Pachymetry Assisted Laser Keratoplasty (PALK): a new approach to an old technique. By Cesar Carriazo MD.
Introduction.
Same corneal disease (e.g. leukomas, irregularities, and dystrophies) may compromise only the anterior portion of the cornea. In that scenario, it is undesirable to treat the unaffected Descemet's membrane and the endothelium. Those patients usually have been treated by lamellar resection of the anterior portion of the cornea, which is replaced by a donor lamellar graft. These techniques are called lamellar keratoplasty or homoplastic lamellar keratomileusis.
The advantages of lamellar keratoplasty (LK) over penetrating keratoplasty (PKP) are: (1) shorter visual rehabilitation; (2) decrease in astigmatism induction; (3) lower risk of infection and graft rejection; (4) lower incidence of intraoperative complications; and (5) the possibility of using stored donor tissue. Despite all these advantages, eyes treated with LK usually do not present the same optical quality achieved with PKP. The host-donor interface is frequently irregular which leads to irregular astigmatism and poor visual outcomes. Additionally, technical difficulties prevent it from being a popular procedure.
In this chapter we present a new technique for lamellar keratoplasty, using excimer laser ablation guided by pachymetric data (pachymetry asisted laser keratoplasty; PALK). This technique has all the advantages of lamellar procedures but it also provides a uniform corneal thickness due to a smooth receptor surface leading to a good optical performance.
Surgical Technique - Pachymetry Assisted Laser Keratoplasty (PALK)
DONOR LENTICULE
In conventional automated lamellar keratoplasties a corneal donor disk is obtained from a whole ocular globe or from an artificial chamber-mounted corneal scleral button, using a microkeratome system without stopper. Corneal scleral buttons must have at least a 16mm diameter to fit an artificial chamber. Thickness and diameters of the corneal lamellar cut can be determined by using different microkeratome heads and suction rings.
In our technique (PALK) we use the same procedure for pathologies like leukomas or irregularities of the corneal anterior surface (<300 microns of depth). In case where the posterior layers of the cornea are compromised or a corneal ecstasy is found, we use a trephine that creates a full thickness donor disc with vertical edges that fulfill the requirements of the patient. The desired thickness of the lenticule is then obtained using an excimer laser (Esiris, Schwind, Germany). The laser is applied onto the endothelial face with the PTL program. The depth of deep stromal ablation is calculated to match the patient's requirement.
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| Diagrammatic illustration of surgical procedure |
HOST CORNEA
This procedure is performed using topical anesthesia since we need the patient's collaboration in order to get the laser fixation. The ablation guided by pachymetry can be achieved manually or automatically. In the manual procedure, the host cornea is marked with violet dye. A marker presenting eight radial and two circular lines is used. After this, we measure the thickness of the cornea in 17 predetermined points using a Sonogage pachymeter. These results, along with clinical and personal data, are transferred to a computer software (Pachy-Link Software). The diameter of the Optical Zone, the pachymetric values, and the position of the 17 points are the most relevant data. If needed, the number of thickness mesasurements can be altered by the surgeon.
In the automatic system we use a slit-lamp-based (via rotating camera) full map pachymeter - the Oculus Pentacam. After this informacion is obtained it is processed directly by the PACHY-LINK software in its own pachymetric map, which automatically generates the ablation profile map that is going to be used on the patient.
The target posterior stromal bed thickness and the optical zone are selected by the surgeon and depend on the preoperative evaluation (e.g. corneal disease, depth of compromised area).
As the excimer laser used works with a flying spot beam, the peripheral borders of the ablation are not perfectly regular, and we recommend using a plastic protector ring of 0.5 mm smaller diameter than the selected ablation zone. This ensures that the peripheral border created is vertical and well defined. Once the posterior stromal bed thickness has been selected the corneal tissue of the receptor cornea is removed by the pachymetry guided laser ablation. This procedure requires an eye tracking system. In patients with keratoconus, this procedure takes between 7 and 8 minutes using the Scwind Esiris laser, which works at 400 Hertz per second.
The donor corneal disk is placed on to the residual stromal bed of the receptor. If the donor corneal disk is thinner than 200um, sutures are not needed. However, when using thicker lenticules sutures are required.
POSTOPERATIVE CLINICAL SIGNS
During the first weeks some posterior folds can be present. In two out of 40 patients in whom the manual system was used we had a micro perforation and obtained a pseudo-chamber in the interface. We have not had any micro perforation cases using the computerized pachymetric map system.
With PALK we did not observe: (1) endothelial rejection (2) endothelial cell loss greater than 10% (in stromal beds of >100um), (3) decentration of the ablation (an indicator that the eye tracking system is suitable for this procedure), (4) intraocular complications (e.g. infections, glaucoma, cataracts), or (5) high postoperative astigmatism. Patients treated with PALK presented a faster postoperative recovery.
The first keratoconus patient was operated in the year 2000. Nowadays, we have performed surgery with the same technique in 55 patients (2006). In the first cases we reached an average visual acuity recovery of 70% due to the fact that there was no thickness uniformity because of technical limitations of the manual system. In the last procedures - which were made automatically - the visual acuities are superior, having patients with 20/20 best spectaclecorrected visual acuity (BSCVA). This occurs once the residual aberrations are corrected.
Another important observation is that visual recovery is inversely proportional to age and directly proportional to the interfaxe integration. The corneal structure reassembled in the immediate postoperative period, and the donor-host interface becomes imperceptible in a couple of months.
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| Pachymetric map in a patient with keratoconus |
DISCUSSION
Excimer lasers are capable of performing corneal ablations with a high degree of accuracy. Excimer laser corneal surgery is currently performed in three different ways: phototherapeutic keratectomy (PTK), photorefractive keratectomy (PRK And LASEK) and laser in situ keratomileusis (LASIK).
In the particular case of PTK, the intention is to smoothen irregular corneal surfaces and/or to remove opaque superficial corneal tissue. In this technique the surgeon chooses the diameter and depth of the desired ablation and in all cases the ablation is made in parallel planes.
Currently, LK can be performed using manual or automated keratectomy. Accidental corneal perforation can occur, specially in manual procedures with deep resections. Corneal irregularities induced by an uneven interface are also of concern.
The PALK technique is a new approach that can be used in cases of severe corneal irregularities of the receptor eye and where the classical techniques are not the best option. The use of automatic multipoint corneal pachymetry allows surgeons to measure laser ablation depth from a fixed surface, avoiding microperforations during the procedure, to obtain a uniform thickness in the host cornea.
By providing a map of the cornela thickness across the ocular globe, multipoint corneal pachymetry effectively provides a cross-sectional view of the corneal bed. With PALK, it is possible to precisely control the depth of the treatment during the ablation of the host area. In the future, the duration of treatment will be reduced because new laser machines higher ablation speeds are being introduced.
Summarizing, we present a new approach for LK technique that we believe is safer and more predictable than manual or automated procedures. To the best of our knowledge, this is the first paper describing pachymetry assisted laser keratoplasty (PALK).
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