Y P Charles1, H Pelletier2, P Hydier3, S Schuller4, J Garnon5, E A Sauleau6, J-P Steib4, P Clavert7. 1. Groupe d'étude de biomécanique ostéo-articulaire de Strasbourg (GEBOAS), fédération de médecine translationnelle de Strasbourg (FMTS), université de Strasbourg, 4, rue Kirschleger, 67085 Strasbourg cedex, France; Service de chirurgie du rachis, hôpitaux universitaires de Strasbourg, 1, place de l'Hôpital, BP 426, 67091 Strasbourg cedex, France. Electronic address: yann.philippe.charles@chru-strasbourg.fr. 2. Institut national des sciences appliquées (INSA), université de Strasbourg, 24, boulevard de la Victoire, 67000 Strasbourg, France; Institut Charles-Sadron, UPR 22 CNRS, université de Strasbourg, 23, rue du Loess, 67000 Strasbourg, France. 3. Institut national des sciences appliquées (INSA), université de Strasbourg, 24, boulevard de la Victoire, 67000 Strasbourg, France. 4. Groupe d'étude de biomécanique ostéo-articulaire de Strasbourg (GEBOAS), fédération de médecine translationnelle de Strasbourg (FMTS), université de Strasbourg, 4, rue Kirschleger, 67085 Strasbourg cedex, France; Service de chirurgie du rachis, hôpitaux universitaires de Strasbourg, 1, place de l'Hôpital, BP 426, 67091 Strasbourg cedex, France. 5. Service de radiologie interventionnelle, hôpitaux universitaires de Strasbourg, 1, place de l'Hôpital, BP 426, 67091 Strasbourg cedex, France. 6. Département de santé publique, hôpitaux universitaires de Strasbourg, 1, place de l'Hôpital, BP 426, 67091 Strasbourg cedex, France. 7. Groupe d'étude de biomécanique ostéo-articulaire de Strasbourg (GEBOAS), fédération de médecine translationnelle de Strasbourg (FMTS), université de Strasbourg, 4, rue Kirschleger, 67085 Strasbourg cedex, France; Institut national des sciences appliquées (INSA), université de Strasbourg, 24, boulevard de la Victoire, 67000 Strasbourg, France; Institut d'anatomie normale, faculté de médecine, université de Strasbourg, 4, rue Kirschleger, 67085 Strasbourg cedex, France.
Abstract
BACKGROUND: Vertebroplasty prefilling or fenestrated pedicle screw augmentation can be used to enhance pullout resistance in elderly patients. It is not clear which method offers the most reliable fixation strength if axial pullout and a bending moment is applied. The purpose of this study is to validate a new in vitro model aimed to reproduce a cut out mechanism of lumbar pedicle screws, to compare fixation strength in elderly spines with different cement augmentation techniques and to analyze factors that might influence the failure pattern. MATERIALS AND METHODS: Six human specimens (82-100 years) were instrumented percutaneously at L2, L3 and L4 by non-augmented screws, vertebroplasty augmentation and fenestrated screws. Cement distribution (2 ml PMMA) was analyzed on CT. Vertebral endplates and the rod were oriented at 45° to the horizontal plane. The vertebral body was held by resin in a cylinder, linked to an unconstrained pivot, on which traction (10 N/s) was applied until rupture. Load-displacement curves were compared to simultaneous video recordings. RESULTS: Median pullout forces were 488.5 N (195-500) for non-augmented screws, 643.5 N (270-1050) for vertebroplasty augmentation and 943.5 N (750-1084) for fenestrated screws. Cement augmentation through fenestrated screws led to significantly higher rupture forces compared to non-augmented screws (P=0.0039). The pullout force after vertebroplasty was variable and linked to cement distribution. A cement bolus around the distal screw tip led to pullout forces similar to non-augmented screws. A proximal cement bolus, as it was observed in fenestrated screws, led to higher pullout resistance. This cement distribution led to vertebral body fractures prior to screw pullout. CONCLUSION: The experimental setup tended to reproduce a pullout mechanism observed on radiographs, combining axial pullout and a bending moment. Cement augmentation with fenestrated screws increased pullout resistance significantly, whereas the fixation strength with the vertebroplasty prefilling method was linked to the cement distribution.
BACKGROUND: Vertebroplasty prefilling or fenestrated pedicle screw augmentation can be used to enhance pullout resistance in elderly patients. It is not clear which method offers the most reliable fixation strength if axial pullout and a bending moment is applied. The purpose of this study is to validate a new in vitro model aimed to reproduce a cut out mechanism of lumbar pedicle screws, to compare fixation strength in elderly spines with different cement augmentation techniques and to analyze factors that might influence the failure pattern. MATERIALS AND METHODS: Six human specimens (82-100 years) were instrumented percutaneously at L2, L3 and L4 by non-augmented screws, vertebroplasty augmentation and fenestrated screws. Cement distribution (2 ml PMMA) was analyzed on CT. Vertebral endplates and the rod were oriented at 45° to the horizontal plane. The vertebral body was held by resin in a cylinder, linked to an unconstrained pivot, on which traction (10 N/s) was applied until rupture. Load-displacement curves were compared to simultaneous video recordings. RESULTS: Median pullout forces were 488.5 N (195-500) for non-augmented screws, 643.5 N (270-1050) for vertebroplasty augmentation and 943.5 N (750-1084) for fenestrated screws. Cement augmentation through fenestrated screws led to significantly higher rupture forces compared to non-augmented screws (P=0.0039). The pullout force after vertebroplasty was variable and linked to cement distribution. A cement bolus around the distal screw tip led to pullout forces similar to non-augmented screws. A proximal cement bolus, as it was observed in fenestrated screws, led to higher pullout resistance. This cement distribution led to vertebral body fractures prior to screw pullout. CONCLUSION: The experimental setup tended to reproduce a pullout mechanism observed on radiographs, combining axial pullout and a bending moment. Cement augmentation with fenestrated screws increased pullout resistance significantly, whereas the fixation strength with the vertebroplasty prefilling method was linked to the cement distribution.