The successful performance of ceramic on ceramic bearings in today's THA can mainly be addressed to the excellent tribological behaviour and the minimal wear of ceramic bearings. The clearance between head and shell plays a major role in this functionality of artificial hip joints. Knowledge about the deformation behaviour of the shell during implantation but also under daily loads is essential to be able to define a minimum clearance of the system. The aim of this work is to establish a tool for determining maximum ceramic shell deformation in order to predict minimum necessary clearance between heads and monolithic ceramic shells. In order to determine the minimum clearance the following in vivo, in vitro and in silico tests were taken into account:
Eight generic metal shells were implanted into cadaveric pelvises of good quality bone realizing an underreaming of 1 mm. Maximum deformation of the metal shells (um) after implantation were determined using an validated optical system. The deformations were measured 10 min. after implantation. The stiffnesses of the metal shells (Cm) were experimentally determined within a two-point-loading frame acc. to ISO 7206-12. The stiffness of a monolithic ceramic shell (Cc) representing common shell designs (outer diameter 46 mm, 3 mm constant wall thickness) were determined acc. to ISO 7206-12 using Finite-Element-Method (FEM). Maximum deformation for the ceramic shells (uc,dl) under daily loading, represented by jogging (5kN, Bergmann et. al), was determined applying FEM. Press-fit forces (Fpf = umCm) can be calculated with the results of test 1 and 2 considering linear elastic material behaviour. Assuming force equilibrium and applying the evaluated stiffness from test 3 the deformation of the ceramic shell (uc) occurring after implantation can be estimated (uc = umCm/Cc). For minimum clearance calculation of a monolithic ceramic shell (uc,lt) in vivo deformation (uc,dl) has to be considered additionally (uc,lt = uc + uc,dl).Introduction
Materials and Methods
Acetabular cup deformation is an important topic in today's THA and was investigated for a variety of metal cup designs (e.g. 1,2,3). Cup deformation caused by press-fit forces can have negative effects on the performance of such systems (e.g. high friction, metal ion release). When considering new materials for monolithic acetabular cups - such as ceramics - detailed knowledge about the deformation behaviour is essential to ensure successful performance. Therefore, the deformation behaviour of monolithic ceramic cups was investigated. Testing was conducted with monolithic ceramic cups (under development, not approved) of size 46mm and 64mm. One cup design of each size had a constant wall thickness of 3.0mm and an offset of 0.0mm (center of rotation on front face level), the other design was lateralized with an offset of 3.5mm (46mm) or 5.0mm (64mm), leading to an increased wall thickness. First, 3 cups of each design were impacted into 1.0mm underreamed Sawbones® blocks (pcf 30, geometry: see (2)). Second, all cups were quasi-statically assembled into the Sawbones® blocks of the same design using a material testing machine. Third, the cups were placed in a two-point-loading frame (acc. to ISO/DIS 7206–12:2014(E)) and a load of up to 1kN was applied. The inner diameter of all cups was measured under unloaded and loaded conditions for all scenarios using a coordinate measurement machine at 9 locations of each cup, 1.5mm below the front face (Fig.1). As the diametrical deformation (unloaded inner diameter – loaded inner diameter) was not normally distributed a Wilcoxon test was performed to statistically analyse the deformation differences of the different cup designs (p<0.05).Introduction
Materials and Methods
Recent literature demonstrates that the assembly load to connect ball head and femoral stem affects the taper junction fretting wear evolution in THR [1]. During assembly the surface profile peaks of the mostly threaded tapers are deformed. This contributes to the taper locking effect. Very little is known about this deformation process and its role in the evolution of fretting and wear. Therefore, this study aimed to experimentally determine the deformation of the profile peaks after the initial assembly process. 36 tapers of three different stem materials acc. to ISO5832-3 (titanium), ISO5832-9 (steel), ISO5832-12 (cobalt chromium) and 36 ceramic ball heads were tested under quasi-static (4kN) and dynamic (impaction) (3.7±0.3kN) axial assembly. Before and after loading 4 surface profiles in 90° offset were measured on each taper. Height differences of profile peaks and areas under profile curves were calculated and compared. Both parameters provide insights into the deformation behavior of the surface structure. Additionally, subsidence of tapers into ball heads was measured and subsidence rates were calculated with regard to varying impaction forces. Due to different thermal expansion coefficients tapers could be disconnected from ball heads by utilizing liquid nitrogen. Thus, further surface damage due to disassembly was avoided. Statistical analysis was performed using a Wilcoxon test (p<0.05).Introduction
Materials and Methods
Modular hip replacement systems use Morse tapers as an interlocking mechanism to connect ball heads to femoral stems. Even though this interlocking mechanism generally performs successfully for decades, failures due to disassociation of the ball head from the stem are reported in the literature. Therefore, this failure mechanism of a possible loosening is usually evaluated in the course of the development of femoral stems. The disassembly force is a possible parameter to characterize the strength of the interlocking mechanism. Thus, the aim of the current study was to examine the impact of different taper parameters on the disassembly force of ceramic ball heads from titanium stem tapers by finite element studies. A 2D axisymmetric finite element model was developed to simulate the disassembly procedure. First ball head and taper were assembled with a force of 4 kN. Afterwards the system was unloaded to simulate the settlement. Disassembly was simulated displacement controlled until no more adhesion between ball head and taper occurred. Isotropic elastic material behavior was modelled for the ceramic ball head while elastic-plastic material behavior was modelled for the titanium taper. Different angular gaps (0.2°, 0.15°, 0.1°, 0.05°, 0°, −0.05°, −0.1°) and different taper topography parameters regarding groove depth (12, 15 µm), groove distance (210, 310 µm) and plateau width (1, 5, 10, 20 µm) were examined. Frictional contact between ball head and taper was modelled.Introduction
Materials and Methods
Modular acetabular liners are fixed in metal shells by a taper locking mechanism. Male tapers of the liner and female tapers of the metal shell have different taper angles resulting in an angular gap. Depending on the specific manufacturing tolerances varying angular gaps may result and, thus, different contact mechanics may be generated that could alter the stresses within the acetabular liner. Therefore, the aim of the current study was to experimentally determine stresses in a ceramic liner depending on different angular gaps under Two ceramic liners were instrumented at the outer contour with five strain gauge (SG) rosettes each (Fig.1). First, metal shells were axially seated in an asymmetric press-fit model with 0.5 mm under-reaming, then liners were assembled with a 2 kN axial load. SG5 was placed at the flat area of the liner, the other four were placed circumferentially in 90 degrees offset on the rear side. SG2 and SG4 were mounted opposite to each other in press-fit direction while SG1 and SG3 were placed in the non-supported direction. Three inclination angles (0°, 30°, 45°) were tested under Introduction
Materials and Methods
Realistic in-vivo loads on knee implants from telemetric analyses were recently published. Impacting an implant, especially a ceramic one, will produce high peak stresses within the component. Data for loads occurring during implantation of a knee implant are scarce. To ensure a safe impaction of ceramic tibial trays the stresses caused by it need to be known. Impaction testing including force measurements (using Kistler piezo load cell 9351B) was performed on a ceramic tibial tray. The same test was simulated by computational analysis using FEM (Finite-Element-Method). Because the forces measured and those calculated by FEM were significantly different, an in vitro impaction study was performed to obtain realistic loads for a ceramic tibial tray. A surgeon was asked to perform heavy hammer blows which may occur during implantation. Using a high speed camera (phantom V7.2) the velocity of the hammer at the time of impaction was determined. Using this parameter instrumented ceramic tibial trays (BPK-S Knee, P. Brehm) were implanted into a biomechanical Sawbones® model. Linear strain gauges were attached to the four fins of the tibial tray as these are the regions of highest stresses. Simulating the surgeon's highest impacts measurements were conducted at a frequency of 1 MHz. The identical hammer was used in this in vitro study and the velocity of the hammer was measured by using the same high speed camera. To investigate the damping effect of bone cement Palacos®R bone cement was used. Only worst-case impacts within the range achieved by the surgeon were applied to evaluate the stress distribution within the ceramic tibial tray.Introduction
Materials and Methods
Deformation of modular acetabular press-fit shells is of much interest for surgeons and manufacturers. Initial fixation is achieved through press-fit between shell and acetabulum with the shell mechanically deforming upon insertion. Shell deformation may disrupt the assembly process of modular systems and may adversely affect integrity and durability of the components and tribology of the bearing. The aim of the study was to show shell deformation as a function of bone and shell stiffness. The stiffness of the generic shells was determined using a uniaxial/ two point loading frame by applying different loads, and the change in dimension was measured by a coordinate measurement machine (CMM). Cadaver lab deformation measurements were done before and after insertion for 32 shells with 2 wall thicknesses and 11 shell sizes using the ATOS Triple Scan III (ATOS) optical system previously validated as a suitable measurement system to perform those measurements. Multiple deformation measurements per cadaver were performed by using both hip sides and stepwise increasing the reamed acetabulum by at least 1 mm, depending on sufficient residual bone stock. The under-reaming was varied between 0mm and 1mm, respectively. From the deformations, the resulting forces on the shells and bone stiffness were calculated assuming force equilibrium as well as linear-elastic material behaviour in each point at the rim of the shell.INTRODUCTION
METHODS
The role and importance of fretting and corrosion in modular hip endoprostheses has become of more and more interest within the last years. Especially bearing couples with large diameters may experience high friction moments leading to an increase of relative micro movements between the surfaces of the taper connections. Recently published studies show that the risk of fretting and corrosion is significantly reduced by using ceramic ball heads compared to metal ball heads. Goal of this study was to investigate the risk of fretting and corrosion as well as possible loosening of large ceramic ball heads with metal sleeves.INTRODUCTION
OBJECTIVES
For a safe design of ceramic components in total hip arthroplasty it is important to know the stress state within each part of the system under in vivo loading scenarios. Besides several design parameters, e.g. diametrical clearance between ball head and liner or angular mismatch in the taper region of metal shell and liner, also physiological factors, like patients' weight or bone quality, influence the stresses within the components. Therefore, the aim of the current study was to experimentally determine the stresses in a ceramic liner varying two of the factors: clearance and inclination angle of the liner. Two ceramic liners were instrumented at the outer contour with five strain gauge (SG) rosettes (measuring grid length: 1.5 mm) on each liner (Fig.1). Metal shells were seated in an asymmetric press-fit Sawbones® model using a 0.5 mm under-reaming, and liners were afterwards axially assembled with a 2 kN load. SG5 was placed at the flat area of the liner, the other four were placed circumferentially in 90 degrees offset on the rear side of the liner. SG2 and SG4 were mounted opposite to each other in press-fit direction (contact of metal shell to the Sawbones® block) whereas SG1 and SG3 were placed in the non-supported direction (no contact of metal shell to the Sawbones® block). Four different inclination angles (0°, 30°, 45°, 60°) were tested under in vivo relevant loads of 4.5 and 11 kN. Two ceramic ball heads were used to examine a mid tolerance clearance and a clearance at the lower tolerance limit. Strain data was converted to stresses and compared using a paired two-sided Wilcoxon Rank Sum Test at an α-level of 0.05.Introduction
Materials and Methods
Deformation of modular acetabular press-fit shells is a topic of much interest for surgeons and manufacturer. Such modular components utilise a titanium shell with a liner manufactured from metal, polyethylene or ceramic. Initial fixation is achieved through a press-fit between shell and acetabulum with the shell mechanically deforming upon insertion. Shell deformation may disrupt the assembly process of inserting the bearing liner into the acetabular shell for modular systems. This may adversely affect the integrity and durability of the components and the tribology of the bearing. Most clinically relevant data to quantify and understand such shell deformation can be achieved by cadaver measurements. ATOS Triple Scan III was identified as a measurement system with the potential to perform those measurements. The study aim was to validate an ATOS Triple Scan III optical measurement system against a co-ordinate measuring machine (CMM) using in-vitro testing and to check capability/ repeatability under cadaver lab conditions.INTRODUCTION
OBJECTIVE
Dislocation is one of the major factors for revision surgery. Current literature states that the usage of larger bearing couples (> 36 mm) have the potential of reducing the risk of dislocation. Smaller ceramic-on-ceramic bearing couples (< 36 mm) have demonstrated very low wear rates. But does the wear behaviour change with increasing diameter? Therefore, the aim of this study was to compare wear rates of larger ceramic-on-ceramic bearing couples for total hip arthroplasty. Wear tests according to ISO 14242 with 36, 40 and 44 mm zirconia platelet toughened alumina (ZPTA) bearings were performed in a servo-hydraulic hip simulator. In total, the specimens were loaded up to 5 million cycles. Wear was measured gravimetrically every million cycles. For each diameter three different combinations regarding clearance and roundness were chosen. One combination represented in tolerance parts (70 μm clearance, < 5 μm roundness). The other two combinations represented parts at the lower end and at twice the upper end of the tolerance band regarding clearance and out of specification parts regarding the roundness.Introduction
Materials and Methods
Modular metal-on-metal hip implants show increased revision rates due to fretting and corrosion at the interface. High frictional torque potentially causes such effects at the head-taper interface, especially for large hip bearings. The aim of this study was to investigate fretting and corrosion of sleeved ceramic heads for large ceramic-on-ceramic (CoC) bearings. The investigated system consists of a ceramic head (ISO 6474-2; BIOLOX® Option), a metal sleeve (Ti-6Al-4V, ISO 5832-3) and different metal stem tapers (Ti-6Al-4V, ISO 5832-3; stainless steel, ISO 5832-1; CoCrMo, ISO 5832-12). Three different test methods were used to assess corrosion behaviour and connection strength of head-sleeve-taper interfaces:
Fretting corrosion acc. to ASTM F1- Corrosion under Frictional torque under severe i like conditions Standardized fretting corrosion tests were carried out. Additionally, a long term test (0.5 mio. cycles) under same conditions was performed. Corrosion effects under 4.5 kN (stair climbing) and 10 kN (stumbling) were determined for three groups. One group was fatigue tested applying 4.5 mio. cycles at 4.5 kN and 0.5 mio. cycles at 10 kN in a corrosive fluid. In parallel two control groups (heads only assembled at same load levels) were stored in the same fluid for same time period. Pull-off tests were performed to detect the effect of corrosion on the interface strength. A new designed test was performed to analyse the connection strength and fretting-corrosion effects on the head-sleeve taper interfaces caused by frictional torque of large CoC bearings (48 mm). Two separate loading conditions were investigated in a hip joint simulator. One created bending torque (pure abduction/adduction), the other set-up applied rotational torque (pure flexion). A static axial force of 3 kN and movements with a frequency of 1 Hz up to 5 mio. cycles in the same corrosive fluid as in the second set of tests were applied for both tests. Surface analysis of the taper and sleeve surfaces was peformed. In order to detect loosening caused by frictional torque, torque-out tests were conducted after simulator testing.INTRODUCTION
METHODS
In knee arthroplasty a ceramic component has several advantages: first, there is no ion release implying a risk for potential allergies. Second, the hardness of the material leads to a scratch resistance which ultimately reduces PE wear over time. In the past, ceramic components in knee applications were limited in the variety of design possibilities due to necessary thickness of the component resulting from the associated fracture risk of ceramics. By the development of an alumina matrix composite material with increased mechanical properties it is possible to develop ceramic knee components which have nearly the same design as a metal component and use the same implantation technique as well as the same instruments. This offers the surgeon the opportunity to choose intraoperatively between metal or ceramic knee components. Extensive in-vitro testing shows that ceramic knee components achieve superior mechanical test results. The reliability of the components is proven by two different burst tests and a fatigue test for both a femoral and a tibial ceramic knee component. The mechanical proof-test was developed by subsequent steps of numerical load/stress analysis and design of an adequate mechanical test equipment. The procedure was organized as follows: Oncologic: Analysis of relevant maximum in-vivo loading conditions Analysis of the “boundary conditions” Finite Element analysis: Identifying regions of highest stress concentration Design analysis and accommodation if necessary Development of an adequate mechanical test equipment which produces stresses comparable to the in-vivo conditions Performing mechanical tests with ceramic femoral components Validation of the test concept: comparison of test results and stress analysis Assign “safety margin”, Establish “proof test”Introduction
Material and method
