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Flexural properties of acrylic bone cements
Track 12. Biomaterials
was only responsible for 52% of the penetration depth in the central zone and 46% in the lateral zone. In conclusion, the majority of cement penetration occurred in the early pressurisation phase of the cement i.e. during its relatively low viscosity stage. 5309 Th, 12:15-12:30 (P41) Flexural properties o f acrylic bone cements I. Knets, V. Krilova, R. Cimdins, L. Berzina, V. Vitins. Institute efBiematerials and Biomechanics of the Riga Technical University, Riga, Latvia Flexural properties of modified acrylic bone cements on the base of PMMA-ethylmethacrylate-triethyleneglycoldimethacrylate have been investigated. Flexural strength and flexural modulus of chemically modified bone cement and bone cement having additives (HA and radio pacifier) were estimated. Acrylic bone cements are applicable for implant fixation in bone tissue. The most of commercially available acrylic bone cements are based upon PMMAMMA system. The aim of present study is further optimisation of bone cement structure and composition and their correlation with flexural properties of developed cements. The new bone cements on the base of PMMA-EMATEGDMA system (ABC) have relatively low polymerisation peak temperature, and chemical modification of this system allows improving also other properties of the cement. The evaluation of mechanical properties, in particular, flexural strength and flexural modulus of modified bone cements is necessary step towards development of bone cement with optimal properties. The comparison of flexural properties of ABC tested and commercial PMMA-MMA cement showed that commercial bone cement had larger values of ultimate strength and modulus of elasticity, but this difference is not great. As to polymerisation peak temperature, there is significant difference between commercial PMMAMMA cement (-80°C) and ABC modified cements (50~0°C). The solid phase apparent density decrease diminishes the flexural modulus of formed bone cement. The introduction of 10% and 18% of HA into solid phase does not influence essentially flexural strength and flexural modulus of ABC bone cements. The introduction of radio pacifier BaSO4 into bone cement leads to flexural strength diminishing. Low polymerisation peak temperature and appropriate mechanical properties of bone cements developed allows to regard the new acrylic bone cements with the 3-D structure as promising biomaterials. 5623 Th, 14:00-14:15 (P44) Interaction of a novel intervertebral disc substitute with the lumbar spine tissue biomechanics - A finite element study J6r6me Noailly 1, D. Lacroix 1, J.A. Planell 1, E. Tanner2, L. Ambrosio 3. 1Reference Centre for Bioengineering of Catalonia, Universitat Politecnica de Catalunya, Barcelona, Spain, 2Department of Materials, Queen Mary University of London, London, UK, 3Institute of Composite and Biomedical Materials-C.N.R, University of Naples "Federico I1", Naples, Italy Because living tissues are responding to their mechanical environment, loading may contribute to catalyse any degenerative process that induces non reversible configurations. Therefore, back pain long term orthopaedic solutions should consider a disc substitute that affects minimally the surrounding tissues. In this study, a composite intervertebral disc prosthesis prototype made of hydrogel, PET fibres and HAPEX was modelled and inserted as a L4-L5 disc substitute in a L3-L5 lumbar spine bi-segment finite element model. The effects of axial compressive forces, sagittal, frontal and axial moments were simulated, and the results with and without prosthesis were compared. Under load control, while the ranges of motion of the intact model compared well with experimental in vitro results, the substitution of the L4-L5 intervertebral disc led to a general loss of the L4-L5 segmental motion greater than 80%. Although the rotations at the L3-L4 level resulted up to 14% higher, neither the facet contact forces, nor the L3-L4 intradiscal pressure or the annulus fibrosus tensile stresses were significantly modified. In the implantation area, the prosthesis pegs increased both bone tensile stresses and strain energy. Finally, under displacement control in flexion, the L3-L4 annulus tensile stresses and intradiscal pressure were increased more than twice. Although the device was too stiff to ensure a physiologic mobility of the treated segment, under load control no traumatic conditions could be predicted at the non treated level. Artificial endplates pegs may favour bone ingrowths, however they also increase the tensile stresses. Finally, displacement control results show that a possible transfer from simulations to clinical long term predictions should involve a better knowledge of the in vivo spine kinematics. Funding from the European Commission is acknowledged (G5RD-2000-CT00267).
12.3. Biomaterial in Biomechanical Applications - Joint ESB Session
$263
4749 Th, 14:15-14:30 (P44) In vitro investigation into the effects o f uni and multi-directional motion on the friction, damage and wear o f innovative chondroplasty materials against articular cartilage E. Northwood, J. Fisher. Institute of Medical and Biological Engineering, University of Leeds, Leeds, UK Understanding wear of the biomaterial/cartilage interface is vital for the development of innovative chondroplasty. The aim of this study was to compare the wear and friction properties of two biphasic [1] materials when sliding against natural articular cartilage under uniaxial reciprocating and multi-directional rotation/reciprocating motions. Two hydrogels, A (water content = 13%, E =36.9 MPa), B (water content =4%, E = 1 4 0 M P a ) were compared to articular cartilage (negative control) and stainless steel (positive control). Friction was measured over 8 hrs by means of a simple geometry wear simulator. A 0.5MPa contact stress was applied through a 0 9 mm cartilage pin over a sliding distance of 10 mm/cycle. During the multi-directional study the pin was also oscillated through ±200 per cycle about the centre, resulting in a Cross Shear Ratio equal to 0.01. All tests were completed in 25% Bovine Serum at 20°C. Mechanical alteration to the surface structure was quantified using surface topography (Talysurf 5, Taylor-Hobson, UK). Articular cartilage produced a constant friction value of 0.06 (±0.01) with and without rotation. Stainless steel produced an increase in friction over time resulting in a peak value of 0.7(±0.02) without rotation, increasing to 0.88 (±0.03) with rotation. Both biphasic hydrogels produced peak friction values lower than the positive control, but higher than cartilage/cartilage. There was no difference between uni and multi-directional motion. Wear of the cartilage pins showed a significant difference between the positive and negative controls. The cartilage pins from the hydrogel demonstrated no statistical change in surface roughness with the application of rotation. The lower friction for phasic specimens regardless of the application of rotation can be attributed to their biphasic properties. The fluid extraction and rehydration at the contact area during the loading cycle results in lower solidto-solid contact and reduces the directional effect. The single phasic materials higher solid-to-solid contact results in higher friction during multidirectional testing. This study illustrated the importance of biphasic properties within the tribology of cartilage substitution materials and future work will focus on the optimisation of biphasic properties such that materials more closely mimic natural cartilage. References
[1] Mow V.C., et al. J. Biomech. Eng 1980; 73-84. 5022 Th, 14:30-14:45 (P44) Evaluation of mechanical properties of synthetic hernia meshes M. Kirilova 1, S. Stoytchev 1, D. Pashkouleva 1, V. Kavardzhikov 1, R. Radev 2. 1Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria, 2Department of General and Operative Surgery, Medical University, Sofia, Bulgria Synthetic mesh prosthesis are widely used in the abdominal hernia surgery. Despite numerous new developments polypropylene remains the most commonly used mesh material. The prosthesis of polypropylene are available in many designs, thickness and size of pores. The present investigation was undertaken to study the mechanical properties of polypropylene (PP) hernia mesh and antimicrobial, polyamid bulgarian mesh Ampoxen. The ultimate goal of the investigation is to improve the compliance and biocompatibility of the synthetic implants. It is believed that the obtained results may serve as a basis for improving the mechanical properties of investigated meshes. Uniaxial tests on flat specimens were performed to determine the mechanical properties of synthetic hernial meshes and fascia transversalis (the posterior wall of inguinal canal, substituted by synthetic meshes in case of inguinal hernia). An investigation was done using Instron type testing device. The experimental data were represented as stress-strain relationships and extrapolated by analytical function. The comparison between samples was accomplished on the bases of the assessment of coefficients in the analytical approximations. It was proved that both kinds of the investigated meshes reveal orthotropic mechanical properties and are much more stiffer than the fascia transversalis. The investigation reveals that the hernia meshes are not properly adapted to the physiology of the abdominal wall. The wide discrepancy between the mechanical properties of synthetic meshes and human fascia transversalis suggests that the properties of this hernia meshes should be improved in order to obtain better postoperative results.
was only responsible for 52% of the penetration depth in the central zone and 46% in the lateral zone. In conclusion, the majority of cement penetration occurred in the early pressurisation phase of the cement i.e. during its relatively low viscosity stage. 5309 Th, 12:15-12:30 (P41) Flexural properties o f acrylic bone cements I. Knets, V. Krilova, R. Cimdins, L. Berzina, V. Vitins. Institute efBiematerials and Biomechanics of the Riga Technical University, Riga, Latvia Flexural properties of modified acrylic bone cements on the base of PMMA-ethylmethacrylate-triethyleneglycoldimethacrylate have been investigated. Flexural strength and flexural modulus of chemically modified bone cement and bone cement having additives (HA and radio pacifier) were estimated. Acrylic bone cements are applicable for implant fixation in bone tissue. The most of commercially available acrylic bone cements are based upon PMMAMMA system. The aim of present study is further optimisation of bone cement structure and composition and their correlation with flexural properties of developed cements. The new bone cements on the base of PMMA-EMATEGDMA system (ABC) have relatively low polymerisation peak temperature, and chemical modification of this system allows improving also other properties of the cement. The evaluation of mechanical properties, in particular, flexural strength and flexural modulus of modified bone cements is necessary step towards development of bone cement with optimal properties. The comparison of flexural properties of ABC tested and commercial PMMA-MMA cement showed that commercial bone cement had larger values of ultimate strength and modulus of elasticity, but this difference is not great. As to polymerisation peak temperature, there is significant difference between commercial PMMAMMA cement (-80°C) and ABC modified cements (50~0°C). The solid phase apparent density decrease diminishes the flexural modulus of formed bone cement. The introduction of 10% and 18% of HA into solid phase does not influence essentially flexural strength and flexural modulus of ABC bone cements. The introduction of radio pacifier BaSO4 into bone cement leads to flexural strength diminishing. Low polymerisation peak temperature and appropriate mechanical properties of bone cements developed allows to regard the new acrylic bone cements with the 3-D structure as promising biomaterials. 5623 Th, 14:00-14:15 (P44) Interaction of a novel intervertebral disc substitute with the lumbar spine tissue biomechanics - A finite element study J6r6me Noailly 1, D. Lacroix 1, J.A. Planell 1, E. Tanner2, L. Ambrosio 3. 1Reference Centre for Bioengineering of Catalonia, Universitat Politecnica de Catalunya, Barcelona, Spain, 2Department of Materials, Queen Mary University of London, London, UK, 3Institute of Composite and Biomedical Materials-C.N.R, University of Naples "Federico I1", Naples, Italy Because living tissues are responding to their mechanical environment, loading may contribute to catalyse any degenerative process that induces non reversible configurations. Therefore, back pain long term orthopaedic solutions should consider a disc substitute that affects minimally the surrounding tissues. In this study, a composite intervertebral disc prosthesis prototype made of hydrogel, PET fibres and HAPEX was modelled and inserted as a L4-L5 disc substitute in a L3-L5 lumbar spine bi-segment finite element model. The effects of axial compressive forces, sagittal, frontal and axial moments were simulated, and the results with and without prosthesis were compared. Under load control, while the ranges of motion of the intact model compared well with experimental in vitro results, the substitution of the L4-L5 intervertebral disc led to a general loss of the L4-L5 segmental motion greater than 80%. Although the rotations at the L3-L4 level resulted up to 14% higher, neither the facet contact forces, nor the L3-L4 intradiscal pressure or the annulus fibrosus tensile stresses were significantly modified. In the implantation area, the prosthesis pegs increased both bone tensile stresses and strain energy. Finally, under displacement control in flexion, the L3-L4 annulus tensile stresses and intradiscal pressure were increased more than twice. Although the device was too stiff to ensure a physiologic mobility of the treated segment, under load control no traumatic conditions could be predicted at the non treated level. Artificial endplates pegs may favour bone ingrowths, however they also increase the tensile stresses. Finally, displacement control results show that a possible transfer from simulations to clinical long term predictions should involve a better knowledge of the in vivo spine kinematics. Funding from the European Commission is acknowledged (G5RD-2000-CT00267).
12.3. Biomaterial in Biomechanical Applications - Joint ESB Session
$263
4749 Th, 14:15-14:30 (P44) In vitro investigation into the effects o f uni and multi-directional motion on the friction, damage and wear o f innovative chondroplasty materials against articular cartilage E. Northwood, J. Fisher. Institute of Medical and Biological Engineering, University of Leeds, Leeds, UK Understanding wear of the biomaterial/cartilage interface is vital for the development of innovative chondroplasty. The aim of this study was to compare the wear and friction properties of two biphasic [1] materials when sliding against natural articular cartilage under uniaxial reciprocating and multi-directional rotation/reciprocating motions. Two hydrogels, A (water content = 13%, E =36.9 MPa), B (water content =4%, E = 1 4 0 M P a ) were compared to articular cartilage (negative control) and stainless steel (positive control). Friction was measured over 8 hrs by means of a simple geometry wear simulator. A 0.5MPa contact stress was applied through a 0 9 mm cartilage pin over a sliding distance of 10 mm/cycle. During the multi-directional study the pin was also oscillated through ±200 per cycle about the centre, resulting in a Cross Shear Ratio equal to 0.01. All tests were completed in 25% Bovine Serum at 20°C. Mechanical alteration to the surface structure was quantified using surface topography (Talysurf 5, Taylor-Hobson, UK). Articular cartilage produced a constant friction value of 0.06 (±0.01) with and without rotation. Stainless steel produced an increase in friction over time resulting in a peak value of 0.7(±0.02) without rotation, increasing to 0.88 (±0.03) with rotation. Both biphasic hydrogels produced peak friction values lower than the positive control, but higher than cartilage/cartilage. There was no difference between uni and multi-directional motion. Wear of the cartilage pins showed a significant difference between the positive and negative controls. The cartilage pins from the hydrogel demonstrated no statistical change in surface roughness with the application of rotation. The lower friction for phasic specimens regardless of the application of rotation can be attributed to their biphasic properties. The fluid extraction and rehydration at the contact area during the loading cycle results in lower solidto-solid contact and reduces the directional effect. The single phasic materials higher solid-to-solid contact results in higher friction during multidirectional testing. This study illustrated the importance of biphasic properties within the tribology of cartilage substitution materials and future work will focus on the optimisation of biphasic properties such that materials more closely mimic natural cartilage. References
[1] Mow V.C., et al. J. Biomech. Eng 1980; 73-84. 5022 Th, 14:30-14:45 (P44) Evaluation of mechanical properties of synthetic hernia meshes M. Kirilova 1, S. Stoytchev 1, D. Pashkouleva 1, V. Kavardzhikov 1, R. Radev 2. 1Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria, 2Department of General and Operative Surgery, Medical University, Sofia, Bulgria Synthetic mesh prosthesis are widely used in the abdominal hernia surgery. Despite numerous new developments polypropylene remains the most commonly used mesh material. The prosthesis of polypropylene are available in many designs, thickness and size of pores. The present investigation was undertaken to study the mechanical properties of polypropylene (PP) hernia mesh and antimicrobial, polyamid bulgarian mesh Ampoxen. The ultimate goal of the investigation is to improve the compliance and biocompatibility of the synthetic implants. It is believed that the obtained results may serve as a basis for improving the mechanical properties of investigated meshes. Uniaxial tests on flat specimens were performed to determine the mechanical properties of synthetic hernial meshes and fascia transversalis (the posterior wall of inguinal canal, substituted by synthetic meshes in case of inguinal hernia). An investigation was done using Instron type testing device. The experimental data were represented as stress-strain relationships and extrapolated by analytical function. The comparison between samples was accomplished on the bases of the assessment of coefficients in the analytical approximations. It was proved that both kinds of the investigated meshes reveal orthotropic mechanical properties and are much more stiffer than the fascia transversalis. The investigation reveals that the hernia meshes are not properly adapted to the physiology of the abdominal wall. The wide discrepancy between the mechanical properties of synthetic meshes and human fascia transversalis suggests that the properties of this hernia meshes should be improved in order to obtain better postoperative results.