This invention pertains in general to microsurgery of tissue, and more specifically to procedures and devices for creating precise openings in tissue of a desired diameter and shape. For example, the procedures and devices can be used in ophthalmic surgery of the anterior lens capsule membrane of an eye.
Lens cataract is the leading cause of blindness worldwide and surgical treatment by cataract removal is the treatment of choice. A cataract is a clouding that develops in the lens of the eye or in its envelope. The creation of areas of opacity in the lens obstructs the passage of light. The lens of the eye is supposed to be transparent. If the lens develops opaque areas, as in a cataract, the lens must be surgically removed. If no lens is present in the eye, heavy corrective glasses are required to focus an image on the retina. The lens, however, can be replaced with an artificial interocular lens (IOL) to provide better vision after cataract removal.
The removal of the lens for replacement with an IOL is a surgical procedure that requires substantial precision. The lens is completely enclosed by a membrane called the lens capsule, so the surgeon must first cut through the capsule to access the lens. It is important to cut the capsule in just the right way. If the lens capsule has been cut correctly, and not damaged during the cataract removal, then it can be used to hold an IOL. The implantation of an IOL requires the creation of an opening in the lens capsule that is precisely centered, sized, and shaped for implant stability and for optimal IOL function. The matching of the lens capsule opening size to the peripheral margins of the IOL is critical. The goal of the surgeon is to create a perfectly circular (e.g., 5.5+/−0.1 mm diameter) hole in the capsule, centered exactly on the optical axis of the eye, with no tears or defects in the edge of the hole. Tears or defects on the edge of the hole make the capsule very weak and vulnerable to losing the ability to hold the IOL properly. Different IOL designs may require a different diameter for the hole (e.g., ranging from 4.5+/−0.1 mm to 5.75+/−0.1 mm), but whatever the prescribed diameter is, the accuracy of the surgeon in actually achieving it is very important for proper outcome of the cataract surgery.
Creating an opening in the lens capsule with this required level of precision is a difficult task for a surgeon controlling and guiding conventional hand-held cutting instruments and attempting to trace a precise circular route on the lens capsule. The present state of the art for performing a capsulotomy (the creation of an opening in the lens capsule) is for the surgeon to manually create a small tear in the anterior region of the lens capsule. With great caution, the surgeon then uses a small needle-like cystotome and/or tweezers to try to extend the edge of the tear so as to follow a circular path of the specified diameter and centered on the optic axis of the eye. In practice it often happens that the hole does not end up circular, or the correct diameter, or centered on the optic axis. There can also be radial tears in the edge of the hole that greatly weaken the capsule. As a result of any of these errors, the capsule may not be able to hold the IOL properly.
A number of devices have attempted to address the capsulotomy problem, but these devices still raise a number of challenging problems. Electrocautery devices have been used in the past to try to burn the lens capsule tissue and/or weaken it enough so that it is possible to then go in with hand-held tweezers and more easily tear out the circular patch of membrane. However, these devices often require massive heating elements to heat up the tissue, and so are rather bulky devices for use in performing delicate capsulotomy procedures on small tissue structures. Further, applying heat to a patient's eye to burn tissue is generally a risky procedure. The heat is often applied for a long time, lengthening the procedure and putting the patient at risk. With these electrocautery instruments, it is necessary to put a great deal of energy into the eye, thereby risking damage to tissue near to the capsule. In addition, the electrocautery devices do not complete the capsulotomy, but instead inconveniently leave the partially burnt or weakened capsule behind, thus requiring yet another step and another tool (e.g., tweezers) to fish out the capsule piece for removal. This adds further time to the procedure and puts the patient at risk by requiring more than one tool to be placed in proximity to the patient's eye.
Mechanical knife devices have also been used for performing capsulotomies. These devices are used to try to cut the capsule membrane with a small knife, applying the same cutting mechanism as would be used with a large handheld knife. The problem with cutting tissue on the microscale level with a knife is that the volume of tissue is so small, it has microscopic stiffness. Therefore, the tissue must be stretched relatively far to build up enough stress to provide the force against the cutting edge of the knife (no matter how sharp) for cutting to occur. The scale of stretching is up to a millimeter, and this distortion is greater than the desired precision (e.g., less than 0.1 mm), so it is not a satisfactory mechanism. Also, in practice, several passes with the knife may have to be made over the same cutting location to actually cut all the way through the membrane. Further, precise microcuts are often not easily reproducible with these microknives.
Given the drawbacks of existing treatment devices/procedures for lens capsule surgery, improved techniques and devices for performing microsurgery are needed.
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