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Conductive keratoplasty useful after cataract surgery, LASIK
This month’s column explains how to use CK to correct astigmatism and how to prevent and treat CK complications.
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Conductive keratoplasty (CK) is based on radiofrequency energy. The controlled release of radiofrequency waves causes shrinkage of corneal collagen. As the treatment is applied as a ring in the mid-peripheral cornea, there is the formation of striae between the spots and a band of contraction with flattening of the mid-peripheral cornea and corresponding steepening of the central cornea (Figure 1). Single pulse deep stromal delivery of the energy is given.
The technique utilizes the electrical properties of the cornea. The stromal temperature rise is induced by impedance to the flow of energy through the corneal collagen and leads to shrinkage of collagen, which occurs at 65° Celsius. A local leukomatous change at the area of application indicates the reaction. The average CK footprint measures approximately 405 µm wide and 509 µm deep. When the tissue temperature reaches 65° Celsius, the collagen starts shrinking without denaturation of proteins. This reaction is self-limiting; as the collagen shrinkage increases, the efficacy of the radio frequency waves decreases and the temperature therefore starts decreasing.
 Amar Agarwal |
The ViewPoint CK System from Refractec is used for CK. The procedure commences by applying topical anesthesia and stabilizing the eyelid with a speculum. With the help of a CK marker, the meridians are marked radially. Each radially marked meridian has three concentric hatch marks, the inner one at 6 mm, the intermediate one at 7 mm and the outermost at 8 mm. Spot placement is defined according to predetermined nomograms. The spots are generally given at a 7-mm zone circle followed by additional spots if needed. The number of spots varies from eight to 32. Radiofrequency energy of 350 MHz is delivered through a thin metal probe, the Keratoplast tip (450 µm ×90 µm), in the peripheral cornea at the predetermined spots. The tip is held perpendicular to the corneal surface. The profile of energy given is 350 MHz, 60% power for 0.6 seconds per spot. The tip provides a uniform cylinder of energy with the depth reaching up to 80%. Deep penetration of the tip is prevented by the Teflon-coated governor. The light-touch technique started by Milne is preferred.
| Pre- and post-CK Orbscan pictures. Note steepening of the central cornea. Images: Agarwal A |
CK in astigmatism
CK in patients with post-LASIK astigmatism has resulted in improved corneal optics and visual acuity. CK can be a viable option in patients for whom further laser procedures are contraindicated.
Intraoperative treatment of astigmatism in patients treated with CK can be done. Flat axis is determined with automated keratometers, and additional spots are given in these points in the flat axis in a 7-mm zone. Intraoperative treatment of astigmatism through the addition of more spots at the minus cylinder or flat axis reduces the degree of induced astigmatism. CK can also be used in astigmatism due to corneal trauma or scarring and after decentered ablation. CK has been tried in corneal ectasias such as keratoconus and pellucid marginal degeneration. Pinelli tried CK in pellucid marginal degeneration with a thin cornea by putting three spots in the flat axis and one spot in the opposite side to counterbalance the tension.
CK in post-cataract surgery
The indication for post-cataract patients is up to 2.25 D of hyperopia and 1.75 D cylinder of hyperopic astigmatism. There should be at least a 1-month gap in the postoperative period between microphakonit (700-µm cataract surgery), 1.5 months with phacoemulsification and 2 months with extracapsular cataract extraction and CK. The patient should have stable refraction on two consecutive refractions at least 1 week apart. The IOL should be well centered, and the pupil should be round and regular. There should also not be any significant irregular astigmatism (Figures 2 and 3).
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CK complications
Induced astigmatism. In conventional CK when the cornea is compressed, a mechanical stretching effect occurs that prevents the tissue from being drawn toward the pulse. This causes post-CK astigmatism. Similarly, when the amount of compression varies from spot to spot, the predictability and refractive outcome change. With the introduction of the light-touch technique, there has been a significant reduction in induced astigmatism. One more cause for induced astigmatism is improper marking of the cornea before surgery or when the spots are not placed symmetrically on the marks. Intraoperatively, one can assess the amount of astigmatism with a handheld keratometer or intraoperative keratometers, and additional treatment spots can be placed. Treatment spots should not overlap.
Relaxation effect. A relaxation effect after the first few months of the procedure has been noted. Patients might require an additional CK or another procedure to overcome this.
Monovision intolerance. This occurs when the refractive difference between the two eyes is more than the patient can tolerate. This problem can be overcome by good patient selection and proper preoperative counseling. In extreme conditions, the patient may require a secondary procedure.
Ablation after CK. When a patient who has undergone CK goes for a laser vision correction procedure such as LASIK, flap creation could damage treated collagen lamellae, which might cause a significant change in the final refraction. Hence, in these patients, it is better to do a surface ablation rather than raising a flap.
Undercorrection. When excessive pressure is uniformly applied to all treatment points, undercorrection occurs. One has to wait at least 6 months before doing an additional procedure.
Overcorrection. When too many spots are applied or if spots are applied on too small a ring, overcorrection can occur. Although the complication is rare, it is better to have the least number of spots as possible. If the overcorrection does not regress after 3 to 6 months postop, surface ablation may need to be done.
For More Information:
- Amar Agarwal, MS, FRCS, FRCOphth is director of Dr. Agarwal’s Group of Eye Hospitals. Dr. Agarwal is author of several books published by SLACK, Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery, and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.
References:
- Agarwal A. Handbook of Ophthalmology. Thorofare, NJ: SLACK Incorporated; 2005.
- Agarwal A. Phaco Nightmares: Conquering Cataract Catastrophes. Thorofare, NJ: SLACK Incorporated; 2006.
- Agarwal A. Refractive Surgery Nightmares: Conquering Refractive Surgery Catastrophes. Thorofare, NJ: SLACK Incorporated; 2007.
- Agarwal A, Agarwal A, Agarwal S. Phacoemulsification. 3rd ed. Thorofare, NJ: SLACK Incorporated; 2004.