Femtosecond Laser versus Microkeratome

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Clear vision following laser vision correction is the result of a perfect execution of a series of interdependent surgical steps. Compromise at any phase will result in a suboptimal result. With Lasik, the technique used to create the Lasik flap is critical to creating a perfect endpoint.

Significant advances have been made in the method of creating that Lasik flap. At Will Vision and Laser Centers we only use the VisuMax® from Carl Zeiss® and the iFS Advanced Femtosecond laser from Johnson and Johnson®, the most advanced systems in the world. The VisuMax® is the fastest femtosecond laser in the world and the only laser precise enough to perform the advanced ReLex SMILE procedure. Our VisuMax® is the only system in the Pacific Northwest. In addition, there is only one other center with an iFS® laser in the region. All other centers are using older technologies that we abandoned years ago.

We have not used the archaic microkeratome since 2002 when the first generation of femtosecond lasers became available. In contrast, most other Lasik centers in the region did not embrace “Bladeless Lasik” or “All Laser LASIK” for years, despite the clear superiority of this technique over conventional Lasik. We suggest this excessive lag in embracing advanced technology and the ongoing use of a microkeratome at many centers today represents a rather tragic commentary on their clinical acumen and ethical judgment.

Frequently we hear the argument that “Bladeless Lasik” is a marketing gimmick or, perhaps even more commonly, patients are presented with a choice of using either the microkeratome (metal blade) or a femtosecond laser based on some ill-conceived tiered pricing menu. Such an approach leaves the patient to determine the safest or best surgical option to use for their own eyes. Such behaviors, in our opinion, can only be driven either by a profound ignorance of the medical facts regarding the challenges presented by the use of both microkeratomes and older femtosecond laser systems or, an unconscionable level of irresponsibility towards the patients that they purport to serve. In our opinion, it is the responsibility of the surgeon to do everything possible to ensure that patients will have the best possible vision for the remainder of their lives – not just until their check clears the bank.

Although it is impossible to discuss the scientific data with any substantive depth on a web site designed for consumption by the general public not versed in scientific jargon, it is worth providing some widely recognized advantages of the newest femtosecond lasers over older technologies including:

  • Reduced Post Lasik Dry Eye
  • Superior Surgical Predictability
  • Superior Healing and Stronger Eye
  • Significantly Less Diffuse Lamellar Keratitis
  • Superior Flap Integrity
  • Marked Reduction of Post Lasik Dry eye / Neurotrophic keratitis

Decreased Post Lasik Dry Eye

One of the most common problems following laser vision correction is “dry eye.” In general, patients predisposed to “dry eye” are post-menopausal women, with women of Asian descent demonstrating the highest risk. Although the majority of these cases improve with time or require only over-the-counter tear supplements, it would be ideal to not deal with this issue at all.

It is widely recognized that patients that undergo Lasik with modern femtosecond lasers such as the VisuMax® and the iFS® have a markedly lower incidence of “dry eye” following Lasik. Having significant personal experience with microkeratomes and multiple generations of femtosecond lasers, our experience validates this general observation. In fact, several excellent peer reviewed studies have been performed documenting the difference in incidence in “dry eye” with the microkeratome versus a femtosecond laser. Steve Wilson’s study at Cole Eye demonstrated approximately 50% reduction in the incidence of “dry eye” using an early generation femtosecond laser compared to the most commonly used microkeratome. We presented a scientific paper at the American Society of Cataract and Refractive Surgery in 2012 that was also published in an Eye World supplement that demonstrated a nearly 600% reduction in the incidence of “dry eye” in post-menopausal women when using the iFS Advanced Femtosecond laser compared to the 4th generation 60 KHz femtosecond laser commonly used throughout the region even today.

Such profound reductions in the incidence and severity of post Lasik “dry eye” have markedly improved the safety of Lasik for everyone, but particularly those at risk for post Lasik “dry eye”. It clearly makes no rational medical sense to place patients at risk for “dry eye” by continuing to use either a microkeratome or an older generation femtosecond laser.

Although the mechanism behind this improvement is not completely understood, in our paper we postulated the following. First, it should be recognized that “dry eye” following Lasik is primarily a neurotrophic keratitis rather than a lack of tear production. This neurotrophic keratitis is caused by severing the nerve fibers that innervate the epithelial layer of the cornea. The interaction between these nerves and the corneal epithelial cell layer maintain a healthy corneal surface. The use of artificial tears primarily serves primarily to mask rather than cure the clinical effects of a neurotrophic keratitis.

A broad base of scientific evidence indicates that regenerating nerves are inhibited by dead (apoptotic) cells in the area and a disruption of the organization of the local collagen fiber orientation (extracellular matrix). It is also well known that microkeratomes cause significantly more keratocyte (corneal cell) death than femtosecond lasers. Moreover, newer femtosecond lasers, by virtue of markedly lower laser pulse energies, cause significantly less local keratocyte (cell) death than earlier generation femtosecond lasers. The same observations occur with respect to the distortion of the local tissue organization where microkeratomes create the largest amount of tissue disorganization and the roughest flap / bed interface while advanced femtosecond lasers cause the least amount of local tissue disruption and the smoothest flap / bed interface.

As a result, regenerating nerves can more easily and more rapidly recover when using advanced generation femtosecond lasers. For the patient, this translates into a significant reduction in the incidence and severity of post Lasik neurotrophic keratitis commonly referred to as “dry eye”.

Superior Surgical Predictability

New femtosecond lasers deliver significantly improved surgical predictability compared to microkeratomes and earlier generation femtosecond lasers. Advanced femtosecond lasers like the VisuMax® and iFS® create the thinnest, most accurate corneal flaps while using markedly less energy than either a microkeratome or older generation femtosecond lasers. As a result, these systems produce the best surgical predictability of any flap creation technology.

It should be obvious that creating the Lasik flap and removing corneal tissue impacts the mechanical and biological properties of the cornea. If significant, these changes can lead to unpredictable changes in corneal shape which in turn causes decreased surgical predictability. Further, the thinnest flap possible will induce the least amount of biomechanical change. In addition, the greatest predictability occurs when the flap is both thin and the flap thickness is uniform throughout. Finally, the least amount of biomechanical change is produced when the health of the corneal tissue is respected. Flap creation using the least amount of laser energy possible produces the least amount of corneal tissue injury and the least amount of local tissue damage and swelling. All of these advantages are provided with the newest femtosecond laser technology.

Superior Healing

Healing following surgery with the iFS® femtosecond laser has been demonstrated to produce superior flap adhesion and a stronger eye compared to microkeratomes and earlier generation femtosecond lasers. This superior healing is the result of a unique reverse bevel sidewall incision available on the iFS® system. In addition, corneas are generally oval shaped rather than being round like a circle. The iFS® is the only system capable of creating an oval flap. By making the flap oval, it is possible to selectively incise only peripherally located cross linked collagen fibers and thereby reduce unwanted biomechanical change. Use of the oval flap architecture automatically places the edge of the flap closer to the edge of the cornea which results in markedly stronger sidewall healing and a significantly stronger eye. Finally, the iFS® flap ovalization feature provides superior exposure of the corneal bed thereby enhancing the predictability of surgical outcomes when treating farsightedness or astigmatism.

Markedly Reduced Incidence of Diffuse Lamellar Keratitis

Diffuse lamellar keratitis (DLK) is a vision threatening complication caused by a shock to corneal physiology due to contaminants, antigens and tissue injuries that aggressively stimulate the eye’s immune system (allergic response). The use of high levels of femtosecond laser energy typical of older generation laser systems or microkeratomes that cause excessive injury to or death of cells in the cornea also cause of DLK. Severe cases of DLK can cause significant compromise of the cornea and vision.

Due primarily to their very low laser energy footprint, DLK is rarely, if ever, observed with use of the VisuMax® or the iFS® Advanced Femtosecond laser systems. By comparison, DLK commonly occurs with use of a microkeratome and, although less common, also occurs with the use of older generation femtosecond lasers.

Superior Flap Integrity both Intraoperatively and Postoperatively

The flap created by theVisuMax® and iFS® Advanced Femtosecond laser has the identical thickness at every point. In stark contrast, flaps created by a microkeratome demonstrate a different thickness at every point. As a result, femtosecond laser flaps are completely optically neutral while microkeratome flaps demonstrate significant optical power.

As previously noted, this difference has an adverse effect on the biomechanical stability of the eye. More critically, if a flap created by a microkeratome is ever compromised by trauma, infection or epithelial ingrowth at any time during the patient’s lifetime and is removed either intentionally or by chance, the eye will be grossly out of focus and will require multiple additional surgeries or a contact lens to create good vision.

In stark contrast, any flap created by the VisuMax® or iFS® laser that becomes anatomically compromised can be removed without any adverse optical or visual effects. In addition, flaps created by a femtosecond laser are significantly more resistant to epithelial ingrowth than flaps created using a microkeratome particularly when performing late enhancements. This provides the option of safely performing a Lasik enhancement for eyes previously operated upon with the iFS® or VisuMax® systems years after the original Lasik procedure.

Side by Side Comparison Table

Click to view the Comparison detail.

Comparison Table

iFS Advanced Femtosecond Laser

VisuMax and iFS Advanced Femtosecond Lasers

Microkeratome

Computerized control of corneal flap thickness

Yes – Surgeon able to control to within 3-4 microns

Yes – Surgeon able to control to within 10 microns

No computer control – Surgeon must rely on manufacturing tolerances on steel blades with typical tolerance of 20 to 30 microns at best

Can create thin flaps to allow larger residual corneal tissue and increase corneal strength

Can accurately and safely create flaps as thin as 90 microns. Flap identical thickness at every point.

Safely create flaps as thin as 100 microns

No control whatsoever on flap thickness. Flap thickness varies throughout the flap

Computerized control of flap diameter

Yes – Surgeon able to control within 100 microns and ovalize the shape to match the natural corneal shape

Yes – Surgeon able to control within 100 microns

No – Surgeon must hope that flap diameter is adequate for the planned ablation

Biomechanical Effects

Thin reproducible flaps have the least adverse biomechanical effects with significantly better vision predictability

Thin flaps improve biomechanics compared to microkeratome but high laser energy causes more tissue edema and mechanical distortion than the iFS system

Unpredictable thick flaps cause significant variability in biomechanical effects resulting in much higher unpredictability of visual results

Flap Ovalization feature allows surgeon to match flap to the oval shape of the cornea

Yes – allows much superior results with large treatments particularly farsightedness and astigmatism

Not available

Impossible

Risk for Dry Eye

Dry eye extremely rare. 6 times less than 60 KHz for high risk patients

Approximately 50% less dry eye than microkeratome but significantly more than iFS system

Highest incidence of dry eye

Computer adjustment of flap centration

Yes – Surgeon able to control centration using software controls

Yes – Surgeon able to control centration using software controls

No – Centration is dependent on surgeon and variability of vacuum positioning of suction ring on the eye. Can result in decentered flap that will compromise the visual result

Flap Characteristics

Planar flap induces few if any new optical aberrations

Planar flap induces few if any new optical aberrations

Meniscus flap architecture induces new visually significant optical aberrations

Consequence of lost or damaged flap

None – eye heals with excellent vision

None – eye heals with excellent vision

Optical disaster – eye will require multiple surgeries to recover if that is even possible

Surface Characteristics of Stromal Bed

Smoothest surface of any femtosecond laser on scanning electron micrographs

Very smooth on scanning electron micrographs

Blade chatter creates irregular surface characteristics on scanning electron micrographs

Sidewall architecture

Sidewall control allows for reverse bevel promotes superior healing, stronger eye and less flap slippage

Vertical sidewall promotes better healing and less flap slippage

Meniscus edge heals less aggressively with increased risk for flap slippage and dislocation

Flap healing and adhesion to the eye

30% stronger than 60 KHz system. Many flaps are difficult to relift within months making the eye more structurally sound

Very good adhesion – Many flaps are difficult to relift within months making the eye more structurally sound

Poor healing and adhesion – flap can be easily lifted many years after the primary procedure

Risk for epithelial ingrowth

Extremely rare if not non-existent

Extremely rare if not non-existent

Common – occurs in 1 to 2% of primary cases and more common in retreatments – can be difficult to eradicate – causes pain, dryness and can compromise vision

Risk for free flap

Highly remote – never reported in over 10 million FS procedures

Highly remote – never reported in over 10 million FS procedures

Yes – if occurs can have severe visual results

Risk for “button hole” flap (flap with a hole in the middle)

No

No

Yes – occurs 1/1000 – requires surgery be aborted and recut in 3 months or otherwise undergo a complex PRK procedure

Risk to patient if vacuum is lost during surgery

No problem – vacuum ring is reinserted and procedure is immediately repeated without incident or sequelae

No problem – vacuum ring is reinserted and procedure is immediately repeated without incident or sequelae

Yes – may cause a defective flap. Treatment must be aborted for at least 3 months and patient may have permanent injury in rare cases

Risk for partial flap or short flap

No problem – vacuum ring is reinserted and procedure can be repeated immediately without incident or sequelae. Reduced energy results in less local cell injury than 60 KHz

No problem – vacuum ring is reinserted and procedure is immediately repeated without incident or sequelae

Yes – occurs 1/2000 – requires that surgery be aborted and recut in 3 months or otherwise undergo complex PRK procedure

Risk for epithelial defect or sloughing

Extremely rare

Extremely rare

Common – occurs in 1 to 2% of primary cases. Results in slow visual recovery, unpredictable refractive endpoints and predisposes to epithelial ingrowth

Risk for Diffuse Lamellar Keratitis (DLK)

Very rare

Uncommon

Common

Risk for Transient Light Sensitivity (TLS)

Transient sensitivity to light – easily treated with eye drops – Extremely rare with new iFS – Does not cause problems with vision, is not permanent and has no long term sequelae

Transient sensitivity to light – easily treated with eye drops – Occurs in about 1% of cases – Does not cause problems with vision, is not permanent and has no long term sequelae

Never Reported

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