- Email: [email protected]
Effect of cement pressurization on stem in-vitro stability with impaction allografting
Track 4. Implants for Trauma and Orthopedics - Joint ESB Track
5041 Mo, 09:15-09:30 (P5) In-vivo wear o f hip surface replacements - a comparison o f retrievals
4.2. Hip Endoprosthetics - Surgical Aspects
and simulator results
4.2. Hip Endoprosthetics
M. Morlock 1, G. Delling 2, W. RiJther2, M. Hahn 2, N. Bishop 1. 1Biomechanics
4.2.1. Surgical Aspects
Section, Hamburg University of Technology, Hamburg, Germany, 2University Hospital Eppendoff, Hamburg, Germany The present revival of hip resurfacing arthroplasty is accompanied by a large number of early failures caused by suboptimal patient selection and implantation technique. 94 retrieved implants (85 heads and 9 cups) were analyzed with respect to wear. The surface geometry of the retrieved bearings was assessed with a coordinate measurement machine. 16 scans through the pole were performed starting and ending at the equator (0.5 mm between points, 1 mm diameter ruby head). To determine wear, the size of the original bearing was estimated. Since the manufacturing tolerances on bearing sphericity are sufficiently low, a perfect sphere was fit to the measured data. This scheme, however, results in errors when all points in the surface measurement of the retrieved bearings are considered since the worn area biases the size and position of the best fit sphere away from regions of the original unworn surface. Therefore points on the worn regions were eliminated (points with highest residuals) from the best fit calculation for the sphere. Revision surgery of the analyzed implants occurred in median 102 days after implantation (min 8, max 1761 days). Mean head diameter was 48.4±4.2 mm. Estimated wear rates for the head depend greatly on inclusion criteria: using all retrievals, the average annual wear rate was 4.16 mm 3. Excluding retrievals for which rim loading was obvious from radiological information, reduced the wear rate to 1.17mm 3 (n=55). Further excluding implants, for which rim loading could not be precluded and implants of older design, reduced the wear rate to 0.0114 mm 3 (n =44). This value is comparable to values reported in the literature for simulator studies. The results of this study should be viewed with great caution since the average time of the implants in situ is short and many patients did not return to normal activity. However, there is a good chance that the expectations regarding minor wear of large diameter metal-on-metal implants are justified - provided correct positioning of the implant components. 7360
Mo, 09:30-09:45 (P5)
Accurate measurement o f low wear volumes in total hip arthroplasty
A. Becker, ~ Dirix, H. Schmotzer. Plus Orthopedics, Aarau, Switzerland Wear in hip replacements is known to be one of the limiting factors to the lifetime of implants. Therefore, both in hard-hard as well as hard-soft bearings, research has been directed to minimize wear. For example in metal-onmetal bearings, careful optimisation of the manufacturing has reduced the steady-state wear rates. In hard-soft bearings, the introduction of crosslinkedpolyethylenes also led to an enormous wear reduction. Whether these reductions increase the lifetime of the implant is still unknown and therefore ask for accurate measurements of these low wear rates, which is a challenge. Therefore, a new approach has been suggested. Instead of measuring circles, the sphere is subdivided and measured in single meridians. The raw data are scanned for worn and virgin areas using a polynomial approximation. Regions of wear are those where the measured radii differ from the approximated virgin radii. Adding all meridians up to the whole sphere allows to calculate the wear volume and the wear contact surface. First, a virtual, "mathematical" ball-head, with an exactly known wear volume and variable accuracy (noise between 1 ~tm and 5 ~tm) of the 3D machine has been used to validate the model. At low linear (<3~tm) and volumetric wear (0.25 mm3), noise of the 3D machine can lead to errors as high as 20%. The new approach has been also applied on "real" metal ball-heads from a metal-on-metal hip simulator test. Again the wear volume is exactly known since it can be derived from the weight losses and compared with the volume measured by the 3D coordinate-machine. In this study, 12 metal ball-heads were first measured on a 3D machine with an accuracy of 3 ~tm and afterwards on a high precision machine with an accuracy below 1 ~tm. At low linear and volumetric wear, the error in the measurements is reduced from 20% to below 5%, showing that new bearing couples with extremely low wear demand for other, and more accurate measurement techniques.
$121
5709 Mo, 11:00-11:15 (P7) Investigating the effect o f surgical approach on the outcome o f total hip arthroplasty G.M. Whatling 1, C.A. Holt 1, L. Jones 1, J.K. Madete 1, H. Dabke 2, P.M. Alderman 2, P. Roberts 2. 1Cardiff School of Engineering, Cardiff
University, Queen's Buildings, Cardiff,, UK, 2Royal Gwent Hospital, Cardiff Road, Newport, UK Hip replacement is a common procedure known to improve quality of life and gait. However postoperatively, gait does not return to normal and it is thought that differing surgical methods may contribute to the extent to which normal gait is achieved. The two main surgical approaches involve either posterior or lateral incisions; affecting different muscular structures surrounding the hip joint. These two approaches have been compared to investigate their effects on postoperative gait. A Dempster-Shafer (DS) classifier was developed to identity the important kinetic and kinematic variables and classify the two groups of patients. 3D motion analysis was performed postoperatively for walking and Trendelenburg tests using QTM Software, Proreflex cameras (Qualisys, Sweden), and Bertec force platforms (Bertec Corporation). Motion data was analysed using a Visual3D 3.34 (C-Motion Inc) biomechanical model of the lower limbs. Joint rotations were defined using a Cardan/Euler sequence and inverse dynamic analysis was used to calculate internal joint moments. This model was subsequently used for 3D kinematic and kinetic analysis. Paired and independent-sample t-tests (SPSS 12.0.2) were applied to the results obtained for 10 subjects to compare: (i) the two approaches and (ii) the operated and non operated leg. This preliminary analysis highlighted several salient parameters including frontal moments, pelvic obliquity and range of motion of the hip joint and pelvis. The gait data from 35 subjects was then analysed using a modified DS based classifier, developed previously to characterise normal and osteoarthritic knee function [Jones 2006]. The value of this generic classifier was explored in terms of providing an objective and visual indicator of postoperative THA recovery and surgical efficacy following lateral and posterior approaches. References
Jones L., Beynon M.J., Holt C.A., Roy S. An application of the Dempster-Shafer theory of evidence to the classification of knee function and detection of improvement due to total knee replacement surgery. J. Biomech, in press. 6011
Mo, 11:15-11:30 (P7)
Effect of cement pressurization on stem in-vitro stability with impaction
allografting C. Albert 1,3, S. Patil 3, H. Frei 3, B. Masri 3, C. Duncan 3, T. Oxland 2,3, G. Fernlund 1,3. Departments of 1Materials Engineering, 2Mechanical
Engineering and 3Orthopaedics, The University of British Columbia, Vancouver, BC, Canada Revision hip replacements are challenging due to bone loss that results from the failure and removal of the primary implant. Impaction allografting (IA) has an appealing potential for bone restoration, however, the graft has not been observed to remodel completely into live bone. Previous research revealed the presence of cement at the host-bone interface, which compromises the potential for graft remodeling. For this reason, a surgical technique yielding lower cement penetration would be favorable for bone restoration, however, the effect of contact between the cement and the host-bone on implant stability is not known. In an effort to minimize the cement penetration into the graft bed during IA, this study compares IA procedures performed with and without cement pressurization and measures the resultant effects on stem in-vitro stability. IA was performed on six pairs of cadaveric femurs using CPT instrumentation. The implant was cemented into one femur from each pair using the conventional pressurized technique (pressure group), while in each corresponding femur the cement was not pressurized (no-pressure group). Physiological loading was simulated on a bi-axial servo-hydraulic testing machine. The threedimensional implant motion relative to the bone was measured using a system comprising six LVDT sensors. The implant's migration (permanent translation) and micromotion (reversible translation) were analyzed with Wilcoxon matched pairs tests. The resultant migration was significantly greater in the no-pressure group, median 231 ~tm (range 54-792 ~tm), compared with the pressure group, 27 ~tm (19-27~tm), p=0.028. The micromotion was also higher in the no-pressure group, 42~tm (22-70 ~tm), compared with 15~tm (10-31 ~tm) in the pressure group, p =0.028. Visual inspection of the cross-sectioned constructs revealed on average less cement penetration in the non-pressurized group.
$122
Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
While eliminating cement pressure in IA could increase the potential for bone incorporation by reducing the cement penetration into the graft bed, our results indicate that implant primary stability would be compromised. 7188 Mo, 11:30-11:45 (P7) Evolution of the residual stresses and temperature during polymerization of bone cement of an idealized hip implant: numerical results N. Nufio 1, M.A. P6rez 2, D. Plamondon 1, A. Matdrala1, J.M. Garcia-Aznar 2, M. Doblar62 . 1D#partement de g6nie de la production automatis#e, Ecole de technologie sup#rieure, Laboratoire de recherche en imagerie et orthop6die, Universit# du Quebec, Montr6al, Canada, 2Group of Structural Mechanics and Materials Modelling, Aragon Institute of Engineering Research (13A), University of Zaragoza, Zaragoza, Spain Polymethylmethacrylate (PMMA), or bone cement, is widely used as a fixation element in total hip replacements. When bone cement polymerizes, residual stresses due to bulk and thermal shrinkage will result, possibly causing micro-cracks within the cement mantle, and at the bone-cement and cementstem interfaces. These factors contribute to the implant loosening and can subsequently lead to revision of the arthroplasty. The aim of this paper is to develop a computational model able to predict the residual stresses and temperature distributions within the cement mantle and at the cement-stem interface during the polymerization process. The model is an adaptation of that used by Baliga et al. (1992), where the rate of heat generation is proportional to the rate of polymerization of PMMA, and the polymerization process is a function of temperature. The most important and novel assumptions are the dependency of the Young modulus evolution on the polymerization degree (Ahmed et al., 1982) considered together with the dependency of the thermal expansion coefficient on the Young modulus (Ahmed et al., 1982) and their incorporation into the computational simulations. The axisymmetric FE model simulates the experiments on idealized cemented hip implants performed at the I~cole de technologie sup6rieure (Montreal, Canada). The numerical results of the residual stresses and temperature distributions at the cementstem interface are compared with the experimental results. The methodology proposed here is a step forward on the numerical simulation of the mechanical behaviour of cemented hip replacements. 7447 Mo, 11:45-12:00 (P8) An intra-operative vibrational method for hip stem insertion endpoint detection and stability assessment - a pilot study L.C. Pastrav 1, S.V.N. Jaecques 1, M. Muller 2, G. Van der Perre 1. 1Division Biomechanics and Engineering Design, Katholieke Universiteit Leuven, Leuven, Belgium, 2Department of Orthopaedic Surgery, University Hospital Leuven, Leuven, Belgium Based mainly on surgeons' experience, the assessment of the primary stability of cementless hip stems still remains a subjective factor and the excessive press-fitting of a total hip replacement (THR) component can be a cause of intra-operative fracture [1]. Vibration analysis can provide objective information about the stability of implant-bone structures [2-4]. This study presents the frequency response function (FRF) change during the hip stem insertion in per-operative conditions. After in vitro studies [3,5] a new per-operative method was designed to assess the stability of cementless hip prosthesis and/or to detect the insertion endpoint: the surgeon inserts the implant in the femoral cavity through hammer blows. After each blow, the FRF of the implant-bone structure is measured directly on the prosthesis neck below 10,000Hz. An FRF shift to higher frequencies indicates an increasing stiffness of the implant-bone structure. The hammering is stopped when the FRF peak positions do not change noticeably anymore. Extra blows will not improve the prosthesis stability but will increase the fracture risk. Volunteer patients were included in this study after informed consent and approval by the institutional review board. After each blow, the FRF change indicates the evolution of the implant stability and the most sensitive frequency band is above 4000 Hz. The correlation coefficient calculated between the FRFs of successive insertion stages provides a quantitative threshold to stop the insertion process. References [1] Meek R.M., et al. J Bone Joint Surg [Am] 2004; 86A(3): 480-5. [2] Meredith N., et al. Clin Oral Impl Res 1997; 8(3): 234-243. [3] Jaecques S.V.N., et al. Proc. ESDA 2004, Manchester, UK, PaperlD 58581, 10pp. [4] Georgiou A.P., Cunningham J.L. Clin Biomech 2001; 16: 315-323. [5] Pastrav L., et al. Proc. TEHNOMUS XIII, 2005, Suceava, Romania, ISBN 973666-154-7, pp. 505-510.
Oral Presentations 6993 Mo, 12:00-12:15 (P8) Influence of residues between metal stem and femoral head on the static fracture load of ceramic implants B. Weisse 1, C. Affolter 1, A. Stutz 1, S. KSbel 2, W. Rieger 2. 1Laboratory for Materials and Engineering, Swiss Federal Laboratories for Material Testing and Research (EMPA), DEtbendorf, Switzerland, 2Metoxit AG, Thayngen, Switzerland The aim of this study was to determine the influence of residues localized between metal stem and alumina femoral head on the static fracture load of the implant. Furthermore, the effects of different deformations applied to the cross section of the stem were studied. The influence of residues (e. g. bone particles, soft tissue and blood drops) on the static fracture load was determined experimentally in accordance to the ISO 7206-10 standard. Such unwanted deposit can occur when a surgeon is inserting modular prosthesis. The effect of these residues was compared to those of 8 mm diameter paper stickers and drops of hardened glue, both were used as a reference system. A decrease of up to 90% of the static fracture load was found when either debris or artefacts of the system were present. Flattening of the stem reduced the fracture load by 25% and a slightly elliptical cross-section of the cone resulted in 50% reduction. All measurements showed that the strength of the system is tremendously reduced if the designed and manufactured interface between stem and ball is altered. This study was carried out in collaboration with Metoxit AG and supported by the Swiss Commission for Technology and Innovation, CTI. 5331 Mo, 12:15-12:30 (P8) Load transfer to the femur by revision hip endoprostheses with distal interlocking option - An experimental analysis M. Ellenrieder 1,2, E. Steinhauser 1, W. Mittelmeier 3, R. Busch 4, R. Gradinger 1. 1KIinik fEtr Orthopaedie und Unfallchirurgie der TU Muenchen, Abt. Biomechanik, Munich, Germany, 2Berufsgenossenschaftliche Unfallklinik Murnau, Murnau, Germany, 3 0rthopaedische Klinik und Poliklinik der Universitaet Restock, Restock, Germany, 41nstitut fEtr Medizinische Statistik und Epidemiologie der TU Muenchen, Munich, Germany Introduction: Hip endoprostheses with long modular stems and distal screw interlocking options are designed for revision operations in case of massive proximal bone defects or periprosthetic fractures. Temporary interlocking should provide axial and rotational stability as a prerequisite for osseous ingrowth. After implant integration the screws can be removed aiming to restore a more physiological, proximal load transfer. Aim of the study: The presented experimental study analyses how newly developed modular revision hip endoprostheses influence the load transfer to the femur with respect to differing stem-stiffness, use of interlocking options and the quality of the proximal bone stock. It was of special interest, if more flexible modular stem extensions can reduce stress-shielding effects. Material and methods: Nine revision hip implants (ESKA Implants, Germany) using three different stiff stem extensions (solid, hollow, hollow-slotted) were implanted in Composite femora (Sawbones Europe, Sweden) with photoelastic coating. Cortical strain values were determined before and after implantation simulating almost physiological loading conditions according to the in-vivo data of Bergmann et al. Statistical evaluation was based on the 99% confidence intervall of the intact femora. Results: With intact proximal bone stock all revision implants caused significant stress-shielding at the medial and lateral aspect of the femur. Proximally, the type of stem extension did not markedly influence the strain values. In the diaphysis strain reduction depended significantly on the stiffness of the stem-extension. With intact proximal femur screw interlocking led to only minor increase of implant-related stress-shielding. Simulating proximal bone deficiency caused a highly significant decrease of cortical strain values. Discussion: Simulating osseous integration of the presented implants ("intact bone"), the most flexible (hollow-slotted) stem extensions caused the least difference to physiological strain pattern after removing the interlocking screws. A proximal femoral defect combined with distally interlocked endoprostheses resulted in far-reaching alterations of strain pattern, regardless of the used stem extension. Therefore, from the clinical view, with respect to the cortical strain changes at the level of the femoral defect, a bone graft should be concerned. This could improve implant integration and avoid implant motion or fatique under dynamic conditions.
5041 Mo, 09:15-09:30 (P5) In-vivo wear o f hip surface replacements - a comparison o f retrievals
4.2. Hip Endoprosthetics - Surgical Aspects
and simulator results
4.2. Hip Endoprosthetics
M. Morlock 1, G. Delling 2, W. RiJther2, M. Hahn 2, N. Bishop 1. 1Biomechanics
4.2.1. Surgical Aspects
Section, Hamburg University of Technology, Hamburg, Germany, 2University Hospital Eppendoff, Hamburg, Germany The present revival of hip resurfacing arthroplasty is accompanied by a large number of early failures caused by suboptimal patient selection and implantation technique. 94 retrieved implants (85 heads and 9 cups) were analyzed with respect to wear. The surface geometry of the retrieved bearings was assessed with a coordinate measurement machine. 16 scans through the pole were performed starting and ending at the equator (0.5 mm between points, 1 mm diameter ruby head). To determine wear, the size of the original bearing was estimated. Since the manufacturing tolerances on bearing sphericity are sufficiently low, a perfect sphere was fit to the measured data. This scheme, however, results in errors when all points in the surface measurement of the retrieved bearings are considered since the worn area biases the size and position of the best fit sphere away from regions of the original unworn surface. Therefore points on the worn regions were eliminated (points with highest residuals) from the best fit calculation for the sphere. Revision surgery of the analyzed implants occurred in median 102 days after implantation (min 8, max 1761 days). Mean head diameter was 48.4±4.2 mm. Estimated wear rates for the head depend greatly on inclusion criteria: using all retrievals, the average annual wear rate was 4.16 mm 3. Excluding retrievals for which rim loading was obvious from radiological information, reduced the wear rate to 1.17mm 3 (n=55). Further excluding implants, for which rim loading could not be precluded and implants of older design, reduced the wear rate to 0.0114 mm 3 (n =44). This value is comparable to values reported in the literature for simulator studies. The results of this study should be viewed with great caution since the average time of the implants in situ is short and many patients did not return to normal activity. However, there is a good chance that the expectations regarding minor wear of large diameter metal-on-metal implants are justified - provided correct positioning of the implant components. 7360
Mo, 09:30-09:45 (P5)
Accurate measurement o f low wear volumes in total hip arthroplasty
A. Becker, ~ Dirix, H. Schmotzer. Plus Orthopedics, Aarau, Switzerland Wear in hip replacements is known to be one of the limiting factors to the lifetime of implants. Therefore, both in hard-hard as well as hard-soft bearings, research has been directed to minimize wear. For example in metal-onmetal bearings, careful optimisation of the manufacturing has reduced the steady-state wear rates. In hard-soft bearings, the introduction of crosslinkedpolyethylenes also led to an enormous wear reduction. Whether these reductions increase the lifetime of the implant is still unknown and therefore ask for accurate measurements of these low wear rates, which is a challenge. Therefore, a new approach has been suggested. Instead of measuring circles, the sphere is subdivided and measured in single meridians. The raw data are scanned for worn and virgin areas using a polynomial approximation. Regions of wear are those where the measured radii differ from the approximated virgin radii. Adding all meridians up to the whole sphere allows to calculate the wear volume and the wear contact surface. First, a virtual, "mathematical" ball-head, with an exactly known wear volume and variable accuracy (noise between 1 ~tm and 5 ~tm) of the 3D machine has been used to validate the model. At low linear (<3~tm) and volumetric wear (0.25 mm3), noise of the 3D machine can lead to errors as high as 20%. The new approach has been also applied on "real" metal ball-heads from a metal-on-metal hip simulator test. Again the wear volume is exactly known since it can be derived from the weight losses and compared with the volume measured by the 3D coordinate-machine. In this study, 12 metal ball-heads were first measured on a 3D machine with an accuracy of 3 ~tm and afterwards on a high precision machine with an accuracy below 1 ~tm. At low linear and volumetric wear, the error in the measurements is reduced from 20% to below 5%, showing that new bearing couples with extremely low wear demand for other, and more accurate measurement techniques.
$121
5709 Mo, 11:00-11:15 (P7) Investigating the effect o f surgical approach on the outcome o f total hip arthroplasty G.M. Whatling 1, C.A. Holt 1, L. Jones 1, J.K. Madete 1, H. Dabke 2, P.M. Alderman 2, P. Roberts 2. 1Cardiff School of Engineering, Cardiff
University, Queen's Buildings, Cardiff,, UK, 2Royal Gwent Hospital, Cardiff Road, Newport, UK Hip replacement is a common procedure known to improve quality of life and gait. However postoperatively, gait does not return to normal and it is thought that differing surgical methods may contribute to the extent to which normal gait is achieved. The two main surgical approaches involve either posterior or lateral incisions; affecting different muscular structures surrounding the hip joint. These two approaches have been compared to investigate their effects on postoperative gait. A Dempster-Shafer (DS) classifier was developed to identity the important kinetic and kinematic variables and classify the two groups of patients. 3D motion analysis was performed postoperatively for walking and Trendelenburg tests using QTM Software, Proreflex cameras (Qualisys, Sweden), and Bertec force platforms (Bertec Corporation). Motion data was analysed using a Visual3D 3.34 (C-Motion Inc) biomechanical model of the lower limbs. Joint rotations were defined using a Cardan/Euler sequence and inverse dynamic analysis was used to calculate internal joint moments. This model was subsequently used for 3D kinematic and kinetic analysis. Paired and independent-sample t-tests (SPSS 12.0.2) were applied to the results obtained for 10 subjects to compare: (i) the two approaches and (ii) the operated and non operated leg. This preliminary analysis highlighted several salient parameters including frontal moments, pelvic obliquity and range of motion of the hip joint and pelvis. The gait data from 35 subjects was then analysed using a modified DS based classifier, developed previously to characterise normal and osteoarthritic knee function [Jones 2006]. The value of this generic classifier was explored in terms of providing an objective and visual indicator of postoperative THA recovery and surgical efficacy following lateral and posterior approaches. References
Jones L., Beynon M.J., Holt C.A., Roy S. An application of the Dempster-Shafer theory of evidence to the classification of knee function and detection of improvement due to total knee replacement surgery. J. Biomech, in press. 6011
Mo, 11:15-11:30 (P7)
Effect of cement pressurization on stem in-vitro stability with impaction
allografting C. Albert 1,3, S. Patil 3, H. Frei 3, B. Masri 3, C. Duncan 3, T. Oxland 2,3, G. Fernlund 1,3. Departments of 1Materials Engineering, 2Mechanical
Engineering and 3Orthopaedics, The University of British Columbia, Vancouver, BC, Canada Revision hip replacements are challenging due to bone loss that results from the failure and removal of the primary implant. Impaction allografting (IA) has an appealing potential for bone restoration, however, the graft has not been observed to remodel completely into live bone. Previous research revealed the presence of cement at the host-bone interface, which compromises the potential for graft remodeling. For this reason, a surgical technique yielding lower cement penetration would be favorable for bone restoration, however, the effect of contact between the cement and the host-bone on implant stability is not known. In an effort to minimize the cement penetration into the graft bed during IA, this study compares IA procedures performed with and without cement pressurization and measures the resultant effects on stem in-vitro stability. IA was performed on six pairs of cadaveric femurs using CPT instrumentation. The implant was cemented into one femur from each pair using the conventional pressurized technique (pressure group), while in each corresponding femur the cement was not pressurized (no-pressure group). Physiological loading was simulated on a bi-axial servo-hydraulic testing machine. The threedimensional implant motion relative to the bone was measured using a system comprising six LVDT sensors. The implant's migration (permanent translation) and micromotion (reversible translation) were analyzed with Wilcoxon matched pairs tests. The resultant migration was significantly greater in the no-pressure group, median 231 ~tm (range 54-792 ~tm), compared with the pressure group, 27 ~tm (19-27~tm), p=0.028. The micromotion was also higher in the no-pressure group, 42~tm (22-70 ~tm), compared with 15~tm (10-31 ~tm) in the pressure group, p =0.028. Visual inspection of the cross-sectioned constructs revealed on average less cement penetration in the non-pressurized group.
$122
Journal of Biomechanics 2006, Vol. 39 (Suppl 1)
While eliminating cement pressure in IA could increase the potential for bone incorporation by reducing the cement penetration into the graft bed, our results indicate that implant primary stability would be compromised. 7188 Mo, 11:30-11:45 (P7) Evolution of the residual stresses and temperature during polymerization of bone cement of an idealized hip implant: numerical results N. Nufio 1, M.A. P6rez 2, D. Plamondon 1, A. Matdrala1, J.M. Garcia-Aznar 2, M. Doblar62 . 1D#partement de g6nie de la production automatis#e, Ecole de technologie sup#rieure, Laboratoire de recherche en imagerie et orthop6die, Universit# du Quebec, Montr6al, Canada, 2Group of Structural Mechanics and Materials Modelling, Aragon Institute of Engineering Research (13A), University of Zaragoza, Zaragoza, Spain Polymethylmethacrylate (PMMA), or bone cement, is widely used as a fixation element in total hip replacements. When bone cement polymerizes, residual stresses due to bulk and thermal shrinkage will result, possibly causing micro-cracks within the cement mantle, and at the bone-cement and cementstem interfaces. These factors contribute to the implant loosening and can subsequently lead to revision of the arthroplasty. The aim of this paper is to develop a computational model able to predict the residual stresses and temperature distributions within the cement mantle and at the cement-stem interface during the polymerization process. The model is an adaptation of that used by Baliga et al. (1992), where the rate of heat generation is proportional to the rate of polymerization of PMMA, and the polymerization process is a function of temperature. The most important and novel assumptions are the dependency of the Young modulus evolution on the polymerization degree (Ahmed et al., 1982) considered together with the dependency of the thermal expansion coefficient on the Young modulus (Ahmed et al., 1982) and their incorporation into the computational simulations. The axisymmetric FE model simulates the experiments on idealized cemented hip implants performed at the I~cole de technologie sup6rieure (Montreal, Canada). The numerical results of the residual stresses and temperature distributions at the cementstem interface are compared with the experimental results. The methodology proposed here is a step forward on the numerical simulation of the mechanical behaviour of cemented hip replacements. 7447 Mo, 11:45-12:00 (P8) An intra-operative vibrational method for hip stem insertion endpoint detection and stability assessment - a pilot study L.C. Pastrav 1, S.V.N. Jaecques 1, M. Muller 2, G. Van der Perre 1. 1Division Biomechanics and Engineering Design, Katholieke Universiteit Leuven, Leuven, Belgium, 2Department of Orthopaedic Surgery, University Hospital Leuven, Leuven, Belgium Based mainly on surgeons' experience, the assessment of the primary stability of cementless hip stems still remains a subjective factor and the excessive press-fitting of a total hip replacement (THR) component can be a cause of intra-operative fracture [1]. Vibration analysis can provide objective information about the stability of implant-bone structures [2-4]. This study presents the frequency response function (FRF) change during the hip stem insertion in per-operative conditions. After in vitro studies [3,5] a new per-operative method was designed to assess the stability of cementless hip prosthesis and/or to detect the insertion endpoint: the surgeon inserts the implant in the femoral cavity through hammer blows. After each blow, the FRF of the implant-bone structure is measured directly on the prosthesis neck below 10,000Hz. An FRF shift to higher frequencies indicates an increasing stiffness of the implant-bone structure. The hammering is stopped when the FRF peak positions do not change noticeably anymore. Extra blows will not improve the prosthesis stability but will increase the fracture risk. Volunteer patients were included in this study after informed consent and approval by the institutional review board. After each blow, the FRF change indicates the evolution of the implant stability and the most sensitive frequency band is above 4000 Hz. The correlation coefficient calculated between the FRFs of successive insertion stages provides a quantitative threshold to stop the insertion process. References [1] Meek R.M., et al. J Bone Joint Surg [Am] 2004; 86A(3): 480-5. [2] Meredith N., et al. Clin Oral Impl Res 1997; 8(3): 234-243. [3] Jaecques S.V.N., et al. Proc. ESDA 2004, Manchester, UK, PaperlD 58581, 10pp. [4] Georgiou A.P., Cunningham J.L. Clin Biomech 2001; 16: 315-323. [5] Pastrav L., et al. Proc. TEHNOMUS XIII, 2005, Suceava, Romania, ISBN 973666-154-7, pp. 505-510.
Oral Presentations 6993 Mo, 12:00-12:15 (P8) Influence of residues between metal stem and femoral head on the static fracture load of ceramic implants B. Weisse 1, C. Affolter 1, A. Stutz 1, S. KSbel 2, W. Rieger 2. 1Laboratory for Materials and Engineering, Swiss Federal Laboratories for Material Testing and Research (EMPA), DEtbendorf, Switzerland, 2Metoxit AG, Thayngen, Switzerland The aim of this study was to determine the influence of residues localized between metal stem and alumina femoral head on the static fracture load of the implant. Furthermore, the effects of different deformations applied to the cross section of the stem were studied. The influence of residues (e. g. bone particles, soft tissue and blood drops) on the static fracture load was determined experimentally in accordance to the ISO 7206-10 standard. Such unwanted deposit can occur when a surgeon is inserting modular prosthesis. The effect of these residues was compared to those of 8 mm diameter paper stickers and drops of hardened glue, both were used as a reference system. A decrease of up to 90% of the static fracture load was found when either debris or artefacts of the system were present. Flattening of the stem reduced the fracture load by 25% and a slightly elliptical cross-section of the cone resulted in 50% reduction. All measurements showed that the strength of the system is tremendously reduced if the designed and manufactured interface between stem and ball is altered. This study was carried out in collaboration with Metoxit AG and supported by the Swiss Commission for Technology and Innovation, CTI. 5331 Mo, 12:15-12:30 (P8) Load transfer to the femur by revision hip endoprostheses with distal interlocking option - An experimental analysis M. Ellenrieder 1,2, E. Steinhauser 1, W. Mittelmeier 3, R. Busch 4, R. Gradinger 1. 1KIinik fEtr Orthopaedie und Unfallchirurgie der TU Muenchen, Abt. Biomechanik, Munich, Germany, 2Berufsgenossenschaftliche Unfallklinik Murnau, Murnau, Germany, 3 0rthopaedische Klinik und Poliklinik der Universitaet Restock, Restock, Germany, 41nstitut fEtr Medizinische Statistik und Epidemiologie der TU Muenchen, Munich, Germany Introduction: Hip endoprostheses with long modular stems and distal screw interlocking options are designed for revision operations in case of massive proximal bone defects or periprosthetic fractures. Temporary interlocking should provide axial and rotational stability as a prerequisite for osseous ingrowth. After implant integration the screws can be removed aiming to restore a more physiological, proximal load transfer. Aim of the study: The presented experimental study analyses how newly developed modular revision hip endoprostheses influence the load transfer to the femur with respect to differing stem-stiffness, use of interlocking options and the quality of the proximal bone stock. It was of special interest, if more flexible modular stem extensions can reduce stress-shielding effects. Material and methods: Nine revision hip implants (ESKA Implants, Germany) using three different stiff stem extensions (solid, hollow, hollow-slotted) were implanted in Composite femora (Sawbones Europe, Sweden) with photoelastic coating. Cortical strain values were determined before and after implantation simulating almost physiological loading conditions according to the in-vivo data of Bergmann et al. Statistical evaluation was based on the 99% confidence intervall of the intact femora. Results: With intact proximal bone stock all revision implants caused significant stress-shielding at the medial and lateral aspect of the femur. Proximally, the type of stem extension did not markedly influence the strain values. In the diaphysis strain reduction depended significantly on the stiffness of the stem-extension. With intact proximal femur screw interlocking led to only minor increase of implant-related stress-shielding. Simulating proximal bone deficiency caused a highly significant decrease of cortical strain values. Discussion: Simulating osseous integration of the presented implants ("intact bone"), the most flexible (hollow-slotted) stem extensions caused the least difference to physiological strain pattern after removing the interlocking screws. A proximal femoral defect combined with distally interlocked endoprostheses resulted in far-reaching alterations of strain pattern, regardless of the used stem extension. Therefore, from the clinical view, with respect to the cortical strain changes at the level of the femoral defect, a bone graft should be concerned. This could improve implant integration and avoid implant motion or fatique under dynamic conditions.