Pullout characteristics of percutaneous pedicle screws with different cement augmentation methods in elderly spines: An in vitro biomechanical study

AbstractBackgroundVertebroplasty 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 methodsSix human specimens (82–100 years) were instrumented percutaneously at L2, L3 and L4 by non-augmented screws, vertebroplasty augmentation and fenestrated screws. Cement distribution (2ml 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 (10N/s) was applied until rupture. Load-displacement curves were compared to simultaneous video recordings.ResultsMedian pullout forces were 488.5N (195–500) for non-augmented screws, 643.5N (270–1050) for vertebroplasty augmentation and 943.5N (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.ConclusionThe 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

The effect of posterior non-fusion instrumentation on segmental shear loading of the lumbar spine

Background : Lumbar stenosis and facet osteoarthritis represent indications for decompression and instrumentation. It is unclear if degenerative spondylolisthesis grade I with a remaining disc height could be an indication for non-fusion instrumentation. The purpose of this study was to determine the influence of a mobile pedicle screw based device on lumbar segmental shear loading, thus simulating the condition of spondylolisthesis. Materials and methods : Six human cadaver specimens were tested in 3 configurations: intact L4–L5 segment, then facetectomy plus undercutting laminectomy, then instrumentation with lesion. A static axial compression of 400 N was applied to the lumbar segment and anterior displacements of L4 on L5 were measured for posterior-anterior shear forces from 0 to 200 N. The slope of the loading curve was assessed to determine shear stiffness. Results : Homogenous load-displacement curves were obtained for all specimens. The average intact anterior displacement was 1.2 mm. After lesion, the displacement increased by 0.6 mm compared to intact (P = 0.032). The instrumentation decreased the displacement by 0.5 mm compared to lesion (P = 0.046). The stiffness's were: 162 N/mm for intact, 106 N/mm for lesion, 148 N/mm for instrumentation. The difference was not significant between instrumented and intact segments (P = 0.591). Conclusions : Facetectomy plus undercutting laminectomy decreases segmental shear stiffness and increases anterior translational L4–L5 displacement. Shear stiffness of the instrumented segment is higher with the device and anterior displacements under shear loading are similar to the intact spine. This condition could theoretically be interesting for the simulation of non-fusion instrumentation in degenerative spondylolisthesis

Approche géographique du recours aux soins en santé mentale

La sectorisation psychiatrique, conception territoriale et démographique de l'organisation des soins, vise par l'implantation des structures de prise en charge hors de l'hôpital, à faciliter l'accès aux soins. Ses directives ont été suivies de manière inégale. L'intérêt est alors de pouvoir identifier les facteurs de « rugosité de l'espace » qui sous-tendent les hétérogénéités voire les inégalités qui affectent la distribution de l'offre et le recours aux soins. Il s'agit d'identifier les liens entre la variabilité spatiale du recours aux structures et les typologies géographiques communales (construites par des techniques d'analyse de données résumant des variables démographiques ou des variables mesurant l'éloignement aux services), les données individuelles des patients, les caractéristiques liées à la structure ... Le recours aux soins est le rapport entre le nombre de patients résidant dans une commune et un nombre attendu de patients. Ce rapport dépend ensuite des covariables dans un modèle généralisé additif. Certaines des variables sont systématiquement incluses (autocorrélation spatiale). Cette méthodologie est appliquée à un centre hospitalier spécialisé accueillant 7.350 patients par an. Les variations du recours aux soins s'expliquent essentiellement par la pathologie des patients mais peu par la typologie des communes de résidence, la densité en médecins généralistes ou psychiatres libéraux. La réflexion actuelle sur les missions de la psychiatrie montre l'importance de repenser l'organisation de l'offre des soins et de mieux prendre en compte sa globalité. Notre méthode est une aide efficace mais doit être complétée par d'autres analyses, notamment d'aménagement du territoire