(a) Micrometer knife, (b) double fixation forceps
An alternative to manual keratotomies with handheld blade has been made available in the form of a Hanna arcitome. This is an adaptation of the Hanna suction corneal trephine. Inside the cowl of the trephine are dual micrometer diamond knives. After suction is applied and the blades penetrate the corneal tissue, they will follow a fixed course of arc length that can be set in advance. The use of the arcitome may provide more reproducible cuts in the corneal tissue.
18.104.22.168 Surgical Planning (Manual Micrometer)
There basically are two approaches which may be employed in the performance of a CRI. The first approach we will refer to is topography guided. In this approach, corneal topography is used to identify the steep meridian or hemi-meridians of the cornea. The length of the CRI will be set to match the general width of the representation of the steep axis as displayed by topography. This technique lends itself well to the performance of asymmetric length incisions to match an asymmetric astigmatism. Additionally, it is not necessary to have both incisions be perfectly orthogonal in the case of an irregularity of the astigmatism. Rather the incisions may be placed corresponding to the position of the steep hemi-meridians indicated by the topography. Regardless of the topographic pattern, however, no incision length should be created longer than 90°. As previously discussed, incisions will be placed either in the graft-host interface or just interior to it, thereby generally giving an optical zone of around 8 mm in diameter. This is an effective technique and lends itself well to the type of astigmatism observed following PKP. It provides a more customized approach to the correction of the astigmatism.
An alternate approach and one which is more traditional employs the use of symmetrical paired incisions. In this case, the incision is performed symmetrically on the steep axis as determined either by keratometry or topography. An approximate nomogram correlating diopters of correction to arc length is shown in Fig. 12.2. It should be noted however that the nomogram is truly just an approximation and that CRI surgery whether topography or nomogram driven does not have nearly the precision or predictability or corneal refractive surgery performed on virgin eyes, especially those cases performed with the excimer laser. The expectation for perfect correction must therefore be tempered in the minds of both the surgeon and the patient.
Manual CRI nomogram
22.214.171.124 Surgical Technique
Obtain a recent corneal topography, and mount this in the operating suite in easy view of the surgeon, and orient in such a fashion that the topographic map will correspond to the surgeon’s perspective through the microscope. If the surgeon is operating from the 12 o’clock position, then the 12 o’clock position of the topography should be at the bottom of the printout.
If employing the topographic CRI approach, mark out the beginning and ending points of the incisions on the topography map, and determine the corresponding degrees for these two points for one or both planned CRIs.
Place a drop of topical anesthetic in the operative eye, position the patient at a slit lamp, and mark the 3, 6, and 9 o’clock positions while the patient is still seated. This will be used for orientation under the microscope to compensate for any possible cyclotorsion which may occur.
Apply a standard prep, and drape to the periocular area including the use of topical Betadine for infection prophylaxis.
Insert speculum. Measure the corneal thickness using the ultrasonic pachymeter along the proposed length of the CRI. If it is not possible to perform the pachymetry at the time of surgery, it should be done immediately before the patient is brought to the surgical suite. Reapply topical anesthetic.
Use a Mendez or equivalent marker to identify the vertical and horizontal meridians of the cornea by lining up the 90 and 180° marks on the Mendez with those previously applied to the cornea while the patient was at the slit lamp.
With the marker still applied to the cornea, identify the degree marks previously determined either by topography or nomogram for the beginning and ending points of the CRI. Use either a super blade or small-caliber needle to create small radial scribe marks in the epithelial surface next to each of these degree marks. Once the appropriate beginning and ending points for the planned CRIs have been marked in this fashion, the Mendez may be removed.
After the scribe marks have been completed, they should be stained with methylene blue to facilitate visualization. This may be applied in liquid form delivered with a small cannula or using a marking pen. Once the dye has been applied, it should be rinsed to remove any excess leaving just the scribe mark visible.
The micrometer blade should now be adjusted to 80% of the measured thickness along the proposed incision site. If there is variability along the incision site, the depth should be set to 80% of the thinnest pachymetry reading. Alternatively, if there is great variability along the CRI path, it is possible to perform the incision with different thickness by removing and readjusting the blade. In this case, it is best to incise the thinner portion of the cornea first and then slightly deepen the blade for the thicker tissue.
Insert the diamond blade at one end of the incision, and plan to move forward in a curved fashion either in the graft-host interface or just anterior and parallel to it. Better penetration to full-set depth may be obtained if the blade is allowed to enter the cornea and rest for a few seconds before beginning the forward movement.
Very long arcuate incisions may be difficult to create. In this case, it is also effective to use multiple stab incisions. In this case, the blade is placed into the corneal tissue and allowed to penetrate to depth. A short segment is created. The blade is then withdrawn, advanced slightly, and an additional cut is placed. This may be continued in a segmental fashion until entire length of the incision has been covered. The blade is then reinserted at the starting point and slowly advanced, cutting through the tissue bridges created by the multiple stab incisions. The end result is essentially the same as a continuous incision but may be more easily produced than along continuous incision.
The globe will need to be fixated firmly during this process. This may be accomplished with either a fixation ring which traverses the entire circumference of the limbus or a double fixation forceps such as Kremer or Bores type. Single-point fixation should not be employed as it does not give sufficient stability to the globe, while the CRI is being created and can lead to an unpredictable incision path. A double fixation forcep is shown in Fig. 12.1b.
Once the first incision is created, the blade is withdrawn, pachymetry adjusted as needed, and the second incision performed according to the pre-marked cornea.
Once both incisions have been created, the incisions are irrigated with BSS.
The incisions should be inspected for any tags or incomplete cuts and also for any evidence of wound leak.
A microdehiscence as evidenced by a slight drop of aqueous escaping under pressure alone may be left. However, a macrodehiscence in which copious aqueous escapes with little or no pressure accompanied by shallowing of the chamber must be sutured immediately.
Once the incisions have been irrigated and found to be free of leaks, compression sutures are placed across the graft-host interface at the site of the previous flat meridian. These bites should be deep to full thickness and very short in length. These are generally placed with 10-O nylon. To facilitate deep bites, a bi-curve needle such as the Ethicon CS-B should be employed.
The tension of the compression suture should be adjusted so as to achieve approximately 50% over correction of the original astigmatism. The compression sutures will serve to both support the wound opposite the CRI and to enhance their effect by creating some pouting or opening of the incision, preventing premature closure and the potential loss of the surgical effect.
The compression sutures should be trimmed and the knots buried into the donor cornea.
The full effect of the procedure is often not obtained at the immediate completion of surgery, and reliance solely upon keratometry or aberrometry can be deceptive. Aberrometry in general may be difficult to use in PKP corneas due to the often irregular optics.
The eye should again be irrigated. An antibiotic and possibly a nonsteroidal drop applied.
The speculum is then removed. A collagen shield or a bandage contact lens is applied for the initial comfort of the patient. Patching is not generally necessary unless a dehiscence has taken place (Video 12.1).
126.96.36.199 Postoperative Care
The patient should receive a combination of antibiotics and steroids for several weeks following the surgery. Generally, the regimen should be similar to that employed for cataract surgery, although I prefer to keep the patient on a small amount of maintenance steroid for about 3 months, as prophylaxis for graft reaction. The compression sutures should generally create about a 50% overcorrection initially. If that is achieved, it should generally be left in place for several months. If the amount of overcorrection is excessive, they may be removed early on.
If there is no initial overcorrection or an under correction, it is likely that the full desired effect will not be achieved. A consideration should be given to enhancing the CRI early on. This should be done again under topography guidance and will generally involve a slight extension of the original CRI. It may also be helpful to slightly deepen the original incisions but not beyond 90% of the measured pachymetry. The healing of the CRI is generally stabilized by about 3–4 months postoperative, and the compression sutures should be removed by that point. Temporary spectacle correction may be attempted prior to that time, but the patient should be made aware of the fact that the manifest refraction will be fluctuating during this period of time.
The technique described above will be slightly different if the mechanical arcitome is employed. This device, for example, uses a smaller optical zone and comes with its own nomogram slightly different than what is described above. The postoperative management however will essentially be the same.
The CRI is an effective technique for managing moderate degrees of postoperative astigmatism. It has the advantage of being customizable with topography guidance. It can be enhanced relatively easily if the full desired effect is not achieved with the initial surgery. It does not however change the overall spherical equivalent. Patients will need to be aware that it is likely that some form of spectacle or contact lens correction will still be required to manage any residual ametropia. The CRI is shown schematically in Fig. 12.3. Clinical photos of a CRI with compression sutures appear in Fig. 12.4a, and topographic maps are shown in Fig. 12.5.
Illustration of CRI procedure
(a) Manual CRI with compression sutures, (b) femtosecond laser CRI, (c) femtosecond laser CRI with asymmetric arc length, (d) wide excision following femtosecond CRI
(a) Post-PKP topography, (b) topography post-CRI incisions, (c) subtraction map of figures (a, b)
12.4.3 Corneal Wedge Resection (Block Resection)
This technique is a less frequently performed method reserved for the correction of fairly high degrees of postoperative astigmatism. It was first described by Troutman as a resection of a wedge-shaped section of tissue from the flatter meridian to try to reduce post-PKP astigmatism .
Over the years, the technique evolved into the creation of a block excision of corneal tissue, usually excising the graft-host interface in the flat meridian. Unlike the relaxing incision, the coupling produced by this procedure is generally 2:1, with 2 diopters of steepening created at the site of the excision to 1 diopter of compensatory flattening, 90 degrees away. This occurs because there is a removal of corneal tissue at the site of the resection. This causes a decrease in the total circumference of the cornea.