Design of upper limb assistive device

Summary

This article summarizes the results of the project which deals with a new approach to designing ortho-prosthetic devices in terms of mechanical solution and innovative design. The work was addressed to a specific patient with congenital malformation of upper limbs called phocomelia. A custom-made compensatory aid was designed on the basis of personal consultations and meetings at specialized workplace; however the aid was built in a way and from parts for other kind of disability. As it turned out, the use of this device is associated with many complications, and therefore as a result, this solution proved as unsatisfactory. Nevertheless, thanks to this compensation aid, it was possible to specify the problems, to reveal important patterns and highlight the needs for development of entirely new and unique aid designed for patients with phocomelia disability. The work is based on these findings and knowledge acquired from bibliographic search in the field of orthotics, prosthetics, assistive devices and prosthetic equipment. A complete redesign was done with the aim to eliminate all detected problems. The presented result is a unique tool for people with specific congenital disability of upper limbs, with working label 4TE.

Key words:
orthoprosthetics – phocomelia – congenital malformation of upper limb – design – redesign – assistive device – handicap

INTRODUCTION 

Phocomelia (from gr. foco melos = seal limb) is a severe congenital malformation defined as a transverse, intercalary defect mostly on upper extremities [1]. It is presented with the hand or its part attached directly to the thorax [2]. Phocomelia is exceedingly rare, with a prevalence of five of 4,024,000 in one series [1].

A phocomelic arm (Fig. 1/2) is characterized by a short length, which is associated with the absence of muscle mass. This combination causes an insufficient force of the arm and hand. The arm is connected directly to the shoulder girdle and has no elbow. Functionality of the entire arm and its manipulation space are significantly limited. A phocomelic hand has fingers, but they are considerably weakened due to missing muscles and tendons in the forearm. The thumb is often huddled in the palm, or is completely missing [3]. In the former case, the thumb is not workable and does not provide support to other fingers.

One of the phocomelia treatments is surgery. During the operation the bone is extended in the arm and the curvature is corrected. The surgery must be performed at a very early age, when tissues are still in development [5]. The case of shoulder joint replacement of a phocomelic patient has also been published. The performance of this surgery does not depend on the patient’s age [6].

The phocomelia patients suffer from a number of problems. The main one is self-sufficiency; patients are dependent on the assistance of another person, which also has a negative impact on their psyche. An insufficient range of manipulation capabilities prevents many routine actions during everyday activities. However phocomelia patients are able to care for themselves. During their life the patients learn how to effectively use the limited function of their arms and hands. Patients are able to write, eat with a spoon and control a mobile phone. A great disadvantage is the limitation of defensive and protective functions, such as a fall protection by hands. Technical support for disabled persons offers a range of modern equipment and facilities, which are characterized by high levels of technological design and material quality and also superior design. The equipment is designed and manufactured according to this general basic rule: The objective has to meet three basic principles [7, 8] according to their priorities: 1. comfort, 2. function, 3. design.

Phocomelia is one of the disabilities for which the compensatory aids have not been yet developed. Patients with phocomelia disability of upper limbs are mostly dependent on the assistance or on alternative and homemade tools.

This article presents a compensatory aid produced by the corporation MS Ortoprotetika from the parts available on the common market. It is the prosthesis primarily designed for other disabilities (amputation, congenital amputation); therefore it is only an alternative solution and its use in phocomelic patients is associated with many problems.

However this prosthesis was a valuable achievement, because it helped to identify problems concerning its use, which proved the need to develop entirely new and unique tools focused on phocomelic patients.   

INPUTS 

The resulting solution was preceded by a series of personal consultations with the patient and his family. Joint visits in the specialized ortho-prosthetic company were a valuable source of skilled information. Several measurements of the patient upper limbs were made with 3D optical scanner ATOS (Fig. 2). Scans served as a basis for creation of the virtual model which was used during computer design of the resulting aid. A plaster cast of the patient’s right hand was made by ING Corporation s.r.o. using casting into the alginate impression material Elastic Cromo, which served as an alternative mass model during the design.

An important input was the prosthetic aid (Fig. 3) customized to our patient, manufactured by specialized producer MS ortoprotetika, s. r. o. It is a myoelectric hand – DynamicArm 12K100. This type is externally driven, i.e. it is not driven by the patient’s muscles. Within commercially available prosthetics, it is the most innovative solution, but also the most expensive and the most susceptible to damage. This prosthesis is controlled by myoelectric signals that are sensed on the patient’s skin by electrodes; it uses biochemical processes in the muscle. This prosthesis is dependent on the occasional battery charge [9, 10].

The DynamicArm 12K100 did not prove useful for patients affected by phocomelia. It unnecessarily restricts the patient, it is problematic to put on, it does not use enough potential movement of his own hands, and its humanized design unnecessarily invokes concerns in the people around. However it served to clarify the basic principles and requirements for the redesign and formation of the new aid in order to effectively eliminate all the imperfections and also to provide additional usability.   

METHODOLOGY 

The design plays an important role in orthoprosthetics; it contributes to a social inclusion of disabled people and is a tool for improvement of their life quality. Redesigning can be understood as a transformation process of already existing thing and creating of something new. It may not necessarily be only a material innovation. In a broader sense it can be understood as an ideological change. Each successful redesign has to bring a change towards a better state and not vice versa. In case of this project, it was a complete change of concept beginning with a mechanical design, fixing on the patient’s body, and finally the proposal of control and management of the aid. The procedure is outlined in the following key activities. 

Key activities

• Evaluation of the benefits and disadvantages of the DynamicArm12K100 prosthesis
• Specification of new parameters
• Determination of a new concept
• Design of concrete structural nodes, ergonomics and design   

RESULTS 

The final form of redesigned aid (Fig. 4 and 5) comes from a deeper reflection on the essence of the problem situation and the need for a more comprehensive solution that would remove all the identified problems. The aid has a working label 4TE [forte]. This designation corresponds to the purpose of the aid, which struggles to change a disadvantage into an advantage, a forte. The resulting concept (see Fig. 4) tries to be a fully valuable tool providing a wide range of positioning while maintaining the natural mobility of the user. It features a very comfortable fit, simple and intuitive control with feedback; the user is aware of the grasp strength of the hands. The great advantage of this solution is independency – the user alone can deploy, operate and remove the aid without much difficulty or without assistance of another person.

The 4TE aid has two independent arms, each with 6 degrees of freedom. The arms are equipped with anthropomorphic hands at the ends. Each hand has 3 degrees of freedom and 10 pressure sensors (Fig. 6). Sensing of the pressure is expressed as a haptic feedback through the control device. Arm movement is controlled by a SpaceNavigator controller. Controllers are positioned in the middle part of the chest, comfortably achieved by the user with phocomelia disability. Thanks to the system of rotating joints connected at the angle of 45°, it is possible to set up the aid into many different positions. The basic position is in the folded state, where the arms are folded inside the backpack. After device activation, arms will be firstly tilted and then set up into the standby position (perpendicular to the body), from which the arms can be arbitrarily controlled using the SpaceNavigator. The telescopes allow reaching distant locations (Fig. 7).

The 4TE assistive device has an anthropomorphic hand with adjustable fingers [11] (see Fig. 6). Two of the three fingers can mutually symmetrically rotate, thus changing the type of grip from cylindrical grip to spherical grip. Wide opening enables to carry flat or broad objects such as a plate. Constriction of the hand results in pinch grip which allows fine manipulation with smaller objects.

The 4TE device is controlled by means of a manual controller (Fig. 8), which operates on the principle of a device called SpaceNavigator. The controller of the 4TE thus enables fast positioning with immediate feedback. The main advantage of the SpaceNavigator is the possibility of a compound movement. In total there are 6 simple moves that are not separated; the controller can move in three directions, tilt in two directions, and rotate around one axis. This allows compounding of moves and due to this it is possible to move along any desired trajectory. The controller therefore allows movement in all three space axes (XYZ) at the same time [12].

   

DISCUSSION 

The assistive device 4TE offers many advantages to patients with phocomelic disability. It was primarily designed to eliminate the disadvantages connected with the use of aids available today for phocomelics, but it is only an alternative solution designed for other types of disabilities, and its use is associated with numerous problems from difficult deployments to unsuitable control, which eventually becomes a burden for the user. 

A summary list of benefits that 4TE aid provides to the user:

• Become independent – no need for further human assistance

It was verified that putting on and wearing of the backpack is an ordinary operation for the patient affected by phocomelia. The user can handle to put on and take off the backpack in a few seconds like a healthy human. Therefore it is not necessary to be assisted by another person in terms of any usual handling of 4TE aid. This is for a phocomelic patient, who is commonly dependent on the help of another person, a very important step in his life, for his psychological well-being and social integration.

• Easy to put-on and take off – easily removable

Simple handling allows the user to remove and put on the 4TE aid easily according to his needs.

• It is not necessary to get undressed when removing or putting on the aid

The 4TE aid is worn like a normal backpack, therefore it is not necessary to get undressed before removing or putting it on, as it is common with myoelectric controlled devices. The whole operation is then much faster and easier.

• Fewer restrictions – more movement

The backpack type of construction does not significantly restrict the patient’s own hands in their natural movement. The 4TE aid does not take too much space in the folded state, its height and width copy the back of the user and the depth of 4TE aid occupies only 105 mm.

• Variability

The 4TE aid can be adapted and used for one-sided handicap, although primarily it is designed for both side handicapped patients. In this case, the battery and the part of the mechanisms will be moved to one side of the bag – towards a healthy hand to provide the counterbalance to the weight of the 4TE arm. Moreover, it is assumed that the 4TE aid can be used as auxiliary equipment for handling the work of a healthy human.

• Intuitive control

Handling of the SpaceNavigator device is very intuitive and fast to learn unlike the myoelectric controlled devices. The 4TE aid is controlled by a similar type of the control device based on the SpaceNavigator. It allows a natural control by the user’s arms. The movement of the aid follows the movement of the navigator; it is a smooth composed motion. Moreover, the controller resists the movement so that the user knows which movement is being performed, even without a visual control.

• Feedback – perception of grip strength

The controller body contains a built-in controller for the grasp of hands. The operation is done with one finger, which is inserted into the hole where its movement is captured and the grasp or the opening of hands is controlled. An internal pneumatic mechanism that transmits the information from pressure sensors located on the hands also provides a feedback – a sense of touch. Thus it is possible to transmit soft and fragile objects without being damaged.

• No negative effect on posture – supports proper posture

The backpack type construction of the aid is the healthiest option for carrying the load. Weight is evenly distributed over the shoulders and does not burden the arms.

• Space-saving

Its small size, compact shape and low weight ensure easy handling, transportation and storability.

• Honest and attractive technical design – no shock for the people around

The appearance of the aid 4TE does not try to hide the disability. At first glance it is clear that this is the high-tech equipment, whose top priority is functionality and flawless service to the user.

Disadvantages:

There is no evidence that the user will be able to handle the straps and buckles of the backpack without problems. Probably it will be necessary to devise a simple system for switching the buckles and handling the straps and this will probably require a long-term training.

The illustrative diagram (Fig. 10) graphically shows a comparison of range of movement from the previous charts, photos and visualizations (see Fig. 8 and 9).

The graph in Fig. 11 summarizes and compares the range of movement for both devices – an average healthy male figure and our patient.

Comparisons of observed and estimated parameters, which are common for DynamicArm 12K100 and 4TE aid, are presented in the following table (Fig. 12). The parameters are of qualitative and quantitative type and are evaluated by a color range from positive to negative ratings. The table clearly shows that the 4TE aid has significantly better ratings for most parameters. It can be stated that undesirable characteristics and negative effects of DynamicArm 12K100 device on the user have been significantly eliminated. The table divides the individual parameters according to three basic principles – comfort, function and appearance (see Introduction), which mutually interact and overlap.

   

CONCLUSION 

This paper presents a new approach to the design and construction of the assistive devices in orthopedic prosthetics.

The result of the work – a design of unique ortho-prosthetic aid-outlined a new direction in the design of aids for people with congenital developmental defect called phocomelia, for whom there is no special equipment currently available on the market. A design approach, which is presented here, is based on the non-conventional conception of the aid as a technical object that does not need to imitate the human body. The goal of the project was not to offer the details in terms of mechanical design. The project endeavors to propose an innovative solution and it is believed that it will serve as the basis for further development in this field and that this first step will contribute to finding an optimal solution for the hard situation of the people affected by phocomelia, or that it will bring a benefit into the areas such as working manipulation.   

Došlo dne 3. 4. 2012.

Do tisku přijato dne 20. 4. 2012. 

Kontaktní adresa:

Ing. Olga Minaříková

Odbor průmyslového design

Ústav konstruování

Fakulta strojního inženýrství

Vysoké učení technické v Brně

Technická 2896/ 2

616 69 Brno

e-mail: yminar03@stud.fme.vutbr.cz