WO2003003953A1

WO2003003953A1 - Rotatable joint stop mechanism

Info

Publication number: WO2003003953A1
Application number: PCT/GB2002/003122
Authority: WO
Inventors: Brian Andrews
Original assignee: University Of Reading
Priority date: 2001-07-06
Filing date: 2002-07-05
Publication date: 2003-01-16
Also published as: GB0116503D0

Abstract

In or for a biomechanical joint, a bracing system comprises: a) a power driven linear-action actuating means incorporating a member which, when said means is actuated, advances or retracts in order to optimise the degree of angular motion permitted to said joint; and b) processing means for controlling the operation of the power driven actuating means for a variety of joint motions and/or loading conditions. Advantageously the linear-action actuating means is a screw.

Description

ROTATABLE JOINT STOP MECHANISM

Field of the Invention

The invention relates to stop mechanisms for rotatable joints and is particularly, though not exclusively applicable to biomechanical joints which are fitted as replacements for the natural joints of the ankle and the like.

Review of Relevant Art Known to the Applicant

Conventional long leg callipers, developed originally to provide unilateral leg stabilisation in cases of weakened quadriceps muscles due to polio, spinal cord injury and so on required mechanical components (uprights, steel bands, mechanical locks etc) around the joint or the thigh to stabilise the knee during stance. They also deliberately prevented the knee from flexing as the leg was swung.

The limitations of this crude brace led in the 1960's to the work of such researchers as Saltiel and Lehneis in developing ankle foot braces. This basic brace is called a floor reaction orthosis (FRO). Its biomechanical action is to stop ankle joint dorsiflexion in approximately the neutral position. This action allows a paralysed leg to be stabilised in extension without the need for muscular action. Its effect is similar to the experience of wearing a ski-boot, where if one leans forward slightly the knee is forced into full extension by the action of the foot to ground reaction force. With the FRO the base of this reaction force can shift to the forefoot region and the force vector can be directed ahead of the knee joint axis and thus mechanically stabilise the leg.

The FRO brace can be applied to standing paraplegic patients. However in a bilateral application, electrical stimulation is needed to stabilise the leg if the ground/foot force vector is to shift behind the knee - as would be the case if the patient were to pull on a door handle for example. Such destabilising events are infrequent and transient in nature. In the case of unilateral application, as in the original applications of Saltiel and Lehneis, the patient can rely on the good leg to prevent collapse. In any application to paraplegics, sensors are needed to detect or even predict incipient knee buckling and to switch on and off the functional electrical stimulation (FES).

Suitable arrangements are described in the literature including:

US patent number 5,121,747 June 16^th 1992 (Hybrid Orthosis) and US patent number 5,609,568 March 11^th 1997 (Ankle Foot Orthosis)

Further work has been carried out by the present inventor(s) in which when predicting incipient knee buckling the sensors within the brace were used to estimate the direction of the ground to foot force vector. If the force vector lay ahead of the knee, the leg was stable and the knee and hip extensors could be automatically switched off otherwise they remained stimulated. Due to the postural biomechanics of quiet standing, the FES could be switched off and the duty cycle of activation of the knee and hip extensor musculature greatly reduced (approximately 6% versus 100% of time when not using the FRO/FES system). Thus muscle fatigue effects could be avoided in the stimulated muscles which enabled paraplegics to remain standing for periods well in excess of an hour compared to a few minutes without the FRO/FES system.

Summary of the Present Invention The present invention is exemplified in a controllable brace joint stopping mechanism (CJS) although as made apparent when defining the field of the invention, this is just one example of any machine joint in which the rotation of the joint must be stopped at precisely controllable position(s) with no effect on motion in the opposite direction and where the mechanism must be physically small, lightweight and low power.

Traditionally, clutches have been used for such purposes e.g. wrapped spring clutches with electromechanical control mechanisms. Other actuators that have been used are electromechanical brakes, magnetic particle or fluid clutches. These mechanisms represent a more bulky and heavier option as well as consuming more power.

A CJS however is required to have a low profile, for cosmetic reasons, and low mass so as not to impede or increase the physical effort of moving the paralysed limb. It is also required to resist very high torques in excess of 100 Nm. Furthermore the electromechanical mechanism should consume minimal power since applications are likely to be battery powered.

The solution therefore requires inventive thought if these conflicting factors are to be balanced.

Scope of the Invention The scope of the invention is defined in the numbered claims which end this specification.

The Specific Embodiment of the Invention

The invention, as exemplified in the embodiment described herein, uses a combination of orthotic and FES technology to allow some of the ambulatory outcomes to be improved over those obtained when using either technique alone. First generation systems used various forms of reciprocating orthoses with open-loop FES. Although useful benefits have been demonstrated, these systems have not been widely used. The braces alone are often overly encumbering and unsuited to donning or doffing from a wheelchair.

Here we disclose a modular hybrid FES system that can be matched to a paraplegic's requirements. In this system, mechanical brace components can be rapidly attached, from the wheelchair, to convert from an ankle foot orthosis (AFO) to knee, ankle, foot orthosis to (KAFO) to hip, knee, ankle, foot orthosis (HKAFO) system as required. We describe the below knee AFO part of the modular system used to prolong upright activity and aid those activities encountered in home and workplace that present difficult wheelchair access.

A novel AFO, of the floor reaction type, is described that includes pressure and motion sensors and can incorporate a docking mechanism to rapidly attach above knee components of the modular system. Each AFO incorporates a low profile, computer controlled, ankle joint. The sensory signals are processed in real-time by microcomputers that control the application of FES and the state of the ankle joint. Artificial Intelligence (Ai) Machine learning techniques are used to predict incipient knee buckling so that FES can be applied only as required in order to avoid knee buckle, minimise induced muscle fatigue and prolong upright activity. The sensory data gained can then be used to determine the phases of gait to automate the system during arnbulation.

Brief Description of the Accompanying Drawings In the drawings which form part of this specification:

Figure 1 shows schematically the basic system; whilst

Figure 2 is again a schematic view of the prototype FRO with the system installed.

Detailed Description of the Illustrated Embodiment

Figure 1 shows a system for a brace joint having uniaxial articulation with a centre of rotation at (9). This prototype was developed using a standard ankle joint supplied by Bekker. The upper steel (8) is embedded into a composite carbon fibre plastic part that is moulded to the shape of the patient's shank so that pressure is applied in the area around the insertion of the patella tendon. The lower steel (11) connects with the rigid composite plastic footplate that inserts inside the shoe. A miniature electric motor/gearbox (supplied by Maxon) (1) is enclosed inside a slotted restraining tube (2). The tube is fixed to the moulded shank part of the brace i.e. fixed with respect to the upper steel (8). The electric motor/gearbox (1) has two radially attached flanges (13) that fit inside an axial slot (12) in the mounting tube (2). This slot and flange arrangement prevents the cylindrical motor/gearbox from rotating axially inside the tube (2). However, the motor/gearbox (1) is free to slide axially inside the tube (2) constrained by the flanges (13) within the guide slots (12).

The output shaft (5) of motor/gearbox (1) connects with a threaded stud (10) through two-in-line universal joints (6) and (7) supplied in this example by Stock Drive Products (USA). These universal joints allow for a degree of misalignment between the axis of the motor and the axis of the ankle joint chamber - this provides flexibility in positioning the motor/gearbox actuator with respect to the ankle joint. Rotation of the motor output shaft (5) causes the threaded stud (10) to screw in or out of a threaded chamber within the ankle joint.

It is envisaged that the rotation of the motor is controlled by processing means. In order to determine the required drive of the motor and the corresponding displacement of the threaded stud, the processing means is fed data from appropriately located sensors. These sensors can be pressure sensors for example located in the sole of the patients shoe. Other sensors may monitor a muscular activity of a particular muscle. The characteristics measured by these sensors will be processed by the processing means while the processing means simultaneously takes into account the sensed screw position and its loading condition. Taking all these into account, the processing means will determine which action is the most appropriate.

As the stud (10) moves in or out of the chamber there will be a similar linear sliding motion of the motor/gearbox (1). This motion is sensed by two Hall effect sensors (3) that detect the linear motion of a small permanent magnet (4) attached to the body of the motor/gearbox (1). Plantar flexion is resisted by a compression spring (not illustrated in the figures) retained in the rear chamber of the joint by a grub screw.

The position of the threaded stud (10) controls the ankle joint dorsifiexion stop. With the stop engaged to prevent dorsifiexion beyond the neutral position, the brace behaves similar to the previous FRO. The Hall effect sensor (3) signals are used as input to a closed loop stud position control system implemented using a microcomputer. In order to minimise the size of the electric motor/gearbox and the required battery power, the control system attempts to position the stud (10) when the joint is detected to be unloaded. This is determined by how much current is drawn by the motor/gearbox (1) - if the joint is loaded this will be higher than when it is not.

The motor (1) current is determined by sensing the voltage that is developed across an in- line resistor. This motor current signal is input to the microcomputer control system.

It is also envisaged within this invention that the monitoring of the voltage can be employed to determine the type of motion of the joint. This data can then be processed to achieve a particular displacement of the screw or threaded stud (10). The control system repeatedly tests for the joint unloaded condition prior to moving the stud (10). Identifying when the joint is unloaded or in a condition other than braced against the screw is particularly advantageous because such a system requires only minimal battery power and therefore can be particularly compact which is also advantageous for aesthetical reasons.

It is also envisaged that if unloading does not occur after a few attempts, then a beeper sounds to indicate to the patient that he should adjust his posture so as to unload the joint and allow the control system to move the stud. The sounder is switched off when the stud has been positioned.

In Figure 2 the prototype floor reaction orthosis is seen with its unilateral computer controlled ankle joint. The thin section carbon fibre composite moulded plastic shank and shoe inerting footplate can be seen outlined in black. The Bekker ankle joint with upper and lower steels is used. The dorsifiexion stop is engaged limiting dorsifiexion to the neutral position i.e. 90 degrees. The PCB next to the ankle joint accommodates the Hall effect position sensors and the PIC microcomputer that implements the dorsifiexion stop position servo system (the other PCB shown is another PIC with additional sensors used for FES control). The brace as illustrated weighs approximately 1.5 lbs (say 0.75 Kg).

The use of a threaded screw allows for a large mechanical advantage in that it cannot be back-driven by ankle joint loading. Thus, large ankle torques can be restrained within a very small mechanism. Since the motor/gearbox has only to overcome friction it can be optimised for size and power consumption. There are two advantages of having an ankle joint in which the stop can be accurately adjusted. Firstly, in previous designs, there was no ankle joint and thus the ankle joint position was fixed once the plastic brace was moulded i.e. no adjustment is possible. This can present a problem if the patient wishes to change shoes with different heel heights as this can affect the biomechanical action - it could reduce the knee extending action making the brace ineffective or it could increase the knee extending force action unnecessarily, which could have potentially long term damaging effects in stretching the joint ligaments. This degree of adjustment can be achieved using a joint, such as the one used in the present prototype, with a manually adjustable stop.

The advantage of having computer control is that adjustment can more easily be made with the patient wearing the brace or alternatively the joint stop can be automatically adjusted using sensors that indicate the inclination of the shank, the ankle angle, the reaction forces developed in the brace and pressures developed between the brace and the limb.

Secondly, in previous designs of brace, once the ankle joint has been set for optimal effect in stabilising the knee it is fixed. However the stabilising effect is only required during standing or during the stance phase of gait. The dorsifiexion stop can impede other ambulatory activities such as walking up a slope or going down steps. In such activities the hard stabilising action of the stop is not required and a free or softer spring resisted joint would be preferred. Therefore, the invention also envisages that the screw or stud means can be adapted to gradually stop the motion of the joint. This could be achieved by cushioning means.

The CJS has a further advantage in that the stop can be quickly removed or applied as required under computer control.

Further Embodiments of the Invention

In the preceding section, the joint described incorporates screw means adapted to advance or retract in order to optimise the degree of angular motion permitted by the joint. The invention envisages other power driven linear action actuating means such as a piston. It may be advantageous in certain circumstances to employ a piston instead of the screw means. For example, it is thought that a piston may be well adapted to achieve a gradual stop. Taking into consideration the above disclosure pertaining to a system with a screw means, the person skilled in the art will be able to derive and adapt the current invention to a piston type of actuating system.

The invention as detailed in the previous section envisages a processing means which would take into account data obtained from sensors placed at a variety of locations of the patient's body. The invention of course extends to the situation where the patient himself inputs the type of motion he intends to follow. For example, the patient could use a remote control to select between walking, standing, walking up hill and walking down hill. Taking the patient's selection into consideration, the processing means would select from a series of pre-determined motions in its memory the required stop position for the joint.

Although the CJS has been described here for an ankle joint it could be used on any joint. Although only one rotary movement of the joint, that of dorsifiexion, is stopped the method could be used to stop both directions of rotation. Although described here for use in orthotics the invention may extend (as stated previously) to any machine joint in which the rotation of the joint must be stopped at precisely controllable positions with little or no practical effect on motion in the opposite direction and where the mechanism must ideally be physically small, light in weight, and require low power consumption only.

None of this, nor the limitations in specific applications of the embodiment described and illustrated, is intended to limit the overall application of the invention which extends wholly within the spirit and scope of the claims which now follow.

Claims

1. In or for a biomechanical joint, a bracing system comprising:

a) a power driven linear-action actuating means incorporating a member which, when said means is actuated, advances or retracts in order to optimise the degree of angular motion permitted to said joint; and

b) processing means for controlling the operation of the power driven actuating means for a variety of joint motions and/or loading conditions.

2. A system according to claim 1 in which the linear-action actuating means is a screw.

3. A system according to claim 1 or claim 2 comprising sensing means adapted to determine the loading conditions on the joint and/or its motion, the data determined by said sensing means being processed for controlling the operation of the power driven actuating means.

4. A system according to either claim 2 or claim 3, wherein the member comprises the screw of the power driven actuating means or an extension thereof.

5. A system according to any of claims 2, 3 and 4 in which the screw actuating means operates via a continuous screw thread.

6. A system according to any preceding claim, wherein the member incorporates means adapted to gradually stop the motion of the joint.

7. A system according to any preceding claim, in which the driving means of the power driven actuating means cuts out when its consumption exceeds a predetermined threshold.

8. In or for a rotatable joint, a system for stopping the rotation of the joint at controllable positions, the system comprising:

a) means for sensing the loading on the joint;

b) a power driven linear-action actuating means incorporating a member which, when said means is actuated, advances or retracts in order to optimise the degree of angular motion permitted to the j oint; and

c) processing means for controlling the operation of the power driven actuating means in response to data supplied by the sensing means.

9. A system according to claim 8 and in which the linear-action actuating means is a screw.

10. A device substantially as described herein with reference to, and as illustrated in any appropriate combination of the text and drawings.