Cup Wear Research Articles

Quantifying the wear of acetabular cups using coordinate metrology

Abstract In an ageing population where individuals demand a pain free and active existence, hip arthroplasty has become one of the most important operations of later life, with developments increasing the longevity of this prosthesis. Accurate assessment of wear is an important way to assess how well hip prostheses perform over time. This paper describes a new protocol to accurately locate and map the wear of acetabular cups. 16 metal-on-metal acetabular cups were tested in a hip simulator for up to 6 million cycles at a range of angles of inclination in order to create a large range of wear volumes. Unworn and worn regions of acetabular cups were digitised in two separate scans using a CMM. The two scans were superimposed in order to allow calculation of maximum depth, wear volume loss and rim damage. Values obtained from the CMM protocol (CMM) were compared to the wear volume determined gravimetrically (G); a strong correlation (CMM = 0.992G−0.504) was established. The CMM protocol established was shown to be a methodology more powerful than many existing geometric protocols which could be used to assess explants and implants tested in vitro.

In Vivo Wear of a Squeaky Alumina-on-Alumina Hip Prosthesis

Alumina-on-alumina bearings for total hip replacement have been in use for over three decades and are associated with good long-term clinical performance1,2. However, recently there have been anecdotal reports of audible noise, usually characterized as squeaking, in ceramic-on-ceramic joints, which have been publicized even in the media3. The reported prevalence of squeaking and noise in these bearings has varied from 10% (as reported by Varnum et al.7, Keurentjes et al.8, and Jarrett et al.9). The etiology of hip squeaking remains unclear and is undoubtedly a multifactorial phenomenon that may involve component femoral neck-cup impingement, microseparation, and subluxation10. Although limited in number, case reports on squeaky ceramic hip bearings appear to reflect this varied etiology. Thus, squeaking was seen in connection with the use of a mismatched zirconia head and alumina cup wear couple11, femoral neck-socket impingement12,13, and acetabular cup abduction and retroversion coupled with leg-length discrepancy14. There is an ongoing debate about whether squeaking of ceramic hips is a cause for concern15 or not16. Regardless of its prevalence, squeaking of ceramic-on-ceramic joints warrants investigation as it may signal abnormal wear of the bearing surfaces, which, in view of the brittle nature of ceramics, may progressively increase in severity. In the present case study, we had the opportunity to examine an alumina-on-alumina hip that was explanted because of severe persistent squeaking with all activities. The purpose of the present study was to determine if the severe noise was accompanied by substantial joint wear. We hypothesized that the squeaking was associated with degradation of the bearing surfaces. We …

Validation of a 3D CT method for measurement of linear wear of acetabular cups

Background We evaluated the accuracy and repeatability of a 3D method for polyethylene acetabular cup wear measurements using computed tomography (CT). We propose that the method be used for clinical in vivo assessment of wear in acetabular cups.Material and methods Ultra-high molecular weight polyethylene cups with a titanium mesh molded on the outside were subjected to wear using a hip simulator. Before and after wear, they were (1) imaged with a CT scanner using a phantom model device, (2) measured using a coordinate measurement machine (CMM), and (3) weighed. CMM was used as the reference method for measurement of femoral head penetration into the cup and for comparison with CT, and gravimetric measurements were used as a reference for both CT and CMM. Femoral head penetration and wear vector angle were studied. The head diameters were also measured with both CMM and CT. The repeatability of the method proposed was evaluated with two repeated measurements using different positions of the phantom in the CT scanner.Results The accuracy of the 3D CT method for evaluation of linear wear was 0.51 mm and the repeatability was 0.39 mm. Repeatability for wear vector angle was 17°.Interpretation This study of metal-meshed hip-simulated acetabular cups shows that CT has the capacity for reliable measurement of linear wear of acetabular cups at a clinically relevant level of accuracy.

Wear Behaviour and Wear Debris Characterization of UHMWPE on Alumina Ceramic, Stainless Steel, CoCrMo and Ti6Al4V Hip Prostheses in a Hip Joint Simulator

Ultra-high molecular-weight polyethylene (UHMWPE) has been used in total hip replacement for the last three decades. Despite the advancements in prosthesis design, the wear of UHMWPE remains a serious clinical problem; the release of wear debris may induce osteolysis and implant loosening. Understanding of wear behavior and wear debris morphology of the polyethylene is essential to improve the reliability of hip joint implants. The investigation in this paper carried out wear simulation tests of UHMWPE on Al2O3, 316L stainless steel, CoCrMo alloy and Ti6Al4V alloy, respectively. The lubrication of plasma solution and bovine serum solution was presented in wear tests. The effect of motion and loading on the wear behavior and wear debris morphology, and the influence of femoral head material and assembly style were studied in order to obtain a better understanding of the morphology of ultra-high molecular weight polyethylene wear particles. It is shown that the wear of UHMWPE acetabular cups against metal femoral heads was significantly higher than that against ceramic heads. The presence of protein in lubricant increases the wear of UHMWPE acetabular cups on Al2O3 heads. The wear rates of UHMWPE in multi-directional motion are approximately 2.5 times of those in uni-directional motion. The size distribution range of the UHMWPE debris particles for all head materials varies from submicron particles up to several hundreds micron. The size distribution range of wear debris particles is not directly related to wear resistance of UHMWPE, but significantly influenced by wear mechanisms. The UHMWPE debris particles produced in hip wear simulation tests are classified as round debris, flake-like debris and stick debris, which are closely related to the primary mechanisms of abrasive wear, adhesive wear and fatigue wear.

Influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations in total hip arthroplasty

Objective To study the influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations, which meets the criteria for desired range of motion (ROM) for activities of daily living in total hip arthroplasty (THA). Methods A three-dimensional generic, parametric and kinematic simulation module of THA was developed to analyze the cup safe-zone and the optimum combination of cup and neck antetorsion. A ROM of flexion ≥ 120°, internal rotation ≥45° at 90° flexion, extension ≥30° and external rotation ≥ 40° was defined as the criteria for desired ROM for activities of daily living. The cup safe-zone was defined as the area that fulfills all the criteria of desired ROM before the neck impinged on the liner of the cup. For a fixed stem-neck (CCD)-angle of 130°, theoretical safe-zones fulfilling the desired ROM were investigated at different general head-neck ratios (GR=2, 2.17, 2.37, 2.61 and 2.92) and femoral anteversions (FA=0°, 10°, 20° and 30°). Results Large GRs greatly increased the size of safe-zones and when the CCD-angle was 130°, a GR>2.37 could further increase the size of safe-zones. There was a complex interplay between the orientation angles of the femoral and acetabular components. When the CCD-angle was 130°, the optimum relationship between operative acetabular ante-version (OA) and femoral antetorsion (FA) could be estimated by the formula: OA=-0.80×FA+47.06, and the minimum allowable operative acetabular inclination (OI min) would be more than 210.5xGR 2255. Conclusions Large GRs greatly increase the size of safe-zones and it is recommended that the GR be more than 2.37 so as to extend the acceptable range of error that surgeons cannot avoid completely during operation. As to the optimum operative acetabular inclination (OI), surgeons need to make a decision combining with other factors, including stress distribution, soft tissue and cup wear conditions, as well as patients' individual situations and demands. The data obtained from this study and the module of THA can be used to assist surgeons to choose and implant appropriate implants.

A New Formulation for the Prediction of Polyethylene Wear in Artificial Hip Joints

Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.

Postmortem Study of Femoral Osteolysis Associated with Metal-on-Metal Articulation in Total Hip Replacement

Improved metal-on-metal articulations were reintroduced in total hip replacement to avoid the osteolysis sometimes seen with conventional ultra-high molecular weight polyethylene bearings. Osteolysis and local lymphocytic infiltration have been reported at revision of some metal-on-metal devices. We report similar and additional results in a study of second-generation metal-on-metal hip implants retrieved post mortem. Components and surrounding tissues were collected post mortem from seven patients with nine total hip replacements (Zweymüller SL stem with an Alloclassic cup) with Metasul metal-on-metal articulations. All available patient information was recorded. Radiographs of the hips were evaluated for osteolysis. Sections of joint capsule as well as of the femoral implant with surrounding bone were reviewed, and energy-dispersive x-ray analysis was used to evaluate the composition of wear products. The amount of wear was measured for each component (nine femoral heads and eight cup inserts), when possible, by a coordinate measurement machine with use of the dimensional method. The patients died between three and ten years after arthroplasty, and six of the seven were asymptomatic at the time of death. One patient, with the highest rate of total wear (i.e., wear of femoral head and acetabular cup; 7.6 microm/yr), had increasing hip pain for one year, and histological analysis confirmed the radiographic findings of osteolysis. For two other patients, histological analysis confirmed the radiographic findings of asymptomatic osteolysis. For three patients, histological analysis revealed osteolysis that had escaped conventional radiographic analysis. Joint capsule tissue showed evidence of metallosis in all hips and local lymphocytic infiltration in eight hips. Energy-dispersive x-ray analysis revealed elements attributable to CoCrMo alloy in all hips and traces of corrosion products in three hips. The postmortem findings of osteolysis and/or lymphocytic infiltration associated with eight clinically well-functioning, low wear devices (a total wear rate of <4 microm/yr) suggest there may be frequent, unappreciated femoral bone loss and local immunological response in patients with second-generation metal-on-metal hip implants. Compared with previous postmortem studies, our findings showed the extent of osteolysis was similar to that with metal-on-polyethylene articulations.

Microstructure analysis and wear behavior of titanium cermet femoral head with hard TiC layer

Titanium cermet was successfully synthesized and formed a thin gradient titanium carbide coating on the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature, while the titanium cermet femoral head was produced. The titanium cermet phase and surface topography were characterized with X-ray diffraction (XRD) and backscattered electron imaging (BSE). And then the wear behavior of titanium cermet femoral head was investigated by using CUMT II artificial joint hip simulator. The surface characterization indicates that carbon effectively diffused into the titanium alloys and formed a hard TiC layer on the Ti6Al4V alloys surface with a micro-porous structure. The artificial hip joint experimental results show that titanium cermet femoral head could not only improve the wear resistance of artificial femoral head, but also decrease the wear of UHMWPE joint cup. In addition, the carburized titanium alloy femoral head could effectively control the UHMWPE debris distribution, and increase the size of UHMWPE debris. All of the results suggest that titanium cermet is a prospective femoral head material in artificial joint.

Total Hip Arthroplasty in Rapidly Destructive Osteoarthritis of the Hip: A Case Series

Rapidly destructive osteoarthritis (RDO) of the hip is a rare condition characterized by rapid joint degeneration and destruction similar to findings of infection, osteonecrosis, or Charcot disease but without a definitive diagnosis. The cause and natural history of RDO are unclear, but total hip arthroplasty has been utilized as a treatment modality due to the severity of the symptoms. We reviewed retrospectively the records of total hip arthroplasties performed between 1990 and 2003 and identified ten hips in eight patients who fit the profile of the diagnosis of RDO. The mean age at time of surgery was 70. Nine hips were treated with total hip arthroplasty with a hybrid configuration; one hip was treated with a non-cemented total hip arthroplasty. Average follow-up was 6 years with no radiographic evidence of acetabular loosening or osteolysis and no evidence of asymmetric cup wear. One femoral component had evidence of loosening but has not been revised. RDO is an idiopathic condition with no single diagnostic laboratory, pathological, or radiographic finding. A complete preoperative work-up for other causes of hip disease prior to arthroplasty for suspected RDO is necessary to rule out treatable disease. Our series of patients with RDO responded well to hybrid and non-cemented total hip arthroplasty with good clinical and radiographic results.

ATR-FTIR as a thickness measurement technique for hydrated polymer-on-polymer coatings.

Hydrated polymer coatings on polymer substrates are common for many biomedical applications, such as tissue engineering constructs, contact lenses, and catheters. The thickness of the coatings can affect the mechanical behavior of the systems and the cellular response, but measuring the coating thickness can be quite challenging using conventional methods. We propose a new method, that is, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to determine the relative thickness, combined with atomic force microscopy to calibrate the ATR-FTIR measurements. This technique was successfully employed to determine the hydrated thickness of a series of crosslinked tetraglyme coatings on ultrahigh molecular weight polyethylene substrates intended to reduce wear of acetabular cups in total hip replacements. The hydrated coatings ranged from 30 to 200 nm thick and were accurately measured despite the relatively high root-mean-square (RMS) roughness of the substrates, 20-35 nm (peak-to-peak roughness 55-100 nm). The calibrated ATR-FTIR technique is a promising new method for measuring the thickness of many other polymer-on-polymer and hydrated coatings.

Cemented Polished Tapered Stems in Patients Less Than 50 Years of Age: A Minimum 10-year Follow-Up

We report the outcome of 58 consecutive polished tapered stems in 47 patients younger than 50 years, with a minimum of 10 years of follow-up. No stems were revised for aseptic loosening or osteolysis. The Harris hip scores were good or excellent in 76% of the patients. There was excellent preservation of proximal bone. Despite higher patient weight and frequent previous surgery, the outcome of polished tapered stems in patients younger than 50 years is comparable to the general population undergoing total hip arthroplasty and superior to other non–polished tapered stem designs in young patients. Cup wear and cup failure were significantly worse in this group, with a higher incidence of periarticular osteolysis. Polished tapered stem designs are the benchmark for comparison to other arthroplasty options in young patients.

Aggressive 3rd-body wear challenge to highly crosslinked polyethylene: A hip simulator model

For patients suffering from arthritic conditions, the dominant type of bearing replacement used in artificial hip joints has been a metal ball running in a hemispherical plastic cup. It has been hypothesized that a major cause of accelerated wear with this combination is a 3rd-body wear mechanism, interacting between the CoCr counterfaces and conventionally crosslinked polyethylene (CXPE) bearings. In addition, over the past 30 years the dominant form of implant fixation to bone has been a polymethyl-methacrylate (PMMA) bone cement. As such, the potentially abrasive wear mechanism may be related to the presence of circulating, micron-sized, particulates of PMMA. One laboratory wear study indicated an unexpected type of roughening mechanism for such bearings in that, compared to ‘clean’ lubricants, wear of CXPE and HXPE cups in abrasive slurry increased 6- and 20-fold, respectively. However the surprising finding was that there was no corresponding change in CoCr roughness. The abrasive mechanism appeared to be a transient formation of PMMA layers on the CoCr counterfaces. Modern cup designs predominantly use highly crosslinked polyethylenes (HXPE), which have a much short clinical history. Thus it is conceivable that wear of the HXPE bearings will also be influenced both by circulating PMMA debris (3rd-body wear) and damaged CoCr counterfaces (2-body wear). The present study re-examined the effects of 3rd-body PMMA particulates on the wear of CXPE and HXPE cups using both 32 and 44 mm diameter CoCr and ceramic balls. The 32 mm CXPE represented the control bearing. The hip simulator wear study was run in two phases to compare the effects of abrasive wear. Conventional polyethylene cups (CXPE: 35 kGy irradiated) and cups highly crosslinked by 75 kGy irradiation (HXPE) typical of modern bearings were run with both CoCr and Al 2O 3 balls as the mating counterfaces. The polyethylene cups were mounted in “inverted” position for the debris studies in a multi-station hip simulator. In phase I, all cups were run in ‘clean’ serum lubricant for 1.5 million load cycles (1.5 Mc). In phase II, the cups were run in an abrasive slurry for 2 Mc duration. A commercial PMMA bone cement powder was used as the “abrasive” medium and the PMMA slurry (5 mg/ml concentration) was added to all lubricant chambers at multiple test durations. Wears assessed by the average bearing weight loss and characterized by linear regression in each test phase. Wear and roughness effects on polyethylene, CoCr and Al 2O 3 bearings was analyzed by reflected light microscopy, white light interferometry and scanning electron microscopy. The total wear study was run to 3.5 Mc duration. Compared to phase-1 wear of control CXPE [32 mm] combinations, the wear of ‘new’ HXPE [44 mm] cups was a 6-fold lower. In the phase-2 study, HXPE cups showed different trends depending on whether CoCr or Al 2O 3 balls were used. Thus the wear of HXPE [44C] decreased by 43%, whereas the wear of HXPE [44M] increased 500%. The wear of the “pre-worn (cup after 10 million cycles of simulator test)” HXPE [44M] was 4-folds higher than the ‘new’ HXPE [44M] cups. In phase-2, the controls averaged 320% higher than the new HXPE [44 mm]. The wear of control CXPE was directly proportional to debris concentration, i.e. with a 50% reduction in slurry concentration, there was a corresponding reduction wear. It was noted that HXPE wear increased only 5-fold compared to the 80-fold previously measured when using double the slurry concentration. This may represent the sensitivity of HXPE to such abrasive wear conditions under the same experimental conditions. The HXPE bearings maintained a superior finish to CXPE (controls). In phase I, the latter's average roughness decreased from 280 to 150 nm whereas ‘pre-worn’ HXPE cup decreased from 180 to 40 nm. During phase-2, the CXPE and HXPE with CoCr bearings averaged 7 and 5 times rougher than with Al 2O 3 combination. A significant correlation was found between phase-2 wear magnitude and resulting cup roughness. Stepwise multiple regression analysis revealed that cup roughness showed the strongest relationship to wear magnitude. Ball roughness was not a factor since CoCr and Al 2O 3 balls maintained a high quality finish (Sa < 12 nm). However, during Phase-2, SEM and interferometry revealed numerous PMMA particles adhering to uncleaned CoCr counterfaces. In contrast the Al 2O 3 surfaces were relatively free of PMMA contamination. Thus use of Al 2O 3 balls minimized the risk of both 2-body and 3rd-body PMMA abrasive wear. We described for the first time that the average cup roughness (Sa) were directly proportional to the wear rate. Therefore the surface finish of the bearings (CoCr, CXPE and HXPE) appears to be an important aspect of the hip wear mechanism.

Randomized Comparison Between 3 Surface Treatments of a Single Anteverted Stem Design: 84 Hips Followed for 5 Years

Eighty patients (84 hips) randomly received an anteverted cobalt-chromium stem (Lubinus SP2; Waldemar Link, Hamburg, Germany) with matte, polymethyl methacrylate–coated, or polished surface (uncollared). Radiostereometry was used to study migration and wear; and the bone mineral density was studied with dual-energy x-ray absorptiometry at 6 months, 1 year, 2 years, and 5 years. The polished stems had subsided 0.1 to 0.2 mm more at 6 months and 0.3 to 0.4 mm more at 5 years than the matte and precoated versions ( P < .0001) and subsided mainly inside the cement mantle. The rotations of the stem and the migration and wear of the cemented cup did not differ between the 3 groups. After 1 and 2 years, the polished stems had lost significantly less bone mineral in Gruen zones 1, 2, 6, and 7 ( P = .004 to .03); but this difference had disappeared after 5 years. The period of improved bone remodeling around the polished version coincided with the early period of increased subsidence, suggesting that stem motions inside the mantle resulted in a favorable loading of the proximal femur. Our observations suggest that alternative shapes could be possible on future polished femoral stem designs.

Comparison of wear of ultra-high molecular weight polyethylene acetabular cups against surface-engineered femoral heads

Alumina ceramic heads have been previously shown to reduce polyethylene wear in comparison to cobalt chrome (CoCr) heads in artificial hip joints. However, there are concerns about the brittle nature of ceramics. It is therefore of interest to investigate ceramic-like coatings on metallic heads. The aim of this study was to compare the friction and wear of ultra-high molecular weight polyethylene (UHMWPE) against alumina ceramic, CoCr, and surface-engineered ceramic-like coatings in a friction simulator and a hip joint simulator. All femoral heads tested were 28 mm diameter and included: Biolox Forte alumina, CoCr, arc evaporative physical vapour deposition (AEPVD) chromium nitride (CrN) coated CoCr, plasma-assisted chemical vapour deposition (PACVD) amorphous diamond-like carbon (aDLC) coated CoCr, sputter CrN coated CoCr, reactive gas controlled arc (RGCA) AEPVD titanium nitride (TiN) coated CoCr, and Graphit-iC coated CoCr. These were articulated against UHMWPE acetabular cups in a friction simulator and a hip joint simulator. Alumina and CoCr gave the lowest wear volumes whereas the sputter coated CrN gave the highest. Alumina also had the lowest friction factor. There was an association between surface parameters and wear. This study indicates that surface topography of surface-engineered femoral heads is more important than friction and wettability in controlling UHMWPE wear.

Tribological Considerations in Primary and Revision Metal-on-Metal Arthroplasty

Metal-on-metal hip bearings undergo biphasic wear, starting with a short period of high wear (bedding-in) and followed by low steady-state wear. Bedding-in is the process by which the cup wears locally to conform to the geometry of the head. This process reduces the maximum contact stress and allows for appropriate lubrication. A critical area of conformance and wear is required for the bearing to reach a low steady-wear state. Cups were analyzed in this study after primary and revision wear scenarios to determine this critical area for this specific bearing. Forty and 56-mm cobalt-chromium resurfacing bearings with 150 and 400-microm clearances were wear tested in a hip simulator for 5 million cycles. The cups underwent an additional 5 million cycles of testing against new heads, simulating a revision scenario. The revision heads were manufactured to cause the highest mismatch with the pre-worn cups, resulting in polar or local annular contact. Cup wear area was determined from weight-loss measurements after each phase of testing. All bearings experienced a biphasic wear performance with a short period of high wear followed by low steady-state wear. A consistent critical area of conformance was reached by all bearings after primary and revision testing conditions, regardless of bearing size, bearing clearance, or contact mode. An area of conformity (wear) reduces contact pressures, is beneficial for lubrication, and is critical to reach a low steady-state wear rate. This study shows that this critical area is consistent regardless of bearing size, clearance, or contact mode. Bearing designs that allow the proper formation of this conformance area should bed-in and reach a low steady-state wear rate.

Effect of head surface roughness and sterilization on wear of UHMWPE acetabular cups

The impact of femoral head surface roughness on wear of gamma-irradiation sterilized (3 MRad in nitrogen, crosslinked) and nonsterilized (not crosslinked) UHMWPE acetabular cups has been evaluated. Gravimetric wear testing was performed in a hip joint simulator for 2 x 10(6) cycles. CoCrMo heads were used with different surface roughness (R(a) = 15 nm and R(a) = 400 nm). The surface roughness after wear test was unchanged for the roughened heads, whereas the initially smooth heads showed a few scratches. The roughened heads increased the wear of the acetabular cups 2-fold. The gamma-irradiated cups tested against rough heads underwent the highest wear. The absorption of water was highest for the gamma-irradiated cups (0.0204% compared to 0.0031% after 85 days). Raman spectroscopy showed small but significant crystallinity changes in the wear zone, where the gamma-irradiated cups with the most extensive abrasion increased in crystallinity, whereas the nonsterilized cups underwent a crystallinity decrease.

Wear tests in a hip joint simulator of different CoCrMo counterfaces on UHMWPE

The objective in this work was to study the effect of different material counterfaces on the Ultra High Molecular Weight Polyethylene (UHMWPE) wear behavior. The materials used as counterfaces were based on CoCrMo: forged with hand polished and mass finished, CoCrMo coating applied on the forged CoCrMo alloy obtained by Physical Vapour Deposition (PVD). A hip joint simulator was designed and built for these studies. The worn surfaces were observed by optical and scanning electron microscopy. The results showed that the hand polished CoCrMo alloy caused the higher UHMWPE wear of the acetabular cups. The CoCrMo coating caused the least UHMWPE wear, while the mass finished CoCrMo alloy caused an intermediate UHMWPE wear. It is shown that the wear rates obtained in this work are closer to clinical studies than to similar hip joints simulator studies.

Optimal orientation of implanted components in total hip arthroplasty with polyethylene on metal articulation

Background In many studies related to the total hip arthroplasty, it has been found that incorrect alignment of the total hip component is one of the major factors influencing mechanical failures. Although various recommendations for cup orientation have been presented, there were few studies that seek to determine a proper orientation of the implants based on the human motion data. The objective of this study is to determine an optimum orientation of the hip implant considering various daily activities. Methods Firstly, the cup orientations free of impingement were calculated for a given set of implant geometric parameters and the required range of motion for daily activities measured in 10 subjects. Next, the optimum values for the cup orientation and stem anteversion avoiding impingement and minimizing cup wear were determined for the proposed motion criteria. Findings Different cup orientation was obtained as optimum for each combination of the neck angles (40° and 50°) and oscillation angles (120° and 135°). The corresponding optimum stem anteversion was also different when different neck angle was used. Interpretation As the margin for the impingement-free orientation of the cup was small, the optimum cup orientation and stem anteversion should be adopted specific for each combination of the neck angle and oscillation angle.

LOAD TRANSMISSION THROUGH ARTIFICIAL HIP JOINTS DUE TO STRESS WAVE LOADING

Since wear of the polyethylene (Ultra High Molecular Weight Polyethylene or UHMWPE) acetabular cup is considered to be the main cause of loosening of the artificial hip joint, the cross-linked UHMWPE with high durability to wear has been developed. This paper deals with impact load transmission through the complex of an artificial hip joint consisting of a UHMWPE acetabular cup (or liner), a metallic femoral head and stem. Impact compressive tests on the complex were performed using the split-Hopkinson pressure bar apparatus. To investigate the effects of material (conventional or cross-linked UHMWPE), size and setting angle of the liner, and test temperature on force transmission, the impact load transmission ratio (ILTR) was experimentally determined. The ILTR decreased with an increase of the setting angle independent of material and size of the liner, and test temperature. The ILTR values at 37°C were larger than those at 24 °C and 60°C. The ILTR also appeared to be affected by the type of material as well as size of the liner.

In vivo change of elastic property in polyethylene acetabular components

Polyethylene is an elastic material. It is known that oxidative degradation of polyethylene occurs after sterilization by means of gamma irradiation. However, there have been few detailed reports with regard to the effects of this degradation on the mechanical property, especially in total hip prostheses. The purpose of this study was to examine the change in mechanical property of irradiated and non-irradiated polyethylene cups after implantation. Fifty-six ultra-high molecular-weight polyethylene (UHMWPE) cups retrieved at revision surgery were evaluated. Thirty-two cups were sterilized by gamma irradiation in air and 24 by ethylene oxide gas (EtO). To evaluate the mechanical property of the cup and its regional distribution, Vickers hardness was measured at nine points at the cross-section of the cups. In the irradiated cups, the hardness increased in proportion to the time from sterilization. This phenomenon was not found in the cups sterilized by EtO. Less change of hardness was observed in the cups sterilized by EtO than in those sterilized by irradiation. The gamma-irradiation in air actually affected the elastic property of cup polyethylene in vivo, although any difference in the wear rate was not detected between two sterilization methods. In cases with accelerated wear of the acetabular cup, other factors affecting wear should also be considered.