LASIK Medical Studies

The incidence and risk factors for developing dry eye after myopic LASIK.

Am J Ophthalmol. 2006 Mar;141(3):438-45.
De Paiva CS, Chen Z, Koch DD, Hamill MB, Manuel FK, Hassan SS, Wilhelmus KR, Pflugfelder SC.

PURPOSE: To determine the incidence of dry eye and its risk factors after myopic laser-assisted in situ keratomileusis (LASIK).

DESIGN: Single-center, prospective randomized clinical trial of 35 adult patients, aged 24 to 54 years, with myopia undergoing LASIK.

METHODS: setting and study population: Participants were randomized to undergo LASIK with a superior or a nasal hinge flap. They were evaluated at 1 week and 1, 3, and 6 months after surgery. intervention: Bilateral LASIK with either a superior-hinge Hansatome microkeratome (n = 17) or a nasal-hinge Amadeus microkeratome (n = 18). main outcome measures: The criterion for dry eye was a total corneal fluorescein staining score >/=3. Visual acuity, ocular surface parameters, and corneal sensitivity were also analyzed. Cox proportional-hazard regression was used to assess rate ratios (RRs) with 95% confidence intervals.

RESULTS: The incidence of dry eye in the nasal- and superior-hinge group was eight (47.06%) of 17 and nine (52.94%) of 17 at 1 week, seven (38.89%) of 18 and seven (41.18%) of 17 at 1 month, four (25%) of 16 and three (17.65%) of 17 at 3 months, and two (12.50%) of 16 and six (35.29%) of 17 at 6 months, respectively. Dry eye was associated with level of preoperative myopia (RR 0.88/each diopter, P = .04), laser-calculated ablation depth (RR 1.01/mum, P = 0.01), and combined ablation depth and flap thickness (RR 1.01/mum, P = 0.01).

CONCLUSIONS: Dry eye occurs commonly after LASIK surgery in patients with no history of dry eye. The risk of developing dry eye is correlated with the degree of preoperative myopia and the depth of laser treatment.

Cohesive tensile strength of human LASIK wounds with histologic, ultrastructural, and clinical correlations.

J Refract Surg. 2005 Sep-Oct;21(5):433-45.
Schmack I, Dawson DG, McCarey BE, Waring GO 3rd, Grossniklaus HE, Edelhauser HF.

PURPOSE: To measure the cohesive tensile strength of human LASIK corneal wounds.

METHODS: Twenty-five human eye bank corneas from 13 donors that had LASIK were cut into 4-mm corneoscleral strips and dissected to expose the interface wound. Using a motorized pulling device, the force required to separate the wound was recorded. Intact and separated specimens were processed for light and electron microscopy. Five normal human eye bank corneas from 5 donors served as controls. A retrospective clinical study was done on 144 eyes that had LASIK flap-lift retreatments, providing clinical correlation.

RESULTS: The mean tensile strength of the central and paracentral LASIK wounds showed minimal change in strength over time after surgery, averaging 2.4% (0.72 +/- 0.33 g/mm) of controls (30.06 +/- 2.93 g/mm). In contrast, the mean peak tensile strength of the flap wound margin gradually increased over time after surgery, reaching maximum values by 3.5 years when the average was 28.1% (8.46 +/- 4.56 g/mm) of controls. Histologic and ultrastructural correlative studies found that the plane of separation always occurred in the lamellar wound, which consisted of a hypocellular primitive stromal scar centrally and paracentrally and a hypercellular fibrotic stromal scar at the flap wound margin. The pathologic correlations demonstrated that the strongest wound margin scars had no epithelial cell ingrowth-the strongest typically being wider or more peripherally located. In contrast, the weakest wound margin scars had epithelial cell ingrowth. The clinical series demonstrated the ability to lift LASIK flaps without complications during retreatments up to 8.4 years after initial surgery, correlating well with the laboratory results.

CONCLUSIONS: The human comeal stroma typically heals after LASIK in a limited and incomplete fashion; this results in a weak, central and paracentral hypocellular primitive stromal scar that averages 2.4% as strong as normal comeal stroma. Conversely, the LASIK flap wound margin heals by producing a 10-fold stronger, peripheral hypercellular fibrotic stromal scar that averages 28.1% as strong as normal comeal stromal, but displays marked variability.

Corneal reinnervation after LASIK: prospective 3-year longitudinal study.

Invest Ophthalmol Vis Sci. 2004 Nov;45(11):3991-6.
Calvillo MP, McLaren JW, Hodge DO, Bourne WM.

PURPOSE: To measure the return of innervation to the cornea during 3 years after LASIK.

METHODS: Seventeen corneas of 11 patients who had undergone LASIK to correct myopia from -2.0 D to -11.0 D were examined by confocal microscopy before surgery, and at 1, 3, 6, 12, 24, and 36 months after surgery. In all available scans, the number of nerve fiber bundles and their density (visible length of nerve per frame area), orientation (mean angle), and depth in the cornea were measured.

RESULTS: The number and density of subbasal nerves decreased >90% in the first month after LASIK. By 6 months these nerves began to recover, and by 2 years they reached densities not significantly different from those before LASIK. Between 2 and 3 years they decreased again, so that at 3 years the numbers remained <60% of the pre-LASIK numbers (P <0.001). In the stromal flap most nerve fiber bundles were also lost after LASIK, and these began recovering by the third month, but by the third year they did not reach their original numbers (P <0.001). In the stromal bed (posterior to the LASIK flap interface), there were no significant changes in nerve number or density. As the subbasal nerves returned, their mean orientation did not change from the predominantly vertical orientation before LASIK. Nerve orientation in the stromal flap and the stromal bed also did not change.

CONCLUSIONS: Both subbasal and stromal corneal nerves in LASIK flaps recover slowly and do not return to preoperative densities by 3 years after LASIK. The numbers of subbasal nerves appear to decrease between 2 and 3 years after LASIK. The orientation of the regenerated subbasal nerves remains predominantly vertical.

Pathologic Findings in Postmortem Corneas After Successful Laser In Situ Keratomileusis.

Cornea. 24(1):92-102, January 2005.
Kramer, Theresa R MD, MBA; Chuckpaiwong, Varintorn MD; Dawson, Daniel G MD; L'Hernault, Nancy; Grossniklaus, Hans E MD; Edelhauser, Henry F PHD

Purpose: To examine the histologic and ultrastructural features of human corneas after successful laser in situ keratomileusis (LASIK).

Methods: Corneas from 48 eyes of 25 postmortem patients were processed for histology and transmission electron microscopy (TEM). The 25 patients had LASIK between 3 months and 7 years prior to death. Evaluation of all 5 layers of the cornea and the LASIK flap interface region was done using routine histology, periodic acid-Schiff (PAS)-stained specimens, toluidine blue-stained thick sections, and TEM.

Results: In patients for whom visual acuity was known, the first postoperative day uncorrected visual acuity was 20/15 to 20/30. In patients for whom clinical records were available, the postoperative corneal topography was normal and clinical examination showed a semicircular ring of haze at the wound margin of the LASIK flap. Histologically, the LASIK flap measured, on average, 142.7 [mu]m (range, 100-200). A spectrum of abnormal histopathologic and ultrastructural findings was present in all corneas. Findings at the flap surface included elongated basal epithelial cells, epithelial hyperplasia, thickening and undulations of the epithelial basement membrane (EBM), and undulations of Bowman's layer. Findings in or adjacent to the wound included collagen lamellar disarray; activated keratocytes; quiescent keratocytes with small vacuoles; epithelial ingrowth; eosinophilic deposits; PAS-positive, electron-dense granular material interspersed with randomly ordered collagen fibrils; increased spacing between collagen fibrils; and widely spaced banded collagen. There was no observable correlation between postoperative intervals and the severity or type of pathologic change except for the accumulation the electron-dense granular material.

Conclusions: Permanent pathologic changes were present in all post-LASIK corneas. These changes were most prevalent in the lamellar interface wound. These changes along with other pathologic alterations in post-LASIK corneas may change the functionality of the cornea after LASIK.

Outcomes of LASIK for myopia with FDA-approved lasers.

Cornea. 2007 Apr;26(3):246-54.
Bailey MD, Zadnik K.
The Ohio State University College of Optometry, Columbus, OH 43210 , USA. mbailey@optometry.osu.edu

PURPOSE: To report expected outcomes of laser in situ keratomileusis (LASIK) for myopia and myopic astigmatism from existing US Food and Drug Administration (FDA) data.

METHODS: Data from Summaries of Safety and Effectiveness for each of the 12 lasers approved by the FDA for LASIK for myopia or myopic astigmatism between 1998 and 2004 were recorded from the FDA Web site. The Cochran-Armitage test for trend was used to determine whether improvements in outcomes occurred with laser technology changes.

RESULTS: For all patients, there was a statistically significant trend toward improvement with improved laser technology in the proportion of patients with uncorrected visual acuity (UCVA) of 20/20 or better, UCVA of 20/40 or better, results within +/-0.50 D of intended correction, results within +/-1.00 D of the intended correction, and night vision symptoms (all P < 0.0002). Because there were preoperative differences across laser types, subgroup analyses were also completed. The results for subgroup analyses (high myopia, low to moderate myopia, spherical myopia, and myopic astigmatism) for visual acuity and refractive error outcomes were similar to results for analyses for all groups combined. Conversely, there was no difference across laser types in the proportion of patients who experienced dry eye symptoms or for the proportion of patients with low to moderate myopia who experienced night vision symptoms that were worse or significantly worse than before LASIK.

CONCLUSIONS: LASIK provides excellent visual acuity and refractive error outcomes. Night vision and dryness symptoms still occur in a significant proportion of patients. Future studies should seek to determine whether additional changes in technology, patient selection criteria, or postoperative treatment could reduce or eliminate these symptoms.

Changes in corneal thickness and curvature after different excimer laser photorefractive procedures and their impact on intraocular pressure measurements

Graefes Arch Clin Exp Ophthalmol. 2005 Dec;243(12):1218-20. Epub 2005 Jul 8.
Svedberg H, Chen E, Hamberg-Nystrom H.
St Erik's Eye Hospital, Karolinska Institutet, Polhemsgatan 50, 112 82, Stockholm, Sweden, enping.chen@sankterik.se.

BACKGROUND: Excimer laser refractive surgery alters the shape and thickness of the cornea by removing central corneal tissue with submicrometer precision. The aim of the study was to analyze the changes in central corneal thickness (CCT) and curvature before and after different excimer laser photorefractive procedures and their possible impact on intraocular pressure (IOP) estimations with Goldmann applanation tonometry.

METHODS: Data on CCT, corneal curvature and IOP readings with Goldmann applanation tonometry before and after excimer laser photorefractive surgery were analyzed retrospectively. The data was further analyzed separately in two subgroups; the photorefractive keratectomy /laser-assisted subepithelial keratomileusis (PRK/LASEK) group and the laser in situ keratomileusis (LASIK) group.

RESULTS: The overall post-operative IOP readings were significantly lower than pre-operative values. There was a significant difference in the lowering of the IOP readings between the two subgroups: LASIK caused a lower IOP reading than PRK/LASEK.

CONCLUSION: The change in corneal thickness and curvature affects the estimation of IOP with Goldmann applanation tonometry after excimer laser photorefractive surgery. The amount of reduction in IOP reading might be influenced by the specific laser surgical procedure. This is of clinical importance in the evaluation of any future glaucoma in the increasing number of patients who undergo photorefractive laser surgery.

Calculation of intraocular lens power after corneal refractive surgery.

Clin Experiment Ophthalmol. 2006 Sep;34(7):640-4.
Chan CC, Hodge C, Lawless M.
Department of Ophthalmology, Royal North Shore Hospital, Sydney, NSW, Australia.

Purpose: Underestimation of required intraocular lens (IOL) power with resultant hyperopia is common in post-corneal refractive surgery eyes. A number of methods to minimize error have been proposed but most studies have been small and theoretical.

Methods: We retrospectively reviewed 34 eyes that had undergone routine phacoemulsification and IOL implantation after photorefractive keratectomy or laser in situ keratomileusis. Sixteen eyes were included in the final analysis. Using known pre- and postoperative data, four methods were used to obtain keratometric values combined with three common IOL formulae (Holladay 2, SRK/T and Hoffer Q) and Koch's published Double-K nomogram. The Double-K method was also used in conjunction with the Holladay 2 formula. Target refractions were calculated and then compared to actual postoperative results.

Results: The Clinical History method at the spectacle plane produced the lowest mean K-values. Shammas adjustment formula combined with the Holladay 2 and Hoffer Q produced results closest to emmetropia. The Double-K methods produced the least number of hyperopic results. Overall, all methods would have resulted in unacceptably high rates of hyperopia and deviation from target refraction.

Conclusions: No method produces acceptably consistent results because modern IOL formulae were designed for presurgical eyes. Accuracy will only be improved when new IOL formulae based on the anatomy of postrefractive eyes become available. Shammas adjustment formula and regression formulae are viable alternatives especially when there is a lack of preoperative data. The Double-K methods are best suited to avoiding a hyperopic surprise.

Refractive power of the cornea.

Compr Ophthalmol Update. 2006 Sep-Oct;7(5):243-51.
Ayres BD, Rapuano CJ.
Cornea Service, Wills Eye Institute, Thomas Jefferson University, Philadelphia, PA.

Corneal refractive surgeries, such as laser in situ keratomileusis and photorefractive keratectomy, have become some of the most commonly performed elective surgical procedures today. Many of the patients undergoing these surgeries are beginning to show signs of cataract formation and are in need of surgical correction. A common problem in the postrefractive patient is accurate prediction of the corneal power for use in intraocular lens calculation.

Residual bed thickness and corneal forward shift after laser in situ keratomileusis.

Cataract Refract Surg. 2004 May;30(5):1067-72.
Miyata K, Tokunaga T, Nakahara M, Ohtani S, Nejima R, Kiuchi T, Kaji Y, Oshika T.
Miyata Eye Hospital, Miyasaki, Japan.

PURPOSE: To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness.

SETTING: Miyata Eye Hospital, Miyazaki, Japan.

METHODS: Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/- 2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface.

RESULTS: The mean residual corneal bed thickness after laser ablation was 388.0 +/- 35.9 microm (range 308 to 489 microm). After surgery, the posterior corneal surface showed a mean forward shift of 46.4 +/- 27.9 microm, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6 +/- 5.7 microm (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B = 0.736, P<.0001) and the preoperative corneal thickness (B = -0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea.

CONCLUSIONS: Even if a residual corneal bed of 300 microm or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.

Long-term results of laser in situ keratomileusis for high myopia: Risk for ectasia

Patrick I. Condon, MCh, FRCS, FRCOphth, Michael O’Keefe, MCh, FRCS, FRCOphth, Perry S. Binder, MD
J Cataract Refract Surg. Vol 33, Apr 2007

Excerpts:

"Several short-term and long-term complications have been reported, with the most worrisome the development of post-LASIK ectasia."

"Although there are many recommendations to avoid ectasia, there have been no definitive studies to establish a 'safe' RSBT to avoid ectasia."

"The incidence of post-LASIK ectasia is yet to be established."

"If we bear in mind that more than 17 million patients globally (34 million eyes) and 8 million (16 million eyes) in the United States have had LASIK to date (source: David Harman, Market Scope, Minnesota, USA) and based on the studies cited above as well as this current study, one would expect to have a possible 20,400 to 112,200 post-LASIK ectasia cases globally and 9600 to 52,000 in the United States alone."