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Load transfer to the femur by revision hip endoprostheses with distal interlocking option — an experimental analysis
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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.
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.