"Performing a refractive procedure on a cornea that is too thin may result in blinding complications." Source: FDA website - When is LASIK not for me?
Iatrogenic keratectasia / ectasia is the most feared and dreaded complication of LASIK. The true rate of ectasia after LASIK is unknown. Ectasia is a particularly insidious complication of LASIK. The medical literature contains reports of late onset ectasia occuring several years after LASIK.
Risk factors for ectasia after LASIK include irregular pre-op corneal astigmatism and thin post-op corneal thickness under the flap. Operating on patients with pre-op thin corneas should be avoided.
There is no cure for ectasia. Treatments for the management of post-LASIK ectasia include hard contact lenses and intraocular pressure-lowering drugs. A corneal transplant may be required. Other experimental treatments include Intacs and UV-riboflavin corneal cross-linking.
What is corneal ectasia?
From The LASIK Report: The cornea is under constant stress from normal intraocular pressure pushing outward. The collagen bands of the cornea provide its form and biomechanical strength. LASIK thins the cornea and severs collagen bands, permanently weakening the cornea. This results in forward bulging of the cornea, which may progress to a condition known as keratectasia, characterized by loss of best corrected vision and possible corneal failure requiring corneal transplant.
The FDA, laser manufacturers, and refractive surgeons are aware of limits on flap thickness, ablation depth, and diameter of the optical zone imposed by corneal biomechanics. When the FDA initially approved lasers for LASIK, it established a minimum of 250 microns of corneal tissue under the flap after LASIK surgery to prevent corneal instability and progressive forward bulging. Subsequent reports in medical literature indicate that 250 microns is not sufficient to ensure corneal biomechanical stability. In response, some surgeons stopped performing LASIK or raised the residual stromal thickness limit in their practices. However, the majority of surgeons continue to observe the 250 micron rule initially established by the FDA, even though this limit has been shown to be insufficient.
The 250 micron rule is often violated inadvertently during surgery, as microkeratomes that cut the LASIK flap are unpredictable and produce flaps of varying thickness. For this reason, flap thickness should be measured intraoperatively. Most surgeons have not incorporated this important measurement into the surgical procedure prior to ablation, which places patients with thicker flaps at increased risk.
Is 250 microns safe?
Corneal thickness before LASIK varies between individuals, with an average of approximately 540 microns. The LASIK flap is generally intended for 160 - 180 microns in thickness, which is about thirty percent of the original thickness of the cornea. It is known that actual flap thickness varies from intended thickness. After the flap is cut and reflected back, the laser ablates corneal tissue to change the refraction of the eye. The amount of tissue ablated per diopter of refractive error varies between lasers. For the purpose of this discussion, let's assume the laser ablates 15 microns per diopter. A patient with myopia of -6 D will have 90 microns of tissue ablated. If the patient had an original corneal thickenss of 540 microns and the surgeon achieved a 160-micron flap, the patient would have 290 microns of corneal tissue remaining under the flap.
When the FDA approved LASIK, it established a minimum of 250 microns of residual stromal thickness (RST) under the flap after surgery. This guideline was just an estimate. No medical study had established that 250 microns was sufficient to prevent the cornea from bulging forward in response to the pressure inside the eye. In the years since LASIK was approved, numerous studies have examined corneal stability after LASIK. There are now many case reports of patients developing ectasia weeks, months or years after LASIK, even in cases of more than 250 microns of RST. In response to the rising number of cases of ectasia after LASIK, some surgeons raised their minimum RST and others abandoned LASIK altogether. There is no cure for post-LASIK ectasia. If ectasia progresses, a corneal transplant is necessary.
The LASIK flap weakens the stronger anterior cornea
LASIK dissects the stroma below Bowman's membrane. Doctors have discovered that the anterior stroma closer to Bowman's membrane is stronger than the deeper layers of the cornea (see article below). This could cause significant problems in the years after LASIK surgery.
The specific architecture of the anterior stroma accounts for maintenance of corneal curvature.
Br J Ophthalmol. 2001 Apr;85(4):437-43.
Müller LJ, Pels E, Vrensen GF.
AIM: To analyse the human corneal stroma in extreme hydration to discover if its structure is responsible for corneal stability.
METHODS: Corneas in several hydration states were used: postmortem control corneas (PM; n=3), corneas left for 1 day in phosphate buffered saline (PBS; n=4), and corneas left for 1 day (n=4), 2 days (n=4), 3 days (n=2), and 4 days (n=4) in deionised water. All corneas were fixed under standardised conditions and processed for light and electron microscopy. In addition, two fresh corneas from the operating theatre were studied which were processed 6 months after storage in sodium cacodylate buffer.
RESULTS: After 1 day in deionised water maximal stromal swelling was reached which did not change up to 4 days. The stroma of deionised water corneas (1400 microm) was much thicker than that of PBS corneas (650 microm) and PM corneas (450 microm). Deionised water treatment led to disappearance of all keratocytes leaving only remnants of nuclei and large interlamellar spaces. In these specimens the distance between the collagen fibres had increased significantly, but the diameter of the collagen fibres did not seem to be affected. A remarkable observation was that the most anterior part of the stroma (100-120 microm) in all deionised water specimens and those stored for 6 months in buffer was not swollen, indicating that the tightly interwoven anterior lamellae are resistant to extreme non-physiological hydration states.
CONCLUSIONS: The rigidity of the most anterior part of the corneal stroma in extreme hydration states points to an important role in maintenance of corneal curvature. Since a large part of this rigid anterior part of the stroma is either removed (PRK) or intersected (LASIK), it is possible that in the long run patients who underwent refractive surgery may be confronted with optical problems.
More scientific articles about post LASIK ectasia
Residual bed thickness and corneal forward shift after laser in situ keratomileusis.
J Cataract Refract Surg. 2004 May;30(5):1067-72.
Miyata K, Tokunaga T, Nakahara M, Ohtani S, Nejima R, Kiuchi T, Kaji Y, Oshika T.
PURPOSE: To prospectively assess the forward shift of the cornea after laser in situ keratomileusis (LASIK) in relation to the residual corneal bed thickness.
SETTING: Miyata Eye Hospital, Miyazaki, Japan.
METHODS: Laser in situ keratomileusis was performed in 164 eyes of 85 patients with a mean myopic refractive error of -5.6 diopters (D) +/- 2.8 (SD) (range -1.25 to -14.5 D). Corneal topography of the posterior corneal surface was obtained using a scanning-slit topography system before and 1 month after surgery. Similar measurements were performed in 20 eyes of 10 normal subjects at an interval of 1 month. The amount of anteroposterior movement of the posterior corneal surface was determined. Multiple regression analysis was used to assess the factors that affected the forward shift of the corneal back surface.
RESULTS: The mean residual corneal bed thickness after laser ablation was 388.0 +/- 35.9 microm (range 308 to 489 microm). After surgery, the posterior corneal surface showed a mean forward shift of 46.4 +/- 27.9 microm, which was significantly larger than the absolute difference of 2 measurements obtained in normal subjects, 2.6 +/- 5.7 microm (P<.0001, Student t test). Variables relevant to the forward shift of the corneal posterior surface were, in order of magnitude of influence, the amount of laser ablation (partial regression coefficient B = 0.736, P<.0001) and the preoperative corneal thickness (B = -0.198, P<.0001). The residual corneal bed thickness was not relevant to the forward shift of the cornea.
CONCLUSIONS: Even if a residual corneal bed of 300 microm or thicker is preserved, anterior bulging of the cornea after LASIK can occur. Eyes with thin corneas and high myopia requiring greater laser ablation are more predisposed to an anterior shift of the cornea.
Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness.
Am J Ophthalmol. 2002 Nov;134(5):771-3.
Ou RJ, Shaw EL, Glasgow BJ.
PURPOSE: To report corneal histopathology associated with keratectasia after laser in situ keratomileusis (LASIK) and to evaluate the thickness of the calculated residual stromal bed in two cases and those in the literature. DESIGN: Interventional case reports.
METHODS: Three eyes of two patients developed keratectasia after LASIK. Corneal specimens after penetrating keratoplasty in one eye of each patient were studied histopathologically, and the residual stromal bed was directly measured. For comparison, residual stromal bed thicknesses were calculated from published cases of keratectasia.
RESULTS: Two eyes of a 26-year-old woman and one eye of a 22-year-old woman developed keratectasia after LASIK. Calculated residual stromal bed thicknesses were 210, 213, and 261 microm. Histologic sections revealed focal scarring in the flap plane. The cornea specimens measured 75 and 118 microm thinner than calculated values immediately after LASIK. Transmission electron microscopy of one case revealed an average lamellar thickness of 0.94 microm. In 28 (49%) of 57 previous cases of keratectasia, the calculated residual stromal bed thicknesses were greater than 250 microm.
CONCLUSIONS: Both the flap and the stromal bed of the cornea may thin after LASIK. A residual stromal bed thickness of 250 microm does not preclude the development of keratectasia after LASIK.
Interferometric technique to measure biomechanical changes in the cornea induced by refractive surgery.
J Cataract Refract Surg. 2005 Jan;31(1):175-84.
Jaycock PD, Lobo L, Ibrahim J, Tyrer J, Marshall J.
Excerpts from the full text:
Given that the corneal stroma consists of lamellae thought to run from limbus to limbus across the corneal arc and that the lamellae consist of organized collagen fibers, loss of lamellar integrity may compromise corneal strength. A significant number of collagen fibers are severed in LASIK compared with a corresponding PRK procedure.4 Hence, the microkeratome flap in LASIK uncouples a significant proportion of the corneal biomechanics, which may affect refractive stability. With the significant reduction in the biomechanical integrity of the cornea, at worst LASIK has the risk for inducing iatrogenic keratectasia5, 6, 7, 8, 9, 10, 11 and at best, a propensity for long-term instability.
Although the incidence of iatrogenic keratectasia appears low at present, LASIK has only been used in clinical practice in recent years and the long-term results are unknown. Despite this potentially serious complication, the popularity of LASIK is growing at a fast pace and more than 5.5 million excimer laser refractive procedures have been performed. Although iatrogenic keratectasia is most common in eyes in which thin beds have been left, cases have been observed when the corneal bed after surgery is thicker than 250 μm, which is regarded by some as an arbitrary safe minimum corneal thickness.
When the microkeratome flap was replaced to cover the stromal bed, the out-of-plane surface movement was of the same order as the movement when the flap was removed, exposing the stromal bed. This indicates that although tissue bulk had been restored, the biomechanical properties remained altered relative to preoperatively because of uncoupling of the collagen fibril array.
The study showed that measurable changes occur in corneal displacement subsequent to microkeratome incisions and such changes are hardly influenced by relocation of the flap. Furthermore, such variations were measured with pressure changes of 0.15 mm Hg (20 Pa), corresponding to a 1% change in IOP. The apparent small difference in out-of-plane displacement (0.3 μm) between the operated and unoperated eyes should be considered in relation to the precision of the measuring technique; that is, 0.01 μm. Thus, these are real effects and should be of concern to the ophthalmic community.
Our results can be summarized as showing (1) a measurable change in out-of-plane forward movement or “bulging” of the cornea with increasing pressure and (2) a region of disharmony coincident with the plane of the microkeratome transsection.
In contrast, biomechanical considerations would predict instability with LASIK. This concept is derived from the finding that no repair transgresses the plane of the microkeratome incision and integrity in this region is established by deposition of ground substances such as fibronectin and tenascin. It is further supported by histological observation of corneas that have keratoplasty after LASIK. In such samples, although the bed remains intact, the collagen fibers within the flap show disorganization and atrophy. This finding supports the concept of reduced strain in the flap because of uncoupling from the stromal bed and atrophy of the fibers tectonically isolated from biomechanical movements engendered by processes such as accommodation. It is unfortunate that few studies have been published on the long-term effects of LASIK. The 1 study that reports 6-year data gives graphic information that is contrary to written claims in that there is a trend toward regression with time. If this trend becomes significant, it further emphasizes the need for a better understanding of the biomechanics of the cornea.
Read more about corneal ectasia after LASIK:Ectasia / Bulging of the Cornea After LASIK