Optimized Ablational Procedures in Perspective: Meditations on Flap Creation

During the last 5 years, advances in the area of flap creation and ablational technology with respect to laser assisted in situ keratomileusis (LASIK) have raised new questions about how and whether to create flaps prior to corrective laser surgery.  New mechanical microkeratomes and femtosecond photo disruption flap creators are the latest devices available for corneal flap procedures, while epithelial insitu-keratomileusis (Epi-LASIK) is a new, optimized surface ablation procedure which creates flaps at an epithelial level.  This article will attempt to determine which of these new techniques and tools produces the best refractive outcomes and which will ensure the most enduring quality of vision for future patients.

Mechanical Microkeratomes

The main features associated with modern microkeratomes is safety and flap predictability.  The latest generation of these devices are represented by the Moria M-2, the Hansatome XP, and the Carriazo Pendular Schwind Microkeratome.  Such microkeratomes have a variability in the flap thickness of about ±10 microns over attempted flap thickness, and their precision is considered superior to the previous generation of microkeratomes.  Mechanical microkeratomes are at this moment in competition with femtosecond technology in terms of flap thickness accuracy, surgical control, and optical flap configuration.  Miniscus flap configurations, which are thinner in the centre than on the periphery, are typical of mechanical microkeratome technology.

Differences in flap thickness between the centre and the periphery of the length and location of the hinge are one of the main causes of bias in the refractive outcomes of mechanical microkeratome LASIK.  The Carriazo Pendular is unique in the way that it offers a planar configuration of the LASIK flap.  The feasibility of this configuration is achieved through the use of a corneal indentation concept while the blade cuts the lamellar flap. The optical performance of the Pendular could be superior to others microkeratomes. 

Femtosecond LASIK

The use of femtosecond technology in LASIK surgery, at this moment available only through the Intralase company, has stirred a considerable amount of interest over the last 3 years.  Femtosecond  LASIK claims to create a flap with planar configuration of the 2 surfaces, leading to a neutral LASIK flap.  Moreover, flap thickness control seems to be optimized as well as flap location and hinge configuration.  While mechanical microkeratomes place the centre of the LASIK flap at the centre of the cornea, femtosecond LASIK places the flap at the centre of the pupil, a detail that renders a considerable amount of advantages when performing customized ablational procedures.

There are a number of reports that seem to place the precision of femtosecond LASIK flap thickness within plus/minus 10 µm of the tentative thickness.  A considerable problem from the beginning has been the increase of corneal wound healing and, especially, the increase of inflammatory complications such as diffuse interface inflammation or deep lamellar keratoplasty (DLK) and the so-called "Good Acuity with Photophobia" (GAP) syndrome, which is also known as Transient Light Sensitivity (TLT).  Both complications have been sharply reduced with the use of lower levels of energy to create the flap and, especially, with the use of the newest generation of Intralase devices based on 30 Khz technology.

EpiLASIK

Practitioners of EpiLASIK claim to be able to create a flap by dissecting the lamina lucida from lamina densa of the basal membrane structure.  To do so, the procedure relies on the same suction ring principle as microkeratomes and  a vibrating dissector that is able to perform the anatomical separation of the basal membrane.  This theory has been partially demonstrated in practice, even though the separation of the basal membrane seems to be at an irregular level, and although the corneal stroma to be invaded is preserved.  Unpublished studies have demonstrated that this principle works well in primary untouched corneas, but any scar or defect at Bowman’s membrane creates unpredictible invasion of the stroma which can reach more than 50 µ thickness (Alió JL. When To Do and Not To Do Epi-LASIK? Paper presented at: XXIII Congress of the European Society of Cataract and Refractive Surgeons (ESCRS); September 10, 2005; Lisbon, Portugal). For this reason, EpiLASIK is indicated in and should only be performed in pristine corneas.

EpiLASIK should not be performed following radial keratotomy (RK), photorefractive keratectomy (PRK), previous laser epithelial keratomileusis (LASEK) procedures, or in the presence of superficial corneal scars, keratoconus (due to the lack of consistency of Bowman’s membrane in some cases), and cases with old superficial keratitis with visible scars.  EpiLASIK claims to decrease the epithelial stroma interaction that leads to the development of haze.  It also claims to decrease the postoperative pain regarding PRK procedures.

In real practice, controversy exists over whether EpiLASIK causes less pain than PRK.  At the same time, the procedure challenges the viability of epithelial cells once they are separated from the corneal stroma at the basal membrane level. Most authors today assume that the epithelial cells are later substituted and die.  EpiLASIK, however, offers a unique opportunity to perform a perfect epithelial dissection, is much less invasive than the traumatic maneouvers to scratch the corneal epithelium, and is more precise than transepithelial PRK.  Whether the epithelial flap plays a role in the postoperative epithelial wound healing or not remains to be seen.

Refractive Outcomes

The ultimate goal of excimer laser surgery is to provide good refractive outcomes and quality of vision.  Specifically, the refractive surgeon is out to achieve spectacle-free correction without secondary symptoms and with the best possible visual performance.  It seems that today there is consistent evidence to support the following statements:

Corneal Biomechanics on Stage

The changes in corneal biomechanics induced by excimer laser surgery and LASIK flap are the main issues involved in the variability of the refractive and corneal aberration outcomes of excimer laser surgery.  Different studies have addressed this issue, but so far there is no final decision on how corneal biomechanics can be better controlled.  In this regard, EpiLASIK seems to cause the least amount of corneal biomechanical changes followed by femtosecond LASIK and standard LASIK (10th Winter Refractive Surgery Meeting of the European Society of Cataract and Refractive Surgeons (ESCRS); February 11, 2006; Monaco).  There is evidence that thin flaps (90-100 um flap thickness) are associated with significantly less corneal biomechanical changes and parallel the outcomes of surface ablational procedures (10th Winter Refractive Surgery Meeting of the European Society of Cataract and Refractive Surgeons (ESCRS); February 11, 2006; Monaco). 

Looking to the Future

Optimized surface ablational procedures have the disadvantage of postoperative pain, slower visual recovery, and less immediate patient satisfaction.  In today’s world, these issues affect the popularity of excimer laser surgery.  For these reasons, it seems that optimized LASIK procedures using high-precision, mechanical microkeratomes capable of producing thin flaps with planar flap configuration and femtosecond technology will be the focus of the future.  Femtosecond LASIK offers a special attraction to patients and surgeons, due to the increased technological and surgical control.  It might be that the future of refractive surgery will revolve around further developments of optimized LASIK procedures which parallel the advantages of today's optimized surface ablational procedures (OSAP).

References

Author Disclosure

The author discloses a financial interest as a consultant or clinical investigator of Intralase, Moria, Schwind, and Bausch & Lomb companies.  He has no proprietary interest in any of the products discussed in this article.