Track 18. Trends in Cranial and Spinal Biomechanics 18.6. Intervertebral Disc Replacement $371

as well the recruitment of motor units whose discharge is not "time-locked" to each stimulus pulse. The augmented H-reflexes may be due in part to potentiated release of neurotransmitter from la afferent terminals induced by the high frequency stimulation. Motor unit discharge may also be sustained by persistent inwards currents in spinal motoneurons and interneurons increasing their responsiveness to synaptic inputs and leading to periods of self-sustained activity in the absence of synaptic drive. The central contribution to electrically- evoked muscle contractions has been shown during stimulation over the triceps surae, tibialis anterior and the wrist flexors. Understanding the mechanism responsible for motoneuron recruitment may provide insights into processes that are part of normal sensorimotor integration in the spinal cord. Pre- and postsynaptic mechanisms within the spinal cord are likely to contribute. The recruitment of spinal motoneurons should occur in the normal physiological recruitment order, with the smallest motoneurons that innervate the most fatigue resistant muscle fibres recruited first. This may be beneficial for using electrical stimulation to restore movement and reduce muscle atrophy.

5002 Tu, 09:00-09:15 (P17) Transcutaneous stimulation technology

T. Keller 1,2, A. Kuhn 1 , M. Lawrence 1,2 . 1Automatic Control Lab., ETH Zurich, Switzerland, 2Spinal Cord Injury Center, University Hospital Balgrist, Zurich, Switzerland

Transcutaneous electrical stimulation (TES) applies a sequence of electrical pulses through the skin surface to artificially generate neural and motor activation in humans. It is a widely applied technique found in physical therapy, sports training and medicine and can be used for muscle atrophy treatment, muscle force training, endurance training, pain treatment, functional movement therapy, and motor function restoration. Recently, novel transcutaneous multi- channel stimulation technologies, which allow automatically controlled spatial and temporal distributions of electrical current fields, are emerging [1]. These new approaches potentially enable an improved control of specific limb articu- lations also under dynamic conditions of multi joint movements. On the other hand number and complexity of control parameters increase dramatically, what makes realistic simulations necessary. We performed finite element modeling (FEM) of the current and potential distri- butions in the proximal arm to evaluate the influence of stimulation parameters on the electrical field generation. The results were applied to nerve activation models (e.g. NEURON [2]) to estimate the impact of changed electrode sizes and positions on motor nerve activations. In TES experiments isometric finger force measurements showed the feasibility of selective muscle activation using the novel dynamic multi-channel stimulation approach. Future developments will have to concentrate on the miniaturization and integration of the multi-channel stimulation technology to a wearable system. Its combination with kinematic and force sensors will create a new generation of transcutaneous neuroprostheses for improved motor function training and restoration.

References [1] Lawrence M., Kirstein T., Keller T. Electrical stimulation of finger flexors using

'virtual electrodes'. In: Proceedings of 8th Vienna international workshop on functional electrical stimulation, Vienna, Austria, September 2004; 8: 191-4.

[2] Hines M.L., Carnevale N.T. The NEURON simulation environment. Neural Computation 1997; 9: 1179-209.

7767 Tu, 09:15-09:30 (P17) FES implantable technology T. Stieglitz, M. Schuettler. University of Freiburg - IMTEK, Department of Microsystems Engineering, Laboratory for Biomedical Microtechnology, Freiburg, Germany

Neuroprosthetic applications often require implantable systems to ensure proper and reliable function during activities of daily living. Nowadays, many systems that interact with the nervous system via electrical stimulation have been established in clinical practice. Cochlea implants restore hearing after loss of hair cells, stimulation of the vagal nerve helps to treat epilepsy or depression and deep brain stimulation suppresses tremor and overcomes dyskinesis in Parkinson's disease patients. Spinal cord stimulation is used for treatment of intractable chronic pain and incontinence. In paralyzed patients electrical stimulation excites muscles to manage the urinary bladder or to establish grasping functions. Latest sensor prostheses even face the challenge of restoring sight in blind subjects. We give an overview of implantable neural implants for sensory, motor and neuromodulation implants and discuss medical requirements and technological challenges and solutions to establish nerve interfaces and electronic system encapsulations for chronic use. Besides precision mechanics implants, latest developments of microtechnological approaches will be presented that allow high channel electrodes and smallest implants. Investigating structural biocom- patibility of materials and implant concepts led to the development of flexible

substrates with integrated electrodes and polymer-based encapsulation layers. Examples of current research comprise retina implants with wireless energy supply, sieve electrodes to investigate regenerating nerves and smart elec- trodes with integrated electronics. The coexistence of electrical and chemical signal processing in the body has been transferred to neural implants recently. The combination of microfluidics for drug application with electrical recording and stimulation opens the opportunity to develop multimodal microimplants for diagnosis and intervention in the future.

5619 Tu, 09:30-09:45 (P17) Moving thoughts - brain-computer interface for control of grasp neuroprostheses in tetraplegic patients

R. Rupp 1 , G. MiJller-Putz 2, R. Scherer 2, G. Pfurtscheller 2, H.J. Gerner 1 . 1 Orthopedic University Hospital II, Heidelberg, Germany, 2Institute for Knowledge Discovery, Laboratory of Brain-Computer Interfaces, Graz University of Technology, Graz, Austria

The complete restoration of movements lost due to an injury of the spinal cord is the greatest desire of physicians, therapists and certainly of the patients themselves. Especially in patients with lesions of the cervical spinal cord every form of improvement of a missing or weak grasp function will result in a large gain of quality of life. Recent technological advancements lead to the establishment of systems for restoration of basic movements in spinal cord in- jured (SCI) persons by means of Functional Electrical Stimulation (FES). While FES systems in the lower extremities for standing/walking have not reached widespread clinical acceptance yet, devices are available for demonstrably improvement of the grasp function in tetraplegic patients with stable, active shoulder function, but missing control of hand and fingers. Especially with the use of implantable systems a long-term stable, easy to handle application is possible. However, the current methods for user driven control of grasp neuroprostheses require a certain amount of preserved voluntary movements i.e. shoulder movements, which are not apparent in very high lesioned SCI patients. For restoration of the upper limb function in this patient group the coupling of neuroprostheses with a Brain-Computer Interface (BCI) based on electroen- cephalographic (EEG) signals is proposed and has been successfully imple- mented in a neuroprosthesis with surface electrodes [1] and in the implantable Freehand system [2]. The tetraplegic patients were able to switch between grasp phases by imaginations of movements. From our results it can be concluded that BCI systems have a high potential for widening the application of grasp neuroprostheses.

References [1] Pfurtscheller G., M(iller G.R., Pfurtscheller J., Gerner H.J., Rupp R. "Thought"

control of functional electrical stimulation to restore hand grasp in a patient with tetraplegia. Neuroscience Letters 2003; 351: 33-36.

[2] MiJller-Putz G.R., Scherer R., Pfurtscheller G., Rupp R. EEG-based neuro- prosthesis control: A step towards clinical practice. Neuroscience Letters 2005; 382(1-2): 169-174.

18.6. Intervertebral Disc Replacement 4412 Tu, 11:00-11:20 (P20) Clinical results of total lumbar disc replacement with ProDisc Ih 3-year results for different indications H.M. Mayer, C.J. Siepe, K. Wiechert, A. Korge. OrthoCenter Munich, Orthopaedic Clinic, Spine Center, Munich, Germany

Study design: Prospective study analyzing mid-term clinical results of total lumbar disc replacement (ProDiscll) for different indications. Objectives: To assess functional outcome after total lumbar disc replacement (TDR) treated for varying indications Summary of background data: Despite its frequent use and increasing popularity, indications and contraindications for TDR have not been defined precisely at this stage and remain a matter of debate, leading to disc replace- ment procedures in a variety of pathologies that have not yet been evaluated and compared separately. Methods: Patients meeting inclusion criteria were evaluated prospectively according to Visual Analogue Scale (VAS), Oswestry Questionnaire, SF-36 and numerous clinical parameters. Indications included degenerative disc disease (DDD), DDD with accompanying soft disc herniation (+NPP), postoperative osteochondrosis following previous discectomy and DDD with presence of Modic-changes. Postoperative improvement was recorded and analyzed for influence of preoperative diagnosis. Results: Overall, 92 patients from 4 groups with a mean follow-up of 34.2 months achieved significant and maintained improvement from preoperative levels (p <0.001). Patients with DDD+NPP achieved results significantly better than patients from the other groups (p < 0.05). Presence of Modic-changes or previous discectomy did not influence outcome negatively. Improvement was achieved for both mono- and bisegmental disc replacements (p <0.05),