Ophthalmic flow converter

Details Ophthalmic flow converter Ophthalmic flow converter

2000-04-13

2003-07-29

Lo, Weilun (Department: 3761)

Surgery

Means for introducing or removing material from body for...

Treating material introduced into or removed from body...

C604S030000, C604S320000, C604S323000, C604S521000, C604S902000, C606S107000, C606S166000, C601S013000

Reexamination Certificate

active

06599271

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
This invention relates to eye surgery machines and in particular, for surgery machine finding application in ophthalmic operations. While the invention is described with particular emphasis in the use of the invention in cataract surgery, those skilled in the art will recognize the wider applicability of the inventive principles discussed hereinafter.
A cataract is the lens of the eye after it has become cloudy. Surgery to remove cataracts has been preformed for many years. Current practice is to remove the cataract by phacoemulsification, replacing the lens with an artificial one. Phacoemulsification utilizes a tube, usually referred to as an ultrasonic needle that is vibrated at ultrasonic frequencies to break up or emulsify the cataract. The needle is driven by a multi-function ultrasound handpiece. The handpiece conventionally includes an ultrasonic motor and fluid channels for aspiration and irrigation of the eye. Aspiration is used to evacuate the lens and other vitreous material from the eye. Irrigation replaces the volume removed by aspiration.
It is vitally important to maintain the intraocular pressure at an appropriate level during ophthalmic operations, by controlling the irrigation and aspiration capacity, thus preventing, in particular, the cornea from collapsing. Corneal collapse occurs when the system fails to supply sufficient inflow to the eye that compensates for the aspirated amount of liquid and/or tissue being removed, so that the device creates a vacuum within the eye itself.
In the past, a bag or bottle is normally used to store irrigation fluid. The fluid is connected to the ultrasound handpiece through flexible surgical tubing. The fluid is delivered to the handpiece under pressure, relative to the surrounding atmosphere. The pressure is developed either by gravity, i.e., hanging the bottle at an elevation greater than the handpiece, by pressurizing the bottle with air, or by using a pumping mechanism, for example. Ultrasound handpieces conventionally have a coaxial fluid flow design along the instrument tip. An irrigation sleeve surrounds the ultrasound needle. The sleeve is normally constructed from a pliable material, for example, silicone and rubber, which is intended to conform somewhat to the shape of the incision in the eye, thereby reducing wound leakage. A connection is made at the handpiece, between the irrigation sleeve and the tubing from the bottle. Fluid enters the eye through ports near the end of the sleeve.
Fluid and tissue are aspirated from the eye through a channel located inside the ultrasound needle. The channel through the ultrasound needle is in fluid communication with a connection on the back of the handpiece. Conventionally, flexible surgical tubing is connected to the handpiece at one end and to the surgery machine at the other end. Vacuum for aspiration is supplied to the tubing by the surgery machine. Surgery machines use various methods for controlling the vacuum level provided for aspiration. They also use various methods for storing the spent fluid and tissue.
During an eye operation, ideally pressure within the eye is maintained at a constant level regardless of fluid flow changes or wound leakage. In practice, pressure can vary a great deal, depending on fluid flow dynamics. One cause for pressure changes is the position of the eye in the fluid flow path. Conventionally, fluid flow restrictions exist between the fluid source, i.e., the irrigation bottle, and the eye. Fluid flow restrictions also exist between the eye and the vacuum or aspiration source. These restrictions cause a pressure drop, which is proportional to fluid flow, as fluid flows from the fluid source through the eye and to the vacuum source. As will be appreciated by those skilled in the art, as fluid flow increases, the pressure inside the eye decreases, it being assumed the fluid pressure at the bottle is a constant.
An additional cause for pressure change is the elastic nature of the tubing connecting the eye to the vacuum source. The diameter of the tubing changes, based on the pressure difference between the inside of the tubing and atmospheric pressure. That is to say, the tubing becomes smaller as vacuum increases. These diameter changes cause the tubing to store energy, damping pressure changes in the tubing. Often during surgery, the tip of the ultrasound needle becomes occluded. With occlusion, the pressure in the anterior chamber of the eye equalizes to the pressure of the fluid source and the pressure inside the ultrasonic needle approaches that of the vacuum source. When the occlusion breaks suddenly, the energy stored in the aspirating tubing causes a surge of fluid to flow, as the tubing returns to the size it was before the occlusion. The flow surge causes the pressure in the anterior chamber of the eye to decrease to a point lower than it would be under constant flow conditions. While this pressure change is momentary, it can be great enough to cause the pressure in the anterior chamber to be less than atmospheric pressure, causing the chamber to collapse.
In addition, any air in the aspiration tubing can exacerbate the pressure problem during surgery. Air bubbles can form from cavitation caused by the ultrasound needle, or from inadequate purging of the aspiration tubing during set up. Because of the compressible nature of air, the bubbles expand under vacuum, then contract as the vacuum is reduced.
Procedurally, fluid flow through the anterior chamber of the eye is used to manipulate tissue within the anterior chamber. As fluid moves, it tends to push items, for example lens material, membranes, and other undesirable debris, towards the tip of the ultrasound needle. To an observer, it appears as if the tip of the needle attracts material toward it. At very low flow rates, the attraction is small, and only things that are close to the tip move towards it. As the fluid flow rate increases, the apparent attraction is greater. At high flow rates, anything in the anterior chamber that is free to move is attracted to the tip of the ultrasound needle. The surgeon takes advantage of this tendency, because the tendency allows the ultrasound tip of the handpiece to remain near the middle of the anterior chamber most of the time, yet provides access to the tissues necessary to accomplish the surgery. The middle of the anterior chamber of the eye generally is considered a safer place to operate, reducing the likelihood of complications from the surgery.
In counter distinction to the attraction discussed above, as ultrasound energy, used to emulsify the lens, is increased, the needle tip tends to push the material intended for emulsification away from the needle tip. This push is greater with higher amounts of ultrasonic energy. However, a higher or greater amount of ultrasound energy is required to emulsify denser harder lenses than is required to emulsify softer lenses. In order to allow efficient transfer of energy to the lens, the lens must be kept against the needle tip. However, when the lens is positioned against the tip of the ultrasound needle, the lens tends to occlude the tip. The pressure difference develops across the lens, forming a holding force against the needle tip. The vacuum level at its source, the irrigation pressure, and the ultrasound needle inside diameter determines the maximum amount of force available to hold the lens. Aspiration assisted phacoemulsification occurs when the holding force generated by the fluid is much greater than that necessary to overcome the opposite push of the ultrasonic source. High vacuum causes stress in the lens to be higher, reducing or eliminating the need for higher ultrasound power to emulsify the lens. I am aware of medical studies that have shown a correlation between total ultrasound power used in phacoemulsification, and cornea endothelial cell loss. Because endothelial cells are not replaced by the body, the loss of the cells can be a serious surgical complication.
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