Potential thermal artifacts in hip joint wear simulators

Surgical cup placement affects the heating up of total joint hip replacements

The long-term success of highly effective total hip arthroplasty (THA) is mainly restricted by aseptic loosening, which is widely associated with friction between the head and cup liner. However, knowledge of the in vivo joint friction and resulting temperature increase is limited. Employing a novel combination of in vivo and in silico technologies, we analyzed the hypothesis that the intraoperatively defined implant orientation defines the individual joint roofing, friction and its associated temperature increase. A total of 38,000 in vivo activity trials from a special group of 10 subjects with instrumented THA implants with an identical material combination were analyzed and showed a significant link between implant orientation, joint kinematics, joint roofing and friction-induced temperature increase but surprisingly not with acting joint contact force magnitude. This combined in vivo and in silico analysis revealed that cup placement in relation to the stem is key to the in vivo joint friction and heating-up of THA. Thus, intraoperative placement, and not only articulating materials, should be the focus of further improvements, especially for young and more active patients.

Wear Simulation of Ceramic-on-Crosslinked Polyethylene Hip Prostheses: A New Non-Oxide Silicon Nitride versus the Gold Standard Composite Oxide Ceramic Femoral Heads

The purpose of the present study was to compare the wear behavior of ceramic-on-vitamin-E-diffused crosslinked polyethylene (Vit-E XLPE) hip bearings employing the gold standard oxide ceramic, zirconia (ZrO₂)-toughened alumina (Al₂O₃) (ZTA, BIOLOX^®delta) and a new non-oxide ceramic, silicon nitride (Si₃N₄, MC^2®). In vitro wear test was performed using a 12-station hip joint simulator. The test was carried out by applying the kinematic inputs and outputs as recommended by ISO 14242-1:2012. Vitamin-E-diffused crosslinked polyethylene (Vit-E XLPE) acetabular liners (E1^®) were coupled with Ø28-mm ZTA and Si₃N₄ femoral heads. XLPE liner weight loss over 5 million cycles (Mc) of testing was compared between the two different bearing couples. Surface topography, phase contents, and residual stresses were analyzed by contact profilometer and Raman microspectroscopy. Vit-E XLPE liners coupled with Si₃N₄ heads produced slightly lower wear rates than identical liners with ZTA heads. The mean wear rates (corrected for fluid absorption) of liners coupled with ZTA and Si₃N₄ heads were 0.53 ± 0.24 and 0.49 ± 0.23 mg/Mc after 5 Mc of simulated gait, respectively. However, after wear testing, the ZTA heads retained a smoother topography and showed fewer surface stresses than the Si₃N₄ ones. Note that no statistically significant differences were found in the above comparisons. This study suggests that the tribochemically formed soft silica layer on the Si₃N₄ heads may have reduced friction and slightly lowered the wear of the Vit-E XLPE liners. Considering also that the toughness of Si₃N₄ is superior to ZTA, the present wear data represent positive news in the future development of long-lasting hip components.

Effect of random variation of input and various daily activities on wear in a hip joint simulator

The ISO 14242-1 standard specifies fixed, simplified, sinusoidal motion and double-peak load cycles for wear testing of total hip prostheses. In order to make the wear simulation more realistic, random variation was added for the first time to the motion and load control signals of a hip joint simulator. For this purpose and for the simulation of various daily activities, computer-controlled, servo-electric drives were mounted on a biaxial hip simulator frame and successfully introduced. Random variation did not result in a statistically significant difference in the wear factor of large diameter VEXLPE liners compared with fixed sinusoidal waveforms. However, level walking according to biomechanical literature surprisingly resulted in a 134 per cent higher, and jogging in a 57 per cent lower wear factor compared with the fixed sinusoidal waveforms. These wear phenomena were likely to be caused by a variation in the lubrication conditions and frictional heating. Simplified motion waveforms may result in an underestimation of wear in walking.

Surface and form alterations in retrieved shoulder hemiarthroplasty

BACKGROUND

Shoulder hemiarthroplasty is a viable treatment for shoulder arthritis, particularly if the natural glenoid is still intact. One of the most common reasons for revision surgery is glenoid erosion, where the cartilage or bone is worn. The aim of the current study was to analyze whether the metallic articular surface of retrieved shoulder hemiarthroplasty is different from new implants. We hypothesized that the surface roughness would increase as a result of the articulation and that metallic wear would be detectable on the implants.

METHODS

Included were 13 retrieved and 3 new shoulder hemiarthroplasties. The surface roughness was measured on different sites of the surface (dome of the head and the rim). The implants were further measured using a coordinate measuring machine to gain information on volumetric wear and geometric alterations. The surface was analyzed with a scanning electron microscope to see marks of wear.

RESULTS

Compared with new implants, the surface roughness on the retrievals was significantly increased. The roughness parameters within the retrieval group were generally higher at the dome of the head than at the rim; however, this difference was not significant. Apart from shape deviations caused by deformation of the retrievals, no volumetric wear was detectable on the heads.

CONCLUSIONS

The current results indicate that the roughness of the surface of the implant and the shape changes occur in vivo and that the material incurs damaged caused by the articulation against the softer cartilage or bone. Increasing roughness of the humeral component may be one cause for glenoid erosion over time.

Release of metal ions from nano CoCrMo wear debris generated from tribo-corrosion processes in artificial hip implants

CoCrMo alloys have been widely used in metal-on-metal (MoM) hip replacements due to their superior wear and corrosion resistance properties. However, metal ions like Co²⁺ and Cr³⁺, or even Cr⁶⁺ released from CoCrMo hip prostheses can induce macrophage apoptotic vs. necrotic mortality and damage the surrounding tissues. Simultaneously, osteolysis induced by the wear debris can be a cause of failure. Nano wear debris is more active than the bulk material, due to its small size. In this study, to accurately analyse the fresh wear debris retrieved from the hip simulator and the interaction between the particles and tribocorrosion of CoCrMo, wear debris was observed without protein digest, using a combined experimental approach involving the employment of TEM and ICP-MS. The results suggest that nanoscale wear debris generated from a hip simulator in bovine serum albumin (BSA) lubrication was Cr-rich, containing crystalline and amorphous structures; meanwhile, without any proteins, the wear particles mostly had an hcp-Co crystalline structure.

PEEK-OPTIMA™ as an alternative to cobalt chrome in the femoral component of total knee replacement: A preliminary study

PEEK-OPTIMA^™ (Invibio Ltd, UK) has been considered as an alternative joint arthroplasty bearing material due to its favourable mechanical properties and the biocompatibility of its wear debris. In this study, the potential to use injection moulded PEEK-OPTIMA^™ as an alternative to cobalt chrome in the femoral component of a total knee replacement was investigated in terms of its wear performance. Experimental wear simulation of three cobalt chrome and three PEEK-OPTIMA^™ femoral components articulating against all-polyethylene tibial components was carried out under two kinematic conditions: 3 million cycles under intermediate kinematics (maximum anterior-posterior displacement of 5 mm) followed by 3 million cycles under high kinematic conditions (anterior-posterior displacement 10 mm). The wear of the GUR1020 ultra-high-molecular-weight polyethylene tibial components was assessed by gravimetric analysis; for both material combinations under each kinematic condition, the mean wear rates were low, that is, below 5 mm³/million cycles. Specifically, under intermediate kinematic conditions, the wear rate of the ultra-high-molecular-weight polyethylene tibial components was 0.96 ± 2.26 mm³/million cycles and 2.44 ± 0.78 mm³/million cycle against cobalt chrome and PEEK-OPTIMA^™ implants, respectively (p = 0.06); under high kinematic conditions, the wear rates were 2.23 ± 1.85 mm³/million cycles and 4.44 ± 2.35 mm³/million cycles, respectively (p = 0.03). Following wear simulation, scratches were apparent on the surface of the PEEK-OPTIMA^™ femoral components. The surface topography of the femoral components was assessed using contacting profilometry and showed a statistically significant increase in measured surface roughness of the PEEK-OPTIMA^™ femoral components compared to the cobalt chrome implants. However, this did not appear to influence the wear rate, which remained linear over the duration of the study. These preliminary findings showed that PEEK-OPTIMA^™ gives promise as an alternative bearing material to cobalt chrome alloy in the femoral component of a total knee replacement with respect to wear performance.

Synovial fluid replication in knee wear testing: an investigation of the fluid volume

Wear testing cannot replicate the variations in wear rates and wear mechanisms seen in vivo, which may be related to differences between in vivo and in vitro conditions. A considerable difference exists between the in vivo synovial fluid volume (few milliliter) and the in vitro substituted bovine serum volume (several hundred milliliter). The aim of this study was to analyze the effects of a reduced fluid volume on the wear behavior in a knee wear simulator study. Four wear tests with decreasing fluid volumes (250, 150, 75, and 45 ml) were carried out. Using a large fluid volume of 250 ml for wear testing resulted in a wear rate of 9.7±1.2 mm3/10(6)  cycles. Decreasing the fluid volume consecutively reduced the wear rate to down to 8.8±1.4 mm3/10(6) for 150 ml (p=1.00), 5.6±1.2 mm3/10(6) for 75 ml (p=0.01), and 1.0±0.2 mm3/10(6) cycles for 45 ml fluid volume (p≤0.01). Additionally, higher serum degradation and larger wear particles were observed with smaller fluid volumes used for testing. This study demonstrates the high relevance of the protein-based lubricant on the wear behavior and the technical limitation to replicate the synovial fluid in simulator tests. Wear testing should be carried out using larger fluid volumes (e.g., 250 ml) to generate physiological relevant wear masses.

How do gait frequency and serum-replacement interval affect polyethylene wear in knee-wear simulator tests?

Polyethylene wear (PE) is known to be a limiting factor in total joint replacements. However, a standardized wear test (e.g. ISO standard) can only replicate the complex in vivo loading condition in a simplified form. In this study, two different parameters were analyzed: (a) Bovine serum, as a substitute for synovial fluid, is typically replaced every 500,000 cycles. However, a continuous regeneration takes place in vivo. How does serum-replacement interval affect the wear rate of total knee replacements? (b) Patients with an artificial joint show reduced gait frequencies compared to standardized testing. What is the influence of a reduced frequency? Three knee wear tests were run: (a) reference test (ISO), (b) testing with a shortened lubricant replacement interval, (c) testing with reduced frequency. The wear behavior was determined based on gravimetric measurements and wear particle analysis. The results showed that the reduced test frequency only had a small effect on wear behavior. Testing with 1 Hz frequency is therefore a valid method for wear testing. However, testing with a shortened replacement interval nearly doubled the wear rate. Wear particle analysis revealed only small differences in wear particle size between the different tests. Wear particles were not linearly released within one replacement interval. The ISO standard should be revised to address the marked effects of lubricant replacement interval on wear rate.

Wear in total knee arthroplasty--just a question of polyethylene?: Metal ion release in total knee arthroplasty

PURPOSE

Biological reactions against wear particles are a common cause for revision in total knee arthroplasty. To date, wear has mainly been attributed to polyethylene. However, the implants have large metallic surfaces that also could potentially lead to metal wear products (metal ions and debris). The aim of this study was to determine the local release of cobalt, chromium, molybdenum and titanium in total knee arthroplasty during a standard knee wear test.

METHODS

Four moderately conforming fixed-bearing implants were subjected to physiological loadings and motions for 5×10(6) walking cycles in a knee wear simulator. Polyethylene wear was determined gravimetrically and the release of metallic wear products was measured using high resolution-inductively coupled plasma-mass spectrometry.

RESULTS

A polyethylene wear rate of 7.28 ± 0.27 mg/10(6) cycles was determined and the cumulative mass of released metals measured 1.63 ± 0.28 mg for cobalt, 0.47 ± 0.06 mg for chromium, 0.42 ± 0.06 mg for molybdenum and 1.28 ± 0.14 mg for titanium.

CONCLUSION

For other metallic implants such as metal-on-metal total hip arthroplasty, the metal wear products can interact with the immune system, potentially leading to immunotoxic effects. In this study about 12 % by weight of the wear products were metallic, and these particles and ions may become clinically relevant for patients sensitive to these materials in particular. Non-metallic materials (e.g. ceramics or suitable coatings) may be considered for an alternative treatment for those patients.

Charnley-Kerboull total hip arthroplasty combining zirconia on polyethylene. A minimum eight-year follow-up prospective study

PURPOSE

Zirconia was introduced in the 1980s for total hip arthroplasty (THA) with the expectation of lower polyethylene wear. The purpose of this prospective study was to evaluate the results of a continuous series of total hip arthroplasties combining a zirconia head with polyethylene socket at a minimum eight-year follow-up.

METHODS

We performed an open prospective clinical trial in 1997. Our study involved 51 consecutive patients (55 hips) with a mean age of 52.5 ± 12 years (range, 25-76 years). All patients had a Charnley-Kerboull all-cemented hip replacement. A 22-mm stabilised yttrium tetragonal polycrystalline zirconia head (Y-TZP) was used in association with moderately cross-linked and annealed polyethylene. Clinical and radiological outcomes were assessed yearly. A survival analysis was performed using revision for any reason as the end-point.

RESULTS

At a minimum eight-year follow-up, 12 patients (13 hips) were lost to follow-up (mean 26.8 months), two patients (two hips) had died, and six patients (six hips) were revised. The remaining 31 patients (34 hips) were alive and had not been revised on either the femoral or acetabular side at a mean follow-up of 117.1 months (range, 96-150 months). Mean functional score at last follow-up was 17.7. Mean linear head penetration was 0.23 mm/year. More than 90% of the remaining hips had signs of periprosthetic osteolysis. Five stems were loosened. The survival at eight years was 87.3% (95% IC: 76.7-97.8).

CONCLUSION

This study confirms earlier short-terms results, and demonstrates that zirconia should no longer be used in THA.

Multifunctional composite coating as a wear-resistant layer for the bearing in total hip joint replacement

In this study, we developed Ti-TiN composite coatings with fine lamellar structures for use as an enhanced wear-resistant layer between the bearing components of the polymer-lined acetabular cup and the metal femoral head of total hip joint replacements (THRs). A plasma spraying deposition method was used to apply the composite coatings, and the thickness of TiN layer in the composite could be controlled by varying the flow rate of N(2) atmospheric gas. The surface properties, such as roughness and hardness, were analyzed, and the friction coefficient (μ) and wear rate (k) were measured using a bovine serum wear test. A biocompatibility test was performed to evaluate the toxicity of the composite coatings. Our experimental results reveal that the friction and wear resistance of composite coatings is superior to that of the metallic implant materials, and they have a higher level of fracture toughness as compared with other ceramic coatings because of a good balance between the hardness of the TiN and the toughness of the Ti. Furthermore, these coatings possessed excellent biocompatibility. The experimental results also demonstrate that the improved wear properties can be attributed to a certain level of unavoidable porosity that is due to the rapid solidification of liquid droplets during the plasma spraying process. The pores in the coating surface play an important role as a lubricant (bovine serum) reservoir, reducing the actual contact area and friction losses.

The effect of lubrication on the friction and wear of Biolox®delta

The performance of total hip-joint replacements depends strongly on the state of lubrication in vivo. In order to test candidate prosthetic materials, in vitro wear testing requires a lubricant that behaves in the same manner as synovial fluid. The current study investigated three lubricants and looked in detail at the lubrication conditions and the consequent effect on ball-on-flat reciprocating wear mechanisms of Biolox®delta against alumina. Biolox®delta, the latest commercial material for artificial hip-joint replacements, is an alumina-matrix composite with improved mechanical properties through the addition of zirconia and other mixed oxides. Three commonly used laboratory lubricants, ultra pure water, 25 vol.% new-born calf serum solution and 1 wt.% carboxymethyl cellulose sodium salt (CMC-Na) solution, were used for the investigation. The lubrication regimes were defined by constructing Stribeck curves. Full fluid-film lubrication was observed for the serum solution whereas full fluid-film and mixed lubrications were observed in both water and the CMC-Na solution. The wear rates in the CMC-Na and new-born calf serum were similar, but were an order of magnitude higher in water. The worn surfaces all exhibited pitting, which is consistent with the transition from mild wear to severe or "stripe" wear. The extent of pitting was greatest in the serum solution, but least in the water. On all worn surfaces, the zirconia appeared to have fully transformed from tetragonal to monoclinic symmetry. However, there was no evidence of microcracking associated with the transformed zirconia. Nevertheless, AFM indicated that zirconia was lost preferentially to the alumina grains during sliding. Thus, the current study has shown conclusively that the wear mechanisms for Biolox®delta clearly depend on the lubricant used, even where wear rates were similar.

The use of self-mating PEEK as an alternative bearing material for cervical disc arthroplasty: a comparison of different simulator inputs and tribological environments

PURPOSE

The hypothesis for this study was that the simulated wear behavior of a hydroxyapatite coated, self-mating PEEK cervical disc arthroplasty device would be dependent on the simulated testing environment.

METHODS

Five groups of devices were evaluated under suggested ASTM and ISO load and motion profiles. The groups utilized different testing frequencies and protein content of simulator fluid, in addition to assessing the potential for third body wear. The average wear rates were determined using linear regression analysis with a generalized estimating equation. Significant differences between groups were determined using the Wald's test.

RESULTS

The simulated wear behavior was shown to be highly dependent on the testing environment, where protein content more than decreasing the cyclic loading frequency resulted in increased wear, but was not dependent on the suggested load and motion profiles. It was demonstrated that a self-mating PEEK cervical disc arthroplasty device has wear rates that are similar to existing material combinations for cervical disc arthroplasty.

CONCLUSIONS

This study showed that at a time when data from retrieval analyses is deficient, it is important to test the wear resistance of cervical disc arthroplasty devices under various conditions. Long-term clinical results and ongoing implant retrievals are required for validation between clinical performance and simulator inputs.

Characterization of protein degradation in serum-based lubricants during simulation wear testing of metal-on-metal hip prostheses

A size exclusion high performance liquid chromatography (SEC-HPLC) method has been developed which is capable of separation and quantitation of bovine serum albumin (BSA) and bovine serum globulin (BSG) components of serum-based lubricant (SBL) solutions. This allowed characterization of the stability profiles of these proteins when acting as lubricants during hip wear simulation, and identification of wear-specific mechanisms of degradation. Using cobalt-chromium metal-on-metal (MOM) hip joints, it was observed that BSA remained stable for up to 3 days (215K cycles) of wear testing after which the protein degraded in a fairly linear fashion. BSG on the other hand, began to degrade immediately and in a linear fashion with a rate constant of 5% per day. Loss of both proteins occurred via the formation of high molecular weight aggregates which precipitated out of solution. No fragmentation of the polypeptide backbone of either protein was observed. Data obtained suggest that protein degradation was not due to microbial contamination, denaturation at the air-water interface, or frictional heating of articulating joint surfaces in these studies. We conclude that the primary source of protein degradation during MOM simulation testing occurs via high shear rates experienced by SBL solutions at articulating surfaces, possibly coupled with metal-protein interactions occurring as new and reactive metal surfaces are generated during wear testing. The development of this analytical methodology will allow new studies to clarify the role of SBL solutions in wear simulation studies and the interactions and lubricating properties of serum proteins with prosthetic surfaces other than MOM.

Y-TZP zirconia run against highly crosslinked UHMWPE tibial inserts: knee simulator wear and phase-transformation studies

BACKGROUND

Zirconia (ZrO(2)) ceramics combined with highly cross-linked polyethylene appears to be a promising approach to minimize wear in artificial knee joints. The wear performance of yttria-stabilized zirconia (YZr) femoral condyles on 7-Mrad tibial inserts was compared in a knee simulator to CoCr bearing on 3.5-Mrad inserts.

METHODS

The knee design was the Bi-Surface type with a 9-year clinical history in Japan (JMM, Japan). A displacement-controlled knee simulator was used with kinematics that included 20 degrees flexion, +/-5 degrees rotation, and 6 mm anterior/posterior translation. Lubricant was alpha-calf serum, test duration was 10 million cycles (10 Mc), and wear was measured by weight-loss techniques. The wear zones were studied by laser interferometry, scanning electron microscopy, and Raman microprobe spectroscopy.

RESULTS

At 10 Mc the wear rates of the CoCr controls averaged 4.5 mm(3)/Mc. This was within 7% of the prior estimate at 5-Mc duration and comparable to Bi-Surface wear data from another laboratory. The CoCr condyles increased in roughness (R(a)) from <50 nm to average R(a) = 250 nm due to linear scratching. The ceramic condyles remained pristine throughout the wear study (R(a) <7 nm). With the YZr/7-Mrad combination, the weight change had a positive slope over at 10 Mc, which meant that the actual polyethylene wear was unmeasurable. Microscopic examinations at 10 Mc showed that the zirconia surfaces were intact and there was no detectable change from tetragonal to monoclinic phase.

INTERPRETATION

Our laboratory knee wear simulation appeared very supportive of the 9-year YZr/PE clinical results with Bi-Surface total knee replacements in Japan.

"Severe" wear challenge to 36 mm mechanically enhanced highly crosslinked polyethylene hip liners

Our purpose was to compare the wear performance of mechanically enhanced 5Mrad highly crosslinked polyethylene (MEP, ArComXL) hip liners to (control) 3Mrad UHMWPE liners (ArCom) in 36 mm head size. As a more severe synergy of clinically relevant test models, we contrasted wear with custom roughened Co-Cr surfaces (Ra 500 nm) to the standard pristine Co-Cr heads (Ra < 20 nm) using a severe microseparation test mode in our hip simulator. We adopted a previously published model to estimate potential biological activity. On new Co-Cr heads, the MEP liners showed a 47% reduction in volumetric wear a 13% reduction in wear particle size and a 27% reduction in Functional Biological Activity (FBA) compared to our control. On rough Co-Cr heads, the MEP liners showed little advantage in terms of volumetric wear compared with the control. However, the MEP liners overall showed a 38% reduction in FBA compared to the control owing to a larger volume fraction of larger particles. Thus overall the MEP liners appeared to offer advantages in terms of reduced FBA indices.

Characterization of the running-in period in total hip resurfacing arthroplasty: an in vivo and in vitro metal ion analysis

BACKGROUND

Metal-on-metal total hip resurfacing arthroplasty is increasingly being performed in young and active patients. Preclinical in vitro testing of implants is usually performed with use of hip simulators, and the serum metal ion concentration is determined for the purpose of monitoring the patients. The goal of this study was to characterize the early running-in period in vivo and in vitro by characterizing metal ion levels.

METHODS

A well-functioning total hip resurfacing prosthesis was implanted in fifteen consecutive patients, and the serum metal ion concentrations in these patients were then determined preoperatively and at intervals during the first postoperative year (at one, six, twelve, twenty-four, and fifty-two weeks). The number of walking cycles was measured with use of a computerized accelerometer in order to compare walking cycles to hip simulator cycles. In vitro, five similar components were investigated for 3 million cycles with use of a hip simulator. Serum samples were obtained at different time points, and wear was measured by quantifying wear particles and ions in the samples. All patient and simulation serum samples were analyzed with use of inductively coupled plasma-mass spectrometry. One simulator implant was investigated with use of scanning electron microscopy.

RESULTS

The serum chromium and cobalt levels of the patients continuously increased during the first six months and showed an insignificant decrease thereafter. The molybdenum concentration was unchanged compared with preoperative values. In contrast, the simulator measurements showed a different wear pattern with a high-wear running-in period and a low-wear steady-state phase. The running-in period was delayed by 300,000 cycles and lasted up to 1 million cycles. Scanning electron microscopic analysis showed a carbon-rich protein film predominantly in the early phases of simulation. Scratches were detected originating from pits filled with aluminum oxide and silicon oxide and from pulled-out carbides that were causing third-body wear.

CONCLUSIONS

The simulator study allowed an exact characterization of the running-in period and showed a delayed onset of running-in wear. In contrast, the clinical data showed a slow increase in measured ion concentrations. These different wear patterns are probably due to the effects of distribution, accumulation, and excretion of particles and ions in vivo.

Wear in molded tibial inserts: knee simulator study of H1900 and GUR1050 polyethylenes

Hi-fax 1900 tibial inserts were used in the IB-1 total knee replacement (TKR) beginning 1978, soon followed by the AGC design. Such direct compression molded (DCM) inserts was relatively immune to oxidation. Unfortunately the Hi-fax 1900 resin (H1900) was taken off the market in year 2004. As an alternate, GUR1050 was introduced in the Vanguard TKR. However there appeared to be little or no wear comparisons of molded inserts. Therefore the study aim was to compare wear performance of GUR1050 to the historical H1900. The hypothesis was that Hi-fax and GUR1050 would show comparable wear performance. The Vanguar was a posterior-cruciate sacrificing design (Biomet Inc.). All tibial inserts were sterilized by gamma-radiation (3.2 Mrad) under argon. A 6-channel, displacement-controlled knee simulator was used with serum lubricant (protein concentration 20 mg/mL). Wear assessments were by gravimetric methods and linear regression techniques. The gross weight-loss trends over 2.5 Mc duration demonstrated excellent linear behavior with good agreement between TKR sets (<+/-10%). Fluid sorption artifacts in control represented less than 5% of gross wear magnitudes. Thus suitable corrections could be made in determining net wear. The H1900 and GUR1050inserts demonstrated net wear-rates of 3.6 and 3.4 mm(3)/Mc, respectively. This difference was not found to be statistically significant. This wear study demonstrated that GUR1050 inserts were indistinguishable from the Hi-fax 1900 in terms of laboratory wear performance, proving our hypothesis. Given the excellent clinical history of DCM Hi-fax 1900, the GUR1050 should be an ideal candidate for TKR.

Preliminary tribological evaluation of nanostructured diamond coatings against ultra-high molecular weight polyethylene

BACKGROUND

Some loss of joint prostheses has been attributed to osteolytic loosening associated with debris from wear of polyethylene articulating against metal alloys. Reduced polyethylene wear has been reported with ceramics serving as an alternative counterface.

METHODS

Nanostructured Diamond (NSD) coatings were deposited onto Ti6Al4V by microwave plasma-assisted chemical vapor deposition, with both hydrogen-rich (H-NSD) and helium-rich (He-NSD) feedgas mixtures. Pin-on-disk wear tests of polyethylene against NSD and CoCr were performed in serum lubrication at body temperature. Scanning electron microscopy was used to examine surface morphology, and nanoindentation was used to determine hardness and modulus of the polyethylene wear surfaces. Raman spectroscopy, surface roughness, and wettability analyses of the NSD coatings were performed.

RESULTS

Raman spectroscopy confirmed sp(2) and sp(3) bonded carbon in the NSD coatings. No significant differences in wear factors were found between polyethylene on H-NSD, He-NSD, and CoCr, despite higher roughness and friction coefficients for the He-NSD and H-NSD coatings, compared with CoCr. Contact angles for the diamond coatings were reduced following the wear tests, indicating that these surfaces became more hydrophilic. Numerous small protuberances were observed on pins articulated against CoCr, and a single, large protuberance was observed in polyethylene-on-NSD. These features were conjectured to be reconsolidated polyethylene particles. Nanoindentation modulus and hardness of the worn polyethylene surfaces were lower for polyethylene-on-diamond than for polyethylene-on-CoCr.

CONCLUSIONS

As a counterface to polyethylene, NSD-coated Ti6Al4V produced wear factors comparable to CoCr in the present pin-on-disk tests, a promising step towards its use in joint replacement bearing applications.

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.

[Early femoral osteolysis following Charnley-Kerboull total hip arthroplasty combining a 22-mm zirconia head on polyethylene]

PURPOSE OF THE STUDY

New bearings have been developed to reduce polyethylene wear. Zirconia ceramic is one proposal with attractive tribologic properties. The purpose of this prospective study was to evaluate the clinical and radiological results at least two years after implantation in a continuous series of total hip arthroplasties combining a zirconia head with polyethylene socket.

MATERIAL AND METHODS

The series included 56total hip arthroplasties performed in 51 patients (30 females and 21 males), mean age 52.2 +/- 12 years (25-76 years). Prostheses were implanted for primary degenerative disease (43%) and dysplasia (27%). The femoral component was a stainless steel stem with a Morse cone measuring 11degrees 25' for 27 hips and 5 degrees 40' for 28 hips. A 22-mm stabilized yttrium tetragonal polycrystalline zirconia head (Y-TZP) was used. The transtrochanteric approach was used for all arthroplaties with implantation of a cemented Charnley-Kerboull prosthesis. Clinical outcome was assessed with the Postel-Merle-d'Aubligné (PMA) score. Acetabular cup wear and periprosthetic osteolysis were measured on successive AP x-rays of the pelvis.

RESULTS

Mean follow-up was 32 months (24-48). None of the patients were lost to follow-up. Mean function score at last follow-up was 17.8 +/- 0.2 (16-18) versus 12.2 +/- 2.6 preoperatively (Wilcoxon, p < 0.0001). There were no cases of implant migration (femoral or acetabular). An endosteal defect by femoral osteolysis in the calcar femoral was observed in 19 of 55 cases. It appeared early between first and second post-operative year and did not progress later. Its surface remained less than 1 cm2 in all cases. We were unable to identify any factor predictive of these osteolytic lesions.

DISCUSSION

The short-term clinical results with this type of arthroplasty was comparable with earlier results described with classical Charnely-Kerboull implants using a metal-polyethylene bearing. The Merkel osteolysis observed in this series occurred early for one-third of the hips with no measurable polyethylene wear. We suggest surgeons should be cautious about using the zirconia head. We are continuing our surveillance of these patients.

Heat generated by knee prostheses

Temperature sensors were placed in 50 knees in 25 patients who had one or both joints replaced. Temperature recordings were made before walking, after walking, and after cycling. The heat generated in healthy, arthritic, and replaced knees was measured. The knee replacements were done using eight different prostheses. A rotating hinge knee prosthesis generated a temperature increase of 7 degrees C in 20 minutes and 9 degrees C in 40 minutes. An unconstrained ceramic femoral prosthesis articulating with a polyethylene tibial prosthesis generated a temperature increase of 4 degrees C compared with a healthy resting knee. The other designs using a cobalt-chrome alloy and high-density polyethylene had temperature increases of 5 degrees-7 degrees C with exercise. Frictional heat generated in a prosthetic knee is not immediately dissipated and may result in wear, creep, and other degenerative processes in the high-density polyethylene. Extended periods of elevated temperature in joints may inhibit cell growth and perhaps contribute to adverse performance via bone resorption or component loosening. Prosthetic knees generate more heat with activity than healthy or arthritic knees. More-constrained knee prostheses generate more heat than less-constrained prostheses. A knee with a ceramic femoral component generates less heat than a knee with the same design using a cobalt-chromium alloy.

Dynamic Analysis of the Resultant Force Acting on the Hip Joint During Level Walking

The present study was designed to determine the resultant force acting on the hip joint during walking using a new dynamic analysis method. Our model utilized joint motion, ground reaction force, and muscle strength data from 18 women (6 normal women aged 20-24 years, 6 normal women aged 50-57 years, and 6 female patients with osteoarthritis, aged 50-66 years). We analyzed the resultant force using the multibody dynamic analysis system. To determine the factors that influence the force acting on the hip, we examined the effect of age and total hip arthroplasty. The maximum resultant force acting on the femoral head was dependent on the subject body weight and correlated with muscle strength and walking speed. The results of this study highlight the agreement between computer simulation analysis and actual measurement of the resultant force acting on the hip. Our results suggest that muscle strength and walking speed are significant determinants of the resultant force acting on the hip.

Protein-mediated boundary lubrication in arthroplasty

Wear of articulated surfaces can be a major lifetime-limiting factor in arthroplasty. In the natural joint, lubrication is effected by the body's natural synovial fluid. Following arthroplasty, and the subsequent reformation of the synovial membrane, a fluid of similar composition surrounds the artificial joint. Synovial fluid contains, among many other constituents, a substantial concentration of the readily adsorbing protein albumin. The ability of human serum albumin to act as a boundary lubricant in joint prostheses has been investigated using a pin-on-disc tribometer. Circular dichroism spectroscopy was employed to follow the temperature- and time-dependent conformational changes of human serum albumin in the model lubricant solution. Effects of protein conformation and polymer surface hydrophilicity on protein adsorption and the resulting friction in the boundary lubrication regime have been investigated. Unfolded proteins preferentially adsorb onto hydrophobic polymer surfaces, where they form a compact, passivating layer and increase sliding friction-an effect that can be largely suppressed by rendering the substrate more hydrophilic. A molecular model for protein-mediated boundary friction is proposed to consolidate the observations. The relevance of the results for in vivo performance and ex vivo hip-joint testing are discussed.

Ceramics in total hip replacement

Alumina-on-alumina total hip arthroplasty has been used for 32 years in Europe. The theoretical advantages of this combination are represented by its remarkable sliding characteristics, its very low wear debris generation, and its improved fracture toughness. These advantages are achieved if the material is processed properly with high density, high purity and small grains. We summarize the results obtained with the alumina-on-alumina combination concerning in vitro and in vivo wear behavior with special emphasis on wear debris characterization and quantification and histologic tissue examinations. Alumina-on-alumina seems to be one of the best choices in young and active patients provided that sound socket fixation is maintained in the long term.

Comparison ofin vitro within vivo characteristics of wear particles from metal-metal hip implants

The purpose of the present study was to compare wear particles isolated from metal-metal (MM) hip implants worn in an orbital bearing simulator with particles from similar MM total hip replacement (THR) implants worn in vivo. Comparison of these particles is important because it will help to assess the overall suitability of this type of hip simulator for reproducing in vivo wear and for producing physiological wear particles suitable for biological studies of in vitro cellular response. Commercial grade components made of ASTM F75 (cast) alloy were evaluated. Simulator tests were performed in 95% bovine calf serum with a 28-mm-diameter implant. Wear particles were collected from 0 to 0.25 million cycles (run-in wear period) and 1.75 to 2 million cycles (steady-state wear period). Tissues from seven patients with MM implants (surface replacement or stem type) were harvested at revision surgeries (after 1-43 months). Metal wear particles were isolated from serum lubricant or tissues using an enzymatic protocol that was previously optimized to minimize particle changes due to reagents. After isolation, particles were centrifuged, embedded in epoxy resin, and characterized by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). Results of EDXA on particles from the hip simulator primarily indicated a predominance of particles containing Cr and O but no Co (most likely chromium oxide particles), and fewer CoCrMo particles presenting varying ratios of Co and Cr. Image analysis of TEM micrographs demonstrated that the majority of the particles from the simulator were round to oval, but a substantial number of needle-shaped particles were also found, especially from 0 to 0.25 Mc. The particles generated from 0 to 0.25 Mc had an average length of 53 nm, whereas those generated from 1.75 to 2 Mc had an average length of 43 nm. In vivo, EDXA and TEM analysis of particles that were retrieved from two patients at 23 and 43 months respectively, revealed that they were the most comparable in composition, average length (57 nm), and shape to particles generated in the hip simulator during the run-in wear period. Because a large clinical retrieval study in the literature suggested that a run-in wear regime might occur in vivo for some 6-36 months, the fidelity of the simulator of the present study was strongly supported. However, some uncertainties existed, including the finding that the particles isolated from the other five patients generated from 1 month up to 15 months (shorter implantation times than the other two patients) were smaller and mostly contained only Cr and O (no Co). In the opinion of the authors, this particular very short term patient group was somewhat atypical. Therefore, despite these uncertainties, the present study was deemed to support the ability of the orbital bearing hip simulator to produce physiological wear particles.

A novel liner locking mechanism enhances retention stability

Acetabular liner retention of a novel design of liner locking was evaluated in static and cyclic endurance modes. The locking mechanism combines geometric form and accurate machining to give high conformity to the acetabular shell and minimise relative motion against the metal shell, minimising debris generation and escape or ingress. Using amended test liners with integral coupling, mean static pullout strength was determined to be 399+/-53 N and lever-out strength 28.03+/-2.8 N m. Cyclic loading of 5 N m for up to 10 million cycles caused no significant reduction in strength, no detectable fretting wear, and the sealing mechanism prevented particle access between the cup interior and the "effective joint space". The stability measured ensures secure and reliable in vivo retention of the liner, comparable with extant component designs using other liner locking mechanisms.

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 degrees C vs. 41 degrees 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 degrees C markedly reduced the bearing temperatures and the protein precipitation. With coolant at 4 degrees 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.

Migration of metal-on-metal versus ceramic-on-polyethylene hip prostheses

In a prospective randomized study, 32 metal-on-metal prostheses and 29 ceramic-on-polyethylene prostheses of similar design were implanted in 59 patients. Radiolucency, cup migration, wear, and function were examined after a minimum of 2 years followup (followup rate, 87%). The purpose of the current study was to evaluate whether higher frictional torque of metal-on-metal will lead to a higher rate of early metal-on-metal cup loosening. A computer-assisted method was used for wear and migration measurements of the acetabular component. Metal-on-metal prostheses migrated in a craniocaudad direction significantly less than ceramic-on-polyethylene prostheses. The mean total migration for both types of prostheses exceeded 1.5 mm at 2 years. Clinically, no difference between the two prostheses regarding activity, pain, or range of motion was found at 2 years. As migration of metal-on-metal prostheses was not higher in comparison with ceramic-on-polyethylene prostheses, the expected higher frictional torque of metal-on-metal prostheses did not increase migration during short-term followup. The different debris produced by both bearings did not influence the short-term results of this study, but might cause different long-term results.

Effects of digestion protocols on the isolation and characterization of metal-metal wear particles. I. Analysis of particle size and shape

Isolation of metal wear particles from hip simulator lubricants or tissues surrounding implants is a challenging problem because of small particle size, their tendency to agglomerate, and their potential for chemical degradation by digestion reagents. To provide realistic measurements of size, shape, and composition of metal wear particles, it is important to optimize particle isolation and minimize particle changes due to the effects of the reagents. In this study (Part I of II), transmission electron microscopy (TEM) was used to examine and compare the effects of different isolation protocols, using enzymes or alkaline solutions, on the size and shape of three different types of cobalt-based alloy particles produced from metal-metal bearings. The effect on particle composition was examined in a subsequent study (Part II). Large particles (<1200 nm) were generated by dry abrasion of CoCrMo alloy against itself and small particles (<300 nm) were generated by hip simulator testing of a metal-metal implant pair in the presence of either distilled-deionized water or a 95% bovine serum solution. The reagents changed particle size and to a lesser extent particle shape. For both large particles and small particles generated in water, the changes in size were more extensive after alkaline than after enzymatic protocols and increased with alkaline concentration and time in solution, up to twofold at 2 h and threefold at 48 h. However, when isolating particles from 95% serum, an initial protective effect of serum proteins and/or lipids was observed. Because of this protective effect, there was no significant difference in particle size and shape for both oval and needle-shaped particles after 2 h in 2N KOH and after enzymatic treatments. However, round particles were significantly smaller after 2 h in 2N KOH than after enzymatic treatments. Particle composition may also have been affected by the 2N KOH treatment, as suggested by a difference in particle contrast under TEM, an issue examined in detail in Part II.

Frictional heating of total hip implants. Part 2: finite element study

Due to higher friction artificial hip joints warm up more than natural joints during walking and other continuous activities. This could lead to thermal damage in the surrounding tissues and be a reason for long-term implant loosening, an effect which has not yet been investigated. In vivo measurements with instrumented implants showed temperatures inside the prosthetic head up to 43.1 degrees C (Part 1 of this work). Based on the experimental data a finite element model was developed to calculate the temperatures in the tissues surrounding the hip implant to determine whether these tissues can heat up to critical levels. Various parameters were investigated which could account for the variations in the measured temperatures in the patients, including the perfusion rate in tissues, the volume of synovial fluid, and different implant materials. We found that the synovial fluid is most endangered by thermal damage and consequent deterioration of lubricating properties. Implants with a cobalt-chromium head and a polyethylene cup are unfavourable as they can elevate the temperature in the synovia to more than 46 degrees C. With regard to thermal properties stems made from cobalt-chromium alloys are superior to titanium stems, by better conducting heat to the femur and minimizing the synovial fluid temperature. Factors determining the temperatures during walking are insufficiently known or cannot be determined in the individual patient. Therefore, the risk of a thermally induced implant loosening cannot currently be estimated. Under unfavourable conditions such a risk exists, however. General improvements of implant materials and clinical studies on the possibility of implant loosening due to high temperatures are therefore required.

Frictional heating of total hip implants. Part 1: measurements in patients

Hip implants heat up due to friction during long lasting, high loading activities like walking. Thermal damage in the surrounding soft and hard tissues and deteriorated lubrication of synovial fluid could contribute to implant loosening. The goal of this study was to determine the implant temperatures in vivo under varying conditions. Temperatures and contact forces in the joints were measured in seven joints of five patients using instrumented prostheses with alumina ceramic heads and telemetry data transmission. The peak temperature in implants with polyethylene cups rose up to 43.1 degrees C after an hour of walking but varied considerably individually. Even higher temperatures at the joints are probable for patients with higher body weight or while jogging. The peak temperature was lower with a ceramic cup, showing the influence of friction in the joint. During cycling the peak temperatures were lower than during walking, proving the effect of force magnitudes on the produced heat. However, no positive correlation was found between force magnitude and maximum temperature during walking. Other individual parameters than just the joint force influence the implant temperatures. Based on the obtained data and the available literature about thermal damage of biological tissues a detrimental effect of friction induced heat on the stability of hip implants cannot be excluded. Because the potential risk for an individual patient cannot be foreseen, the use and improvement of low friction implant materials is important.