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Mechanical stability of augmented spinal segments
Track 4. Implants for Trauma and Orthopedics-Joint ESB Track 6978 We-Th, no. 50 (P59) Mechanical stability of augmented spinal segments G. Huber 1, L. M~Jller-Bergen 1,2, J. Heinze 2, C. Eggers 2, K. PiJschel 3, M.M. Morlock 1. 1Biomechanic Section, Hamburg University of Technology,
Hamburg, Germany, 2 Trauma Department, Asklepios Clinic St. Georg, Hamburg, Germany, 3Legal Medicine Department, University Hospital Eppendoff, Hamburg, Germany A successful treatment for fractured osteoporotic vertebrae is augmentation with bone cement. Augmentation is also performed as prophylactic treatment for critical and adjacent vertebrae. The Kyphoplasty procedure (Kyphon, Sunnyvale, CA) uses a balloon to restore the vertebral height and to create a defined space for the cement. In Vertebroplasty (DePuy AcroMed, Raynham, MA) no balloon is used. In this study it was investigated whether the load bearing capacity of the augmented and the adjacent vertebra is different for the two methods. 24 complete human lumbar spines (female, >60y) were harvested for in-vitro testing. Specimens were assigned to four groups (Vertebroplasty/Kyphoplasty, fractured/non-fractured) based on the bone mineral density (QCT). L1 and L5 were potted in metal holders and mounted in a servo-hydraulic test rig (MTS, Eden Prairie, MN). Defined compression fractures at level L3 were produced in 50% of the specimens by static flexion-compression. During static loading L1-L2 and L4-L5 were supported by plaster cast. Plaster casts were removed afterwards. All L3 vertebrae were then augmented. Force controlled dynamic flexion-compression loading (4 Hz) starting from 100 N was increased by 100 N every 1000 cycles until a fracture occurred. Predominantly L2 fractured during dynamic loading. Prophylactically treated L3 vertebrae did not fail at all. In three of the four groups, vertebral fractures occurred at a load of about 1500 N. The fracture load for the Kyphoplasty group with pre-fractured L3 only reached about 1000N. However, for this group, the force necessary to produce the original L3 fracture was clearly lower. Taking this covariance into account, no significant differences in mechanical stability between the four groups could be observed. Prophylactic treatment of a vertebral body seems to prevent its fracture during dynamic loading. The study was supported by the City of Hamburg, DePuy AcroMed, and Kyphon 5032 We-Th, no. 51 (P59) Development o f a new implant to correct scoliosis by means of segmental translation A. Oltra 1, L.A. P6rez Mill~n 2, C. Atienza 1, J.L. Peris 1, F. Moll~ 1, J. Montero 1, J. S~nchez 1, J. Prat 1. 1Institute of Biomechanics of Valencia, Valencia, Spain,
2,La Fe" Hospital of Valencia, Valencia, Spain Surgical treatment of scoliosis has evolved during last times due to the appearance of new surgical protocols based on derotation of the injured spine or based on translation of vertebrae with sublaminar wires, both of them with neurological risk. A new spinal fixation system with a new surgical protocol, has been developed to correct scoliotic or kyphotic deformities through a segmental translation with laminar clamp without invading the neural canal and favouring vertebral derotation minimizing the neurological risk. The spinal fixation system TRANSPINE ® is composed by transpedicular screws and laminar or pediculo-laminar clamp to create a strong fixation in the limits of the deformity. In the apex region of the deformity, laminar clamps formed by the connection of supralaminar and sublaminar hooks will be implanted so that correction will be carried out translating vertebrae with cables joined to the clamps. Once the spinal fixation system TRANSPINE ® was designed, a complete mechanical evaluation has been performed to check the correct mechanical behaviour of the components. Furthermore, tests for assessing the usability and advantages of the system have been developed. Results showed that the TRANSPINE ® system and its components and assemblies fulfil properly the requirements related to this kind of spinal fixation in static and fatigue tests. After two years follow-up, more than twenty patients treated with this spinal fixation system demonstrate a high degree of correction in the sagittal plane and in the coronal plane without complications related to the new surgical protocol. Reduction of neurological risk during and after the surgery has been observed. 4563 We-Th, no. 52 (P59) Finite element analysis of four different lumbar interbody fusion cage designs W.-R Chen 1, C.-Y. Lin 1, S.-H. Chen 2, C.-H. Chien 1,3. 1Department of
Biomedical Engineering, Chung Yuan Christian University, Chungli, Taiwan, 2Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Kaohsiung, Taiwan, 3Dpartment of Orthopaedic Surgery, Ten-Chen General Hospital, Yangmei, Taiwan Initial promise of stand-alone interbody fusion cage to treat chronic backache and restore disc height has not been maintained. The current study used finite
4.4. Spinal Implants
$523
element analyses (FEA) to investigate the stress distribution in four different interbody fusion systems (dual paralleled cage, single large cage, treaded cylinder cage, and two-part cage consisting of transpezoidal box and threaded cylinder). The load transfer mechanism on the cage and surrounding tissues was evaluated. Computed tomography (CT) scans of an L3-L4 motion segment of a healthy young male were obtained to create the intact L3-L4 finite element model. Four different cages were implanted respectively in the intact L3-4 model. Compressive preload of 150 N was applied and friction coefficient of 0.4 was set for the interface between the interbody implants and vertebral body. The L3 vertebral body was subjected to 10 Nm moment of flexion, extension, lateral bending and torsion, respectively and the L4 bottom surface was constrained. Finite element analyses were performed with or without posterior pedicle screw implant and tested under 80 lordotic alignment to investigate their relative importance on the segmental response. The changes of predicted stress on the facet joint, vertebral rotation angle and maximum displacement of the cage on the endplate were obtained under afore-mentioned Ioadings. Simulation results predicted less facet contact stress and displacement under the maximal contact area between the implant and vertebral body. When supplemented with pedicle screw implant on each model to share the load, the displacement of cage on the endplate became less. The stand-alone twopart fusion cage has minimal displacement, as compared to the other three cages either adding with posterior fixation or not and the cage together with pedicle screw fixation can provide better initial stability following operation. 4509 We-Th, no. 53 (P59) Design considerations for a multi-specimen lumbar spinal disc fatigue and wear simulator J. Lusk 1, G. McKewan 1, J. Price 2. 1Bose Corp. - ESG, Eden Prairie, MN,
USA, 2Theken Disc, Akron, OH, USA Introduction: Recent history has seen the progression from fixation devices to intervertebral disc (IVD) replacement devices in an attempt to restore and/or maintain the physiologic motion of spinal segments. In an attempt to regulate and test these devices, ASTM and ISO have developed testing methods, in the form of guides or standards, to determine the wear exhibited by the device following a long-term dynamic test. Methods: A single-station wear simulator prototype was developed as a proof of concept for a 6-station system. The simulator was designed to accommodate both ASTM and ISO guidelines and apply axial compression and rotation, flexion-extension, and lateral bending to an intervertebral device. Results: The single-station wear simulator makes use of four axial actuators to provide the appropriate loading and rotations on the device. It is capable of applying these rotations independently or simultaneously at frequencies up to 2 Hz in a variety of testing protocols and acquiring data for each channel. In addition, an X-Y stage was designed to be placed above or below the device to limit or allow passive shear translations. A six-component load cell was placed below the specimen chamber to acquire date for the forces and moments applied to the specimen. Discussion and Conclusions: The single-station prototype is now being used for testing of an IVD replacement device and has already run in excess of five million cycles. Development of a 6-station version with a load-soak station is underway. The key requirements are meeting the forces, rotations, frequencies, and waveforms that are desired to perform these tests in an accurate and timely fashion per the ASTM and ISO test methods. References [1] Nydegger T. "Flexible" Stabilization of the Spine - Is this the future? In: Trans. of the World Congress of Biomechanics, 2002. [2] ASTM WK454. Standard test guide for the functional and kinematic wear assessment of total disc prostheses. In: Draft 9, 2005. [3] ISO/DIS 18192-1. Implants for surgery-Wear of total intervertebral spinal disc prostheses, 2005. 4042 We-Th, no. 54 (P59) Comparison of transpedicular screw fixation in soft and cured kyphoplasty cement O. Linhardt 1, C. L~ring 1, J. Matussek 1, C. Hamberger 37523°, W. Plitz 2, j. Grifka 1 1Orthopaedic Department of University Regensburg, Germany,
2Orthopaedic Clinic of Ludwig-Maximilians-University Munich, Grol3hadem Clinic, Munich, Germany The goal of this cadavar study was to compare the stability of pedicle screws after implantation in soft or cured kyphoplasty cement. For this purpose, pedicle screws were inserted in a total of 30 thoracolumbar vertebrae of 10 different human specimens: 10 screws were implanted in nonaugmented vertebrae (group 1), each 10 screws were placed in soft (group 2) and cured (group 3) cement. Pedicle screws were then evaluated for biomechanical axial pullout resistance.
Hamburg, Germany, 2 Trauma Department, Asklepios Clinic St. Georg, Hamburg, Germany, 3Legal Medicine Department, University Hospital Eppendoff, Hamburg, Germany A successful treatment for fractured osteoporotic vertebrae is augmentation with bone cement. Augmentation is also performed as prophylactic treatment for critical and adjacent vertebrae. The Kyphoplasty procedure (Kyphon, Sunnyvale, CA) uses a balloon to restore the vertebral height and to create a defined space for the cement. In Vertebroplasty (DePuy AcroMed, Raynham, MA) no balloon is used. In this study it was investigated whether the load bearing capacity of the augmented and the adjacent vertebra is different for the two methods. 24 complete human lumbar spines (female, >60y) were harvested for in-vitro testing. Specimens were assigned to four groups (Vertebroplasty/Kyphoplasty, fractured/non-fractured) based on the bone mineral density (QCT). L1 and L5 were potted in metal holders and mounted in a servo-hydraulic test rig (MTS, Eden Prairie, MN). Defined compression fractures at level L3 were produced in 50% of the specimens by static flexion-compression. During static loading L1-L2 and L4-L5 were supported by plaster cast. Plaster casts were removed afterwards. All L3 vertebrae were then augmented. Force controlled dynamic flexion-compression loading (4 Hz) starting from 100 N was increased by 100 N every 1000 cycles until a fracture occurred. Predominantly L2 fractured during dynamic loading. Prophylactically treated L3 vertebrae did not fail at all. In three of the four groups, vertebral fractures occurred at a load of about 1500 N. The fracture load for the Kyphoplasty group with pre-fractured L3 only reached about 1000N. However, for this group, the force necessary to produce the original L3 fracture was clearly lower. Taking this covariance into account, no significant differences in mechanical stability between the four groups could be observed. Prophylactic treatment of a vertebral body seems to prevent its fracture during dynamic loading. The study was supported by the City of Hamburg, DePuy AcroMed, and Kyphon 5032 We-Th, no. 51 (P59) Development o f a new implant to correct scoliosis by means of segmental translation A. Oltra 1, L.A. P6rez Mill~n 2, C. Atienza 1, J.L. Peris 1, F. Moll~ 1, J. Montero 1, J. S~nchez 1, J. Prat 1. 1Institute of Biomechanics of Valencia, Valencia, Spain,
2,La Fe" Hospital of Valencia, Valencia, Spain Surgical treatment of scoliosis has evolved during last times due to the appearance of new surgical protocols based on derotation of the injured spine or based on translation of vertebrae with sublaminar wires, both of them with neurological risk. A new spinal fixation system with a new surgical protocol, has been developed to correct scoliotic or kyphotic deformities through a segmental translation with laminar clamp without invading the neural canal and favouring vertebral derotation minimizing the neurological risk. The spinal fixation system TRANSPINE ® is composed by transpedicular screws and laminar or pediculo-laminar clamp to create a strong fixation in the limits of the deformity. In the apex region of the deformity, laminar clamps formed by the connection of supralaminar and sublaminar hooks will be implanted so that correction will be carried out translating vertebrae with cables joined to the clamps. Once the spinal fixation system TRANSPINE ® was designed, a complete mechanical evaluation has been performed to check the correct mechanical behaviour of the components. Furthermore, tests for assessing the usability and advantages of the system have been developed. Results showed that the TRANSPINE ® system and its components and assemblies fulfil properly the requirements related to this kind of spinal fixation in static and fatigue tests. After two years follow-up, more than twenty patients treated with this spinal fixation system demonstrate a high degree of correction in the sagittal plane and in the coronal plane without complications related to the new surgical protocol. Reduction of neurological risk during and after the surgery has been observed. 4563 We-Th, no. 52 (P59) Finite element analysis of four different lumbar interbody fusion cage designs W.-R Chen 1, C.-Y. Lin 1, S.-H. Chen 2, C.-H. Chien 1,3. 1Department of
Biomedical Engineering, Chung Yuan Christian University, Chungli, Taiwan, 2Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Kaohsiung, Taiwan, 3Dpartment of Orthopaedic Surgery, Ten-Chen General Hospital, Yangmei, Taiwan Initial promise of stand-alone interbody fusion cage to treat chronic backache and restore disc height has not been maintained. The current study used finite
4.4. Spinal Implants
$523
element analyses (FEA) to investigate the stress distribution in four different interbody fusion systems (dual paralleled cage, single large cage, treaded cylinder cage, and two-part cage consisting of transpezoidal box and threaded cylinder). The load transfer mechanism on the cage and surrounding tissues was evaluated. Computed tomography (CT) scans of an L3-L4 motion segment of a healthy young male were obtained to create the intact L3-L4 finite element model. Four different cages were implanted respectively in the intact L3-4 model. Compressive preload of 150 N was applied and friction coefficient of 0.4 was set for the interface between the interbody implants and vertebral body. The L3 vertebral body was subjected to 10 Nm moment of flexion, extension, lateral bending and torsion, respectively and the L4 bottom surface was constrained. Finite element analyses were performed with or without posterior pedicle screw implant and tested under 80 lordotic alignment to investigate their relative importance on the segmental response. The changes of predicted stress on the facet joint, vertebral rotation angle and maximum displacement of the cage on the endplate were obtained under afore-mentioned Ioadings. Simulation results predicted less facet contact stress and displacement under the maximal contact area between the implant and vertebral body. When supplemented with pedicle screw implant on each model to share the load, the displacement of cage on the endplate became less. The stand-alone twopart fusion cage has minimal displacement, as compared to the other three cages either adding with posterior fixation or not and the cage together with pedicle screw fixation can provide better initial stability following operation. 4509 We-Th, no. 53 (P59) Design considerations for a multi-specimen lumbar spinal disc fatigue and wear simulator J. Lusk 1, G. McKewan 1, J. Price 2. 1Bose Corp. - ESG, Eden Prairie, MN,
USA, 2Theken Disc, Akron, OH, USA Introduction: Recent history has seen the progression from fixation devices to intervertebral disc (IVD) replacement devices in an attempt to restore and/or maintain the physiologic motion of spinal segments. In an attempt to regulate and test these devices, ASTM and ISO have developed testing methods, in the form of guides or standards, to determine the wear exhibited by the device following a long-term dynamic test. Methods: A single-station wear simulator prototype was developed as a proof of concept for a 6-station system. The simulator was designed to accommodate both ASTM and ISO guidelines and apply axial compression and rotation, flexion-extension, and lateral bending to an intervertebral device. Results: The single-station wear simulator makes use of four axial actuators to provide the appropriate loading and rotations on the device. It is capable of applying these rotations independently or simultaneously at frequencies up to 2 Hz in a variety of testing protocols and acquiring data for each channel. In addition, an X-Y stage was designed to be placed above or below the device to limit or allow passive shear translations. A six-component load cell was placed below the specimen chamber to acquire date for the forces and moments applied to the specimen. Discussion and Conclusions: The single-station prototype is now being used for testing of an IVD replacement device and has already run in excess of five million cycles. Development of a 6-station version with a load-soak station is underway. The key requirements are meeting the forces, rotations, frequencies, and waveforms that are desired to perform these tests in an accurate and timely fashion per the ASTM and ISO test methods. References [1] Nydegger T. "Flexible" Stabilization of the Spine - Is this the future? In: Trans. of the World Congress of Biomechanics, 2002. [2] ASTM WK454. Standard test guide for the functional and kinematic wear assessment of total disc prostheses. In: Draft 9, 2005. [3] ISO/DIS 18192-1. Implants for surgery-Wear of total intervertebral spinal disc prostheses, 2005. 4042 We-Th, no. 54 (P59) Comparison of transpedicular screw fixation in soft and cured kyphoplasty cement O. Linhardt 1, C. L~ring 1, J. Matussek 1, C. Hamberger 37523°, W. Plitz 2, j. Grifka 1 1Orthopaedic Department of University Regensburg, Germany,
2Orthopaedic Clinic of Ludwig-Maximilians-University Munich, Grol3hadem Clinic, Munich, Germany The goal of this cadavar study was to compare the stability of pedicle screws after implantation in soft or cured kyphoplasty cement. For this purpose, pedicle screws were inserted in a total of 30 thoracolumbar vertebrae of 10 different human specimens: 10 screws were implanted in nonaugmented vertebrae (group 1), each 10 screws were placed in soft (group 2) and cured (group 3) cement. Pedicle screws were then evaluated for biomechanical axial pullout resistance.