WOUND HEALING AFFECTINGVISUAL OUTCOME IN LASIK
Director Netradhama Eye Hospital, Bangalore.
Wound healing forms an integral part of the final visual outcome of refractive surgery. Poor healing is often due to poor patient selection or an inadvertent intra-operative event. LASIK involves the creation of a corneal flap the depth of which is upto one-third to one-fourth of the total corneal thickness. Laser ablation is performed within the corneal stromal bed after the hinged flap is raised. Healing of this flap is an essential part of the visual outcome.
Anatomy of the corneal stroma
The corneal stroma forms the bed on which excimer laser photoablation is performed. It consists of extracellular matrix components including collagen fibres, proteoglycans and keratocytes (specialized forms of fibrocytes that maintain the corneal stroma by synthesizing the corneal stromal extracellular matrix components.
Healing patterns can be discussed under
Extracellular matrix component alterations
Role of collagen fibres
Within 24 hours after excimer laser treatment the keratocytes in the treated anterior stroma disappear by a mechanism called Apoptosis or programmed cell death, which is initiated by secretion of interleukin IL-K secreted from regenerative corneal epithelial cells.
Following is the sequence of events:
• enclosed with floppy
Extracellular matrix and collagen fibres
Collagen fibrils normally run parallel to the corneal surface. Immediately after excimer laser treatment the surface structure of the corneal stromal bed is not smooth, due to multiple elevations composed of collagen fibrils. An irregularly ablated surface of the cornea may induce increased wound healing responses and corneal scar formation. New generation excimer lasers specifically address the issue of uniformity of the stromal bed. Four weeks after the treatment, the surface of the corneal stroma becomes smooth with newly synthesized collagen fibrils on its surface. Visual performance is significantly dependent on the stromal wound healing. Patients may experience fluctuations in vision for the first few weeks due to corneal healing at the cellular level. Due to the structural changes in the collagen fibrils of the corneal stroma there may be an increase in light scattering, which is clinically recognized as corneal haze. This is a transient feature. As scar tissue is gradually replaced by the normal components of the corneal stroma by a process called ‘remodelling’, the corneal stroma becomes clear.
Wound healing affecting visual outcome
There are various factors affecting the visual outcome following LASIK. They may be either flap-related or excimer laser-related.
Flap related factors
1. Epithelial ingrowth
2. Stromal melting
It is the extremely rare occurrence of proliferation of epithelial cells under the corneal flap after LASIK. It is usually observed in the first two or three weeks after LASIK. Epithelial ingrowth and its progression may be central or peripheral, rapidly growing, stationary or self-limiting.
The epithelial ingrowth is removed by re-lifting the flap and scraping away the epithelial cells. This is indicated only if a progression is seen beyond 2 mm from the flap edge or if it affects visual acuity.
It is the destruction of normal stromal connection structure with thinning or abscess formation of portions of the flap. It is extremely rare and is generally caused by poor patient selection - all conditions associated with poor blinking or wetting by the tear film such as filamentary keratoconjunctivitis and giant cell keratoconjunctivitis.
Post operative advantages of a large flap
There is a larger area of adherence to underlying tissue, resulting in a more stable cornea. Re-epithelialization is rapid as the new epithelium forms peripherally with minimum foreign body sensation.
Excimer laser related
3. Non-specific diffuse intra-lamellar-keratitis
Over or under correction may occur in 1-2% of patients. They may be attributed to
—Full correction of myopia/myopic astigmatism in elderly patients in whom the refractive response to ablation is higher due to collagen maturity and low regression effect due to lower elasticity with age.
—Underestimating the counter effect of the astigmatism ablation during a surgical plan to fully correct high astigmatism. The counter effect of the ablation leads to corneal flattening generally in the opposite axis.
—Occasionally laser beam calibration or ambient air conditions may alter results.
The term used for an unstable refractive result even after 6 months.
Regression may occur due to a combination of epithelial hyperplasia and remodelling of the stroma. The epithelium becomes thicker in response to flattening of the cornea. Large optical zones can reduce or limit the factors that cause central epithelial hyperplasia and present a smooth and more progressive profile of the anterior corneal curvature. Central fibrosis can occur in patients with severe myopia under the ablation area. This signals that active stromal reshaping is taking place. If a single zone technique is used to correct severe myopia and the optical zone is too small to conserve stromal tissue, regression will be greater as compared to large optical zones or with multi-zone treatments.
It is important to differentiate regression from corneal ectasia or iatrogenic keratoconus. These occur when an insufficient thickness of corneal stromal tissue is left post-treatment, causing the cornea to bulge outwards. To be considered in high myopes in whom the regression is not stabilized. It is of utmost importance that the residual stromal bed be at least 250 m thick.
Non-specific Diffuse lamellar keratitis
It is an inflammatory keratitis wherein sterile inflammation of the interface (under the corneal flap) is present secondary to inflammation. PMN granulocytic neutrophils are found. The hypotheses postulated are:
1. Polymorphonuclear inflammatory reaction in response to a corneal insult
2. Immune reaction to antigens or toxins in the interface
3. Inflammation due to reaction of the stroma or toxicity from UV light
4. An idiosyncratic reaction to any substance present in the stroma.
Trauma and epithelial defects are associated with this syndrome and may be risk factors. Topical steroids are the mainstay of treatment, by preventing degranulation by PMN enzyme cells. Good results are achieved by cleaning the flap alone.
LASIK involves creation of a corneal flap and laser ablation of the exposed corneal stromal bed. Various flap-related and excimer laser-related factors may delay wound healing, some of which are due to immune-mediated or cellular reactions to the remodelling of the cornea and occasionally poor patient selection. A procedure that is performed meticulously with the best technology coupled with surgeon skill and adherence to guidelines results in very minimal reaction, a safe and predictable treatment and a stable post-operative refractive status.