Metallic resurfacing systems have been widely used until pseudotumors and ALTR have been clinically found and related to excessive wear of these metal-on-metal hip systems. Hence, surgeons widely abandoned the use of resurfacing systems. Meanwhile, there is a ceramic on ceramic (CoC) resurfacing system (Embody, London, UK) made of zirconia toughened alumina (BIOLOX® Combined experimental and numerical results were used to determine the deformation of the ceramic shell. In a cadaver lab, the resulting deformations after impaction of generic metal shells have been measured, see e.g. [1] for the method of measurement. The maximum deformation has been chosen for further calculation. Additionally, the stiffness of both generic metal and ceramic shells has been measured using ISO 7206–12. The deformation of the ceramic shells were then calculated by the equation where uc and um are the deformations of the ceramic and the metal shell, respectively, and Km and Kc are the respective stiffnesses. Additionally, in a finite element simulation, the resulting deformation of the ceramic shell under in-vivo conditions was calculated and superposed with uc. The resulting deformation was used as the minimum value of the clearance for the ceramic resurfacing system.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
Ceramic hip components are known for their superior material properties and longevity. In comparison to other materials commonly used, ceramics have a very low friction coefficient and a high fracture load. However, even though in-vivo fractures of ceramic ball heads are a relatively rare occurrence compared to other reasons for revision, they are of concern to the surgeon using ceramic components. The goal of this work was to evaluate the most probable causes for fracture and to quantify the influence of the metal taper contamination and shell deformation, respectively. An experimental set-up imitating the in-vivo loading situation was used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper contamination. 58 ceramic ball heads made of pure alumina were loaded until fracture under various conditions. Parameters under investigation were the inclination of the insert, the loading velocity, and the contamination of the interface between taper and ball head.INTRODUCTION
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
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
The reported revision rate for THA is below 10% at 10 years. Major factors for revision are aseptic loosening or dislocation of the articulating components. CoC bearings in total hip arthroplasty (THA) have demonstrated very low wear rates. Due to producing the least number of wear particles of any articular bearing used for THA, osteolysis is very rarely observed. Zirconia-platelet toughened alumina (ZPTA) has improved toughness and bending strength while maintaining all other advantageous properties of alumina. Consequently, its clinical fracture rate is minimal and wear resistance is superior to alumina. Since a trend exists towards the usage of larger bearings the aim of this study was to compare the tribological behavior of different ZPTA/ZPTA THAs with respect to their ball head diameter.INTRODUCTION
OBJECTIVES
Concerns have been raised that deformation of
acetabular shells may disrupt the assembly process of modular prostheses.
In this study we aimed to examine the effect that the strength of
bone has on the amount of deformation of the acetabular shell. The
hypothesis was that stronger bone would result in greater deformation.
A total of 17 acetabular shells were inserted into the acetabula
of eight cadavers, and deformation was measured using an optical
measuring system. Cores of bone from the femoral head were taken
from each cadaver and compressed using a materials testing machine.
The highest peak modulus and yield stress for each cadaver were used
to represent the strength of the bone and compared with the values
for the deformation and the surgeon’s subjective assessment of the
hardness of the bone. The mean deformation of the shell was 129
µm (3 to 340). No correlation was found between deformation and
either the maximum peak modulus (r² = 0.011, t = 0.426, p = 0.676) or
the yield stress (r² = 0.024, t = 0.614, p = 0.549) of the bone.
Although no correlation was found between the strength of the bone
and deformation, the values for the deformation observed could be
sufficient to disrupt the assembly process of modular acetabular
components. Cite this article:
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
In order to obtain a secure taper connection it is advised to clean and dry the metal cup before assembling a ceramic insert. A slight axial tap using a plastic impactor completes the insertion procedure. There are a few reported cases that the taper connection failed intraoperatively although it was inserted and impacted as recommended. A conceivable reason seems to be a high amount of fluid in the gap between insert and cup (e.g. from rinsing process, blood) that prevent the insert from being securely fixed due to its incompressibility. Cups embedded in a cast resin have been used in an appropriate impaction test setup. Four different amounts of 1.75% polyvinyl pyrrolidone solution with comparable viscosity to that of blood were filled into the metal cups (figure 1). To obtain reference values, tests were made with dry metal cups (0%), too. Three different The fluid
cannot escape from the gap can permeate through a low permeable screen cloth can permeate through a high permeable screen cloth. The screen cloth should represent different cancellous bone densities. Ten Ceramic inserts of each size (28 and 36 mm) made of pure alumina (BIOLOX® INTRODUCTION:
Methods:
Since over 40 years, ceramics are known for their excellent biocompatibility, extremely low wear rates and excellent wettability. This would make a ceramic-on-Polyethylene bearing also a beneficial combination for a knee implant if potential strength issues could be overcome. A mechanical proof-test for a ceramic femoral knee implant component was developed by subsequent steps of numerical load/stress analysis and design of adequate mechanical test equipment. The procedure was organized as follows:
Analysis of maximum in-vivo loading condition and distinguish between alternating regular loading with a high cycle number during life time and irregular worst case loading. The relevant regular loading is represented by rising from a chair and normal walking. The most critical irregular worst cases are stumbling or impact loading. The load transfer, stress distribution and the anticipated cycle number during life-time are distinguished and taken into account for the development of the test concept. Analysis of the “boundary conditions,” i.e. the fixation of the ceramic prosthesis on the bone identifying the worst-case conditions Finite Element analysis: Identifying regions of highest stress concentration at variable external loading Design analysis and accommodation if necessary From step 3 it is evident that stress concentration is mainly generated by geometric features, e.g. the shape of the corners at the interface to the cement. Significant reduction of stress concentration was achieved by some minor corrections of design details.
Development of an adequate mechanical test equipment which produces stresses comparable to the in-vivo conditions and performing of mechanical tests with ceramic femoral components Assign “ Establish “INTRODUCTION
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
Ceramic hip components are known for their superior material properties concerning the invivo loading situation. In comparison to other commonly used materials, ceramics have a very low friction coefficient and a high fracture load. However, there are a few reported occasions of in-vivo fracture of ceramic ball heads. An experimental set-up imitating the in-vivo loading situation is used to analyze different scenarios that may lead to the fracture of the ball heads, such as dynamic loading, edge loading and the metal taper condition. It will be shown that even the worst-case set-up does not lead to fracture loads if the interface between ceramic ball head and metal taper is clean and dry. In contrast, certain disturbances/impurities of this interface can cause a further reduction of the fracture load. Ceramic ball heads made of pure alumina have been loaded until fracture under various conditions. The angle between the loading direction and the metal taper equals 35°, the ceramic ball is mounted in an alumina insert. Parameters under investigation were the inclination of the insert, the loading rate, and the condition of taper and ball head (contamination of the interface between taper and ball with adipose and osseous tissue; stripe wear on the outside of the ball head). Altogether 58 specimens (all alumina heads mounted on a titanium taper) have been tested, To resemble the position of the human acetabulum during walking and standing up, the inclination of the insert was chosen to differ between 45° (walking) and 80° (standing up). A variation of the loading speed is also tested, with a maximal speed in the range of the in-vivo loading rate (chosen parameters: 0,5 kN/sec and 25 kN/sec). For fabric samples, bovine femur (corticalis) and porcine adipose tissue were used. All fractured ball heads were statistically analyzed regarding the appearance of fracture in general, the fracture origin, and the metal transfer in the cone of the ceramic ball head. The behavior of the ball heads for the different scenarios shows a great variation: If the inclination of the insert equals 45°, it is not pos sible to break the ceramic ball head at all because of the high plastic deformation of the metal taper. In case of edge loading, the fracture load drops to 20 kN for 28-12/14 S ball heads and 36 kN for 28-12/14 L ball heads. The loading rate and the contamination of the interface between ball head and taper with adipose tissue have no measurable influence on this value. The largest effect on the fracture load has a contamination with osseous tissue. The fracture load decreases to 32% compared to the value measured without the contamination. A minimal fracture load of approximately 8 kN (KK 28-12/14 L) was measured. Statistical analysis shows that the fracture load depends linearly on the stiffness of the system (ball heads 28-12/14 S). Because none of the other parts changes during the experiments, the cause of the change in stiffness is most likely due to a change of the friction coefficient between ball head and taper: A reduced stiffness indicates a lower friction coefficient which results in higher normal forces in the ball head and, therefore, leads to lower fracture loads. This theory is supported by numerical calculations. The influence of edge loading and contamination of the interface between taper and ball with osseous tissue on the fracture load can be shown. If the insert has a high inclination angle, high bending forces are applied to the ball head amplifying the effect of edge loading. It should be accentuated, that the minimum fracture load of a ball head without contamination of the interface is still twice as high as the maximum forces measured in-vivo. Contamination with osseous tissue leads to a minimum fracture load of approximately eight times of the body weight, a value being close to the maximum forces ever measured invivo. Therefore, diligence is recommended during the implantation of the ceramic hip components in order to avoid disturbances of this interface. Because the reduction of the stiffness results in a reduction of the fracture load, the lubrication of the taper should be avoided.
