Conductive Keratoplasty and the Lighttouch Technique
Conductive keratoplasty (CK) is a corneal steepening procedure for hyperopia correction and presbyopia management. In April 2002, the US Food and Drug Administration (FDA) approved CK for 0.75 D to 3.00 D of hyperopia. In March 2004, it was approved to correct presbyopia symptoms by inducing 1.00 D to 2.00 D of myopia in the nondominant eye.
With 200 million people aged ≥40 years in the European Union and 130 million in the United States, interest in this refractive segment remains high. The recent innovation of the CK Lighttouch (Refractec, Irvine, California) technique has facilitated more predictable outcomes with significantly less induced astigmatism than conventional CK, and >150,000 procedures have been performed. CK is now the leading nonlaser refractive technique in the United States.
CK is a radiofrequency-based collagen shrinkage procedure (See sidebar entitled History of Clinical Thermokerato-plasty). Mendez and Mendez-Noble first used the procedure in 1993.1 The Viewpoint CK's (Refractec) handheld probe delivers a low-energy high-frequency current (ie, radio frequency [RF] 350 kHz) directly into the paracentral corneal stroma using a keratoplasty tip (Keratoplast; Refractec) with a 90-µm diameter and a 450-µm length.
Electric impedance to the RF energy causes stromal temperature rise and collagen shrinkage. The temperature range is between 50ºC and 70ºC (See sidebar entitled Biochemistry of Corneal Steepening). As the fibrils contract, striae form between the spots and produce a circumferential band of tightening. Central corneal curvature increases, thereby reducing spherical hyperopia or inducing unilateral myopia to correct presbyopia.
PRESBYOPIA CORRECTION, BLENDED VISION
Challenges in presbyopia correction are both practical and theoretical. Clinical approaches to provide useful distance and near vision are based upon various optical arrangements (eg, IOLs/contact lenses with multifocal or bifocal properties or monovision via surgery or contact lenses).
CK correction produces optical factors that may improve near vision above surgical or lenticular monovision. This effect has been coined blended vision to distinguish it from monovision. Corneal multifocality, altered higher order aberrations (especially spherical aberration and vertical coma), surgically induced astigmatism and implications of effectivity from corneal steepening versus correction at the spectacle plane may play a role. As described by Hersh,2 the newly reformed cornea is not simply the original cornea with a different power: Changes are complex, and optical predictability remains to be assessed.
EVOLUTION OF THE LIGHTTOUCH CK TECHNIQUE
Traditionally during CK, the cornea is deeply compressed as RF energy is delivered (ie, as in FDA trials). I and other surgeons using this original technique noticed a significant variability in total spherical response compared with the FDA nomogram. Additionally, induced astigmatism was a frequent problem, requiring CK correction in ≤40% of cases.
Rick Milne, MD, of Columbia, South Carolina, had patients who consistently under-responded compared with the FDA nomogram. Glenn Strauss, MD, of Tyler, Texas, however, produced results consistently in line with those of the FDA by using comparatively less corneal compression with the Keratoplast tip during CK spot application. Dr. Milne theorized that the degree of corneal compression and stretching of collagen fibrils by the tip may impact shrinkage, leading to reduction of effect and outcome variability.
Dr. Milne refined his technique into an application now called Lighttouch CK and shared his findings with me and other CK surgeons. He aims to have minimal compression (creating a 1-mm to 2-mm dimple on the cornea) at the time of RF application. The degree and consistency of compression applied for each treatment spot are critical components of success.
Since summer 2004, I performed >1,500 Lighttouch procedures and share an identical nomogram with Dr. Milne. Additionally, my incidence of correction of induced astigmatism has fallen from 40% to 2%.
LIGHTTOUCH EXPLAINED
The conventional CK technique places a degree of corneal compression as RF energy is introduced at each spot location. Compression of the cornea causes mechanical stretching of the corneal fibers that resist the natural tendency of the tissue being drawn toward the heat generated by the pulse of energy. With greater amounts of corneal indentation, less corneal steepening takes place. It seems reasonable that variable indentation and stretching of collagen fibrils affect the degree of fibril shrinkage. This may explain the varied response to standard CK treatments and clarify frequent astigmatism in some surgeons' hands, despite an apparently consistent CK technique. With the Lighttouch technique, the greatly reduced compression permits a higher degree of collagen fibril contraction (ie, during the 0.6 seconds RF energy pulse) and a more robust response.
CREATES A GUIDE TEMPLATE
Following topical anethesia and gentle drying of the corneal surface, a CK marker creates a guide template with methylene blue or gentian violet; it is essential to center the ring template on the pupil. At each designated treatment point, the Keratoplast tip is inserted firmly into the stroma following a diagonal sequence.
Pressure is applied to indent the cornea and ensure adequate seating of the tip, and the ring light reflex is displaced from the treatment spot. Pressure is gradually released until only a trace of indentation is on the corneal surface. I prefer to elevate IOP with bimanual pressure application to the speculum. This provides a counterforce to the pressure of the tip, ensuring reliable contact with the cornea (ie, 1 mm to 2 mm of indentation). Additionally, the surgeon's fingers and speculum assist in stabilizing the eye and handpiece. While CK RF energy is delivered, the corneal surface will be depressed. You must maintain pressure and gently follow so that the Keratoplast probe does not pull away from the surface. A bimanual technique with balanced counterforces between the left and right hand assists in this maneuver.
According to information from Refractec, the Lighttouch technique produces a more robust response than conventional CK. Figure 1 illustrates the 1-mm to 2-mm dimple produced by the Lighttouch technique versus the 5-mm dimple from conventional CK. The FDA-approved standard CK nomogram calls for eight, 16, 24 or 32 spots that are applied as rings of eight. The Lighttouch nomogram used by myself and Dr. Milne — based on >1,500 treatments I have performed — uses eight or 16 spots to achieve the same results. The average correction for each stage is seen in Table 1. This nomogram can be used for primary treatments, retreatments and hyperopic PRK/LASIK eyes. You will, however, need to develop and establish your own personal nomogram.
No loss of stability with the Lighttouch technique has been noted to date, however, approximately 10% of patients do not achieve sufficient near vision and require an additional eight spots. Mark the original CK spots at the slit lamp (Figure 2) and apply the new treatment under the microscope. Place spots either offset by 22º on the original ring diameter or as an additional ring at a different diameter, depending on the original pattern.
With both Lighttouch and conventional CK techniques, according to my personal series and communication with other surgeons, 4% to 5% of patients show significant regression 9 to 12 months after correction. Some patients may achieve stable correction with an additional eight spots, while I have managed others very successfully with a surface laser enhancement.
The incidence of induced astigmatism is greatly reduced with Lighttouch (eg, 40% to 2% in my series). Placing one or two spots on the axis of the negative cylinder will eliminate the astigmatism 90% of the time. With Lighttouch, one spot at the 8-mm diameter will correct approximately 1.00 D of astigmatism.
Nearly 5 million people in the United States have undergone LASIK surgery since 1997. At an average age of 42 years, nearly 44% are now presbyopic and must once again wear spectacles. A third FDA trial, assessing the efficacy of CK for the post-LASIK patient, is underway.
Daniel S. Durrie, MD, of Durrie Vision, Overland Park, Kansas, has presented early FDA-approval data on Lighttouch with eight spots at 8 mm. Patients achieved a mean refractive change of 1.22 D and high satisfaction ratings. To date, the safety profile is excellent with no flap complications or loss of BCVA.
I have performed >70 CK procedures on post-LASIK and PRK patients. My results have been very good; the standard nomogram may be used for hyperopes and mild myopes, whereas a reduced treatment plan is required for corrected moderate to high myopes, especially post-LASIK cases that exhibit an increased response.
CK has found its place for the correction of presbyopia in emmetropes and very low hyperopes. I expect CK will also find acceptance in the treatment of postexcimer presbyopes. The Lighttouch technique is a significant step forward because of its reduction in postoperative discomfort (ie, due to fewer treatment spots), greater predictability and a dramatic reduction in the incidence of induced astigmatism.
David Allamby FRCS, FRCOphth, is from the Focus Reading Vision Clinic, in London. Dr. Allamby states that he has no financial interest in the products or companies mentioned. Dr. Allamby may be reached at david.allamby@focusclinics.com.
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