Femoral Balls Research Articles

Comparison of zirconia degradation in dental implants and femoral balls: an X-ray diffraction and nanoindentation study

BackgroundNew tetragonal zirconia polycrystal dental implants stabilized with yttria (Y-TZP) have appeared in the implantology market in the form of single piece or two-piece zircona implant system. These new type of implants improve the aesthetical properties compared to conventional commercially pure (c.p.) titanium used for implants, although the long term mechanical behavior of these new implants is not yet well known. In orthopaedics, the application of zirconia as femoral balls presented an important controversial use due to the premature fracture once implanted. Y-TZP dental implants can be affected by hydrothermal degradation and its behavior should be analysed to avoid a premature fracture. The scientific question behind the study is to analyse if the degradation mechanism observed in orthopaedics applications of Y-TZP is similar to that of Y-TZP for dental applications.Materials and methodsFor this purpose, 30 original Y-TZP dental implants and 42 Y-TZP femoral balls fractured in vivo have been studied. Dental implants were submitted to an accelerated hydrothermal degradation to compare with the femoral balls fractured in vivo. Phase transformation as well as the mechanical behaviour of the degraded samples was studied by X ray diffraction and nanoindentation tests, respectively.ResultsResults have shown that the fracture mechanism of dental implants does not resemble the mechanism observed in orthopaedic samples, presenting a good long-term behaviour.ConclusionThe results ensure the good performance of zirconia dental implants, because the degradation of the ceramic is very limited and does not affect the mechanical properties.

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.

Increasing the wear resistance of UHMWPE acetabular cups by adding natural biocompatible particles

Ultra-high molecular weight polyethylene (UHMWPE) is a kind of biomaterial applied as the acetabular cup in artificial hip joints. However, wear debris generation of UHMWPE materials after their replacements in human body has been proved to induce aseptic loosening and osteolysis, which is the main cause of long-term failure of total hip joints composed of UHMWPE and hard femoral balls. In this paper, natural coral (NC) particles in the size within 50 μm were applied as reinforcement fillers into UHMWPE to increase the wear resistance of UHMWPE composites as artificial acetabular cups. The micro-hardness and scratch resistance of UHMWPE/NC composites were studied. A hip joint wear simulator was operated to investigate the wear behavior of UHMWPE/NC composites against CoCrMo balls, the wear tests were lubricated by 25% bovine serum solutions. The wear mass loss and wear debris distribution through one million test cycles were examined in the paper. It is shown that the adding of NC particles in UHMWPE resulted in the enhancement of micro-hardness and scratching resistance of the UHMWPE/NC composites. The micro-hardness of UHMWPE/NC composites increased with natural coral contents in the linear relationship. The scratch coefficients of UHMWPE/NC composites increased in contents range of 10 wt% NC particles, the scratch depth decreased with the increasing NC contents. The wear resistance of UHMWPE/NC composites increased with the increasing contents of natural coral particles. It was found that the relation of wear mass loss of UHMWPE acetabular cup to the micro-hardness followed a negative power law. The wear mechanism of UHMWPE/NC composites was mainly controlled by adhesive wear. The NC contents in UHMWPE changed the severity of adhesive wear. The investigation of UHMWPE wear debris revealed that adding of NC particles in UHMWPE resulted in variations of size distribution of UHMWPE wear debris. Because micro-hardness of natural coral materials was much lower compared to CoCrMo alloy, few metal debris from CoCrMo ball were captured in the filter paper.

Third-body abrasive wear challenge of 32 mm conventional and 44 mm highly crosslinked polyethylene liners in a hip simulator model

Hip simulator studies have shown that wear in the polyethylene liners used for total hip replacements increased with the larger-diameter femoral balls and could also be exacerbated by third-body abrasion. However, they also indicated that the more highly cross-linked polyethylene (HXPE) bearings were more wear resistant than conventional polyethylene (CXPE) bearings. Unfortunately the HXPE bearings appeared to be particularly sensitive to adverse wear conditions. One simulator study in particular indicated that poly(methyl methacrylate) (PMMA) debris increased wear sixfold by means of two-body abrasive interactions rather than the supposed third-body abrasion or roughening effects of the Co-Cr surfaces. There has been no confirmation of such novel theories. Therefore the goal of this study was to investigate the sensitivity of large-diameter HXPE bearings to the third-body PMMA wear challenge in a hip simulator model. An orbital hip simulator was used in standard test mode with a physiological load profile. The 32 mm control liners were machined from moulded GUR1050 and gamma irradiated to 35 kGy under nitrogen (CXPE). The 44 mm liners were also from moulded blanks, gamma irradiated to 75 kGy, machined to shape, given a proprietary heat treatment, and sterilized by gas plasma (HXPE). As in the published simulator model, the study was conducted in three phases. In phase 1, all cups were run in standard ('clean') lubricant for 1.5 x 10(6) cycles duration. In phase 2, three CXPE cups and six HXPE cups were run for 2 x 10(6) cycles with a slurry of PMMA particles added to the lubricant. In phase 3, the implants were again run in 'clean' lubricant for 2 x 10(6) cycles duration. In addition, three HXPE cups were run as wear controls for 5.5 x 10(6) cycles duration in clean lubricant. In phase-1, the HXPE liners demonstrated twelvefold reduced wear compared with the CXPE controls. The 32 mm and 44 mm Co-Cr balls were judged of comparable roughnesses. However, the surface finish of HXPE liners was superior to that of CXPE liners. In phase-2 abrasion, wear rates increased sixfold and eighty-fold for CXPE and HXPE bearings respectively. These data confirmed that HXPE bearings were particularly sensitive to 'severe' test modes. The Co-Cr balls revealed numerous surface patches representing transferred PMMA with average transient roughness increased to 25 nm and 212 nm for the 32 mm and 44 mm balls respectively. These PMMA patches produced an aggressive two-body abrasion wear of the polyethylene. After cleaning, the ball roughness returned to near normal. Therefore the Co-Cr roughness was not an issue in this severe test mode. In phase 3, the wear decreased to near the index values of phase 1, while liner roughness dropped by more than 90 per cent. The control CXPE liners now demonstrated twice the wear of the HXPE, as would be predicted comparing the diameter and cross-linking algorithms. No previous study has correlated polyethylene roughness profiles to wear performance. In phase 2, PMMA abrasion created significant damage to the polyethylene surfaces. The average roughness Sa of CXPE liners increased to 3.6 microm, a twenty-four-fold increase with some scratches up to 40 microm deep. The HXPE roughness also increased but only to 1.5 microm, a ninefold increase. The scratch indices Sz and Sp for HXPE surfaces were also 50 per cent less severe than on CXPE surfaces. However, within 2 x 10(6) cycles duration of phase 3, all liners had recovered to virtually their original surface finish in phase 1. In all test phases, the surface finish of the HXPE liners remained superior to control liners. These experimental data confirmed many of the results from the previous simulator study with the PMMA abrasion models. Thus the 44 mm liners appeared an excellent clinical alternative to the smaller ball designs used in total hip replacements.

Surface-Gradient Cross-linked Polyethylene Acetabular Cups

Two methods were developed and evaluated for cross-linking the bearing surface of a polyethylene acetabular cup to a limited depth, in order to improve its resistance to wear without degrading the mechanical properties of the bulk of the component. In the first method, low-energy electron beams were used to cross-link only the bearing surface of the cups to a maximum depth of about 2 mm. The cups then were annealed at 100 degrees C in vacuum for 3 or 6 days to reduce the residual free radicals, and the resultant resistance to oxidation was compared by artificially aging the cups at 80 degrees C in air. Chemically cross-linked surface layers were produced by coating the bearing surfaces of the cups with a thin layer of polyethylene powder mixed with 1% weight peroxide, and compressing them at 6.9 MPa (1000 psi) and 170 degrees C. This resulted in a cross-linked surface layer that extended about 3 mm deep, with a gradual transition to conventional (noncross-linked) polyethylene in the bulk of the implant. In hip simulator wear tests with highly polished (implant quality) femoral balls, both types of surface cross-linking were found to improve markedly the wear resistance of the acetabular cups. In tests with roughened femoral balls, the wear rates were much higher and were comparable to those obtained with similarly roughened balls against noncross-linked polyethylene cups in a previous study, indicating that the full benefit of cross-linking may not be realized under conditions of severe third-body abrasion. Nevertheless, these results show a promising approach for optimizing the wear resistance and the bulk mechanical properties of polyethylene components in total joint arthroplasty.

Comparison of Diamond‐Like‐Carbon and Alumina‐Oxide articulating with Polyethylene in Total Hip Arthroplasty

AbstractTo reduce wear in joint bearings of total hip arthroplasty (THA) is the most important issue for improving long term results and implant survival. Due to low wear rates and excellent tribological features in simulator tests Diamond‐Like‐Carbon coating (DLC) of femoral balls is still discussed as an alternative articulation in THA. This clinical prospective study compares survivorship of DLC‐coated femoral heads and of Aluminia‐Oxide‐ (Al2O3) heads articulating with Polyethylene (PE).Over a period of two years 101 THA with DLC‐coated heads and PE cups (DLC‐group) and another 101 THA consisting of Al2O3 heads (Al2O3‐group) and PE cups as well were implanted. Both articulations were based on the same type of cementless hip joint prosthesis. All hips were implanted by one surgeon in consecutive series consisting of 51 Al2O3 and 101 DLC‐articulations and further 50 Al2O3. All perioperative and follow‐up data was processed with SPSS®. Survival of THA in both groups was evaluated according to Kaplan‐Meier survivorship analysis with an intervall of 90 months (range:78‐101). Qualitative surface analysis was performed in nineteen retrieved DLC‐heads which were revised for aseptic loosening using field scanning electron microscopy (FE‐SEM, XL 30 SFEG Philips, Eindoven NL).178 patients (88.2 %) were evaluated for follow‐up. Fourteen patients died meantimes (nine DLC, five Al2O3) with the implant components not revised. Ten patients (five DLC, five Al2O3) were lost to follow‐up. Both groups were comparable regarding patient age, weight and indications for THA with a normal distribution. Survivorship analysis for aseptic loosening 8.5 years following implantation resulted in a significant difference between both groups with a 54 % survival for DLC/PE compared to 88 % for Al2O3/PE bearings (p <0.001). No correlation to variables as age, gender or bodyweight could be detected. Surfaces of nineteen retrieved DLC‐heads showed numerous smallest pits of the diamond‐carbon layers in different quantity. SEM showed delamination of the carbon layer which caused excessive debris of polyethylene and in some cases even of the metallic substrate of the heads.Despite modern manufacturing technology and excellent experimental results for its tribochemical characteristics and wear, even “new” DLC‐coating of femoral heads is to be considered critically due to very high rates of clinical failure.

The effect of frictional heating and forced cooling on the serum lubricant and wear of UHMW polyethylene cups against cobalt–chromium and zirconia balls

Hip simulator tests of femoral balls of cobalt–chromium alloy or zirconia against acetabular cups of UHMW polyethylene were run with and without a coolant circulated inside the femoral balls. Without cooling, the wear of polyethylene against zirconia was about 48% lower than with cobalt–chromium alloy, but the steady-state temperature of the zirconia ball was higher (55°C vs. 41°C), and there was more precipitation of protein from the serum, which sometimes formed an adherent layer on the surface of the zirconia. Circulating coolant at 1–20°C markedly reduced the bearing temperatures and the protein precipitation. With coolant at 4°C, wear of the polyethylene against cobalt–chromium alloy was about 26% lower than against zirconia, but the macroscopic and microscopic appearance of the worn polyethylene surfaces were unlike that typically generated in vivo. With or without coolant, the morphology of the polyethylene wear debris was comparable to that generated in vivo, but the ratio of fibrillar to granular debris was higher at the reduced temperature. These results suggested that circulating coolant at an appropriate temperature could avoid overheating (due to non-stop running of the simulator), preventing excessive protein precipitation while providing wear surfaces and wear debris with morphologies closely comparable to those generated in vivo.

Potential thermal artifacts in hip joint wear simulators.

Frictional heat was monitored during wear tests of ultrahigh molecular weight polyethylene acetabular cups bearing against femoral balls of metal or ceramic in a hip simulator, using bovine serum as a lubricant. About 1 to 2 h of continuous cycling were required for the temperature in the zone of contact between the cup and ball to rise to its maximum steady value, and this equilibrium temperature was markedly higher with increased load and/or cycling rate. Frictional heating caused substantial precipitation of the proteins from the serum and, in some of the tests running at 1.5 or 2 Hz, an adherent proteinaceous layer was observed attached to the surface of the balls. The maximum temperature was also substantially higher in tests run with the cup mounted above the ball rather than below. Surprisingly, the tests running at higher frictional torque and temperature (i.e., those with the most protein precipitation and/or adherent layers) produced the least wear of the polyethylene. This might have been due to the solid proteins that formed a protective layer between the ball and cup. Because patients with hip prostheses typically do not walk for hours without rest, the maximum temperatures in vivo are likely to be much lower than those reached in the hip simulator. Therefore, the affects of protein precipitation on the resultant wear properties of the materials should be considered potential artifacts of the hip simulator tests. Increasing the volume of the lubricant bath reduced the maximum temperatures for tests running at 1.5 Hz but had little affect at 2 Hz. Reducing the cycling rate is an effective way to avoid overheating of the specimens, but this necessarily extends the time required to complete a test.

Frictional heating of bearing materials tested in a hip joint wear simulator.

In a hip simulator wear test using bovine serum as a lubricant, the heat generated by ball-cup friction may cause precipitation of the proteins from the lubricant. The resultant accumulation of a solid layer of precipitated protein between the ball and cup could artificially protect the bearing surfaces from wear, in a manner that does not occur in vivo. Alternatively, the gradual depletion of the soluble proteins could interfere with their ability to act as boundary lubricants on the bearing surfaces, thereby artificially increasing the wear rate. Because the rate of protein precipitation may depend on the maximum temperature at the bearing surfaces during sliding, rather than the mean temperature of the bulk lubricant, this study determined the transient surface temperatures using an array of thermocouples embedded in acetabular cups of GUR 415 ultra-high molecular weight polyethylene (UHMWPE) and femoral balls of metal or ceramic, in conjunction with a finite element model of the temperature distribution. The prostheses were tested at one cycle/s under a Paul-type, physiological load profile with 2030 N maximum force, with the load cycle synchronized to the motion cycle. The steady state temperatures of the bulk lubricant were 38 degrees C for the zirconia balls, 36 degrees C for the cobalt-chromium and 33 degrees C for the alumina. However, the corresponding surface temperatures of the polyethylene, calculated with the finite element model, were 99 degrees C with zirconia ceramic, 60 degrees C with cobalt-chromium alloy, and 45 degrees C with alumina ceramic. The rank order of the surface temperatures corresponded to the relative amounts of protein that were precipitated in the test chambers during wear tests with these materials.

Comparison of Alumina-Polyethylene and Metal-Polyethylene in Clinical Trials

The dimensional changes of hip sockets of Müller-type total endoprostheses is the subject of this article. Regular anteroposterior roentgenographs of the pelvis were taken to determine the orientation of the center of the prosthetic head in relation to the wire marker of the polyethylene cup. Three different materials used for the femoral balls and matched with polyethylene as socket material were investigated, and the results of the displacement of the ball into the socket were compared. Both creep and wear contribute to the dimensional changes of the hip sockets; the proportional amount of each mechanism is not known. Data from laboratory examinations suggest a relatively high rate of creep in the first six months after implantation. With longer periods, the dimensional changes are predominately caused by wear. In the beginning of joint function, measurements show a high rate of the yearly dimensional changes. The head shifts up to 0.5 mm per year and diminishes after five years to rates of 0.1-0.2 mm, respectively. All dimensional changes that exceed a shift of the head of 0.2 mm per year are considered to be unfavorable and to contribute to loosening of the implants. Using metallic balls (Protasul-2), 64% had a wear rate of less than 0.2 mm; of those using Prostasul-10, 77% had lower rates than 0.2 mm. In patients where ceramic balls were implanted, the displacement rate was below 0.2 mm per year in 95%. Therefore, ceramic seems to be the most favorable material.