Advantages of ceramic materials for hip joint prosthesis are recognized to be high hardness, scratch resistance, improved wettability, lower friction and lower wear than CoCr surfaces. Recent studies suggest the use of ceramic femoral head reduce fretting corrosion at stem taper junction compared to metal-on-metal taper junction[1]. Continuous improvement of ceramic materials for orthopedic lead to the development of a resurfacing ceramic-on-ceramic hip joint prosthesis. The main differences of resurfacing heads respect to standard heads are their anatomical dimension and the empty shape suitable to cover the femoral bone and to connect with the resurfacing stem. Ceramic is essentially a brittle material and its strength is influenced by the minimum thickness in the stressed area. Ceramic resurfacing head minimum thickness is comparable with ceramic revision head already on the market. The aim of this study is to develop a mechanical pre-clinical analysis verification process for the newly developed system. The empty shape of the ceramic resurfacing head may influence its strength in a crush loading scenario. Although this is not a physiological condition this test represents the most severe loading for a resurfacing head. Also comparative analysis can be done considering the yield point of conventional metal resurfacing heads reported by the FDA Center for Devices and Radiological Health. For this reason a static unsupported head strength test is performed by applying a compressive load perpendicular to the head axis along the equatorial plane[2](Fig.1). Resurfacing ceramic head made in ZTA is suitable both for a resurfacing stem and an adaptor to be coupled with a standard stem. Mechanical test was performed on worst case resurfacing head size both with resurfacing stem and standard stem based and on FE non linear analysis performed in ANSYS 17.2 according the following material properties: ZTA ceramic (modulus of elasticity E, Poisson ratio ν and density ρ of 348GPa, 0.23 and 4.25g/cm3 respectively), and Ti6Al4V (E=114GPa, ν=0.33 and ρ=4.43g/cm3). For comparison purposes unsupported test was performed on standard head Ø28#S both in Biolox®Delta and Biolox®Forte ceramic. At least three components were used for each test and the average values was compared with predicates[2]. Static compressive load was applied with MTS hydraulic actuators with load cell of 100kN.Introduction
Materials and methods
Modularity allows surgeons to use femoral heads of various materials, diameters and offsets to achieve the best possible outcome, nevertheless the fretting corrosion behaviour of modular junctions can be significantly affected. The aim of this study was to assess physiological friction moment and lubrication ratio in order to compare various tribological materials against different bearing sizes. This data is important as lubrication will affect the friction, wear and torque generated which may lead directly to the production of debris or to enhanced corrosion at modular junctions. Hip joints were tested in lubricant condition on a hip simulator following the ISO14242-3 configuration. Three samples for each combinations were examined: 1) 36mm metal-on-metal made in CoCrMo 2) 36mm ceramic-on-ceramic made in ZTA 3) 58mm resurfacing metal-on-metal made in CoCrMo 4) 57mm resurfacing ceramic-on-ceramic made in ZTA. Preconditioning and dynamic loading steps were spaced out by rest periods (Fig. 1) and the entire series was repeated three times for each combination. Strains were measured on the Ti6Al4V neck's femoral stem with three couples of biaxial strain gauges and were converted into friction moments by means of analytical formulas. Mean maximum moment M and lubrication ratio λ were calculated. MSTART-UP and MTURN-OFF were respectively the first three and last three peak moment sampled for each consecutive step.Introduction
Materials and methods
Complications of metal-on-metal hip resurfacing, leading to implant failure, include femoral notching, neck fracture, and avascular necrosis. Revision arthroplasty options include femoral-only revision with a head, however mis-matching radial clearance could accelerate metal ion release. Alternatively, revision of a well-fixed acetabular component could lead to further bone loss, complicating revision surgery. We have developed a ceramic hip resurfacing system with a titanium-ceramic taper junction; taking advantage of the low frictional torque and wear rates that ceramic affords. Taking a revision scenario into account, the ceramic head has a deep female taper for the resurfacing stem, but also a superficial tapered rim. Should revision to this resurfacing be required, any femoral stem with a 12/14 taper can be implanted, onto which a dual taper adaptor is attached. The outer diameter of the taper adaptor then becomes the male taper for the superficial taper of the ceramic head; ultimately allowing retention of the acetabular component. In an in-vitro model, we have compared the fretting corrosion of this taper adaptor to existing revision taper options: a titanium-cobalt chrome (Ti-CoCr) taper junction, and a titanium-titanium sleeve-ceramic (Ti-Ti-Cer) taper junction. To simulate gait, sinusoidal cyclical loads between 300N-2300N, at a frequency of 3Hz was applied to different neck offsets generating different bending moments and torques. Bending moment and frictional torque were tested separately. An electrochemical assessment using potentiostatic tests at an applied potential of 200mV, was used to measure the fretting current (μA) and current amplitude (μA). In a short term 1000 cycle test with bending moment, four neck lengths (short to x-long) were applied. For frictional torque, four increments of increasing torque (2-4-6-8Nm) were applied. In a long-term test using the taper adaptor, the combination of worst-case scenario of bending and torque were applied, and fretting currents measured every million cycles, up to 10 million cycles.Background
Methods
Hip resurfacing has advantages for the young active patient with arthritis; maintaining a large range of motion, preserving bone stock, and reduced dislocation risk. However high serum metal ion levels with metal-on-metal resurfacing, and their clinical implications, has led to a decline in the use of hip resurfacing. Ceramic bearing surfaces display the lowest frictional torque and excellent wear rates. Recent developments have enabled large, strong ceramic materials to be used as resurfacing components. Any wear debris that is generated from these articulations is inert. However an all-ceramic hip resurfacing could be at risk of fracture at the head-stem junction. A new ceramic hip resurfacing system with a titanium-ceramic modular taper junction has been developed. The introduction of a taper introduces the potential for fretting corrosion; we sought to determine the extent of this in an in-vitro model, and compared this prosthesis to the conventional 12/14 titanium-cobalt chrome (Ti6Al4V-CoCr) taper junction. To simulate the gait cycle, sinusoidal cyclical loads between 300N-2300N, at a frequency of 3Hz, were applied to different head-neck offsets generating different bending moments and torques. The effect of increasing the bending moment and frictional torque were tested separately. Furthermore, the resurfacing head was mounted in a fixture held with just the stem, thus representing complete bone resorption under the head. An electrochemical assessment using potentiostatic tests at an applied potential of 200mV, was used to measure the fretting current (μA) and current amplitude (μA). In a short-term 1000 cycle test, six neck lengths (short to xxx-long) of the Ti6Al4V-CoCr taper were compared to the standard neutral (concentric), and 3mm A/P offset stem options for the resurfacing design. To represent frictional torque, four increments of increasing torque (2-4-6-8Nm) were applied to both tapers. In a long term test with the resurfacing stem, the worst-case scenario of the eccentric offset option and 8Nm of torque were applied, and potentiostatic measurements were taken every million cycles, up to 10 million cycles.Background
Methods
