Eye surgery apparatus with vacuum surge suppressor

Collapse of the cornea into the cavity formed when material such as the lens is removed from an eye in the irrigation-aspiration method is minimized by incorporating a pressure responsive, variable flow resistance element in the aspiration line of the irrigation-aspiration apparatus. A preferred pressure responsive, variable resistance element is a length of thin walled tubing which is easily collapsed to smaller flow path area in response to increasing negative pressure in the aspiration line.

This invention relates to methods and means for accomplishing surgical 
removal of material from the eye with increased safety. 
BACKGROUND 
A variety of procedures are currently being employed to accomplish removal 
of cataracts and other material from within the eye. One of those 
procedures is often referred to as an irrigation-aspiration method. It 
involves breaking, tearing or otherwise dividing the material to be 
removed into small fragments and aspirating those fragments from the eye 
into a fluid flow line by which they are carried away. Fluid pressure 
levels in the apparatus by which that procedure is practiced are dictated 
by the need to preserve pressure balance within the eye. More 
particularly, in the case of cataract removal, it is important to prevent 
or minimize collapse of the cornea into the cavity formed by removal of 
lens material. Difficulty in preventing such collapse has tended to 
discourage some surgeons from the use of the irrigation-aspiration method 
notwithstanding that it's successful use can result in substantially less 
discomfort to patients in the post operative period. 
In the method, an incision is made on or about the corneal margin to gain 
access to the lens. A tool in the form of a probe which houses the outlet 
end of a fluid supply line and the inlet end of an aspiration line is 
inserted into the opening under the cornea. Some means is provided for 
dividing the lens material into small fragments. The means for dividing 
eye material may comprise no more than a cutting edge at or near the flow 
tube openings. More commonly an ultrasonically driven tool is included to 
aid in division of the material to be removed. The task is to separate 
that material and divide it into small pieces, aspirate that material from 
the eye and to replace the material so removed with water, actually a 
balanced, salt in water solution. The opening in which the probe is 
inserted is small, 6 millimeters or less, and the opening tends to close 
so that a substantially closed cavity is formed as lens material is 
removed. The water is disposed under the cornea and serves to prevent 
corneal collapse into the cavity. It is the water in that cavity that is 
used to aspirate and carry away lens material. Accordingly the flow of 
water to the lens cavity must equal the flow of water being removed by 
aspiration augmented by enough water to account for increase in lens 
cavity size. That flow ratio must be maintained while maintaining enough 
water in the lens cavity to prevent the cornea's collapse. The inner 
surface of the cornea is covered by a layer of irreplaceable endothelium 
cells. Collapse of the cornea would bring those cells into destructive 
contact with the removal tool. In the case of a sonically activated 
removal tool, collapse of the cornea into contact with the tool could 
result in puncture of the cornea. To accomplish the pressure balance to 
prevent those catastrophes requires precise control of supply water 
pressure and aspiration pressure. In practice, positive supply pressure is 
achieved by elevating the supply water container. Aspiration pressure is 
negative whereby the pressure at the lens cavity is close to atmospheric 
pressure. 
Two primary factors tend to upset the desired pressure level at the eye. 
Negative pressure in the aspiration circuit is developed by a pump in most 
practical systems. Peristatic pumps are usually used but whatever the pump 
form, small cyclic variations in negative pressure occur and tend to make 
the cornea oscillate over the cavity being formed by the lens. A greater 
and more troublesome pressure variation occurs when an occlusion or 
partial occlusion of the aspiration opening is overcome. Occlusion occurs 
when a piece of eye material too large to pass through the aspiration 
inlet is drawn to it. Pressure in the aspiration line is forced more 
negative by the aspiration pump until the blocking material is divided or 
drawn into the inlet and the occlusion is cleared. When that occurs, the 
negative aspiration pressure, now greater in absolute value than the 
supply pressure, evacuates the lens cavity and collapses the cornea. In 
practice the supply water includes entrained air and there may be bubbles 
in the supply. The mass of the water and the compliance of the air line, 
coupled with the reduced flow resistance as the occlusion is overcome, act 
as an under damped oscillatory system in which the cornea may be vibrated 
violently as a function of the size of the cavity. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method and an 
improved means by which to conduct eye surgery with greater safety; 
Still another object is to provide such improvements in a way that is 
inexpensive, is applicable to all current brands of irrigation-aspiration 
systems apparatus and requires less, not more, skill in performing such 
procedures as cataract removal. 
These and other objects and advantages of the invention are realized in 
part by the provision of a method and apparatus which convert what is a 
normally underdamped irrigation-aspiration system to an overdamped system 
when an occlusion is cleared. It can be described as the method of 
minimizing corneal collapse during material removal from an eye by the 
irrigation-aspiration procedure using irrigation-aspiration apparatus 
which method comprises the steps of: 
sensing an increase in negative pressure in the aspiration line of said 
apparatus; and 
increasing flow resistance in said aspiration line as an incident to having 
sensed said increase. 
Stated another way, the objects and advantages of the invention stem, in 
part, from the provision of an article of manufacture for inclusion in the 
aspiration line of an irrigation-aspiration apparatus for conducting eye 
surgery by the irrigation-aspiration method the article comprising a flow 
tube the walls of which have sufficient renitence to maintain the flow 
path open at negative internal pressure values corresponding to the 
positive internal pressure values of the irrigation line of said 
apparatus, said walls being responsive to more negative internal pressures 
to deform to reduce the cross-sectional area of said flow path. 
A tool is provided in the preferred form which comprises dividing means for 
dividing material found in the interior of an eye and two flow paths each 
having openings proximate to one another and to said dividing means. The 
tool further comprises a variable flow resisting element responsive to 
increasing negative pressure to increase it's resistance to flow connected 
in series with one of said flow paths.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The apparatus of FIG. 1 includes a source of irrigation fluid, a balanced 
salt in water solution, referred to here as water. It includes a tool for 
removing material from the inner eye, a supply line arranged to conduct 
water under positive pressure from the source to the tool, a means for 
developing a negative pressure and an aspiration line arranged to utilize 
the negative pressure to conduct water and removed material from the tool. 
The elements thus far described form an irrigation-aspiration system of 
conventional form. In this embodiment the source is the water container 
10. The supply line is numbered 12. The tool is numbered 14 and the 
aspiration line is numbered 16. The supply line ends at 18 and the 
aspiration line begins at 20 in the tool 14. 
The tool is mounted on a handle which is not shown here both because the 
handle per se does not form part of the invention and for the sake of 
clarity. The tool performs several functions. It must divide and sever the 
lens or other material to be removed. To that end it is formed with 
cutting edges at it's forward extremity at 22. Some tools incorporate 
ultrasonically activated elements to facilitate material division. The 
lens or other material is to be removed by aspiration into and along the 
aspiration line. The vehicle for aspiration is water which is supplied 
from the source 10 by line 12. The water is supplied at positive pressure 
by elevating the water container 10. In this case it is mounted on a pole 
30. In a typical case it is mounted about 30 inches above the working 
level of the tool. The operating surgeon may control the supply pressure 
by having the bottle elevated or lowered in some degree. Negative pressure 
for aspiration is developed by any convenient means and in the case of 
most systems is developed by connecting the aspiration line to a pump. 
Peristaltic pumps are commonly used and it is such a pump that is depicted 
at 32 in FIG. 1. 
The cross-sectional area of each of the supply and aspiration line ends is 
necessarily small because those ends must be placed in close proximity to 
the cutting edges and they and the cutting edges are inserted through an 
edge opening in the cornea into the lens to be viewed with a microscope 
during the removal phase of the procedure. That is depicted schematically 
in FIG. 2 wherein the end of the tool is numbered 24. It extends under the 
cornea 26 into the lens 28. 
The conventional irrigation-aspiration apparatus includes a normally open, 
solenoid actuated clamp valve which may be mounted on the pump unit. The 
supply line is placed in the clamp structure such that it is pinched 
closed when the solenoid is closed by operation of a foot switch. Another 
valve, responsive to operation of the foot switch and located in an air 
inlet line, opens the aspiration line to atmosphere. They are also the 
only protection in the conventional system against the consequences of a 
step function increase in negative pressure in the cavity from which 
material is being removed. The primary occasion for such an increase is 
the sudden relief of a blockage of the aspiration tube inlet. If a corneal 
collapse is observed, the surgeon can pinch off the water supply, open the 
aspiration line to relieve the negative pressure, and stop the pump. 
However, those expedients do not solve the problem for at least two 
reasons. In practice the only means available to the surgeon for clearing 
a blockage are mechanical manipulation of the tool and the suction force 
at the aspiration line end. To use the suction force requires closing the 
air inlet line and operating the pump. Pressure in the eye cavity will 
drop immediately when the occlusion is cleared and pinching off the line 
and stopping the pump will ordinarily not prevent either the drop or the 
resulting collapse of the cornea. The other reason is that the only means 
the surgeon has to known that negative pressure must be reduced is to 
observe a corneal collapse which is akin to closing the barn door after 
the horse is gone. The supply line valve is numbered 70. The air inlet 
passage to the aspiration line is numbered 72 and it's air inlet valve is 
numbered 74. 
The pressure variations attending an occlusion in the conventional system 
and the occlusion's relief are described graphically in FIG. 4 where y 
distance represents the magnitude of pressure and the x distance 
represents, in turn, a period prior to an occlusion of the aspiration line 
input, the time of an occlusion, a period over which it persists followed 
by the time of sudden relief of the occlusion and, finally, a period 
following relief. The supply and aspiration line ends have small flow path 
areas which present relative high resistance to flow. That factor serves 
to limit the rate of pressure change in the cavity formed by removal of 
lens material. Typical supply and aspiration lines have an inside diameter 
up to about 2 millimeters. However, the usual pump is capable of 
developing negative pressure up to about 500 millimeters of mercury to 
insure that the removed material will be aspirated through the small 
aspiration inlet. The upper dashed line in FIGS. 4 and 5 represents the 
pressure at which water arrives for discharge into the cavity. It's 
magnitude is determined by the height of the supply container. Accordingly 
it's magnitude is substantially constant. The lower dashed line of the 
graphs represents the negative pressure at the aspirating line inlet 
downstream from the occlusion and, in the case of FIG. 5, upstream from 
the vacuum controlled resistor. The dotted line in FIG. 5 represents 
pressure in the aspiration line downstream from the vacuum controlled 
resistor. If adjusted properly, the negative pressure at the aspiration 
line inlet balances the positive supply pressure so that cavity pressure, 
the solid line, corresponds to ambient atmospheric pressure in the absence 
of an occlusion. When occlusion occurs, flow resistance at the aspiration 
inlet approaches infinity. The pump pulls down the pressure in the 
aspiration line to a value substantially below normal operating value. The 
cavity pressure increases somewhat but the increase is limited because 
water leaks from the cavity via the incision. 
When the occlusion is cleared, the sum of the positive supply pressure and 
the aspirating line pressure swings sharply negative and water is sucked 
from the cavity. The net negative pressure is overcome by initial inrush 
of water into the cavity line input so the cavity pressure rises rapidly 
to become less negative. The effect is to produce a decremental 
oscillation of cavity pressure. Oscillation magnitude, frequency and decay 
rate depend upon the mass and compliance of the water and the system 
elements and thus, in part, on cavity volume. The pressure excursions in 
the cavity effect collapse and flutter of the cornea. 
That collapse and oscillation is largely overcome in the invention by the 
inclusion of a vacuum responsive, variable flow resistance element in the 
aspiration line. Such an element, a vacuum controlled resistor, is 
included in FIG. 1 where it is identified by the numeral 40. It forms part 
of the tool 14 in the preferred form. The effectiveness of the element is 
greatest when it is close to the aspiration line inlet. 
Element 40 may have a variety of forms. Flow resistance in a flow line 
varies with cross-sectional area, flow path shape, obstruction 
configuration, length of restrictions and some other factors. It is 
currently preferred to employ a flow section in the aspiration line which 
will deform in response to increase in negative pressure to reduce the 
cross-sectional area of the flow path and which tends to reform to 
increase flow area when aspirating line pressure becomes less negative. 
The currently preferred element having that quality is depicted in FIG. 3 
and is the element 40 of FIG. 1. It is shown in cross-section taken on a 
plane which contains its longitudinal axis. It is an elongated cylindrical 
tube having reduced outer wall diameter and it has reduced wall thickness, 
except at its ends. In relaxed condition the renitence of the tube forces 
it to substantially cylindrical shape. The cross-sectional area is 
approximately equal to the cross-sectional flow area, of the remainder of 
the aspiration line other than at the tool. In preferred form the section 
of reduced wall thickness is at least one quarter of an inch long and more 
preferably is about one inch long. The wall thickness in the reduced area 
is from 0.020 inches to 0.001 inches. It has a cross sectional flow area 
in relaxed condition between 0.0005 to 0.05 square inches and is formed of 
an inert elastomeric material, preferably a silicon rubber. It should 
collapse nearly completely at some negative pressure such that the 
pressure upstream from the vacuum controlled resistor does not exceed a 
value during an occlusion that will draw down the pressure in the eye 
cavity to about 50 millimeters of mercury below atmospheric pressure after 
the occlusion is cleared. The cornea may collapse if the negative pressure 
exceeds the value. Selection of the response characteristics is based on 
balancing the need for sufficient negative pressure at the cavity to 
overcome occlusions against the advantage of limiting the negative 
pressure surge on clearance by increasing aspiration line flow resistance. 
It is possible to find a workable design by conventional computation 
methods but only a minimum of experimentation is needed to find an 
entirely suitable wall thickness and reduced section length for any 
commercially available elastomeric tubing. If the vacuum controlled 
resistor is allowed to close the aspiration line completely, suction at 
the aspiration line inlet will not increase further. If the occlusion has 
not been cleared, suction is no longer available to clear it. The line 
must remain open in some degree to achieve clearance. As long as the line 
remains open in some degree, suction force is available by Pascal's Law to 
help clear the occlusion. The ideal element is one that collapses readily 
under negative pressure greater in absolute value than the positive supply 
pressure but which does not close the flow path completely unless the 
blockage is so severe that unduly high negative pressure would be 
developed upstream from the element. That complete closure feature need 
not be incorporated in the design of the element but it is included in the 
preferred embodiment as a gross safety measure. The pump controller can be 
arranged to cease pumping if negative pressure becomes excessive. Complete 
closure of the resistance element is then useful only in the event of 
failure of the pump controller to limit suction. 
FIG. 5 illustrates that effect of the negative pressure responsive, 
variable resistance of the invention is to convert the system that is 
normally underdamped at the eye cavity to an overdamped system immediately 
on release of an occlusion. Again the pressure of the irrigation fluid, 
the supply water, is substantially constant before, during and after 
overcoming an occlusion with aspiration pressure. Following an occlusion, 
pressure in the cavity increases but the increase is slight because the 
water escapes at the incision from the space under the cornea. When an 
occlusion occurs, the pump "draws down" the aspiration line. The degree in 
which the pressure downstream from the cavity is permitted to go negative 
is determined by the negative pressure at which the variable resistance 
device, element 40, shuts off communication in the line. Comparison of 
FIG. 5 with FIG. 4 shows that the variable resistance device 40 has 
limited the negative pressure of the aspiration line upstream from the 
device to a lesser value in FIG. 5. When the occlusion is cleared in FIG. 
5, the pressure of water in the eye cavity and the renitence of the 
variable resistance element tend to force the flow path of the variable 
resistance device to increased area and lower resistance. However, 
negative pressure downstream toward the pump tends to draw the device's 
walls to smaller area. Consequently, the device presents a considerably 
higher flow resistance at the beginning of the transition period which 
slowly decreases to a smaller value as the fluid flows through it. The net 
result is reduced rate of pressure change in the eye cavity and a much 
smaller and non-oscillatory negative pressure excursion and that 
translates into minimal corneal collapse. 
The scale lines and the dashed cavity pressure lines in FIGS. 4 and 5 
correspond substantially to oscilloscope scale lines and traces recorded 
during actual test of a system with the vacuum controlled resistor in the 
case of FIG. 5 and without it in the case of FIG. 4. The bottle was 
mounted 70 centimeters above the tool. The supply and aspiration lines had 
an inside diameter of 2 millimeters. The vacuum controlled resistor had a 
relaxed inside diameter of 0.06 inches. It had a wall thickness reduced to 
0.005 inches over a length of one inch and was made of medium hardness, 
silicone rubber flow tube. It was located in series in the aspiration line 
eight inches from to aspiration line inlet. In FIGS. 4 and 5 the interval 
between adjacent vertical scale lines represents one volt or 40 
millimeters of mercury. The interval between horizontal scale lines is 0.2 
seconds. In both Figures the occlusion was cleared at 0.4 seconds. In FIG. 
4 the eye cavity pressure dropped to about negative 90 millimeters of 
mercury. In FIG. 5, inclusion of the vacuum controlled resistor limited 
the drop in the eye cavity to less than 20 millimeters of Mercury. In FIG. 
4, the undulations in the curve after 1 second result from pressure 
variation produced in the aspiration line by the peristaltic pump. 
Inclusion of the vacuum controlled resistor smoothes out those variations 
as shown by the solid line in FIG. 5. 
In the experiment depicted in FIG. 5, the negative pressure in the 
aspiration line upstream from the vacuum controlled resistor reached about 
250 millimeters of mercury. Increasing the wall thickness of the resistor 
to 0.006 inches resulted in an reduction of the peak pressure to negative 
300 millimeters and an increase to a wall thickness to 0.007 inches 
resulted in a peak pressure reduction to about 350 millimeters of mercury. 
The ratio of change in peak negative pressure to wall thickness change can 
be reduced substantially, and the wall thickness dimension made less 
critical, by connecting two or more vacuum control resistors in parallel 
as shown in FIG. 6. In some cases the parallel arrangement may be 
preferred. 
Another variation is shown in FIGS. 7 and 8. Here the exterior of the 
vacuum controlled resistor 300 is subjected to the pressure in a chamber 
302 which is included in series in the water supply line 304. By this 
arrangement, the effect of the vacuum controlled resistor is made more 
uniform despite variation in the height of the supply water container. The 
degree in which flow resistance is increased is a joint function of the 
negative pressure in the aspiration line and the positive pressure in the 
supply line. 
The particular embodiments described herein and which are shown in the 
drawings represent what are considered to be the best embodiments and 
modes of practicing the invention. However, it is to be understood that 
other embodiments and modes of practicing the invention are possible and 
the scope of the invention is not to be considered to be limited to what 
is shown and specifically described is to be limited instead by the scope 
of the appended claims. In this connection the term "vacuum controlled 
resistor" is intended to be a generic description rather than a designator 
of only the particular form shown and described.