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Orthopaedic Proceedings
Vol. 102-B, Issue SUPP_2 | Pages 90 - 90
1 Feb 2020
Haeussler K Pandorf T
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Introduction

Lipped liners have the potential to decrease the rate of revision for instability after total hip replacement since they increase the jumping distance in the direction of the lip. However, the elevated lip also may reduce the Range of Motion and may lead to early impingement of the femoral stem on the liner. It is unclear whether the use of a lipped liner has an impact on the level of lever-out moments or the contact stresses. Therefore, the aim of the current study was to calculate these values for lipped liners and compare these results to a conventional liner geometry.

Materials and Methods

3D Finite Element studies were conducted comparing a ceramic lipped liner prototype and a ceramic conventional liner both made from BIOLOX®delta. The bearing diameter was 36 mm. To apply loading, a test taper made of titanium alloy was bonded to a femoral head, also made from BIOLOX®delta. Titanium was modeled with a bilinear isotropic hardening law. For the bearing contact a coefficient of friction of both 0.09 or 0.3 was assumed to model a well and poorly lubricated system. Frictionless contact was modeled between taper and liner. Pre-load was varied between 500 N and 1500 N and applied along the taper axis. While keeping pre-load constant, lever-out force was applied perpendicular to the taper axis until subluxation occurred. Liners were fixed at the taper region. Lever-out moment, equivalent plastic strain and von Mises stress of the taper, bearing contact area and contact area between taper and liner was evaluated.


Orthopaedic Proceedings
Vol. 102-B, Issue SUPP_2 | Pages 89 - 89
1 Feb 2020
Haeussler K Pandorf T
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Introduction

The process of wear and corrosion at the head-neck junction of a total hip replacement is initiated when the femoral head and stem are joined together during surgery. To date, the effects of the surface topography of the femoral head and metal stem on the contact mechanics during assembly and thus on tribology and fretting corrosion during service life of the implant are not well understood. Therefore, the objective of this study was to investigate the influence of the surface topography of the metal stem taper on contact mechanics and wear during assembly of the head-neck junction using Finite Element models.

Materials and Methods

2D axisymmetric Finite Element models were developed consisting of a simplified head-neck junction incorporating the surface topography of a threaded stem taper to investigate axial assembly with 1 kN. Subsequently, a base model and three modifications of the base model in terms of profile peak height and plateau width of the stem taper topography and femoral head taper angle were calculated. To account for the wear process during assembly a law based on the Archard equation was implemented. Femoral head was modeled as ceramic (linear-elastic), taper material was either modeled as titanium, stainless steel or cobalt-chromium (all elastic-plastic). Wear volume, contact area, taper subsidence, equivalent plastic strain, von Mises stress, engagement length and crevice width was analyzed.


Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_4 | Pages 15 - 15
1 Apr 2019
Pandorf T Preuss R
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Introduction

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®delta, CeramTec, Plochingen, Germany) in a clinical safety study. Even though conventional CoC hip systems are known for their excellent wear behavior, it has to be ensured that intraoperative and in-vivo deformations of the ceramic acetabular cup do not infringe the proper functionality of the system. The method of determining the minimum clearance of such a system will be presented here.

Materials and Methods

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.


Orthopaedic Proceedings
Vol. 101-B, Issue SUPP_4 | Pages 29 - 29
1 Apr 2019
Haeussler K Haefner L Butenschoen L Pandorf T
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Introduction

Hip stem taper wear and corrosion is a multifactorial process involving mechanical, chemical and biological damage modes. For the most cases it seems likely that the mechanically driven fretting wear is accompanied by other damage modes like pitting corrosion, galvanic corrosion or metal transfer. Recent retrieval studies have reported that the taper surface topography may affect taper damage resulting from fretting and corrosion [1]. Therefore, the current study aimed to examine effects of different taper topography parameters and material combinations on taper mechanics and results regarding wear and corrosion have been investigated.

Materials and Methods

Combined experimental and numerical studies were conducted using titanium, cobalt-chromium and stainless steel generic tapers (Figure1). Uniaxial tensile tests were performed to determine the mechanical properties of the materials examined. For the taper studies macro-geometry of ceramic ball heads (BIOLOX®delta) and tapers were characterized using a coordinate measuring machine, and assembly experiments according to ISO7206-10 were conducted up to 4kN. Before and after loading, taper subsidence was quantified by assembly height measurements. Taper micro-geometry, taper surface deformation, and contact area were determined by profilometry. Initial numerical studies determined coefficients of friction for the three material combinations. Macro- and micro-geometries of the tapers were modelled, and taper subsidence and assembly load served as boundary conditions. Further studies used simplified models to examine effects of varying profile depths and angular gaps on surface deformation, taper subsidence, contact area, engagement length and pull-off force.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_6 | Pages 61 - 61
1 Apr 2018
Upmann C Eisele M Pandorf T
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Introduction

Ceramic ball heads are well known in hip arthroplasty for their superior tribology performance and high burst strength. To assess the ball head performance and the in-vivo fracture risk Pandorf et al 2008 examined the burst strength of BIOLOX®forte (pure aluminium oxide ceramic, CeramTec GmbH, Plochingen, Germany) ball heads on clean standard test tapers and contaminated test tapers. They found that fat tissue and scratches are reducing the burst strength to 40% and to 20% of the reference burst strength, respectively. The aim of this work is to investigate if BIOLOX®delta (alumina matrix ceramic, CeramTec GmbH, Plochingen, Germany) ball heads show a similar behaviour as BIOLOX®forte ball heads with respect to taper contamination.

Materials and Methods

Each test series consisted of n=5 BIOLOX®delta 28–12/14 L ball heads and n=5 metal test tapers (Ti-6Al-4V, ISO 5832-3). For the reference series the metal tapers remained untouched representing the CeramTec standard test procedure. For the fluid series the ball heads were filled up with tap water or calf blood serum. For the solid series the metal test tapers were contaminated with small particles of bovine bone, commercially available bone cement and porcine fat tissue in the engagement zone. A chisel and a slight hammer tap were used to scratch the proximal region of the metal test taper. The ball heads were then manually attached to the contaminated metal test tapers without further force appliances. An apparatus according to ISO 7206-10 was used for burst testing. The tests were performed at CeramTec in-house test laboratory.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_6 | Pages 36 - 36
1 Apr 2018
Häußler K Häfner L Pandorf T
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Introduction

Frictional behavior and, therefore, the coefficient of friction (CoF) play an important role in the evolution of fretting wear. Several studies investigated fretting at the ball head-taper junction with a remarkable variation in the CoF (0.15 to 0.55). This may be due to different material couplings, surface topographies or macro-geometries. Since the results of Finite Element (FE) models are strongly dependent on the choice of CoF it is crucial to determine the correct CoF for a speci?c system. Therefore, this study aimed to determine the CoF for the interface between ceramic ball heads and metal tapers.

Materials and Methods

Three groups of taper-ball head couplings were investigated (n=18 titanium (Ti), n=18 cobalt chromium (CoCr), n=18 steel tapers (SS)). Line profiles of the taper surfaces were measured and tapers and ball heads were assembled using different loads (2, 4, 6 kN). Tapers were disassembled from ball heads by using liquid nitrogen, surface topography was remeasured and the effective contact area was determined. Another set of measurements was conducted (n=5 tapers per taper material) to measure the contact pressure. Here, pressure sensitive films were placed between tapers and ball heads during assembly. Using the effective contact area and contact pressure the CoF was calculated.


Orthopaedic Proceedings
Vol. 100-B, Issue SUPP_5 | Pages 85 - 85
1 Apr 2018
Flohr M Freutel M Pandorf T
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Introduction

The successful performance of ceramic on ceramic bearings in today's THA can mainly be addressed to the excellent tribological behaviour and the minimal wear of ceramic bearings. The clearance between head and shell plays a major role in this functionality of artificial hip joints. Knowledge about the deformation behaviour of the shell during implantation but also under daily loads is essential to be able to define a minimum clearance of the system. The aim of this work is to establish a tool for determining maximum ceramic shell deformation in order to predict minimum necessary clearance between heads and monolithic ceramic shells.

Materials and Methods

In order to determine the minimum clearance the following in vivo, in vitro and in silico tests were taken into account:

Eight generic metal shells were implanted into cadaveric pelvises of good quality bone realizing an underreaming of 1 mm. Maximum deformation of the metal shells (um) after implantation were determined using an validated optical system. The deformations were measured 10 min. after implantation.

The stiffnesses of the metal shells (Cm) were experimentally determined within a two-point-loading frame acc. to ISO 7206-12.

The stiffness of a monolithic ceramic shell (Cc) representing common shell designs (outer diameter 46 mm, 3 mm constant wall thickness) were determined acc. to ISO 7206-12 using Finite-Element-Method (FEM).

Maximum deformation for the ceramic shells (uc,dl) under daily loading, represented by jogging (5kN, Bergmann et. al), was determined applying FEM.

Press-fit forces (Fpf = umCm) can be calculated with the results of test 1 and 2 considering linear elastic material behaviour. Assuming force equilibrium and applying the evaluated stiffness from test 3 the deformation of the ceramic shell (uc) occurring after implantation can be estimated (uc = umCm/Cc). For minimum clearance calculation of a monolithic ceramic shell (uc,lt) in vivo deformation (uc,dl) has to be considered additionally (uc,lt = uc + uc,dl).


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_12 | Pages 12 - 12
1 Jun 2017
Holland J Deehan D Manning W Pandorf T
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Modular un-cemented acetabular components are used in over 50% of UK hip replacements. Mal-seating of hard liners has been reported as a cause of failure which may be a result of errors in assembly, but also could be affected by deformation of the acetabular shell on insertion. Little information exists on in vivo shell deformation. Previous work has confirmed the importance of shell diameter and thickness upon shell behaviour, but mostly using single measurements in models or cold cadavers.

Exploration of deformation and its relaxation over the first twenty minutes after implantation of eight generic metal cups at body temperature.

Using a previously validated cadaveric model at controlled physiological temperature with standardised surgical technique, we tested the null hypothesis that there was no consistency for time dependent or directional change in deformation for a standard metal shell inserted under controlled conditions into the hip joint. Eight custom made titanium alloy (TiAl6V4) cups were implanted into 4 cadavers (8 hips). Time dependent cup deformation was determined using the previously validated ATOS Triple Scan III (ATOS) optical measurement system. The pattern of change in the shape of the surgically implanted cup was measured at 3 time points after insertion.

We found consistency for quantitative and directional deformation of the shells. There was consistency for relaxation of the deformation with time. Immediate mean change in cup radius was 104μm (sd 32, range 67–153) relaxing to mean 96 μm (sd 32, range 63–150) after 10 minutes and mean 92 μm (sd 28, range 66–138) after 20 minutes.

This work shows the time dependent deformation and relaxation of acetabular titanium shells and may aid determining the optimal time for insertion of the inner liner at surgery.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_3 | Pages 121 - 121
1 Feb 2017
Flohr M Freutel M Halasch C Pandorf T Streicher R
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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.

Materials and Methods

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).


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_9 | Pages 108 - 108
1 May 2016
Pandorf T Preuss R Streicher R
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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.

METHODS

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.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_7 | Pages 115 - 115
1 May 2016
Dold P Pandorf T Flohr M Preuss R Bone M Holland J Deehan D
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INTRODUCTION

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.

METHODS

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.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_3 | Pages 124 - 124
1 Jan 2016
Porporati AA Pandorf T Flohr M Preuss R Streicher R
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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.

OBJECTIVES

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.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_3 | Pages 123 - 123
1 Jan 2016
Porporati AA Pandorf T Haeussler K Streicher R Preuss R Kaddick C
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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.

OBJECTIVES

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.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXXVII | Pages 229 - 229
1 Sep 2012
Masson B Pandorf T
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Introduction

In total hip arthroplasty ceramic on ceramic bearing couples are used more and more frequently and on a wordwide basis. The main reason of this choice is reduction of wear debris and osteolysis. The tribological properties and the mechanical behaviour of the implanted ceramic must remain the same throughout the patient's life.

The aim of this study was to evaluate the resistance of Alumina Matrix Composite to environmental degradation.

Material and method

The alumina matrix composite or BIOLOX ® delta is manufactured in Germany by CeramTec. It is made up of 80 vol.% Al2O3, 17 vol.% Yttria Stabilized ZrO2 and 3vol.% strontium aluminate platelets. The zirconia grains account for 1.3 mol.% of the Yttria content.

Accelerated aging tests in water steam at 142°C, 134°C, 121°C, and 105°C were performed to evaluate the aging kinetics of the composite.

X-ray diffraction was used to determine the monoclinic phase content on the material surface. Phase transformation is associated with weakness and increase in roughness of zirconia ceramic implants.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXXVII | Pages 316 - 316
1 Sep 2012
Pandorf T Preuss R Flohr M Upmann C
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Introduction

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.

Material and method

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”


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_IV | Pages 471 - 471
1 Nov 2011
Pandorf T Preuss R
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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.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_II | Pages 221 - 221
1 May 2011
Masson B Pandorf T
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The introduction of ceramics in total hip arthroplasty contributed significantly to the wear reduction of poly-ethylene and in consequence reduced osteolysis and loosening. This great benefit has been demonstrated in several clinical observations. In a recent study from Norway, the wear of a 28mm alumina and a CrCo ball head against Ultra High Molecular Weight Polyethylene (UHMWPE) after 10 years is compared using the RSA method of wear measurement.

It was concluded that the considerable reduced wear for ceramic ball heads in comparison to CrCo ball heads is a great advantage in hip arthroplasty.

A first prospective, randomized study with a 15 years follow up has been presented recently in the EFORT 2009. The comparison of wear of polyethylene between alumina and metal ball head shows a reduction of 44% penetration (linear wear) with the alumina-polyethylene bearing surface. In order to offer improved mechanical resistance and tribological qualities than alumina whilst maintaining structural stability, a new generation of alumina matrix composite (BIOLOX®delta) has been used in orthopedics since 2001. The topic of this study is to demonstrate the excellent wear performance of the alumina ceramic composite against polyethylene, compared to alumina/PE in vivo.

Methods: The BIOLOX®delta-PE bearing has been tested on a six station hip simulator (Endolob, Rosenheim) according to ISO/DIS 14242. The newborn calf serum was replaced every 0.5 million cycles and the test was stopped after 5 million cycles. Weight was measured using a high precision balance (Sartorius BP 211D)

Results and Discussion: After 5 million cycles, the insert surface appeared polished with fine scratching on the whole contact area. The wear rates calculated by linear interpolation were 13,52 mg per million cycles. (Standard deviation 0,60). The wear rate measured for BIOLOX®delta against UHMWPE was 13,52 mg per million cycles.

In general, the wear rate can be regarded as small compared to other hip simulator tests using ceramic against polyethylene couplings. When comparing the results for BIOLOX®forte on polyethylene with the same 28mm diameter and same testing parameter, we observed 26,57 +/− 3,55mg/million and 16,08+/−2,31 mg/million, respectively. The BIOLOX®delta on UHMWPE bearing shows improved wear behavior with a much lower wear rate.

Conclusion: This study demonstrates the very low in vitro wear of the Alumina ceramic composite on UHMPE compared to ball heads made of pure alumina. Based on this results and the clinical performance of the alumina-UHMPE bearing from the literature, we can expect a further reduction of wear for the BIOLOX®delta on UHMWPE in vivo that will increase the survival rate of the total hip arthroplasty.


Orthopaedic Proceedings
Vol. 92-B, Issue SUPP_I | Pages 98 - 98
1 Mar 2010
Pandorf T
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In almost all countries performing Total Hip Replacement (THR), dislocation is one of the major reasons for revision. Hence, in the last years the trend to larger bearings has been observed, following an improve in the bearing materials, the operation technique, and fixation techniques of stem and shell. Larger bearings allow for more range of motion and higher stability than conventional 28 mm bearing couples, leading to a better postoperative mobility. On the other hand, size limitations on the acetabular side are given by the anatomy of the human pelvic bone as well as the deformation and fracture behaviour of the used artificial materials. Therefore, the best solution to be achieved provides a maximum physiological outcome along with a minimised risk of intraoperative and in-vivo failures.

Investigating the wall thickness of the metal shell which is press-fitted in the human pelvic bone, the general trend towards a smaller wall thickness yielding an increased compliance can be observed with larger bearing diameters. This may lead to deformations of the metal shell making it difficult for the surgeon to properly introduce the insert. Hence, taking into account that a proper seating of the insert is absolutely necessary when using a ceramic insert in order to avoid point loads, operation time may strongly increase especially when minimal invasive surgery technique is used. With decreasing overall wall thickness of the acetabular components the volumetric stresses increase by definition. Therefore, an optimal component coupling between insert and metal shell is necessary in order to avoid point loads and resulting stress concentrations. With pre-assembled systems, this optimal coupling is reached by the force-controlled insertion of the insert in the metal shell without any prior deformation of the shell. This procedure enables to design acetabular components with a much lower overall wall thickness than conventional systems. As an example, in the case of the DELTA motion system, this overall wall thickness has been decreased to 5 mm allowing e.g. for a usage of a 36 mm bearing couple together with a 46 mm outer diameter of the metal shell. Additionally, the coating of the metal shell allows for direct bone ingrowth. Problems involved with larger bearing diameters may also arise from higher wear rates inducing possibly osteolysis and aseptic loosening. Investigations concerning the wear behaviour of large ceramic bearings have shown that there is no increase in the wear volume with increasing diameter.


Orthopaedic Proceedings
Vol. 92-B, Issue SUPP_I | Pages 176 - 176
1 Mar 2010
Pandorf T
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Modern Total Hip Replacement (THR) is in general one of the most successful surgical treatments although the functional requirements of modern patients are more and more demanding. Challenges arise from an extended life-span, a higher activity level requiring more sophisticated artificial materials, and a larger required range-of-motion (ROM) caused by the younger patients’ eagerness to continue a sporty lifestyle. The design criteria for modern THR resulting from these patient demands also depend on the anatomical conditions as well as the socio-cultural circumstances of the patients.

Asian people require in general a higher ROM due to their habit to squat during daily activities which is not common in western societies. The outcome of a THR regarding the ROM is influenced by the size of the bearing couple, the design of the acetabular component, the head-to-neck ratio, and the implantation angles. In the case of a wrongly designed or a misaligned component, e. g. a verticalised socket, subluxations and impingement might occur leading to edge-loading between the ball head and the insert. This leads in all material couplings to problems: in hard-soft couplings (ceramic or metal ball head and polyethylene insert) to strongly increased polyethylene wear, in hard-hard bearings (metal-on-metal or ceramic-on-ceramic) to point loading followed by stripe wear and, in the case of a metal-on-metal coupling, a much higher metal ion level in the blood. Therefore, an appropriate choice of the prosthesis design together with the necessary surgeon’s diligence is necessary to avoid this kind of complication.

Other important design challenges come from possible anatomical differences between different ethnical groups. It has been shown that the “asian knee” has a different mean thickness in anterior-posterior as well as medio-lateral direction compared to caucasian. As another example, an extensive study of mexican people has shown a significantly different femur geometry concerning the height of the Trochanter major compared to the cross section of the femoral axis and the neck axis. For asian people it is widely accepted that the mean femoral size is smaller. The nonobservance of these geometrical factors in implant design may again lead to higher wear rates or subluxation and impingement followed by dislocation.