Neuromuscular electrical stimulation and electron-tactile stimulation in rehabilitation of artificial prehension and proprioception in tetraplegic patients

Abstracts

Esse trabalho discute o uso da estimulação elétrica na reabilitação sensoriomotora de membros superiores paralisados. A restauração da função motora de preensão foi obtida pela aplicação da estimulação elétrica neuromuscular, em seqüências de ativação adequadas a realização de atividades do cotidiano como comer, beber, escrever e digitar. Uma luva instrumentalizada com sensores de força possibilitou quantificar o padrão de movimento exercido artificialmente. Esse sistema foi utilizado como alça de realimentação para a restauração de uma propriocepção através da aplicação da estimulação eletrotáctil, possibilitando a evocação de sensações tácteis codificadas, relacionadas ao movimento artificial. A integração sensoriomotora se deu pela aplicação simultânea dos sistemas desenvolvidos, possibilitando desde a restauração de padrões funcionais de preensão, até o reconhecimento do padrão de movimento exercido através das sensações evocadas artificialmente.

Estimulação Elétrica Neuromuscular; Estimulação Eletrotáctil; Restauração de Movimentos; Preensão; Propriocepção; Tetraplegia

ARTIGO ORIGINAL

Neuromuscular electrical stimulation and electron-tactile stimulation in rehabilitation of artificial prehension and proprioception in tetraplegic patients

Maria Claudia Ferrari de Castro^I; Alberto Cliquet Jr.^II

^IPhD in Biomedical Engineering - Faculdade de Engenharia Elétrica e de Computação of UNICAMP and Post-Doc as Electric Engineering Department of Escola de Engenharia de São Carlos (USP)

^IITitular Professor of Universidade de São Paulo (USP) at Electrical Engineering Department of Escola de Engenharia de São Carlos e Professor Titular at Orthopedics and Traumatology Department of Faculdade de Ciências Médicas da Universidade Estadual de Campinas (UNICAMP)

^{Address for correspondence}

SUMMARY

This paper discusses the use of electrical stimulation in upper limb sensorial and motor rehabilitation. Neuromuscular electrical stimulation (NMES) was used aiming to restore motor hand function by means of muscle activation sequences to perform daily living activities such as drinking, eating, writing and typewriting. Custom made gloves instrumented with force transducers were used aiming quantitative evaluation of the artificially generated movement. This system was used as a sensorial feedback supplier for an artificial proprioception system. Encoded tactile sensation relating to artificially generated movements was provided by electron-tactile stimulation. The results showed that the sensorial-motor integration attained yielded both functional movement restoration and the recognition of artificial grasp force patterns, in order to allow the neuroprosthetic system to become closer to the biologic system.

Key words - Neuromuscular Electrical Stimulation. Electrotactile Stimulation. Movement Restoration. Grasp. Proprioception. Tetraplegia.

INTRODUCTION

Activities based on object prehension and manipulation, even though are usual in common daily activities, involve a number of complex movements requiring activation of several muscle units in a time-space correct sequence. Ability and agility of upper limbs is so a reflex of the ability of nervous system to plan, coordinate and execute these movements. ^(1,2).

Spine cord injuries generally break the functional communication between upper motor controlling centers and muscle below the injury level, so preventing commands to come from supra-medullary centers and reach target muscle, as well as proprioceptive information, necessary for system feedback, can’t reach controlling centers, resulting in palsy. In case of injuries affecting levels from 5^th to 6^th cervical vertebrae (C5-C6), even though potential movement of arm is preserved (shoulder and elbow), upper limb function is hindered by lack of control of intrinsic and extrinsic hand muscles making difficult or impossible prehension of needed objects for common daily activities.

Neuromuscular Electric Stimulation (NMES) is a neural activation technique aiming to get muscle contractions with use of low current levels. Adoption of this technique doesn’t expect a neurologic restoration but to give means to an artificial restoration of motor function. Controlled activation of nervous fibers innervating specific muscles in a time-space correct sequence can be used for generation of functional patterns of prehension, contributing for reintegration of upper limbs in performing common daily activities ^(3-9).

On the other hand, it is not only motor activity that is compromised, but also the sensorial one. From a practical point of view of the daily use of NMES, this limits autonomy and performance of system users to coordinate the artificially obtained movement, mainly in prehension restoration. In this case, it is necessary just not to start a sequence of preset activation, but also to graduate the exercised force, aiming at a necessary and enough force for prehension of the different objects. An excessive force, besides could damage the object, contributes to accelerate the process of muscular fatigue, one of the main factors that make unfeasible the daily use of NMES. In that way, it is necessary also the restoration of the proprioceptive function, feeding back the individual with parameters that characterize the motor reaction artificially obtained, addressing the adjusts that may be necessary. Tactile function has been studied to serve as a supplementary sensorial input, and can help or even replace other sensorial function. Electron-tactile stimulation has been shown to be a promising technique for implementation of a supplementary communication through tactile function. The information should be coded through stimulation parameter variation that will be interpreted, allowing the individual to link the evoked tactile sensation to the information intended to transmit ^(10,13,14).

This work is intended to look at both problems, focusing sensorial-motor integration in prehension restoration in spine injuried patients, and presenting development and application of an artificial systems based on neuromuscular electric stimulation and electron-tactile stimulation.

MATERIALS AND METHODS

Aiming prehension movements restoration, it was developed an specific system for clinic and laboratory usage. The stimulator is controlled by a computer, making possible the elaboration and application of different strategies of stimulation and allowing coordinated activation of up to 8 muscle units. The output sign corresponds to groups of tension monophasic pulses, with square shaped waves, maximum width of 300 s and frequency of 20 Hz. The stimulation is released trans-cutaneously according to preset sequence and temporization, through self-adherent surface electrodes positioned at selected muscles motor points.

Sequences of neural activation were made aiming palmar prehension restoration (Table 1) and lateral prehension (Table 2). These two patterns (Figure 1) taken together make possible to perform most of the daily activities. Muscle selection was based on anatomical, kinesiologic and electron-myographic studies in normal individuals and on viability of stimulation with surface electrodes, resulting in 6 motor units: Radial Carpal Extensor (RCE), Common Finger Extensor (CFE), Superficial Finger Flexor (SFF), Thumb Abductor (TA), Thumb Opponent (TO) and Lumbricalis (L). In order to coordinate these muscles activation, the movement was divided into 4 sub-phases described as: opening, positioning, prehension and release of the object. In each one of these phases, the muscles responsible for the intended function were activated according to an specific temporization, except the prehension sub-phase, which has a variable duration, according to the activity to be performed.

A third sequence, based on L and SFF was defined aiming an adequate positioning of fingers for using a computer keyboard, which is currently a very common activity, but, for not involving objects prehension can not be performed through the previous sequences.

With the objective of restoring force proprioception two systems were developed. The first one, capable of detecting and quantifying the strength applied during palmar prehension, consists in a Lycra glove with strength sensors (FSRs - force sensing resistors) positioned on palmar aspect of medial phalanx of indicator and medium fingers and lateral aspect of distal thumb phalanx ^(8,15,16). The second system, able to encode prehension force variation information through electron-tactile stimulation, is based on application of the phi-tactile phenomenon, according to which a tactile image in movement is evoked over the skin from three pairs of electrodes when stimuli amplitude applied in sequence has a complementary variation. Preliminary experiments demonstrated that the best results are achieved with square pulses of 100 Hz, modulated in amplitude by an elliptical involving signal of 1 Hz frequency, maintaining a 180 degree discrepancy between consecutive channels. Application of this particular configuration of electron-tactile stimulation results in the perception of an ellipsis as if a pencil was exercising pressure against skin drawing dynamically the figure (Figure 2) ^(12,13).

The second phase corresponds to elaboration of a code that links the evoked image (ellipsis) and variations of prehension strength, a parameter that should be transmitted feeding back the artificial movement restoration system user. The amplitude of stimulation for each one of the three stimulation channels, proportional to the detected force for each one of the sensors of the special glove, allowing to transmit the strength pattern applied, either qualitatively, identifying the force distribution among the monitored fingers, or quantitatively specifying the magnitude of the applied force.

Sensorial-motor integration can be described according to Figure 3. The neuromuscular electric stimulation system is applied aiming artificial restoration of prehension pattern. The special glove detects and quantifies the exercised force for each one of the monitored fingers, thus defining the amplitude of stimulation of each one of the electron-tactile stimulator channels. The application of this stimulation in a region with preserved sensation (posterior shoulder region) results in a tactile sensation that should be associated to the pattern of prehension strength through a defined code.

RESULTS

Application of the NMES with defined activation sequence resulted in achieving functional patterns of palmar and lateral prehension, making possible to perform common daily activities such as eating, drinking, writing and typewriting. (Figure 4).

Activation of RCE aimed to obtain a functional position of the hand. CFE and TA give a hand opening useful for introduction of liberation of an object. For activation of palmar prehension, L activation flexes metacarpophalangeal joints starting approximation of the fingers to the object, while SFF and TO warrant prehension. In lateral prehension SFF activation prepares the hand to receive the object at lateral indicator aspect, while TO in the next phase, warrants prehension.

An adequate final motor response was possible for the definition of the specific stimulation amplitudes and adequate to each of motor units, resulting functional patterns close to normal ones ^(14,15). Once the object is fixed and stable in the hand, the individual can, using voluntary control of elbow and shoulder, manipulate it and perform the intended activity.

The relation between stimulation amplitude of the motor units is an important parameter and determinant of the quality of motor response and of the intended movement. L activation, according to the level of applied stimulation, can simultaneously activate, by mirror effect of current lines, the index finger extensor, giving a different hand configuration. Even though limiting the performance of palmar prehension obtained by the sequence defined in Table 1, this new configuration can be used for other objectives. Once the index is extended, SFF stimulation results only in extension of the other fingers. The resultant configuration, seen in Figure 4, can be used for typing a computer keyboard.

In relation to the force proprioception restoration system, the application of the defined code resulted in variations of the perception of the pattern image, characterized by interruptions in the ellipsis tracking in the region correspondent to the finger which was not exercising force or even the differentiation of different levels of pressure perceived over the skin by each one of the fingers (Figure 5). This analogy proven to be of easy interpretation by the individuals, becoming efficacious in transmission of the force pattern artificially exercised. Individuals with lesions below C6 could clearly describe the force pattern artificially exercised by means of identification and interpretation of the tactile image evoked over the skin by the application of electron-tactile stimulation.

On the other hand, the stimulated region, due to monopolar configuration of the electrodes, exceeds the sensorial preserved area in individuals with injury above C6, so that individuals with injury between C5 and C6 presented a partial perception of the image, corresponding to the superior region of the ellipsis. The inferior region could not be clearly identified, so limiting the efficacy of the code used in information transmission.

DISCUSSION

Even though hand movements are widely complex and varied, it is possible to restrict them to palmar and lateral prehension, which together can allow performing most of daily activities. Application of NMES makes feasible to artificially restore these movements, helping spine cord injuried patients to get back their independence.

The developed NMES system was adequate to clinical and laboratorial use, and can be of help in rehabilitation process of upper limbs. The presented sequences allowed individuals to demonstrate their ability to hold and release daily use objects, allowing to perform activities such as eating, drinking, writing and typewriting.

Besides this, the use of surface electrodes, considered inadequate to restore prehension did not show to be limiting of system performance, since are correctly placed at motor points, only restricting the number of motor units available for stimulation. The quality of artificially obtained movement determinant patterns are correspondent to physiological condition of the motor units, activation sequence between involved motor units and the stimulation amplitude relation between the involved motor units in prehension pattern generation.

However, pro a daily use of NMES it is necessary to develop a functional system which is autonomic and micro-processed, allowing the individual, through a command mechanism, to control and coordinate the movement, temporizing and graduating the motor response according to the needs of the intended activity.

On the other hand, movement control presupposes an evaluation of the defining parameters. Once medullary injury affects not only motor function, but also the sensorial one, it is necessary to simultaneously implement an artificial proprioception, directing precise and consciously any action of the individual over the system, giving means to adjust the motor response to the intended movement.

Prehension movement can be characterized according to the fingers position and exercised strength. The first parameter can easily be monitored through visual feed back, what is not possible to be performed about prehension strength. In this context, electron-tactile stimulation, based on the phi-tactile phenomenon, demonstrated, due to the used code, to be a promising technique in artificial restoration of prehension force proprioception, since the stimulated region has preserved sensorial function. The use of stimulation amplitude as a parameter for information transference allowed creating different evoked image over the skin patterns that could be easily interpreted in qualitative and quantitative description of the force pattern exercised.

Yet individuals with injuries at C5 presented sensorial deficit in 50% of the stimulated region, limiting evoked image perception. In these cases alteration in transference codes should be provided, or the use of a bipolar configuration of the electrodes, restricting the region to be used.

CONCLUSIONS

Restoration of prehension patterns and force proprioception by the use of NMES and electron-tactile stimulation shows that sensorial-motor integration is not only feasible but fundamental to improve performance and functionality of restoration of prehension movements, in an attempt to make artificial movement system closer to the physiologic one.

ACKNOWLEDGEMENTS

The authors would like to thank CNPq and FAPESP for financial support to this work.

REFERÊNCIAS

Work performed at Department of Orthopedics and Traumatology of Hospital das Clínicas da Faculdade de Ciências Médicas of the Universidade Estadual de Campinas (UNICAMP).

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Publication Dates

• Publication in this collection
  20 Feb 2006
• Date of issue
  Sept 2001