Increasing the performance of total hip replacement prostheses through functionally graded material innovation and design

1. Cytotoxicity testing of the Al2O3 and ZrO2 starting powders was performed according to ISO 10993-5, in the framework of GLP, using two test systems : one established cell line (Balb 3T3) and primary human cell cultures (human osteogenic cells arising from bone marrow stroma) (HOBM). Three parameters were assessed at 24 h and 72 h: cell viability through cell counting, cell mitochondria activity trough MTT test, and necrosis versus apoptosis.This study pointed out the difficulties in interpreting the results : one has to take into account the specific surface area of the powders, toxic ion dissolution specially from additive, or dopants, using AAS when possible, the event of phagocytosis depending on the chemistry of the powder particles which might alter cell growth and phenotype expression. One has to focus on the difference between a cytostatic and a cytotoxic effect. Moreover it pointed out the differences in sensitivity to cytotoxic or cytostatic elements between established cell lines (Balb 3T3), which are very useful for screening purpose and human primary cell cultures (Ex : HOBM) arising from the implantation site. Human cells were shown much more sensitive to powder particles than Balb 3T3. 2. Cytocompatibility testing (HOBM cell growth and phenotype expression) of Al2O3 and ZrO2 starting powders let us show that, although powder particles undergo phatocytosis, HOBM cell growth and differentiation (ALP acitivity, collagen synthesis) was not significantly altered except for high concentrations (100 µg/mL). At the contrary HOBM challenged with FGM aluminas wear debris generated by a hip stimulator phagocytosed partially wear debris and exhibit altered functions resulting in a partial loss of their capacity to proliferate and differentiate. Moreover the mode of generation of wear debris seems to be a critical point. These different results obtained with starting powders and wear debris arising from the final component gives food for thought and needs to go deeper into: size, surface chemistry depending on generation mode. It could be very useful in risk analysis study of ceramics.

Small-angle neutron scattering (SANS) was used in a rather original way for microporosity characterization in functionally graded and laminated Al2O3/Y-ZrO2 ceramics. The porosity characteristics (volume fraction, size distribution, specific surface) of these materials were obtained by SANS with a high degree of accuracy. This non-destructive technique yields information on the total volume fraction of residual pores (both open and closed). The SANS experiments bring novel and valuable information on influence of the HIP process on porosity and the relation between the detected internal stresses and porosity.

Normally, the prosthetic femoral heads are coupled with the stems by means of a conical coupling. It is important that after assembly the head subsequently remains immobile on the neck because the movement of the ceramic component on the metal taper of the stem will cause the metal component to wear. It is essential that the strength of the fixation between the head and the neck is sufficient to withstand the torque likely to be transmitted through the prosthesis in use. The loads acting on the heads are often very high (even more then three times the body weight), cyclically variable. Because of the characteristics of brittleness, ceramics don't allow any rate of local plastic deformation (possible with metals). It is therefore recommended to design a head-stem conic coupling in which the stress condition will never be critical for ceramic, to avoid the risk of breakage of the femoral head coupled with prosthetic acetabulum in the pelvis. The purpose of this work is the optimisation of the coupling between neck and femoral head. In order to do that, FEM analysis performed with I-DEAS and MSC MARC software of the FGM ball head coupled with different stem tapers by incorporating the residual stresses calculated by HUT with ABAQUS software were done.

The actual production process of ceramic hip joint heads ("ball heads") and cup inserts ("liners") includes spray drying and green machining. The electrophoretic deposition process (EPD) would allow the elimination of theses processes. In addition, the EPD process would allow to advantageously combining the properties of alumina and zirconia in a "functionally graded material". Alumina would act as the wear resisting outer layer, whereas zirconia would be used as a strong core material. By an appropriate combination of these materials a compression stress would be set up in the alumina surface thus increasing the load bearing capability of the component and, as a consequence, further increasing the � already high � reliability of ceramic components in total hip joint replacement. The contribution of CeramTec was: - characterizing structural and especially strength properties of homogeneous and graded test specimens and components. The strength results on ball heads are promising, both EPD and SSC technology must be further improved. The current stage of development has to be classified as "Experimental development stage".

The main task of the NPI participant in the BIOGRAD project was the neutron diffraction characterization of graded Al2O3/Y-ZrO2 ceramics. Neutron diffraction methods of determination of internal strains and stresses in structural materials have found a lot of applications in material sciences and engineering. However, application of the convention neutron diffraction method has been found rather difficult in the studied multiphase system with a gradient of phase composition. The diffraction methods use the crystal lattice as a built-in strain gauge and the corresponding information on stress state is then received from each phase separately. From technological point of view, the most important characteristics of graded ceramics are the distribution of the macroscopic residual stresses. These stresses result mainly from the phase specific stresses after cooling from sintering temperature and could be varied by an appropriate selection of the phase gradient. The used diffraction method provides information on lattice strains, which are detected in each constituent phase individually. The experimentally observed value of lattice strains reflects superposition of both phase specific strains and macroscopic residual strains. Within the BIOGRAD project, the new concept of determination both stress/strain components, i.e. the macroscopic residual stress and the phase specific stresses was developed.

The electrical field drop over a deposit during EPD influences the deposition rate. Due to the large practical consequences of a potential drop over the deposit, a procedure was developed to calculate it from current-conductivity measurements during EPD. The suspension composition determines whether an extra potential drop over the deposit is present or not. Alumina suspensions based on MEK and n-butylamine showed no potential drop over the deposit, while suspensions based on ethanol and an acid caused an extra potential drop over the deposit. The potential drop over the electrode is determined by the interparticle interactions in the deposit, i.e, the surface charge and potential of the pore wall and the Debye screening length that is related to the ionic strength. For ethanol-based suspensions with HNO3 addition, the specific deposit resistivity and the concomitant potential drop over the deposit was found to increase with increasing potential in the centre of the pores of the deposit, as calculated from the zeta potential of the particles in suspension, the Debye screening length and the measured average pore size. An increased pore potential inhibits ion movement through the pores of the deposit, increasing the resistivity of and the potential drop over the deposit. The very thin double layer relative to the pore diameter explained the absence of an extra potential drop for MEK, n-butylamine suspensions. These findings allow developing suspensions for specific applications. Suspensions with a high potential drop over the deposit are suitable for thin homogeneous coatings on rough surfaces, while suspensions with a low potential drop are suitable for EPD of thick bulk components. A mathematic model was developed to describe the deposition yield during electrophoretic deposition, taking into account the influence of the powder concentration in the suspension, the deposition efficiency and accounting a changing electrical field during deposition. By means of this new equation, a relation was found between the electrophoretic mobility and the deposition efficiency for ethanol based alumina suspensions. The deposition efficiency increased with decreasing electrophoretic mobility (zeta potential).

A thermo- elasto- viscoplastic model developed and adapted for the sintering of alumina- zirconia FGMs. With the model distribution of temperature, density, strain rates and displacements, normal (radial and axial) stresses and shear stresses can be obtained. The model uses analytical contribution as micro mechanical analysis and experimental feedback to validate the calculation results and to ensure the most adequate description of the materials state during and after processing. The model was coupled with heat transfer and materials viscosity. Heating non-uniformity has been taking into account and instant temperature distributions were also obtained. The model was implemented for ABAQUS/Standard software for fully coupled thermal displacement analysis. The materials properties have been collected from experiments and integrated into a special user subroutine. It is possible to adapt the model for other commercial software such as FEMLAB. With the model, it is possible to evaluate critical stresses distribution, optimise processing route (temperature program, green density, gradient composition etc.) to obtain targeted properties. Also, a reverse task is possible - to calculate green part dimensions using the most desired parameters in final state. This is essential for the design and shaping of green bodies before sintering.

Bodycote and K.U.Leuven optimised the sintering schedule of conventional sintering + HIPing allowing full densification of medical grade Al2O3-ZrO2 powder based FGM discs. The densification cycle was also applicable for the sequentially slip casted components from IJS. Pressure less sintering is performed for 1 h in air at 1550°C, followed by post-HIPing at 1390°C for 20 min in 140 MPa Ar. The final grain size of the Al2O3 surface layer of the components remains below 2 µm.

The small amount of wear generated by hip components hampers its precise assessment by gravimetric measurements. Thus, the possibility of applying the Thin Layer Activation (TLA) technique to ceramic components was taken into account during the last months of the project. The obtained results demonstrate that the activation of alumina is possible. The employment of TLA allows a highly sensitive measurement of wear, with a resolution in the sub-nanogram range, and it represents a mean to investigate the dissolution of material due to a liquid environment. Future investigations on this technique will provide quantitative data about the wear of ceramic hip components.

The actual production process of ceramic hip joint heads ("ball heads") and cup inserts ("liners") includes spray drying and green machining. The electrophoretic deposition process (EPD) would allow the elimination of theses processes. In addition, the EPD process would allow to advantageously combining the properties of alumina and zirconia in a "functionally graded material". Alumina would act as the wear resisting outer layer, whereas zirconia would be used as a strong core material. By an appropriate combination of these materials a compression stress would be set up in the alumina surface thus increasing the load bearing capability of the component and, as a consequence, further increasing the � already high � reliability of ceramic components in total hip joint replacement. The contribution of CeramTec was: - the hard machining, i.e. grinding and polishing of sintered near net shape blanks of homogeneous and graded test specimens and components Development of grinding and polishing of ball head blanks produced by EPD and by SSC process was completed successfully. Some of the ball head blanks broke during hard machining due to problems which have to be solved within the development of the EPD- and SSC-technology as well as in the design development. Ball heads which did survive the hard machining were supplied to the project partners and to the mechanical testing at CeramTec.

In order to successfully shape green ceramics with complex geometry by means of electrophoretic deposition, a rigid and accurate mechanical electrode-positioning device is essential. Additionally, the design of the deposition and counter electrodes is crucial to manufacture near-net-shape components. K.U.Leuven designed and built a rigid computerised positioning and EPD device composed of; a sledge, linear motor, drivers, computer interfaces, power supply, peristaltic pumps, thermocouple and temperature control unit. Electrical field calculations were performed to assist the design of proper electrodes to shape ball-heads for hip-prosthesis. Basic know-how was obtained on the optimal flow of the suspension through the deposition cell. The set-up allows to automatically running pre-programmed EPD experiments with on-line logging of the voltage, current, suspension temperature and conductivity during the experiment. Due to the complex geometry of the ball-head shape, a sequence of electrodes has to be used in order to achieve a near-net-shaped ball-head deposit. The shaping capabilities of the EPD processing technology and set-up have been demonstrated for homogeneous Al2O3 and functionally graded ZrO2-Al2O3 ball-heads and liners. The potential applicability of the EPD technique is not limited to the Al2O3-ZrO2 system, but covers the broad range of ceramic, metals and polymer materials, providing a suitable suspension is developed. The shape of the electrodes used, which can be designed according to the envisaged application, defines the shaping potential of the developed EPD set-up.

The actual production process of ceramic hip joint heads ("ball heads") and cup inserts ("liners") includes spray drying and green machining. The electrophoretic deposition process (EPD) would allow the elimination of theses processes. In addition, the EPD process would allow to advantageously combining the properties of alumina and zirconia in a functionally graded material. Alumina would act as the wear resisting outer layer, whereas zirconia would be used as a strong core material. By an appropriate combination of these materials a compression stress would be set up in the alumina surface thus increasing the load bearing capability of the component and, as a consequence, further increasing the - already high - reliability of ceramic components in total hip joint replacement. The contribution of CeramTec was: The hard machining, i.e. grinding and polishing of sintered near net shape blanks of homogeneous and graded test specimens and components Development of grinding and polishing of ball head blanks produced by EPD and by SSC process was completed successfully. Some of the ball head blanks broke during hard machining due to problems, which have to be solved within the development of the EPD- and SSC-technology as well as in the design development. Ball heads, which did survive the hard machining, were supplied to the project partners and to the mechanical testing at CeramTec.

The task of the EC-JRC-IHCP was to evaluate the tribological behaviour of FGM disks performing screening wear tests on a 2-stations "reciprocating pin-on-flat" (RPOF) machine. Alumina pins were employed as counterparts. Preliminary studies were carried out using "Biolox forte" discs and pins. Since there is no standard or guideline, which describes RPOF tests on ceramic-ceramic couples (due to the difficulty to avoid edge contact and the risk of pin spallation) these first results allowed to build up a comparison base for the evaluation of the FGM performances. Moreover, they led to the design of a special sample holder that was able to grant a flat contact between pins and disks. Through laser profilometer and the gravimetric wear measurements, the effectiveness of the pin holder has been proved and the tribological behaviour of the FGM disks has been assessed to be equivalent to that of the reference material (Biolox forte (TM)).

EC-JRC-IHCP performed wear test of Biolox forte(TM) hip components using a Biaxial Rocking Motion (BRM) hip joint simulator. The results obtained, following the ISO 14242 standard, led to the definition of the provisions that has to be taken against the operational sources of error.

K.U.Leuven developed a procedure for the colloidal processing of free standing Al2O3-ZrO2 cylindrical discs with a gradient composition by means of electrophoretic deposition (EPD). The capability to produce thick Al2O3-ZrO2 FGM discs was explored. The EPD processing parameters and subsequent sintering schedules were optimised. In order to deposit thick Al2O3-ZrO2 plates, it was necessary to develop a suitable organic suspension with adequate stability. A mathematical model was established for the EPD process, allowing to calculate the composition gradient in plate-shaped FGM materials from the starting composition of the suspensions, the EPD operating parameters and the powder specific EPD characteristics. The model was essential to calculate the experimental parameters to engineer the symmetric gradient profiles in the green deposits. Defect-free Al2O3/ZrO2 FGM discs with a diameter up to 55 mm and a thickness of more than 5 mm were successfully produced.A procedure was described for the manufacturing of Al2O3/ZrO2 composite plates with an outer layer of pure Al2O3, a central homogeneous Al2O3/ZrO2 composite layer and intermediate continuously graded layers by means of EPD from a suspension of acetone and butylamine and subsequent sintering in air. The experimentally obtained gradients were found to be in excellent agreement with the composition profile, calculated according to a developed EPD-model, based on the EPD parameters and the EPD characteristics of the powders. By careful engineering of the thickness of the homogeneous regions and the slope of the profiles in the graded layers in the FGM disc, it is possible to generate and modify the compressive stress in the outer pure alumina layer of the Al2O3/ZrO2 FGM discs.

In Biograd-Dip the aim was to prepare ceramic functionally graded ball-heads using a technique alternative to electrophoretic deposition. The basic idea was to use an adapted dip-coating technique that combines the principles of several conventional shaping methods: a porous core material is dipped into aqueous suspensions with gradually changing compositions. After encouraging preliminary experiments with samples of simple geometry, several problems appeared in shaping of larger discs and ball-heads. For this reason we implemented another shaping technique: sequential slip casting, where the part is built layer by layer by casting of the suspensions with different composition in porous mould. In numerous experiments, conditions for preparation of green parts without major defects were determined. Special attention was paid to the most critical phases - drying and sintering. Using the suspensions with adapted properties, resulting in similar green densities of the layers, step-graded discs and ball-heads were prepared. The results of the analysis showed, that the residual compressive stress at the alumina surface are typically 100-150 MPa that yield in a decreased wear rate. The test step-graded ball-heads that survived the machining reached the proposed burst strength. Reliability is, however, still low that indicates a need of optimisation of the process with special attention given to elimination of defects.

K.U.Leuven developed a drying technique to avoid the formation of drying cracks in complex shaped components with a non-uniform wall-thickness, colloidally processed from aqueous or non-aqueous media. In this technique, the solvent is pressed out of the green wet body under isostatic pressure. For this purpose, the wet component is packed in a plastic bag and cold isostatically pressed. To absorb the solvent, absorbing paper is inserted in the plastic bag around the component. Under the influence of a small isostatic pressure of about 5 MPa, the excess solvent contained in the component is accumulated by the absorbing material reducing the residual amount of solvent in the component below the critical level. The isostatic pressure prevents crack formation in the deposit during drying, allows a rapid solvent removal and increases the green strength.

Exploitable results

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