The CoCrMo large bearings had shown a high failure rate, because of metal ion and particle release. Alumina matrix composite (AMC) ball heads have shown to mitigate such phenomena. The aim of this study was to investigate the leaching properties of AMC clinically as well as experimentally. Two patient groups were compared: a control group (n=15) without any implant (Controls) and 15 Patients with unilateral treatment with Biolox delta ceramic-on-ceramic (CoC). Whole-blood samples of Controls and Patients (after 3 and 12 months from treatment with CoC) were measured by means of trace element analysis using a HR-ICPMS. The leaching behaviour of BIOLOX delta was also analysed in-vitro: five Biolox delta heads and five CoCrMo heads were immersed in serum for seven days at 37°C. Aluminium, cobalt, chromium and strontium were detected based on HR-ICPMS.Background
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
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
Wear, aseptic loosening, dislocation, corrosion and prosthetic joint infection (PJI) are major factors leading to revision of THA. The effect of using ceramic components to address these issues was investigated to determine their behaviour and potential benefit.INTRODUCTION
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
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
Third body wear caused by contaminated bearing environment with debris that may have been generated by a worn or fractured revised bearing coupling, but also originated by generation of metal/cement particles during the primary or revision surgery, may be a relevant issue for the implant life. To evaluate the wear behavior of a last generation alumina matrix ceramic composite (AMC) bearing in a worst case scenario consisting of highly contaminated test lubricant with alumina particles in a hip joint simulator study.Introduction
Objectives
Wettability of bearing couples has always been related to the tribological performance of implants, and it is understood to affect lubrication of surfaces. So far researchers fail to understand the real mechanisms governing the lubrication process of prostheses. Different models attempt to explain the phenomena, but more research is needed. To add more difficulties, some classical measuring techniques have provided inaccurate values of surface properties. For instance, wettability may seem a priori a simple technique capable of providing easy-to-read cost-effective information. However, ignoring surface preparation may lead to wrong values of wettability and mis-understanding of the results. The dependence of wettability of commercial bearing materials used in arthroplasty has been studied as a function of the cleaning procedure, showing the variability of the results, and providing a series of guidelines to understand and perform wettability measurements.INTRODUCTION
OBJECTIVES
Ceramics are excellently suited for applications in arthroplasty, mainly total hip, knee and shoulder replacement. As the most prominent representative of this demanding type of material, BIOLOX® To assess the influence of INTRODUCTION
OBJECTIVES
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
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
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
As allergic reactions to implant wear are gaining more attention [4], the incorporation of ceramic materials to device design appears to be a promising development. In particular, ceramic femoral components of total knee replacements have been designed to produce less wear under standard [1] and adverse [5] implant conditions. Whereas the wear reduction effect of ceramics is generally accepted for hip implants, the corresponding effect for knee implants is not proven. Ezzet et al. reported a wear reduction of 42% for standard wear conditions [2] and of 55% for adverse wear conditions [3] when compared to a geometrically identical CoCr femoral component. In contrast to these findings, an analysis of the EndoLab® database has indicated wear rates of ceramic knee implants that are comparable to traditional low wear material couplings (Figure 1), and are within the range of clinically established devices. The purpose of this study was to directly compare two TKR designs, one fixed bearing and one mobile bearing, each made of traditional CoCr to one made of alumina matrix composite (BIOLOX® For the fixed bearing groups, a mean wear rate of 12.01 mg per million cycles (StdDev. 3.28) was determined for the CoCr implant and of 1.78 mg per million cycles (StdDev. 0.40) for the BIOLOX® Based upon the EndoLab® experience the ceramic total knee replacements tested perform as good as the best performing metallic total knee replacements. However it can be concluded that for the two implant systems tested the wear rate is reduced by more than 50% by using ceramic on polyethylene articulation when compared to an identical cobald crome design.
From a tribological point of view and clinical experience, a ceramic-on-ceramic bearing represents the best treatment option after rare cases of ceramic component fracture in total hip arthroplasty (THA). Fractured ceramic components potentially leave small ceramic fragments in the joint capsule which might become embedded in PE acetabular liners. This in vitro study compared for the first time the wear behaviour of femoral ball heads made of ceramic and metal tested with PE liners in the presence of ceramic third-body debris. The contamination of the test environment with third-body ceramic debris, insertion of ceramic fragments into the PE liners and implementation of continuous subluxation simulated a worst-case scenario after revision of a fractured ceramic component.Introduction
Purpose
Pin-on-disk studies have demonstrated the role that cross-shear plays in polyethylene wear. It has been found that applying shear stresses on the polyethylene surface in multiple directions will increase wear rates significantly compared to linear sliding. Hip and knee joint replacements utilize polyethylene as a bearing surface and are subjected to cross-shear motions to various degrees. This is the mechanism that produces wear particles in hip and knee arthroplasty bearings and if excessive may lead to osteolysis, implant loosening, and failure. The amount of cross-shear is dependent on the bearing diameter and the angular motion exerted onto the bearing due to the gait of the patient. This study will determine the effect of sliding curvature (angular change per linear sliding distance) on the wear rate of polyethylene. Virgin polyethylene blocks were machined with a 28mm diameter bearing surface and against 28mm cobalt chromium femoral heads in a hip simulator. Dynamic loading was applied simulating walking gait but the motion differed between testing groups. Typical walking gait testing utilizes 23° biaxial rocking motion, in this study, 10°, 15°, 20°, and 23° biaxial rocking motions resulting in various sliding curvatures. Sliding motion path is described in Figure 1 and is a function of the bearing radius and the rocking angle. With increased rocking angle, the sliding distance reduces per cycle and the sliding path becomes more curved (more angular change per linear distance of sliding). Despite a significant increase in sliding distance at higher rocking angles, wear rates were relatively unchanged and ranged from 57mm3/mc to 62mm3/mc. Wear rates per millimeter increased exponentially with reduced sliding arc radius (smaller rocking angle) as shown in Figure 2. This study suggests that wear of polyethylene is highly dependent on sliding path curvature. The sliding path is largely a function of the bearing diameter and the patient activity. Large bearing diameter implants have been recently introduced to increase joint stability. Sliding distance increases proportional to the bearing radius which has led to some concerns regarding increased wear in larger bearings. However, in vitro wear studies have not shown this trend. Increased bearing diameter also increases the sliding path curvature which this study has shown to cause a reduction in wear roughly proportional to the radius of the bearing. Therefore, the increase in wear due to sliding distance is offset by the reduction in wear caused by the sliding curvature resulting in no significant change in wear with increased bearing diameter. Curved sliding path causes a change in surface shear direction which has been shown to increase wear of polyethylene. This study confirms that increased cross-shear in the form of more angular change per linear sliding distance can increase wear of polyethylene exponentially
