DRAINAGE DEVICE AND METHODS

Abstract

A drainage device for use in an eye to drain aqueous humour so as to reduce intraocular pressure or for treating glaucoma. The device has a multi-lumen tube having a first end, a second end opposite the first end, and a plurality of lumen extending between the first end and the second end. At least one of the lumen is sealed at the first end. A flow through the multi-lumen tube is adjusted by forming at least one aperture open in one of the lumen through a wall of the tube and/or sealing at least one aperture open in one of the lumen. The tube has a longitudinal axis through the first end and the second end, and an outer surface extending between the first end and the second end. A cross-section perpendicular to the longitudinal axis has a non-circular shape at the outer surface with an aspect ratio of at least 3:1.

Claims

1. A drainage device for use in an eye to drain aqueous humour so as to reduce intraocular pressure, the device comprising a flexible multi-lumen tube having a first end, a second end opposite the first end, a longitudinal axis through the first end and the second end, a plurality of lumen extending between the first end and the second end, and an outer surface extending between the first end and the second end, wherein a cross-section perpendicular to the longitudinal axis has a non-circular shape at the outer surface with an aspect ratio of at least 3:1. width:height and/or an eccentricity between approximately 0.6 and approximately 0.98.

2. A drainage device according to claim 1, wherein the cross-section shape at the outer surface is an ellipse.

3. A drainage device according to claim 1, wherein the multi-lumen tube is anisotropic in bending about two axes each perpendicular to the longitudinal axis.

4. A drainage device according to claim 3, wherein the multi-lumen tube has a width and a height, and the plurality of lumen are spaced in the width dimension, and the tube has greater bending flexibility in a plane including the height dimension than in a plane including the width dimension.

5. A drainage device for use in an eye to drain aqueous humour so as to reduce intraocular pressure, the device comprising a multi-lumen tube having a first end, a second end opposite the first end, and a plurality of lumen extending between the first end and the second end, wherein at least one of the lumen is sealed at the first end, and further comprising at least one aperture open in said at least one of the lumen through a sidewall of the tube and located along the length of the tube between the first end and the second end, wherein the at least one aperture fluidly connects the second end of the tube to outside the tube through said lumen.

6. A drainage device according to claim 5, wherein a distance from the second end to the aperture is selected to provide predetermined resistance to fluid flow through the device.

7. A drainage device according to claim 5, further comprising a plurality of the apertures.

8. A draining device according to claim 7, wherein a plurality of the lumen each have at least one of the apertures discretely through the sidewall.

9. A drainage device according to claim 7, wherein said at least one of the lumen has a plurality of the apertures spaced along the length of the tube.

10. A drainage device according to claim 1, wherein said at least one of the lumen has an internal diameter selected to provide predetermined resistance to fluid flow through the device.

11. A drainage device according to claim 1, wherein the tube is flexible.

12. A drainage device according to claim 1, wherein each lumen has a diameter of between approximately 40 microns to approximately 200 microns.

13. A drainage device according to claim 1, wherein each lumen has a substantially constant cross section along the length of the tube.

14. A drainage device according to claim 1, wherein the tube length is between approximately 5 mm to approximately 30 mm.

15. A drainage device according to claim 1, wherein the tube width is between approximately 0.5 mm to approximately 3 mm.

16. A drainage device according to claim 1, wherein a maximum height of the tube is approximately 500 microns or less.

17. A drainage device according to claim 1, wherein two or more of the lumen have different internal diameters.

18. A drainage device according to claim 1, wherein one or more of the lumen have a substantially circular cross section.

19. A drainage device according to claim 1, wherein the tube includes biocompatible and/or biostable material.

20. A drainage device according to claim 1, wherein the tube includes at least one of plastics material and silicone.

21. A drainage device according to claim 1, wherein a sidewall of the tube has a thickness of between approximately 5 microns to approximately 200 microns.

22. A drainage device according to claim 1, wherein the tube includes transparent or translucent material.

23. A drainage device according to claim 1, wherein each lumen is valveless.

24. A drainage device according to claim 1, wherein the first end of the tube has a bevelled edge.

25. A drainage device according to claim 1, further comprising generally planar extensions projecting from the tube intermediate the first and second ends.

26. (canceled)

27. A drainage device according to claim 1, further comprising a plate adapted to locate on the eye, and wherein the first end of the multi-lumen tube opens in the underside of the plate.

28. A method of manufacturing a drainage device for use in an eye to drain aqueous humour so as to reduce intraocular pressure, comprising providing a multi-lumen tube having a first end, a second end opposite the first end, a plurality of lumen extending between the first end and the second end, and adjusting a flow through the multi-lumen tube by forming at least one aperture open in one of the lumen through a wall of the tube and/or sealing at least one aperture open in one of the lumen.

29. A method according to claim 28, wherein forming the aperture comprises forming the aperture through a sidewall of the tube.

30. A method according to claim 28, wherein at least one of the lumen is sealed at the first end to provide an end wall, and forming the aperture comprises forming the aperture through the end wall of the sealed first end.

31. A method according to claim 30, wherein sealing the aperture comprises either closing an open first end of the lumen or closing an aperture through a sidewall of the tube.

32. A method according to claim 28, wherein the aperture is formed by laser cutting.

33. A method according to claim 28, wherein the multi-lumen tube is made by extrusion, drawing or injection moulding, wherein extrusion comprises extruding a multi-lumen preform through a die, optionally stretching the preform in the longitudinal direction to reduce the lumen diameter and cutting to a desired tube length.

34. A method according to claim 28, for manufacturing a drainage device.

35. A method for treating glaucoma or controlling intraocular pressure in a patient's eye with a drainage device, wherein the drainage device comprises a flexible multi-lumen tube having a first end, a second end opposite the first end, a longitudinal axis through the first end and the second end, a plurality of lumen extending between the first end and the second end, and an outer surface extending between the first end and the second end, wherein a cross-section perpendicular to the longitudinal axis has a non-circular shape at the outer surface with an aspect ratio of at least 3:1 width:height and/or an eccentricity between approximately 0.6 and approximately 0.98, the method comprising positioning the first end of the drainage device in the anterior chamber of the patient's eye, and positioning the second end of the drainage device in the subconjunctival space of the patient's eye.

36. A method according to claim 35, further comprising opening one or more apertures in one or more of the lumens to control the flow rate of aqueous humour through the drainage device.

37. A method for preparing a drainage device for surgery, the drainage device comprising a multi-lumen tube having a first end a second end opposite the first end, a plurality of lumen extending between the first end and the second end, wherein at least one of the lumen is sealed at the first end, and further comprising at least one aperture open in said at least one of the lumen through a sidewall of the tube and located along the length of the tube between the first end and the second end, wherein the at least one aperture fluidly connects the second end of the tube to outside the tube through said lumen, the method comprising: comparing an intraocular pressure measurement obtained from a patient with a threshold to calculate the required drop in intraocular pressure, and opening the at least one aperture in the at least one lumen to control the flow rate of aqueous humour through the drainage device and provide the required drop in intraocular pressure.

38. A kit comprising a drainage device according to claim 1 and complimentary forceps and/or blade and/or inserter.

39. A drainage device according to claim 1, further comprising at least one aperture open in at least one of the lumen through a sidewall of the tube and located along the length of the tube between the first end and the second end, wherein the at least one aperture fluidly connects the second end of the tube to outside the tube through said lumen.

40. A drainage device according to claim 39, further comprising generally planar extensions projecting from the tube intermediate the first and second ends.

41. A drainage device according to claim 40, wherein the at least one aperture is located between the generally planar extensions and the first end.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0061] FIG. 1 shows a glaucoma drainage device (GDD) connecting the anterior chamber of the eye to a bleb in the subconjuctival space;

[0062] FIGS. 2 to 9 show various views of the GDD;

[0063] FIGS. 10(a)-(c) show various examples of the GDD with apertures in different locations to adjust the pressure drop though the GDD;

[0064] FIGS. 11 and 12 show the pressure drop for the examples of FIGS. 10(a)-(c);

[0065] FIGS. 13(a)-(f) show the change in pressure drop though an aperture of different diameters;

[0066] FIGS. 14(a)-(d) show various alternative cross sections for the GDD; and

[0067] FIG. 15 shows a plate for use with the GDD.

[0068] FIG. 16 shows a GDD divided into three parts: A, B and C.

[0069] FIG. 17 shows Finite-Element Analysis (FEA) simulation of a GDD with force applied at the back of the tube when held in place at the wings before bending (a), and after bending (b).

[0070] FIG. 18 shows a simulation of a conjunctival map for mitomycin-c (MMC) application during trabeculectomy surgery (Khaw et al. Dev. Opthalmol. 2017, 59:15-35).

[0071] FIG. 19 shows deflections and associated Von Mises stress of conjunctival flaps. (a)-(b): 1 mm indentation by 1 mm wide; (c)-(d): 0.5 mm indentation by 0.5 mm wide; (e)-(f): 0.25 mm indentation by 0.25 mm wide; and (g)-(h): 0.125 mm wide. Each indentation is 2.5 mm long.

[0072] FIG. 21 shows (a) elliptical tubes ranging from eccentricity of 0 (circular tube) to 0.98 (an example of the GDD described herein) with (b): the definition of height (H) and width (b) of elliptical tube.

[0073] FIG. 22 shows a model of an incision shape when a circular tube is inserted through it.

[0074] FIG. 23 shows a cross-section of tubes of different eccentricity with a 0.500 mm incision shown at the outer boundary. All dimensions are in mm.

[0075] FIG. 24 shows computational fluid dynamics analysis of the initial set-up (a) and flow pathlines (b) for a circular tube of external diameter 0.2 mm, length 3 mm and a lumen diameter of 0.05 mm. The incision is 0.2 mm high and 0.5 mm wide and 3 mm long. The flow rate was fixed at 2 μl/min and the pressure drop was less than 0.1 mmHg.

[0076] FIG. 25 shows computational fluid dynamics analysis of the initial set-up (a) and flow pathlines (b) for an elliptical tube (H=0.2 mm, b=0.438 mm and length of 3 mm) with a lumen diameter of 0.05 mm. The incision is 0.2 mm high, 0.5 mm wide and 3 mm long. The flow rate was fixed at 2 μl/min and the pressure drop was around 3 mmHg.

[0077] FIG. 26 shows computational fluid dynamics analysis of the initial set-up (a) and flow pathlines (b) for an elliptical tube (H=0.2 mm, b=0.492 mm and length of 3 mm) with a lumen diameter of 0.05 mm. The incision is 0.2 mm high, 0.5 mm wide and 3 mm long. The flow rate was fixed at 2 μl/min and the pressure drop was around 5 mmHg.

[0078] FIG. 27 shows computational fluid dynamics analysis of pathlines around an elliptical rod (H=0.2 mm, b=0.492 mm and length of 3 mm) inserted into an incision of 0.2 mm high, 0.5 mm wide and 3 mm long. The flow rate was fixed at 2 μl/min and the pressure drop was in excess of 400 mmHg.

[0079] FIG. 28 shows (a) variation of the second moment area ratio I.sub.ex/I.sub.rx, with b/H and (b) example of deflection of the GDD along the y axis.

[0080] FIG. 29 shows (a) variation of the second moment area ratio I.sub.ey/I.sub.ry, with b/H and (b) example of deflection of the GDD along the x axis.

[0081] FIG. 30 shows Von Mises Stresses of a circular cylinder of diameter 0.2 mm (a) and elliptical (0.2 mm by 1 mm) when bent with a force of 1 mN.

[0082] FIG. 31 shows a schematic of a tube being deflected upward where exiting from the incision into the subconjunctival space (a) and the corresponding Finite-Element Analysis (FEA) model for a circular tube of diameter 0.2 mm (b) and an elliptical tube of height 0.2 mm and width 1 mm (c) deflected upward by 1 mm.

[0083] FIG. 32 shows Von Mises Stress along the tube corresponding to the deflections mentioned in FIG. 31 for the circular tube in (a-b) and elliptical tube (c-d).

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0084] FIG. 1 illustrates schematically an eye, e.g. of a human, showing the lens 1, retina 2, optic nerve 3, cornea 4, sclera 5 and anterior chamber 6. A glaucoma drainage device (GDD) 7 is used to fluidly connect the anterior chamber 6 to a bleb 8 in the subconjunctival space following trabeculectomy so as to lower the intra-ocular pressure (IOP). The GDD 7 in accordance with a first embodiment is shown in detail in FIGS. 2 to 9.

Shape of the Device

[0085] As shown in FIG. 2, the GDD 7 is a triple-lumen tube having a first end 11 and a second end 12 opposite the first end. FIG. 3 is an ‘X-ray’ type image showing three lumens 13, 14, 15 extending from the first end to the second end. When in use, the first end 11 is for locating in the anterior chamber 6 and the second end is for locating in the subconjunctival space to discharge aqueous humour into the bleb 8.

[0086] The tube has a first end face 16 at the first end 11, and a second end face 17 at the second end 12. As best shown in FIG. 4, the first end face 16 is closed (sealed) to all three lumens 13, 14, 15 and, as best shown in FIG. 5, the second end face 17 is open to all three lumen 13, 14, 15.

[0087] The lumen 13, 14, 15 extend in the longitudinal direction of the tube 10. The central lumen 14 has a larger diameter than the outer lumen 13, 15.

[0088] The tube 10 has an outer surface 18 extending between the first and second end faces 16, 17. The tube 10 has sidewalls between the outer surface 18 and the respective lumen 13, 14, 15. The outer surface 18 has an oval shaped cross section, having an aspect ratio (width to height) of at least 3:1.

[0089] As best shown in FIGS. 6, 8 and 9, one lumen 13 has an aperture 20 open through the sidewall 21 near the first end 11 so as to fluidly connect the second end 12 to the outside of the tube 10 through the lumen 13. The location of the aperture 20 is selected to provide a fluid path between the aperture 20 and the open second end of the lumen 13 to provide a predetermined pressure drop across the GDD 7, as will be described in detail later. The aperture 20 may be formed by lasering the exterior of the tube, for example, as will be described in detail later.

[0090] Adopting an oval cross-section configuration helps to accommodate the three lumens 13, 14, 15 of different sizes while maintaining a small tube 10 height by placing the lumens 13, 14, 15 laterally next to each other. A round tube where all lumens are located in the centre of the tube would increase the sidewall thickness of the central lumen 14. This has implications for lasering the exterior of the lumen to create the aperture 20.

[0091] The oval cross-section also helps reduce any lateral movement when in-situ. A circular cross section tube would require the same force to deflect longitudinally and laterally. However, the oval shape makes it relatively more difficult to deflect the tube in bending laterally compared to longitudinally, indeed the ratio between lateral and longitudinal deflection is (width/height).sup.2. In the embodiment illustrated in FIGS. 2 to 9, the oval tube 10 with height=0.2 mm and width=0.9 mm, requires 20.25 times more force to deflect the tube laterally than longitudinally.

[0092] Stabilising generally planar extensions (or ‘wings’) 19 project from the outer surface 18 intermediate the first and second ends 11, 12. In the illustrated embodiment the wings 19 are located 4.5 mm behind the first end 11 of the tube 10. The ‘wings’ 19 or more generally a part to minimise tube movement (notches along the tube 10 could also be possible) help the GDD 7 to stay in place behind the limbus by preventing the tube 10 to slide in the anterior chamber 6.

[0093] The first end 11 is bevelled in the longitudinal direction. In the illustrated embodiment of FIGS. 2 to 9 the bevel angle is approximately 27° to give a bevel length of 0.4 mm or twice the height of the tube 10. The force to insert the GDD 7 into the conjunctiva is reduced when the first end 11 of the tube 10 is bevelled.

[0094] Finally, to minimise the damage of the tube 10 to the surrounding tissues when the tube is implanted, the first end 11 of the tube 10 is tipped (rounded). The second end 12 of the tube may also be tipped if convenient.

Flow Resistance of the Device

[0095] The GDD 7 controls the pressure drop through its lumens 13, 14, 15. A classic Hagen-Poiseuille law describes pressure drop through circular tube or lumen as shown in Equation 1:

[00001]$\begin{array}{cc}\delta .Math..Math.P=\frac{128.Math..Math.\upsilon .Math..Math.\mathrm{Lq}}{\pi .Math..Math.D^4}& \left(1\right)\end{array}$

with δP is the pressure drop though the lumen, v, the dynamic viscosity of the fluid, q, the flow rate and D, the diameter of the lumen and L, the length of the lumen. The resistance of the lumen is independent of the flow rate and is only defined by geometrical parameters and the dynamic viscosity of the fluid. Equation (1) can be rewritten in terms of resistance (R) as P=Q×R with R defined as:

[00002]$\begin{array}{cc}R=\frac{128.Math..Math.\mathrm{vL}}{\pi .Math..Math.D^4}& \left(2\right)\end{array}$

[0096] The diameter of lumen 13 is selected to provide a pressure drop of no less than 5 mmHg at 2 μl/min and a temperature of 36.7° C. Taking a tolerance for the lumen diameter of 3 μm, over a distance of 7.4 mm (to take into account the bevel length of 0.4 mm+a maximum plugged length of 0.2 mm), the maximum lumen diameter was calculated to be 57 μm such as to give a pressure drop of 5 mmHg at 2 μl/min and a temperature of 36.7° C. Any diameter above that value has been calculated to give a pressure drop lower than 5 mmHg. The diameter of the lumen 13 (and lumen 15) is therefore 54+/−3 μm.

[0097] If the IOP builds up in time due to different reasons such as an increase of resistance from the bleb 8, the GDD 7 offers the possibility to open lumen 15 by lasering above the tube 10 creating another aperture 20 to form a fluid path from the device exterior to the second end 12. When opening the lumen 15 at the same location as aperture 20 in lumen 13, the pressure drop is exactly 50% of that using aperture 20 in lumen 13 alone as each resistance acts in parallel.

[0098] The middle lumen 14 has a diameter of 110 μm. The middle lumen diameter is selected to reduce the pressure drop through the GDD 7 as much as possible without having an excessively thin sidewall 21 around the middle lumen 14. Ideally the GDD 7 has near zero flow resistance when the middle lumen 14 is opened. For example, middle lumen 14 may be opened in the case that the IOP of the patient should be reduced as much as possible using the GDD 7.

[0099] The choice of the diameter of the middle lumen 14 may be selected to ensure the pressure drop though the GDD 7 relatively low and ideally below 0.5 mmHg for 2 μl/min. With the middle lumen diameter of 110 μm, a pressure drop of approximately 0.3 mmHg is achieved when all three lumens 13, 14, 15 are opened at the first end 11 of the tube 10.

Resistance Adjustment Potential

[0100] The GDD 7 offers the possibility to alter the resistance of the device by lasering multiple apertures along a single lumen or by lasering more than one lumens or a combination of both. When lasering along a single lumen, the resistance of the device is proportional to the length of the tube as shown in Equation (2) (all other parameters being equal) and therefore the pressure drop is entirely affected by the length of the lumen. When opening more than one lumen in parallel, the resistance of N lumen adds up following Equation (3):

[00003]$\begin{array}{cc}{R}_{\mathrm{tot}={.Math.}_1^N}.Math.\frac{1}{\frac{1}{R_N}}& \left(3\right)\end{array}$

[0101] The final pressure drop given by the GDD 7 is δP=QR.sub.tot. Altering the resistance of the GDD 7 opens up minute modifications of the pressure drop given by the GDD. The GDD offers the possibility to open any of the three lumens 13, 14, 15 up to 3.5 mm from the first end 11 of the device (for a minimum lumen length of 4.5 mm).

[0102] FIGS. 10(a) to (c) illustrate 18 variants of the GDD 7 each numbered 1-18 and each having a different configuration of the aperture(s) 20 open in one or more of the lumen 13, 14, 15.

[0103] Table 1 shows the resistance of the device as well as the pressure drop (at 2 μl/min) for locations lasered 1 mm apart along each lumen.

TABLE-US-00001 TABLE 1 Resistance and pressure drop at 2 μl/min of the GDD 7 when lasered along and across each lumen. No. Resistance δP (mmHg) at 2 μl/min  1.  ≈100% ≈6.3  2.   ≈87% ≈5.5  3.   ≈73% ≈4.6  4.   ≈60% ≈3.8  5.   ≈50% ≈3.2  6.   ≈46% ≈2.9  7.   ≈43% ≈2.7  8.   ≈42% ≈2.7  9.   ≈40% ≈2.5 10.   ≈38% ≈2.4 11.   ≈37% ≈2.3 12.   ≈35% ≈2.2 13.   ≈33% ≈2.1 14.   ≈30% ≈1.9 15. ≈0.05% ≈0.3 16. ≈0.05% ≈0.3 17. ≈0.04% ≈0.2 18. ≈0.03% ≈0.2

[0104] Position 1 corresponds to a lumen length L of 7.5 mm, position 2 to L=6.5 mm, position 3 to L=5.5 mm and position 4 to L=4.5 mm. The resistance of each lumen is decreased by a maximum of 40% when opening the lumen at position 4, but the combination of lumens working in parallel achieve a larger pressure reduction up to approximately 0.02 mmHg at 2 μl/min and 36.7° C. when each lumen are opened at the position 4. Positions 1, 2, 3, and 4 are hypothetical locations, but any locations in between each of the positions are also possible giving an infinite control of the resistance of the GDD 7 and hence the IOP of the patient.

[0105] FIG. 11 shows the pressure drop at 2 μl/min referring to the different configurations reported in FIGS. 10(a) to (c) and Table 1 along one of lumens 13 or 15. FIG. 12 shows the pressure drop at 2 μl/min referring to the different configurations reported in FIGS. 10(a) to (c) and Table 1 along all three lumens 13, 14, 15. The curves in FIG. 12 are coded corresponding to the pressure drop decrease when opening one lumen shown in solid line, dashed line to two open lumens and dot-dashed line to three open lumens. It is important to note that any pressure drop δP value shown by the three curves is achievable. It is intended that the GDD 7 could provide any continuous resistance decrease from 100% to 0%. However for the selected diameters of the illustrated embodiment there are some gaps.

[0106] Opening the first lumen (13 or 15) enables the decrease of the resistance of the device by up to 40%; opening the two side lumens (13 and 15) enables the decrease of the resistance by between approximately 50 and 70%; opening the two side lumens and the middle lumen (13, 14 and 15) enables the decrease of the resistance by between approximately 99.5 and 99.7%. It is worth noting that opening one side lumen and the middle lumen will also decrease the resistance between approximately 99.5 and 99.7%.

[0107] The GDD 7 offers the possibility to alter the resistance and hence the pressure drop by simply lasering along each lumen 13, 14, 15. The GDD 7 may include silicon or plastics material such as polyurethane. The apertures may be formed by laser cutting. A YAG or Argon laser may be used, for example. Alternatively, the aperture may be formed by puncturing each lumen as necessary to achieve the desired flow rate.

[0108] The multi-lumen tube 10 may be made by extruding a multi-lumen preform through a die, stretching the preform in the longitudinal direction to reduce the lumen diameter, and cutting to a desired tube length. The extruded material may be a plastics material. Stretching the extruded preform may achieve small diameter lumen not achievable with a directly extruded product. Suitable plastics may include polycarbonate, phosphorycholine hydrogel, polyether block amide, polycarbonate based polyurethanes, aliphatic based polyurethanes and nylon, for example. A biocompatible material, or biocompatible coating, may be used.

[0109] Moulding the device from silicon material may be advantageous in that the generally planar extensions or ‘wings’ 19 can be co-moulded with the tube. Extruding the tube means that the extensions need to be attached later.

[0110] Lasering along the GDD 7 gives the ability to alter the flow resistance. Lumens are lasered from the top, therefore Poiseuille law does not fully apply along the apertures 20 (holes) created by lasering as they have approximately the same width and length and a correction need to be added. Flows through holes may be considered as the combination of two flows, the Sampson flow and the Poiseuille flow. For a typical hole of radius equal to 50 μm and a length equal to 30 μm, the Sampson component is twice the Poiseuille flow, therefore it is important to consider both parameters when looking at flows through holes. It is expected that the minimum sidewall 21 thickness may be around 25 μm or less.

[0111] FIGS. 13(a) to (f) shows the pressure drop for increasing hole diameter, i.e. 5 to 10 μm reported in FIG. 13(a), 10 to 20 μm in FIG. 13(b), 20 to 40 μm in FIG. 13(c), 40 to 60 μm in FIG. 13(d), 60 to 100 μm in FIG. 13(e) and 100 to 160 μm in FIG. 13(f). It can be seen that for an aperture 20 diameter above approximately 26 μm the pressure drop introduced by the aperture is less than half of 1 mmHg.

[0112] Whilst in the illustrated embodiment above the GDD 7 has an oval shape cross-section it is useful to consider other GDD cross-section shapes that could accommodate more than one lumen as shown in FIGS. 14(a)-(e). FIG. 14(a) shows a tube 110 with rectangular cross-section with four lumens. FIG. 14(b) shows a tube 210 with a section having variable thickness and four lumens located adjacent respective upper and lower troughs between respective upper and lower ridges. FIG. 14(c) shows a tube 310 having a circular cross section with five lumens arranged radially around only an upper portion of the tube so as to be substantially equidistant from the centre of the section. FIG. 14(d) shows a tube 410 having a crescent shaped cross-section with three lumens, two of which are circular of different diameters and one of which is elliptical. It will be appreciated that the invention may employ a variety of cross sections and a variety of lumens of different number, shape, size, etc. FIG. 14(e) shows a tube 510 having an aspect ratio (width to height) of at least about 7:1 and having three circular lumens arranged side-by-side, with a larger diameter lumen in the centre and smaller diameter lumens to either side. Other suitable lumen cross section shapes could be used as discussed above. The lumens of tube 510 are arranged asymmetrically within the tube so as to be closer to the surface that is uppermost when the device is in use. In the embodiment shown in FIG. 14(e) tube 510 has a total width of approximately 1.49 mm to approximately 1.57 mm and a maximum height of approximately 200 microns. The central lumen has a diameter equal to approximately 60% of the maximum height of the tube, while the two side lumens have diameters equal to approximately 40% to approximately 50% of the diameter of the central lumen. The distance from the centre of the central lumen to the centre of each of the side lumens is equal to approximately 6 to approximately 7 times the diameters of the side lumens. The distance from the centre of each of the side lumens to the outer edge of the tube is equal to approximately 6.5 to approximately 7.5 times the diameters of the side lumens.

[0113] FIG. 15 shows a plate 500 for use with a multi-lumen tube 501. The multi-lumen tube is near identical to the tube 10 of the GDD 7 but for the absence of the wings 19. In all other respects the tube 10 and variants of it as described above may be used with the plate 500. The tube 501 opens in the underside of the plate 500 which can be secured to the patient's eye to stabilise the tube 501. The plate 500 may be used instead of the wings 19 of the GDD 7.

[0114] Any glaucoma device exerts locally stretching and compressive forces on the surrounding tissue when implanted in an human eye. It is therefore extremely important to minimise as much as possible these forces to prolong the life of the device and avoid any excessive scarring. There should be extra care on the device shape to achieve these goals. When inserted, the tube may be divided into three different zones (Parts A, B and C) with different requirements to optimise the interaction of the tube with the surrounding tissue as shown in FIG. 16.

[0115] Part A represents the part of the tube exposed to the conjunctival tissue. Part B is the section of the tube inside the scleral tunnel to maintain a seal between the anterior chamber and the subconjunctival tissue. Finally, Part C corresponds to the part of the tube inside the anterior chamber that will periodically flap due to blinking, eye saccades and head movements.

[0116] Part A—Minimising Conjunctival Tissue Stress

[0117] When the tube is in-situ, the back of the tube exposed to the conjunctival tissue (after the wings) is being deflected downwards to hold it in place along the surface of the eye. An example of the tube deflection is shown in FIG. 17 where the back of the GDD (posterior to the wings) is deflected downward by approximately 1.6 mm.

[0118] This will result in a contact zone with the tissue under pressure that needs to be minimised to reduce the trauma of the conjunctiva tissue as much as possible: Firstly, the indentation depth into the conjunctival tissue needs to be minimised to reduce the trauma on the tissue. Secondly, the maximum contact pressure for a fixed depth of penetration needs also to be minimised to reduce the stress on the tissue.

[0119] Tube indentation depth in the conjunctival tissue

[0120] We used finite-element analysis to illustrate this point. We have modelled a 0.2 mm thick conjunctival flap as shown in FIG. 18.

[0121] Four indentation depths of respectively 1 mm depth by 1 mm wide, 0.5 mm depth by 0.5 mm wide, 0.25 mm depth by 0.25 mm wide and 0.125 mm depth by 0.125 mm wide were created locally. These indentation depths correspond to diameters of tubes resting on the sclera with the conjunctival tissue covering it. The length of the tube interacting with the conjunctival flap is 2.5 mm.

[0122] In FIG. 19, it can be seen that as the diameter of the tube decreases (a, c, e, g), the deflection of the conjunctival flap is reduced as expected. It can also be seen that the pyramidal shape of the tissue is greatly reduced as the tube external diameter is reduced. Indeed, this is reflected on the stress shown on the corresponding plots for 1 mm deflection in (b), 0.5 mm deflection in (d), 0.25 mm deflection in (f) and 0.125 mm deflection in (h). For example, reducing the tube diameter from 0.5 mm to 0.25 mm halves approximately the maximum Von Mises stress from 950 Pa to 415 Pa. It is therefore important to reduce the tube diameter to reduce the trauma on the conjunctiva. The first requirement of the tube is to have a small diameter to reduce the indentation into the conjunctival tissue.

[0123] Maximum contact pressure on the conjunctival tissue

[0124] The interaction between a tube and the subconjunctival tissue can be modelled as the interaction between a cylinder and a flat surface. Indeed, locally the curvature of the eyeball is between 1 to 2 orders of magnitude larger than the tube radius, the conjunctival tissue can be treated as a planar section. As shown in FIG. 20, the cylinder has a radius R and a length L, and is pushed with a force F towards the conjunctival tissue.

[0125] The contact surface has a width a defined by:

[00004]$\begin{array}{cc}a=\sqrt{\frac{8.Math..Math.\mathrm{FR}}{\pi .Math..Math.E^{*}}}& \left(1\right)\end{array}$

[0126] with E* defined as:

[00005]$\begin{array}{cc}{E}^{*}=\frac{1-v_1^2}{E_1}+\frac{1-v_2^2}{E_2}& \left(2\right)\end{array}$

[0127] with E.sub.1 and E.sub.2, the elastic Young's modulii of the tube and the conjunctival tissue and v.sub.1, v.sub.2, the associated Poisson's ratios. Therefore, it can be seen that with increasing tube radius, the width of the surface contact increases as expected as the square root of the tube radius. The maximum pressure of contact is obtained at the centre of the surface contact and is defined as:

[00006]$\begin{array}{cc}{p}_{\max }=\sqrt{\frac{{\mathrm{FE}}^{*}}{\pi .Math..Math.\mathrm{LR}}}& \left(3\right)\end{array}$

[0128] The maximum contact pressure is proportional to √{square root over (1/R)} and can be reduced by increasing the radius of the tube for a fixed indentation. Therefore, it is important to increase the radius of the tube to decrease the maximum contact pressure exerted by the tube on the conjunctiva. The only way to increase the contact area of the tube with the tissue while keeping a fixed indentation length is by adopting an elliptical shape to increase the width of the tube while keeping its height constant as shown in FIG. 21(a). In other words, it consists in increasing the eccentricity of the tube, e, which is defined as:

[00007]$\begin{array}{cc}e=\sqrt{1-\frac{H^2}{b^2}}& \left(4\right)\end{array}$

[0129] with H being the height of the tube (or twice the semiminor axe), and b, the width of the tube (or twice the semimajor axe) as shown in FIG. 21(b).

[0130] It is therefore important to increase the eccentricity of the tube to reduce the maximum contact pressure between the tube and the conjunctival tissue. The GDD described herein preferably has an eccentricity of approximately 0.98.

[0131] Part B: Minimising Scleral Incision Seal

[0132] Each tube in glaucoma surgery must be inserted through an incision that connects the anterior chamber to other part of the eye: the subconjunctival or suprachoroidal space. However, incision in the tissue is usually made using a knife and this gives a straight cut. Therefore, a circular tube placed in a horizontal cut will result in the tissue being stretched upwards, resulting in leaks around the outside of the tube circular tube as the incision tends to be mainly elliptical as shown in FIG. 22.

[0133] In first approximation, the incision can be modelled with an elliptical shape. We simulated a flow of 2 μl/min going through a 3 mm long tube with a lumen of 50 μm of increasing eccentricity inserted into an incision of 3 mm length (0.2 mm high and 0.5 mm wide) and recorded the pressure drop across the set-up. We chose 5 different shapes of tube with an height of 0.2 mm and width of 0.2 mm (e=0), 0.25 mm (e=0.6), 0.375 mm (e=0.85), 0.438 mm (e=0.89) and 0.492 mm (e=0.91) as shown in FIG. 23. All tubes have a lumen diameter of 0.05 mm.

[0134] Monitoring the pressure drop for all configurations gives us an indication of leakage. The pressure drop should be around 5-6 mmHg if the incision is sealed properly as the flow should only be going through the tube, any lower value means there is leakage. We found that for the first three configuration (b=0.2, 0.25 and 0.375 mm), the pressure drop through the set up was well below 1 mmHg, showing important leakage. However, for the fourth configuration (0.438 mm), the pressure drop was around 3 mmHg, which is approximately half of the correct value, denoting that flow is going through the tube as well as the space created by the incision. When the width of the tube was 0.492 mm (fifth configuration), the pressure drop was correct at around 5-6 mmHg, showing that the flow was going predominantly through the tube.

[0135] The pathlines of particles seeded in the anterior chamber are plotted to show: (i) where there is important leakage (FIG. 24); (ii) where the flow goes through the tube and the space created by the incision (FIG. 25); (iii) where the flow goes mainly through the tube (FIG. 26).

[0136] We conducted additional simulations for the configuration shown in FIG. 26, where we plugged the lumen and calculated the pressure drop required to push aqueous humour fluid (identical viscosity) at a flow rate at 2 μl/min through the gap in the incision between the tube and the tissue as shown in FIG. 27. The pressure obtained was in excess of 400 mmHg with the lumens of the tube closed. This demonstrates that the flow tends to go preferably through an open lumen which is the path of least resistance, hence there was minimal leakage on the outside of the tube for this eccentricity.

[0137] These simulations hold true for any incision dimension with the same outcome. Elliptical tubes inserted into incisions tend to reduce leakage due to being similar shape to the incision which has been made.

[0138] Part C: Minimising Tube Flapping in the Anterior Chamber

[0139] Flapping of an end of a tube in the anterior chamber of the eye can cause trauma to the corneal epithelium. The elliptical shape of the tube defined in the previous section will reduce the deflection of the tube in the anterior chamber for a fixed force. If we simplified the tube as being held in place at the wings at the limbus section, the potential displacement of the tube in the anterior part of the chamber can be modelled as a cantilever beam held in place at the wing section. In that case, the small deflection of cantilever beams A is calculated as:

[00008]$\begin{array}{cc}\Delta =\frac{L^3.Math.F}{3.Math.E_1.Math.I}& \left(5\right)\end{array}$

[0140] with F: the force exerted on the tube, E.sub.1: the Young's Modulus of the tube material, I: the second moment of area and L: the length of the tube being displaced. The parameter I characterises the section geometry of the tube in relation to a given axis, depending on the direction of the deflection. The higher the value of I, the lower the deflection of the tube for a given force. The tube in the anterior chamber can flap in any direction. We will concentrate on the main two directions and any other direction will be a combination of these two directions.

[0141] First, we look at the displacement of the tube normal to the surface of the cornea. In that direction, the second moment of area for a cylinder:

[00009]$I_{\mathrm{ex}}=\frac{\text{?}}{\text{?}}..Math.\text{?}.Math.\text{indicates text missing or illegible when filed}.Math.$

with D being the outer diameter. For an elliptical shape, the second moment of area is given by

[00010]$I_{\mathrm{ex}}=\frac{\text{?}}{\text{?}},.Math.\text{?}.Math.\text{indicates text missing or illegible when filed}.Math.$

with H, the height of the oval and b, the width of the tube as shown in FIG. 21(b). For a similar force, Young's Modulus and length, the ratio of the displacement (Equation 5) of the cylindrical tube versus the elliptical tube can be rewritten solely using the second moment of area I. We assume that the cylindrical and elliptical tube have the same height (D=H). In that case, the ratio of deflection of a circular tube over an elliptical tube for a tube of identical length, L, Young's Modulus, E, and force, F, can be rewritten entirely from Equation 5 as:

[00011]$\begin{array}{cc}\frac{\text{?}}{\text{?}}=\frac{b}{H}& \left(6\right)\\ \text{?}.Math.\text{indicates text missing or illegible when filed}.Math.& \end{array}$

[0142] The ratio on deflection only depends on two factors: the width of the elliptical tube over the height or diameter of the circular tube. For example, if the tube has a diameter D=0.2 mm with D=H and the elliptical tube has a width of 1 mm, we have I.sub.ex/I.sub.rx=5. It means that the GDD described herein reduces the deflection of the elliptical tube normal to the cornea by a factor of 5 compared to a circular tube (D=0.2 mm) according to equation (5) for a fixed force. Further characterisation of this ratio is shown in FIG. 28(a) and an illustration of the deflection from the GDD is shown in FIG. 28(b).

[0143] Secondly, lateral displacements of the tube should also be reduced to avoid any rubbing of the epithelium of the cornea by the tube. Indeed, a circular tube anchored at one point can move in any direction (normal and parallel to the cornea) with the same force. However, for an elliptical tube, this is not the case. We minimise any lateral displacement by increasing the second moment of area relative to parallel displacement. The force described in Equation (5) is still valid and we simply need to modify the second moment of area to reflect lateral displacement. For a circular tube, the second moment of area is unchanged and equal to

[00012]$\text{?}=\frac{\text{?}}{\text{?}}..Math.\text{?}.Math.\text{indicates text missing or illegible when filed}.Math.$

For the elliptical tube, it becomes

[00013]$\text{?}=\frac{\text{?}}{\text{?}}..Math.\text{?}.Math.\text{indicates text missing or illegible when filed}.Math.$

Therefore, if we assume D=H, we now have for lateral displacement:

[00014]$\begin{array}{cc}\frac{\text{?}}{\text{?}}=\frac{b^3}{H^3}& \left(7\right)\\ \text{?}.Math.\text{indicates text missing or illegible when filed}.Math.& \end{array}$

[0144] The ratio is plotted in FIG. 29(a) with an illustration of the deflection from the GDD shown in FIG. 29(b).

[0145] This ratio is proportional to b.sup.3/H.sup.3 and is equal to 125 when b=1 mm and H=0.2 mm. The GDD reduces the deflection of the elliptical tube parallel to the cornea by a factor of 125 compared to a circular tube (D=0.2 mm) according to equation (5) for a given force.

[0146] In conclusion, adopting an elliptical shape significantly reduces the flapping of the tube in any direction which can damage the corneal epithelium layer over time.

[0147] Part A-B-C: Minimising Tube Internal Stress

[0148] General Tube Bending

[0149] It is important to minimise the stress that the tube is under when implanted. Indeed, the tube is under constant stress as it is bent when implanted. This is a different requirement from minimising the surface contact pressure and concerns the internal stress that the tube undergoes when implanted. The shape is instrumental in reducing internal stress. In FIG. 30, it can be seen that for the same deflection force, the internal stress of the elliptical shape of an example of the GDD of eccentricity 0.98 (H=0.2 mm and b=1 mm) is reduced to a third of that of a circular tube of the same height (D=0.2 mm). This illustrates that an elliptical shape also reduces the internal stress of the tube and could therefore prolong the life of the device since it undergoes relatively lower stress.

[0150] Specific Tube Bending through Incision

[0151] When the tube exits from the incision into the subconjunctival space, the tube is under stress at the point of exit shown by an arrow in FIG. 31(a). It is important to reduce the internal stress of the tube as it may remain in that position for decades. Two Finite-Element Analysis have been conducted, with a circular tube of diameter 0.2 mm and an elliptical tube of height 0.2 mm and width 1 mm (eccentricity of 0.98 similar to the GDD), and an upward deflection of 1 mm as shown in FIG. 31(b) and (c), respectively.

[0152] The corresponding tube internal stresses of the deflections shown in FIG. 31 are plotted in FIG. 32. It can be seen that the maximum internal stress of the tube is reduced by approximately 25% from 3.75 MPa to around 2.8 MPa when a circular tube of 0.2 mm diameter is compared to an elliptical tube of height 0.2 mm and width 1 mm, similar to the GDD. Therefore, an elliptical shape reduces internal stress of the tube when it exits the incision into the subconjunctival shape.

[0153] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.