Conductive keratoplasty: Effective and safe with maintained results

ByDaniel S. Durrie, MD

Conductive keratoplasty procedures are increasing in popularity. Within the first year following approval of CK for hyperopia by the Food and Drug Administration in 2002, more than 14,000 CK procedures were performed in the United States. The ViewPoint CK System (Refractec, Inc., Irvine, Calif.) was then used to perform NearVision CK in a second study for presbyopic patients and, in 2004, was the first FDA-approved treatment for near vision in patients who are presbyopic with emmetropia and low hyperopia.¹ To qualify for CK, patients must possess a spherical hyperopia of 0.75 D to 3 D and up to 0.75 D of astigmatism and must be at least 40 years of age.² Surgeons have found that the best patients for CK are the “plano presbyopes” within ± 0.75 D of plano. Since the baby boomer generation is approaching the onset of or is already experiencing presbyopia, the interest in CK procedures is rapidly growing and more than 100,000 procedures have been performed.

The CK procedure is easy to perform: a topical anesthetic is administered and a lid speculum is inserted. The eye is then marked and radiofrequency energy is applied in rings on the cornea (Figure 1A). Shrinking of the cornea’s periphery subsequently steepens the central cornea. The goal of surgery is a round, more prolate peripheral cornea (Figure 1B). To achieve optimal results, the eye must be marked in the exact center of the pupil and standard moisture must be maintained.

	Figure 1A: Example of conventional NearVision CK nomograms and sequence (16 and 24 spots).	Figure 1B: The surgical goal of CK is a round, more prolate cornea.
	Figures courtesy of Daniel S. Durrie, MD

Is CK effective?

In the FDA presbyopic clinical trial, CK was performed in 188 eyes of 150 subjects with a mean age of 53.³ The mean intended correction was 1.58 D ± 0.67 D, with a correction range of 0.75 D to 3 D.⁴ Results were recorded at 1, 3, 6, 9, 12 and 24 months postoperatively.

Testing at 6 months postoperatively showed 85% of all patients had binocular distance vision uncorrected visual acuity (UCVA) of 20/20 or better along with J3 or better near vision. Also at 6 months postoperatively, 86% of patients had manifest refractive spherical equivalent (MRSE) changes of ± 0.50 D between 3 and 6 months postoperatively.⁵

Also at 6 months postoperatively, patients experienced improvements in their ability to read computer screens, menus, newspapers, magazines and fine print without glasses. According to patients surveyed, distance vision, as well as near vision, improved. No contrast sensitivity was lost between preoperative best spectacle-corrected visual acuity (BSCVA) and postoperative UCVA at distance. Depth perception was maintained and patient satisfaction was also high.⁴ At 12 months, 98% of patients noted improvement in quality of vision and 84% were “satisfied” or “very satisfied” with CK results.⁴

Additional findings – my personal CK results

To determine if and how long CK results were maintained, I tested nine patients who participated in the FDA trial at 12 through 26 months postoperatively (Figure 2). Compared to preoperatively, these patients gained on average 7.1 ± 2.9 lines in near UCVA in the CK-corrected nondominant eye. No change was seen in the untreated dominant eye. Patients also gained 0.33 ± 1.7 lines in distance UCVA in the treated eye and experienced no change in the dominant eye. The 7-line gain in near vision and the slight gain in distance vision led to high patient satisfaction.

	Figure 2: Preoperative and postoperative characteristics of nine patients from a comparative study of presbyopes.

Presbyopia and hyperopia are progressive conditions that typically become problematic after the age of 40. As people age, the crystalline lens changes and retinal function decreases. Although CK cannot prevent these alterations, the CK procedures I have performed resulted in patients who remained glasses-free for more than 2 years. When patients eventually needed glasses, they wore them less frequently.

CK vs. monovision contacts

Because CK results in no loss of subjective or contrast sensitivity and better distance vision than other monovision treatments, my colleagues and I executed a comparative study. Ten 50-year-old plano-presbyopes were adapted to 0.75 D, 1.5 D and 2.5 D monovision contact lenses in their nondominant eye over at least 1 week and were then tested at each power. Participants then underwent CK in the same eye and were tested postoperatively at 1 week, 1 month and 3 months.

We found the mean near UCVA for the non-treated, dominant eye was unchanged through all stages of contact lenses and post-CK. Near vision for the treated eye and for both eyes together improved as the power of contact lenses increased, nearing 20/20 with the 2.5 D lens. Near UCVA at 1 week post-CK and at 1 month post-CK vision was on par with the 2.5 D contact lens (Figure 3).

	Figure 3: The mean near UCVA pre- and postoperatively in untreated and treated eyes.

The mean distance UCVA for the non-treated eye and both eyes together was maintained at approximately 20/20 with all contact len powers and post-CK. However, distance vision in the treated eye decreased from 20/20 pre-monovision to nearly 20/300 as the contact lens power increased (Figure 4).

	Figure 4: The mean distance UCVA pre- and postoperatively in untreated and treated eyes.

	Figure 5: Postoperative patient distance vision ratings.

Post CK, the distance vision was maintained between 20/60 and 20/30. Patients rated their distance vision as being worse with the 2.5-D lens (Figure 5). At 3 months post-CK, patients rated their distance vision equal to their pre-monovision distance vision. Similar results were reported in terms of visual clarity, halos, glare, haze and contrast sensitivity. The worst results were reported with the highest contact lens power and the best results were reported at 3 months post-CK, where vision was comparable to their pre-monovision quality. At 3 months post-CK, 100% of patients could read newspaper headlines, dashboards, computer screens, watches and cellular phones. Throughout post-CK follow-ups, 89% of patients could read medicine bottle labels, compared with 78% with the 2.5-D contact lens. Subjectively, every patient preferred CK to the contact lens. Thus, the important difference that was observed in the subjective and objective testing was that CK was able to obtain near vision equivalent to the 2.5-D contact lens with superior distance vision.

Theories behind CK optical results

– Daniel S. Durrie, MD

I have several theories on the reasons as to why CK offers no loss of depth perception and better distance vision than other monovision treatments. One possibility is that a CK-induced change in spherical aberration of the cornea better “matches” the age-induced change in the spherical aberration of the lens. The natural young lens has a negative spherical aberration, which it loses with age. The cornea has a positive spherical aberration. As the lens loses its negative spherical aberration, CK reduces the cornea’s positive spherical aberration to better “match” the aging lens. This “matching” may explain why distance vision, as well as near vision, improves in CK-treated patients. The improved distance vision in CK patients may also be attributed to mild residual accommodation or potential “good” aberrations induced by CK. Further research is needed to determine the specific mechanics of these changes in aberration and their effect on vision.

Conclusion

CK has been proven to be effective and safe,⁶ and vision improvements have been determined to last for at least 1 year and up to 26 months post-CK. Although CK cannot prevent age-related alterations in lenses and retinal function, patients often remain spectacle-free for up to 2 years. If spectacles become necessary, patients use them less frequently than preoperatively. Contrast sensitivity and depth perception are maintained post-CK and, unlike other monovision treatments, the procedure does not significantly affect distance vision while improving near vision in patients. Theories attempting to determine the causes of the differences in contrast sensitivity, depth perception and distance UCVA have been proposed, but more research is needed to determine the mechanics behind CK’s success in these areas vs. other monovision treatments.

References

Refractec, Inc. Available at www.refractec.com/US/150.asp. Accessed March 31, 2005.

Food and Drug Administration. ViewPoint CK System – P010018. Rockville, MD: Dept of Health and Human Services; April 11, 2002. Available at http://www.fda.gov/cdrh/pdf/P010018a.pdf. Accessed March 31, 2005.

Guttman, C. CK Blended Vision- a new approach to presbyopia. Available at www.escrs.org/eurotimes/December2003/CK_blended.asp. Accessed April 1, 2005.

Refractec, Inc. professional labeling. Available at www.fda.gov/cdrh/pdf/P010018s005c.pdf. Accessed April 25, 2005.

McDonald MB, Durrie D, Asbell P, Maloney R, Nichamin L. Treatment of presbyopia with conductive keratoplasty: six-month results of the 1-year United States FDA clinical trial. Cornea. 2004;23(7):661-668.

McDonald MB, Hersh PS, Manche EE, Maloney RK, Davidorf J, Sabry M. Conductive Keratoplasty United States Investigators Group. Conductive keratoplasty for the correction of low to moderate hyperopia: U.S. clinical trial 1-year results on 355 eyes. Ophthalmology. 2002;109:1978-1989.