Ординатура / Офтальмология / Английские материалы / Ocular Periphery and Disorders_Dartt, Bex, Amore_2011

region, for example, damage caused by limbal stem cell deficiencies, results in serious corneal surface problems such as persistent epithelial defects, conjunctivalization with superficial vascularization, keratinization, scarring, etc., with an associated severe loss of vision.

In order to rescue such damaged ocular surfaces, surgical modalities have been developed over the past 20 years that are aimed at reconstructing the diseased ocular surface epithelium. In this article, we explain the surgical treatment for total stem cell deficiency.

Corneal Epithelial Transplantation for Total Stem Cell Deficiency

In Vivo Expansion of Corneal Epithelial Cells

(Keratolimbal Allografts)

History and concept of ocular surface reconstruction

The concept of ocular surface reconstruction was first introduced via an autologous conjunctival transplantation for unilateral chemical injury reported in 1977 by Thoft. Thereafter, Thoft described a new technique known as keratoepithelioplasty (KEP), which involves the transplantation of peripheral corneal lenticules harvested from donor tissue for the treatment of severe ocular surface diseases. Following this development, autografts or allografts of limbal transplantations were developed to improve the outcome of ocular surface reconstruction. These surgical procedures, which involve the utilization of donor limbal stem cells in conjunction with the peripheral corneal lenticules, are now classified as keratolimbal allografts (KLALs), a kind of in vivo expansion of limbal stem cells.

Indications

KLAL is a procedure in which limbal tissue with peripheral cornea is obtained from donor eyes and transplanted to the recipient eyes. KLAL is performed to treat severe bilateral ocular surface disorders combined with limbal

stem cell deficiencies. This procedure is also applicable to patients with unilateral disease who do not wish to use limbal tissue obtained from their healthy eye. KLAL is a surgery most suitable for diseases with total stem cell deficiency with less inflammation and less conjunctival cicatrization, such as ocular surface tumors (conjunctival intraepithelial neoplasia or squamous cell carcinoma) or aniridic keratopathy. For cases involving stem cell deficiency with severe inflammation, such as those resulting from chemical injury, Stevens–Johnson syndrome (SJS), or ocular cicatricial pemphigoid (OCP), a KLAL is applicable if the inflammation can be well controlled prior to surgery by steroid and immunosuppressive treatment and if conjunctival involvement is not severe. When the conjunctival scarring is severe, amniotic membrane (AM) transplantation combined with a KLAL is performed to reconstruct the conjunctival fornix (Figure 1).

Surgical procedure

For KLAL, fresh donor corneoscleral tissue preserved at 4 C is used. Ideally, the donor tissue should be as fresh as possible when used, with surgery being performed within 6 days after preparation. After the central cornea of the donor tissue is excised with a 7.0–7.5-mm trephine, the peripheral cornea with scleral rim is sectioned into four to five pieces of lamellar grafts (lenticules). The residual corneoscleral rim after conventional penetrating keratoplasty is also useful. Under a surgical microscope, the excess peripheral scleral tissue of each lenticule is removed by scissors. Then, the posterior two-thirds of the corneal stroma that is attached by Descemet’s membrane and the corneal endothelium are removed by lamellar dissection using spring scissors. After trimming the edge of each lenticule by spring scissors, the lenticules are placed onto the limbal area of the patient’s eye and secured with two to three interrupted sutures per lenticule using 10-0 nylon. Immediately after surgery, a therapeutic soft contact lens is placed on the ocular surface to prevent donor epithelial damage and promote smooth corneal epithelial healing.

Postoperative management

Compared to conventional penetrating keratoplasty, limbal allografts are at significantly higher risk for immunological rejection. Postoperatively, 0.1% dexamethasone and 0.05% cyclosporine A should be instilled topically, and dry-eye patients should receive preservative-free artificial tears. Systemic corticosteroids (betamethasone, 1 mg per day), cyclosporine A (3 mg per kg of body weight per day), and cyclophosphamide (1 mg per day), as well as mycophenolate mofetil (1 g per day), in some cases, should be used to prevent postoperative inflammation and immunological rejection. Systemic immunosuppression as described above should be used for at least 6 months postoperatively, after which it can be gradually reduced depending on clinical characteristics (Table 1). In many cases, it is necessary to administer a low dose of cyclosporine A (1 mg per kg of body weight per day) for up to 2–3 years.

A therapeutic soft contact lens should be used for several years after surgery, changing the lens once every 2–4 weeks, as it has been shown that continuous coverage of the corneal surface including the limbal area by use of a soft contact lens is effective for preventing immunological rejection as well as mechanical damage of the corneal epithelium. Although the working mechanism is unknown, it is speculated that the continuous soft contact lens wear may prevent the exposure of the donor limbal tissue to host immunocompetent lymphocytes in tears.

Other surgical procedures

Conjunctival limbal autograft (CLAU) is a procedure used for a unilateral stem cell deficiency, in which limbal tissue attached to a conjunctival carrier is transplanted from the healthy eye of the patient. The biggest advantage of this procedure is that no immunosuppression is required for an autograft. Living-related conjunctival limbal allograft (LR-CLAL) is a similar surgery to CLAU, yet in this procedure, a living relative of the patient is the source of the limbal

Table 1 Immunosuppressive treatment after all-corneal epithelial transplantation

tissue used for transplantation. LR-CLAL is applicable for bilateral stem cell deficiency. A major concern associated with both CLAU and LR-CLAL is the risk of stem cell deficiency of the donor eye. Prior to the surgery, it is vital to exclude any possibility of limbal stem cell damage in the donor eye, and it is important to continue observing the condition of the donor eye through postsurgical follow-up. Compared to the KLAL, the amount of limbal tissue that can be taken from the donor eye is limited in CLAU and LRCLAL. Thus, partial stem cell deficiency is the appropriate condition for the use of those two procedures.

Ex Vivo Expansion of Corneal Epithelial Cells

History and concepts

There is no doubt that corneal epithelial transplantations, including limbal autografts and KLALs, have helped to improve the outcome of ocular surface reconstruction in a number of situations. However, in severe ocular surface diseases, such as SJS or OCP, severe inflammation interferes with in vivo epithelial healing and results in a persistent epithelial defect. An alternative concept to the in vivo expansion of corneal epithelial cells is the ex vivo expansion of corneal epithelium using a tissue engineering technique.

For this purpose, many investigators endeavored to reconstruct corneal epithelial sheets on carrier materials, such as collagen sheets, and corneal stromal carriers to create stratified corneal epithelial cell layers. Some groups tried to reconstruct not only the epithelial sheet, but also three layers of corneal tissue – a corneal equivalent – using cell-line cells supported by natural and synthetic polymers. This kind of corneal equivalent is now ready to be used for testing toxicity and drug efficacy, but it is not ready for clinical application.

Despite the potential drawbacks of cultivated corneal epithelial transplantation, its first clinical application was demonstrated in 1997. A method was developed to reconstruct stratified corneal epithelial cell sheets on petrolatum gauze or a soft contact lens as a carrier. Two patients, who had unilateral chemical burns, were treated by transplanting cultivated corneal epithelial cells taken from the limbus of the healthy contralateral eye. The wellestablished keratinocyte-culturing method, which involves the use of 3T3 feeder layers to help maintain epithelial stem cells, was used.

AM as a suitable carrier for corneal epithelial cell culture

Researchers soon realized the potential of AM as a carrier for corneal epithelial stem cell culture. AM is the innermost layer of the fetal membrane, and it is composed of a monolayer of amniotic epithelial cells, a thick basement membrane, and an avascular stroma. AM has been used for several years in a range of ocular surgeries, with or

without limbal transplantation, and has proven to be useful for the treatment of thermal and chemical injuries, severe pterygium, persistent or deep corneal ulcers, OCP, SJS, and other limbal stem cell deficiencies.

AM is known to have many unique characteristics that are beneficial to ocular surface reconstruction. Notably, AM inhibits conjunctival fibroblasts by suppressing the transforming-growth-factor-beta signaling system, and it also prevents myofibroblastic differentiation of normal fibroblasts. Furthermore, the normal differentiation of conjunctival epithelial cells is encouraged after AM transplantation. The basement membrane of AM is reported to resemble that of the conjunctival epithelium as well as that of corneal epithelium. In addition, growth factors, such as epidermal growth factor (EGF), keratinocyte growth factor (KGF), and hepatocyte growth factor (HGF), detected in AM may also play a role in accelerated epithelialization after AM transplantation. A high and therapeutic level of nerve growth factor (NGF) is also reported to be present in AM. It has been reported that AM has antiinflammatory effects by inducing the suppression of interleukin 1a and interleukin 1b in limbal epithelial cells, as well as by trapping and preventing polymorphonuclear cells from infiltrating into corneal stroma.

Based on these interesting clinical and laboratory findings, preserved AM is considered to be one of the most appropriate carrier materials for transplantation of cultivated corneal epithelial cells. Although there is still debate surrounding its use, including the merits and demerits of the denuding process for amniotic epithelial cells and controversy regarding methods, the clinical results of cultivated corneal epithelial transplantation using an AM carrier are encouraging.

Cell culture procedure

Denuded AM is useful to promote the prompt migration of corneal epithelial cells in vitro, and AM has the potential to make well-stratified and differentiated corneal epithelial

cell layers that express corneal-epithelium-specific keratins K3 and K12. In our clinical experiences, we have found that the ocular surface condition of candidates for cultivated corneal epithelial transplantation is very severe, often accompanied by complications such as severe aqueousdeficient dry eye and eyelid abnormality. For these patients, we consider it essential to transplant well-stratified epithelial cell layers that have developed barrier functions with well-controlled proliferative activity in basal cells and differentiated superficial cells. For this purpose, a culture system using an air-lifting method to promote epithelial cells via tight-junction formation was developed. By airlifting, we have obtained cultivated epithelial cell sheets with smaller intercellular spaces in the superficial cells and with an epithelial barrier function. We have also attempted to transplant cultivated corneal epithelial cells, including limbal stem cells, and have developed a cellsuspension culture system capable of supplying cultivated corneal epithelial sheets that are well developed, potentially allowing the transplantation of more corneal epithelial stem cells.

Indications

In 1999, the Institutional Review Board of Kyoto Prefectural University of Medicine, Kyoto, Japan, approved the transplantation of cultivated corneal epithelial cell sheets. The use of cultivated corneal epithelial sheet transplantation was restricted to those patients who had poor visual prognosis with conventional corneal epithelial transplantation, such as a KLAL. Thus, cultivated corneal epithelial transplantation was performed on 39 eyes of 36 patients with total stem cell deficiencies such as severe chemical injury, SJS, and OCP (Figure 2). While the acute-phase eyes with persistent epithelial defects received cultivated corneal epithelial transplantation for the purpose of covering the corneal surface, alleviating intensive inflammation, and avoiding complications that accompany persistent epithelial defects, the chronic-phase eyes

received cultivated corneal epithelial transplantation to obtain better visual function.

Surgical procedure

In our surgical procedure, scarred conjunctival tissue overlying the ocular surface from the cornea is removed up to approximately 3 mm outside of the limbus. After removing the subconjunctival tissue, the small tips of several microsponges containing 0.04% mitomycin C are placed in the subconjunctival space adjacent to the cornea for 5 min and vigorous saline washing is then performed to prevent the development of subconjunctival fibrosis after surgery. A cultivated corneal epithelial sheet on AM is then transplanted onto the corneal surface and sutured using 10-0 nylon. A therapeutic soft contact lens is then applied. For the chronic-phase eyes with corneal stromal scarring, lamellar keratoplasty is first performed with the use of preserved donor grafts to replace the scarred corneal stroma, followed by cultivated corneal epithelial transplantation.

Postoperative management

Postoperatively, 0.1% dexamethasone and 0.05% cyclosporine A are instilled topically, and dry-eye patients receive preservative-free artificial tears. Systemic corticosteroids (betamethasone, 1 mg per day), cyclosporine A (3 mg per kg of body weight per day), and cyclophosphamide (1 mg per day), as well as mycophenolate mofetil (1 g per day), in some cases, should be used to prevent postoperative inflammation and immunological rejections. Systemic immunosuppression as described above should be used for at least 6 months postoperatively, after which it can be gradually reduced depending on clinical characteristics (Table 1). In many cases, it is necessary to administer a low dose of cyclosporine A (1 mg per kg of body weight per day) for up to 2–3 years.

Clinical outcome of allogeneic cultivated corneal epithelial transplantation

The epithelial integrity was satisfactory in all cases, as evidenced by the fact that the transplanted corneal epithelium did not stain with sodium fluorescein just after being transferred onto the ocular surface during surgery. In addition, in every case there was no epithelial damage to the transplanted corneal epithelium 48 h after transplantation. The transplanted AM did not disturb the visual acuity, and clarity increased day by day. Surprisingly, the preoperative ocular surface inflammation, which had not been controlled by conventional treatment, decreased rapidly after surgery in all of the acute-phase patients.

In the chronic-phase eyes, the long-term visual prognosis and epithelial stability were varied in the three kinds of diseases discussed below. In the case of severe chemical injury, the transplanted corneal epithelium was clear and stable up to 8 years after transplantation, and very little

conjunctival inflammation was present during the entire postoperative period. On the other hand, in patients with Stevens–Johnson syndrome, mild to moderate ocular surface inflammation occurred several months after surgery, and then decreased during the following 18 months. Whereas subconjunctival fibrosis had not progressed in the eyes with SJS, conjunctival scarring such as symblepharon and shortening of the fornix had progressed in the eyes with OCP. In most of the chronic-phase patients with SJS and OCP, the phenotypes of ocular surface cells on AM gradually changed from donor to host epithelial cells over a couple of years; however, subepithelial scarring and neovascularization did not progress. In other words, host conjunctival epithelium replacement on AM occurred without scarring. This phenomenon is considered to be partly due to a mild rejection of the transplanted corneal epithelial cells. Although graft survival was not very long in some eyes in these chronic cases, the ocular surface maintained its transparency and the patients obtained a better visual function than before surgery. It is possible to perform regrafting of cultivated corneal epithelium in which the severity of epithelial opacity progressed after an episode of rejection or persistent conjunctival inflammation.

In unilateral cases, autologous cultivated corneal epithelial transplantation is applicable. There is less damage to the contralateral eye than has been the case with limbal autografts, and the cultivated corneal epithelial sheets formed well-stratified epithelial layers from the very small amount of limbal tissue. After a substantial followup period, the transplanted epithelium remained transparent and stable, and the patient achieved good visual acuity with no complications in the healthy contralateral eye.

Ex Vivo Expansion of Oral Mucosal Epithelial

Cells

Concept

Due to the fact that severe ocular surface diseases are usually bilateral, allogeneic corneal epithelial transplantation (either KLAL or cultivated corneal epithelial transplantation) is normally performed. However, these procedures not only require sufficient donor tissue, but they also are accompanied by the risk of rejection; therefore, prolonged immunosuppression is required that severely affects the clinical results. With these drawbacks in mind, we have established cultivated oral mucosal epithelial transplantation using autologous tissue.

Cell culture procedure

Small oral biopsies (approximately 2–3 mm in size) are obtained from the oral cavity under local anesthesia. The biopsy specimens are then incubated with enzymatic reagents, such as dispase and trypsin – ethylenediaminetetraacetic acid (EDTA), to separate the cells from the underlying connective tissue. The resultant single-cell suspension

of oral mucosal epithelial cells is then co-cultured for 2–3 weeks on a denuded AM carrier with inactivated 3T3 fibroblasts. Toward the end of the culture period, an airlifting technique is used to facilitate epithelial differentiation and stratification. The oral mucosal epithelial cells cultivated on AM will show five to six layers of stratification and appear very similar to in vivo normal corneal epithelium. The cultivated oral mucosal epithelial sheet will show nonkeratinized, mucosal-specific keratins 4 and 13, and corneaspecific keratin 3; however, keratinization-related keratin 1 or keratin 10 will not be detectable. Under appropriate culture conditions, oral mucosal epithelial cells cultivated on AM have the potential ability to differentiate into cornealike epithelial cells.

Indications

In our clinical trials, we applied cultivated oral mucosal epithelial transplantation in two different forms of surgery with a total of 50 eyes. One form of surgery was reconstruction of the corneal surface of a severe bilateral corneal stem cell deficiency, using a cultivated oral mucosal epithelial sheet instead of allogeneic corneal epithelium. The other was reconstruction of the conjunctival fornix in patients with OCP, SJS, and chemical and thermal burns (Figure 3).

Surgical procedure and postoperative medications

The surgical procedure is almost the same as that of cultivated corneal epithelial sheet transplantation. After complete removal of damaged tissue on the corneal surface and subconjunctival fibroblastic tissue, residual subconjunctival tissue is treated for 5 min with 0.04% mitomycin C, followed by vigorous repeated washing with saline in order to suppress the excessive preoperative inflammation and subconjunctival fibrosis. Then, the cultivated oral mucosal epithelial sheet on AM is transplanted onto the corneal surface and secured with 10-0 nylon sutures at the limbus.

The integrity of the cultivated oral mucosal epithelium is then confirmed via intraoperative fluorescein staining, and a therapeutic soft contact lens is applied. Postoperatively, topical antibiotics and corticosteroids are usually applied. Due to the fact that it is an autograft, immunosuppressives are not necessary, except for corticosteroids and cyclosporine to control the inflammation of the original disease.

Clinical outcome of cultivated autologous oral mucosal epithelial transplantation

Using slit-lamp examination with fluorescein staining, the survival of the transplanted epithelium can be confirmed 48 h after surgery. An epithelial phenotype of transplanted cultivated oral mucosal epithelium will be somewhat distinguishable from the conjunctival epithelium by fluorescein staining. Our preliminary data show the successful survival of autologous cultivated oral mucosal epithelium on the ocular surface without returning to an in vivo oral tissue phenotype, as was previously the case with oral mucosal transplantation. This major difference can be explained by the elimination of the subconjunctival fibrous tissue and vascular component in oral mucosa during the tissue culture system. It is possible that AM has some effect on this phenomenon as well. One adverse effect of this procedure is that the transplanted cultivated oral mucosal epithelium can sometimes show some neovascularization in the peripheral cornea with epithelial thickening. For cases with poor visual recovery due to the optical corneal opacity, the two-step surgical combination of cultivated autologous oral mucosal epithelial transplantation followed by penetrating keratoplasty is advised.

Cultivated oral mucosal epithelial transplantation is also useful for reconstruction of the conjunctival fornix; this form of surgery is successful in cases of cicatricial pemphigoid, chemical injury, etc. However, it is important to be aware of abnormal postoperative fibrovascular proliferation caused by primary diseases, which is still critical to the long-term prognosis.

Phototherapeutic Keratectomy for Corneal Epithelial Disorders

Phototherapeutic keratectomy (PTK) using an excimer laser is a good therapeutic tool for a variety of corneal surface disorders, including corneal degenerations and dystrophies, epithelial adherence problems, persistent epithelial defects, corneal irregularities, and superficial stromal scars. PTK, with or without manual superficial keratectomy, can make the patient’s corneal surface smooth and can effectively improve visual acuity or relieve symptoms such as pain, glare, and tearing.

PTK for Corneal Epithelial Defect

Indications

Recurrent epithelial erosions associated with posttraumatic or epithelial basement dystrophy resistant to the conventional therapy, including lubricative medications, bandage soft contact lens, or epithelial debridement, are good indications for PTK. PTK is also very effective for Schield ulcer seen in patients with either vernal or atopic keratoconjunctivitis.

Surgical procedure

Prior to an excimer laser abrasion to the surface, any disadherent epithelium adjacent to the epithelial defect, but not degenerated epithelium, should be removed by gentle manual debridement. Then, a 10–20-0m abrasion should be performed either focally or diffusely depending on the area for treatment. The area to be abraded should encompass the area of epithelial defect and include 1 mm of adjacent cornea. Application of artificial-tear eye drops just prior to laser abrasion will enhance the smoothness of the cornea surface.

Postoperative management

After the laser abrasion, a disposable bandage contact lens is applied to enhance reepithelialization and reduce the pain. For the best results, the patient should wear the contact leans for up to 3 months to establish permanent epithelial-basement-membrane adhesions. Topical antibiotics and corticosteroids are essential to prevent infection and decrease postoperative inflammation. We suggest the instillation of 0.1% betamethasone and 0.3% levofloxacin 4 times per day for 2 weeks, followed by a taperingoff of the dosage.

Tectonic Lamellar Keratoplasty for Peripheral Corneal Ulcers

Concept

Lamellar keratoplasty is an operation in which diseased corneal tissue is removed and replaced by lamellar

corneal tissue from a donor. The procedure is performed either to improve vision (optical keratoplasty) or to provide structural support for the cornea (tectonic keratoplasty). Lamellar keratoplasty can be performed to replace just a portion of the corneal thickness when the endothelium is healthy. Depending on the location of the corneal abnormality, it may be sufficient to replace just the anterior layers with lamellar keratoplasty, or the full thickness of the corneal stroma without endothelium, with deep lamellar keratoplasty. Postoperative care involving the use of appropriate immunosuppressive therapy often influences the results of optic keratoplasty as well as tectonic keratoplasty.

Rheumatoid Arthritis

Peripheral corneal ulceration is associated with scleral or episcleral inflammation in rheumatoid arthritis (RA) patients. When corneal thinning progresses or perforation occurs in reaction to conventional steroid therapy, KEP combined with peripheral lamellar keratoplasty is effective. In RA patients, paracentral corneal perforation is also often observed. Small-size lamellar keratoplasty is a good surgical treatment for paracentral corneal perforation. However, it is important to bear in mind that the prognosis for penetrating keratoplasty is not good for RA patients.

Preand postoperative antirheumatic therapies improve surgical results. We recommend the application of therapeutic soft contact lenses at the conclusion of surgery; these should then be continuously used for several years to avoid the infiltration of immunoreactive cells from tears and prevent the recurrence of ulceration. Topical corticosteroids and antibiotic drops should be applied 4 times per day. The frequency should then be decreased as inflammatory signs subside to a level of two to three drops per day for several months. Careful removal of the sutures is performed during the first and second postoperative months to avoid epithelial damage. To avoid the recurrence of the original disease, corticosteroid drops should be continued once or twice per day for a number of years.

Mooren’s Ulcer

For the treatment of severe Mooren’s ulcer, which does not respond to steroids or immunosuppressive therapy using cyclosporine A, one surgical option which is available is KEP, with or without lamellar keratoplasty. By transplanting the Bowman’s layer with a thin corneal stroma onto the sclera adjacent to the ulcerated area, inflamed conjunctival tissue is unable to invade the corneal surface and cellular infiltration of the ulcerated peripheral cornea is prevented (Figure 4). Preand postoperative anti-inflammatory therapy, including systemic cyclosporine A, is important for the achievement of good surgical results. In our cornea service, we administer oral

cyclosporine A (100–200 mg per day) and oral betamethasone (2–4 mg), as well as topical 0.1% dexamethasone (4 times per day) and antibiotics (0.3% ofloxacin).

As mentioned earlier, careful removal of the sutures is performed during the first and second postoperative months to avoid epithelial damage. To avoid the recurrence of the original disease, the extended use of therapeutic soft contact lenses and corticosteroid eye drops (1–2 times per day) should be continued for as many years as possible.

See also: Contact Lenses; Corneal Epithelium: Cell Biology and Basic Science; Corneal Epithelium: Wound Healing Junctions, Attachment to Stroma Receptors, Matrix Metalloproteinases, Intracellular Communications; Defense Mechanisms of Tears and Ocular Surface; Lids: Anatomy, Pathophysiology, Mucocutaneous Junction; Stem Cells of the Ocular Surface.

Further Reading

Das, S. and Seitz, B. (2008). Recurrent corneal erosion syndrome.

Survey of Ophthalmolgy 53: 3–15.

Dua, H. S. and Azuara-Blanco, A. (1999). Amniotic membrane transplantation. British Journal Ophtalmology 83: 748–752.

Holland, E. J., Schwartz, G. S., and Nordlund, M. L. (2005). Surgical technique for ocular surface reconstruction. In: Krachmer, J. H., Mannis, M. J., and Holland, E. J. (eds.) Cornea, 2nd edn.,

pp. 1799–1812. Philadelphia, PA: Elsevier Mosby. Kenyon, K. R. (1989). Limbal autograft transplantation for

chemical and thermal burns. Developments in Ophthalmology

18: 53–58.

Kinoshita, S., Koizumi, N., and Nakamura, T. (2004). Transplantable cultivated mucosal epithelial sheet for ocular surface reconstruction.

Experimental Eye Research 78: 483–491.

Kinoshita, S., Koizumi, N., Sotozono, C., et al. (2004). Concept and clinical application of cultivated epithelial transplantation for ocular surface disorders. Ocularsurface 2: 21–33.

Kinoshita, S., Ohashi, Y., Ohji, M., and Manabe, R. (1991). Long-term results of keratoepithelioplasty in Mooren’s ulcer. Ophthalmology 98: 438–445.

Glossary

Aberrations – Phase deviations from the ideal wave front measured at the pupil plane. Aberrometers measure local ray deviations, which are fitted to the derivatives of the wave aberration, usually expressed as a Zernike polynomial expansion. A relevant aberration in the human eye (and particularly after standard refractive surgery) is spherical aberration (which results in peripheral rays converging on a different plane than central rays). A common optical quality metric derived from the wave aberration is the root mean square (RMS) wave-front error.

Ablation profile – Corneal tissue that needs to be removed at each location to produce the desired change in corneal power. Generally, the ablation-profile equation is converted into

the number of laser pulses to be applied at each location.

Asphericity – Parameter used to describe the deviation of the anterior corneal surface from a sphere. The corneal surface can be fitted to a conic section using the apical radius of curvature and the eccentricity e (variation of this curve with distance from the apex). Asphericity Q is defined as –e2, with the surface represented by the following equation: (X2+Y2)+(1+Q)Z2 2ZR = 0. For a sphere Q = 0, a typical cornea shows an asphericity Q = 0.26; a surface with zero spherical aberration should have an asphericity Q = 0.52; an oblate surface (exhibiting positive spherical aberration) will have

Q > 0; and a prolate surface (exhibiting low positive spherical aberration or negative) will have Q < 0. Beer–Lambert law – Law governing the photoablation of the corneal tissue by excimer laser. The depth of ablated material is proportional to the logarithm of the laser fluence (relative to the ablation threshold).

Contrast-sensitivity function (CSF) – The contrast-sensitivity function represents the minimum subjectively discernible contrast as a function of spatial frequency. It typically has an inverted-U shape, peaking at around 4 cycles per degree (c/deg), with sensitivity decreasing on either side of the peak. The shape of the CSF is determined by the properties of the visual neurons and the optical aberrations of the eye. Other factors affecting the

CSF are pupil diameter and luminance. CSF is a more sensitive measure of changes in visual quality following a change in the optics (such as that produced by refractive surgery) than visual acuity. Excimer laser – Laser producing stimulated emission after electrical discharge forming dimers or complexes, emitting typically ultraviolet (UV) light. Lasers applied in refractive surgery use a combination of Argon (as inert compound) and fluorine (reactive gas) and emit at 193 nm. The excimer lasers are well suited to remove exceptionally fine layers of surface material (particularly, biological matter and organic compounds) by disrupting the molecular

bonds of the tissue, through ablation rather than burning, leaving the remainder of the material almost intact. Lasers used in refractive surgery

have fluences typically ranging between 120 and 400 mJ cm 2.

Laser-assisted in situ keratomileusis (LASIK) –

Corneal refractive surgery technique which involves the creation of a thin flap on the cornea, folding it to enable remodeling of the tissue underneath with laser and repositioning the flap back after the corneal ablation has been performed.

Modulation-transfer function (MTF) – Optical function representing the contrast degradation by an optical system as a function of spatial frequency. Factors affecting the MTF are diffraction (pupil size), optical aberrations, scattering, and wavelength. The ocular MTF is a low-pass function, with a cut-off frequency at around 70 c/deg.

Munnerlyn formula – Equation on which standard ablation algorithms for corneal refractive surgery are based. The corneal tissue to be removed is a lenticule with an anterior radius of curvature equal to the preoperative corneal radius and the posterior radius of curvature equal to the postoperative corneal radius (easily related with the attempted correction). A parabolic approximation of the Munnerlyn formula states that the depth of the ablation (in microns) per diopter of refractive change is equal to the square of the optical ablation zone measured in millimeters, divided by 3.

Optical zone – Area of the cornea where the ablation algorithm is applied. For the same attempted

correction, smaller optical zones require less ablation depth. However, small optical zones can severely compromise vision if they are smaller than pupil diameter. A transition zone around the optical zone is created to avoid an abrupt step between treated and untreated areas.

Short Historical Background of Corneal

Refractive Surgery

Corneal refractive surgery has become, in recent years, a popular surgical approach to correct ametropia. Table 1 depicts highlights of refractive surgery history in chronological order. The first surgical procedure that achieved a change in corneal power by relaxation through corneal incision was radial keratotomy (RK). First attempts of incisional corneal surgery date from 1885 onward.

The technique evolved through the 1930s and 1940s to the early 1980s when Fyodorov developed a systematic and more predictable RK procedure that he applied to thousands of patients. In the 1960s, Barraquer invented keratomileusis – the first lamellar surgical technique. This technique consisted of separating a thin layer of the superficial corneal tissue using a microkeratome, removing a small piece of cornea, which was frozen and then reshaped using a lathe, and suturing it back into place. However, it was only in the late 1980s, when excimer lasers were developed at IBM, that their excellent properties for micromachining of biological tissue and organic materials were identified by Srinivasan. In the 1980s, Stephen Trokel applied, for the first time, an argonfluoride excimer laser to remove tissue in bovine corneas, following previous mechanical removal of the outer layer of the cornea (corneal epithelium) to treat ametropia, thus giving birth to photorefractive keratectomy (PRK). However, PRK was limited by unpredictability in higher ranges of refractive error and higher risk of corneal haze after surgery. In the 1990s, Pallikaris combined these two techniques (keratomileusis and PRK), creating the laser-assisted in situ keratomileusis (LASIK), which has become the most popular refractive surgery technique (see the section titled ‘The LASIK procedure’). Today, faster lasers, larger spot areas, bladeless flap creation, intraoperative pachymetry, and wave-front-optimized and wave-front-guided techniques have significantly improved the reliability of the procedure compared to that of 1991. Nonetheless, the fundamental limitations of excimer lasers, the limited corneal thickness (particularly in the presence of the flap), and undesirable destruction of corneal nerves have spawned research into many alternatives to standard LASIK, including laser-assisted subepithelial keratomileusis (LASEK) or Epi-LASIK, which aim at combining the advantages of surface ablations such as PRKwith those of LASIK surgery. Although safety and efficacy, and refraction predictability of PRK and LASIK are high, complaints of decreased vision and glare in mesopic and scotopic light levels, that is, night-vision problems, exist. Haze, halos, and increased optical aberrations are attributed to cause visual degradation, particularly in eyes that had undergone high refraction corrections. Several questions are still open today: proper transfer of the ablation profile to the cornea, wound healing, biological response, corneal biomechanics, microstructural stromal changes, and longterm healing. The implementation of aberrometry in refractive surgery has meant a turning point in the history of laser refractive surgery since – along with other technological advances including improvements in surgical lasers (such as flying spot lasers), ablation algorithms, and eyetracking – the measurement of ocular wave aberrations has opened the potential for improved refractive surgery, aiming not only at correcting refractive errors but also to minimize optical aberrations of the eye.

The LASIK Procedure

Figure 1 illustrates a LASIK procedure. In this technique, a hinged flap is created by means of a microkeratome, and folded back to leave the stroma exposed. To create the flap, a corneal suction ring is applied to the eye, holding the eye in place. Once the eye is immobilized, the flap is created. This process is achieved with a mechanical microkeratome using a metal blade, or more recently a femtosecond laser microkeratome. A hinge is left at one end of this flap. The flap is folded back, revealing the stroma, the middle section of the cornea. An excimer laser is then used to photoablate the stroma. For treatment of myopia, the central cornea is flattened and experiences a deeper ablation than the periphery. For a hyperopic treatment, the outer area of the optical zone experiences deeper ablation than the central area resulting in a conelike corneal profile. After the laser has reshaped the stromal layer, the LASIK flap is carefully repositioned over the treatment area by the surgeon and checked for the presence of air bubbles, debris, and proper fit on the eye.

Figure 1 Illustration of a LASIK procedure. A flap is lifted and the exposed stroma ablated with excimer laser pulses according to the programmed ablation profile following which the flap is repositioned.

The flap remains in position by natural adhesion until healing is completed.

Corneal Refractive Surgery, Wound

Healing, and Haze

The corneal response to laser refractive surgery induces keratocyte apoptosis immediately following the procedure. Proliferation and migration of keratocytes begins within 12–24 h, giving rise to activated keratocytes and myofibroblasts, which are critical components in the wound-healing cascade. Myofibroblasts and newly synthesized extracellular matrix play a major role in haze formation and regression due to stromal remodeling. The timing, intensity, and spatial distribution of wound healing vary significantly between LASIK and PRK. PRK involves injury on a broader area and removal of the epithelium, epithelial basement membrane, Bowman’s layer, and a portion of the anterior stroma, while LASIK leaves these structures relatively undisturbed, except at the flap margin. Figure 2 depicts immunohistology microscopic images of excised corneas in an avian model following PRK (5, 15, and 30 days following surgery), showing the distribution of myofibroblasts.

Refractive regression is a major challenge following PRK for myopia, hyperopia, and astigmatism, especially for high levels of correction, and is both more common and more pronounced than the regression following LASIK. The source of regression is attributed to differential changes in the thickness of the cornea due to a combination of stromal remodeling and epithelial hyperplasia. The intensity of the corneal response is related to the magnitude of attempted treatment.

Corneal Refractive Surgery, Optical

Aberrations, and Visual Quality

One of the most important side effects of standard refractive surgery is the induction of higher-order aberrations. Early