$370 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

been separately investigated only in three planes; sagittal (flexion/extension), transverse (torsion) and frontal (bilateral bending). The purpose of the present study was to determine the load response of the spine to pure bending moments in all three planes simultaneously before and after treatment of the intervertebral disc. The goals of this investigation were; first, to compare the flexibility of the spinal motion segemtns under different loading regimes; second, to investigate the effect of a new hydrogel materials to restore disc mechanical properties. Sixteen motion segments from two spines (T9/T10, T11/T12, L1/L2, L3/L4) were prepared for biomechanical testing. The top and bottom of the spinal segments were permanently potted in casting cement and rigidly fixed to the robotic testing system at their neutral position. Each specimen was tested in a load feedback control to determine the path in response to pure moments [Nm.] of 10 (without preload) and 7.5, 10, and 12.5 (with 600 N axial preload). For each case, the path of least resistance (minimum shear and compressive load) was determined by flexing the specimen in eight planes (separated by 22.5 degrees in transverse plane) while maintaining the overall bending moment constant. This protocol was repeated for the remaining three loading cases. For treatment study, the specimens were tested pre- and post treatment using the path of 10 Nm. (without preload). For the treatment group, the nucleus pulposus were injected with 2 ml of hydrogel (© Medtronic, Inc. USA). The motion paths under different loading regimes (for each segment and within each spine) and also the effect of hydrogel on the shear forces and bending moment for each path were compared. The degree of degeneration of the intervertebral disc for each specimen was assessed after the experiment.

5841 Mo, 16:45-17:00 (P13) Approach for maxillofacial reconstructions based on reverse engineering and rapid prototyping

M. Kalaidjieva 1 , P. Polihronov 2, E. Tosheva 1 , L.C. Hieu 3, '~ Toshev 1 . 1Institute of Mechanics and Biomechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria, 2 Maxillofacial Surgery Hospital, Sofia, Bulgaria, 3School of Engineering, Cardiff University, Cardiff, UK

Reverse Engineering (RE) is an up-to-date methodology for constructing 3D CAD model of physical parts by digitizing an existing part and then using that model to reproduce the part. Tools digitizing an existing part could be: Coordinate Measuring Machine, Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). CT imaging for maxillofacial surgery is common practice now. Software that produces pseudo-3D images from sets of 2D axial computed tomograms was used for examining different maxillofacial diseases or traumas. The resulting 3D models can facilitate the process of diagnosis and the operations planning. The aim of the study is to present an approach to use CT (or MRI) data for patients after maxillofacial tumor resection, to obtain 3D computer models using appropriate software and to use them for ProEngineer TM CAD design of the needed implant parts. The computer designed parts are easily to be used for manufacturing on the base of Rapid Prototyping (RP) as a new technology that takes a 3D computer model and builds real part by building layers upon layer of material. RP technology has gained a great amount of medical attention, particularly in Orthotics & Prosthetics. The use of biocompatible materials in RP is of decisive importance for the fabrication of medical products. Until now, practically, there is a lack of such biocompatible materials. The determination of long-term toxic effect is of great importance. Practical application of the proposed approach was demonstrated for some patients after maxillofacial tumor resection. The prefabricated im- plants were well-matched with defects and it was proved that they can play an important role in the diagnosis and simulation of operation for repairing. The real surgical application of the RP implants depends on suitable biocompatible materials and the results of long-term clinical evaluation. Acknowledgments: The study was supported by the Bulgarian National Re- search Fund (Grant 1407/04) and 6FP 4M NoE

5021 Mo, 17:00-17:15 (P13) Biomechanics of the cervical spine after fusion and arthroplasty

F. Galbusera 1 , M.T. Raimondi 1 , M. Sassi 2, M. Fornari 2, R. Assietti 3. 1Laboratory of Biological Structure Mechanics, Department of Structural Engineering, Politecnico di Milano, Milan, Italy, 2 Department of Neurosurgery, Istituto Ortopedico Galeazzi, Milan, Italy, 3Department of Neurosurgery, Ospedale Fatebenefratelli e Oftalmico, Milan, Italy

The target of the present work is the investigation of the influence of spinal fusion and arthroplasty on the biomechanics of the cervical spine. To this purpose, a nonlinear finite element model of the intact lower cervical spine (C3-C7) was built and validated against literature data in different load cases: pure compression, flexion-extension, lateral bending and axial rotation [1,2]. The model was modified in order to investigate the biomechanics of fusion and arthroplasty procedures at the C5-C6 level. To model cervical fusion, the intervertebral space was filled with solid elements with the elastic properties of

trabecular bone. Arthroplasty was modeled considering a representative full- constrained prosthesis which fixes the center of rotation of the C5-C6 segment in a specific point. Three different centers of rotation were considered: at the center of the C6 upper endplate, at the midpoint between the center and the posterior margin, at the midpoint between the center and the anterior margin. The same loading conditions used with the intact model were applied to the models including fusion and arthroplasty. The calculated global segment motion resulted preserved for the arthroplasty models and minorly reduced for the fusion model, with respect to the intact model, for all the loading conditions. The average intradiscal pressure was calculated for all the models, and resulted similar, considering a specified global motion, for the intact and the arthroplasty models, while increased for the fusion model. The present study quantitatively investigates the alteration on the biomechanics of the adjacent segments induced by fusion and shows that arthroplasty can preserve the biomechanics of the implanted and the adjacent levels.

References [1] Maurel N., Lavaste E, Skalli W. J Biomech 1997; 20(9): 921-931. [2] Wheeldon J.A., Pintar EA., Knowles S., Yoganandan N. J Biomech 2006; 39:

375-380.

18.5. Neuroprosthetics 6447 Tu, 08:15-08:45 (P17) Neuroprostheses for restoration of motor functions - Overview

J. Quintern. Neurological Hospital Bad Aibling, Bad Aibling, Germany

Neuroprotheses by means of functional electrical stimulation (FES) may be used to restore lost motor functions after lesions in the central nervous system (CNS). The lacking or disturbed signals from the CNS to the peripheral nerves and muscles are replaced by low-frequency trains of electrical stimulation pulses delivered to the lower motor neurons via surface electrodes, percu- taneous electrodes, or fully implanted electrodes. One of the most challenging applications of FES is stance and gait in patients with complete thoracic spinal cord injuries (SCI). However, due to several unsolved problems only therapeutic walking and no functional walking can be achieved with these neuroprostheses. Main problems are rapid fatigue of electrically stimulated muscles and lacking equilibrium control. Not only external forces, but also reflex induced muscle contractions and the preserved voluntary upper body movements interfere with the FES induced movements. Feedback control strategies which consider voluntary movements as a dis- turbance variable which has to be compensated by the neuroprostheses have not yet been successful. Cooperative control strategies which support voluntary movements induced by the patient are a more promising approach. In patients with disturbed proprioception sensory substitution systems may improve coordination. A temporary therapeutic use of neuroprostheses plays an increasing role in the rehabilitation of hemiparetic patients after stroke or other lesions in the brain and patients with incomplete SCI. The neuroprostheses enables an efficient gait training in severely affected patients early after the lesion. Several randomized controlled clinical studies have shown positive effects of gait training with FES compared to conventional therapies. In selected tetraplegic patients upper extremity neuroprostheses can restore the grip function. Comparable neuroprostheses may also be used for functional training of the affected arm of hemiparetic patients. In patients with partial control of shoulder and elbow and lacking control of wrist and fingers grasping can be trained by multichannel stimulation of lower arm and hand muscles. However, further studies are needed to compare this novel kind of therapy with robot supported training and other repetitive training strategies.

7174 Tu, 08:45-09:00 (P17) Central and peripheral contributions to contractions evoked by tetanic electrical stimulation of human muscle D.E Collins, J.C. Dean, O. Lagerquist, L.M. Yates. Human Neurophysiology Laboratory, Faculty of Physical Education and Recreation, Centre for Neuroscience, University of Alberta, Edmonton, Alberta, Canada

Electrical stimulation of human muscle can be used to restore movement and reduce muscle atrophy for persons with movement disorders. The evoked contractions are thought to arise from a peripheral mechanism whereby muscle fibres are recruited by the activation of motor axons beneath the stimulating electrodes. This recruits preferentially muscle fibres that fatigue most rapidly. However, during stimulation using wide pulse widths (1 ms) and relatively high frequencies (up to 100 Hz) additional torque develops. This "extra" torque is absent when the nerve to the muscle is blocked proximal to the stimulating electrodes and thus is due to a central mechanism involving spinal motoneu- rons recruited by reflex inputs to the spinal cord. This synaptic activation of motoneurons should recruit the muscle fibres that are most fatigue resistant. The central mechanism is associated with increased transmission through the H-reflex pathway (an electrically-evoked analogue of the stretch reflex)