decrease in BMD with more modest losses in Gruen zone 1. All patients demonstrated an increase in BMD in zones 2 through 6 at latest follow-up, except for 1 patient in Gruen zone 6. BMD changes were not limited to the first 2 years of follow-up. Conclusion: This small follow-up cohort study found excellent long-term clinical results, no plain radiographic signs of notable stress shielding, and general maintenance of BMD at a follow-up of over 20 years for this isoelastic stem. Long-term bone remodeling after implantation of the isoelastic stem resulted in increased BMD in Gruen zones 2 through 6, suggesting that composite implant designs may still have a role in THA.

No Difference in Conventional Polyethylene Wear Between Yttria-stabilized Zirconia and Cobalt-chromium-molybdenum Femoral Heads at 10 Years.

BACKGROUND: Concerns have arisen regarding deterioration of wear properties of yttria-stabilized zirconia (YSZ) femoral head on conventional polyethylene (PE) bearings due to YSZ phase transformation. QUESTIONS/PURPOSES: The purpose of this study was to determine if there is a difference in long-term PE wear properties between YSZ and cobalt-chromium-molybdenum (Co-Cr-Mo) femoral heads. METHODS: Ten-year radiographic wear assessment was performed on a cohort of patients enrolled in a prospective randomized clinical trial comparing total hip arthroplasty with YSZ or Co-Cr-Mo femoral heads on conventional, non-cross-linked PE. RESULTS: PE linear wear, annualized wear, and steady-state wear rates remained low and similar between groups. No cases of osteolysis were observed. CONCLUSIONS: Measured conventional PE wear was similar between YSZ and Co-Cr-Mo femoral heads with the steady-state wear rates for both remaining below the generally accepted threshold at which osteolysis typically occurs. Whether clinically relevant phase transformation with YSZ femoral heads occurs is uncertain; however, the use of YSZ femoral heads in this study was not associated with increased PE wear, osteolysis, or deterioration of wear properties.

Reassessment of computerized wear measurement for total hip arthroplasty with correction for projectional image distortion: a brief follow-up report.

BACKGROUND: Wear of the ultra-high molecular weight polyethylene articular surface has been recognized as a major factor threatening the long-term success of total hip arthroplasty. Manual techniques that have been used to measure femoral head penetration into the polyethylene have been plagued with poor reproducibility and limited accuracy. Using a previously described phantom model simulating an unworn total hip arthroplasty, we previously demonstrated significant limitations in the accuracy of several widely used computerized wear measurement programs. A major component of these inaccuracies is projectional distortion of the femoral head and acetabular shell on the radiograph. These inaccuracies can be "corrected for" mathematically. METHODS: In the present follow-up study, we evaluated a widely used hip wear measurement software program (Hip Analysis Suite version 8.0.3.0) that corrects for these projectional errors with use of our previously described "zero wear" phantom model. A cementless metal-backed acetabular component was evaluated radiographically at three different cephalocaudad locations with respect to a radiopaque centering target. At all three positions, the cup was aligned in three different angles of planar abduction (35 degrees , 45 degrees , 55 degrees ) and four angles of planar anteversion (10 degrees , 20 degrees , 30 degrees , 40 degrees ). The accuracy and reproducibility of Hip Analysis Suite version 8.0.3.0 were determined and compared with the results obtained with the earlier version, Hip Analysis Suite version 4.0. RESULTS: Hip Analysis Suite version 8.0.3.0 was significantly more accurate than Hip Analysis Suite version 4.0 for determining linear wear and volumetric wear. Hip Analysis Suite version 8.0.3.0 was significantly more accurate for determining femoral head penetration at the different cephalocaudad acetabular positions and over the range of acetabular component anteversion and abduction angles in comparison with Hip Analysis Suite version 4.0. CONCLUSIONS: With use of the same methodology that was used to evaluate earlier versions of Hip Analysis Suite, the present study showed improvement in the accuracy of wear measurement with Hip Analysis Suite version 8.0.3.0 as compared with Hip Analysis Suite version 4.0. CLINICAL RELEVANCE: On the basis of the results of the present study, Hip Analysis Suite version 8.0.3.0 appears to fulfill the needs for a user-friendly, efficient, and accurate method of assessment of polyethylene wear following total hip arthroplasty.