Augmentation of the atrophic maxillary sinus floor: graft stiffness, implant shape and length.

This study investigates the characteristics of load transmission to bone of alternative treatments for posterior maxilla edentulism with relatively limited available bone volume. Implant shape (conical and cylindrical), augmentation technique and the effect of bone-graft stiffness were taken into consideration. The finite element models of the atrophic sinus implanted with short implant were compared to two grafted-sinus models implanted with longer implants, engaged bicortically. Bone-graft stiffness was varied to describe different stages of graft-maturation (from short-term to long-term). Stress and load distributions due to axial and bending loads were compared on the bony structures. In the short-term, axial force is supported almost equally by the cortical layers and the trabecular core, while a bending load is mainly supported by the crestal cortical layer and secondarily by the cortical floor, the bone-graft supported a negligible load. Bicortical engagement produces higher load transfer to the cortical floor under axial load. In the long-term, as the stiffness of the bone-graft increases, the load is transferred progressively towards the grafted region, progressively unloading other structures, particularly the internal cortical layer.

Revision surgery after PSO failure with rod breakage: a comparison of different techniques.

STUDY DESIGN: Author experience and literature review. OBJECTIVES: To compare different revision techniques in the treatment of implant failure after pedicle subtraction osteotomy (PSO). The complication rate of pedicle subtraction osteotomy is substantially higher than other corrective procedures available for the treatment of spinal sagittal imbalance: in particular, hardware failures and mechanical complications affect this technique and their biomechanical explanation is still purely speculative. METHODS: The author's experience and the literature regarding the revision techniques for PSO failures are discussed. RESULTS: In this paper, eight consecutive revision cases due to rod breakage after PSO surgery are reported. In our experience, the main goals are to restore the spinal balance, through a posterior approach (correction and hardware revision and implementation) and to get a solid anterior fusion (both through a traditional anterior approach or minimally invasive transpsoas approach). CONCLUSION: The efficacy of PSO should be balanced with the high risk of the procedure reported in the literature. Management of revision surgery after PSO may require the addition of anterior column support to maintain correction and reduce complications.

Parametric FE mesh generation: application to the cervical spine.

A voxel-based reconstruction algorithm, targeted at the generation of finite element (FE) meshes of structures with schematic geometry, is presented. The algorithm is able to generate three dimensional fully hexahedral FE meshes of structures composed of volumes with a schematic geometry. In order to be meshed, each volume must be described in terms of a set of surfaces which enclose the volume. Due to its schematic nature, the method allows the generation of fully parameterized FE models, thus it facilitates the investigation of the mechanical relevance of geometrical parameters by speeding up the mesh generation process. The algorithm was employed in the automatic construction of an FE model of the C3-C7 spinal segment with schematic geometry, made up exclusively of hexahedral elements. Non-linear simulations were carried out in different loading conditions: flexion- extension, lateral bending and axial rotation. The results were compared to data retrieved from the literature in order to ensure the validity of the model. Moment-rotation curves for each loading condition were determined. The range of motion was obtained for each spinal unit and loading condition. Both principal and coupled rotations were determined in lateral bending and axial rotation, for each spinal unit. The intradiscal pressure was also computed in the nucleus pulposus, for all the intervertebral levels. Geometrical parameterization of the models offers potential for the biomechanical investigation of pathologic conditions and surgical procedures, such as spinal fusion and disc arthroplasty, even on a patient-specific basis.

Computational modeling of combined cell population dynamics and oxygen transport in engineered tissue subject to interstitial perfusion.

This work presents a computational model of tissue growth under interstitial perfusion inside a tissue engineering bioreactor. The model accounts both for the cell population dynamics, using a model based on cellular automata, and for the hydrodynamic microenvironment imposed by the bioreactor, using a model based on the Lattice-Boltzmann equation and the convection-diffusion equation. The conditions of static culture versus perfused culture were compared, by including the population dynamics along with oxygen diffusion, convective transport and consumption. The model is able to deal with arbitrary complex geometries of the spatial domain; in the present work, the domain modeled was the void space of a porous scaffold for tissue-engineered cartilage. The cell population dynamics algorithm provided results which qualitatively resembled population dynamics patterns observed in experimental studies, and these results were in good quantitative agreement with previous computational studies. Simulation of oxygen transport and consumption showed the fundamental contribution of convective transport in maintaining a high level of oxygen concentration in the whole spatial domain of the scaffold. The model was designed with the aim to be computationally efficient and easily expandable, i.e. to allow straightforward implementability of further models of complex biological phenomena of increasing scientific interest in tissue engineering, such as chemotaxis, extracellular matrix deposition and effect of mechanical stimulation.

237 ACoA aneurysms clipped or embolized. Outcomes measurement using the De Santis-CESE assessment tool.

AIM: The aim of this retrospective study was to demonstrate the difference in patient outcomes after treatment for bleeding endocranial aneurysms when evaluated with methods based on different assessment criteria. METHODS: The outcome of 237 patients, 141 of which were operated on for anterior communicating artery aneurysm and 96 embolized, was assessed by a new method developed by De Santis. The patients operated on were assessed by the Glasgow Outcome Scale (GOS) and Rank Disability Scale (RDS) and the results of the latter were compared with the new method, the De Santis-CESE (Clinical Emotional Social Evaluation) method, which consists of a clinical evaluation and a numeric scoring system based on seven standard points. Comparison between the three methods showed significantly different outcomes. Patients who underwent surgical operation showed changes in character and behaviour, whereas the others showed cognitive, emotional and sexual habit changes. CONCLUSION: Compared with the GOS and RDS instruments, the CESE method showed significant differences in patient outcome assessment, particularly regarding best outcomes. These differences may be due to the greater sensitivity of the CESE method over the other two scales. Furthermore, surgical patients seemed to achieve a better outcome than endovascular patients. The authors intend to conduct a prospective study to test the results obtained in this retrospective study.

Biomechanics of the C5-C6 spinal unit before and after placement of a disc prosthesis.

The study consists of a biomechanical comparison between the intact C5-C6 spinal segment and the same segment implanted with the Bryan artificial disc prosthesis (Medtronic Ltd., Memphis, TN, USA), by the use of the finite element (FE) method. Our target is the prediction of the influence of prosthesis placement on the resulting mechanics of the C5-C6 spine unit. A FE model of the intact C5-C6 segment was built, employing realistic models of the vertebrae, disc and ligaments. Simulations were conducted imposing a compression preload combined to a flexion/extension moment, a pure lateral bending moment and a pure torsion moment, and the calculated results were compared to data from literature. The model was then modified to include the Bryan cervical disc prosthesis, and the simulations were repeated. The location of the instantaneous center of rotation (ICR) of C5 with respect to C6 throughout flexion/extension was calculated in both models. In general, the moment-rotation curves obtained from the disc prosthesis-implanted model were comparable to the curves obtained from the intact model, except for a slightly greater stiffness induced by the artificial disc. The position of the calculated ICRs was rather stable throughout flexion-extension and was generally confined to a small area, qualitatively matching the corresponding physiological region, in both models. These results imply that the Bryan disc prosthesis allows to correctly reproduce a physiological flexion/extension at the implanted level. The results of this study have quantified aspects that may assist in optimizing cervical disc replacement primarily from a biomechanical point of view.

Multibody modeling of the cervical spine in the simulation of flexion-extension after disc arthroplasty.

This paper presents a three-dimensional (3D) multibody model of the cervical spine implanted with an artificial disc. The model was used to predict prosthesis placement influence on the resulting cervical kinematics in a series of patients. The vertebral tract modeled was the C2-C7, and the vertebral geometries were reconstructed from computed tomography (CT) images. The model was used to simulate the flexion-extension motion of the cervical spine in 10 patients implanted with the Prestige commercial disc prosthesis at a single level. For each patient, a geometrical model of the prosthesis was scaled and included in the multibody model to match the size and positioning of the actual prosthesis, as assessed on post-operative radiographs. Simulations of complete flexion-extension were carried out for each patient, and the main parameters relevant to the motion of the vertebral bodies were calculated and compared to data measured from dynamic post-operative radiographs. At the implanted level, the simulated ranges of motion generally agreed with the measured ones, with an average deviation <2 degrees. In addition, the simulated relative angles between vertebral bodies agreed with the measured ones, with minor average differ-ences of 1.2, 1.8 and 2.1 degrees in full flexion, neutral alignment and full extension, respectively. The cervical kinematics after prosthesis placement was influenced both by the design of the artificial joint and by surgical positioning. Therefore, the model presented can be used both to support pre-operative planning for disc arthroplasty and in the optimization of new prostheses design.