A lexicon for wear of metal-on-metal hip prostheses.

Research on metal-on-metal (MoM) hip bearings has generated an extensive vocabulary to describe the wear processes and resultant surface damage. However, a lack of consistency and some redundancy exist in the current terminology. To facilitate the understanding of MoM tribology and to enhance communication of results among researchers and clinicians, we propose four categories of wear terminology: wear modes refer to the in vivo conditions under which the wear occurred; wear mechanisms refer to fundamental wear processes (adhesion, abrasion, fatigue, and tribochemical reactions); wear damage refers to the resultant changes in the morphology and/or composition of the surfaces; and wear features refer to the specific wear phenomena that are described in terms of the relevant modes, mechanisms, and damage. Clarifying examples are presented, but it is expected that terms will be added to the lexicon as new mechanisms and types of damage are identified. Corrosion refers to electrochemical processes that can remove or add material and thus also generate damage. Corrosion can act alone or may interact with mechanical wear. Examples of corrosion damage are also presented. However, an in-depth discussion of the many types of corrosion and their effects is beyond the scope of the present wear lexicon.

Accuracy of methods for calculating volumetric wear from coordinate measuring machine data of retrieved metal-on-metal hip joint implants.

This study compared the accuracy and sensitivity of several numerical methods employing spherical or plane triangles for calculating the volumetric wear of retrieved metal-on-metal hip joint implants from coordinate measuring machine measurements. Five methods, one using spherical triangles and four using plane triangles to represent the bearing and the best-fit surfaces, were assessed and compared on a perfect hemisphere model and a hemi-ellipsoid model (i.e. unworn models), computer-generated wear models and wear-tested femoral balls, with point spacings of 0.5, 1, 2 and 3 mm. The results showed that the algorithm (Method 1) employing spherical triangles to represent the bearing surface and to scale the mesh to the best-fit surfaces produced adequate accuracy for the wear volume with point spacings of 0.5, 1, 2 and 3 mm. The algorithms (Methods 2-4) using plane triangles to represent the bearing surface and to scale the mesh to the best-fit surface also produced accuracies that were comparable to that with spherical triangles. In contrast, if the bearing surface was represented with a mesh of plane triangles and the best-fit surface was taken as a smooth surface without discretization (Method 5), the algorithm produced much lower accuracy with a point spacing of 0.5 mm than Methods 1-4 with a point spacing of 3 mm.

Fracture of a cross-linked polyethylene liner: a multifactorial issue.

A limited number of reports have detailed the cause of fracture of a highly cross-linked polyethylene liner. Typically, the fractures have occurred in a region of thin and/or unsupported polyethylene, in association with superiorly directed edge loading conditions secondary to an excessively inclinated acetabular component. This case report details an unusual fracture mechanism of a 5-mrad cross-linked liner caused by horizontal loading conditions. The report details several factors that were felt to be etiologic including the specific liner locking mechanism. The treatment options are discussed.

Wear versus thickness and other features of 5-Mrad crosslinked UHMWPE acetabular liners.

BACKGROUND: The low wear rates of crosslinked polyethylenes provide the potential to use larger diameters to resist dislocation. However, this requires the use of thinner liners in the acetabular component, with concern that higher contact stresses will increase wear, offsetting the benefits of the crosslinking. QUESTIONS/PURPOSES: We asked the following questions: Is the wear of conventional and crosslinked polyethylene liners affected by ball diameter, rigidity of backing, and liner thickness? Are the stresses in the liner affected by thickness? METHODS: Wear rates were measured in a hip simulator and stresses were calculated using finite element modeling. RESULTS: Without crosslinking, the wear rate was 4% to 10% greater with a 36-mm diameter than a 28-mm diameter. With crosslinking, wear was 9% lower with a 36-mm diameter without metal backing and 4% greater with metal backing. Reducing the thickness from 6 mm to 3 mm increased the contact stress by 46%, but the wear rate decreased by 19%. CONCLUSIONS: The reduction in wear with 5 Mrad of crosslinking was not offset by increasing the diameter from 28 mm to 36 mm or by using a liner as thin as 3 mm. CLINICAL RELEVANCE: The results indicate, for a properly positioned 5-Mrad crosslinked acetabular component and within the range of dimensions evaluated, neither wear nor stresses in the polyethylene are limiting factors in the use of larger-diameter, thinner cups to resist dislocation.

Metal wear particles: What we know, what we do not know, and why.

The importance of wear particle characterization for orthopaedic implants has long been established in the hip and knee arthroplasty literature. With the increasing use of motion preservation implants in the spine, the characterization of wear debris, particularly metallic nature, is gaining importance. An accurate morphological analysis of wear particles provides for both a complete characterization of the biocompatibility of the implant material and its wear products, and an in-depth understanding of the wear mechanisms, ion release, and associated corrosive activity related to the wear particles. In this paper, we present an overview of the most commonly-used published protocols for the isolation and characterization of metal wear particles, and highlight the limitations and uncertainties inherent to metal particle analysis.

How have wear testing and joint simulator studies helped to discriminate among materials and designs?

Historically, hip joint simulators most often have been used to model wear of a bearing surface against a bearing surface. These simulators have provided highly accurate predictions of the in vivo wear of a broad spectrum of bearing materials, including cross-linked polyethylenes, metal-on-metal, ceramic-on-ceramic, and others in development. In recent years, more severe conditions have been successfully modeled, including jogging, stair climbing, ball-cup micro separation, third-body abrasion, and neck-socket impingement. These tests have served to identify improved materials and to eliminate some with inadequate wear resistance prior to their clinical use. Simulation of the knee joint is inherently more complex than it is for the hip. It is more difficult to compare the results of laboratory tests with actual clinical performance, due to the lack of accurate in vivo measures of wear. Nevertheless, knee simulators, based on force control or motion control, have successfully reproduced the type of surface damage that occurs in vivo (eg, burnishing, scratching, pitting) as well as the size and shapes of the resultant wear particles. Knee simulators have been used to compare molded versus machined polyethylene components, highly cross-linked polyethylenes, fixed versus mobile bearings, and oxidized zirconia and other materials, under optimal conditions as well as more severe wear modes, such as malalignment, higher loading and activity levels, and third-body roughening.

The lexicon of polyethylene wear in artificial joints.

The analysis of wear on polyethylene components that have been retrieved after use in patients has provided invaluable understanding of how wear occurs in vivo, and how it may be minimized through improved materials and implant design. The great number of such studies that have been published over the past three decades has lead to an extensive vocabulary to describe the tribology of prosthetic joints. However, these also have led to some confusion, due to the occasional misuse of terms from classical tribology, along with the use of multiple terms to describe the same wear phenomenon, and vice versa. The author has proposed that our understanding of wear in artificial joints may be enhanced by recognizing that there are four general subject areas: Modes, Mechanisms, Damage and Debris. Wear Mode 1 occurs when the two bearing surfaces are articulating against each other in the manner intended by the implant designer. Mode 2 occurs when a bearing surface articulates against a non-bearing surface. Mode 3 occurs when third-body abrasive particles have become entrapped between the two bearing surfaces, and Mode 4 occurs when two non-bearing surfaces are wearing against each other. The least wear occurs in Mode 1, whereas severe wear typically occurs in Modes 2, 3 and 4. The classical wear mechanisms that apply to prosthetic joints include adhesion, abrasion and fatigue. These can occur in varying amounts in either of the four wear modes. As used in the literature for the past three decades, wear "damage" can best be defined as the change surface texture or morphology that is caused by the action of the wear mechanisms. Although a wide variety of terms have been used, an overview of the literature indicates that about eight terms have been sufficient to describe the types of damage that occur on retrieved polyethylene components, i.e., burnishing, abrasion, scratches, plastic deformation, cracks, pits, delamination, and embedded third bodies. The author suggests that, as far as possible, investigators endeavor to limit their descriptions of surface damage to these terms and, importantly, to clearly and consistently distinguish the classical wear mechanisms from the types of damage produced by those mechanisms. Wear debris refers to the billions of particles, some measuring in nanometers, that are generated by the wear mechanisms, and that initiate biological reactions, such as osteolysis, that may lead to the failure of the implant. As the methods for recovering wear debris from joint fluids and tissues are improved, investigators are using a growing number of terms to describe them. As with the types of damage, it will be important in the coming years to maximize clarity and minimize redundancy of the vocabulary in this important area of research.

Accuracy of measurement of polyethylene wear with use of radiographs of total hip replacements.

BACKGROUND: Although a number of methods are used to estimate polyethylene liner wear from radiographs of total hip replacements, there is no consensus with regard to the accuracy of these methods. The purpose of this study was to compare the accuracy of several such measurement methods with use of both laboratory radiographs and routine clinical radiographs. METHODS: A phantom apparatus was designed to simulate random values of three-dimensional wear, with varying degrees of cup abduction and anteversion, and to obtain anteroposterior and cross-table lateral radiographs with each value. Wear was measured with use of the Charnley duoradiographic method, the Livermore method, and the method described by Dorr and Wan, as well as with use of PolyWare and Hip32 software packages, both with and without three-dimensional measurements. Clinical wear was measured from conventional radiographs made prior to revision surgery in fourteen patients and was compared with wear measured directly from the retrieved liners with use of a coordinate measuring machine. RESULTS: With laboratory radiographs, median errors were 0.1 mm with the Livermore method and both computerized methods, 0.23 mm with the Charnley method, and 1.7 mm with the method of Dorr and Wan. Maximum errors were between 0.6 mm (Livermore) and 4.3 mm (Dorr and Wan). In contrast, with use of clinical radiographs, median errors ranged between 0.2 mm (Hip32) and 0.6 mm (Dorr and Wan). Maximum errors ranged between 1.8 mm (Dorr and Wan) and 2.5 mm (Livermore). CONCLUSIONS: With laboratory radiographs, computerized methods of polyethylene wear measurement offered distinctly greater accuracy than did manual methods; however, with clinical radiographs, they offered only slightly better accuracy. Although the increased accuracy of computerized methods may be necessary in research settings, manual methods provided sufficient accuracy for routine clinical assessment of wear.

Long-term radiographic changes in cemented total hip arthroplasty with six designs of femoral components.

Measurements were made from annual follow-up radiographs, obtained over 27.6 years, of 860 cemented total hip arthroplasties implanted by one surgeon. Femoral components were made of stainless steel or titanium alloy, were non-modular, and were all fixed with cement, and acetabular cups were all-polyethylene and were fixed with cement. Radiographic outcome was correlated with the shape and material of the femoral component. Specifically, throughout the follow-up, stems made of titanium alloy were at greater risk of developing bone-cement radiolucent lines than those made of stainless steel, the difference ranging from approximately 10-50 percent at 2-10 years of follow-up. Similarly, titanium alloy stems were at greater risk of developing endosteal scalloping, indicating osteolytic lesions. Among the stainless steel Charnley cobra and straight-narrow Charnley stems, none developed cement fracture, only one became radiographically loose and one developed endosteal scalloping. The differences in the risk of developing radiolucent lines, cement fracture and progressive loosening among these stems were correlated with the relative rigidity of the femoral stems, and were generally consistent with the predictions made heretofore using finite element models, although differences in stem surface finish and femoral ball size and material could have also influenced the results.

Interaction of oxidation and crosslinking in gamma-irradiated ultrahigh molecular-weight polyethylene.

The interaction between oxidation and crosslinking in gamma-irradiated ultrahigh molecular-weight polyethylene with and without artificial aging was studied. The effect of the atmosphere during irradiation (air vs. low oxygen) occurred primarily within about 0.5 mm of the surface, that is, the depth to which oxygen had diffused when the polyethylene specimen was machined and when it was irradiated. Irradiation in the presence of oxygen induced oxidation instead of crosslinking, so that the level of crosslinking achieved was lower than that which normally would occur at the same dose in the absence of oxygen. Subsequent artificial aging reduced the gel content (crosslinking) and had a maximal effect on the surface and subsurface regions for the gamma-air and gamma-low oxygen polyethylenes, respectively. Thus the storage environments and durations prior to irradiation and prior to artificial aging must be taken into account when attempting to duplicate the oxidation-crosslinking profiles that occur with actual implants in clinical use. In addition, the oxidation mechanisms initiated by the artificial aging method used in this study (i.e., heating in air to 80 degrees C) initiated somewhat different oxidative reactions from those that occur during prolonged shelf life at room temperature or in vivo. In particular, the formation of a peak of oxidation below the free surface of the polyethylene is due to the combined effects of the distribution of residual free radicals and the diffusion gradient of the oxygen. The interactive relationship between oxidation and crosslinking characterized in the present study provides a fundamental basis for understanding the wear behavior of gamma-sterilized components in past clinical use. It also provides guidelines for the development of polyethylenes with improved resistance to oxidation and wear, with particular relevance to estimation of the amount of crosslinking need- ed to potentially eliminate the clinical problem of osteolysis.