E. Viguier

Effects of freezing on the biomechanics of the intervertebral disc@@@Effets de la congélation sur la biomécanique du disque intervertébral

SummaryStorage of anatomic specimens is possible only if there is a reliable method for preservation of the tissues. The establishment of such a procedure is thus of twofold importance: clinical (transplantation of segments of the vertebral column) and experimental (research and teaching programs). Simple freezing at −18° C is the simplest and least expensive method for storing spinal specimens compared to other modes of storage such as cryo-preservation and lyophilization. Does this mode of storage affect the biomechanics of frozen anatomic specimens, in particular those of the intervertebral disc? This experiment dealt with the comparative biomechanical analysis before and after three months of freezing of 19 segments of the sheep vertebral column (4 functional C4–C5 units, 3 C7-T1 units, 6 T13-L1 units and 6 L5–L67 units). The results showed that there was no significant difference (risk of error 5%) between frozen and fresh segments of vertebral column in terms of amplitude and rigidity, except for the C7-T12 segment where the conditions of validity of the statistical tests were not met. The results of this experiment allowed us to validate a biomechanical model to assay the effects of freezing.RésuméLe stockage des pièces anatomiques nest possible que sil existe un mode de conservation fiable des tissus biologiques. La détermination dun tel procédé présente donc un double intérêt clinique (transplantation de segments de colonne vertébrale) et expérimental (programme denseignement et de recherche). La congélation simple à −18° C est, parmi les autres modes de conservation tels que la cryopréservation et la lyophilisation, le moyen le plus simple et le moins coûteux de conserver des pièces rachidiennes. Ce mode de conservation altère t-il la biomécanique de pièces anatomiques congelées et tout particulièrement celle du disque intervertébral ? Cette expérimentation a porté sur lanalyse biomécanique comparée avant et après trois mois de congélation de 19 segments de colonne vertébrale de mouton (4 unités fonctionnelles C4–C5, 3 unités C7-T1, 6 unités T13-L1 et 6 unités L5–L6). Les résultats montrent quil nexiste pas de différence significative (avec un risque derreur de 5 %) entre les segments de colonne vertébrale congelés et frais en terme damplitude et de rigidité à lexception du segment C7-T1 où les conditions de validité des tests statistiques ne sont pas réunies. Les résultats de cette expérimentation permettent de valider ainsi un modèle biomécanique congelé.

Experimental Model of Posterolateral Spinal Arthrodesis in Sheep: Part 1. Experimental Procedures and Results With Autologous Bone Graft

Objectives The authors evaluated the reliability in obtaining a posterolateral spinal arthrodesis (PSA) with autologous bone graft. Summary of Background Data Posterolateral spinal arthrodesis using autogenous cancellous bone graft is the most simple and efficient technique to get a spinal graft. No extensive biomechanical study of PSA is available. Thus, an experimental model of PSA is needed. Methods Eleven sheep underwent lumbar autologous bone grafts and Cotrel-Dubousset instrumentations, and four sheep were used as controls. Sacrifice and biomechanical evaluation of the lumbar spines were performed after 1 year. Results All grafts appeared continuous. A large decrease of flexibility (in rotation and in translation) was found in grafted spines in every direction. Failure in extension occurred at a mean value of 35.26 ± 3.71 Nm. Conclusion A constant and homogenous PSA appears to be obtained in sheep under conditions close to the human surgery.

Micro-computed tomography evaluation of vertebral end-plate trabecular bone changes in a porcine asymmetric vertebral tether

We conducted a micro‐CT analysis of subchondral bone of the vertebral end‐plates after application of compressive stress. Thoracic and lumbar vertebral units were instrumented by carrying out left asymmetric tether in eleven 4‐week‐old pigs. After 3 months of growth, instrumented units and control units were harvested. Micro‐CT study of subchondral bone was performed on one central and two lateral specimens (fixated side and non‐fixated side). In control units, bone volume fraction (BV/TV), number of trabeculae (Tb.N), trabecular thickness (Tb.Th), and degree of anisotropy (DA) were significantly higher, whereas intertrabecular space (Tb.Sp) was significantly lower in center than in periphery. No significant difference between the fixated and non‐fixated sides was found. In instrumented units, BV/TV, Tb.N, Tb.Th, and DA were significantly higher in center than in periphery. BV/TV, Tb.N, and Conn.D were significantly higher in fixated than in non‐fixated side, while Tb.Sp was significantly lower. We noted BV/TV, Tb.N, and Tb.Th significantly lower, and Tb.Sp significantly higher, in the instrumented levels. This study showed, in instrumented units, two opposing processes generating a reorganization of the trabecular network. First, an osteolytic process (decrease in BV/TV, Tb.N, Tb.Th) by stress‐shielding, greater in center and on non‐fixated side. Second, an osteogenic process (higher BV/TV, Tb.N, Conn.D, and lower Tb.Sp) due to the compressive loading induced by growth on the fixated side. This study demonstrates the densification of the trabecular bone tissue of the vertebral end‐plates after compressive loading, and illustrates the potential risks of excessively rigid spinal instrumentation which may induce premature osteopenia.

Micro-computed tomography study of the subchondral bone of the vertebral endplates in a porcine model: correlations with histomorphometric parameters.

PurposeSubchondral bone (SCB) of the vertebral endplates (VEP) is the principal site of changes in vertebral trabecular microarchitecture secondary to intervertebral disc degeneration. However, the microstructure of this region has not yet been clearly characterized.MethodsOne thoracic and one lumbar vertebral unit (vertebra-disc-vertebra) was removed in nine pigs aged 4xa0months. Three samples (one central and two laterals) were taken from each VEP. Micro-CT examination and histomorphometric measurements of the subchondral trabecular bone of the VEP were carried out. Correlations between micro-CT and histological parameters were sought.ResultsTrabecular network was significantly denser [increased bone volume fraction (BV/TV) and trabecular number (Tb.N), decreased intertrabecular separation (Tb.Sp)] in the cranial endplates of the vertebral units. It was also significantly denser and less well organized [increased degree of anisotropy (DA)] in the centre of the VEP. The thickness of the cartilage endplate (CEP), SCB and growth cartilage were significantly lower in the centre of the VEP. There was a significant negative correlation between BV/TV, Tb.N and DA with the thicknesses of the CEP and SCB whereas Tb.Sp was positively correlated with these two parameters.ConclusionWe observed densification of the trabecular network in the centre of the VEP overlying the nucleus pulposus, partly related to thinner hyaline cartilage. Densification is associated with more anisotropic architecture that could cause lower mechanical strength in this area. This study provides new information on the microarchitecture of the SCB of the VEP which will make it possible to validate future models.