Minimal essential standards for reporting studies of refractive surgical procedures.
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Humans↗
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Development and evaluation of refractive surgical procedures.
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Clinical Trials as Topic↗
Surgical procedures performed after refractive surgery.
BACKGROUND: Refractive surgical procedures have been performed for over one decade. The complications that cause visual disability are abnormalities in wound healing, residual ametropia, and/or severe irregular astigmatism. Many of these complications can be manifested clinically in terms of visually disabling problems (loss of best-corrected vision, glare, monocular diplopia, etc). Some of these complications require additional standard ophthalmic or refractive surgical procedures to restore vision. However, since these secondary procedures have been performed in only a few cases, we known little about their outcome. METHODS: We report the histopathologic analysis of 132 cases that have undergone a secondary surgical procedure following a primary keratorefractive procedure. We attempt to correlate the problems encountered with such secondary intervention with histopathologic information obtained from specimens that have been submitted to our pathology laboratory and that had undergone secondary surgical procedures. RESULTS: Many specimens displayed abnormalities in wound healing associated with visual difficulties such as loss of best corrected acuity, glare, and under- and overcorrection. A short time from the initial procedure to the time of secondary intervention was common. CONCLUSIONS: The combination of a secondary surgical or keratorefractive procedure can produce unpredictable refractive results. Judicious application of a refractive procedure and an appropriate time period before a second procedure is performed may reduce unexpected refractive complications.
Adult↗
Physiologic analysis of corneal healing after epikeratophakia.
The evaluation of epithelial permeability, stromal transparency, and endothelial cell density is essential to determine the resumption of normal function in each corneal physiologic unit after refractive surgical procedures. The authors report the results of a prospective study conducted in 55 consecutive patients undergoing epikeratophakia using prelathed, lyophilized tissue lenses. Epithelial permeability was evaluated by means of fluorophotometry preoperatively and 1, 2, 4, 8, 12, 24, and 52 weeks after epikeratophakia. With few exceptions, Scheimpflug photography also was performed at the same examination times to assess stromal optical density. Endothelial cell counts were performed in each patient preoperatively and between 6 and 12 months postoperatively. The epithelial barrier function resumed normal values within 8 weeks after epikeratophakia. The optical density of both donor lenticule and recipient corneas was initially increased but returned to values comparable with those of unoperated corneas by 12 weeks postoperatively. Endothelial cell density was not affected by epikeratophakia. These results confirm the authors' preliminary observation that epikeratophakia allows a relatively quick recovery of normal corneal functions and should prompt investigators to demonstrate the safety of other refractive surgical procedures in a similar way.
Aged↗
On the computer-aided and optimal design of keratorefractive surgery.
BACKGROUND: Several recent papers have discussed the use of engineering-based computer methods for the analysis of keratorefractive surgical procedures. What has been lacking is a broader view of the role of engineering analysis in keratorefractive surgery. This article demonstrates how these various analysis methods can be coupled to provide a comprehensive methodology for the design of refractive surgical procedures. METHODS: A structural model of the eye, based on a linearly elastic, transversely isotropic finite element formulation is coupled to a full-eye optical model. The optical errors due to refractive keratotomy are estimated by ray tracing through the optical model and measuring the position of the resulting focal plane relative to the retina. Computer-based optimization methods are employed to determine the surgical parameters necessary to correct myopia for a given set of surgical design goals. RESULTS: Results based on a hypothetical eye demonstrate agreement with clinical trends. Radial keratotomies are designed that eliminate refractive error while minimizing invasiveness in one case and maximizing the optical zone size in another. It is also shown that there is significant potential to customize this process on a patient-by-patient basis using clinically measured data. CONCLUSIONS: We present an overview of the research necessary to bring this approach to fruition. While only a first step, the methodology presented in this article has the potential to increase the predictability of keratorefractive surgery by substantially increasing both the quality and the quantity of the information available to the refractive surgeon preoperatively.
Biomechanical Phenomena↗
Current applications of the Corneal Modeling System.
The ability of the Corneal Modeling System to incorporate curvature and thickness data into a single mathematical construct of the cornea is an important achievement. As experience with this and similar instruments is gained, a number of important issues in corneal topography can be addressed. Snellen visual acuity, refraction, and keratometric changes do not fully reveal the effects of refractive surgical procedures. Detailed analysis of corneal topography will prove necessary to understanding the quality of vision resulting from surgery, as it can explain the disparity between uncorrected visual acuity and residual refractive error after radial keratotomy. Computer-assisted topographic measurement devices will undoubtedly play a particularly important role in the design and evaluation of current and future refractive procedures. Similarly, mild topographic changes (eg, subclinical keratoconus) can be detected that are not apparent with refraction, biomicroscopy, or keratometry. The author reviews his personal experience with one instrument.
Astigmatism↗
One-year results of excimer laser photorefractive keratectomy for myopia.
BACKGROUND: Excimer laser photorefractive keratectomy for the correction of myopia is presently under investigation in the United States by the Food and Drug Administration (FDA). The Phase II-B FDA study is being conducted on 75 normally sighted myopic eyes utilizing three currently available excimer lasers. This report presents the 1-year results on 12 myopic eyes treated with the VISX excimer laser system at the Ellis Eye Center at Cedars-Sinai Medical Center in Los Angeles under the Phase II-B FDA protocol. METHODS: Twelve eyes of 12 patients with myopia between -1.75 and -5.00 diopters underwent 193 nm argon/fluoride excimer laser photorefractive keratectomy. The epithelium was mechanically removed, and fixation was accomplished with a suction ring which provided nitrogen flow across the corneal surface. The computer controlled corneal ablations were 5.00 mm in diameter and were accomplished with an iris diaphragm closing from large to small. RESULTS: The preoperative spherical equivalent myopia was -3.50 D (SD = 1.02) and the postoperative myopia was -0.25 (SD = 0.48). Eleven of the 12 patients achieved an uncorrected visual acuity of 20/30 or better and were corrected to within +/- 0.50 D of emmetropia. All corneas demonstrated a mild reticular subepithelial haze which was barely visible at 1 year. There were no vision-threatening complications and none of the eyes experienced a loss of best corrected visual acuity. CONCLUSIONS: In this small trial, the excimer laser appears to be capable of accurately changing the refractive power of the cornea for the correction of myopia with minimal side effects. Only when larger numbers of patients undergo the procedure will we be able to determine the safety and efficacy of photorefractive keratectomy as a refractive surgical procedure.
Adult↗
Prospective evaluation of radial keratotomy. Photokeratoscope corneal topography.
Preoperative and postoperative corneascope photographs of 368 myopic patients undergoing radial keratotomy in the Prospective Evaluation of Radial Keratotomy (PERK) study were optically scanned and digitized. A high-resolution scanning system was developed in order to quantify the preoperative and postoperative corneal shape accurately. Careful analysis of the 72 data points in the nine representative rings demonstrated that corneal topography is best represented by radius of curvature from the center to the periphery. The normal myopic cornea flattens approximately +0.28 mm from the center to the periphery, demonstrating the cornea's aspheric nature. More highly myopic patients in the PERK population (-4.50 to -8.00 diopters [D]) demonstrated corneas that are 0.08 to 0.10 mm steeper than the less myopic population (-2.00 to -3.12 D). Optical zone, patient age, and gender are all correlated to changes in corneal topography after radial keratotomy. In more myopic populations, men have corneas which are flatter than those of women by 0.09 to 0.11 mm in all rings represented on corneoscopy. Highly myopic males also experience more corneal flattening after 3.0-mm optical zone radial keratotomy. Regardless of the optical zone used in radial keratotomy, the resulting corneal topography flattens in all rings. However, the ratio of millimeters of radius of curvature change to diopters of correction is consistent for each ring. The dioptric change observed after radial keratotomy corresponds closely with the millimeters of flattening at the respective rings being examined. The central rings flatten 0.166-mm radius of curvature per diopter of refractive alteration obtained. The largest degree of corneal flattening occurs centrally, 0.72 mm, in the more highly myopic patients who underwent 3-mm optical zone radial keratotomy. The use of smaller optical zones in radial keratotomy produces larger changes in the radius of curvature and, consequently, in the amount of refraction than when larger optical zones are used. When compared with younger patients, older patients with 3.0, 3.5, and 4.0 optical zone radial keratotomies experience more central and peripheral corneal flattening. This study of the corneal topography of the myopic population demonstrates that the refractive change resulting from radial keratotomy is related to alterations in corneal topography. The use of similar modifications of the corneal surface may be effective for newer refractive surgical procedures.
Adult↗
Complications of radial and transverse keratotomy.
Radial keratotomy for myopia and transverse keratotomy for astigmatism are the most commonly performed refractive surgical procedures. A decade of experience with modern techniques has produced considerable literature on the complications of keratotomy. Vision-threatening complications (bacterial keratitis, traumatic rupture of the globe through weakened keratotomy scars, endophthalmitis, cataract formation from surgical trauma to the lens) are quite rare, occurring in less than 1% of eyes in published series. The most common side effects affect most patients in the first few months after surgery: pain for 24 to 48 hours, transient glare and light sensitivity, and fluctuating visual acuity. The most common persistent complications are overcorrection and undercorrection. Persistent irregular astigmatism occurs in almost all cases in the region of the incision scars, but it is rarely severe enough to reduce spectacle acuity. Most individuals have mild glare, but this is rarely disabling. Diurnal variation of refraction in visual acuity occurs commonly, but the magnitude of the fluctuation is seldom enough to require multiple pairs of spectacles. Longterm refractive stability occurs in approximately half of eyes by six months, but approximately one in four eyes will experience continued change over six months to four years. Complications, such as scarring from intersecting keratotomy incisions, irregular astigmatism resulting from multiple reoperations, and overcorrections with the attendant early onset of symptomatic presbyopia are becoming much less frequent.
Contact Lenses↗
Gel injection adjustable keratoplasty.
Gel injection adjustable keratoplasty (GIAK) is a new refractive surgical procedure designed for the correction of myopia by injection of a gel substance into the peripheral corneal stroma. This paper describes the GIAK technique and reports the results obtained in 21 fresh cadaveric eyes using the procedure. After a deep interlamellar canal has been dissected with a helicoid spatula surrounding the visual axis, the gel is injected under keratometric control. In the 1st group of 14 eyes, the degree of correction varied from 2.2 to 12.8 D; there was a direct relationship between the amount of gel injected and the keratometric change. In the 2nd group of 7 eyes, the adjustability of the procedure was demonstrated. Through partial extraction of the gel and subsequent modification of the corneal curvature, the previously induced keratometric changes could be reversed or altered to a specific extent. Following the initial injection of gel to a targeted flattest meridian power of 35 D, an average value of 35.8 +/- 0.5 D was achieved in these eyes. We subsequently attempted to increase the flattest meridian to 40 D by partial removal of the gel and achieved a mean value of 40.2 +/- 0.4 D. Average presurgical astigmatism of 1.497 +/- 0.737 D was reduced to a postsurgical reading of 0.941 +/- 0.590 D (P = 0.005, Student's paired t-test), indicating an autocorrection by autodistribution of the gel inside the canaliculus (Laplace's law). GIAK is a simple, inexpensive procedure designed for the correction of myopia that has the added advantage of reducing preexisting astigmatism without encroaching on the visual axis.
Astigmatism↗
Corneal sensitivity after photorefractive keratectomy.
Corneal anesthesia or hypesthesia can complicate refractive surgical procedures such as epikeratophakia and radial keratotomy. An esthesiometer was used to measure the corneal sensitivity in unoperated-on corneas and fellow corneas after excimer laser photorefractive keratectomy. Decrease in corneal sensitivity was noted within six postoperative weeks, with mean sensitivity being 75.2% +/- 13.3% of normal. Within the first three postoperative months, the patients operated on for correction of compound astigmatism recovered 95.7% +/- 5.3% of the corneal sensitivity, whereas the patients operated on for correction of severe myopia recovered 86.2% +/- 11.2% (P = .07). None of the patients had delayed epithelial healing or recurrent corneal erosions during the time of decreased corneal sensitivity. In otherwise normal myopic eyes, photorefractive keratectomy measurably reduced corneal sensitivity for several postoperative weeks.
Adult↗
Keratocyte activity in wound healing after epikeratophakia in rabbits.
Epikeratophakia is a refractive surgical procedure for the correction of aphakia, high myopia, or keratoconus. To solve clinical problems associated with epikeratophakia, a basic knowledge of its postoperative healing process is needed. The authors investigated keratocyte activities, particularly cell proliferation and collagen synthesis, during wound healing after epikeratophakia in rabbits. Epikeratophakia was done on rabbit corneas with a homologous cryolathed keratolens. Ten, 16, 28, 45, 63, 90, 254, and 360 days after the operation, the corneas were excised, labeled with either 3H-thymidine (10 microCi/ml) or 3H-proline (10 microCi/ml) for 4 hr and examined histologically and by autoradiography. Keratocytes in keratolenses were killed during the freezing process. On postoperative day 10, a few keratocytes migrated to the edge of the keratolens from the host stroma. On days 16 and 28, keratocytes in the keratolens and host stroma near the junction between the host and the keratolens incorporated 3H-thymidine, suggesting active proliferation. The proliferating activity was no longer seen after day 45. The repopulation of keratocytes was almost complete on day 90 and gradually returned to normal through day 360. Keratocytes in the keratolens and host stroma beneath the keratolens showed a higher 3H-proline incorporation than the control from days 16-254 with the highest activity at around 4-9 weeks after surgery. These results suggest that remodeling of collagen fibers continues for a long postoperative period after epikeratophakia.
Animals↗
Pupil diameter and the principal ray.
Placement of the surgical zone is critical in refractive procedures that alter a portion of the corneal curve. An improperly centered optical zone may produce glare, decrease best corrected visual acuity, and decrease contrast sensitivity. For proper placement, the new surface should be centered around the line of sight, which is the principal ray from the object of regard that passes through the image of the patient's pupil as projected on the cornea. This point is not necessarily at the geometric center of the cornea and is found by locating the center of the pupil while the patient is maintaining fixation coaxially with the surgeon. However, the pupil does not dilate concentrically and its geometric center moves as the pupil diameter changes. We have found a shift up to 0.7 mm in the geometric center of the pupil as it dilates. Therefore, centration of an ablated or a radial keratotomy zone is most efficiently done when the diameter of the modified corneal optical zone is centered around the line of sight and is superimposed upon the entrance pupil. This will minimize extension of the edge of the large pupil beyond the ablated zone and reduce unwanted secondary optical effects from degrading vision.
Adult↗
Barraquer lecture. What we have learned about corneal wound healing from refractive surgery.
The refractive corneal surgical procedures achieve their effects by modifying the corneal curvature of previously unoperated healthy eyes. Combinations of corneal incisions, excisions, or the introduction of optical interfaces are utilized to achieve the intended refractive effect(s). Each of the currently practiced refractive procedures modifies the wound healing response of the cornea in ways that are different from the standard corneal wound healing studies using sutured wounds as a model. The wound healing response of the cornea to each of the refractive procedures can be responsible for some of the visual complications that have been documented to occur after these procedures. An understanding of the factor(s) associated with the wound healing response and our subsequent ability to control those factors may establish refractive corneal surgery as a major field in ophthalmology.
Animals↗
[Is there a future for the Excimer laser in refractive surgery?].
The refractive surgery concerns all the surgical procedures implicated in the refractive power change of the cornea. Its clinical results are just known, and a new physical procedure is becoming capable to replace surgery: the Excimer laser. Without any instrumental contact with the corneal surface, the laser beam is able to remodel the corneal tissue, and to treat astigmatism, myopia, hypermetropia. Millions of people could be treated by such a laser, and could leave their glasses. Biological effects on rabbit and monkeys are presented. An argon fluoride excimer laser (193 nm) with a moving slit delivery system was used to perform anterior myopic keratomileusis in both eyes of 37 rabbits and 15 monkeys. Histological analysis of the corneas was made after ablation and at intervals up to 20 months. By slit examination at the longer follow up time, 60% of treated rabbits and 40% of treated monkeys keep a clear cornea, but the others had central spotty subepithelial haze. Light and electron microscopy documented corneal healing. In the clear corneas a good reconstitution of the epithelium, its basal lamina was observed, and anterior stromal corneas contained few active fibrocytes with a good preservation of the connective lamellar structure. On the contrary, in the cornea with opacification focal areas of 20 microns thick subepithelial scarring were present and the interface between epithelial cells and anterior stroma remained disturbed by incomplete, disrupted or duplicated basal lamina. Differences between the responses of monkeys, rabbits corneas to the same photoablation procedure remain unclear.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Lans lecture. Lamellar refractive surgery: where have we been and where are we going?
In summary, the greatest challenge for lamellar refractive surgery is in the treatment of myopia. There are and always will be patients who are satisfied with a partial correction of myopia. Many of these patients can be served quite adequately by currently available refractive techniques, but a technique in which level of accuracy is adequate for the treatment of myopia in general is not yet available. Lamellar refractive surgery--where have we been and where are we going? We have seen the evolution of the epikeratoplasty procedure for almost a decade and continue to see procedural changes which may increase the accuracy and stability of the procedure, such as alternative modalities for tissue preservation and alternative methods for surgical attachment. It remains potentially useful in several areas of refractive surgery, but its inaccuracy in the treatment of myopia remains a significant problem. Will the epithelial healing problems and postoperative care be too cumbersome for the general ophthalmologist? Will the excimer laser fill the needs of myopic refractive surgery? Personally, I do not feel that any biological lens, be it a cornea which has had incisions as with radial keratotomy, or a cornea which is remolded in one way or another as with the excimer laser, will produce predictable refractive results to the extent that is required to satisfy the general needs of the myopic population. Hydrogel keratophakia is in its infancy, but it holds the potential of being a far more accurate procedure because of the ability to interchange lenses to further refine the refractive result. Refractive surgery will in the next decade achieve a level of sophistication far greater than what we have seen to date. Lans could not have dreamed of the remarkable evolution in refractive surgical innovation in the past century. It is hoped that we will not have to wait that long to create a readily available solution to the problem of refractive surgery for myopia.
Animals↗
The corneal disk: a new keratoscopic device.
An inexpensive instrument for rapid intraoperative or postoperative evaluation of corneal astigmatism is presented. Its ease of use and availability make it an ideal adjunct to cataract and corneal surgical procedures in which minimal postoperative astigmatism is desired.
Astigmatism↗
Radial keratotomy: eleven-year experiences.
During the period from 1980 to 1991, Radial Keratotomy (RK) had been done by the author under topical anesthesia in more than 10,000 cases, mostly on both eyes at the same time. Those patients had myopia with a preoperative refractive error between 1.5 and 20.0 diopters (D). The surgical technique consisted of 4, 8, 16 incisions using a diamond knife with micrometer and the diameter of the central clear zone was mostly 3.0 mm and determined by preoperative refractive error. Many different procedures were tried to improve the effect of RK, including redeepening intentional microperforations at 1.5 mm, 3 mm, 5 mm distance from the corneal center. But these procedures had no significant effect 6 months after the operation. Complications such as infection, glaucoma, cataract, etc. were almost none. Results of RK on long term follow up showed hyperoptic tendency, and the visual fluctuation were none or minimal.
Follow-Up Studies↗