FES for hand grasp and release

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Technology for the restoration of hand function in tetraplegic individuals has been under development for over three decades (Billian and Gorman, 1992) and has now entered the clinical environment (Peckham et al., 2001). The objective of the use of FES in hands of tetraplegic individuals is to restore grasp, hold and release, thereby increasing independence in performance of functional tasks.

Tetraplegic hand grasp systems have focused on the C5 and C6 level SCI patient populations. These individuals have adequate voluntary strength in the proximal muscles (i.e., deltoid, rotator cuff, biceps) to move their hand in a functional space. Those with C4 level injury have also participated in limited laboratory-based investigations of FES systems (Nathan and Ohry, 1990), but the results of these studies have been limited. Patients injured at the C7 and lower levels have multiple voluntarily active forearm muscles which can be used to motor new functions by means of tendon transfer surgery (Keith et al., 1996).

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□ implanted components □ External components

Sensor feedback electrode

Implantable receiver stimulator in-line connectors

Electrodes '

| Triceps electrode

Transmitting coil lapW

Shoulder controller

External control unit

Figure 9.3. Components of an implantable upper extremity neuroprosthesis. On the left of the diagram are the implanted components, which include the implant stimulator, electrode leads, epimysial electrodes, and in some cases a sensory electrode to provide a form of sensory feedback. Not shown are implantable intramuscular electrodes which are an available option. On the right are the external components of the neuroprosthesis, which include a shoulder position controller incorporating the device's on/off switch, an ECU, and a transmitting coil.

Two commercially available FDA approved upper extremity neuroprostheses have been developed. One of them is a surface system and one is implanted. These devices will be discussed separately.

Freehand system

The Freehand System® consists of an external joint position transducer/controller, a rechargeable programmable external control unit (ECU) and an implantable eight-channel stimulator/receiver attached via flexible wires to epimysial disc electrodes (Fig. 9.3). The user controls the system through small movements of either the shoulder or wrist. The joint position transducer is typically mounted on the skin from sternum to contralateral shoulder or across the ipsilateral wrist. The ECU uses this signal to power proportional control of hand grasp and release. Communication between the ECU and the implantable stimulator, which is located in a surgical pocket in the upper chest, is through radio-frequency coupling. The system can be programmed through a personal computer interface to individualize the grasps as well as the shoulder control (Kilgore et al., 1997). Individuals can choose to use either a palmar or lateral prehension grasp pattern. This is helpful in handling either large objects or small objects respectively. The current system is intended to provide unilateral hand grasp only. Results from the multicenter trial that led to FDA approval in 1997 are discussed below (Peckham et al., 2001).

Sixty-one C5 or C6 patients were enrolled. All of the patients had one or more concurrent surgical procedures to augment hand function. A total of 128 cumulative implant years were evaluated. Summary pinch force measurements with and without the neuroprosthesis are shown in Fig. 9.4. Pinch force in both lateral and palmar prehension improved with the neuroprosthesis. The small improvement seen post-operatively with the neuroprosthesis turned off can be attributed to tendon synchronization. All patients realized some improvement in pinch force measurement in at least one grasp pattern with the neuroprosthesis.

A grasp and release test was used to evaluate each subject's ability to grasp, move, and release six standardized objects. The number of completed moves in 30 s achieved by subjects increased significantly for the heavier objects with the neuroprosthesis.

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