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Conductive keratoplasty: versatile, stable procedure for increasing corneal curvature
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Yet another new refractive surgery technology has emerged, and it may be close to Food and Drug Administration (FDA) approval. The ViewPoint CK System, being developed by Refractec, is considered a collagen shrinkage procedure for correcting hyperopia. Conductive keratoplasty (CK) may be one of the most versatile and stable procedures for increasing corneal curvature.
The device
---Conductive keratoplasty: The tip of the probe is introduced perpendicularly into the corneal stroma at each point. Because the tip is surrounded by a collar or stop, it penetrates only 450 µm at each spot, which is approximately 80% of the peripheral stromal depth.
The ViewPoint CK System is an advanced, non-laser approach for the potential treatment of hyperopia, presbyopia and astigmatism. Phase 3 clinical trials for spherical hyperopia have been underway since February 1999, and FDA approval is anticipated this year. The system includes a micro-thin probe 450 µm in length and thinner than a human hair. The probe is disposable and is attached to a handle about the size of a small laser pointer. This handle is connected to a power source that provides current to the tip and allows for a portable unit.
Procedure
Eight well-defined points are indicated on the patient’s eye with a placement marker along the circumference of a circle around the peripheral cornea. The tip is introduced perpendicularly into the corneal stroma at each point. Because the tip is surrounded by a collar or stop, it penetrates only 450 µm at each spot, which is approximately 80% of the peripheral stromal depth.
Once the tip is inserted, a controlled release of radio-frequency energy is delivered. Stromal collagen fibrils reach an optimal temperature, which in turn shrinks the surrounding tissue. As the tip introduces energy, which in turn heats the corneal tissue, the tip itself remains cool. The cylindrical footprint that remains is very consistent from point to point along the circular treatment zone.
Eight treatment spots are placed along the periphery of the cornea at a diameter of 6 mm, 7 mm, 8 mm or all three diameters depending on the correction needed. Striae develop between treatment spots, creating a band of tightening. Similar to the purse string analogy, this band of tightening is what causes the central cornea to steepen. Investigational studies indicate this procedure can correct up to 3 D of hyperopia.
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Patient experience
Immediately after the procedure, patients experience some minor foreign-body sensation, which subsides within the first 24 hours. As with other hyperopic procedures, there is an initial overshoot of refractive effect of less than .75 D, leaving the patient slightly more myopic than the final target. By month 1, the patient should be within 0.50 D of his or her intended correction.
Present studies show no difference in effectiveness in patients requiring a +0.75 D correction compared with those requiring a +3.00 D correction. There also seems to be no difference in effect between men and women.
Although anecdotal at this point, the procedure does not appear to disrupt corneal nerve integrity, which is often the case with LASIK procedures. Additionally, ocular dryness is minimal, and most patients do not experience dryness after 1 week postop.
Longevity/stability
Based on present clinical results, we feel this procedure will be more stable and predictable than laser thermokeratoplasty. The greater stability of the CK procedure and the minimal initial overshoot, when compared to other collagen shrinkage procedures, is probably due to two factors.
First, CK provides a more optimal temperature for permanent collagen shrinkage. If temperatures in the stroma do not reach an optimal level, the collagen will rehydrate and regain its original configuration. This would result in a regression of effect. Only when the optimal temperature is reached is permanent collagen shrinkage obtained. If the temperature is too high the collagen starts to melt and relax and it may necrose. Scarring and regression again may occur.
The other factor is the homogeneity of the energy delivered. The energy is delivered uniformly throughout the column because the probe is inserted into the stroma. With laser thermal procedures, greater energy is used for greater effects and each spot has a gradient heat effect from the surface inward; therefore, varying temperatures occur throughout the stromal matrix. With CK, uniform collagen shrinkage occurs up to 80% of the stromal depth at each point. This may also contribute to the consistency and stability of the procedure.
Other positive aspects of CK
Many aspects of CK make it an appealing procedure and solution for hyperopic corrections. Because of the nature of the procedure and the minimal equipment involved, CK can be performed in a minor surgical suite or even at a slit lamp. The CK system device is by far much less expensive to purchase, operate and maintain than laser devices. The safety of the LASIK suction ring on the eye has been discussed. Studies are not clear on the possible negative effects of this situation. Because no suction ring is placed on the eye with CK, possible risks associated with increased intraocular pressure are reduced.
Eye movement is of no concern with CK because the surgeon controls probe placement. Untouched central cornea increases safety and decreases the chance of lost contrast sensitivity. Because there is less nerve disruption with CK, patients do not experience typical post-procedure dry eye often seen with lamellar procedures.
After further studies, CK could potentially be used for other applications. With select placement of spots, corneal steepening could be performed along one specific axis for astigmatic corrections. Low levels of cylinder correction sometimes are needed postoperatively in cataract patients or after other types of refractive surgery. This would allow for a simple in-office enhancement procedure without flattening a corneal axis. CK could also be performed for overcorrections in LASIK, photorefractive keratectomy and cataract surgery.
Helping your patients make a well-informed decision about refractive surgery is only possible when the knowledge you have as a clinician is complete. CK offers a safe, stable and accurate option for correcting hyperopia and presbyopia. We believe that once the FDA approves this technology many surgeons will incorporate it into their refractive surgery armamentarium not only for low levels of hyperopia, but for many other future applications.
For Your Information:
- Steven H. Linn, OD, is the refractive clinic director at Hoopes Vision Correction Center in Salt Lake City and was recently educational director at Hunkeler Eye Centers in Kansas City. Dr. Linn can be reached at Hoopes Vision Correction Center, 10011 S. Centennial Pkwy., Suite 400, Sandy, UT 84070; (801) 568-0200; fax: (801) 563-0200. Dr. Linn has no direct financial interest in the products mentioned in this article. He is a paid consultant for Refractec.
- Paul M. Karpecki, OD, is director of research for the Novamed/Hunkeler Eye Study Center and is the clinical director of cornea and refractive surgery for the Hunkeler Eye Center. He is also the residency director of the Corneal and Refractive Surgery Residency fellowship program affiliated with the Pennsylvania College of Optometry and a faculty member of the Kansas University Department of Ophthalmology, where he heads the refractive surgery clinic program for residents. He may be contacted at The Eye Center of Kansas City, 5520 College Blvd., Overland Park, KS 66211; (816) 931-4733; fax: (816) 931-9498; e-mail: pkarpecki@novamed.com. Dr. Karpecki has no direct financial interest in the products mentioned in this article. He is a paid consultant for Refractec.
- Refractec can be contacted at 3 Jenner, Suite 140, Irvine, CA 92618; (949) 784-2600; (800) 752-9544; fax: (949) 784-2601; Web site: www.refractec.com.