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BOOK EXCERPT
A Complete Surgical Guide for Correcting Astigmatism
James P. Gills, M.D.

CHAPTER THREE
Treating Astigmatism With Conductive Keratoplasty
Jason E. Stahl, MD; and Daniel S. Durrie, MD

Introduction

In 1898, Lans published animal experiments documenting that heating the peripheral cornea could induce central steepening.1 His findings were supported by others in the early 20th century.2-4 The result of these reports helped demonstrate that it was possible to surgically alter the shape of the astigmatic cornea by steepening the flat axis by shrinking the collagen of the cornea with heat, a procedure called thermal keratoplasty.

In 1964, Stringer and Parr5 reported that the disintegration of the helical structure of corneal collagen occurs when heated to approximately 55° to 58° C. This breakdown in the collagen structure allows the individual collagen fibers to contract by as much as one third of their original size. This contraction results in localized shrinking of collagen in the areas where heat is applied, thereby reshaping of the corneal curvature. On this basis, application of heat to the peripheral cornea would induce steepening of the central cornea.

Several thermal keratoplasty techniques for shrinking peripheral corneal collagen have been attempted to steepen the central cornea for refractive indications. In the 1980s, Fyodorov and collaborators evaluated the use of a wire heated up to 600° C and inserted into the cornea at up to 95% depth to treat hyperopia.6-9 The extreme temperatures and non-uniform heating of the corneal stroma resulted in stromal necrosis, poor predictability, and substantial regression of the initial effect. 10-11

Mainster12 theorized that laser thermal keratoplasty (LTK) would deliver heat to the stroma in a more uniform fashion with less damage to the epithelium and endothelium. In the early 1990s, the first contact probe holmium yttrium-aluminum-garnet (Ho:YAG) LTK procedure (Summit Technology, Waltham, Mass) was developed.13 FDA clinical trials of Summit contact LTK demonstrated poor predictability and regression of effect. Non-contact Ho:YAG LTK (Sunrise Technologies, Fremont, Calif) was approved for the treatment of hyperopia in 2000. The initial overcorrection and significant regression of effect have been problematic for this technique. Diode LTK (Prolaser Medical Systems, Inc, Dusseldorf, Germany) has demonstrated effective ness to steepen the cornea in animal, cadaver, and blind eyes.14-16

Conductive keratoplasty, or CK (Refractec, Inc, Irvine, Calif), is a new thermokeratoplasty technology that received FDA approval in April 2002 for the correction of 0.75 D to 3.00 D of hyperopia. CK utilizes radiofrequency waves to shrink collagen and is currently under investigation for the treatment of astigmatism.

Figure 3-1
Figure 3-1. ViewPoint CK system: Console, probe, and specula. Photo courtesy of Refractec, Inc.

Figure 3-2
Figure 3-2. CK probe inserted into previously placed corneal marks on the midperipheral cornea. Photo courtesy of Refractec, Inc.

Figure 3-3
Figure 3-3. Nomogram for the correction of 0.75 D to 3.0 D of astigmatism. The nomogram is indicated within each circle with the sequence application shown on the right.

Conductive Keratoplasty

CK, developed by Mendez,17 is a laserless thermokeratoplasty technique that uses radiofrequency energy to shrink stromal collagen. The ViewPoint CK system (Figure 3-1) (Refractec, Inc, Irvine, Calif) uses a probe (Figure 3-2) to deliver low energy, high frequency (350 kHz) current directly into the midperipheral corneal stroma at eight to 32 treatment spots (Figure 3-3). Striae form between the spots (Figures 3-4a and 3-4b), producing a band of tightening that increases the central corneal curvature, thereby reducing spherical hyperopia (Figure 3-5).

Figure3-4a
Figure 3-4a. Slit lamp photo of CK spots with striae.

Figure 3-4b
Figure 3-4b. Slit lamp photo of CK spots with striae.

Figure 3-5
Figure 3-5. CK produces a band of tightening that increases the central corneal curvature reducing hyperopia.

Figure 3-6
Figure 3-6. The CK probe with Keratoplast tip (90 µm wide, 450 µm long) with coated stop at the distal end (shown next to a 7-0 suture). Photo courtesy Refractec, Inc.

CK utilizes the unique conductive properties of the cornea. As the current flows through the tip, the surrounding corneal tissue creates resistance to the energy resulting in gentle, well-controlled homogenous heat production. The stainless steel Keratoplast tip (Refractec, Inc, Irvine, Calif) (Figure 3-6), with a diameter of 90 µm and length of 450 µm, penetrates the cornea, delivering the current equally from corneal surface to the end of the tip. The collagen surrounding the entire length of the tip is exposed to the same temperature, creating a column or cylindrical footprint that extends deep into the stroma at approximately 80% depth (Figures 3-7a and 3-7b).

Figure3-7a Figure 3-7b

Figure 3-7 a & b. Pig cornea histology 1 week after treatment shows a CK spot with a cylindrical footprint approximately 80% of corneal thickness.

In contrast, the Ho:YAG LTK procedures apply pulsed infrared energy onto the corneal surface producing an axial gradient with the highest temperatures at the corneal surface. These techniques result in a cone-shaped footprint13,18 at only approximately 50% of corneal depth.18 The leading criticism of the Sunrise LTK technique has been the regression of the initial effect. A comparison of stability values for CK and Sunrise LTK suggests that less regression occurs with CK and that stability is achieved earlier with CK.19 These results may be attributed to the deeper cylindrical collagen shrinkage that is achieved with CK.

The shape and depth of the footprint of collagen shrinkage techniques appears to be extremely important for the long-term stability of these procedures. We were interested in evaluating the CK and LTK footprints in patients 12 months after surgery. We performed ultrasound biomicroscopy (UBM) studies on these patients 1 year after CK and LTK. The UBM of the CK spots (Figure 3-8) shows a deep cylindrical footprint while the LTK spots (Figure 3-9) show a shallow conical footprint.

Figure 3-8
Figure 3-8. UBM of CK spot 12 months postoperative with the arrow indicating the deep cylindrical footprint.

Figure 3-9
Figure 3-9. UBM of LTK spot 12 months postoperative with arrow indicating the shallow conical footprint.

The FDA has approved CK for the temporary reduction of spherical hyperopia in patients who have 0.75 D to 3.25 D of cycloplegic spherical hyperopia with less than 0.75 D of refractive astigmatism (minus cylinder) and a cycloplegic spherical equivalent of +0.75 D to +3.00 D. Patients must be 40 years of age or older with a documented stability of refraction for the prior 12 months, as demonstrated by a change of less than 0.50 D in spherical and cylindrical components of manifest refraction.

Potential off-label applications of the CK technology include the treatment of astigmatism, presbyopia, over- or undercorrections following photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK), and finally refining post-cataract outcomes.

Treating Astigmatism With Thermokeratoplasty

The principles of astigmatic keratotomy (AK) are well known to corneal surgeons. AK incisions placed across the steep corneal meridian (paralleling the axis of plus-cylinder refraction) causes relaxation of the collagen that results in central flattening of that meridian and steepening of the unincised meridian 90 degrees away. These incisions weaken the cornea leaving it susceptible to rupture with trauma.

Compression sutures placed in the flat meridian (paralleling the axis of minus-cylinder refraction) demonstrate that tightening collagen causes central steepening of that meridian. Theoretically, a thermokeratoplasty procedure that places treatment spots on the flat meridian would correct astigmatism by tightening the collagen, resulting in central steepening of that meridian.

Holmium:YAG laser thermokeratoplasty for the correction of astigmatism has been evaluated by Thompson.20 He reported the 6-month results of the FDA phase 1 clinical trial for the correction of astigmatism from 1.5 D to 4 D with the Summit contact LTK device. Twenty-six eyes were treated with two LTK spots placed on each side of the flat meridian at the 8.5-mm optical zone. The astigmatic correction ranged from 0.4 D to 3.98 D with a mean correction of 1.69 D. Thompson noted a trend toward greater astigmatic correction with increasing age and toward a myopic shift in the spherical equivalent. He reported the regression of astigmatic LTK was greater than that seen with hyperopic corrections. We speculate that the regression of effect was likely related to the cone-shaped footprint produced by the LTK procedure.

Treating Astigmatism With Conductive Keratoplasty

There is great interest in utilizing CK technology for the treatment of astigmatism. The deep penetrating cylindrical footprint of CK may result in a more predictable astigmatic correction than previously attempted thermokeratoplasty techniques. We have recently started an institiution review board-approved prospective clinical trial on CK for the treatment of astigmatism with and without hyperopia. We would like to share our initial thoughts on four different strategies for using CK for the correction of astigmatism alone and hyperopia with astigmatism.

Preoperative Refraction and/or Topography

Evaluation of the preoperative refraction and/or topography identifies the flat meridian and helps to formulate a “game plan” for surgery. When treating hyperopia and astigmatism, it is advisable to rotate the corneal marks for the hyperopic part of the treatment such that the flat meridian is left unmarked. These open areas will be used for placing additional spots for the astigmatic correction. The placement of the spots for astigmatic correction may be made at different optical zones with increasing astigmatic effect as the spots are placed more centrally. Nomograms for the correction of astigmatism that instruct on the number of spots and their optical zone placement have yet to be developed and will require a large, controlled study to analyze the many variables.

Intraoperative Keratometry

The second approach is to enter the operating room with a general understanding of the astigmatism and proceed to use intraoperative qualitative keratometry to decide the location of additional spots for astigmatism. Based on the refraction and topography, it is advisable to consider where you might add spots for astigmatic correction and leave these areas open when placing your marks as previously stated above. Using a qualitative keratometer, such as the Mastel ring (Mastel Precision, Rapid City, SD) or something as simple as the round end of a safety pin, produces a qualitative ring. The surgeon uses this ring to assess if there is significant astigmatism present at the end of the procedure. The ring will be oval (astigmatic) (Figures 3-10a and b) if significant astigmatism is present and we place additional spots (Figures 3-11a and b) in the flat meridian until the ring is round (spherical). The photo in Figure 3-12 shows a cornea that was treated with CK for mixed astigmatism. It is important to make sure that the lid speculum is not causing induced astigmatism that would influence the intraoperative keratometry results.

Figure 3-10a Figure 3-10b

Figures 3-10a and b. Intraoperative keratometry (A) with correlating diagram (B). The oval ring indicates astigmatism.
Figure 3-11a Figure 3-11b

Figures 3-11a and b. Intraoperative keratometry with correlating diagram shows a spherical ring after astigmatism treated with CK spots placed on the flat meridian.

Case Presentation

The intraoperative keratometry technique was used to treat a 48-year-old female. Her uncorrected visual acuity was 20/50 with a refraction of +2.50 -2.00 x 47, and a best-corrected visual acuity of 20/20. She underwent CK with eight treatment spots placed at the 7-mm optical zone and two additional treatment spots placed to produce a round keratometry ring. Her preoperative Orbscan (Bausch & Lomb, Salt Lake City, Utah) topography reveals the astigmatism with the flat meridian at approximately 45 degrees. Figures 3-13a and b show the treatment pattern placed on top of the pre- and postoperative Orbscan topography to demonstrate the placement of the spots to correct the astigmatism by steepening the flat meridian. Her uncorrected visual acuity is 20/20 with a refraction of +0.25 D sphere. Longer follow-up is needed to determine stability.

Figure 3-12
Figure 3-12. Clinical photo of a patient treated with CK spots placed on flat meridian for the correction of mixed astigmatism.

Postoperative Keratometry and/or Topography

A third approach to correcting astigmatism with CK is to initially perform a symmetrical hyperopic correction based on the preoperative refraction and topography. At the end of the procedure the patient is taken for either postoperative keratometry or topography readings. Based on this postoperative information, the patient is taken back to the operating room for additional spots for astigmatic correction.

We have not found the immediate postoperative topography to be useful. During the FDA hyperopia clinical trials we performed Orbscan topography 1 hour after surgery on all patients. We did not correlate the immediate postoperative Orbscan topography with postoperative induced astigmatism in these patients. In contrast, using immediate postoperative keratometry to guide astigmatic treatment has been reported to be a useful technique (Marguerite McDonald, MD, personal communication).

Figure 3-13a
Figure 3-13a. Preoperative Orbscan topography demonstrating spot pattern to treat +2.50 –2.00 x 047. The arrows show the two spots that were added on the flat meridian using intraoperative keratometry to correct astigmatism.

Figure 3-13b
Figure 3-13b. Note the spherical central steepening and large optical zone in the postoperative topography.

Delayed Correction of Astigmatism

When treating hyperopia and astigmatism, another option is to initially perform only the symmetrical hyperopia treatment initially. At approximately the 1-month examination, treatment spots can be added in clinic at the slit lamp to correct residual astigmatism. Patients usually experience minimal postoperative discomfort with the CK procedure, which makes adding one or two spots during a follow-up visit quite easy for the patient. The postoperative refraction, topography, and keratometry help guide the placement of these additional spots.

Conclusion

CK is an emerging thermokeratoplasty technology that recently received approval for the treatment of mild to moderate spherical hyperopia. Early studies suggest that CK can be used to correct astigmatism by steepening the flat meridian. Controlled clinical trials will need to be conducted to study the predictability and stability of CK for the correction of astigmatism. Our CK astigmatism clinical trial is ongoing and we look forward to presenting further results in the near future.

References

  1. Lans L. Experimentelle untersuchungen über die entstehung von astigmatismus durch nicht perforirende corneawunden. Albrecht von Graefes Arch Klin Exp Ophthalmol. 1898;45:117-152.
  2. Terrien F. Dystrophic marginale symmétrique des deux corneés avec astigmatisme regular. Consecutif et guerison par la cauterization ignee. Arch Ophthalmol (Paris). 1900;20:12.
  3. Wray C. Case of 6 D of hypermetropic astigmatism cured by the cautery. Trans Ophthalmol Soc UK. 1914;34:109-110.
  4. O’Connor R. Corneal cautery for high myopic astigmatism. Am J Ophthalmol. 1933;16:337.
  5. Stringer H, Parr J. Shrinkage temperature of eye collagen. Nature. 1964;204:1307.
  6. Caster AI. The Fyodorov technique of hyperopia correction by thermal coagulation: a preliminary report. J Refract Surg. 1988;4:105-108.
  7. Neumann A, Sanders D, Salz J. Refract Corneal Surg. 1989;5:50-54.
  8. Neumann A, Fyodorov S, Sanders D. Radial thermokeratoplasty for the correction of hyperopia. Refract Corneal Surg. 1990;6:404-412.
  9. Neumann A, Sanders D, Raanan M, DeLuca M. Hyperopic thermokeratoplasty: clinical evaluation. J Cataract Refract Surg. 1991;17:830-838.
  10. Feldman ST, Ellis W, Frucht-Pery J, Chayet A, Brown SI. Regression of effect following radial thermokeratoplasty in humans. Refract Corneal Surg. 1989;5:288-291.
  11. Thompson VM, Seiler T, Durrie DS, Cavanaugh TB. Holmium:Yag laser thermokeratoplasty for hyperopia and astigmatism: an overview. Refract Corneal Surg. 1993;9(2 Suppl):S134-S137.
  12. Mainster MA. Ophthalmic applications of infrared lasers–thermal considerations. Invest Ophthalmol Vis Sci. 1979;18:414-420.
  13. Seiler T, Matallana M, Bende T. Laser thermokeratoplasty by means of a pulsed holmium:YAG laser for hyperopic correction. Refract Corneal Surg. 1990;6:335-339.
  14. Brinkmann R, Koop N, Geerling G, et al. Diode laser thermokeratoplasty: application strategy and dosimetry. J Cataract Refract Surg. 1998;24:1195-1207.
  15. Bende T, Jean B, Oltrup T. Laser thermal keratoplasty using a continuous wave diode laser. J Refract Surg. 1999;15:154-158.
  16. Geerling G, Koop N, Brinkmann R, et al. Continuous-wave diode laser thermokeratoplasty: first clinical experience in blind human eyes. J Cataract Refract Surg. 1999;25:32-40.
  17. Mendez A, Mendez AN. Conductive keratoplasty for the correction of hyperopia. In: Sher NA, ed. Surgery for Hyperopia and Presbyopia. Baltimore, Md: Williams & Wilkins; 1997:163-171.
  18. Koch DD, Kohnen T, Anderson JA, et al. Histologic changes and wound healing response following 10 pulse noncontact holmium:YAG laser thermal keratoplasty. J Refract Surg. 1996:12:623-634.
  19. McDonald M, Hersh PS, Manche EE, Maloney RK, Davidorf J, Sabry M. Conductive keratoplasty for the correction of low to moderate hyperopia: 1-year results on the first 54 eyes. Ophthalmology. 2002;109:637-649.
  20. Thompson VM. Holmium:YAG laser thermokeratoplasty for correction of astigmatism. Refract Corneal Surg. 1994;10:293.

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