AN ACOUSTIC DIVERTER FOR IMPROVED SAFETY DURING OPHTHAMLMIC LASER TREATMENTS

Abstract

The present invention belongs to the field of instruments for examination and treatment of human eyes. The invention relates to an acoustic diverter, which is used during laser treatment of eyes based on photodisruption within the eye, such as capsulotomy, iridotomy and vitreolysis. The present invention achieves the task of limiting or preventing acoustic waves to be focused in the eye so that the threshold negative pressures for injuries is not reached by providing an acoustic diverter, which has a geometry of an anterior surface concave, flat or slightly convex such that it prevents or decreases refocusing of acoustic waves back into the eye, and wherein the acoustic diverter is made of any biocompatible and sterilisable material transparent for the visible and near IR light. The preferred material has acoustic impedance between 1.4 to 1.7×10.sup.6 kg/(m.sup.2.Math.s) and/or attenuation coefficient more than 1 cm.sup.−1 at 10 MHz in order to ensure improved dispersion or attenuation of acoustic waves.

Claims

1. An acoustic diverter for dispersing of acoustic waves to limit or prevent their focusing in the eye, which is designed as a one-piece body having a posterior surface for contacting the eye and an anterior surface, wherein the geometry of the anterior surface is concave, flat or slightly convex with a radius of curvature more than 25 mm, such that it prevents or decreases refocusing of acoustic waves back into the eye, and wherein the acoustic diverter is made of any material having the following properties: transparency for the visible and near IR light, has to be autoclavable or sterilisable, and has to be biocompatible.

2. The acoustic diverter according to claim 1, wherein it is made of PMMA, glass or plastic.

3. The acoustic diverter according to claim 1, wherein it is made of a material having acoustic impedance between 1.4 and 1.7×10.sup.6 kg/m.sup.2s.

4. The acoustic diverter according to claim 3, wherein the material is polymethylpentene (PMP).

5. The acoustic diverter according to claim 1, wherein it is made of a material that has attenuation coefficient more than 1 cm.sup.−1 at 10 MHz.

6. The acoustic diverter according to claim 1, wherein it is made of a material having acoustic impedance between 1.4 and 1.7×10.sup.6 kg/(m.sup.2s) and attenuation coefficient more than 1 cm.sup.−1 at 10 MHz.

7. An acoustic diverter for dispersing of the acoustic waves to limit or prevent their focusing in the eye, which is designed as a one-piece body having a posterior surface for contacting the eye and an anterior surface, wherein the acoustic diverter is made of any material having the following properties: transparency for visible and near IR light, autoclavable or sterilisable, and biocompatible, with acoustic impedance between 1.4 and 1.7×10.sup.6 kg/(m.sup.2s) and attenuation coefficient more than 1 cm.sup.−1 at 10 MHz.

8. The acoustic diverter according to claim 7, wherein the material is PMP.

9. The acoustic diverter according to any of the preceding claims, wherein the diverter covers the air-exposed part of the sclera and the entire cornea.

10. The acoustic diverter according to any of the preceding claims, wherein it is combined with one or more additional lenses to form a lens assembly, wherein the diverter and lenses are separated by air space.

11. The acoustic diverter according to any of the preceding claims, wherein the entry lens and optional intermediate lens is made of the same material as the acoustic diverter or any other suitable material.

12. The acoustic diverter according to any of the preceding claims, wherein all surfaces have an optical coating for visible and IR light.

13. Use of the acoustic diverter according to any of the preceding claims in ophthalmology.

14. Use according to claim 13 in laser treatments of eyes based on photodisruption, preferably during iridotomy, capsulotomy or vitreolysis.

15. Use of the acoustic diverter according to any claim from 13 to 14, with a photodisruptor.

16. An acoustic diverter for use in photodisruption based laser treatments in which optical breakdown occurs for dispersing of acoustic waves to limit or prevent their focusing in the eye, which is designed as a one-piece body having a posterior surface for contacting the eye and an anterior surface, wherein the geometry of the anterior surface is concave, flat or slightly convex with a radius of curvature more than 25 mm, such that it prevents or decreases refocusing of acoustic waves back into the eye, and wherein the acoustic diverter is made of any material having the following properties: transparency for the visible and near IR light, has to be autoclavable or sterilisable, and has to be biocompatible, has acoustic impedance between 1.4 and 1.7×10.sup.6 kg/(m.sup.2s) and has acoustic attenuation coefficient more than 1 cm.sup.−1 at 10 MHz.

17. The acoustic diverter according to claim 16, wherein the material is polymethylpentene (PMP).

18. An acoustic diverter for use in photodisruption based laser treatments for dispersing of the acoustic waves to limit or prevent their focusing in the eye, which is designed as a one-piece body having a posterior surface for contacting the eye and a concave anterior surface, wherein the acoustic diverter is made of any material having the following properties: transparency for visible and near IR light, autoclavable or sterilisable, and biocompatible, and is selected in the group consisting of PMMA, glass, plastic and polymethylpentene (PMP).

19. The acoustic diverter according to any of the preceding claims, wherein the diverter covers the air-exposed part of the sclera and the entire cornea.

20. The acoustic diverter according to any of the preceding claims, wherein it is combined with one or more additional lenses to form a lens assembly, wherein the diverter and lenses are separated by air space.

21. The acoustic diverter according to any of the preceding claims, wherein the entry lens and optional intermediate lens is made of the same material as the acoustic diverter or any other suitable material.

22. The acoustic diverter according to any of the preceding claims, wherein all surfaces have an optical coating for visible and IR light.

23. Use of the acoustic diverter according to any of the preceding claims in ophthalmology.

24. The acoustic diverter according to any claim from 16 to 22 for use in iridotomy, capsulotomy or vitreolysis.

25. The acoustic diverter according to any claim from 16 to 22 for use with a photodisruptor.

Description

DESCRIPTION OF THE INVENTION

[0014] The invention will be described based on figures, which show:

[0015] FIG. 1: Acoustic wave propagation within the eye during ophthalmic surgery at approximately a) 0 μs, b) 3 μs, c) 9 μs, d) 11 μs and e) 14 μs after photoionization

[0016] FIG. 2a: Acoustic diverter according to the invention with a concave anterior surface

[0017] FIG. 2b: Acoustic diverter according to the invention with a flat anterior surface

[0018] FIG. 2c: Acoustic diverter according to the invention with a slightly convex anterior surface

[0019] FIG. 3: Acoustic wave propagation with the acoustic diverter according to the invention at approximately a) 0 μs, b) 3 μs, c) 8 μs, d) 10 μs and e) 13 μs after photoionization

[0020] FIG. 4: Acoustic wave propagation with a typical ophthalmic lens not suitable for reducing undesirable acoustic wave effects at approximately a) 0 μs, b) 3 μs, c) 8 μs, d) 14 μs and e) 18 μs after photoionization

[0021] FIG. 5: Acoustic diverter with additional entry lens

[0022] The present invention aims to provide an acoustic diverter for decreasing risk of damage to eye tissues. The acoustic diverter is manually placed onto the cornea of the eye during the laser treatment. The task of the acoustic diverter is to prevent or at least minimize damaging effects of acoustic waves emitted during the ophthalmic laser treatments, particularly those based on photodisruption.

[0023] The present invention achieves the above discussed task by providing an acoustic diverter, which has to have a geometry such that it prevents or decreases refocusing of acoustic waves back into the eye. The diverter is designed as a one-piece body having a surface to contact the eye and a surface away from the eye as shown in FIGS. 2a-2c. The surface of the acoustic diverter, which is in contact with the cornea of the eye is called the posterior surface. Conversely, the surface away from the eye is called the anterior surface. The geometry, more precisely the shape of the anterior surface of the acoustic diverter, influences whether the wave will be focused or dispersed.

[0024] The geometry of the acoustic diverter has to be such that the posterior surface is concave in order to adapt to average cornea curvature and the anterior surface is concave, flat or slightly convex (with a radius of more than 25 mm) in order to disperse the acoustic waves, so that focusing does not occur within the eye. Using such acoustic diverter, the negative peak pressures during the propagation of the pressure waves never exceed the injury threshold, thus preventing or at least minimizing eye injuries. The preferred geometry is that the anterior surface is concave, as this geometry provides the best dispersion of waves.

[0025] It is desirable that the acoustic diverter covers as much of the air-exposed surface of the eye as possible. Specifically, the air-exposed part of the sclera and the entire cornea, in order to prevent excessive contact between the eye and air, because this interface is subjected to significant wave reflection due to the differences in acoustic impendences of air and eye structures. Namely, acoustic waves are almost fully reflected and polarity-inverted from compressions to rarefactions when reflection takes place at the tissue-air interface.

[0026] FIG. 2a shows a preferred embodiment of the acoustic diverter 6 according to the present invention placed onto the human eye 7, in which the right surface of the diverter is posterior 6a and in contact with the cornea and sclera 7a, while the left surface of the diverter is anterior 6b. This embodiment has a concave anterior surface 6b. FIGS. 2b and 2c show possible embodiments of the acoustic diverter having the anterior surface 6b′ and 6b″ flat or slightly convex with radius more than 25 mm, respectively.

[0027] FIG. 3 shows the propagation of acoustic waves when the acoustic diverter according to the invention is used. The compression waves 8, which are transmitted through the cornea 9a into the acoustic diverter 10 are reflected from the concave anterior surface 10a of the diverter back into the eye as diverging rarefaction 8a. Because of the divergent propagation, the rarefaction negative amplitude is not sufficient enough to cause damage to the eye structures after entering the eye 9.

[0028] The diverter may be made of any suitable material that has the following properties: [0029] transparency for the visible and near IR light, [0030] has to be autoclavable or sterilisable, and [0031] has to be biocompatible.

[0032] Such materials are glass, different plastics, polymethylmethacrylate (PMMA), polymethylpentene polymer (PMP), etc.

[0033] The properties of the material, namely its acoustic impedance Z, determine the reflected and transmitted amount of the acoustic wave at the diverter-cornea surface, and what is their pressure ratio with respect to the incoming wave. The reflection coefficient for the acoustic wave pressure amplitude on the interface between two materials can be calculated using the following formula:

r=(Z.sub.1−Z.sub.2)/(Z.sub.1+Z.sub.2)

[0034] in which r is the acoustic pressure reflection coefficient, Z.sub.1 is the acoustic impedance of the material through which the wave initially propagates, Z.sub.2 is the acoustic impedance of the material into which the wave is transmitted. In this case, Z.sub.1 is related to the eye tissue, while Z.sub.2 corresponds to the material of the acoustic diverter.

[0035] If the diverter is made of a material with acoustic impedance differing from the acoustic impedance of eye tissues (ranging from 1.4 to 1.7×10.sup.6 kg/(m.sup.2.Math.s)) a part of the wave will be reflected. The acoustic impedance values for glass and PMMA are around 13×10.sup.6 kg/(m.sup.2.Math.s) and 3.3×10.sup.6 kg/(m.sup.2.Math.s), respectively. In case the material of the acoustic diverter is PMMA, the acoustic pressure reflection coefficient is r 35%, which means that the pressure amplitude of the reflected acoustic wave is 35% of the incident acoustic wave. In the case of glass, this reflection is approximately 80%. Due to the reflection from the contact surface between the eye and the acoustic diverter made of glass or PMMA, refocusing of the pressure wave within the eye still occurs. This means that both materials at least partly reduce the amplitude of the reflected acoustic waves, which reduces invasiveness of laser treatments based on photodisruption. If there is no element placed on the cornea, the amount of reflected wave will be almost 100%, since the acoustic impedance of air is 0.0004×10.sup.6 kg/(m.sup.2.Math.s). Table 1 lists the acoustic pressure reflection coefficient for different materials used as an acoustic diverter.

TABLE-US-00001 TABLE 1 Amount of reflected acoustic wave for different materials Acoustic pressure reflection Acoustic diverter material coefficient No diverter (Air) Almost 100% Glass 80% PMMA 35% PMP Close to 0%

[0036] In order to ensure the transmission of acoustic waves without reflections, the acoustic diverter is preferably made of a material with similar acoustic impedance as eye tissue, preferably the cornea. The human cornea has acoustic impedance of 1.7×10.sup.6 kg/(m.sup.2.Math.s). When the material of the acoustic diverter features an acoustic impedance similar to the eye (between 1.4 and 1.7×10.sup.6 kg/(m.sup.2s), the amount of reflected waves is almost 0. Hence, such material is the preferred candidate for acoustic diverter according to the invention, as focusing of acoustic waves will be prevented to a larger extent compared to when using glass or PMMA. A preferred example of a suitable material is polymethylpentene (PMP). PMP is a kind of a polyolefin and this expression covers different variants of the polymer, such as methylpentene homopolymer, or copolymers of 4-methylpentene-1 with olefinic monomers such as ethylene, propylene, butylene and higher olefins. It is commercially available from Mitsui Chemicals as TPX™-RT18. The acoustic impedance of PMP ranges from 1.46 to 1.70×10.sup.6 kg/(m.sup.2.Math.s) at temperatures ranging from 25° C. to 37° C., which is thus very close or equal to an acoustic impedance of the human eye tissues, making it particularly suitable for the acoustic diverter according to the invention. The synergistic effect of the acoustic diverter geometry and the material with acoustic impedance between 1.4 and 1.7×10.sup.6 kg/(m.sup.2.Math.s) is that acoustic waves are not focused back into the eye but are effectively dispersed. Consequently, the negative peak pressures never exceed the threshold of −4 MPa, which means that eye injuries are prevented or minimized.

[0037] In addition to the geometry or to the geometry and the suitable acoustic impedance, large acoustic attenuation of the material is preferred, because in this case the waves are absorbed within the diverter and cannot be reflected from the anterior surface of the diverter towards the eye. It is thus desirable that the material used to manufacture the acoustic diverter has a large attenuation coefficient near the acoustic wave frequency of 10 MHz, which is the dominant frequency of shock waves encountered in ophthalmic surgeries. The attenuation coefficient should be preferably more than 1 cm.sup.−1, which ensures that the majority of the wave is absorbed upon travelling the acoustic diverter back and forth. This way the effects of the reflected acoustic waves from the anterior surface of the diverter are further reduced.

[0038] According to the previous two paragraphs, in the ideal case, the acoustic diverter is made of the material which has an acoustic impedance in the range from 1.4 to 1.7×10.sup.6 kg/(m.sup.2.Math.s) and attenuation coefficient of more than 1 cm.sup.−1.

[0039] According to calculations, an acoustic diverter made of the material with acoustic impedance in the range from 1.4 to 1.7×10.sup.6 kg/(m.sup.2.Math.s) and having an attenuation coefficient of more than 1 cm.sup.−1 excluding the above-mentioned geometry of the anterior surface already ensures sufficient damping of the acoustic waves, because the acoustic waves would already have been sufficiently absorbed. Thus, the acoustic diverter with an arbitrary shape of the anterior surface (concave, convex or flat) made from the material with an acoustic impedance in the range from 1.4 to 1.7×10.sup.6 kg/(m.sup.2.Math.s) and attenuation coefficient larger than 1 cm.sup.−1 represents an alternative solution of the technical problem.

[0040] Usually, a physician uses a special contact ophthalmic lens while performing a treatment. Such ophthalmic lenses are specialized for a particular treatment, such as capsulotomy, iridotomy, vitreolysis. They are usually made of glass or PMMA material. However, they do not prevent the refocusing of the acoustic waves back into the eye due to the inappropriate geometry and material used as shown in FIG. 4. A portion of the compression 11b is reflected already at the posterior surface 11a. The anterior surface 11 of these ophthalmic lenses is convex, which results in rarefaction focusing 12 back into the eye. Rarefaction focusing may also result in microbubble formation 13 in the laser illuminated parts of the eye. There exist some highly specialized lenses, like the ones according to WO2009094214 and CH694936A5, which can have non-convex anterior surface. However, they are not appropriate for any of the relevant photodisruption-based treatments, since they are all intended for diagnostics and treatment of peripheral regions of the eye and retina.

[0041] The present invention also provides a possibility to add to the acoustic diverter additional lenses in order to provide a collective system of lenses with an effective focal length most suitable for a particular treatment as shown in FIG. 5. The acoustic diverter 14 and any possible additional lenses 16 may be made of the same material as the acoustic diverter described above or of a different material, provided that the material is suitable for making ophthalmology lenses, such as optical glass or polymeric material. Possible polymeric materials are PMMA, polycarbonate, polystyrene . . . . Preferably, the diverter 14 and any additional lenses 16 are made of a material having acoustic impedance 1.46 to 1.70×10.sup.6 kg/(m.sup.2.Math.s), most preferably said elements are made of PMP. Additionally, the surfaces of the acoustic diverter and possible additional lenses may have an optical antireflection coating for visible and IR light. The most preferred embodiment has an optical antireflection coating on all surfaces.

[0042] In the embodiment, in which the acoustic diverter according to the invention is combined with an additional anterior lens, both parts are separated by air space 15. The separation between both elements is provided with any suitable distancing element, such as a distancing O-ring with the appropriate width or other suitable spacing elements in a well-known manner. There may be more than one lens following the acoustic diverter. In the embodiment with intermediate lenses, the above-mentioned air space is provided between the contact and the intermediate lens in the same way. The function of this air gap is to prevent any transmission of the acoustic waves through the anterior surface of the acoustic diverter to the additional lenses.

[0043] The acoustic diverter is intended for the use in ophthalmology, in particular for the use in photodisruption laser treatments, such as capsulotomy, iridotomy and vitreolysis. The acoustic diverter according to the invention ensures that the acoustic waves are effectively dispersed so that they are not focused in the eye. Consequently, the negative peak pressures do not exceed the injury threshold, thus preventing or at least minimizing eye injuries. This is achieved by a specific geometry and/or specific material of the acoustic diverter.