Penetrating Keratoplasty or PK can be one of the most challenging procedures for the general ophthalmologist. Like all other surgical procedures it consists of a series of steps that are unforgiving and must be completed relatively perfectly to obtain an acceptable result. Failure at any one of the steps can result in high astigmatism, spherical error and even graft failure. Completed correctly, the result can look like the example above, a clear graft with barely discernible margins, astigmatism less than one diopter and an uncorrected vision of 20/25. I am going to present a series of posts that describes how we get to this result.
Today's lesson is on suturing the PK. Suturing is difficult for most ophthalmologists because it is done so infrequently in other procedures. Obviously it is the cornerstone of PK. A number of decisions need to be made prior to attempting the suturing. We must decide on the type of needle, the suture material, the desired suture pattern, the length of the bites and when to remove the sutures postoperatively. Each of these decisions impinges on the others and must be made properly.
First let us examine the different types of needle points. In the figure above we can see four different types of points. To the left is the original round pointed needle. This is rarely used in ophthalmology do to the fact that it does not cut through the tissue but instead relies on tearing and requires a great deal of force if it works at all. The result is distortion of tissue and it is completely unusable on an “open” eye. This is the needle used in early cataract surgeries, in which pre-placed sutures were performed prior to entering the eye. The needle to the far right is a flat cutting tip, a spatula needle. It is meant to find a level in the tissue and cut along a tissue plane. I use this needle with a 6-0 silk to place the scleral ring prior to trephining. The advantage is that it does not tend to perforate as for example the second or third needle. The second needle from the right, the reverse cutting tip, is used for the actual corneal sutures, it is sharp, penetrates easily with little force, results in little tissue disruption and the suture tension is against the flat side of the tract.
The next decision is to choose the needle shape. A circular needle will make a long bite, while in PK we prefer short bites. In the early days, penetrating keratoplasty was associated with glaucoma, due to the long bites of circular needles putting stress on the trabecular meshwork. Additionally, after trephination with an 8mm trephine there really is not much room on the recipient cornea to place the suture. For these reasons a compound curve needle is the needle of choice. It allows short deep bites which are ideal for PK. In the 1980's Dr Troutman developed the TG6-C compound curve needle for Ethicon specifically for PK and it or it's equivalent is the preferred needle for PK.
It is instructive to remember that eye surgeries used to be performed with closed eye needles. Dr Troutman related a story to me that the finest nylon available in the early days of PK came from German women's nylon stockings. It was the fellows responsibility to unravel a thread from the stocking and thread the needle prior to surgery! Thankfully all needles are now swaged to the suture and disposable, giving a fresh sharp needle for each surgery.
Now we turn to the suture material. Absorbable sutures are clearly out of the question since we leave sutures in PK for at least a year. Braided sutures like silk are also unsuitable since the braid can allow capillary action and allow bacteria into the wound and eye. Monofilament sutures come in two main materials, nylon and prolene. A fair amount of discussion has occurred over the years over the optimal monofilament suture for corneal transplants. To make the right decision we need to know how the suture interacts with corneal tissue. If we use a metal wire to suture the cornea, the inextensible wire will rather quickly cheesewire through the cornea, resulting in decreased compression across the wound and poor wound healing. What we really want is an elastic material that has approximately the same elasticity as corneal tissue, flexing with with the cornea while maintaining tension across the corneal wound. Prolene is relatively inelastic, cheesewiring through corneal tissue. 11-0 Nylon is certainly elastic but too much, providing little wound compression. Amazingly, 10-0 nylon is almost a perfect fit, providing enough elasticity to flex with the cornea (avoiding significant cheesewiring), maintaining tension across the wound and it lasts a long time in the eye. It gets even better, in the first month 10-0 nylon cheesewires just a little into the top and bottom if the cornea, leaving it buried in the cornea and thus closing a potential route of infection.
Now that we have chosen the needle and suture material, we should discuss needle handling. The needle holder should have fine jaws to accommodate the tiny needle. The shank of the needle is round at the swage and becomes flattened just below. If the needle holder is positioned on the round part, it will twist in use, making needle insertion unpredictable. Instead, grasp the needle on the flattened portion. Also, alway avoid grasping the needle anywhere other than the shank. In effect, this is a knife and manipulation along the sharp edges will dull the needle. Instead, grasp the suture thread above the needle with a forceps, balance the needle vertically and grasp the needle on the shank.
The choice of needle holder and forceps is crucial in the speedy and atraumatic completion of the surgery. Because you will be placing sutures at odd and sometimes uncomfortable angles, the needle holder and forceps should be short. This prevents interference with the nose and othe features of the surgical surface. As mentioned before, the needle holder should have fine jaws and most importantly it should be non-locking. Corneal transplantation requires rapid opening and closing of the jaws, locking needle holders are just awkward. The Troutman needle holder seen above has some additional features. The round short handle allows rotation of the needle in place, facilitating suture placement. Curved jaws on a needle holder make suture placement more difficult. The Pierse atraumatic tissue forceps are also short and straight with special jaws that don't catch on the suture and are gentle on the corneal tissue.
The previous decisions regarding needle and instrument selection can be seen and summarized in the above figures. In figure A the donor cornea is grasped with the atraumatic Pierse forceps and the Ethicon TG6-C needle is driven completely through the cornea. In figure B the needle is rotated into position for a short bite. The tight radius of the fishhook needle allows for the short bite. In figure C the host cornea is supported with the Pierse forceps prior to engaging the needle. The needle is driven straight up through the host cornea as seen in figure D. As seen in figure E, the needle is grasped with the needle holder and pulled through as seen on figure F. The result is a through and through atraumatic, short suture bite. Supporting both the entry and exit of the needle is essential to prevent disruption and possible damage to the globe. Done correctly, the donor and host tissue should not experience movement and/or distortion.
The depth of corneal sutures has also been the subject of past debate. Some surgeons have advocated suture depth at 80% of the thickness of the cornea. When tied, these suture bites compress the upper cornea and splay the lower cornea, creating a gap termed by Dr Troutman the posterior lambda. This gap is filled with endothelium, draining endothelium even after suture removal and resulting in a limited graft lifetime. Even worse, this posterior lambda effect weakens the wound, resulting in astigmatism and an unstable refractive result. Early surgeons were concerned that a through and through penetrating corneal suture might provide a route of leakage and/or bacterial infection. Actual experience has shown that these concerns were unfounded. The 10-0 nylon cheesewires slightly into the wound, closing any external access. In my 25 year experience with this technique, I have never had leakage from the sutures. The advantage is secure closure from the top to bottom of the cornea and dramatically increased lifetimes of the corneal graft. Of course if you use the wrong needle or handle the tissue roughly with a bad needle insertion, anything can happen. The answer is to practice, practice, practice.
When the suture has been placed, it needs to be tied off. The slip knot is the preferred knot because it is adjustable and results in a smaller knot that buries easily. While I am going to devote a post to suture handling and the slip knot, you can find a full description on my web site here.
Troutman RC, McGregor W: New compound curved needle TG6-C Plus, Ophthalmology, Instrument and Book Suppl 92(8):84-86, 1985
The Slip Knot: http://buzard.info/page64.html
Corneal Astigmatism Etiology, Prevention and Management: by Dr Buzard and Dr Troutman, 1992 Mosby: http://www.amazon.com/Corneal-Astigmatism-Etiology-Prevention-Management/dp/0801655315/ref=cm_cmu_up_thanks_hdr