Femtosecond Lasers & Mechanical Microkeratomes

Photodisruption is the use of a femtosecond infrared laser at about 1035 nm to cut the LASIK flap rather than using the blade of a microkeratome.

There are 4 makes of femtosecond laser on the market as of Oct 2008. The one we can use is one made by Ziemer. There is an independent article written by Dr Omid Kermani from Cologne which is useful to read and tells of his group's first year experience of the Intralase system. I have scanned it from a magazine (March 2006) with 3 pages:

I have been trained and certified by Intralase.


In the ESCRS congress in Berlin in September 2008, there were a couple of papers comparing mechanical and femtosecond lasers. No difference was found between them.

     

The main practical difference might be that there are slightly less complications with femtosecond lasers rather than mechanical microkeratomes. My own opinion, having used the Intralase system is that both systems have an extremely low rate of complications and these vary in nature between the two systems. The bulletin board for certified Intralase surgeons seems to have complications more often than the manufacturer indicates in their sales literature.

Dr Jose Geull, a highly respected surgeon from Barcelona, says in a recent article (Eurotimes 4 April 2009) "visual outcomes with both microkeratome and femtosecond laser flaps are virtually identical....I think that the question about which technology is better for flap creation is really a market-driven question"

I use the Morial microkeratome system, which gives us the choice of flaps of various thicknesses - 100, 110, 130 and 150 microns. I use both the Moria M2 and the Moria One Use Plus systems. The new Moria SBK (Sub Bowmans Keratomeilusis) microkeratome head cuts 100 micron thin flaps.

Performance equivalent to IntraLase 60kHz - ESCRS Congress Berlin 2008

During the Refractive Free Paper Session, Osama Ibrahim, MD (Alexandria, Egypt) presented his “1-year outcomes with Moria One Use-Plus SBK for thin-flap LASIK” compared to his own experience with IntraLase 60kHz (AMO)4. In series of first 151 consecutive eyes of 83 patients, Dr Ibrahim obtained equivalent accuracy and predictability in flap thickness to femto-SBK.

 

Keratome

Surgeon

No. of eyes

Intended flap thickness (microns)

Accuracy (microns)

Standard Deviation (microns)

Range (microns)

One Use - Plus SBK

Osama Ibrahim, MD

151

100

108

+/- 11

77-123

 

Osama Ibrahim, MD

112

100

110

+/- 12

94-124

 

In the January 2009 issue of Cataract & Refractive Surgery Today Europe, Daniel Casado Rodríguez, MD (Madrid, Spain) reported the results in a series of 200 consecutive eyes treated with the One Use-Plus SBK5:

  • 100 eyes with the single-use plastic rings
  • 100 eyes with the reusable metallic rings.

With either type of ring, the One Use-Plus SBK provided excellent, consistent 100-micron flap thickness (measured by ultrasound pachymetry):

 

 

Number of Eyes

Intended flap thickness (microns)

Accuracy (microns)

Predictability (microns)

Range
(microns)

Metal Rings

100 100 99.7 +/- 13.2 73-135

Single Use Rings

100 100 98.7 +/- 10.7 77-122

 

The Moria One Use-Plus SBK microkeratome with:

  • a single-use ring (left)
  • a metallic ring (right)

With the use of an anterior segment Optical Coherence Tomography technology (Visante®, Carl Zeiss Meditec, Jena, Germany), flap thickness was assessed horizontally and vertically, at 1.5 and 3mm from the apex nasally and temporally. The planar profile was demonstrated with both types of rings.


 

On the basis of these and other studies Moria claims that the SBK microkeratome head has the following characteristics:

  • Thin, 100-micron planar flaps,
  • Accuracy, predictability and reproducibility equivalent to Femto-SBK
  • Stromal bed of a unsurpassed smoothness
  • Faster visual recovery than with Femto-SBK
  • No femto complications
  • …and at a fraction of the cost of a Femto-SBK

We are just starting to use this particular head and will see if our results match these (April 2009)


 

 

How a Femtosecond Laser Works:

Intralase Laser:

The excimer laser, which is ultraviolet, works on the surface and is called "photoablation"

These femtosecond lasers, which are infrared, work in the middle of the cornea and is called "photodisruption" Thousands of very small bubbles are joined together to make a cornea flap very precisely and safely.

The laser presses lightly on the eye and then is activated to make the corneal flap. One of the advantages of the flap as opposed to a flap made by a microkeratome is that it is the same thickness throughout:

Protocol for Femtosecond

Note: Femtosecond is the generic name. IntraLase is the trade name.

Aftercare

The aftercare regime is the same as it would be for LASIK performed with a microkeratome.

History of Femtosecond

The first femtosecond initiated LASIK procedure in the US was carried out in April 2000. Follow up data for Intralase is available for the last four years.

Complications & Considerations

Retreatment after Femtosecond

The retreatment procedure is the same regardless of whether the original treatment was with a mechanical keratome or with the femtosecond laser. Retreatment can be carried out as soon as the prescription is stable. 95% of flaps can be lifted after two years. Retreatment rates after femtosecond are less by 30% - 40% compared to retreatment rates after a mechanical microkeratome.

DLK

Inflammation as seen in post LASIK with microkeratome patients is not seen with the femtosecond. A less serious peripheral inflammation is seen. This is the cause of transient light sensitivity. See below

Transient Light Sensitivity

This was first seen in patients in 2003. It occurs 2 - 6 weeks post operatively. It is usually treated with steroids and clears up after about 1 - 4 weeks. This occurred in up to 20% of patients in some studies with high initial energy settings on the femtosecond laser. Following reductions in laser energy settings and based on collected cases, it is thought to occur with about 1% of patients.

Minor Flap Complications

Sometimes there can be wrinkles in the flap or debris under the flap and the surgeon may need to lift the flap on one of the early post-operative aftercares. The laser will not have to be applied again at this point.

Incomplete Flaps & Buttonholes

If there is loss of suction with the IntraLase procedure while the flap is being cut, suction can be reapplied and the cutting can continue. Because of this, the femtosecond laser does not create incomplete flaps or buttonholes (where the centre of the flap remains uncut) and is particularly good for patients with tight eyelids and deep set eyes.

Flap Thickness

The thickness of femtosecond flaps is more uniform and predictable than flaps created with a mechanical microkeratome. Numerous studies have confirmed this including a study on 1000 consecutive LASIK flaps created with the IntraLase laser carried out by Perry Binder. This means that some patients with thinner corneas, who may previously have been unsuitable for LASIK may now be suitable. The ability to more accurately predict flap thicknesses means that surgeons can more accurately predict residual stromal bed. Maintaining a residual stromal bed of >300 microns, helps to prevent the formation of post Lasik ectasia (forward bulging of the cornea).

Night Glare

Patients who have their LASIK flap cut with the femtosecond laser rather than a mechanical blade can be expected to have slightly less problems with night glare (haloes) for two reasons. The femtosecond laser's accuracy in cutting flaps means that a greater area of the eye's front surface is available for treatment. Large treatment zones are associated with less night vision problems. Also in a study made of nine patients where one eye had a flap created with a microkeratome and the other was created with IntraLase, the IntraLase created flap created less higher order aberrations. It is these aberrations which are usually responsible for most night vision problems.

Epithelial Defects and Corneal Abrasions

In a retrospective study of 375 eyes by Stonecipher and Kezirian in 2003, no epithelial defects were observed among patients who had had flaps cut with the IntraLase laser. In a retrospective study done on 404 patients who had LASIK flaps cut with a microkeratome between Jan 2002 and June 2002 8% had some form of epithelial defect.

Dry Eyes

A mechanical microkeratome blade enters the cornea at a relatively shallow angle and ends deep in the highly innervated central. This causes damage to the nerve endings, which as a result can cause dry eyes. The femtosecond flap enters the cornea at a steeper angle believed to preserve deep corneal innervation and thus dry eye tends to be less of a problem after femtosecond LASIK. In a study of 300 eyes cut with a microkeratome and 300 eyes cut with Intralase, 55% of the microkeratome patients complained of dry eyes whereas only 15% of IntraLase patients did.

Epithelial Ingrowth

The difference in flap structure means that epithelial ingrowth is less likely in flaps created with the femtosecond laser compared to those created with a mechanical blade (1 in 10,000).

Reading Spectacles

If a patient is fully corrected for distance then they will still require reading glasses in their 40s regardless of procedure.

Infection

Infection following LASIK treatment is very rare (1 in 2,500). It is possible that it may be even rarer after femtosecond due to its sterile sys


Transient Light Sensitivity Syndrome

TLSS describes a constellation of symptoms that can occur following LASIK with femtosecond laser flap creation. Patients with TLSS generally present with light sensitivity that is out of the ordinary, good UCVA and minimal slit lamp findings 2-6 weeks after uncomplicated LASIK. All patients responded to topical steroids, although improvement with Restasis (cyclosporine ophthalmic solution 0.05%, Allergan) has also been reported in one centre.

Based on collected cases, an incidence of approximately 1% was identified. The highest incidence rates occurred with the highest initial energy settings and with reduction in the number of cases following reductions in laser energy settings by 20-60%. In best documented series there was an approximately 5 fold reduction in incidence to less than 0.2% following an approximately 20% lowering of surgical energies.