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A Foot Rotary Mechanism for the Modular Below-Knee Prosthesis

Al-Turaiki, M.H.S., B.Sc. (Hons). Ph.D.

Historical Background

In 1973, the Riyadh Medical Rehabilitation Centre was established in Saudi Arabia for the purpose of providing prostheses, orthoses, and other walking aids to physically handicapped persons. In 1986, an Orthopaedic Bio-engineer was appointed to join the Medical Committee which then consisted of an orthopaedic surgeon, an orthotist-prosthetist, and a physiotherapist. Consequently, a joint Unit for Research and Development in Prosthetics and Orthotics was established. This Unit is now being jointly run by the College of Medicine, College of Applied Medical Sciences of King Saud University, and Riyadh Medical Rehabilitation Centre of Ministry of Health.

Although the Rehabilitation Centre provides service primarily for the Central Region of the Kingdom of Saudi Arabia, it is not unusual to find a considerable number of patients coming from all over the Kingdom to the Centre for treatment.

Therefore, the incidence of different cases (Figure 1) , the incidence of below-knee amputations (Figure 2) , and amputation types (Figure 3) are to a certain extent representative of the whole of the Kingdom.

Origins and Objectives

Many below-knee amputees,3 who have been fitted with PTB modular prostheses (Figure 4) , complain of painful knees and of strain on their hips and back when sitting during performance of prayers. A simple force analysis of the body in the sitting position (Figure 5) shows that the body's center of gravity is displaced anteriorly with the weight being supported by the anterior aspect of the knee and the artificial forefoot.

Therefore, the main objective of this work was to incorporate in the prosthesis a reliable foot rotator to alleviate this problem by uniformly distributing the load to the entire artificial limb (Figure 6) . This will help many muslims all over the world with below-knee amputations to carry out their religious duties with minimal inconvenience as well as making the prosthesis behave as a normal leg with respect to foot rotation (Figure 7) .

Method

Subjects

it was decided that only one subject would be involved in this investigation. The selection was made according to the following criteria. The patient must be:

  1. an unilateral below-knee amputee wearing a patellar tendon bearing below-knee modular prosthesis;

  2. in good general health and with no skin problems with the residual limb;

  3. willing to participate in this experimental work for a duration of three months.

Design Criteria

The goal was to provide the below-knee amputee with a modified PTB prosthesis and facilitate comfortable sitting during prayers (Figure 8) .

In making such a modification, we were striving for the following:

  • function and reliability

  • safety

  • light-weigh

    t

  • frictionless

  • rust-resistance

  • durability

  • local manufacture

  • low cost

  • easy fitting

  • not affecting aesthetic value

  • silent

Also, in designing and constructing the device, several practical considerations were:

  • no machining required

  • use of standardized parts

  • possibility of adjustment

  • simplified assembly

  • maintenance free

Material

Other than standard parts, mild steel was used to make various parts of the device, since it has several desirable characteristics: sufficiently strong, readily available, inexpensive, and easily machined. Other properties, such as rust resistance, were considered when making the final prototypes.

Description

The basic purpose of the foot-rotary mechanism was to permit safe and smooth yet controlled, internal-external axial rotation (+/- 90 degrees) of the foot relative to the shank piece of an Otto Bock modular PTB below-knee prosthesis.

The rotator (Figure 9) is interposed in the foot shank interface (between 4 and 6) and consists of low friction devices in the form of bearings13 and a pin-hole locking mechanism11,16 to prevent any rotation during walking.

The annulus (lower half of mechanism) is fastened to the foot by means of a connector (e). The three holes in the annulus are made to receive the pin (i) which is used to stop rotation. The pin is (i) loaded with a spring (g) and can be safely fixed to the shank piece (c). Said pin (i) can be disengaged by pulling it out using the cable control handle attached to the pulling cord (a).

The prosthesis, according to the invention, allows the foot component (7) to be positioned pointing anteriorly, medially, or laterally as desired. This is achieved by simply pulling the control handle up, pressing the foot component (7) against the ground and rotating the leg with respect to the fixed foot. Frictionless motion is achieved through the use of a thrust bearing (13) attached to the annulus (16) via a bolt (14) and nut (15).

The upper half of the mechanism is fastened to the modular shank piece (c) via a flange (f) with locking screws (o). This part also has the bearing housing (h) and locking pin assembly (i,g).

The upper half of the mechanism is fastened to the lower half via a locking screw through a hole (p).

Discussion and Conclusion

The approach described has been found to be valuable for alleviating the problems faced by muslim below-knee amputees while offering their prayers. Another advantage of this method is the fact that no modification is necessary for incorporating the rotary mechanism into the modular PTB prosthesis because most components are standard parts, available locally. This makes the system more cost effective. The prototype made proved to be an effective and safe device for the purpose of this experimental work. However, if this mechanism is to be adopted as a standard part of the PTB prosthesis, more work has to be carried out on the selection of material and reduction of the number of components required.


Al-Turaiki, M.H.S., B.Sc. (Hons), Ph.D., is Assistant Professor of Orthopaedic Bioengineering at College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia. He is also Director of The Joint Centre for Research in Prosthetics and Orthotics at Riyadh Medical Rehabilitation Centre, Saudi Arabia.

References:

  1. Al-Turaiki, M.; Y. Haggag; M. Zarrar; and L. AL-Falahi, "Design, Construction, and Assessment of a Simple Foot Rotator for the Conventional BK Prostheses of Praying Muslims," presented at the Fourth International Conference on Biomedical Engineering, National University of Singapore, Faculty of Engineering and Faculty of Medicine, IV-6, 1987, pp. 1-15.
  2. "Statistical Review," published by Department of Statistics, Ministry of Health, Riyadh, Saudi Arabia, 1987.
  3. Al-Turaiki, M., "A Ten-Year Experience of Rehabilitation Engineering Services at Riyadh Medical Rehabilitation Centre" Proceedings of the Symposium on Rehabilitation held at Riyadh Central Hospital, Riyadh, Saudi Arabia, 1986.