| Abstract |
Chapter 1. Introduction.
The surgical correction of hyperopia and hyperopic astigmatism has always been a challenge for refractive surgeons. The efficacy, quality of vision and most im-portantly safety, provided by the existing laser surgical techniques are often not satis-factory.
On the other hand, thermokeratoplasty procedures are minimally invasive, do not require corneal tissue removal, and operate outside the central clear zone of the cornea.
The goal of conductive keratoplasty, which is a thermokeratoplasty technique, is to produce structural changes in the three-dimensional grating of collagen in the stroma of the cornea. These changes are achieved through the elevation of the tissue temperature. The increased tissue temperature is induced by electric impedance in the flow of energy through the collagen fibrils. Controlled-release radiofrequency energy (350 KHz) is delivered to the depth of 500 μm with the help of a Keratoplast tip ViewPoint™ CK (Refractec, Inc., Irvine, CA, USA). With the increase of the tissue temperature the collagen fibrils shrink by 30% of their original length. The application of the energy takes place in the periphery of the cornea where the increase of tempera-ture causes “corneal shrinkage” and flattening of the corneal surface, the fact that leads to a comparative increase of curvature in the central optically active part of the cornea.
This new technology is currently used for the correction of low to moderate hyperopia and most recently presbyopia in patients of 45 years and older. In the course of the current study conductive keratoplasty was used to primarily treat hyper-opic astigmatism through the application of an original astigmatic nomogram.
The current study reports on the results of conductive keratoplasty treatment of primary and secondary astigmatism trough the application of the original astigmatic nomogram, the safety profile of the new technique obtained trough human and animal histological studies, as well as the expansion of the existing nomogram for the spheri-cal corrections to higher degrees of hyperopia.
The purpose of the current thesis is to study the shrinkage of the corneal tissue with the means of conductive keratoplasty in order to produce safe, predictable and effective treatment of low to moderate hyperopia as well as hyperopic astigmatism.
In order to achieve this goal the technique was applied for the treatment of low to moderate hyperopia in individuals over 45 years of age. The safety of the treatment was evaluated through the histological studies of CK-treated human and rabbit cor-neas. The technique was applied for the treatment of higher levels of hyperopia due to the modification of the surgical technique as well as for the treatment of primary and secondary astigmatism.
Chapter 2. Review of the literature.
Hyperopia and hyperopic astigmatism can be corrected non-surgically with glasses or contact lenses or surgically with the means of refractive surgery procedures. Most of the surgical techniques proposed for the correction of hyperopia and hyper-opic astigmatism seem to be promising initially, but tend to be abandoned shortly due to multiple and serious complications
Examples of such procedures include keratophakia, hexagonal keratotomy and automated lamellar keratoplasty. The most common reasons for a surgical technique to be abandoned were the extensive trauma of the cornea, high incidence of irregular astigmatism, lack of efficacy, regression or the combination of the above. The newer surgical techniques for the correction of hyperopia and hyperopic astigmatism include arcuate keratotomy, photorefractive keratectomy and laser-assisted in situ keratomil-eusis. These newest versions are more effective and predictable, but also cause a number of serious complications such as infectious keratitis, diffuse lamellar keratitis, flap complications, epithelial ingrowth which could lead to irregular astigmatism, corneal ectasia, and dry eye. A decentered ablation could lead to irregular astigma-tism, regression of the refractive effect, low quality of vision, loss of best-corrected visual acuity. Lack of safety of the above procedures led to the quest for minimally invasive, safer, but effective and predictable techniques for the correction of hyper-opia and astigmatism. As a response to this need the laser thermokeratoplasty and conductive keratoplasty treatments evolved greatly.
Chapter 3. Patients and methods.
Experimental studies:
Α)The study of morphologic changes of human corneas induced by conductive kerato-plasty. The purpose of the study was to determine the morphologic changes in human corneas over time following radiofrequency-based conductive keratoplasty (CK) treatment. In this observational case series six human corneas of six patients (5 male and 1 female) with localized peripheral keratoconus underwent CK treatment fol-lowed by penetrating keratoplasty. Three spots were applied in the periphery of each cornea (6 mm optical zone). Corneal buttons were examined with light and electron microscopy at different postoperative intervals up to six months post-CK.
Β) Histological study of rabbit corneas following conductive keratoplasty application on the previously created corneal flap. After a flap creation in 4 right eyes of 4 rab-bits, 8 CK spots were applied in the optical zone of 7 mm. The application of the spots and the sacrifice of the rabbits occurred 1 month after the flap was created. The safety of this CK application over a previously created flap was studied with the means of light and electron microscopy.
C) Histological study of rabbit corneas with the light touch and standard CK surgical techniques. 19 right eyes of 19 rabbits were treated with 8 spots of conductive kerato-plasty in the 7 mm optical zone. In 10 eyes the surgical technique was performed with the standard touch and in 9 eyes with the light touch. The animals were followed for 6 months and sacrificed. The safety and stability of these two CK surgical techniques was studied with the means of light and electron microscopy.
Clinical studies:
Α) Correction of low to moderate hyperopia. The purpose of the study was to assess the safety, efficacy, predictability and stability of conductive keratoplasty for the treatment of low to moderate hyperopia with a follow-up of 30 month. In this prospec-tive, nonrandomized, noncontrolled single-center study 38 eyes of 26 patients (13 fe-male and 13 male) were treated for hyperopia (up to +3.25 D) with a Refractec View-Point™ CK system and followed for at least 30 months. All eyes were treated with the current conductive keratoplasty (CK) nomogram for the treatment of spherical hy-
peropia. The treatment consisted of 8 to 32 spots applied to the periphery of the cor-nea. Mean age was 50.3 ± 8.8 SD years (range 31-71).
Β) Correction of hyperopic astigmatism: The purpose of the study was to evaluate safety, efficacy, predictability and stability of conductive keratoplasty for the treat-ment of primary and secondary hyperopic astigmatism. In this prospective, nonran-domized, noncontrolled single-center study 47 eyes of 34 patients (15 female and 19 male) were treated for hyperopic astigmatism (up to + 3.50 D) with a Refractec ViewPoint™ CK system and followed for 24 ± 0.6 SD months. The treatment consis-ted of 4 to 36 spots applied to the periphery of the cornea. Mean age was 48.5 ± 9.7 SD years, range 25 to 68 years.
C) Changes of intraocular pressure following conductive keratoplasty.
The purpose of the protocol was to study the changes of intraocular pressure up to 11 months following a conductive keratoplasty treatment. 32 eyes of 18 patients (10 male and 8 female) were treated with conductive keratoplasty. The mean age of the group was 53.5 ± 9.7 SD (45 to 66) years. The operated eyes had hyperopia of + 1.00 to + 4.50 D. The treatment consisted of 8 - 32 spots in the optical zones of 6 - 8 mm.
Chapter 4. Development of the original nomogram for the correction of primary and secondary astigmatism with the means do conductive keratoplasty. This chapter includes a comparative analysis of 4 nomogram types suggested for the cor-rection of hyperopic astigmatism. The analysis describes the pros and cons of all the types of the astigmatic nomogram. The most effective and predictable nomogram is selected to be tested and applied for the correction of hyperopic astigmatism.
Chapter 5. Results.
A) The study of morphologic changes of human corneas induced by conductive kera-toplasty. In samples assessed on days one and three post-CK, small areas of detach-ment between the basal layer of epithelial cells and Bowman’s layer were observed. At one week after the CK procedure the epithelium appeared almost normal. Endothe-lium and Descemet’s membrane had no alterations. In all samples thermally induced misconfiguration of collagen fibers, described as “crumpled” changes of collagen lay-ers, was observed reaching 75-80% of the stromal depth. The area of alterations had a cylindrical shape with a diameter of 120 microns.
B) Histological study of rabbit corneas following conductive keratoplasty application on the previously created corneal flap. The application of CK spots over the previ-ously created corneal flap did not cause shrinkage, oedema, detachment or misplace-ment of the flap. He condition of the Descemet’s membrane and endothelium beneath the treated area was normal.
C) Histological study of rabbit corneas with the light touch and standard CK surgical techniques We observed more pronounced and deeper changes of collagen within the corneal stroma with the standard CK technique. The depth of the alterations with the standard technique was 330 μm vs. 300 μm with the light-touch CK technique. The spot diameter was larger with the light-touch technique compared to the standard technique (120 μm vs. 110 μm).
D) Correction of low to moderate hyperopia.
Preoperatively, mean manifest refraction spherical equivalent (MRSE) was +1.89 D ± 0.6 SD (range +1.00 to +3.25 D), at 12 months it was -0.06 D ± 0.8 SD and -0.02 D ± 0.7 SD at 30 months. At 30 months mean MRSE was within ± 0.50 D in 68 %, within ± 1.00 D in 92 % and within ± 2.00 D in all eyes. At 30 months uncorrected visual acuity was 20/20 or better in 52.5 % and 20/40 or better in 89 % of the eyes. No eye lost ≥ 2 Snellen lines nor had an induced cylinder ≥ 2.00 D.
E) Correction of hyperopic astigmatism:
Preoperatively, mean manifest refraction spherical equivalent (MRSE) was + 2.11 D ± 0.88 SD (range - 0.50 to + 4.13 D), at 12 months it was - 0.52 D ± 0.73 SD, and - 0.50 D ± 0.77 SD at 24 months. At 24 months mean MRSE was within ± 0.50 D in 61 %, within ± 1.00 D in 83 % and within ± 2.00 D in all eyes. At 24 months uncorrected visual acuity was 20/20 or better in 37 % and 20/40 or better in 93 % of the eyes. By the end of the follow-up no eye lost ≥ 2 Snellen lines nor had an induced cylinder ≥ 1.50 D.
F) Changes of intraocular pressure following conductive keratoplasty.
We observed a statistically significant reduction (p < .001) of the measured intraocu-lar pressure following conductive keratoplasty. Preoperatively the intraocular pressure was measured 14.22 ± 1.64 mm Hg. At 11 months the intraocular pressure was meas-ured 12.66 ± 2.21 mm Hg. There was no correlation of the observed intraocular pres-sure changes with the age, gender or attempted correction and keratometry readings of the patients.
Chapter 6. Discussion of the results and conclusions.
In the course of the study an original nomogram for the treatment of hyperopic astigmatism with the means of conductive keratoplasty was developed and applied. The suggested nomogram provided effective, predictable and stable correction of hy-peropic astigmatism. The safety of conductive keratoplasty as observed within the his-tological studies of human and animal corneas was often higher than with the excimer laser treatments. The technique can be safely used over a previously created LASIK flap. Conductive keratoplasty can be used for the treatments of higher hyperopias with the light-touch surgical technique, as proved with a relevant histological study.
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