JACKSON: Corneal Section for Cataract Extraction.
145
THE CORNEAL SECTION FOR CATARACT EXTRACTION AND THE CATARACT KNIFE. By EDWARD JACKSON, M.D., DENVER, COLO.
The cataract knife is an instrument designed to accomplish a mechanical effect, under such definite conditions that it is capable of very accurate adaptation to its purpose. The size of the cornea, the size of the incision to be made for the escape of the lens, and the distance that the point of the knife can be thrust forward without sticking the side of the nose, are so constant that the dimensions of the knife should be made to conform accurately to them. The corneal section for cataract extraction, whatever its preferred size, shape, or position, should be as smooth as possible, and should be accomplished with the least possible mechanical disturbance of the eye. This smoothness is secured by making the section, so far as possible, by a single forward thrust of the knife. The mechanical disturbance of the eyeball is to be avoided by making the exertion of force in cutting the cornea as uniform as possible, and reducing to the minimum the turning effect, that has to be opposed by fixation of the globe. To understand the problem of keeping uniform the force exerted, we must consider the relation of different parts of the corneal section to the edge of the cataract knife. Let us take first the case of a corneal section made with a narrow Graefe knife, which has for the greater part its edge and back parallel. While such a knife is makin'g the puncture, while the tapering point is entering the cornea, a certain amount of force is required. But when the full width of the knife has entered, the resistance to its forward thrust ceases; only to begin again with the counterpuncture. At the completion of the. counter-puncture it will OPH.- 10
i46 JACKSON Corneal Section for Cataract Extraction. again cease, unless the operator promptly begins that upward (or downward) pressure, designed to divide the remaining bridge of corneal tissue. This pressure, or force, tending to rotate the eyeball upward (or downward),- must then increase until the edge of the knife passes from the anterior chamber entirely into corneal tissue, after which time it rapidly diminishes with the length of the bridge of tissue the knife is dividing. The greater extent of tissue presented to the edge of the knife as it leaves the anterior chamber is a point of much practical importance. By using either the puncture or the counter-puncture as a sort of fulcrum, and cutting chiefly with the other part of the knife, these variations of resistance can be reduced. Some operators seek to overcome the difficulty by giving a peculiar sweep of the knife. But it cannot be wholly avoided, when using such a knife, without at least two distinct changes in the direction of the motion. Many operators have come to prefer wider knives than Graefe suggested, and these, although not perfectly proportioned, possess some of the advantages of the knife to be presently described. The fact is that Graefe's plan, of making the cataract knife transfix the eyeball and then cut out, lost the chief reason for its existence when we began to operate under local anaesthesia, and so gained the full co-operation of the patient. Beer's knife, and its various modifications, designed to complete the section by a single forward thrust, gave a smooth incision without angles, and with no intermission in the resistance. But this resistance continued to increase until the edge of the knife had passed out of the anterior chamber. Then the decrease was sudden. The knife I showed before this Society some years ago (see Transactions, i888, p. 62) was open to the same criticism. But it prevented the escape of aqueous until the practical completion *of the incision, and aided in fixation by the counter pressure of the back. So I continued to use it until three years ago; when the form to which I now call attention was adopted, because to the above advantages it added the securing of uniformity of the resistance to the simple forward thrust.
JACKSON: Corneal Section for Cataract Extraction. T47 The essential characteristics of the knife now presented are: The blade has a maximum width barely sufficient to complete the desired corneal section. The maximum width is reached 20 mm. from the point. At io mm. from the point fully twothirds of this maximum width is reached. From the point to the io mm. line the edge of the knife is straight. From the I0 mm. line it begins to curve, so that at the 20 mm. line the edge is parallel to the back. The back is straight throughout. The blade is flat and is as thin as is compatible with sufficient rigidity and strength. Its outline is shown in Fig. I, in which the dotted lines indicate distances of I0 mm. and 20 mm. from the point.
.U
3m.
m.
The particular width of the knife must be determined by the depth of the corneal section it is desired to make. The dimensions given in the figure are for nearly the maximum corneal flap of 4.5 to 5 mm. For a 3 mm. flap the maximum width of blade should be 3 mm. or a little less, and the width at io mm. from the point should be 2 mm. By holding such a knife with its widest part in front of the cornea with its cutting edge opposite the corneal margin, where the center of the incision is to come, the back will indicate quite accurately the points for the puncture and counter-puncture. The knife is to be entered at the point for the puncture with its plane in the plane of the proposed section, and its straight back pointing toward the point for the counter-puncture, and
148 JACKSON: Corneal Section for Cataract Extraction. then pushed steadily forward. In Fig. 2 the solid outline represents the position of the knife at the completion of the counterpuncture. The boundaries of the corneal section are shown by
the heavy lines, the section of the external surface by the solid line, and of the internal surface by the broken line. The back of the knife, by its counter-pressure, so balances the pressure of the cutting edge that there results only a slight ten-
dency of the eyeball to rotate. * This tendency is to turn in the direction of the forward thrust. It is best met by placing the fixation forceps just outside the limbus, close below the counterpuncture. The forward thrust of the knife is continued until the section is completed, or until the widest part of the blade has fairly entered the cornea. If the section made is a smaller one than the knife is capable of cutting, it will be finished before the Widest part of the knife has quite entered the cornea. If it be deeper than the maximum width of the knife, the end of the forward thrust, indicated by the broken line in Fig. 2, leaves a thin bridge of tissue undivided. This is best divided as the knife is withdrawn, by rocking the knife about the point of puncture so as to complete the section with the part of the cutting edge near the point.
145
THE CORNEAL SECTION FOR CATARACT EXTRACTION AND THE CATARACT KNIFE. By EDWARD JACKSON, M.D., DENVER, COLO.
The cataract knife is an instrument designed to accomplish a mechanical effect, under such definite conditions that it is capable of very accurate adaptation to its purpose. The size of the cornea, the size of the incision to be made for the escape of the lens, and the distance that the point of the knife can be thrust forward without sticking the side of the nose, are so constant that the dimensions of the knife should be made to conform accurately to them. The corneal section for cataract extraction, whatever its preferred size, shape, or position, should be as smooth as possible, and should be accomplished with the least possible mechanical disturbance of the eye. This smoothness is secured by making the section, so far as possible, by a single forward thrust of the knife. The mechanical disturbance of the eyeball is to be avoided by making the exertion of force in cutting the cornea as uniform as possible, and reducing to the minimum the turning effect, that has to be opposed by fixation of the globe. To understand the problem of keeping uniform the force exerted, we must consider the relation of different parts of the corneal section to the edge of the cataract knife. Let us take first the case of a corneal section made with a narrow Graefe knife, which has for the greater part its edge and back parallel. While such a knife is makin'g the puncture, while the tapering point is entering the cornea, a certain amount of force is required. But when the full width of the knife has entered, the resistance to its forward thrust ceases; only to begin again with the counterpuncture. At the completion of the. counter-puncture it will OPH.- 10
i46 JACKSON Corneal Section for Cataract Extraction. again cease, unless the operator promptly begins that upward (or downward) pressure, designed to divide the remaining bridge of corneal tissue. This pressure, or force, tending to rotate the eyeball upward (or downward),- must then increase until the edge of the knife passes from the anterior chamber entirely into corneal tissue, after which time it rapidly diminishes with the length of the bridge of tissue the knife is dividing. The greater extent of tissue presented to the edge of the knife as it leaves the anterior chamber is a point of much practical importance. By using either the puncture or the counter-puncture as a sort of fulcrum, and cutting chiefly with the other part of the knife, these variations of resistance can be reduced. Some operators seek to overcome the difficulty by giving a peculiar sweep of the knife. But it cannot be wholly avoided, when using such a knife, without at least two distinct changes in the direction of the motion. Many operators have come to prefer wider knives than Graefe suggested, and these, although not perfectly proportioned, possess some of the advantages of the knife to be presently described. The fact is that Graefe's plan, of making the cataract knife transfix the eyeball and then cut out, lost the chief reason for its existence when we began to operate under local anaesthesia, and so gained the full co-operation of the patient. Beer's knife, and its various modifications, designed to complete the section by a single forward thrust, gave a smooth incision without angles, and with no intermission in the resistance. But this resistance continued to increase until the edge of the knife had passed out of the anterior chamber. Then the decrease was sudden. The knife I showed before this Society some years ago (see Transactions, i888, p. 62) was open to the same criticism. But it prevented the escape of aqueous until the practical completion *of the incision, and aided in fixation by the counter pressure of the back. So I continued to use it until three years ago; when the form to which I now call attention was adopted, because to the above advantages it added the securing of uniformity of the resistance to the simple forward thrust.
JACKSON: Corneal Section for Cataract Extraction. T47 The essential characteristics of the knife now presented are: The blade has a maximum width barely sufficient to complete the desired corneal section. The maximum width is reached 20 mm. from the point. At io mm. from the point fully twothirds of this maximum width is reached. From the point to the io mm. line the edge of the knife is straight. From the I0 mm. line it begins to curve, so that at the 20 mm. line the edge is parallel to the back. The back is straight throughout. The blade is flat and is as thin as is compatible with sufficient rigidity and strength. Its outline is shown in Fig. I, in which the dotted lines indicate distances of I0 mm. and 20 mm. from the point.
.U
3m.
m.
The particular width of the knife must be determined by the depth of the corneal section it is desired to make. The dimensions given in the figure are for nearly the maximum corneal flap of 4.5 to 5 mm. For a 3 mm. flap the maximum width of blade should be 3 mm. or a little less, and the width at io mm. from the point should be 2 mm. By holding such a knife with its widest part in front of the cornea with its cutting edge opposite the corneal margin, where the center of the incision is to come, the back will indicate quite accurately the points for the puncture and counter-puncture. The knife is to be entered at the point for the puncture with its plane in the plane of the proposed section, and its straight back pointing toward the point for the counter-puncture, and
148 JACKSON: Corneal Section for Cataract Extraction. then pushed steadily forward. In Fig. 2 the solid outline represents the position of the knife at the completion of the counterpuncture. The boundaries of the corneal section are shown by
the heavy lines, the section of the external surface by the solid line, and of the internal surface by the broken line. The back of the knife, by its counter-pressure, so balances the pressure of the cutting edge that there results only a slight ten-
dency of the eyeball to rotate. * This tendency is to turn in the direction of the forward thrust. It is best met by placing the fixation forceps just outside the limbus, close below the counterpuncture. The forward thrust of the knife is continued until the section is completed, or until the widest part of the blade has fairly entered the cornea. If the section made is a smaller one than the knife is capable of cutting, it will be finished before the Widest part of the knife has quite entered the cornea. If it be deeper than the maximum width of the knife, the end of the forward thrust, indicated by the broken line in Fig. 2, leaves a thin bridge of tissue undivided. This is best divided as the knife is withdrawn, by rocking the knife about the point of puncture so as to complete the section with the part of the cutting edge near the point.
