Technique for measuring laser in situ keratomileusis flap thickness using the IntraLase laser

A Comparative Study of Laser In Situ Keratomileusis Flaps Created Using Single-Functional Versus Multifunctional Femtosecond Laser for Refractive Surgery

PURPOSE

To compare laser in situ keratomileusis flaps created at the programmed target thickness of 120 μm using the LenSx multifunctional and the FS200 single-functional femtosecond lasers as evaluated by anterior segment optical coherence tomography (AS-OCT).

METHODS

This was a prospective, comparative, consecutive, blinded, nonrandomized study. Patients with stable refraction for over a year were consecutively allocated to the LenSx group or the FS200 group (n = 66; 33 patients in each group). All the patients underwent refractive surgery in both eyes. Previous eye surgery, ocular pathology associated with refractive errors, pregnancy, breastfeeding, and use of medication that causes ocular adverse effects constituted the exclusion criteria. Corneal topography, corneal tomography, dilated fundoscopy, applanation tonometry, ultrasonic pachymetry, dynamic and static refraction, visual acuity, and AS-OCT were evaluated before and after surgery. AS-OCT flap thickness was measured at 20 points on each cornea.

RESULTS

In the LenSx group, AS-OCT flap thickness differed significantly from the target thickness at 2 of 20 points (mean differences of 2.106 and 1.803 μm). In the FS200 group, 6 of 20 measurements differed significantly (mean differences ranging from 1.121 to 2.121 μm). The 2 lasers were similarly successful in creating regular and uniform flaps. The agreement between the 2 blinded examiners regarding the AS-OCT flap thickness measurements was excellent (intraclass correlation coefficient >0.75) in both groups.

CONCLUSIONS

Both femtosecond lasers were safe and capable of creating highly reproducible, uniform, and regular flaps at the target preoperative thickness of 120 μm. The LenSx multifunctional femtosecond laser offers the advantage of allowing both cataract and corneal surgeries.

Direct and Indirect Flap Measurements in Femtosecond Laser-Assisted In Situ Keratomileusis

PURPOSE

To compare direct and indirect LASIK flap thickness measurements using ultrasound and Scheimpflug technology.

METHODS

Eighty-two eyes treated with laser-assisted in situ keratomileusis refractive surgery using a femtosecond laser (IntraLase FS150) were prospectively included in the study. Flap thickness was set to 115 μm. Corneal flap thickness was measured using the direct method-ie, ultrasound pachymetry immediately after flap construction in the presence of cavitation bubbles-and indirect methods, with subtraction of intraoperative post-lift corneal thickness measured using ultrasound pachymetry (intrastroma) from preoperative central corneal thickness using ultrasound (Indirect-US) or Scheimpflug thinnest pachymetry (Indirect-Scheimpflug).

RESULTS

Mean flap thickness was overestimated using the indirect methods, Indirect-US and Indirect-Scheimpflug (122.6 ± 24.5 μm and 128.1 ± 26.1 μm, respectively; P < 0.0060 and P < 0.0001, respectively). There were no significant correlations between the direct and indirect methods. Indirect-Scheimpflug was significantly higher (P = 0.0122) than Indirect-US. The closest average flap thickness compared with the set parameter of 115 μm was that of the direct method (115.6 ± 8.6 μm; 95% confidence interval: -1.3 to 2.5; P = 0.5163). The direct method provided the lowest SD of all groups (SD: 8.64).

CONCLUSIONS

The direct method of flap thickness measurement was the most comparable to the set parameter compared with the indirect subtraction methods. Additional studies are needed to determine which method allows for the most accurate measurement of flap thickness.

Intraoperative anterior segment optical coherence tomography: a novel assessment tool during deep anterior lamellar keratoplasty

PURPOSE

To describe the use of high-resolution anterior segment optical coherence tomography (AS-OCT) during deep anterior lamellar keratoplasty (DALK).

DESIGN

Prospective case series.

METHODS

At the Singapore National Eye Centre we performed sequential intraoperative AS-OCT scans using iVue 100-2 (Optovue, Fremont, CA, USA) during various DALK techniques.

RESULTS

In case 1 (corneal scarring) the OCT images helped to guide manual dissection, showed the depth of the needle track before air injection, and identified the location of a small bubble when the big bubble failed. In cases 2 (macular dystrophy) and 3 (lattice), viscodissection was undertaken, but in case 2 the OCT showed the viscoelastic trapped intrastromally, compared to case 3, in which the viscobubble dissection was successful. In case 4 (irregular corneal thinning and scarring), AS-OCT enabled accurate decision on initial trephination and guided dissection. In case 5 (keratoconus), the OCT showed the achieved big-bubble and detached Descemet membrane. Case 6 was a repeat DALK; the OCT guided the manual dissection of the residual stroma underlying the failed graft. In case 7, the OCT showed an intrastromal retention of fluid that was not detectable by the operating microscope due to diffuse scarring following alkaline injury. The OCT helped to assess the location of the Descemet membrane and guided the manual dissection.

CONCLUSIONS

Intraoperative high-definition AS-OCT obtained good-quality images of the cornea during DALK and proved useful in various cases of DALK to help the surgeon decide on a number of surgical steps.

Comparison of the predictability, uniformity and stability of a laser in situ keratomileusis corneal flap created with a VisuMax femtosecond laser or a Moria microkeratome

This prospective study compared the predictability, uniformity and stability of laser in situ keratomileusis corneal flap thickness created by a femtosecond laser or a classic microkeratome. Twenty-five femtosecond laser (VisuMax, Carl Zeiss Meditec) flaps and 38 microkeratome (Moria M3) flaps were measured using anterior segment optical coherence tomography at 1 week, 1 month and 6 months postoperatively. Flap thickness at seven points on each of the four meridians was calculated. At 6 months, VisuMax flaps showed better prediction than Moria flaps for central flap thickness. The standard deviation within individual flaps was smaller for VisuMax flaps and their index of symmetry was better. The mean thicknesses among the four eccentricities in the VisuMax flaps were the same, while Moria flaps were thicker at the 3-mm radius compared with the centre. The VisuMax femtosecond laser created corneal flaps with better predictability and uniformity, and similar reproducibility and stability, compared with the microkeratome.

Anterior segment optical coherence tomography measurement of LASIK flaps: femtosecond laser vs microkeratome

PURPOSE

To compare the uniformity and accuracy of LASIK flaps created with a femtosecond laser versus a mechanical microkeratome using optical coherence tomography (OCT).

METHODS

A prospective study was performed on 72 consecutive patients who underwent LASIK in both eyes in alternating fashion with either the AMO IntraLase FS60 (72 eyes) or the Moria M2 microkeratome (72 eyes). One month after surgery, anterior segment OCT was performed on each eye to measure flap thickness at 20 locations and the results were assessed for uniformity and accuracy.

RESULTS

At 1 month after surgery, the microkeratome group flap ranges were greater than those found in the femtosecond laser group (P<.05). The meridian and radial flap uniformity in the femtosecond laser group were better, showing an almost planar configuration, than the meniscus-shaped flaps created with the microkeratome. Comparison of the nasal and temporal flap thickness revealed more regularity in the femtosecond laser group than the microkeratome group. The maximum deviation from the intended flap thicknesses was 7 μm in the femtosecond laser group compared to 26 μm in the microkeratome group. During the 1440 measurements of the 72 eyes, a difference >20 μm was observed in 0.42% of eyes in the femtosecond laser group and 15% of eyes in the microkeratome group.

CONCLUSIONS

LASIK flaps made with the IntraLase FS60 were more uniform with more accurate thickness than those created by the Moria M2 microkeratome.

Flap thickness reproducibility in laser in situ keratomileusis with a femtosecond laser: optical coherence tomography measurement

PURPOSE

To compare ultrasound (US) pachymetry, Orbscan, and optical coherence tomography (OCT) measurements of the central corneal thickness (CCT) before laser in situ keratomileusis (LASIK) and evaluate the reproducibility of flaps created with the IntraLase femtosecond laser (IntraLase, Inc.) using OCT.

SETTING

Department of Ophthalmology, IIsan Paik Hospital, Goyang, Korea.

METHODS

Central corneal thickness was measured using OCT, US pachymetry, and Orbscan in 59 eyes of 30 patients before femtosecond laser LASIK. The postoperative corneal flap thickness, measured using OCT, was compared with the preoperative intended thickness.

RESULTS

Optical coherence tomography, US pachymetry, and Orbscan measurements provided similar CCT values (P>.05). Corneal thickness values obtained using US or Orbscan correlated well with those obtained by OCT, with the correlation coefficient ranging from 0.804 to 0.889 (P<.05). The OCT measurements showed no significant difference between the postoperative flap thickness and the intended flap thickness (P>.05).

CONCLUSIONS

Optical coherence tomography was comparable to US pachymetry and Orbscan in cornea thickness measurement. Optical coherence tomography was easy and relatively accurate to use preoperatively and in the early postoperative period. The femtosecond laser created highly reproducible flaps that corresponded with the preoperative intended thickness.

Comparative Study of Stromal Bed Quality by Using Mechanical, IntraLase Femtosecond Laser 15- and 30-kHz Microkeratomes

PURPOSE

To evaluate corneal stromal bed quality of lamellar keratectomy in laser in situ keratomileusis (LASIK) procedures by using mechanical and femtosecond laser microkeratomes.

METHODS

LASIK corneal flaps were created in 9 fresh human globes not suitable for transplantation. We grouped the samples into 3 different groups of 3 globes each. Group 1 was the control group, in which the flaps were created using a Hansatome microkeratome with a 160-microm head. Groups 2 and 3 consisted of flaps created at the 110-microm flap thickness setting using the IntraLase 15- and 30-kHz femtosecond laser, respectively. All the flaps were removed, and the corneal stromal beds were prepared for scanning electron microscopy (SEM). Qualitative surface roughness (QlSR) of the SEM images was graded using a roughness grading scale from 1 to 5 by 3 masked observers. Quantitative surface roughness (QnSR) of the SEM images was also assessed using software designed for roughness analysis. Mann-Whitney nonparametric statistical analysis was performed to compare groups.

RESULTS

There was no statistically significant difference in QlSR and QnSR scores between group 2 and group 1 (brand-new blade only). Group 3 30-kHz IntraLase samples showed a smoother stromal bed compared with group 1 and 2 samples. This was a statistically significant difference for QlSR (QlSR = 1.1 +/- 0.17, P < 0.001) and showed borderline significance for QnSR (QnSR = 24.4 +/- 0.96, P = 0.05).

CONCLUSIONS

The IntraLase 15-kHz femtosecond laser 110-microm flaps and the Hansatome 160-microm head using a new blade both produced smooth, good-quality, compact stromal beds as assessed qualitatively by masked observers and quantitatively by image analysis software. The 30-kHz femotsecond laser permits a tighter spot/line separation and lower energy per pulse, which creates smoother corneal stromal beds.

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

PURPOSE

To ascertain the long-term stability of laser in situ keratomileusis (LASIK) in highly myopic eyes.

SETTING

Clinical practice office-based surgery.

METHOD

Charts of eyes with high myopia who had LASIK surgery by the same surgeon between 1994 and 2000 were reviewed in 2003, and patients were given an appointment for follow-up examinations. In these highly myopic eyes, surgery was originally performed to create undercorrections with or without decreasing the ablation diameters to maximally conserve the residual stromal bed thickness.

RESULTS

Of the 107 eyes with myopia between -10.00 diopters (D) and -35.00 D reviewed and operated on in a 3-year period between 1994 and 1998, 35 eyes of 31 patients had a single enhancement procedure. One case of ectasia as a result of excessive tissue removal occurred in a patient with a preoperative refraction of -28.00 D. Of the 107 eyes reviewed, 78 (73%) were examined after 5 years, 68 (63%) after 7 years, and 15 (14%) between 9 years and 11 years.

CONCLUSIONS

Operating on eyes with highly myopic refractive errors and removing substantial tissue thickness did not produce ectasia in this series. Although high myopia has been considered a risk factor for post-LASIK ectasia, adherence to proper screening and intraoperative pachymetry appears to decrease the risk.