Model-based development of neuroprosthesis for paraplegic patients.

In paraplegic patients with upper motor neuron lesions the signal path from the central nervous system to the muscles is interrupted. Functional electrical stimulation applied to the lower motor neurons can replace the lacking signals. A so-called neuroprosthesis may be used to restore motor function in paraplegic patients on the basis of functional electrical stimulation. However, the control of multiple joints is difficult due to the complexity, nonlinearity, and time-variance of the system involved. Furthermore, effects such as muscle fatigue, spasticity, and limited force in the stimulated muscle further complicate the control task. Mathematical models of the human musculoskeletal system can support the development of neuroprosthesis. In this article a detailed overview of the existing work in the literature is given and two examples developed by the author are presented that give an insight into model-based development of neuroprosthesis for paraplegic patients. It is shown that modelling the musculoskeletal system can provide better understanding of muscular force production and movement coordination principles. Models can also be used to design and test stimulation patterns and feedback control strategies. Additionally, model components can be implemented in a controller to improve control performance. Eventually, the use of musculoskeletal models for neuroprosthesis design may help to avoid internal disturbances such as fatigue and optimize muscular force output. Furthermore, better controller quality can be obtained than in previous empirical approaches. In addition, the number of experimental tests to be performed with human subjects can be reduced. It is concluded that mathematical models play an increasing role in the development of reliable closed-loop controlled, lower extremity neuroprostheses.     

Deafferentation of the urinary bladder and implantation of a sacral anterior root stimulator (SARS) for treatment of the neurogenic bladder in paraplegic patients.

Since 25 years electrical stimulation has become an established and widely acknowledged therapy option. Today, FES is widely employed, e.g. for cardiostimulation, diaphragm stimulation, kinetotherapy, for treatment of tremor in Parkinson patients, and finally for bladder stimulation in patients with bladder voiding dysfunctions. Brindley was the first researcher who succeeded in stimulating the spinal nerves via implanted electrodes in an animal model. In the years 1978/79 Brindley implanted five paraplegic patients with so-called sacral anterior root stimulators; all of them were able to void under stimulation. This method of sacral anterior root stimulation (SARS) proved an alternative to frequent one-way catheterisation for patients with severe voiding dysfunctions, without achieving complete continence, however. The following study is to provide an overview over the latest insights in the context of implanting sacral anterior root stimulators; it discusses the preconditions required for such interventions and presents criteria to decide in which cases there is a contraindication for sacral deafferentation of the posterior roots. Moreover, it contrasts advantages and disadvantages of the intradural and extradural implantation methods and presents the currently available long-term follow-up results with SDAF and SARS for treatment of bladder voiding dysfunctions.     

Computer-controlled temperature multiplexer for thermal dosimetry studies

Thermistor probes are widely used for mapping thermal distributions in hyperthermia research. To facilitate multi-probe mapping, a computer-controlled multiplexer for automated thermistor impedance measurement and temperature conversion is described. A complete hardware interface schematic is shown. An algorithm which calculates individual temperature values as a function of thermistor impedance is tested. Total error is quantified by calculating the absolute error between the theoretical temperatures obtained from the algorithm and the experimental calibration temperatures obtained from a National Bureau of Standards referenced mercury thermometer. If calibration intervals of 1.00° C or less are used, the total error of the system is less than 0.10° C.     

A suspensory jacket permitting long-term sitting in paraplegic patients

A thoracic suspensory jacket has been used in four patients with recurring ischial pressure sores to transfer the site of skin pressure from the insensate ischial area to the sensory area over the rib cage. This modality is not an alternative to surgery, but rather an adjunct to prevent an early recurrence after an ischial pressure sore has been repaired with a proper surgical procedure.     

Symmetry of FES responses in the lower extremities of paraplegic patients

Due to natural or artificial obstacles, gait is a less automatic and periodic process than it would appear when studying normal walking on the level. Pre-programmed functional electrical stimulation (FES) sequences, therefore, do not appear to be a suitable approach to the control of multichannel electrical stimulators in the restoration of paraplegic walking. Walking in paraplegic subjects must be, to a large extent, under voluntary control. To lessen the burden of this control, the symmetry of walking can be taken into account. Symmetric motion of the legs requires symmetric FES actuation. Symmetry of FES responses was studied in a group of 10 paraplegic subjects who had all undergone the FES training program. Recruitment curve, fatigue index and twitch delay were assessed. An average 80% symmetry was found in all parameters measured, thus allowing a reduction of complexity of control approach for FES locomotor aids.     

Energetics of paraplegic walking

A group of 35 paraplegic subjects using reciprocating walking orthoses have been examined in order to gain an insight into the potential functional benefits of using such devices. Measurements have been made of walking speeds and of the energy costs of ambulation using an established technique based on heart rate recordings. It was found that orthotically aided walking for paraplegics was slow and energy costly compared with both normal walking and wheelchair propulsion and, as it additionally requires the use of a walking aid in both hands, cannot be considered to confer the functional benefits frequently claimed for it. Nevertheless, the majority of the subjects studied liked their orthosis and did well in it, with many subjectively reporting improvements in mobility and independence.     

Computer-controlled direct scanning gel chromatography system

We have developed a system for direct scanning gel chromatography which is under direct control of a SYM-1 microprocessor which is in turn under control of a PDP 1104. Each scan consisting of ～200 data points of a 20-cm column may be obtained in as little as 3.5 s. Up to 100 scans may be obtained automatically without operator attention. A series of data manipulation programs have been written to allow determination of centroid migration rates, time difference chromatography calculations, etc. In conjunction with a mass transport simulation program it is possible to rapidly fit observed chromatography profiles. Use of the system to determine dispersion coefficients is described.     

The use of tissue expansion in lower extremity wounds in paraplegic patients

A report of the use of tissue expansion in the lower extremity of paraplegic patients is presented with over 1-year follow-up in two cases. Expansion was accomplished without difficulty and without compromise of skin integrity when performed carefully and slowly. It is important to observe the overlying skin for capillary refill. Pain, which may limit expansion in neurologically intact patients, is not a factor in paraplegics. By using tissue expansion, more risky, longer procedures using myocutaneous flaps or microvascular tissue transfers may be unnecessary, and reliable, durable, full-thickness coverage is obtained in two short procedures.     

Computer-controlled bedside monitoring and infusion system.

A mult-channel system for continuous bedside monitoring of glucose, pH pCO2 and automatic computer-controlled infusion of insulin, glucose, potassium, and bicarbonate solutions is described. Regulatory algorithms are presented for correction of metabolic acidosis and hyperglycemia with and without acidosis.     

Computerized 22-channel stimulator for limb movement

The Nucleus 22-channel implantable hearing prosthesis (Cochlear Pty. Ltd., Lane Cove, Sydney, Australia) has been modified by computer programming (MOCO, Inc., Scituate, Mass. USA) into a functional electrical stimulator. Individual or multiple channels can be sequenced and adjusted for their amplitude, width and frequency of the pulse so that activation of single and multiple nerves can be achieved. Sciatic nerve branches (lateral and medial) of anesthetized rabbits were stimulated to produce single contractions or co-contraction at the ankle and simultaneous bilateral joint movements. The spiral (Helix) electrode was also found suitable in these experiments. The external equipment is being re-designed for a wearable ambulation unit to be used in paraplegic subjects.