Analysis of wear and oxidation on retrieved bipolar polyethylene liner

Efficacy of vitamin E for mechanical damage and oxidation of polyethylene rim by stem neck impingement

BACKGROUND

The aim of this study is to determine the influence of crosslinking and addition of 0.3 wt% vitamin E in a polyethylene rim on its mechanical damage and oxidation caused by impingement.

METHODS

Six ultrahigh-molecular weight polyethylene samples were studied (control; crosslinked; vitamin-E containing; crosslinked and vitamin-E containing; aged control; and aged crosslinked and vitamin-E containing). Crosslinking was attained by irradiation with a 300 kGy electron beam; vitamin E incorporation was at 0.3 wt%; and aging was performed through forced oxidation for 14 days. Resistance to impingement was evaluated by stereoscopic observations, three-dimensional measurements, and oxidation measurements by Fourier transform infrared spectroscopy.

FINDINGS

Rim breakage (delamination and fracture) due to impingement was observed only for the aged control specimen. In contrast, crosslinked specimens containing vitamin E showed no failure of the rim after aging. The addition of vitamin E to polyethylene suppressed its oxidation and reduced the oxidation caused by crosslinking or impingement. The impingement resistance of the control sample deteriorated upon oxidation, whereas that of vitamin E-containing crosslinked polyethylene remained high due to the antioxidant property of vitamin E.

INTERPRETATION

Vitamin E-containing polyethylene showed a reduced risk of wear/breakage of polyethylene rims by impingement.

Relationship between pull-out strength and oscillation angle in bipolar cups: an in vitro study

PURPOSE

To compare the past and present bipolar hip arthroplasty (BHA) models in terms of balance between pull-out strength and oscillation angle (OA).

METHODS

The pull-out strength and OA of 8 BHA models were compared: UPF-II, IBC, and Tandem XLPE (Smith & Nephew); Ringloc x (Biomet); J-FX (DePuy); Bipolar (Nakashima Medical); Multipolar (Zimmer); and Centrax (Stryker).

RESULTS

Respectively for the UPF-II, IBC, Tandem, Ringloc, J-FX, Nakashima Bipolar, Multipolar, and Centrax, the mean pull-out strength was 2219 N, 3303 N, 1503 N, 951 N, 1453 N, 1856 N, 1536 N, and 753 N, whereas the mean OA was 54.2°, 53.8°, 64.0°, 73.2°, 63.0°, 65.4°, 55.6°, and 75.4°. The OA was lower in the integrated types. For pull-out strength of the locking mechanism, the integrated type (IBC and Nakashima) was stronger than the metal or polyethylene ring-lock type (all others). The pull-out strength and OA were negatively correlated (r= -0.881, p=0.007), and the balance between the 2 varied for different models.

CONCLUSION

There is a trade-off between the pullout strength and OA; optimal balance between the 2 should be based on each patient's need.

Identification of femoral head center of bipolar hemiarthroplasy in radiostereometric analysis with elementary geometrical shape models

Elementary geometrical shape (EGS) models are useful in radiostereometric analysis (RSA) on hip stems because tantalum markers attached to the stems can be omitted. In order to create an EGS model of a femoral stem, the center of the femoral head has to be identified. The contour of the femoral head is recommended to be used. However, the contour of the femoral head cannot be detected exclusively by computer if it is combined with a bipolar head or a metal cup. We therefore hypothesized that the contour of the outer head of bipolar hemiarthroplasty can be included in the EGS model as well as the femoral head contour. We calculated the time required for the detection of the contour, the precision of analysis and the stem micromotion at 2 years using the two different methods in the same picture set and compared the results. The detection of the bipolar head contour was 10 times faster than that of the femoral head contour. The precision for subsidence was 0.16 mm in EGS RSA with the femoral head contour, and 0.15 mm with the bipolar head contour (p=0.68). The precisions were comparable and clinically acceptable. There was no significant difference between the results of the 2-year micromotion with the two different methods. We conclude that this new method is applicable to measure stem micromotion of hemi-arthroplasty with EGS RSA and the method facilitates the Radiostereometric analysis.

Effect of component design in retrieved bipolar hip hemiarthroplasty systems

Primary articulation of bipolar hemiarthroplasty systems is at the femoral head-liner interface. The purpose of this study was to compare observed damage modes on 36 retrieved bipolar systems with implant, demographic, intraoperative, and radiographic data to elucidate the effects of component design, specifically locking mechanism, on clinical performance. Retrieved bipolar hip hemiarthroplasty systems of 3 different design types were obtained, disassembled, and evaluated macro- and microscopically for varying modes of wear, including abrasion, burnishing, embedding, scratching, and pitting. Clinical record review and radiographic analysis were performed by a senior orthopedic surgery resident. Average bipolar hip hemiarthroplasty system term of service was 46 months (range, 0.27-187 months). All devices contained wear debris captured within the articulating space between the femoral head and liner. In 31% of patients without infection, lucency was observed on immediate prerevision radiographs. The system with a leaf locking mechanism showed significantly increased radiographically observed osteolysis (P=.03) compared with a system with a stopper ring locking mechanism. In addition, implant design and observed damage modes, including pitting and third-body particle embedding, were significantly associated with radiographically observed osteolysis.

In vivo oxidation in remelted highly cross-linked retrievals

BACKGROUND

Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests.

METHODS

Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air.

RESULTS

The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo.

CONCLUSIONS

Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.

Wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners

This study analyzed the wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners. The geometric patterns of the wear surface were first classified, then wear volume formulae were derived by integral calculus. SolidWorks((R)) software or published formulae were used to verify the accuracy of the proposed formulae. The analytical results showed that the wear shape of the liner can be categorized into seven wear patterns, including the special case of wear at 90 degrees , and the seven corresponding wear formulae were derived. In addition, wear of the cylindrical elongation might add considerably to the volume loss of the liner, depending on the height and shape of the elongation and the depth and direction of the linear penetration, being maximally 21% in the investigated model. The proposed wear formulae and patterns will be useful for more accurate performance evaluation of existing hip components implanted in patients and for the designing of new hip components.