Effectiveness and safety of two types of tools used in LASIK (a type of refractive surgery) for nearsightedness

What was the aim of this review?
The aim of this Cochrane Review was to assess whether two types of tools for surgical correction of vision (laser-assisted in-situ keratomileusis [LASIK]) are effective and safe in people with nearsighted vision. One tool uses a high-precision blade (mechanical microkeratome) and the other tool uses infrared waves (femtosecond laser).

What was studied in this review?
Nearsightedness is a medical condition in which people can see objects near to them clearly but objects farther away are blurry. As of 2010, nearsightedness affected approximately two billion people worldwide. Nearsightedness can be treated by using either glasses, contact lenses, or surgery. Surgery for nearsightedness changes the shape of the transparent structure in the front part of the eye (cornea). LASIK is the most common surgical procedure used to correct nearsightedness.

The LASIK procedure creates a flap in the cornea in order to reshape it. This flap can be made by either a mechanical microkeratome or a femtosecond laser. The mechanical microkeratome uses a blade to make the flap and the femtosecond laser uses a laser to make the flap. The flaps made by the two methods are different in thickness and structure, and the side effects resulting from each method are also different. We collected and analyzed all relevant studies to answer this question. We found 11 studies that included 943 participants (1691 eyes).

What were the main results?
There is no evidence of a difference in vision outcomes between using mechanical microkeratome or femtosecond laser. The certainty of evidence is low. However, there may be a difference in side effects between the two methods. The mechanical microkeratome group had more cases of dry eye and the femtosecond laser group had more cases of swelling of the cornea. Overall, due to the low-certainty evidence presented, it is difficult to draw a general conclusion regarding the effectiveness and safety of these two tools.

How up-to-date was this review?
The authors searched for trials that had been published up to 22 February 2019.

Authors' conclusions: 

Regarding the visual acuity outcomes, there may be no difference between LASIK with mechanical microkeratome and LASIK with femtosecond laser. Dry eye and diffuse lamellar keratitis are likely adverse events with mechanical microkeratome and femtosecond laser, respectively. The evidence is uncertain regarding corneal haze and epithelial ingrowth as adverse events of each intervention. The limited number of outcomes reported in the included trials, some with potentially significant risk of bias, makes it difficult to draw a firm conclusion regarding the effectiveness and safety of the interventions investigated in this review.

Read the full abstract...
Background: 

Laser-assisted in-situ keratomileusis (LASIK) is a surgical procedure that corrects refractive errors. This technique creates a flap of the outermost parts of the cornea (epithelium, bowman layer, and anterior stroma) to expose the middle part of the cornea (stromal bed) and reshape it with excimer laser using photoablation. The flaps can be created by a mechanical microkeratome or a femtosecond laser.

Objectives: 

To compare the effectiveness and safety of mechanical microkeratome versus femtosecond laser in LASIK for adults with myopia.

Search strategy: 

We searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register) (2019, Issue 2); Ovid MEDLINE; Embase; PubMed; LILACS; ClinicalTrials.gov and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP). We used no date or language restrictions. We searched the reference lists of included trials. We searched the electronic databases on 22 February 2019.

Selection criteria: 

We included randomized controlled trials (RCTs) of LASIK with a mechanical microkeratome compared to a femtosecond laser in people aged 18 years or older with more than 0.5 diopters of myopia or myopic astigmatism.

Data collection and analysis: 

We used standard methodological procedures expected by Cochrane.

Main results: 

We included 16 records from 11 trials enrolling 943 adults (1691 eyes) with spherical or spherocylindrical myopia, who were suitable candidates for LASIK. Five hundred and forty-seven participants (824 eyes) received LASIK with a mechanical microkeratome and 588 participants (867 eyes) with a femtosecond laser. Each trial included between nine and 360 participants. In six trials, the same participants received both interventions. Overall, the trials were at an uncertain risk of bias for most domains.

At 12 months, data from one trial (42 eyes) indicates no difference in the mean uncorrected visual acuity (logMAR scale) between LASIK with a mechanical microkeratome and LASIK with a femtosecond laser (mean difference (MD) –0.01, 95% confidence interval (CI) –0.06 to 0.04; low-certainty evidence). Similar findings were observed at 12 months after surgery, regarding participants achieving 0.5 diopters within target refraction (risk ratio (RR) 0.97, 95% CI 0.85 to 1.11; 1 trial, 79 eyes; low-certainty evidence) as well as mean spherical equivalent of the refractive error 12 months after surgery (MD 0.09, 95% CI –0.01 to 0.19; 3 trials, 168 eyes [92 participants]; low-certainty evidence).

Based on data from three trials (134 eyes, 66 participants), mechanical microkeratome was associated with lower risk of diffuse lamellar keratitis compared with femtosecond laser (RR 0.27, 95% CI 0.10 to 0.78; low-certainty evidence). Thus, diffuse lamellar keratitis was a more common adverse event with femtosecond laser than with mechanical microkeratome, decreasing from an assumed rate of 209 per 1000 people in the femtosecond laser group to 56 per 1000 people in the mechanical microkeratome group. Data from one trial (183 eyes, 183 participants) indicates that dry eye as an adverse event may be more common with mechanical microkeratome than with femtosecond laser, increasing from an assumed rate of 80 per 1000 people in the femtosecond laser group to 457 per 1000 people in the mechanical microkeratome group (RR 5.74, 95% CI 2.92 to 11.29; low-certainty evidence). There was no evidence of a difference between the two groups for corneal haze (RR 0.33, 95% CI 0.01 to 7.96; 1 trial, 43 eyes) and epithelial ingrowth (RR 1.04, 95% CI 0.11 to 9.42; 2 trials, 102 eyes [50 participants]). The certainty of evidence for both outcomes was very low.

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