In numerical simulations of skeletal muscle contractions, geometric information is of major importance. The aim of the present study was to determine whether the diffusion tensor imaging (DTI) technique is suitable to obtain valid input with regard to skeletal muscle fibre direction. The accuracy of the DTI method was therefore studied by comparison of DTI fibre directions in the rat tibialis anterior muscle with fascicle striation patterns visible on high-resolution magnetic resonance imaging (MRI) and with fibre directions in an actual longitudinal section (ALS) through the same muscle. The results showed an excellent qualitative agreement between high-resolution MRI and DTI. Despite less accurate quantitative comparison with ALS, it was concluded that DTI does indeed measure skeletal muscle fibre direction. After the experiment, it was possible to determine an appropriate voxel size (0n9 mm$) that provided enough resolution and acceptable accuracy (5m) to use DTI fibre directions in biomechanical analyses. Muscle deformation during contraction, resulting from a finite element simulation with a mesh that was directly generated from the experimental data, has been presented.
The spinal cord follows the straightest line through the imposed geometry of the spinal canal. Accordingly, there is relatively more posterior separation of the cord and surrounding thecal tissue at midthoracic levels in the apex of the thoracic kyphosis. Placing a patient in a position that accentuates the thoracic curvature of the spine (ie,sitting head-down) increases the posterior separation of the spinal cord and dural sheath at thoracic levels.
The experimental delivery of spinal anaesthetics to the desired heights in the body, even above the termination of the spinal cord (thoracic level), has been shown to be potentially very valuable. Since there is no blockade of the lower extremities, little caudal spread, a significantly larger portion of the body experiences no venal dilation, and may offer a compensatory buffer to adverse changes in blood pressure intra-operatively. Further, the dosing of the anaesthetic is exceedingly low, given the highly specific block to only certain nerve function along a section of the cord. Thirdly, the degree of muscle relaxation achievable without central or peripheral respiratory or circulatory depression is superior to that with general anaesthesia.Results from Magnetic Resonance Imaging (MRI) studies indicate that the spinal cord lies anteriorly within its thecal boundaries in the apex of the thoracic curve. Intrathecal injections, therefore, at thoracic levels may have a greater absolute margin of error before needle contact with neural tissue -although the consequences of inadvertent contact are possibly more disastrous.The thoracic CSE technique has been practised in twelve patients with cardiovascular and/or pulmonary problems. Despite bad haemodynamic situations and/or severe end-stage lung problems, abdominal surgery (i.e. cholecystectomy, bowel and vascular surgery) could be performed successfully with the thoracic CSE method, with low impact on the patient. Anaesthetic care of the patients would have been more difficult, with consequently larger impacts on haemodynamic and pulmonary function, should these surgeries have occurred under general anaesthesia -the usual anaesthetic technique of choice.CSE techniques can be used in the thoracic region in patients who otherwise would receive general anaesthesia. High risk patients, with limited cardio-respiratory reserves, present challenges to the anaesthesiologist. Using the thoracic CSE technique in the thoracic space is extending the boundaries of regional anaesthesia.
Deformation of the extradural space and the possibility of impression upon the dural sac during atlanto-axial rotation are investigated. Atlanto-axial rotation leads to a reduction in the cross-sectional area of the bony spinal canal of approximately 40%. Atlanto-axial rotation was recorded by endocanalar views from a video camera fixed inside the skull of six unembalmed cadavers. Axial thin-section T1-weighted MRI slice sets were acquired from three volunteers (mid-position and maximal left and right rotation of the head and cervical spine). The axial cross-sectional areas of the bony spinal canal, dural sac and spinal cord were measured. In two other persons post-gadolinium contrast-enhanced T1-weighted MRI volume scans with fat-suppression prepulse were acquired (mid-position and rotation) to determine venous contents of the extradural space. The 50:50 ratio between left and right extradural halves in mid-position changed to an ipsilateral:contralateral ratio of 20:80 in maximum rotation at the level just above the lateral C1-C2 joints. Directly below these joints the opposite occurred. The post-contrast studies showed an enhancing internal vertebral venous plexus (IVVP), which almost completely occupied the extradural space at the atlanto-axial level. This could not be shown in the cadaver experiments, because of absence of blood and cerebrospinal fluid (CSF) pressure. During atlanto-axial rotation blood displacement in the IVVP allows major deformations of the extradural space. This prevents dural sac impression.
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