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 behaviour in total ankle replacement: a comparison between an in vitro simulation and retrieved prostheses

BACKGROUND

To minimise wear of the meniscal component in total ankle replacement, a three-component artificial joint has recently been developed. This new prosthesis has convex spherical tibial and anticlastic talar metal components with non-anatomic but ligament-compatible shapes in the sagittal plane, and a fully conforming ultra-high-molecular-weight-polyethylene meniscal component inserted in between. The in vitro wear of meniscal components can be assessed using a four-station joint simulator. The study was aimed at comparing wear patterns obtained in vitro with those observed in implant retrievals with the same design.

METHODS

The wear tests were run in a joint wear simulator at a frequency of 1.1 Hz for two million cycles. Three bearings within corresponding metal components were subjected to flexion/extension (range 0-58 degrees), anterior-posterior translation (0-5.2 mm), internal-external rotation (-1.9 degrees to +5.7 degrees), and a maximum axial load of 2.6 KN. These conditions were taken from the most recent findings in ankle joint mechanics. Three prostheses of the same type were harvested from patients due to replacement failures not associated with the device, 24, 24 and 9 months, respectively, after implantation. The in vitro worn components and the three retrievals were analysed by using a scanning electron microscope, a Coordinate Measuring Machine, and micro-Raman spectroscopy.

FINDINGS

Visual and microscopic observations, analyses, and Raman crystallinity-based measurements showed similarity between the patterns generated experimentally in the wear simulator and those seen in retrievals with similar wear life.

INTERPRETATION

A joint wear simulator like the one used in this study, once configured properly, appears to be suitable to assess wear rates also in total ankle prostheses.

Implant fixation in knee replacement: preliminary in vitro comparison of ceramic and metal cemented femoral components

Improved wear resistance in total knee replacement (TKR) is a suitable goal. Whereas the use of metal components is well established, mechanical loosening in recently introduced ceramic components are a cause of concern. The scope of this work was to test in vitro whether ceramic TKR femoral components are more prone to mechanical loosening than metal ones. Composite femurs were implanted with commercially available TKR metal components, and with ceramic components having identical shape to the metal ones. Implanted femurs were tested on a knee simulator for up to 5 x 10(-6) cycles. Inducible micromotions and permanent migrations were recorded throughout the test. The cement layers were inspected for signs of damage or fracture. Micromotions and migrations were similar for metal and ceramic components: their magnitude and trend over time indicated that no implant was becoming loose. When there were statistically significant differences, the ceramic components were more stable than the metal ones. When the cement layers were inspected, a few short cracks were observed; most such cracks appeared during the first cycles, while no further damage occurred in the rest of the test. The type of damage found for both the metal and the ceramic components is compatible with well-fixed implants after long-term cycling. Altogether, no remarkable difference was found between the metal and ceramic components. Therefore, this study rejects the hypothesis that ceramic TKR femoral components are more prone to mechanical loosening. Although this study had a limited sample size, it provides novel pre-clinical indications about the potential of ceramic TKR femoral components.

Predictive role of the Λ ratio in the evaluation of metal-on-metal total hip replacement

The wear of metal-on-metal bearings is affected by various design parameters, such as the clearance or surface roughness. It would be very useful to have a significant indicator of wear according to these design parameters, such as the Λ ratio. Three different batches of cast high- and low-carbon cobalt—chromium hip implants (28 mm, 32 mm, and 36 mm diameters) were tested in a hip joint simulator for 2 × 106 cycles. Bovine calf serum was used as lubricant, and the samples were weighed at regular intervals during the test. The predictive role of the Λ ratio on the wear behaviour was investigated. Three different configurations were tested to explore the wear rate for a broad range of Λ ratios. The results of these studies clearly showed that the femoral heads of 36 mm diameter had the best wear behaviour with respect to the other two smaller configurations tested. From a predictive point of view, the Λ ratios associated with the configurations tested could clearly indicate that the femoral heads of 36 mm diameter worked in the mixed-lubrication regime (Λ>1); all the smallest configurations (28 mm size) had λ< 1, thus showing their aptitude to work in the boundary lubrication regime, with substantially higher volume depletion due to wear. The Λ values associated with the 32 mm size varied in a range around 1 (0.95<Λ<1.16), suggesting the possibility of operating in the mixed-lubrication regime.

Long-term implant—bone fixation of the femoral component in total knee replacement

Success of total knee replacement (TKR) depends on the prosthetic design. Aseptic loosening of the femoral component is a significant failure mode that has received little attention. Despite the clinical relevance of failures, no protocol is available to test long-term implant—bone fixation of TKR in vitro. The scope of this work was to develop and validate a protocol to assess pre-clinically the fixation of TKR femoral components. An in vitro protocol was designed to apply a simplified but relevant loading profile using a 6-degrees-of-freedom knee simulator for 1 000 000 cycles. Implant—bone inducible micromotions and permanent migrations were measured at three locations throughout the test. After test completion, fatigue damage in the cement was quantified. The developed protocol was successfully applied to a commercial TKR. Additional tests were performed to exclude artefacts due to swelling or creep of the composite femur models. The components migrated distally; they tilted towards valgus in the frontal plane and in extension in the sagittal plane. The migration patterns were consistent with clinical roentgen-stereophotogrammetric recordings with TKR. Additional indicators were proposed that could quantify the tendency to loosen/stabilize. The type and amount of damage found in the cement, as well as the migration patterns, were consistent with clinical experience with the specific TKR investigated. The proposed pre-clinical test yielded repeatable results, which were consistent with the clinical literature. Therefore, its relevance and reliability was proved.

Investigation on wear of knee prostheses under fixed kinematic conditions

Although polyethylene components have been used in the human joint for over 30 years, wear simulation studies are fundamental to assess wear resistance of total joint replacements. This assessment will help to obtain quality control and acquire further knowledge of the tribological processes that involve joint prostheses. As a result, the risk of implant failure of innovative prostheses will be reduced. The aim of the present study was to investigate the influence on wear of the polyethylene tibial inserts against metallic femoral components using a knee simulator following a standardized kinematics protocol (ISO/CD 14243-3.2). Four intact "small" size specimens of the tibial MP 913 were tested in a four-station knee wear simulator for two million cycles. The volumetric weight loss for the ultra-high-molecular-weight-polyethylene (UHMWPE) tibial inserts was 44 +/- 2, 45 +/- 3, and 47 +/- 3 mg for the specimens #1-3, respectively. This study demonstrated a good repeatability among the stations of the knee simulator obtaining weight loss values congruent with those found by other authors using similar test conditions.

Advanced nanocomposite materials for orthopaedic applications. I. A long-term in vitro wear study of zirconia-toughened alumina

The use of ceramic-on-ceramic (alumina- and zirconia-based) couplings in hip joint prostheses has been reported to produce lower wear rates than other combinations (i.e., metal-on-polyethylene and ceramic-on-polyethylene). The addition of zirconia into an alumina matrix (zirconia-toughened alumina, ZTA) has been reported to result in an enhancement of flexural strength, fracture toughness, and fatigue resistance. The development of new processing routes in nonaqueous media has allowed to obtain high-density ZTA nanocomposites with a very homogeneous microstructure and a significantly smaller and narrower particle-size distribution of zirconia than conventional powder mixing methods. The aim of the present study was to set up and validate a new ZTA nanocomposite by testing its biocompatibility and wear behavior in a hip-joint simulator in comparison with commercial alumina and experimental alumina specimens. The primary osteoblast proliferation onto ZTA nanocomposite samples was found to be not significantly different from that onto commercial alumina samples. After 7 million cycles, no significant differences were observed between the wear behaviors of the three sets of cups. In this light, it can be affirmed that ZTA nanocomposite materials can offer the option of improving the lifetime and reliability of ceramic joint prostheses.

Thermomechanical analysis of ultra-high molecular weight polyethylene-metal hip prostheses

In order to predict the frictional heating and the contact stresses between the polyethylene cup and the metallic ball-head forming the articulation of a hip prosthesis a three-dimensional finite element model was developed and calculated. The non-linear model includes a fully coupled thermomechanical formulation of the mechanical properties of the ultra-high-molecular-weight polyethylene, and a large-sliding Coulomb frictional contact between the two components. The model predicts the temperature of the polyethylene with an accuracy that was tested by comparing the model predictions with the temperature measurements. The temperature measurements were taken by thermocouples placed on the cup surface, the head surface and the inside of the thermostatic bath, during a complete test within a hip joint wear simulator. The model was found to be very accurate, predicting the measured temperatures with an accuracy better than 2 per cent. The temperature peak (51 degrees C) was predicted at the contact surface. The model results indicate that frictional heat is mostly dissipated through the metallic ball-head. The full coupling between the thermal and the mechanical conditions used in this study appears to be necessary if accurate predictions of the polyethylene deformation are required.

Larger diameter bearings reduce wear in metal-on-metal hip implants

Metal-on-metal hip arthroplasty has the longest clinical history of all total arthroplasties. We asked whether large diameter femoral heads would result in less wear than those with small diameters. We also asked if there is a threshold diameter that ensures good wear behavior. We tested three batches of cast high-carbon cobalt-chromium-molybdenum hip implants (28 mm, 36 mm, and 54 mm diameters) in a hip simulator for 5 million cycles. We used bovine serum as lubricant and weighed the samples at regular intervals during testing. The 28-mm configuration had almost twice the wear of the 54-mm configuration, but we observed no difference between the 36-mm and the 54-mm configurations. The similarity in the wear performances of the larger configurations supports the presence of a threshold diameter that ensures good wear behavior.

Oxidation in ultrahigh molecular weight polyethylene and cross-linked polyethylene acetabular cups tested against roughened femoral heads in a hip joint simulator

This study was aimed at comparing the oxidative degradation of commercial acetabular cups made of cross-linked polyethylene (XLPE) and conventional ultrahigh molecular weight polyethylene (UHMWPE). After testing against deliberately scratched CoCrMo femoral heads in a hip joint simulator, the cups, microtomed parallel to the articulating surface, were analyzed by IR spectroscopy. Due to the potential for artifacts caused by absorbed contaminants, the IR spectra were compared only after hexane extraction; actually, XLPE was found to absorb more serum than UHMWPE. The two sets of unworn acetabular cups showed different oxidation patterns with consequently different distributions of carbonyl species; unworn XLPE was characterized by lower contents of carbonyl species and hydrogen-bonded alcohols and higher contents of trans-vinylene species than unworn UHMWPE. Upon simulator testing, UHMWPE showed more significant changes in oxidation indexes and distribution of carbonyl compounds than XLPE, confirming a better wear behavior for XLPE under the adopted testing conditions.

Meniscal wear at a three-component total ankle prosthesis by a knee joint simulator

Despite the fundamental value of wear simulation studies to assess wear resistance of total joint replacements, neither specialised simulators nor established external conditions are available for the human ankle joint. The aim of the present study was to verify the suitability of a knee wear simulator to assess wear rates in ankle prostheses, and to report preliminary this rate for a novel three-component total ankle replacement design. Four intact 'small' size specimens of the Box ankle were analysed in a four-station knee wear simulator. Special component-to-actuator holders were manufactured and starting spatial alignment of the three-components was sought. Consistent load and motion cycles representing conditions at the ankle joint replaced exactly with the prosthesis design under analysis were taken from a corresponding mechanical model of the stance phase of walking. The weight loss for the three specimens, after two million cycles, was 32.68, 14.78, and 62.28mg which correspond to a linear penetration of 0.018, 0.008, and 0.034mm per million-cycle, respectively for the specimens #1, #2, and #3. The knee wear simulator was able to reproduce load-motion patterns typical of a replaced ankle. Motion of the meniscal bearing in between the tibial and talar components was smooth, this component remaining in place and in complete congruence with the metal components throughout the test.

The predictive Power of Surface Profile Parameters on the Amount of Wear Measured In Vitro on Metal-On-Polyethylene Artificial Hip Joints

Various studies report a weak correlation between the average surface roughness Ra of metallic ballheads and the observed wear rate of the polyethylene cups coupled to them. The aim of this study is to verify, with controlled in vitro experiments, whether other parameters such as the total roughness Rt, and the skewness Rsk are better predictors of the polyethylene wear when the metallic heads present a surface conditioning that varies substantially from specimen to specimen, as is usually the case with retrieved specimens. Twelve CoCrMo (cobalt-chromium-molybdenum) ballheads were intentionally damaged in order to reproduce a wide spectrum of surface conditioning and were then subjected to the standard wear test against polyethylene cups, using a hip joint wear simulator. After 2 × 106 cycles the weight lost by the cups was assessed with a gravimetric procedure, and the surface roughness of the metallic ballheads was qualified in terms of Ra, Rt, and Rsk. The various parameters were correlated to the weight loss using a linear regression analysis. The skewness Rsk showed a coefficient linear regression R2 = 0.80, while the average roughness Ra, used in most previous studies, presented an R2 = 0.56. It was concluded that when specimens with substantially different surface conditioning are compared, as in retrievals analysis, it is also important to report the skewness Rsk so that qualify the surface roughness of the specimens can be qualified.

Wear behaviour of cross-linked polyethylene assessed in vitro under severe conditions

The polyethylene (PE) for hip implants presents serious clinical problems; the production of debris may induce adverse tissue reactions that may lead to extensive bone loss around the implant and consequently osteolysis and implant loosening. Several attempts have been made to improve the wear properties of ultra-high molecular weight polyethylene (UHMWPE). More recently the attention of various researchers has been focused on cross-linked polyethylene (XLPE), due to its improved wear resistance with respect to conventional UHMWPE. This study was aimed at comparing the wear performances of clinically available acetabular liners (Zimmer Inc.) made of electron beam XLPE and conventional UHMWPE. To evaluate the influence of the material properties on wear, conventional UHMWPE and XLPE acetabular cups were tested against deliberately scratched CoCrMo femoral heads (Ra = 0.12-0.14 microm) in a hip joint wear simulator run for 3 million cycles with bovine calf serum as lubricant. Gravimetric measurements revealed significant differences between the wear behaviours of the two sets of acetabular cups: XLPE exhibited a wear rate about 40 times lower than conventional UHMWPE. Raman spectroscopy coupled to partial least-squares analysis was used to evaluate the possible crystallinity changes induced by mechanical stress (and thus the material wear resistance): only the UHMWPE cup which showed the highest weight loss displayed significant crystallinity changes. These results were correlated to the thickness of the plasticity-induced damage layer. The wear debris produced during the tests were isolated according to a validated protocol and imaged by scanning electron microscopy . The wear particles produced by XLPE were smaller than those produced by UHMWPE; the latter were observed as fibrillar and agglomerated particles. The mean equivalent circle diameter was 0.71 and 0.26 microm for UHMWPE and XLPE, respectively.

An easy technique to digest and isolate UHMWPE wear particles from a hip joint simulator

The size and morphology of polyethylene wear particles isolated from in vitro tests were analysed in this study. There are some major controversies emerging in the literature that will only be resolved by careful particles analysis. Since it is difficult to identify the factors that affect size and morphology of the wear particles, the goal of this study was to compare four different polymer wear debris isolation techniques (base method and acid treatment) under identical conditions in a hip joint simulator to obtain polyethylene wear particles. We investigated achieving particulate isolation by using a different approach to the one reported in the literature that involved an easier and streamline method of particle debris isolation. The new method, compared to the previous one, used a strong base, normal centrifugation, and filtration to digest the serum constituents thus isolating the polyethylene particle debris from lubricant. In all four methods the isolation involved some or all of the following steps: density gradient separation, centrifugation or ultracentrifugation, and washing. However, a requirement for all these techniques was that the lubricant had to be devoid of organic compounds in order to effectively image and count the particle debris. The results from these studies clearly show that this new method of particle isolation is easier and more streamlined than the three methods analysed and reported in literature because it did not involve ultracentrifugation and is quicker.

Sodium-azide versus ProClin 300: influence on the morphology of UHMWPE particles generated in laboratory tests

Ultra-high molecular-weight polyethylene (UHMWPE) has been used in total joint replacement for the last three decades and is currently the best polymer available for this use. Nevertheless, the wear of UHMWPE remains a serious clinical problem. Polyethylene wear debris has been identified as a cause of osteolysis and a major factor reducing the life of the total hip arthroplasty. Debris generated at the articular surfaces enters the periprostethic tissue where it is phagocyted by macrophages. Studies have shown that particles in the 0.1-10microm size range are particularly important in causing adverse cellular reactions resulting in osteolysis. The morphology, size, mass, and number of wear particles produced in a hip joint simulator are influenced by the tribological conditions used during the experiment. This paper shows that the morphology of the UHMWPE particles generated in vitro is influenced by the type of lubricant used. This study compared, quantitatively and qualitatively, particles generated in vitro using bovine calf serum as lubricant with two different preservatives: sodium azide and ProClin 300. No significant difference was observed with regards to wear between the two types of lubricant used. Quantitative analysis of the wear particles showed that particles generated in serum with sodium azide were not [corrected] morphologically different from those produced in serum with ProClin 300 [corrected]

The performance of gamma- and EtO-sterilised UHMWPE acetabular cups tested under severe simulator conditions. Part 2: wear particle characteristics with isolation protocols

Ultra-high molecular-weight polyethylene (UHMWPE) has been used in total joint replacement for the last three decades. Despite the recent advancements in prosthesis design, the wear of UHMWPE remains a serious clinical problem; the release of wear debris may induce osteolysis and implant loosening. Controlling the quality of the polyethylene is essential to improve its wear resistance and any potential adverse effect caused by processing, manufacturing or sterilisation should be avoided. To evaluate the influence of the sterilisation method (gamma-irradiation and ethylene oxide (EtO)-treatment) and third-body particles, gamma- and EtO-sterilised UHMWPE acetabular cups were tested against CoCrMo femoral heads in a hip joint simulator run for 2.5million cycles in bovine calf serum in the presence of third-body polymethylmethacrylate (PMMA) particles. A method not requiring ultra-centrifugation has been proposed for the isolation of polyethylene wear debris from the serum lubricant. SEM analysis allowed debris shape and morphology to be determined, and the wear mechanism operating in this study to be hypothesised. The morphological features of the wear debris were in agreement with clinical findings, enabling the hip simulator function to be validated. Micro-Raman spectroscopy coupled to PLS analysis showed that the mechanical friction during in vitro tests induced significant crystallinity changes in all the cups. The most significant changes were observed for the EtO-sterilised cups, which showed the highest weight loss.

Investigations on the wear behaviour of the temporary PMMA-based hip Spacer-G

Total hip replacement has become one of the most successful orthopaedic procedures. However, complications due to infections may give serious problems and have devastating consequences for the hip implant. The use of a temporary three-dimensional polymethylmethacrylate (PMMA) cement spacer may be an alternative to solve infections in hip implants, improving the lives of patients awaiting reimplantation. In order to evaluate their wear behaviour, five PMMA Spacer-G femoral heads were tested against five post-mortem pelves in a hip joint simulator with bovine calf serum as lubricant. The surface of the worn spacers was characterized by scanning electron microscopy (SEM) analysis; all the samples revealed a similar morphology, showing areas characterized by different degrees of wear. Particle debris was isolated from the lubricant and PMMA particles and bone fractions were quantified. The amount of debris was found to be higher than where no-temporary prostheses were used. However, this result is acceptable since wear debris is removed by lavage irrigation when the Spacer-G is explanted. On the basis of these data, it is considered that the use of the cement Spacer-G could be a promising approach to the treatment of complicated infections of the hip joint. Therefore, Spacer-G is worthy of further research.

Surface characterization and debris analysis of ceramic pairings after ten million cycles on a hip joint simulator

This study was aimed at characterizing the ceramic wear particles produced during tests on a hip joint wear simulator of up to 10 million cycles. Alumina and alumina-zirconia composites were studied as commercial or potential hip joint products respectively. No ceramic particles could be observed, even after a careful isolation procedure. This confirms the low wear rate found for these materials in previous works (of the order of tenths of milligrams per million cycles). Surface characterization was conducted by means of scanning electron microscopy. It confirms the low wear regime of ceramic pairings and allows ceramic wear debris morphology to be defined. The effect of microstructure on surface wear is discussed.

The performance of gamma- and EtO-sterilised UHWMPE acetabular cups tested under severe simulator conditions. Part 1: role of the third-body wear process

Due to its excellent combination of properties, ultra-high-molecular-weight-polyethylene has been used for the last 30 years in the replacement of damaged articulating cartilage for total joint replacement surgery. However, in some cases, wear, failure and delamination have been observed. Polyethylene performance may be affected by oxidation during consolidation of the resin, sterilisation of the finished specimens and post-irradiation storage. In order to evaluate the influence of the sterilisation method (gamma-irradiation and ethylene oxide(EtO)-treatment) and third-body particles on the ultra-high-molecular-weight-polyethylene wear behaviour, gamma- and EtO-sterilised ultra-high-molecular-weight-polyethylene acetabular cups were tested against CoCrMo femoral heads in a hip joint simulator run for 2.5 million cycles in bovine calf serum in the presence of third-body PMMA particles. Weight loss measurements revealed that the gamma-sterilised acetabular cups exhibited a significantly lower wear rate than those EtO-sterilised. Moreover, significant differences were found for each type of sterilisation between the gravimetric wear trends obtained until 2.5 million cycles in the presence and in the absence of PMMA particles.

Effects of the sterilisation method on the wear of UHMWPE acetabular cups tested in a hip joint simulator

Ultra-high molecular-weight-polyethylene is the most commonly used bearing material in total joint replacement. Wear of polyethylene is a Serious Clinical problem that limits the longevity of orthopaedic implants. Information on degradative changes in the material properties and on the methods used for the sterilisation of polyethylene may help in the selection process of orthopaedic implants with the best wear resistance. This study was performed to investigate the effects of the sterilisation method (gamma irradiation and ethylene oxide treatment) on the wear and on the changes in physical properties of polyethylene acetabular cups. At this purpose, gamma-sterilised and ethylene oxide (EtO)-sterilised acetabular cups were tested against CoCr femoral heads in a hip joint simulator run for 5 million cycles in bovine calf serum. The crystallinity of the cups was evaluated by micro-Raman spectroscopy as a function of the inner surface position. The partial least square calibration was used to correlate the Raman spectra with the crystallinity of the polymer measured by differential scanning calorimetry. The analysis performed on soak control acetabular cups demonstrated that the gamma-sterilised cups are significantly more crystalline than the EtO-sterilised ones. The mean crystallinity values obtained for the gamma-sterilised and EtO-sterilised soak control cups were 65.0% and 63.4%, respectively. Weight loss measurements revealed that the gamma-sterilised acetabular cups exhibited a lower wear rate than that by EtO-sterilised. Thc Raman results obtained on gamma-sterilised and EtO-sterilised acetabular cups showed that the changes in surface crystallinity were mainly caused by irradiation rather than by the mechanical friction during the in vitro tests.

Isolation and morphological characterisation of UHMWPE wear debris generated in vitro

Wear tests are generally carried out on materials used in prosthetic hip implants, in order to obtain a better understanding of the tribological processes involved and improve the quality control of joint prostheses, directed towards reducing the risk of implant failure of innovative prostheses. Ceramic femoral heads of mixed alumina-zirconia oxides as well as zirconia and alumina single oxide heads were tested against UHMWPE acetabular cups in a hip joint simulator. Polyethylene cups and ceramic femoral heads were mounted in a simulator apparatus moving according to a sinusoidal function, under load and in the presence of bovine calf serum as lubricant. Wear particles were isolated from the bovine calf serum collected during the wear tests. An easy to follow method was used to separate the wear particles from the lubricant. Chemical digestive methods were used to separate the wear particles from the lubricant and the isolated particles were studied using scanning electron microscopy. The morphologies of the polyethylene debris showed considerable differences, both in size and shape of the particles, as a function of the coupled head material.

Severe wear from retrieved alumina-on-alumina coupled implant: a case report

This study details the in vivo wear behavior of an alumina acetabular cup and a femoral head on a retrieved non-cemented hip prosthesis. A commercial alumina ceramic-on-ceramic prosthesis was retrieved from a patient previously treated for bilateral hip arthrosis in "coxa profunda". Massive wear was found on the retrieved alumina ceramic head and acetabular cup. The total measured penetration depth was 1.9 mm while the total calculated weight loss for the acetabular cup was 6.06 g. The study underlines the head-cup instability caused by cup loosening as major cause of severe ceramic wear.

Fluid absorption study in ultra-high molecular weight polyethylene (UHMWPE) sterilized and unsterilized acetabular cups

The weight gain due to fluid absorption was measured in gamma-sterilized, ethylene oxide (ETO) gas-sterilized, and unsterilized ultra-high molecular weight (UHMW) polyethylene acetabular cups. After about 2 months the total average fluid absorption gain of the polyethylene cups was: 1.76 +/- 0.45 mg (average +/- standard deviation) for the unsterilized polyethylene cups, 2.81 +/- 0.95 mg for the gamma-sterilized polyethylene cups and 1.51 +/- 0.34 for the ETO gas-sterilized polyethylene cups. There was little difference in fluid absorption between the sterilized and unsterilized specimens. In particular, the gamma-sterilized acetabular cups absorbed more than the ETO gas-sterilized and the unsterilized cups. However, the weight gain was higher in serum compared with that in water.

Mixed oxides prosthetic ceramic ball heads. Part 3: effect of the ZrO2 fraction on the wear of ceramic on ceramic hip joint prostheses. A long-term in vitro wear study

Using ceramic materials it is possible to obtain a number of beneficial mechanical properties such as considerable hardness, good chemical resistance, high tensile strength, and a good fracture toughness. The use of ceramic-on-ceramic as bearing surfaces for hip joint prostheses has been reported to produce a lower wear rate than other combinations (i.e. metal-on-polyethylene and ceramic-on-polyethylene) in total hip artroplasty. These advantages may increase the life expectancy of hip implants and improve the life of patients. Two new types of mixed-oxide ceramics (alumina and yttria-stabilised zirconia) femoral heads and acetabular cups containing different ratios of alumina and zirconia were compared with pure commercial alumina in terms of wear behaviour in a hip joint simulator. Hip joint wear simulator studies were carried out with a full-peak load of 2030 N and a frequency of 1 Hz in bovine calf serum. After 10 million cycles the measured weight loss of all specimens was very low. However, the experimental results did not show any significant difference between the new experimental mixed-oxide ceramics prototype and the commercial ceramic material couplings.

An in vitro investigation of diamond-like carbon as a femoral head coating

Wear of polyethylene acetabular components of hip implants is a significant clinical problem. In prosthetic hip surgery, polyethylene wear is identified as a factor that limits the life of the implant; it is known that the production of debris can cause adverse tissue reactions that may lead to extensive bone loss around the implant, and consequently loosening of the fixation. A new class of so-called Diamond-Like Carbon coatings, applied to titanium femoral heads was compared to ceramic and metallic heads in terms of wear behavior against UHMWPE using a hip joint simulator with a bovine calf serum lubricant. A thin film of Diamond-Like Carbon was deposited directly onto titanium (Ti6Al4V) head using chemical vapor deposition. The wear of polyethylene coupled with Diamond-Like Carbon coated femoral heads was comparable to that obtained with the polyethylene coupled with commercial alumina femoral heads.

Leg length measurement: a new method to assure the correct leg length in total hip arthroplasty

The precise measurement of leg length plays an important role in total hip arthroplasty. Leg length inequality occurs frequently after total hip arthroplasty and may cause patient discomfort. Current clinical methods used for measuring leg length are not accurate enough to meet the demands of precision required for hip replacement. The aim of this study was to examine the validity of determining leg length differences using an ultrasound system. The proposed system measures the distance between three points, in millimetres, so that the difference between preoperative and postoperative measurements gives an indication about the leg length. The mean ultrasound variation observed in in vitro measurements showed a relative error of 1.7% (range: 52-133 mm) that means a leg length inequality of about 0.4 mm (range: 52-133 mm). The method is non-invasive (ultrasound is not limited by radiation hazards), easy, quick to use, and can be used for standard clinical screening.

Alumina femoral head fracture: an in vitro study

A fracture of a ceramic femoral head is reported in this study. Fractures of ceramic femoral heads are uncommon and reports on this complication are rare. After 3 million cycles, on a twelve station hip simulator that tested alumina femoral head against polyethylene acetabular cup, fracture of the ceramic ball was observed. The retrieved specimen consisted of three large ceramic fragments from the same ceramic femoral head, a polyethylene acetabular cup and a stainless steel jig. Careful and detailed examination of the removed components was made. The fracture of the ceramic ball resulted in damage to the metal taper of the jig component which was fixed into the simulator.

Mixed-oxides prosthetic ceramic ball heads. Part 2: effect of the ZrO2 fraction on the wear of ceramic on ceramic joints

Three different types of mixed-oxides ceramic ball heads have been investigated for their wear behaviour against acetabular cups of the same materials in a hip joint simulator. Mixed-oxides ceramics have been indicated in literature as a promising compromise between strength and wear but no reports are available on the influence of a percentage of zirconia in a ceramic femoral head when sliding against itself. Mixed-oxides ceramic acetabular cups and femoral heads were tested on a simulator apparatus with a sinusoidal load in presence of bovine calf serum. The experimental results did not show any significant difference between the experimental and commercial ceramic material couplings. These results were found to be in accord with those developed in Part 1.

Hip prothesis: an in vitro wear protocol based on a comparison between gravimetric and profilometric analysis

Ultra high molecular weight polyethylene acetabular cups were analysed by means of a shadowgraph method (using a profile projector) to measure linear wear. The results were compared with those of previous wear tests performed on a hip joint simulator. Twelve polyethylene acetabular cups were analysed. The specimens were evaluated visually for evidence of polyethylene wear. Examination of the polyethylene inner surface did not reveal evidence of surface failure such as delamination, fatigue cracks or scratches. Volumetric wear was calculated using a formula based on dimensional change due to the penetration of the femoral head in the acetabular cup. It was found to be of the same order of magnitude as the wear obtained in in vitro experimental tests.

Mixed oxides prosthetic ceramic ball heads. Part 1: effect of the ZrO2 fraction on the wear of ceramic on polythylene joints

Although mixed oxides ceramics have been indicated in the literature as a promising compromise between strength and wear, to the authors' knowledge no reports are available on the influence of the percentage of zirconia in ceramic femoral heads when sliding against polyethylene cups. Two types of mixed oxides ceramic ball heads (alumina plus, respectively, 60 and 80% of zirconia) were compared to pure zirconia and pure alumina heads in terms of wear behaviour against UHMWPE in a hip joint simulator. Polyethylene cups and ceramic femoral heads were fixed on a simulator apparatus with a sinusoidal movement and load in presence of bovine calf serum. The experimental results did not show significant difference between the two experimental ceramic materials or in comparison with pure materials. Considering that all specimens, regardless of the material, had the same level of surface roughness, this roughness factor seems to have a more relevant role than the mix of oxides used to manufacture the ceramic head. Wear tests are conducted on materials used in prosthetic hip implants in order to obtain quality control and to acquire further knowledge of the tribological processes that involve joint prostheses, therefore reducing the risk of implant failure of innovative prostheses.

Metal-UHWMPE (Ultra High Molecular Weight Polyethylene) wear: experimental testing

Testing conducted on a hip joint simulator verified the Charnley hypothesis that the diameter of the prosthetic head is proportional to wear. The study was conducted by pairing heads in Ortinox TM with acetabular cups in polyethylene at high molecular weight (UHMWPE). A comparison between the different sizes of the heads (26 and 30 mm in diameter) showed a different trend in the wear rate; in particular, the head with a smaller diameter behaves better tribologically than the head with a larger diameter.

Wear of the epiphysis-acetabulum in total hip arthroplasty: a review of the literature

Problems related to the use of total hip arthroplasty and the properties of the materials used to make prostheses have encouraged wear phenomena of the epiphysis-acetabulum combination of international importance and acknowledgement. Based on a review of the literature, the authors analyze the most recent years of study of wear phenomena and discuss the choice of materials to be used in total hip arthroplasty implant, with particular interest in any potential defects.

A computerized system for radiographical evaluation in total hip arthroplasty

In the field of orthopaedic surgery radiographical image evaluation is frequently used to determine possible pathological processes. For total hip arthroplasty in particular, it is important to evaluate both preoperative femoral morphology and post-operative prosthesis performance in terms of bone adhesion to the implant and, consequently, loosening or stability of prosthesis components. References to a method called Roentgen stereophotogrammetric analysis (RSA) exist in the literature; however, the authors note that this method is expensive and requires the insertion of markers during the operation. Here we present an alternative computerized method for radiographical evaluation which we call radiographical evaluation system in total hip arthroplasty (RESTHA). The accuracy and the repeatability of the method have been evaluated using a medium-sized pelvis-femur system made of composite material. A mean error of +/- 2 mm has been associated to each experimental point. A post-operative inquiry case study is presented to indicate the applicability of the method.

A computerized morphometric evaluation of x-ray films for preoperative planning of hip arthroplasty

Methods for measuring metric size on radiograms constitute an instrument of proven utility. For example, when preoperatively evaluating hip arthroplasty the diameter of the medullary canal and the cervicodiaphyseal angle, must be measured in order to determine the center of rotation of the femoral head, and to establish the flare index of the diaphyseal canal. These results may be obtained by using a computer-controlled graphic table to place the coordinates for the areas of greater anatomical and physiological importance on the radiologic image. Thus, a calculation of distances, anatomical axes and angles is obtained immediately, accurately defining the morphometry of the joint. In this study, the anteroposterior preoperative radiographic views of 87 femurs in 84 patients were evaluated by this method. The values provided by the morphometric analysis were then related to sex, age and weight. The diaphyseal canal was classified by typology for the preoperative planning of hip arthroplasty.