Background

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.

Introduction

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.

Introduction

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.

INTRODUCTION

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

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.

Introduction

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 in vivo like loading conditions.

Introduction

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.

Introduction

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.

INTRODUCTION

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.

INTRODUCTION

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^®delta is widely used and very successful in the market for more than 10 years. The ability of zirconia phase transformation (t-ZrO₂→m-ZrO₂) in zirconia-platelet toughened alumina (ZPTA) ceramics is an indispensable prerequisite for their excellent mechanical properties. The degree of stabilization of the zirconia tetragonal phase at body temperature is essential for the desired toughening mechanism. Y₂O₃ is the most widely used t-ZrO₂ chemical stabilizer; also microstructure and grain size contribute to t-ZrO₂ phase stabilization. Stabilization must be achieved such that no material degradation will occur in body environment, i.e. in aqueous liquid (synovia), which is known to potentially trigger phase transformation at the surface of ceramic components. In this study, it is shown how phase stabilization in BIOLOX^®delta as a reference material is excellently balanced by means of optimal mechanical performance and environmental stability.

INTRODUCTION

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.

Introduction

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.

INTRODUCTION

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.

Introduction

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.

INTRODUCTION:

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.

INTRODUCTION

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.

INTRODUCTION

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.

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®delta, CeramTec, Germany) ceramic material. The BPK-S Rotating Platform System (Peter-Brehm, Germany; Figure 2) and the MULTIGEN PLUS fixed bearing (Lima, Italy) were knee simulator wear tested according to ISO 14243-1 (2002). A total of three specimens plus one loaded soak control for each group (four groups in total) was subjected to 5 million standard gait cycles. The anterior-posterior (AP) and internal-external rotational (IE) motion of the implants resulting from the external load application of this force controlled test was recorded continuously. Wear was determined gravimetrically. The surface appearance of contact areas was analyzed by light microscopy and particle analysis was performed according to ISO 18129. For the mobile bearing groups, a mean wear rate of 2.47 mg per million cycles (StdDev. 0.38) was determined for the CoCr implant and of 1.10 mg per million cycles (StdDev. 0.46) for the BIOLOX®deltaimplant (Figure 3). The total AP and IE motion of the two groups did not differ. However, motion during stance phase was considerably higher for the ceramic group, indicating reduced frictional resistance (data not shown).

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®delta implant.

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.

Introduction

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.

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

Several options for high demand/high activity patients for bearings in THA exist. Each of them faces certain known and unknown risks of failure. There is a remarked trend to bigger diameter heads to reduce the incidence of dislocation for such patients. While combinations with hard-on-hard bearings have been used in such incidences, a Polyethylene (PE) option is desirable due to its less sensitivity to edge loading and price.

A highly crosslinked sequentially annealed PE of the 3rd generation was prepared by sequentially crosslinking with appropriate annealing steps with a cumulative dose of 90 kGy and subsequent gas plasma sterilization. The structure of this material was determined using TEM, DSC and SAXS. Free radicals and oxidation was determined by ESR and IR spectroscopy. Mechanical evaluation in the unaged and aged condition were performed by quasi-static, dynamic and functional dynamic tests in comparison with negative controls. Wear testing was performed by ball-on-plate tests and hip joint simulators. PE inserts of various internal diameters up to 44mm and thicknesses of 4-8mm in comparison with a historic inert gas irradiation sterilized PE as negative control. These tests have been carried out at 3 institutions using different set-up and protocols. To look at worst case scenarios the simulator testing was done in an impingement mode and fatigue tests of the thinnest components where performed in 2 different fatigue set-ups up to 10 million cycles.

The structure and crystallinity of the sequentially crosslinked PE were comparable to the controls. The radical concentration was reduced by more than 95% due to the sequential process employed and consequently the oxidation level after artificially aging remained at the level of untreated PE. 5 year storage data confirmed the stability of this polymer. All mechanical testing revealed the maintenance of the properties at the same level as the controls. The screening wear test revealed that the high sliding stress used in this set-up had no effect on the sequentially crosslinked PE even when aged, while the controls showed fatigue wear after a short time of testing.

The decrease in volumetric wear compared to a negative control (28 mm head size) was on average 90% in volumetric independent of the head size and thickness of the PE liner. This result was confirmed by the studies at 2 other institutions with a wear reduction of 86 and 95% respectively. Impingement increased the wear rate marginally, without causing any fractures or failures of the components. The analysis of the wear particles from the simulator studies showed a marked decrease in number with close similarity in appearance and morphology to that from the control tests. Fatigue testing even in a luxation model showed no negative effect on the impact on the rim after 10 million cycles also with the thinnest components.

Highly crosslinked, sequentially annealed PE from the perspective of tribological and fatigue testing can be used safely even in impingement and luxation situations. Other factors in the clinical usage of thin liners may play a role and need to be investigated further.

It is accepted that larger diameter heads are more difficult to dislocate due to the increased distance the head has to travel to come out of the cup. Currently larger femoral heads are being used for their resistance to dislocation however, there remains little reporting on the effect of design of cup on jump distance. Monoblock metal on metal cups, which were designed for hip resurfacing are typically less than a hemisphere internally in order to increase the range of motion (ROM) needed when the femoral neck is retained. This does however also reduce the jump distance. We investigated several designs of cup with a variety of head sizes in order to compare ROM using a computer range of motion tool and a two dimensional jump distance with the cup at 45 degrees inclination.

Jump distances were calculated for: Internally hemispheric cups in 28, 32 and 36mm bearing diameters; 28, 40 and 44mm polyethylene liners which were hemispheric but with an additional 2mm cylinder and a 0.7mm chamfer at the equator (Trident, Stryker, Mahwah, USA); 38, 48 and 54mm monoblock metal on metal resurfacing cups with a 3.5mm offset (BHR, Smith and Nephew, Memphis, USA); 40, 48, 58 dual mobility cups with an anatomic rim (Restoration ADM, Stryker, Mahwah, USA)

Range of motion modeling was carried out using custom-written software according to a previously published method2 with 5 degrees of pelvic tilt and a standard femoral component. For the present study, range of motion was assessed on a standard stem with a 132° neck angle. Inclination of the cup was set to 45° and anteversion to 20°. For each implant tested, the total ROM was computed in flexion/extension, ab/adduction, and int/external rotation.

Components tested for range of motion were: Trident 32, 36, 40 and 44mm Internal Diameter; Hemispheric 28 and 32mm Internal Diameter cups; MITCH TRH MoM Monoblock Resurfacing Cup (Stryker EMEA, Montreux, Switzerland) 46mm cup bearing diameter with a 2.75mm offset bore; Dual Mobility 40, 46 and 58mm cups. The metal on metal monoblock cups had a very high range of motion but a 48mm head has only a similar jump distance to a hemispheric 36mm design. The designs with the cylinder and chamfer have a markedly higher jump distance than their hemispheric equivalents but slightly reduced ROM. Interestingly, the dual mobility design has almost double the jump distance of an equivalently sized metal on metal resurfacing type cup and a higher jump distance than an equivalent head size in a conventional unipolar design. The dual mobility design has similar ROM to a 40mm head in the hemisphere with cylinder and chamfer design. ROM is slightly higher in the hemispheric and sub-hemispheric designs but this model does not take into account bony or soft tissue impingement. The role of design of ace-tabular component has a great effect on the range of motion and jump distance of bearings.

Introduction: Clinical experience has shown that addressing variations in bone morphology is important in the development of successful hip implant designs. Numerous studies of femoral bone morphology have been published utilizing various techniques. This study has developed a method which consistently measures large quantities of 3-dimensional digital femura geometry segmented from computed tomography (CT) scans and can accurately make anatomical measurements from these images

Methods: CT images of left femora on five hundred fifty six left femura (57% male, 43% female), consisting of 69% Caucasian, 16% Asian and 14% unknown were analyzed. The average age was 66 years, ranging from 40 to 93 years. Segmentation of the outer cortical, inner cortical, and marrow boundaries were consistently performed over all CT scans. The positions identified on the reference bone are transformed to the equivalent position on the clinical bone images, from which the dimensional data is extracted and stored. The mediolateral width (MLW), medial offset (MO) and lateral offset (LO) were measured in 10mm increments, ranging from 20mm above the lesser trochanter (LT) to 130mm below the lesser trochanter. The canal flare index was defined as a ratio of the mediolateral width at a section 20mm above the lesser trochanter to the mediolateral width at the isthmus level.

Results: The mean mediolateral width at 20mm above the lesser trochanter was 47.0 ± 4.5 (35.1–61.8; n=556). Noble reported 45.4 ± 5.3 (31.0–60.0; n=200), Husmann reported in a neck oriented study 46.3 ± 6.9 (27.6–63.6; n=310) and Laine reported 47.1 ± 4.9 (n=50). The mean medial offset at a section 20mm above the lesser trochanter was 25.1 ± 2.9 (16.7–33.4). In the study by Husmann, a mean of 25.0 ± 5.2 (9.4–45.5) was reported. The mean canal flare index was 4.49 ±.8. Noble reported a mean canal flare index of 3.80 ±.074, Husmann 3.81 ±.83 and Laine 4.3 ±.93.

Discussion: In general, the study showed minor differences to published data of proximal bone morphology. However, this more powerful study has shown that there is a higher mean canal flare index than determined by Noble and a similar mean canal flare index as determined by Laine. As reported by Laine, the canal flare index varies significantly with the placement of measurements in the canal. In this study the measurements were performed in a plane oriented by the femoral neck as a hip stem would be placed. The CFI over the isthmus width showed a greater correlation than previously shown by Noble. The novel software tool allows for anatomical measurements that can be applied to an unlimited population size enabling further applications and studies.

The introduction of highly crosslinked PE with improved wear performance has allowed for the marketing of thin liners. Previous studies have shown that steep angles reduce femoral head coverage thereby decreasing contact area and can subject the acetabular rim to excessive stresses. This can be especially concerning for thinner PE constructs. Previous work with thicker (9.9mm) non-crosslinked PE show a correlation of decreased wear with increased abduction angle. Therefore, the objective of this study was to isolate and examine the effects of varying cup abduction angles on the wear of a thin second generation highly crosslinked polyethylene. Five sets of sequentially crosslinked Trident^® design inserts with a wall thickness of 3.9mm were evaluated. Sequentially crosslinked liners were machined from compression molded GUR1020 UHMWPE that had been γ-irradiated followed by annealing 3 times (X3). Testing was conducted using a hip joint simulator for 3 million cycles. All cups were fixed, positioned superiorly at a neutral version angle, and divided into five groups of varying inclination angles: 0°, 20°, 30°, 50° and 70°. A physiological load was applied to each couple at a rate of 1Hz using Alpha Calf Fraction serum. Weight was converted to volume and plotted as a function of cycle count. In addition, all PE inserts were microscopically analyzed for any gross damage and areas of deformation. Wear rates plotted against inclination angle exhibited poor correlation between wear rate and angle (R2=0.253). Student’s t-tests revealed significant differences (p< 0.05) between 0° and 70°, and between 50° and 70° angles. There was no statistical differences for any of the other tested angles. Visual inspection of the tested liners revealed wear scars of increased areas of polishing on inserts positioned at lower abduction angles. No deformation, cracking or pitting of the liners was observed. Visual inspection of the liners revealed an increase in overall area of polishing with a reduction in abduction angle. This indicates that load is concentrated over a smaller area for higher angles resulting in increased contact stress for steeper cups; however, this did not translate into a correlation of high abduction angle and high wear. These results do not correlate with our previous work, however that study was conducted on smaller diameter thicker non-highly crosslinked material. We believe the difference in results is due to fundamental material response. Although visual burnishing indicates a trend in contact area, there may be a role of deformation in the results. Future work will involve finite element analysis to study these differences. The results in this study suggests that the sequentially crosslinked polyethylene is able to maintain its low wear characteristics at various abduction angles even with a thin (3.9 mm) liner.

Steep angles (> 55°) reduce femoral head coverage decreasing contact area and can subject the acetabular rim to excessive stresses. In the case of metal-metal implants it has been shown that at steep angles there is no bedding-in of the implants and run-away wear occurs. The dual mobility bearing concept mates a metal femoral head with a polyethylene liner that is free to articulate inside a polished metal shell. Previous work has shown acetabular wear can be minimized with this design, possibly through reduction of total amount of cross-shear motion in the joint. An additional potential benefit may exist through the maintenance of conforming contact and head coverage even under high inclination angle. This study evaluates the influence of inclination angle on the wear performance of three hip bearing designs. Four sets of dual mobility implants, three sets of metal-on-metal hip implants, and five sets of fixed hip implants were evaluated per inclination angle. All polyethylene components were made of GUR 1020 UHMWPE that was sequentially crosslinked and annealed three times (X3). The MoM components were fabricated from high carbon cast CoCr as per ASTM F75 (no heat treatment). A hip joint simulator was used for testing for a total of 2.5 million cycles with the cups oriented at either 35° or 65° of abduction. Testing was run at 1Hz following Paul curve physiologic loading and statistical analysis was performed using the Student’s t-test (p< 0.05). results for the 35 degrees of inclination angle condition show no statistical difference between any of the testing combinations with X3 polyethylene showing immeasurable wear. At this angle wear of the MoM devices was similar, although ion levels were not measured. results for the 65 degree condition showed an increase for the fixed PE and MoM systems. The increase in fixed PE bearing wear is consistent with previous findings and still within noise level values. The increase in MoM wear was substantial with both heads and cups showing scratches and abrasion damage related to edge contact. There is a statistically significant wear rate reduction (p< 0.05) of over 94% for both the dual mobility and fixed bearing PE constructs when compared to MoM. When comparing wear rates of the dual mobility system to the standard fixed acetabular bearing, the dual mobility device exhibited an 85% (p< 0.05) reduction in wear rate. The results of this study support our hypothesis that acetabular wear at high angles can be diminished through design. This is likely due to maintenance of the nature of the primary inner bearing contact regardless of shell positioning. Based on these results this dual mobility construct can be expected to outperform a conventional fixed construct and a metal-on-metal construct in terms of wear at high inclination angles, without any of the metal ion release concerns.

A number of densitometry studies have reported dramatic density losses in the acetabular region after uncemented Total Hip Arthroplasty (THA)1,2. However the mechanical implication of such loss is not yet known. This study aims to perform a mechanical analysis with patient specific Finite Element (FE) models to find out how the stress distribution affects the Bone Mineral Density (BMD) changes after uncemented THA.

An existing patient CT dataset collected for a densitometry study was used to generate patient-specific FE models with a previously validated FE mesh generation method3. Boundary and loading conditions included the hip joint force and the forces of 21 muscles attached to the pelvic bone at eight characteristic phases of a gait cycle 4. Tensile and compressive components of principal stresses were calculated after each simulation.

In general, both compressive and tensile principal stresses decreased after uncemented THA but the magnitude of decrease for tensile stresses was much greater than compressive stresses. The changes in tensile stresses were matched with BMD loss patterns. In particular, the densitometry study revealed that areas dorsal to the prosthesis lost more bone density than areas ventral to the prosthesis1. The stress distribution pattern showed that such areas experienced high tensile stress initially and then a dramatic decrease in their magnitude while their compressive stresses remained relatively unchanged. On the other hand, the regions where BMD was maintained - the areas superior to the cup - experienced high compressive stresses initially, which remained relatively high three years after the surgery.

Although it is a result from one patient, results suggest that changes to tensile and compressive stresses might influence BMD differently after uncemented THA. Our hypothesis is that regions with high tensile stress experience bone loss while BMD of the regions with high compressive stress are maintained. More patient datasets are being processed to test this hypothesis. Findings from this study can explain the phenomena of retroacetabular osteolysis, late migration and implant failure of press-fit cups observed in long-term clinical studies.

In recent years, some attempts have been made to develop a method that generates finite element (FE) models of the femur and pelvis using CT. However, due to the complex bone geometry, most of these methods require an excessive amount of CT radiation dosage. Here we describe a method for generating accurate patient-specific FE models of the total hip using a small number of CT scans in order to reduce radiation exposure.

A previously reported method for autogenerating patient-specific FE models of the femur was extended to include the pelvis. CT osteodensitometry was performed on 3 patients who had hip replacement surgery and patient-specific FE models of the total hip were generated. The pelvis was generated with a new technique that incorporated a mesh morphing method called ‘host mesh fitting’. It used an existing generic mesh and then morphed it to reflect the patient specific geometry. This can be used to morph the whole pelvis, but our patient dataset was limited to the acetabulum. An algorithm was developed that automated all the procedures involved in the fitting process.

Average error between the fitted mesh and patient specific data sets for the femur was less than 1mm. The error for the pelvis was about 2.5mm. This was when a total 18 CT scans with 10mm gap were used – 12 of the femur, and 6 of the pelvis. There was no element distortion and a smooth element surface was achieved.

Previously, we reported a new method for automatically generating a FE model of the femur with as few CT scans as possible. Here we describe a technique that customizes a generic pelvis mesh to patient-specific data sets. Thus we have developed a novel hybrid technique which can generate an accurate FE model of the total hip using significantly less CT scans.

An automated method of generating FE models for the total hip with reduced CT radiation exposure will be a valuable clinical tool for surgeons.

In recent years, some attempts have been made to develop a method that generates finite element (FE) models of the femur and pelvis using CT. However, due to the complex bone geometry, most of these methods require an excessive amount of CT radiation dosage. Here we describe a method for generating accurate patient-specific FE models of the total hip using a small number of CT scans in order to reduce radiation exposure.

A previously reported method for autogenerating patient-specific FE models of the femur was extended to include the pelvis. CT osteodensitometry was performed on 3 patients who had hip replacement surgery and patient-specific FE models of the total hip were generated. The pelvis was generated with a new technique that incorporated a mesh morphing method called ‘host mesh fitting’. It used an existing generic mesh and then morphed it to reflect the patient specific geometry. This can be used to morph the whole pelvis, but our patient dataset was limited to the acetabulum. An algorithm was developed that automated all the procedures involved in the fitting process.

Average error between the fitted mesh and patient specific data sets for the femur was less than 1mm. The error for the pelvis was about 2.5mm. This was when a total 18 CT scans with 10mm gap were used – 12 of the femur, and 6 of the pelvis. There was no element distortion and a smooth element surface was achieved.

Previously, we reported a new method for automatically generating a FE model of the femur with as few CT scans as possible. Here we describe a technique that customizes a generic pelvis mesh to patient-specific data sets. Thus we have developed a novel hybrid technique which can generate an accurate FE model of the total hip using significantly less CT scans.

Kinematic evaluation of the knee after total joint arthroplasty plays an important role to analyze and understand the post operative outcome of the surgical procedure. The objective of the study was to quantify in vivo kinematics of two different knee designs (dual radius, single radius) by combining video fluoroscopy and helical axis of motion analysis.

3D position of the finite helical axis (FHA) of the displacement of the tibial component of the prosthesis relative the femoral component during a knee extension from 55° to 20° flexion underweight bearing conditions was computed. The motion data were extracted from in vivo fluoroscopy measurement. Angular deviations as angles between each FHA and the mediolateral axis of the femoral component of the prosthesis, and the localization deviation as the distance between each FHA and the center of the femoral component of the prosthesis were calculated. The median and the interquartile range (IQR) of the angular deviation and the localization deviation were computed. Non-parametric Wilcoxon test compared the values of the two designs.

The angular and localization deviations of the dual radius design were bigger than of the single radius design. Median localization deviation, IQR Angle deviation, IQR localization deviation showed highly significant differences between the two designs (p< 0.01).

Compared to the dual radius design the single radius design modified the knee kinematics in vivo. Since it is asingle axis design FHA is therefore concentrated near this unique single axis. On the contrary the dual radius design has two axes, and the FHA floated between these two axes.

Introduction and Aims: The extremely low wear rates of third generation alumina-alumina bearings in traditional hip simulators are not reflected in vivo. Separation of the bearing during swing phase and edge loading with heel strike is reported to account for this discrepancy.

Method: We have had the opportunity to visually inspect 21 bearings at re-operation from a group of 1588 hip arthroplasties with third generation alumina ceramic-ceramic bearings. Re-operations were for heterotopic ossification (one), loosening (three), femoral fracture (six), psoas tendonitis (six), sepsis (three) and dislocation (two). There were no re-operations for bearing failure. Sixteen of these 21 bearings (16 heads and 12 inserts) were retrieved and analysed. We mapped the location and we measured the volume of the wear and we performed microscopy and measured roughness of worn and unworn areas.

Results: Eleven bearings had visual evidence of edge loading wear, making an incidence of 52% in the 21 patients having re-operations. These 11 bearings and five visually undamaged bearings were analysed. The wear on the insert was always located at the rim indicating edge loading. The location and orientation of the stripe on the head was not consistent with subluxation during normal gait but was consistent with subluxation and edge loading with the hip flexed at 90 degrees. The average wear volume was 0.7mm3 per year (heads plus liners). Longer service bearings had signs under SEM of repolishing of the wear area suggesting that the process of edge loading wear will be self-limiting. The heads without a wear scar showed very little damage: under SEM, a slight relief polishing of individual grains and minor pitting was noted.

Conclusion: The subluxation causing the stripe wear in these patients did not occur during normal walking gait. It probably occurred with rising from a chair. Simulator testing of third generation alumina-alumina components must include edge loading if it is to give a realistic indication of in vivo performance. Alumina-alumina bearings remain an excellent option for total hip arthroplasty, however more work is required to understand the clinical consequences.

Sagittal knee implant design, together with soft tissue and alignment, determines the kinematics of an artificial knee joint. A single-radius design was thought to improve the kinematics and biomechanics of a knee joint prosthesis and therefore also improve rehabilitation. Two total knee joint prosthesis designs, differing only in their sagittal geometry, were compared in vivo.

To determine the three-dimensional kinematics and difference between a multi-radius and single-radius implants, six patients, all one-year postoperative, were subjected to video-fluoroscopy while walking on a treadmill, stepping up and down a 20-cm step and doing deep lunges.

In a clinical evaluation, differences in range of motion, functional knee score, 40-cm chair raise and anterior pain at 6 weeks and 3, 6 and 12 months were compared in 86 patients with multi-radius and 108 patients with single-radius implants. The age of the patients in the two groups was similar and ranged from 68 to 70 years.

Fluoroscopically-determined flexion was 105° in the multi-radius group and 123° in the single-radius group (p < 0.01). External rotation and lateral condyle movement was statistically similar. The single-radius group did not exhibit paradoxical motion of the medial condyle and had less overall movement. The objective knee scores did not differ significantly (p > 0.05). Patients in the single-radius group gained flexion significantly faster (p < 0.001). After one year, there was no difference between the groups. Three months postoperatively, 72% of the single-radius group could rise from a chair without using their arms, compared to 40% of the multi-radius group (p < 0.001). Although this improved in both groups, it remained superior in the single-radius group. Anterior knee pain was present in 59% of the multi-radius group and in only 18% of the single-radius group at three months (p < 0.001). At one-year follow-up, 4% of the single-radius and 29% of the multi-radius groups respectively complained of anterior knee pain (p < 0.001).

A single-radius sagittal design knee prosthesis leads to faster rehabilitation better and kinematics than a multi-radius design. The reduced movement of the condyles on the polyethylene insert should result in less long-term wear.

Introduction: Alumina exhibits excellent hardness and wear properties, however it is a brittle material with an inherent risk of fracture. Therefore, the feasibility of a new family of Alumina based ceramics with improved toughness for hip joint articulation applications was investigated.

Materials and methods: The addition of 25% Zirconia to Alumina during the manufacturing process to achieve the objective has been proposed. Two types of Zirconia Toughened Alumina (ZTA) ceramics were analysed; one binary and the other pentary by composition. Following tests were used: structural analysis, mechanical testing of components, determination of hardness (HV), fl exural strength (ASTM C1161), indentation fracture toughness, X-ray diffraction (XRD), aging (accelerated and real-time) and wear simulator testing. The test data was analysed by descriptive statistics.

Results: The structure of the two ZTAs is similar with small-grained Zirconia dispersed in a matrix of larger grained Alumina. X-ray diffraction analysis showed no phase transformation after accelerated and real-time aging and the strength values did not change. Flexural strength was statistically significant increased by > 50% over Alumina. The indentation fracture toughness was also increased by up to 50% while the hardness of the ZTA ceramics was not affected. The wear testing showed that ZTA – ZTA couples articulating against themselves produce not significant lower wear than Alumina – Alumina couples, but the combination of ZTA ball-heads with Alumina inserts produced significantly lower wear rates, also in micro-separation.

Conclusions: The toughness and bending strength of the Alumina was successfully increased while all other properties of the Alumina were maintained. No change in properties after aging was observed and the wear properties of the ZTA were lower wear than for Alumina. Zirconia Toughened Alumina looks promising for the next generation of fracture and wear resistant ceramic bearings.

The reported revision rate of total hip arthroplasties (THAs) due to wear and osteolysis is around 10% at 10 years. However, the actual rate is probably higher: the incidence of osteolysis is reported to be 10% to 45%. Apart from design improvements, improved or new materials and/or and combinations are important in reducing particle-induced osteolysis, especially in young and active patients.

Wear reduction of up to 40% after inert gas sterilisation of polyethylene (PE) has been demonstrated, both in vitro and in vivo. An effective means of providing further increases in wear resistance is to cross-link PE extensively. Early clinical results of non-melt-annealed PE at three years showed wear reduction of up to 85% compared to inert gas radiation-sterilised PE.

In hip joint simulator investigations, bearings with a ceramic ball-head articulating against a composite cup demonstrated wear rates similar to those of ceramic-ceramic bearings. The wear particles are benign. Clinical data collected over two years suggest no disadvantages compared to the standard articulation controls.

The wear resistance of alumina-alumina articulation has been enhanced. In-vitro investigation demonstrated that even with a cup inclination of 60° the wear rate is not increased. The effect of micro-separation of the artificial joint is also minimised. Several prospective multi-centre alumina-alumina studies have shown no additional complications with this articulation. However, alumina is a brittle material with an inherent risk of fracture. The addition of 25% zirconia to alumina (ZTA) in the manufacturing process improves its fracture resistance, increasing its strength by more than 50%, while maintaining its other properties. The wear properties of ZTA are even better than that of alumina, especially in micro-separation articulation mode.

Highly cross-linked and optimised PE and composite technology are promising concepts in address wear particle-induced osteolysis.

Aims: The purpose of this study was to assess the effect of changes in peripheral attachment on stresses and displacements at the liner-shell interface. Methods: Three dimensional þnite element models were constructed of two acetabular cup designs for a liner with a 32 mm inner diameter, a liner thickness of 5 mm, and a shell thickness of 4 mm. An additional set of models was constructed with a 28 mm head diameter, corresponding to a liner thickness of 7 mm. 16 sequential quasistatic loading steps were used to describe the stance phase of a patientñs gait cycle. Results: Changes in the design had a larger inßuence on the backside relative motion during the gait cycle than load magnitude. However, changes in the design had a smaller inßuence on the backside contact stress, von Mises stress, or radial extrusion into screw holes. Reduction in head size from 32 to 28 mm diameter resulted in a slight decrease in screw hole extrusion. Conclusions: In this study, changes in the acetabular cup design, including screw hole placement and increased peripheral interlocking, were shown to decrease relative motion at the liner-shell interface, but the peak liner-shell contact stresses, backside von Mises stresses, and radial screw hole extrusion were less signiþcantly changed.

Aims: The purpose of this study was to assess the effect of gaps between the polyethylene liners and the metal acetabular shells used in two generations of acetabular component design. Methods: Finite element models were developed for two generations of acetabular component. The three variables assessed were: design (Mark I versus II); liner thickness (5, 8, and 11 mm); and gap size (0., 0.1, and 0.3 mm). 16 sequential quasistatic loading steps, coupled with ßexion/extension of the femoral head, were used to describe the stance phase of a patientñs gait cycle. Results: Gaps of less than 0.1 mm between the acetabular liner and the supporting metal shell will close under loading typical for gait, but only for smaller Ðsized acetabular cups. A gap of 0.1 mm seems to be at the edge of the range where rim loading, versus dome loading, occurs. Gaps of 0.1 to 0.3 mm between a polyethylene acetabular liner and the metal shell can produce ßuid pumping of approximately 100 to 250 microliters in each gait cycle. Conclusions: The changes from the 1st to the 2nd generation of this acetabular component led to important advantages. Indeed, due to an improvement of the liner conformity and the locking mechanism, backside micromotion, ßuid volume displaced, liner stresses and liner-shell contact stresses were strongly diminished.

Fluid pressure generated in the hip during activity has been implicated in component loosening. Animal studies show both the adverse effect of direct pressure on osteocytes and the resorption of bone subjected to cyclic loads. Pressure fluctuation measured in contained pelvic osteolytic lesions during manipulation of the hip at revision surgery suggests cyclic pressure may have a direct effect on bone resorption leading to pelvic osteolysis.

To determine the cause of pressure fluctuation in pelvic structures supporting a hip implant, we conducted an experimental and numerical analysis of relative motion at modular interfaces of acetabular cups as load was applied and removed.

We showed that for polyethylene bearing inserts supported primarily at the rim, the application of cyclic load caused cyclic motion between the insert and the inside surface of the acetabular shell. In a fluid environment, this motion can generate cyclic pressure pulses that may be applied to bone directly through the holes in the shell provided for screw fixation.

We conclude that motion at modular interfaces of acetabular components may contribute to pelvic osteolysis. Our hypothesis is that the motion of a bearing liner under cyclic load can produce fluctuating pressure pulses that are applied to bone directly through screw holes. In addition, the pulses may aid the transport of polyethylene wear debris particles into fixation interfaces. It is possible that lytic lesions previously associated with backside wear of the liner may be related to pumping of joint fluid by the liner.

Late aseptic loosening of total hip arthroplasty (THA) components due to wear debris especially sub-micron Polyethylene induced osteolysis has been identified to be the major cause for revision. Therefore, the use of wear resistant designs and materials is imperative for the long-term success of articulating implants. One of the most promising articulations for THA regarding extremely low wear is the hard-on-hard Alumina/Alumina combination with a long history in orthopaedics accumulating to now 30 years of experience.

Alumina Ceramic: Aluminum-Oxide Ceramic (Alumina, Al2O3, ISO 6474) is an extremely hard material that can only be scratched and also machined and polished by diamond. It has excellent mechanical properties but is brittle as are most ceramic materials and can, therefore, not be used for highly stressed implants like an artificial hip joint stem. Due to its ionic structure it is hydrophilic allowing liquids to bound to and lubricate its surface. Alumina’s hardness allows a polishing to a low roughness, which is entirely inverse and therefore able to create lubricating “pools.”

Density, purity and grain size in combination with an optimised manufacturing process are crucial for the final properties of Alumina and have been sub-optimal at the early pioneering times. Also the design has to be adapted to the critical properties of this ceramic.

The biological activity of Alumina is graded as bio-inert and no direct osteointegration is to be expected, also proven by clinical experience. Alpha Alumina is bio-stable and practically insoluble in the body environment. Therefore, ageing or any systemic reaction in the human body with this ceramic is of no concern.

First Generation Alumina Ceramic: Boutin has introduced Alumina ceramic components for articulation with itself for THA in 1970, followed by Mittelmeier and other surgeons. First reports of its combination with UHMWPE cups date back to 1972. More than one million Alumina heads have been worldwide implanted since then. Problems regarding fracture of the head were mostly encountered with collared heads of one design and the cup (especially if placed at > 50 deg abduction position). Run-away wear in case of edge contact have been reported with these first generation implants.

A comprehensive retrieval study using a systematic analysis of two different designs confirmed that Alumina/ Alumina couples have a low clinical wear rate and identified the main risk factors that can lead to early failure of such devices. The wear rate of those historic Alumina / Alumina articulations is in the range of less than 5 microns per year. This is one to two orders of magnitude less than for any articulation with Polyethylene cups. Reaction to Alumina wear particles, mostly encountered on catastrophic failure of the historic implant designs is sparse and mostly benign.

Third Generation Alumina Ceramic: Significant improvements in material properties and quality have been made since its introduction. The third generation of Alumina has been introduced in 1994 and was a further evolution regarding material properties, manufacturing, quality control and design. Refinement of purity, grain size and manufacturing results in improved fracture and also wear resistance, which decreased from low to almost immeasurable values. With the evolution of this new generation ceramic the problems of the past have been successfully addressed. Following improvements are characteristic for a third generation Alumina:

Improved purity and reduced grain size (Figure 1) for better properties

These improvements result in a significant increase in mechanical properties. The risk of head fracture has been reduced for this 3rd generation Alumina ceramic heads from 1 per 500 (0.2%) to 1 per 25 000 (0.025%).

The wear resistance of the couple Alumina/Alumina has also been enhanced which was measured in simulator tests to be around 1 micron per year. Tribological investigation involved a series of screening, pendulum and anatomical hip simulator tests with actual Alumina/ Alumina components in respect to the effect of clearance and cup angle (45° & 60°) in a series of tests for up to 5 million cycles. Adverse testing conditions for Alumina e.g. dilution of lubricant, dry and water, high load in swing phase, stop-start, etc in ascending aggressiveness (each at 1 million cycles) have also been investigated. No significant difference in wear volume was found comparing clearance or cup angle for all components tested. A new simulator test set up using a microseparation mode during every single cycle was also run for 5 million cycles combining Alumina from one single manufacturer and also mixing Alumina’s from different manufacturers. The wear in all cases was low and lower than for the first generation Alumina’s.

A series of implantations with Alumina/Alumina articulation has been performed since November 1996 according to a prospective multicentric FDA IDE protocol comparing the same implant with CoCr metal heads/PE combinations. Short-term results demonstrate no early complications with this third generation Alumina/Alumina articulation if implanted correctly. The benefit of the dramatically reduced wear rate will show only after longer-term follow-up.

Conclusion: Alumina/Alumina articulation demonstrates the lowest wear rate of all available material combinations for THA in the laboratory as well as in clinical praxis. The bulk material as well as its particles is bio-inert, therefore, giving no concern for adverse biological reactions.

Problems with earlier designs of Alumina/Alumina articulation have been successfully addressed by taking the identified risk parameters into consideration. Components of the third generation Alumina ceramic and design have been extensively tested. All components pass the mechanical tests. Alumina heads and liners of the third generation in the size and under the conditions tested are safe and efficient. Their wear rate even under the influence of adverse condition is minimal.

Further clinical trials parallel the application of this superior articulation couple in an innovative and modern design.Alumina/Alumina is, therefore, the material combination of choice for the active patient with high life expectancy.