WO2005002479A1

WO2005002479A1 - Surgical device

Info

Publication number: WO2005002479A1
Application number: PCT/GB2004/002844
Authority: WO
Inventors: Ajay Kapoor; Mohamad Jabir; David I. Fletcher
Original assignee: University Of Sheffield
Priority date: 2003-07-02
Filing date: 2004-07-01
Publication date: 2005-01-13
Also published as: GB0315449D0; GB2403413A

Abstract

A surgical device comprising a surgical instrument and a means for measuring the distance of the implement from a surface. The means for measuring the distance from the surface comprises feedback means for providing a surgeon with information on the position of the surgical instrument. The feedback means can provide an audible, visual or tactile signal. The means for measuring can be provided with a means for measuring optical change, for example hue.

Description

Surgical Device

This invention relates to surgical devices, and more particularly to a surgical device having a position sensing capability.

In modern surgical, especially micro-surgical, techniques, it is frequently necessary for the surgeon to be able to determine the precise location of a surgical implement during a surgical procedure. X-ray techniques, for example, are commonly used for this purpose. However, the skill of the surgeon is still fundamental for many surgical procedures and the estimation of distance of a surgical implement from a vulnerable area is still a matter of judgement.

Cataract is the term used generally to describe the loss of transparency of the eye lens. Cataracts are caused when the protein constituent of the lens becomes denatured and clumps together causing clouds in certain areas of the lens. Early cataracts can be corrected by wearing stronger eyeglasses. However, as the cataract matures, surgery is the only option to correct vision.

One surgical technique employed to remove cataracts is extracapsular extraction. This method requires a 12mm incision to be made in the eye, through which the lens is extracted. An intraocular lens is then inserted into the lens capsule. Once the lens Is located in position, multiple sutures seal the eye. The technique can cause rupture of the lens capsule, in particular rupture' of the posterior capsule resulting in vitreous loss, as well as astigmatism due to loose or tight stitches.

Phaco-emulsification has been developed in order to overcome the complications resulting from extracapsular extraction. The first step of phacoemulsification is making an incision around 3mm in length in the edge of the cornea. Next, the anterior capsule is removed in a process called ^λcaρsulorrhexis" . The following step is the injection of a viscoelastic material that inflates the anterior chamber to facilitate the operation, and helps to protect the corneal endothelium. A hollow ultrasonic phacoemulsification needle or probe (herein called a "phaco-tip") is then inserted through the incision, which produces high frequency movements (40000- 60000 times per second) and fragments and emulsifies the cataract lens. Flow of fluid through the same needle then aspirates the tiny pieces of the lens.

Using both the cutting power of the ultrasonic vibration, and the sharp edge of the phaco-tip, the process of breaking the lens is begun by making two grooves at right angles across its front surface, dividing it into four quadrants. The quadrants are then free to move, making emulsification and aspiration easier. Making the grooves requires great care in cutting through only a proportion of the cataract lens. If the surgeon cuts all the way through the lens, the posterior capsule may be damaged. The posterior capsule has to be kept intact to suspend the replacement intraocular lens (IOL) and to prevent the prolapse of the vitreous body into the anterior chamber, which can lead to complications such as macular oedema and glaucoma.

The most common complication in phacoemulsification cataract surgery is rupture of the posterior capsule. This rupture can happen accidentally at the time when the surgeon uses the phaco-tip to make the grooves in the lens at the start of its removal. If the tip exceeds the full depth of the lens nucleus, damage to the posterior capsule can occur almost immediately. This happens because the surgeon cannot know the depth of the nucleus and the position of the posterior capsule prior to the operation. Such errors are particularly significant in inexperienced hands and during training.

During the grooving process to break up the lens an experienced surgeon will typically insert the phaco-tip to a depth of up to 75-90% of the total nucleus depth (depending on whether the nucleus is hard or soft) to avoid tearing the posterior capsule. Estimation of the depth of penetration is usually gauged subjectively by the amount of light reflected from the retina, which increases as the groove deepens, and through use of binocular vision via the operating microscope. The resistance to insertion of the tip does not change appreciably with depth, so there is no force feedback available to guide the surgeon.

The provision of an instrument for fragmenting and removing a cataract with low risk damage to the capsule wall has been reported in US 5,730,718. The specification provides a surface-discriminating fragmenting tool that fragments and permits ^' aspiration of high mass, rough surface, rigid tissue, without damaging nearby smooth, flexible, low mass walls. The tool does not eliminate the danger of penetrating the posterior capsule as a result of surgeon error.

US 5,540,690 discloses a phaco-shield for placement over an eye prior to surgery to prevent contact between the leading edge of the phaco-probe and the posterior capsule of the eye. The shield partially surrounds the nucleus of the lens to be shattered. The phaco-probe is disposed within the shield, and designed so that it never extends beyond the outer extremities of the shield. The shield severely limits the freedom of the surgeon to operate within the lens capsule, the inability to move the phaco- probe within the eye potentially impacting upon the success of the surgery.

It will be apparent that in cataract removal and many other surgical techniques the surgeon is in need of better information about the location of the surgical implement .

In a first aspect, the present invention provides a surgical device comprising a surgical implement and means for measuring the distance of the implement from a surface .

The invention is particularly applicable to phacoemulsification devices and will henceforth be more particularly described with reference thereto. It is to be understood, however, that the invention may equally be applied to other ultrasonic probes, laser devices, cutting knives and other surgical implements in appropriate circumstances. Preferably, the surgical device comprises an ultrasonic vibratable probe comprising a phaco-tip and means for measuring the distance of the phaco-tip from a surface.

Preferably the means for measuring the distance of the implement from the surface comprises feedback means for providing the surgeon with real-time, preferably continuous, information on the position of the surgical instrument. Such information can be visual or tactile, but is preferably aural, so that the attention of the surgeon is not distracted by having to look at, for example, a screen. Alternatively, if the surgical technique involves the use of a microscope, the information could be displayed in the view piece or view finder of the microscope.

In one embodiment, the feedback means provides the surgeon with information on the position of the surgical instrument only when the distance between the implement and the surface reaches a specified figure.

The means for measuring the distance of the implement from the surface can comprise any suitable means for measuring a physical change. In one embodiment, such a change can be, for example, a change in electrical impedance, or a change in acoustic impedance. Alternatively, a triangulation method may be employed, in which the relative position of a source, which can be an image of the implement, or a magnetic source, is measured.

Preferably, however, the means for measuring the distance of the implement from the surface comprises means for detecting an optical change. Such an optical change can be, for example, a change in light intensity, a change in colour intensity, or a change in light pattern. Preferably the optical change is a change in hue.

According to a preferred aspect of the present invention there is provided a surgical device for eye surgery, the eye comprising a cornea, an iris and a lens, the lens comprising an anterior capsule, a posterior capsule and lens matter, and especially for cataract removal, which comprises : a) a surgical implement; b) means for detecting changes in hue of light visible through the eye lens during surgery; and c) feedback means wherein the feedback means receives an output from the detecting means and provides a signal to a surgeon controlling the surgical instrument indicating the distance of the implement from the posterior capsule. In a further aspect, the invention also provides a method of reducing rupturing of the posterior capsule during eye surgery comprising: a) measuring changes in hue of light visible through an eye lens during surgery; and b) providing feedback to a surgeon controlling a surgical implement about the changes in hue; wherein a change in hue indicates a change in distance between the surgical implement and the posterior capsule.

By hue, in this specification, is meant a measure of the relative amounts of the additive primary colours that contribute to the colour of an object. By additive primary colours is meant the spectral colours red, green and blue .

In a preferred embodiment of the invention, an increase in redness of hue indicates a reduction in distance between the surgical implement and the posterior capsule.

In a further preferred embodiment of the invention, means for recording the hue are provided. The recording means may operate intermittently, but is preferably continuous, and preferably operates in real time. Any suitable means for recording visible hue can be used, for example, a video recorder, or similar device. Preferably the device comprises means for capturing the hue visible through an eye lens during surgery, said means comprising a camera, especially a digital camera, providing a signal output of captured images to a processor.

In a further preferred embodiment of the invention, said processor comprises a PC (computer) or similar processing means. The PC or similar processing means preferably processes information concerning changes in hue, for example an image signal from a digital camera, continuously in order to obtain a real time reading. Alternatively, the PC or similar processing means may take signal readings at specified intervals. For example, the PC or similar processing means may read the RGB values (as hereinafter defined) of the image signal, the intensity value of the image signal, or, preferably, the HSV (hue, saturation value) Colour Space values (as hereinafter defined) of the image signal.

In a further preferred embodiment of the invention, the feedback means emits an audible signal. Preferably said audible signal is in the form of a beep and said signal is fed into an ear-piece worn by the surgeon. The beep can be intermittent or continuous.

In a preferred embodiment of the invention, an increase in redness of hue results in an increase in frequency or pitch of the audible signal.

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying Drawings, in which:

Figure 1 shows a cross sectional view of an eye; Figure 2 shows a cross sectional schematic representation of a phaco-probe and phaco-tip in the lens capsule of the eye; Figure 3 shows a cross sectional view of the eye of figure 1, the eye having a ruptured posterior capsule and vitreous loss; Figure 4 shows a schematic representation of a first device according to a first embodiment of the invention; Figure 5 shows a schematic representation of a ' second¹ device according to a second aspect of the invention; Figure 6 shows an image of an eye and indicates positions from which light intensity measurements can be made. Points 1 to 4 defined the outer edges of the image. Point 5 defined the centre of the lens, point 6 ^' its outer edge, and point 7 the outer edge of the iris. Circles were defined using these points to represent the lens and iris (shown in black) ; and Figure 7 shows graphically the changes in light intensity and colour throughout a typical cataract removal. Total operation duration of 160 frames, lasting just over • 5 minutes. 7(a) red, green and blue light components, 7 (b) hue, saturation and value for^' the whole of the captured image, 7(c) hue results for the reference area only (note the limited range of hue relative to that for the images as a whole) , and 7 (d) the radius of the pupil within the captured image .

Referring to figure 1, an eye is illustrated generally at 1. A lens 2 is positioned behind a cornea 3, an anterior 8a chamber 4 and an iris 5. The lens 2 comprises a lens capsule 6 comprising a front anterior capsule 7 and a rear posterior capsule 8. Vitreous fluid 12 fills the chamber adjacent to the posterior capsule 8. A retina 19 is located so that the lens can focus light thereon.

During cataract surgery using phaco-emulsification, a phaco-probe 9 having a phaco-tip 20, is inserted into the lens capsule through a small incision 10, as illustrated in figure 2. The phaco-tip 20 emits an ultrasonic sound wave into the lens matter that fragments and emulsifies the lens. The emulsified lens 2 is then aspirated through a suction channel 11 in the phaco-tip 20. Figure 3 illustrates a common complication that results from removal of the lens 2 using a phaco-tip 20. The phaco-tip 20 ruptures the posterior capsule 8 resulting in loss of vitreous fluid 12.

An embodiment of a first device according to the invention is illustrated schematically in figure 4. The device comprises a camera 15 for receiving images from lens 2 of the eye 1 during eye surgery. The camera 15 detects the hue of the light visible through the lens 2 during the course of the surgery.

The camera 15 produces captured images that are input to a processor 17, comprising a processing means 33 and a signal generating means 18. The images are input into the processing means 33 through a cable 16. Optionally the images from the camera can be sent to a recorder 34 before entering the processor 17. The processing means 33 is provided with image processing software that measures changes in hue of the light visible through the lens during the course of the surgery and produces an output that varies according to the change measured.

The output from the processing means 33 is fed to the signal generating means 18. The signal generating means in turn generates an output to an ear-piece 21, via a 10 cable or wireless emission means, which provides an audible signal which alerts the surgeon to changes in the output of the processor 17 and thereby to changes in hue visible through the lens. Through simple training procedures the surgeon can readily learn to recognise changes in frequency and/or pitch and/or volume of the audible signal and relate these to the distance of the phaco-tip from the posterior capsule. In one embodiment, the output from the camera is fed directly to a processing device, for example a PC, which processes the output and provides a real time signal directly to the feedback means . In another embodiment of the invention, the system also comprises recording means, for example, a video recorder. The video recorder, records real time images of the lens of the eye during surgery. The recorded images are used as input for a PC provided with a video card and the appropriate software to measure changes in hue visible through the lens, which serves as the processing means.

The device can measure changes in hue visible through the lens in a variety of possible ways. Firstly the images can be read as RGB (Red-Green-Blue) images. The intensity of each colour relative to one another can be used to measure changes in hue. Alternatively, the intensity of one colour can be used to measure changes in intensity of that colour alone, for example changes in intensity of red hue.

Alternatively, the PC can read the images as GREY images. The intensity values of the additive primary colours are combined to give one value for average intensity, rather than three separate values for each colour. 11

In a preferred device and method according to the invention the PC is adapted to read the images using HSV colour space analysis (Hue, Saturation, Value) . HSV is a colour model that describes colours in terms of hue (^λλtint", the colour) saturation ("shade", the difference between grey shades and strong colour) and value ("luminance", or overall brightness) . Using this analysis the scale of hue varies from 0 to 1.0, with corresponding colours varying from red, through to yellow, green, cyan, blue, magenta and back to red. Saturation is measured on a scale of 0 to 1.0, the corresponding hues varying from shades of grey (unsaturated) to those having no white component (fully saturated) . Finally, value measures the brightness varying from 0 to 1.0, hues being brightest at 1.0.

By way of example only, the device can be used in a method to reduce rupture of the posterior capsule during cataract surgery, by alerting the surgeon to the proximity of the phaco-tip to the posterior capsule in the following manner.

As the surgery progresses the hue visible through the lens changes due to emulsification and aspiration of the lens. In particular, the intensity of red in the hue increases relative to blue and green. This is due to the retina becoming increasingly visible as the lens is emulsified. A video recorder records real 'time images of the lens during the surgery and inputs the images to a PC for processing. The PC, using RGB analysis, measures changes in intensity of hue, in particular changes in intensity of red hue. The changes in intensity are used to provide an audible signal, in the form of a beep, which increases in frequency as the intensity of the red 12 hue increases. As such the surgeon receives an audible signal that alerts him to the amount of lens removed and thus the proximity of the phaco-tip to the posterior capsule .

A second embodiment of a device according to the invention and a method according to the invention will now be described, by way of example only, in Example 1. EXAMPLE 1

Ten phacoemulsification cataract removal operations were recorded in a VHS analogue format with a camera fitted to the operating microscope. To digitise the images for processing on the computer the tapes were played ^• in a video recorder and received by a video capture card, which was Installed in a PC. Images captured were stored in AVI format. Images were captured at a rate of one image per two seconds, producing around 125 images for a 5 minute video clip. The length of the relevant parts of the operations ranged from 5 to 7 minutes. This time excludes the process of insertion of the artificial lens. Figure 5 shows a schematic diagram of the device and the process of analysis of the images from the operating theatre.

To begin the analysis the area of interest in the image is selected to include the pupil and iris. The pupil area was selected because this site is the principle area of interest. The iris area was selected as a reference for which changes in intensity of the image pixels would occur due to changes in ambient lighting during the operation, but not due to the operation and cataract removal itself. 13 The measurements made included light intensity and the size of the selected regions, with the pupil assumed to be represented by a circle and the iris reference area as a ring. Figure 6 illustrates the seven steps that were taken to obtain the measurements from each frame. The first four points specify the dimensions of the whole image. The fifth point specifies the centre of the pupil. The sixth point specifies the diameter of the pupil and the internal diameter of the reference area. The seventh point specifies the external edge of the reference area.

Results of the analysis of each operation were recorded in spreadsheets. The parameters measured were red, green and blue light intensities (RGB) , hue, saturation and colour value data (HSV) and the radius of the selected regions . For further information on these parameters see O'Quinn D, "Photoshop in a Nutshell" O'Reilly (1999). Details of the HSV system can be obtained from A R Smith (1978) "Color Gamut Transform Pairs" SIGGRAPH '78, published in Computer Graphics 12(3), pp 12 - 19.

Each cataract operation was divided into three phases to simplify the analysis. The first phase is anterior capsule removal (capsulorrhexis) . The second phase is grooving of the cataract lens to divide it into quadrants for removal (nucleus sculpting) , and the third phase is segment removal. Results of a typical operation are illustrated in Figure 7.

Red, green and blue light intensity

Figure 7 shows the light intensity levels of the selected regions of the images captured during the operation, split into red, green and blue components. The changes 14 in the RGB intensities throughout the operation were not consistent between operations . There were dramatic increases and decreases in RGB levels at times, resulting from light scattered from the surface of the cornea. The reflected light increases or decreases depending on the location of the patient's eye, resulting in RGB intensity values that change unpredictably throughout the operation.

The effect on the RGB values of the changes in radius of the lens within the captured image due to changes in camera zoom are also significant. Figure 7 illustrates the lens radius which starts high during removal of the anterior capsule (capsulorrhexis) for which the surgeon zooms in closely on the eye. The camera zoom is then decreased to give a better view of the whole procedure of the operation during the nucleus sculpting and segment removal stages, thus making the lens radius smaller within the captured image. When the camera zoom is decreased there is a decrease in light intensity within the captured image. Although the changes in zoom are made only before the process of nucleus sculpting, and not during this critical stage, the sensitivity of RGB values to camera zoom limits their usefulness for analysis. Attempts were made to normalise the changes in RGB values seen in the lens using those for the iris, which are affected by changes in zoom and ambient lighting but are not otherwise altered during the operation. However, correction in this way did not wholly resolve the problems satisfactorily.

Hue, saturation and value

Hue, saturation and value (HSV) results are plotted in Figure 7. The hue represents the colour "tint" present 15 in the image, and is separate from the overall brightness. Use of the HSV colour model therefore enables the isolation of colour changes from changes in brightness due to ambient lighting or camera zoom, which primarily affect the "value" results. Using the RGB system it was harder to distinguish the difference between colour change (change in the ratio of RGB values) from brightness change (change in the magnitude of the RGB values while retaining the same ratio between them) .

The insensitivity of hue values to camera zoom is illustrated by the virtually constant mean level of hue for the iris throughout the operations (i.e. neglecting scatter on individual readings) shown in Figure 3c. Variation of "saturation" and "value" were sensitive to zoom and external lighting to a similar degree to the RGB values .

Capsulorrhexis

For all operations analysed the hue values were stable during the process of removing the anterior capsule (capsulorrhexis) as shown by the hue line in Figure 7, approximately frames 0-50. This stability is believed to be due to the fact that only the anterior capsule is removed during this process. The anterior capsule is a very thin, transparent and colourless membrane, so no change in hue would be expected upon its removal . Although hue values are generally stable, there are some frames where there are "blips" away from the steady values. These changes are generally associated with insertion of surgical tools. Scattering of light by the tool surface changes not only the brightness of the image but also changes the hue (i.e. the colour tint). 16 Nucleus Sculpting

The hue curves for all operations generally show a gradual increase during the nucleus-sculpting portion of the operation. See Figure 7 from frame 56 to 120. These changes correspond to removal of the cataractous lens material, which brings the retina into view and hence produces a change in the hue of the image. It is this change that is the key to the use of image analysis as an aid to preventing posterior capsule rupture, which is most likely to occur during the process of nucleus sculpting.

Segment removal

This section of the operation deals with removal of the four quadrants formed by nucleus sculpting. During this stage the interior of the eye is revealed, giving a decrease' in the hue value seen in the last 20-30 frames of each operation.

The operations demonstrated that the hue, representing the tint of the colour, showed changes that could be

^• correlated with the process of emulsification of the lens. Changes in the colour of light captured via the operating microscope can accordingly be used to detect the thickness of the lens remaining, thereby helping to prevent posterior capsule rupture. The detected changes can be used to provide an output to a suitable signal generating means, for example, an ear-piece. The earpiece can emit an audible signal that alerts the surgeon to the amount of lens removed and thus the proximity of the phaco-tip to the posterior capsule. The reader's attention is directed to all papers and 17 documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims :

1. A surgical device comprising a surgical implement and means for measuring the distance of the implement from a surface.

2. A surgical device according to claim 1, wherein the surgical implement comprises a phaco-emulsification device .

3. A surgical device according to claim 1 or 2, wherein the surgical implement comprises an ultrasonic probe.

4. A surgical device according to any one of the preceding claims, wherein the means for measuring the distance of the implement from the surface comprises feedback means for providing the surgeon with information on the position of the surgical implement.

5. A surgical device according to claim 4, wherein the feedback means provides an audible signal.

6. A surgical device according to any one of the preceding claims, wherein the means for measuring the distance of the implement from the surface comprises means for detecting an optical change.

7. A surgical device according to claim 6, wherein the optical change detected is a change in hue.

8. A surgical device according to any of the preceding claims, for eye surgery, the eye comprising a lens, a cornea, an iris, an anterior capsule and a posterior capsule, which comprises : a) a surgical implement; 19 b) means for detecting changes in hue in light visible through the eye lens during surgery; and c) feedback means wherein the feedback means receives an output from the detecting means and provides a signal to a surgeon controlling the surgical implement indicating the distance of the implement from the posterior capsule.

9. A surgical device for reducing the risk of rupturing of the posterior capsule during eye surgery comprising: a) means for capturing an image of the hue of light visible through an eye lens during surgery; b) means for measuring changes in hue of light visible through the eye lens during surgery; and c) feedback means wherein the captured image of the hue is used as an input for the means for measuring changes in hue and the means for measuring changes in hue provides an input to the feedback means which in turn provides a signal to alert a surgeon controlling the surgical device to said change.

10. A surgical device according to claim 9, wherein an increase in redness of hue indicates a reduction in distance between the surgical implement and the posterior capsule.

11. A surgical device according to any one of claims 8 to 10, wherein said eye surgery is cataract surgery.

12. A surgical device as claimed in claim 11, wherein said cataract surgery comprises phaco-emulsification. 20

13. A surgical device according to claim 9, wherein the feedback means provide a real time signal to the surgeon.

14. A surgical device as claimed in claim 6, which also comprises means for recording the optical change.

15. A surgical device as claimed in claim 14, wherein the means for recording is a video recorder.

16. A device as claimed in any preceding claim, wherein said means for measuring or detecting comprises a computer .

17. A device as claimed in claim 16, wherein said computer reads the HSV Colour Space values of the recording at specified intervals.

18. A device as claimed in claim 5, wherein said audible signal is fed into an earpiece adapted to be worn by a surgeon.

19. A device as claimed in claim 18, wherein an increase in redness of hue results in an increase in frequency of audible signal.

20. A method of reducing the risk of rupturing of the posterior capsule during eye surgery comprising: d) measuring changes in hue visible through an eye lens during surgery; and e) providing feedback to a surgeon controlling a surgical implement about the changes in hue wherein a change in hue indicates a change in 21 distance between the surgical implement and the posterior capsule.

21. A method according to claim 20, wherein an increase in redness of hue indicates a decrease in the distance between the surgical implement and the posterior capsule .

22. A method as claimed in claim 20 or 21, wherein said eye surgery is cataract surgery.

23. A method as claimed in claim 22, wherein said cataract surgery comprises phaco-emulsification.

24. A method as claimed in any of claims 20 to 23, wherein said measuring of changes in hue comprises recording images of the eye lens during surgery and processing the recording to provide an output informative about changes in hue.

25. A method as claimed in any one of claims 20 to 24, wherein said measuring of changes in hue comprises reading the HSV Colour Space values of the recording continuously or at specified intervals.

26. A method according to claim 24 or 25, wherein said output is input to a feedback device adapted to provide feedback to the surgeon about changes in hue.

27. A method according to claim 26, wherein said feedback device emits an audible signal and comprises an earpiece adapted to be worn by the surgeon.

28. A position measuring device for a surgical instrument for measuring the distance of the implement 22 from a surface.

29. A position measuring device according to claim 28, wherein said device comprises a feedback means for providing a surgeon with information on the position of the surgical implement.

30. A position measuring device according to claim 29, wherein the feedback means provides an audible signal.

31. A position measuring device according to any of claims 28 to 30, wherein the means for measuring the distance of the implement from the surface comprises means for detecting an optical change.

32. A position measuring device according to claim 31, wherein the optical change detected is a change in hue.

33. A surgical device substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings.

34. A surgical device substantially as hereinbefore described.

35. A method of reducing the risk of rupturing of the posterior capsule . during eye surgery substantially as described in the Example.

36. A method of reducing the risk of rupturing of the posterior capsule during eye surgery substantially as hereinbefore described.