WO2003054834A1

WO2003054834A1 - Simulating haptic feedback

Info

Publication number: WO2003054834A1
Application number: PCT/AU2002/001737
Authority: WO
Inventors: Harry Owen; Karen Reynolds; Olivia Pallotta; Jodie Wilson
Original assignee: Flinders Technologies Pty Ltd
Priority date: 2001-12-20
Filing date: 2002-12-20
Publication date: 2003-07-03
Also published as: AUPR965001A0

Abstract

A simulation device simulates haptic feedback which results when an elongate member advances through tissue. The simulation device includes an elongate member receiving component (8) into which the elongate member (1) is inserted; a depth determining component (14) for determining the depth of advancement of the elongate member; a resistive force determining component which determines the resistive force required to simulate haptic feedback; and a resistive force generating component (16) for generating the resistive force and applying it to the elongate member, and pre-determined values of resistive force for a depth of advancement of an elongate member in real tissue.

Description

Simulating Haptic Feedback

Field of the Invention

This invention relates to simulating haptic feedback which results when an elongate member advances through tissue. It relates particularly but not exclusively to a simulation device for simulating haptic feedback during an epidural procedure and a method of simulating such haptic feedback.

Background to the Invention Epidural anaesthesia is a procedure involving the injection of a local anaesthetic into the epidural space of the spinal column. It is used during surgery or in childbirth to relieve pain and restrict sensation. This is a very delicate procedure which involves the insertion of a needle into the lumbar region of the lower back. As the needle passes through the ligaments in the lumbar region, tactile clues are generated. These clues must then be interpreted to locate successfully the epidural space before the needle overshoots, penetrating the area too deeply and puncturing the durai membrane. Once the epidural space is located, a catheter is inserted and the anaesthetic administered to relieve pain. Most forms of epidural anaesthesia are administered towards the base of the spine which is below the base of the spinal cord in most adults. As the epidural needle advances toward the epidural space, it first passes through a number of layers. Firstly, the needle punctures the skin, subcutaneous fat and then the supraspinous ligament that links the outside of the spinous processes. The next layer is located approximately at the depth of the spinous processes and contains the interspinous ligament.

One test which can be used to aid in locating the epidural space is the "loss of resistance test". This test uses the density and toughness characteristics of the ligamentum flavum which is the last layer of resistance before the epidural space is penetrated. Fluid from a syringe cannot be injected while the tip of the epidural needle is in a region of high density such as the ligamentum flavum. As a result, the ligamentum flavum provides feedback to the anaesthetist administering the epidural, in the form of resistance to penetration. Once the ligamentum flavum has been reached, a "loss of resistance" syringe filled with saline is attached to the hub of the needle. This syringe is specifically designed for the loss of resistance technique of epidural injection which has a freely moving plunger. If the needle tip is correctly located in the ligamentum flavum, then no saline can be injected. If the needle tip is still within the softer ligaments, then a reasonable pressure on the syringe plunger will inject some of the saline solution, indicating that the needle has not penetrated the tissue deeply enough.

While in the ligamentum flavum, the needle/syringe combination is slowly advanced with pressure being applied to the plunger of the syringe. This gives continuous feedback regarding the positioning of the tip of the needle. When the tip of the needle enters the epidural space, there is a sudden loss of resistance to injection and the saline in the syringe passes freely into the space. Thus, if executed correctly, the "loss of resistance" technique successfully indicates that the epidural space has been reached.

Finally, the syringe is removed from the epidural needle and a catheter is passed into the epidural space. The needle is then also removed and anaesthetic can be continuously or repeatedly administered without further injection. A spinal injection, which is similar to a spinal tap, is an extension of the epidural technique. For a spinal injection, the needle passes into the dura mater to inject or withdraw fluid. Since no catheter is required, a smaller needle is used.

To administer a standard epidural to the lumbar region, the patient is positioned in a hunched posture either lying down or sitting up in order to flex the spine. This maximises the peripheral spacing of the vertebrae and permits location of the bony protrusions. The midpoint of the chosen interspace is found and marked and an epidural needle is advanced through the skin and softer tissues until the denser ligamentum flavum is reached. Detection of this increased resistive force on the needle requires good tactile perception and experience.

Many anaesthetists use a well established grip where the hands are braced on the back to prevent patient movement from affecting the needle advancement as the needle penetrates through the first layers of tissue. Cues which are used to confirm that the needle tip has reached the ligamentum flavum are: a) the needle is no longer able to wobble as is the case when it is penetrating the softer tissue. Once the tough ligamentum flavum has been penetrated, the needle position and angle are fixed; and b) a greater force must be applied by the anaesthetist to push through the thickness of the ligament.

There are very few ways that a trainee can practice the procedure of administering epidural anaesthesia before attempting it on a patient. Two methods which are frequently used to convey the feeling of penetrating the ligamentum flavum are injecting a thick-skinned orange or an eraser, since it is not ethical to use live animals for practice and cadavers are expensive and do not provide accurate haptic feedback for the procedure.

As a result, trainees gain most of their experience from performing epidural procedures on actual patients. Clearly, inexperienced operators have a higher chance of encountering complications. Therefore, this type of training has a very high risk associated with it. It is also time consuming, opportunities are limited and anaesthetists have no way to repeat previous cases to improve their technique. Risks associated with improper administering of epidural anaesthesia include failure to block sensation, backache, infection, headache, intravascular injection, total spinal injection, neurological injury, brain damage and death. The most common complication is a severe headache, which is caused by a dural puncture. Dural punctures occur in approximately in 1.7% of cases in teaching hospitals, 85% of which will result in headache. This occurs as a result of the tip of the epidural needle puncturing the dura mater causing cerebral spinal fluid (CSF) to leak into the epidural space, causing the headache.

Summary of the Invention

According to a first aspect of the invention, there is provided a simulation device for simulating haptic feedback which results when an elongate member advances through tissue, the simulation device including: (a) an elongate member receiving component into which the elongate member is inserted;

(b) a depth determining component for determining the depth of advancement of the elongate member; (c) a resistive force determining component which determines the resistive force required to simulate haptic feedback; and

(d) a resistive force generating component for generating the resistive force and applying it to the elongate member; wherein the resistive force is determined by considering: (i) the depth of advancement of the elongate member; and

(ii) pre-determined values of force and depth of advancement of an elongate member in real tissue.

The elongate member receiving component may be any component which allows the elongate member to be inserted to a depth which closely represents the depth of penetration of the skin and subcutaneous layers in real tissue. It is preferred that the elongate member receiving component permits a displacement of less than 5mm. Further, it is preferred that the elongate member receiving component includes a cutaneous tissue simulating component which substantially simulates haptic feedback which results when an elongate member, such as a needle, punctures skin. The cutaneous tissue simulating component may be in the form of two sealing "0"-rings or one or more rubber layers, or any other one or more sheaths or layers which simulate haptic feedback characteristics of skin punctures using needles or other elongate members. It is also preferred that the elongate member receiving component prevents leakage of fluid from the elongate member once it has been inserted into the simulation device. This is especially preferred when the tissue for which haptic feedback is being simulated is spinal tissue, and the elongate member has reached a depth, which is less than that which corresponds to the depth of the ligamentum flavum.

It is preferred that the elongate member receiving component includes a valve such as a solenoid valve and a reservoir, so that on opening the solenoid valve, fluid from within the elongate member receiving component flows into the reservoir. This creates a haptic sensation akin to a "loss of resistance" which corresponds to the haptic feedback of a needle entering the epidural space of the spine.

The depth determining component may be any device which is capable of determining the depth of penetration of the elongate member into tissue for which haptic feedback is being simulated. The resistive force determining component uses at least the output from the depth determining component to ascertain a resistive force to be applied to the elongate member to simulate the haptic feedback.

In one suitable arrangement, a linear displacement transducer is used in conjunction with a hydraulic system to determine the depth of the elongate member. Preferably, the force applied by a user to the elongate member is also measured. This may be done using a force/pressure or any other appropriate type of transducer such as flow and/or pressure transducers within the fluid flow of the hydraulic system. Alternatively, the depth determining component may include an optical rotary encoder (ORE) which provides information about the position of a platform along a rotatable lead screw via a hydraulic system. In this particular arrangement, as the elongate member advances, it presses against the first end of a hydraulic system which causes the second end of the hydraulic system to move. The second end of the hydraulic system is mounted on the platform, which moves along the rotatable lead screw, which turns to accommodate the movement. This movement is monitored by the ORE which is preferably configured to provide a voltage output corresponding to the amount of lead screw rotation . It is preferred that the voltage output is in the form of a number of pulses or a change in analogue voltage output.

Preferably, the resistive force determining component is a microprocessor which uses the output from the depth determining component (e.g. the linear displacement transducer or the ORE) to determine the depth of penetration of the elongate member. This may be used in addition to outputs from force/pressure or other transducers which measure the magnitude of the force applied by the user to the elongate member. Outputs from these devices can then be used to ascertain a resistive force to be applied to the elongate member to accurately simulate haptic feedback experienced in a real procedure. It is preferred that the resistive force is determined using data which has been recorded from actual procedures in which needles have been inserted into real tissue. The determination may be made by applying a formula which is executed by a microprocessor or using a look-up table to determine the appropriate resistive force to be applied. The resistive force generating component may be any component which is capable of generating a force which, when applied to the elongate member, resists further advancement to simulate resistance of an elongate member such as a needle advancing through real tissue. It is preferred that the resistive force generating component includes a fluid pump or a motor such as a servomotor with a lead screw and platform as previously described.

Preferably, a microprocessor causes the fluid pump to force a particular volume of fluid back into the hydraulic system thereby causing the elongate member to resist further advancement. Alternatively, the microprocessor may cause the motor and lead screw to which it is connected to rotate, thereby exerting a force, via the platform and hydraulic system upon the elongate member, causing it to resist further advancement.

In another preferred embodiment of the invention, the simulation device includes a tissue variation component which enables variation of the resistive force in order to simulate different tissue characteristics which include one or more of the following characteristics:

(a) ligament thickness;

(b) ligament hardness or softness;

(c) ligament elasticity;

(d) bone thickness; (e) bone density; ^*

(f) fluid-filled spaces;

(g) the age of a patient whose tissue is being simulated;

(h) the health status of a patient whose tissue may be simulated, the health status including pregnancy, abnormal subcutaneous fat layer thickness, or any other condition which affects the haptic feedback which results from advancement into the tissue by an elongate member.

The tissue variation component may simulate different tissue characteristics using a look-up table with at least one entry for each of the different tissue characteristics. Alternatively, the tissue variation component may include formulae which are used in association with the resistive force determining component to determine an appropriate resistive force for simulating realistic haptic feedback for particular real tissue characteristics. It is especially preferred that the tissue variation component simulates haptic feedback for tissue characteristics which are either selected from a plurality of tissue characteristics, or generated at random.

The tissue characteristics may be selected using any selection procedure. For example, this may involve a rotating dial, a multi-level switch or selecting options from a menu of tissue characteristics which are presented using a display unit.

Preferably, a gimbal mechanism is included to couple the elongate member receiving component to other components of the simulation device such as the hydraulic system. The gimbal mechanism facilitates alteration of the angle at which the elongate member enters the simulation device. Preferably, this occurs by coupling the elongate member to the receiving component in such a way that the receiving component is free to move with the elongate member as its angle of attack is changed. In a preferred embodiment, the gimbal mechanism is attached to the first end of the hydraulic system and as the angle is varied, the entire first end of the hydraulic system moves with changes in angle of attack. In such an embodiment, the first end of the hydraulic system may be attached to the second end of the hydraulic system by way of one or more flexible tubes which contain fluid. In one embodiment, the second end of the hydraulic system is a fluid pump.

It is preferred that the simulation device further includes a force sensing component for sensing the force applied to the elongate member as it advances. The force sensing component may be any suitable device. Preferably, the force sensing component is a force-sensitive resistor (FSR) since FSRs are generally of thin construction with sizes which are appropriate for use in applications such as the present invention However, the force sensing component may include, strain gauges, load cells, pressure transducers, other force-sensitive resistors or any other suitable device.

Preferably, the output of the force sensing component is modelled using a mathematical function or other formula which can then be used as an input to the resistive force determining component, in addition to the output from the depth determining component. This enables the resistive force required to simulate haptic feedback to be determined, more accurately since the profile of the force applied to a needle used in an epidural procedure is known to vary with the depth of insertion of the needle into the tissue. The variation in the force required for the elongate member to advance through the various layers of tissues is affected by several factors including the velocity with which the elongate member advances and the age, gender and health of the patient. Therefore, in a preferred embodiment of the invention, the resistive force is determined by additionally considering the force with which the elongate member advances, as determined by the force sensing component.

In yet another embodiment, the simulation device further includes a fluid receiving compartment for simulating a loss of resistance which is observed when the tip of the elongate member moves from a higher density tissue to a lower density tissue, by allowing transfer of liquid from the elongate member into the elongate member receiving component. In such a preferred embodiment, the elongate member receiving component includes a reservoir for collecting fluid as it is released from the elongate member. It is preferred that incorporation of the fluid receiving compartment simulates a loss of resistance which permits use of the "loss of resistance test" which is often applied in actual epidural procedures to locate particular layers of tissue such as the epidural space.

In this embodiment of the invention, it is preferred that the fluid receiving compartment includes a solenoid valve which is under the control of a microprocessor. The microprocessor opens the valve according to commands with which the microprocessor has been programmed. Where the haptic feedback being simulated relates to an epidural procedure being administered in the lumbar region of the spine, the commands controlling the solenoid valve permit opening when the elongate member has reached a depth of advancement corresponding to the depth of the ligamentum flavum. In still another embodiment of the invention, the simulation device further includes a tissue signalling component which signals to the user of the simulation device when the elongate member has penetrated particular layers of tissue. These layers of tissue may include one or more of the following: (a) subcutaneous fat; (b) supraspinous ligaments;

(c) intraspinous ligaments;

(d) the epidural space;

(e) the dura mater; and (f) bone.

The signal may be any signal which makes the user aware of the layer which is being penetrated by the elongate member, such as an audible signal or a visual signal or a combination of the two. The visual signal may be in the form of one or more lights being illuminated in one or more sequences or colours. In a preferred embodiment, the signal includes a display using a string of characters to present the name of the tissue layer, which layer the elongate member has penetrated.

According to a second aspect of the invention, there is provided a method of simulating haptic feedback which is present when an elongate member advances through a tissue, the method including:

(a) monitoring the depth of advancement by the elongate member;

(b) determining a resistive force to be applied to the elongate member which simulates the haptic feedback; and

(c) applying the resistive force to the elongate member; wherein the resistive force is determined by considering:

(i) the depth of advancement of the elongate member; and (ii) pre-determined values of resistive force for a depth of advancement of an elongate member in real tissue.

The depth of advancement of the elongate member may be monitored using any device which is capable of determining the depth to which the elongate member has advanced. It is preferred that the depth of advancement is monitored using a linear displacement transducer. Alternatively, an Optical Rotary Encoder (ORE) may be used which determines how many times a lead screw rotates as the elongate member advances. The resistive force to be applied to the elongate member may be determined using a device such as a microcontroller or computer which is able to ascertain the resistive force to be applied to the elongate member to simulate realistically haptic feedback which would result if the elongate member was to advance through real tissue. The resistive force may be applied to the elongate member using any suitable force-application method. Preferably, a fluid pump is used to controllably maintain and/or generate pressure within a hydraulic system which is sufficient to apply an appropriate resistive force to the elongate member. Alternatively, the resistive force may be applied by a motor whose rotational motion is transformed into linear motion by way of a lead screw with a platform to which a second end of a hydraulic system is mounted. In such an embodiment, rotation of the motor causes the second end of the hydraulic system to move so that a first end of the hydraulic system shifts toward the elongate member, resulting in a force being exerted on the elongate member thereby resisting further advancement.

Preferably, the resistive force is determined by additionally considering the force with which the elongate member advances.

In another embodiment of the invention, where the tissue for which haptic feedback is being simulated is spinal tissue, the method includes the additional step of fixing the angle at which the elongate member advances when the tip of the elongate member has advanced to a depth which corresponds to the ligamentum flavum in real spinal tissue. The angle of the elongate member may be fixed using any suitable means which prevents the angle of insertion of the elongate member from altering once it has advanced to the appropriate depth.

It is preferred that if the elongate member first advances at an angle, a gimbal mechanism is used to facilitate that angle and any subsequent variation in it. Accordingly, parts of the simulation device are coupled to the elongate member in such a way that the feedback can be generated as if there were no angular deviation. Mechanical devices such as slotted bars and clamps may be used to fix the angle at which the elongate member advances. It is preferred that mechanical fixing is controlled by a microprocessor which in turn uses as one of its inputs the monitored depth of advancement of the elongate member, This enables the depth of the ligamentum flavum to be accurately determined and the angle of advancement fixed accordingly.

In yet another embodiment of the invention, the method further includes the additional step of varying the resistive force, thereby simulating variation in tissue characteristics including: (a) ligament thickness; (b) ligament hardness or softness;

(c) ligament elasticity;

(d) bone thickness;

(e) bone density; (f) fluid-filled spaces;

(g) the age of a patient whose tissue is being simulated; (h) the health status of a patient whose tissue may be simulated, the health status including pregnancy, abnormal subcutaneous fat layer thickness, or any other condition which affects the haptic feedback which results from advancement into the tissue by an elongate member.

The haptic feedback for a simulated variation in tissue characteristics may be selected from a plurality of known tissue characteristics or may be generated at random.

In still another embodiment of the invention, the method includes the additional step of signalling when particular depths of advancement have been reached by the elongate member, the depths of advancement including penetration of the following layers of tissue:

(a) subcutaneous fat;

(b) supraspinous ligaments; (c) intraspinous ligaments;

(d) the epidural space;

(e) the dura mater; and

(f) bone.

Signaling may occur by visual and/or audible means, in addition to the tactile (or haptic) feedback which is provided. Audible signals may be in the form of different tones being made to indicate penetration of different tissue layers. In addition or as an alternative, a light may be illuminated, the colour of which may be indicative of the tissue layer into which the elongate member has advanced. In a preferred embodiment, the signalling involves presenting the user with messages on a visual display unit which uses a string of characters to present the name of the tissue layer into which the elongate member has advanced.

According to a third aspect of the invention, there is provided computer software for controlling components of a haptic feedback simulating device, the device simulating forces which result from an elongate member penetrating real tissue, the software including:

(a) pre-determined tissue data relating to actual tissue depths and actual resistive forces which are experienced by an actual elongate member as it reaches those tissue depths in real tissue;

(b) a data input component for receiving data relating to the depth of an elongate member in the simulating device;

(c) a resistive force determining component for determining the resistive force to be applied to the elongate member to simulate the haptic feedback; and (d) a motor control component for controlling the speed and direction of rotation of a motor.

The actual data may be any data which relates to the haptic feedback which can be detected when an elongate member is used to penetrate tissue. It is preferred that the actual data is representative of a large population of subjects for whom the relevant haptic feedback has been obtained during procedures where their tissue has been penetrated with an elongate member such as a needle. In a preferred embodiment, the actual data relates to spinal tissue. In a particularly preferred embodiment, the actual data relates to tissue located in the lumbar area of the spine. The data input component may be any device configurable to receive data relating to the depth to which the elongate member has advanced. It is preferred that the data input component includes a microprocessor which has an analog-to-digital converter (ADC). Preferably, the data input component is also configurable to receive data relating to the force applied to the elongate member as it advances.

The resistive force determining software component may be any suitable software component for determining the resistive force to be applied to the elongate member to simulate haptic feedback representative of an elongate member advancing through real tissue. Preferably, the resistive force determining component uses pre-determined values of applied force for a particular depth of advancement of an elongate member in real tissue to ascertain the resistive force to be applied. The resistive force determining component may also use the measured force which is applied to the elongate member to determine a more accurate force to be applied. The motor control component preferably includes instructions to run the motor at speeds appropriate for applying haptic feedback to the elongate member. It is preferred that the motor controller uses pulse width modulation (PWM) to control the speed and direction of the motor, facilitating digital control. The motor can be used as a fluid pump or to produce resistance via a platform and lead screw arrangement.

In another embodiment of the invention, the computer software further includes a tissue characteristics variation component which varies the haptic feedback to simulate variations in tissue characteristics which may be seen in real subjects. Preferably, the tissue characteristics variation component generates tissue characteristics which may be selected from a plurality of tissue characteristics or generated at random. The tissue characteristics variation component may include a set of resistive force values which can be observed when an elongate member penetrates tissue with particular characteristics at a certain depth. Alternatively, the tissue characteristics variation component may store functions or formulae which are used to determine the resistive force.

In yet another embodiment of the invention, the computer software further includes a fluid retention control component for controlling retention of fluid in the elongate member. This component may include commands for controlling a solenoid valve or other device which is used to prevent fluid from moving out of the elongate member. It is preferred that where the haptic feedback simulating device is used to simulate feedback which can be observed when an elongate member penetrates spinal tissue, the fluid is prevented from leaking out until the elongate member is at a depth corresponding to that of the ligamentum flavum. This enables use of the "loss of resistance" test which is frequently used to perform real epidural procedures.

In still another embodiment of the invention, the computer software further includes a display control component for controlling the display of information relating to the haptic feedback simulating device, the information including but not limited to, when the device is ready for use, instructions for operating the haptic feedback simulating device and the depth of advancement of the elongate member.

It is preferred that the display control component includes a microprocessor to control presentation of strings of characters on a display device, in accordance with instructions which are executed by the microprocessor.

Brief Description of the Drawings

The invention will herein after be described in greater detail by reference to the attached drawings. It is to be understood that the particularity of the drawings does not supersede the generality of the preceding description of the invention. Figure 1 is an illustration of one embodiment of the invention.

Figure 2 is an illustration of another embodiment of the invention.

Figure 3 is a flow diagram presenting steps involved in the operation of one embodiment of the invention.

Figure 4 is a table of values indicating tissue-type thickness and the general range of variation which can be seen between patients.

Detailed Description

Referring firstly to Figure 1 , a simulation device for simulating haptic feedback resulting from an elongate member, such as epidural needle 1 , advancing through tissue is shown. The simulation device includes elongate member receiving component shown in the form of a needle capture system 3 into which the elongate member is inserted and depth determining component shown in the form of Optical Rotary Encoder (ORE) 14 for determining the depth of advancement of the elongate member. Resistive force determining component shown in the form of microprocessor 7 determines the resistive force required to simulate haptic feedback and resistive force generating component shown as motor 16 generates the resistive force which is applied to needle 1 through platform 12 and lead screw 13. The resistive force is determined by considering the depth of advancement of needle 1 and pre- determined values of force and depth of advancement of needle 1 in real tissue.

Epidural needle 1 is targeted toward a small area of simulated skin between bony spinal protrusions 2. When needle 1 penetrates needle capture system 3, a haptic sensation which is similar to that of a skin puncture is created by a cutaneous tissue simulating component embodied in the form of two "0"-rings 4. Leakage of fluid from needle 1 is prevented by way of normally closed solenoid valve 5. Solenoid valve 5 acts as a switch, wherein when it is open, fluid within needle capture system 3 may be injected and flow into a fluid receiving component shown as reservoir 6. Solenoid valve 5 is controlled by microprocessor 7, which provides a signal which opens valve 5 when data collected by microprocessor 7 indicates that the needle has been inserted to a depth which corresponds to the epidural space.

Figure 1 also shows a hydraulic system which includes first syringe 8 and second syringe 9, interconnected by flexible tubing 10. Gimbal mechanism 17 accommodates angular insertion of needle 1 and is attached to force transducer 11 , which in turn contacts plunger 8a of first syringe 8. Such a configuration enables gimbal 17, which is moveable in 2 planes, to align first syringe 8 with the angle of entry of needle 1 , if the angle of entry of needle 1 is not perpendicular to plunger 8a. However, second syringe 9 remains in a substantially fixed position.

As needle 1 advances to the right of the system illustrated in Figure 1, force transducer 11 produces an output which reflects the magnitude of the force with which needle 1 advances. As needle 1 advances deeper, plunger 8a of first syringe 8 moves to the right, tending to push fluid out of first syringe 8 into tubing 10 and then into syringe 9. This results in the hydraulic system tending to push plunger 9a of second syringe 9 outward.

Plunger 9a is mounted on platform 12 which is attached to lead screw 13. Lead screw 13 converts the translational movement resulting from the advancement of epidural needle 1 into rotational movement. This rotational movement of lead screw 13 is detected and monitored by Optical Rotary Encoder (ORE) 14, which produces a voltage output corresponding to the direction and degree of rotation which has been produced. In such an arrangement, it is preferred that the pitch of lead screw 13 is selected so that very small translational movements by needle 1 , preferably in the order of 10^"4 mm, are detectable. ORE 14 is calibrated so that the output can be used by microprocessor 7 to monitor the depth of penetration of needle 1.

Referring now to the embodiment illustrated in Figure 2, the hydraulic system includes first syringe 8 and fluid pump 19, are interconnected by flexible tubing 10. Gimbal mechanism 17 is attached to force transducer 11 , which in turn contacts plunger 8a of first syringe 8 again enabling gimbal 17 to align first syringe 8 with the angle of entry of needle 1. Fluid pump 19 remains in a fixed position. As needle 1 advances to the right of the system illustrated in Figure 2, force transducer 11 produces an output which reflects the magnitude of the force with which needle 1 advances. Linear displacement transducer (LDT) 20 measures the depth of advancement of needle 1 by monitoring the amount by which plunger 8a moves as needle 1 advances.

Microprocessor 7 is responsible for the execution of commands which control various components of the simulation device, in addition to receiving and processing data which arrives at microprocessor 7 from peripheral components including force transducer 11 , ORE , 14 or LDT 20 and control unit 18. Commands with which microprocessor 7 is programmed may be varied to suit the purposes for which the simulation device is used, in terms of the visual or audio feedback which is provided to the user and the messages and instructions which are presented. Figure 3 illustrates steps which may be performed using software which is executed on microprocessor 7.

When the system is turned on, microprocessor 7 is powered up, as are separately powered peripheral components. Microprocessor 7 then sends instructions which initialise each of the peripheral components, and sends character strings to liquid crystal display (LCD) 15, which provide instructions to the user regarding operation of the simulation device. The system may be configured to generate random "patients" wherein haptic feedback for different tissue characteristics is randomly simulated. The user then inserts needle 1 into the simulation device, and applies a force so that needle 1 advances. Microprocessor 7 then controls motor 16 or fluid pump 19 so that further advancement of needle 1 is resisted to simulate realistically tissues which are penetrated during an epidural procedure.

Epidural procedures can be performed at a number of different speeds depending on the experience and style of the anaesthetist. It has been shown that the average speed of administering epidural anaesthesia via a needle or catheter insertion device is approximately 60mm. min^"1. However, it may be performed at speeds of up to 150mm. min^"1. Accordingly, a gear ratio which reduces the speed of motor 16 to a range which is closely matched to the speed with which the epidural needle is inserted should be selected. Microprocessor 7 receives on one of its input pins the output from ORE 14 or LDT 20. On another of the input pins of microprocessor 7 output from force transducer 11 is received and output from LDT 20 is received on yet another input pin. Together, these inputs are used by microprocessor 7 to determine the resistive force to be applied to needle 1. This information is then used to further ascertain the duration and speed at which motor 16 or fluid pump 19 is run to generated the appropriate resistance and simulate realistic haptic feedback. In this embodiment of the invention, motor 16 or fluid pump 19 may be controlled using pulse width modulation (PWM), which facilitates use of digital control signals provided directly by microprocessor 7. PWM varies the duty cycle of motor 16 or fluid pump 19 resulting in varied speed of operation. In a preferred embodiment, microprocessor 7 facilitates bi-directional motor control by including two PWM modules.

Microprocessor 7 determines the direction in which motor 16 or fluid pump 19 should operate in accordance with input which is provided from ORE 14 or LDT 20.

Rules which microprocessor 7 uses to determine the speed at which motor 16 or fluid pump 19 should run are devised from data collected during iso-velocity epidural procedures performed on porcine samples and verified using measurements taken during epidural procedures performed on recently deceased cadavers. A characteristic curve of force data plotted as a function of depth of advancement of an epidural needle is shown in Brett, P. N et al, 1997, "Simulation of Resistance Forces acting on Surgical Needles" Proc. Instn Mech Engrs, vol 211 , Part H, which is hereby incorporated herein by reference. Other plots of force versus depth may be used upon collection of further data from which an array of force and corresponding tissue depth values can be devised and employed by microprocessor 7.

In the embodiment of the invention indicated in the flow diagram illustrated in Figure 3, tissue characteristics for different patients are generated at random. As an example, in general, a more obese patient has ligaments and cutaneous layers of the same thickness as patients within a normal weight range. However, the subcutaneous fat layer is generally thicker. Accordingly, the force-depth values corresponding to particular tissues, such as subcutaneous fat must be adjusted. In order to access the force data, microprocessor 7 may be programmed with a function containing a 'for' loop based on the value of an array counter in microprocessor 7. The array counter may be incremented for each iteration of the 'for' loop; as long as the next depth value in the array is greater than or equal to the current depth of needle 1. When a value in the array is located which is less than or equal to the current depth of needle 1 , a corresponding force value determined using the data presented in Figure 3 is returned by the function. To perform random generation of patient types, a random number generator may be used to select one patient type from a predetermined range. The force and depth data for different patient types can be determined using data which has been collected in previous studies. Such studies have indicated that the maximum force experienced when the tip of the needle is in the ligamentum flavum varies from 22.5N to 30.4N and the thickness of the interspinous and supraspinous ligaments varies from 17mm to 47.5mm. Other data which can be used in the generation of resistive forces used in simulating haptic feedback for different patient types are available in Bromage, P.R., 1978, "Epidural Analgesia", Saunders Co., USA, and Chestnut, D.H., 1999, "Obstetric Anesthesia, Principles and Practice", 2^nd Ed., Mosby, USA, both of which are hereby incorporated herein by reference and are presented in the table in Figure 4.

Variation in tissue thicknesses which exist between patients may be simulated in many ways. In one embodiment of the invention, force values may be multiplied by a pre-determined number which is within set limits and is randomly generated, the limits determined by known patient characteristics. Sample data representing different patient characteristics is presented in the table in Figure 4. However, any other data which represents variation in tissue thickness for different patients may also be used. Other techniques which may be used for varying the simulated tissue thickness include storing separate sets of array values for each patient type which is available. However, this does not facilitate random combinations of characteristics, such as pregnancy combined with proportionately low levels of sub cutaneous fat, or an obese elderly patient who may have thicker subcutaneous fat layers combined with ligaments which have been hardened due to calcification. It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing from the ambit of the present invention.

Claims

CLAIMS :

1. A simulation device for simulating haptic feedback which results when an elongate member advances through tissue, the simulation device including: (a) an elongate member receiving component into which the elongate member is inserted;

(b) a depth determining component for determining the depth of advancement of the elongate member;

(c) a resistive force determining component which determines the resistive force required to simulate haptic feedback; and

(d) a resistive force generating component for generating the resistive force and applying it to the elongate member; wherein the resistive force is determined by considering: (i) the depth of advancement of the elongate member; and (ii) pre-determined values of resistive force for a depth of advancement of an elongate member in real tissue.

2. A simulation device according to claim 1 further including a tissue variation component which enables variation of the resistive force to simulate different tissue characteristics which include one or more of:

(a) ligament thickness;

(b) ligament hardness or softness;

(c) ligament elasticity;

(d) bone thickness; (e) bone density;

(f) fluid-filled spaces;

(g) the age of a patient for whom haptic feedback is simulated;

(h) the health status of a patient for whom haptic feedback is simulated, the health status including pregnancy, abnormal subcutaneous fat layer thickness, or any other condition which affects haptic feedback resulting from advancement into tissue by an elongate member.

3. A simulation device according to claim 2 wherein the tissue variation component is used to vary the resistive force to simulate different tissue characteristics which are:

(a) selected from a plurality of tissue characteristics; or

(b) generated at random.

4. A simulation device according to claim 1 further including a gimbal mechanism for coupling the elongate member receiving component to other components of the simulation device.

5. A simulation device according to claim 1 further including a cutaneous tissue simulating component for simulating haptic feedback which results when an elongate member pierces skin.

6. A simulation device according to claim 1 further including a force sensing component for sensing the force applied to the elongate member as it advances.

7. A simulation device according to claim 6 wherein the resistive force is calculated by additionally considering the force with which the elongate member advances.

8. A simulation device according to claim 1 further including a fluid receiving compartment for simulating a loss of resistance which is observed when the tip of an elongate member moves from higher density tissue to lower density tissue by facilitating transfer of liquid from the elongate member into the elongate member receiving component.

9. A simulation device according to claim 1 further including a tissue signalling component which signals to a user of the simulation device when the elongate member has penetrated particular tissue layers including:

(a) subcutaneous fat;

(b) supraspinous ligaments;

(c) intraspinous ligaments; (d) the epidural space;

(e) the dura mater; and

(f) bone.

10. A method of simulating haptic feedback which is present when an elongate member advances through a tissue, the method including:

(a) monitoring the depth of advancement by the elongate member;

(b) determining a resistive force to be applied to the elongate member which simulates the haptic feedback; and (c) applying the resistive force to the elongate member; wherein the resistive force is determined by considering: (i) the depth of advancement of the elongate member; and (ii) pre-determined values of resistive force for a depth of advancement of an elongate member in real tissue.

11. A method of simulating haptic feedback according to claim 10 further including the additional step of monitoring the force with which the elongate member advances.

12. A method of simulating haptic feedback according to claim 10 wherein the resistive force is determined by additionally considering the force with which the elongate member advances.

13. A method of simulating haptic feedback according to claim 10 wherein the tissue being simulated is spinal tissue and the method includes the additional step of fixing the angle at which the elongate member advances, when the tip of the elongate member has advanced to a depth which corresponds to the ligamentum flavum in real spinal tissue.

14. A method of simulating haptic feedback according to claim 10 including varying the resistive force to simulate different tissue characteristics which include one or more of: a) ligament thickness; (b) ligament hardness or softness; (c) ligament elasticity;

(d) bone thickness;

(e) bone density;

(f) fluid-filled spaces;

(g) the age of a patient for whom haptic feedback is simulated; and

(h) the health status of a patient for whom haptic feedback is simulated, the health status including pregnancy, abnormal subcutaneous fat layer thickness, or any other condition which affects the haptic feedback which results from advancement into the tissue by an elongate member.

15. A method of simulating haptic feedback according to claim 14 wherein the different tissue characteristics are:

(a) selected from a plurality of known tissue characteristics; or

(b) generated at random.

16. A method of simulating haptic feedback according to claim 10 including the additional step of signalling when particular depths of advancement have been reached by the elongate member, the depths of advancement including penetration of the following layers of tissue: (a) subcutaneous fat;

(b) supraspinous ligaments;

(c) intraspinous ligaments;

(d) the epidural space;

(e) the dura mater; and (f) bone.

17. Computer software for controlling components of a haptic feedback simulating device, the device simulating forces which result from an elongate member penetrating tissue, the software including: (a) pre-determined tissue data relating to actual tissue depths and actual resistive forces which are experienced by an actual elongate member as it reaches those tissue depths in real tissue;

(b) a data input component for receiving data relating to the depth of an elongate member in the simulating device; (c) a resistive force determining component for determining the resistive force to be applied to the elongate member to simulate the haptic feedback; and (d) a motor control component for controlling the speed and direction of rotation of a motor.

18. Computer software according to claim 17 further including a tissue characteristics variation component which varies the simulated haptic feedback to simulate variations in tissue characteristics including one or more of:

(a) ligament thickness; (b) ligament hardness or softness;

(c) ligament elasticity;

(d) bone thickness;

(e) bone density;

(f) fluid-filled spaces; (g) characteristics affected by the age of a patient for whom haptic feedback is simulated;

(h) characteristics affected by the health status of a patient for whom haptic feedback is simulated, the health status including pregnancy, abnormal subcutaneous fat layer thickness, or any other condition which affects tissue characteristics.

19. Computer software according to claim 18 wherein the tissue characteristics variation component generates tissue characteristics which are: (a) selected from a plurality of tissue characteristics; or (b) generated at random.

20. Computer software according to claim 17 further including a fluid retention control component for substantially controlling retention of fluid in the elongate member.

21. Computer software according to claim 17 further including a display control component which controls the display of information relating to the haptic feedback simulating device including: (a) when the device is ready for use; (b) instructions for operating the haptic feedback simulating device; and

(c) the depth of penetration of the elongate member.

22. Computer software according to claim 21 wherein the display control component further provides information which, when the tissue characteristics relate to spinal tissue, indicate the depth of the elongate member, the depth indicative of penetration of layers of tissue including:

(a) subcutaneous fat;

(b) supraspinous ligaments; (c) intraspinous ligaments;

(d) the epidural space;

(e) the dura mater; and

(f) bone.