5. The knife-needle of Knapp (Fig. [34]), which is so generally used for capsulotomy, is unfortunately not well adapted for iridotomy. The point is too oval, the cutting edge is too much bellied, and the blade is too short (5 mm.). It will not easily puncture a dense iris-membrane, and the long sawing incision can not be well executed, because the short blade either persists in slipping out of the iris incision or else allows the membrane to ride up on the shank, in either case interfering with the completion of the operation.
6. Sichel’s iridotome (Fig. [35]) closely resembles Knapp’s knife-needle, and although specially designed for this purpose, has the same faults, an oval point and a bellied edge. On the other hand, the blade is too long (11 mm.) to be easily manipulated in the anterior chamber.
7. The Hays knife-needle (Fig. [36]), as suggested in the early part of this paper, has the same general shape as Cheselden’s instrument, although much smaller. It was devised by Dr. Isaac Hays, an early surgeon of the Wills Hospital, and, although not well known to the profession at large, has been in constant use by the staff of that hospital for more than half a century. I may be pardoned for briefly quoting the original description of the instrument as published by Hays[31] in 1855:
“This instrument from the point to the head, near the handle (a to b, Fig. [36]), is six-tenths of an inch, its cutting edge (a to c) is nearly four-tenths of an inch. The back is straight to near the point, where it is truncated so as to make the point stronger, but at the same time leaving it very acute, and the edge of this truncated portion of the back is made to cut. The remainder of the back is simply rounded off. The cutting edge is perfectly straight and is made to cut up to the part where the instrument becomes round, c. This portion requires to be carefully constructed, so that as the instrument enters the eye it shall fill up the incision, and thus prevent the escape of the aqueous humor.”
Fig. 4.—Cheselden’s knife-needle (after Sharpe).
Fig. 37.—Ziegler’s model of knife-needle.
Fig. 36.—Hays’ knife-needle, exact size and enlarged (Hays).