Amniotic membrane transplantation (AMT) has been reportedly used for a variety of ocular surface problems including persistent corneal epithelial defects (PED), shield ulcer of vernal keratoconjunctivitis, partial limbal stem cell deficiency (LSCD) and conjunctival defects following excision of surface tumors and pterygia.1 Significant progress has been made in ocular surface reconstruction procedures and understanding limbal stem cell disease. The strategies of managing limbal deficiency include cadaveric or living related limbal transplantation in bilateral LSCD; simple debridement, AMT and allotransplantation for unilateral LSCD.2 The latest approach to limbal stem cell transplantation is based on cultivation of limbal stem cells on denuded amniotic membrane (AM).3 Preserved human amnion has been successfully used as a biological bandage, promoter of epithelialization, inhibitor of inflammation and angiogenesis, as well as a carrier for ex vivo cultured limbal stem cells. However, there remains a lack of evidence that AMT is better than the existing treatment modalities for the various clinical conditions described herein. The purpose of this review is to present a summary of current ophthalmic indications and outcome for AMT.
Scientific Basis of Clinical Application
The human AM is the innermost layer of the placenta. Histologically the amnion is a 0.02 mm to 0.5 mm five-layered membrane, composed of three basic layers.
- Epithelial monolayer
- Thick basement membrane
- Avascular, hypocellular stromal matrix
The epithelium consists of a single layer of cuboidal cells with a large number of microvilli on the apical surface. The basement membrane is a thin layer composed of a network of reticular fibers. Histochemically the basement membrane closely resembles that of the conjunctiva.4 The compact layer contributes to the tensile strength of the membrane. The fibroblast layer is the thickest layer of the AM made up of a loose fibroblast network. The outermost layer of the amnion is the spongy layer.
The basement membrane is one of the thickest membranes found in human tissue. This layer is resistant to current cryopreservation techniques. The structural integrity, transparency and elasticity of the amniotic basement membrane makes it currently the most widely accepted tissue replacement for ocular surface reconstruction. It is known to promote epithelial cell migration, adhesion and differentiation. It is an ideal substrate for supporting the growth of epithelial progenitor cells by prolonging their lifespan, maintaining their clonigenicity and preventing epithelial cell apoptosis.5 This action explains why AMT facilitates epithelialization for PED with stromal ulceration.6 In tissue cultures AM supports epithelial cells grown from explant cultures and maintains their normal morphology and differentiation. The resultant cultured epithelium can be transplanted with the AM to reconstruct damaged corneas.7 The AM can be used to promote non-goblet cell differentiation of the conjunctival epithelium.8
The basement membrane of the AM, cornea and conjunctiva contain collagen types IV, V and VII, in addition to fibronectin and laminin.4 Though the laminins are very effective in facilitating corneal epithelial cell adhesion Type V collagen helps in the epithelial cell anchorage to the stroma.9
AM produces basic fibroblast, hepatocyte and transforming growth factor (TGF). These growth factors can stimulate epithelialization and modulate proliferation and differentiation of stromal fibroblasts.10
The AM stromal matrix, rich in fetal hyaluronic acid suppresses TGF B signaling, proliferation and myofibroblastic differentiation of normal corneal and limbal fibroblasts as well as normal conjunctival and pterygium fibroblasts.11 This action explains why AMT helps reduce scars during conjunctival surface reconstruction, prevents recurrent scarring after pterygium removal and reduces corneal haze following photorefractive keratectomy. The stromal matrix also suppresses expression of certain inflammatory cytokines that originate from the ocular surface epithelia, including interleukin 1a, IL -2, IL-8, interferon γ, tumor necrosis factor-β, basic fibroblast growth factor and platelet derived growth factor.12 The AM attracts and sequesters inflammatory cells infiltrating the ocular surface and contains various forms of protease inhibitors.13 This may explain some of its anti-inflammatory properties.
Aminotic membrane graft (AMG) procurement, processing and preservation
Amniotic membrane is obtained from prospective donors undergoing Caesarean section, who are negative for communicable diseases including HIV, hepatitis and syphilis. Different protocols exist for the processing and storage.1415 According to Kim et al.15 the placenta is cleaned with balanced salt solution containing a cocktail of antibiotics (50 mg/ml penicillin, 50 µg/ml streptomycin, 100 mg/ml of neomycin as well as 2.5 mg/ml of amphotericin B) under sterile conditions. The amnion is separated from the chorion by blunt dissection. The separated membranes are cut in different sizes placed on nitrocellulose paper strips with the epithelial side up. Dulbecco Modified Eagles Medium/glycerol (1:1) is used for cryopreservation and the tissues are frozen at -80 degrees until further use.16171819 Amnion stored in 50-85% glycerol is reliable and effective for over a year, with the added advantage of antibacterial properties.20 Human AM deprived of amniotic epithelial cells by incubation with EDTA when freeze dried, vacuum packed and sterilized with gamma-irradiation at 25kGy retained most of the physical, biological and morphologic characteristics of cryopreserved AM.21 Lyophilized AMs were found to be impermeable to different strains of bacteria - Bacillus, Escherichia coli, Pseudomonas, Citrobacter, Flavimonas and Staphylococcus. The results indicate that AMs processed by air-drying are stable and can be stored under different environmental conditions without compromising their clinical performance.22
Fresh versus preserved AM
Both fresh and preserved AM have been found to function equally well when transplanted onto the ocular surface.23 However, there are certain concerns when using fresh AM. Ideally, serologic tests on the maternal donor must be done both at the time of procurement of the donor tissue and again six months later. This dual testing eliminates the slightest risk of disease transmission. With the fresh AM the time interval from tissue procurement to transplantation is short and prevents repeat testing of the donor. Patients have to be brought to the hospital at a short notice unlike with preserved AM, which allows more flexibility in scheduling surgery.23 A distinct disadvantage is wastage of unused tissue with non-preserved AM as opposed to frozen AM where up to 30 grafts can be prepared from one placenta.
The epithelial cells in fresh or preserved AMG are nonviable. The viability of amniotic epithelial cells may be associated with low-grade inflammatory response.19 The drawback with the preserved AM is the need for a -70o refrigerator, which precludes its use outside big institutions.
Principles of surgery
The main objectives of AMT are ocular surface reconstruction, promotion of epithelialization, providing symptomatic relief and reducing inflammation. There are three basic principles upon which the final technique is individualized.
Inlay or graft technique: When the AMG is tailored to the size of the defect and is meant to act as a scaffold for the epithelial cells, which then merges with the host tissue, it is referred to as a graft.24 The AM is secured with its basement membrane or epithelial side up to allow migration of the surrounding epithelial cells on the membrane.
Overlay or patch technique: When the AM is used akin to a biological contact lens in order to protect the healing surface defect beneath, it is referred to as a patch.2526 A patch also reduces inflammation by its barrier effect against the chemical mediators from the tear film. When used as patch the membrane is secured with its epithelial side up and it either falls off or is removed.
Filling-in or layered technique: In this technique the entire depth of an ulcer crater is filled with small pieces of AM trimmed to the size of the defect.27 A larger graft is sutured to the edges of the defect in an inlay fashion and an additional patch may help in preserving the deeper layers for a longer duration.28
The preferred surgical orientation of the AM on the ocular surface is with the epithelial side up. The stromal surface can be identified by the presence of vitreous-like strands that can be raised with a sponge. Hu et al.29 elaborated that AM stains with indocyanine green, rose bengal, trypan blue and lissamine green B. The dyes stain both the epithelial and stromal surfaces and may be useful in identifying the edges and wrinkles in the graft. Intraoperative staining with lissamine green B may be a simple and effective way to assist surgeons in the proper handling of AM, while fluorescein staining has no role.29
Corneal surface reconstruction
Non-absorbable sutures are used to anchor AMGs to the cornea. A single sheet of AM may be applied as an inlay graft or overlay patch and anchored to the superficial cornea with multiple interrupted 10-0 nylon monofilament sutures. A Weckcel sponge or blade is used to remove all cellular debris or exudates from the base of the defect in case of PEDs, shield ulcers and ulcerative keratitis. Loose epithelium surrounding an epithelial defect or over an area of bullous keratopathy is debrided using a fine forceps and a straight crescent blade [Fig 1 and 2]. The size of the graft should be at least 1 mm larger than the defect. The sutures must be placed circumferentially or parallel to the cut edge of the graft in an interrupted or continuous manner. The suture knots must be cut short and knots buried in corneal tissue.
If AM is used to fill in deep corneal ulcers, descemetoceles or perforations, a multilayered approach is preferred. Small pieces of AM may be layered into the defect or a single sheet may be folded on itself twice (blanket fold). In either case a larger patch is anchored over the entire defect in an overlay fashion as shown in Figs 3 and 4.
Conjunctival surface reconstruction
Vicryl sutures are used to anchor AM to the conjunctiva. Given the rapid healing ability of the conjunctiva, 8-0 or, 9-0 or 10-0 vicryl may be used for this purpose. The essence of the surgical technique in each of the indications is adequate dissection and removal of pathological subconjunctival tissue.
In order to anchor a sheet of AM to the fornix two sets of double armed 4-0 chromic gut sutures on a cutting needle may be used. The needles are passed from the AM surface through the inferior fornix, via the full-thickness of the eyelid and exit through the eyelid skin. The two needles of each of the two sets of sutures are passed through two segments of an encircling band and then tied.30
Ocular surface reconstruction
Extensive ocular surface damage seen in severe grades of chemical injury, Stevens Johnson syndrome (SJS) and ocular cicatricial pemphigoid warrants sequential surface reconstruction. It is important to ensure that all fibrotic tissue is meticulously dissected and removed from the corneal and conjunctival surfaces. The AMG must be a continuous sheet devoid of buttonholes. The lower lid is everted with a large chalazion clamp. A large sheet of AM is placed on the ocular surface and it is first anchored to the inner surface of the everted lower lid close to the lid margin using multiple interrupted 10-0 vicryl sutures. The anchorage to the inferior fornix is as described above. A continuous encircling 10-0 nylon suture is used to anchor the membrane at the limbus or the peripheral 360o cornea. In addition, multiple interrupted vicryl sutures are placed to attach the membrane to the inner lid surface, beyond the inferior fornix and onto the bulbar conjunctiva [Figs 5 and 6].
Indications of AMT in ocular surgery
Conjunctival surface reconstruction
Ocular surface squamous neoplasia (OSSN)
Corneal surface reconstruction
Non-healing stromal ulcers
Substrate for ex vivo expansion of limbal stem cells
Outcome of AMT
In SJS, ocular cicatricial pemphigoid and toxic epidermal necrolysis, immune-mediated inflammation must be controlled prior to surgery. Johnet al. first reported the beneficial effects of AMT in the acute stage of toxic epidermal necrosis.31 Honavar et al. evaluated the role of AMT as a preliminary step in the sequential management of SJS.32 Creditable improvements in the ocular surface were measured in terms of greater patient comfort, reduced surface inflammation, decrease in the severity of vascularization and absence of recurrent corneal erosions.
In 9/10 patients there was a significant improvement in the ocular surface with deepening of fornices. Barabino et al. noted a slight deterioration in the clinical effects with time owing to the ongoing surface inflammation33 and deep-seated conjunctival34 injury.
Chemical and thermal injury
Joseph et al. reported that AMT was not found to be useful in the restoration of the ocular surface in Grade IV burns.35 In very severe ocular burns involving 360o of the limbus and entire conjunctiva there is probably a total loss of epithelial stem cells, leaving little resource for the amnion to allow regeneration. Although AMT in eyes with acute ocular burns has advantages in terms of pain relief and rapid epithelialization in moderate grade burns, no definite benefit of AMT over medical therapy alone has been reported with respect to severe ocular burns.36 This contrasts with the series in which Meller et al. performed AMT within two weeks after injury in 13 eyes and concluded that AMT is effective in promoting re-epithelialization and reducing inflammation in the acute stage of chemical injury, thereby preventing scarring sequelae in the late stages.37 Performing AMT during the first 7-10 days following acute burns maximizes the effects of the treatment. Associated lid deformities, symblephara and conjunctival foreshortening complicate management of chemical injury in the late stages. AMT alone gives satisfactory results in partial LSCD [Figs 7-9]. In total LSCD it may be used as an adjunct to limbal stem cell transplantation.
Results of AMT for bullous keratopathy have been rather conflicting. Its efficacy has been studied in the palliative management of symptomatic bullous keratopathy with poor visual potential.383940 AMT may also be performed as a temporary measure in patients waiting for corneal transplantation and intolerant to bandage contact lens (BCL). However, long-term relief from AMT needs to be studied and compared with other modalities
Conjunctival tumors and OSSN'S
Amniotic membrane transplantation has been reported to be successful in conjunctival surface reconstruction after excision of benign as well as malignant tumors such as conjunctival melanomas, lymphomas and OSSN. When used as a graft to cover the conjunctival wound it provides a substrate for the migration of conjunctival epithelial cells. Surface lesions are particularly challenging when they arise multifocally or extend over large areas and warrant an extensive conjunctivectomy [Figs 10 and 11]. The advantages of AMT over conjunctival autografts and mucous membrane grafts in this scenario, include superior postoperative cosmesis, absence of donor site morbidity complicating the harvest of mucosal and conjunctival autografts (CAG) and the ability to clinically monitor local recurrence of tumor beneath the transparent AMG.41 Combined therapeutic approaches consisting of extensive tumor removal, cryotherapy, topical mitomycin C and AM allograft can be effective in the management of diffuse conjunctival melanomas arising from primary acquired melanosis (PAM).4243 Successful treatment of complex choristoma by excision and AMT was reported by Sangwan et al.44
PED signify varying degrees of LSCD and chronic inflammation. AM serves to provide a basement membrane substrate for the migration and adhesion of epithelial cells when used as an inlay graft. When used as an overlay patch [Figs 12 and 13] it facilitates epithelialization in a fashion akin to a BCL and by providing a barrier against inflammatory cells and mediators. The AM, being continuously moistened by tears, provides adequate hydration to the regenerating epithelium and protects it from the abrasive effect of an abnormal palpebral conjunctiva.45
Amniotic membrane transplantation may be considered an alternative method for treating PEDs that are refractory to conventional treatment such as lubrication, elimination of toxic drugs, BCL and punctal occlusion. Although results have been promising in the epithelialization of PEDs from various causes, early detachment of the patch remains a major problem despite the use of multiple sutures or a protective BCL.46
Pterygium excision with a CAG has gained worldwide acceptance as the most favorable technique as it has proven to be both safe and effective in reducing pterygium recurrence. The benefits of using AMT in pterygium surgery were first reported by Prabhasawat et al.47 They reported a recurrence rate of 10.9% for primary pterygium following excision with AMT in a prospective study, which was further reduced to 3% by modifying the surgical technique. They concluded that AM could serve as a useful alternative to conjunctival grafts when there exists a very large conjunctival defect to cover in primary double-headed pterygium, in previous multiple failed surgeries or in the context of preserving superior bulbar conjunctiva for future glaucoma surgeries.
Earlier reports on the use of AMG for recurrent pterygia, showed unsatisfactory results, with reported recurrence rates of 37.5% and 25% respectively.47 Following primary and recurrent unilateral pterygium excision respectively, recurrences were noted in 46 (19.4%) and one (33.3%) eyes after bare sclera technique, in five (16.7%) and 0 after primary closure, in 28 (26.7%) and 0 with AMG, in 42 (12.2%) and five (31.3%) with CAG and in nine (17.3%) and two (40%) with conjunctival limbal autograft (CLAG) [Figs 14 and 15].48 Solomon et al. reported in a non-comparative study that double-layered AMG combined with the intraoperative injection of triamcinolone significantly reduced the recurrence rates to 3% for primary and 9.5% for recurrent pterygia, a result that is comparable with that after CAG.49 Results of AMT as a first line measure for recurrent pterygia have been less favorable in comparison to CAG. A recurrence rate of 37.5% was documented following excision with AMT, which was considerably higher than the 9.1% obtained with CAG.50 Combining AMT with CLAG may be employed in the management of recurrent pterygia [Figs 16 and 17]. A combined approach including pterygium excision, AMT, CLAG and application of mitomycin C was reported to be beneficial in the management of chronically recurring pterygium in young patients.51
Recently, a larger study (47 versus 48 eyes) with a longer follow-up, found that single-layered AMG alone reduced the recurrence rate to 12.5%. The addition of intraoperative mitomycin C did not further reduce the recurrence rate.50
Shield ulcers of vernal keratoconjunctivitis
Severe shield ulcers that do not respond to surgical debridement and BCL may be eradicated with superficial keratectomy or excimer photo therapeutic keratectomy (PTK).
Amniotic membrane transplantation combined with surgical debridement is an effective alternative modality in the management of these ulcers.52 The renewed basement membrane promotes epithelialization, reinforces cellular adhesion and prevents epithelial apoptosis.
The surgical procedure involves complete debridement of the mucous plaque and cellular debris from the ulcer base and edge. The surrounding loose epithelium is gently peeled off until normal adherent epithelium is reached. The AMG is tailored to be a millimeter larger than the defect and sutured with 10-0 nylon interrupted sutures. Using this technique Sridhar et al.52 achieved a success rate of 94.7% with shield ulcers [Figs 18 and 19].
Although performed in an uncontrolled and non-randomized series of patients, studies indicate that AMT shows promise in selected cases for the restoration of the ocular surface and reduction of stromal inflammation in ulcerative keratitis. Amniotic membrane transplantation can be considered an effective alternative for treating persistent neurotrophic ulcers, non-traumatic corneal perforations and descemetoceles.5354 It can serve as a permanent therapy or as a temporizing measure until the inflammation has subsided and a definitive reconstructive procedure can be performed. Being a relatively simple procedure without risks of allograft rejection it could be particularly useful when faced with shortage of donor corneas. Chen et al. performed AMT after treatment of Pseudomonas keratitis with fortified antibiotics for at least one week. In 5/6 cases, after AMT it was noted that the progression of the lesions and stromal loss was limited.55 In a series of seven patients with herpes simplex virus or varicella zoster-induced severe ulcerative keratitis, 5/7 eyes healed after the first AMT.56 The stromal defect was filled up with multiple layers of AM. In eyes with deep corneal ulcers multilayer technique proved to be better than monolayer procedure.272857 More recently Hick et al. demonstrated the advantages of fibrin glue in association with AMT in the management of perforations of up to 3 mm in diameter. The combination had a success rate of 92% in perforated ulcers compared to 74% in non-perforated eyes with AMT alone.58
Lid and orbital surgery
There are limited reports on the application of AMT in oculoplastic procedures. Ti et al. evaluated the role of AM in the correction of cicatricial entropion.59 Park et al. successfully managed a patient with cicatricial entropion due to SJS using autologous allogeneic AM, in an attempt to reduce disease transmission.60 It has also been applied as a punctal patch for punctal occlusion in the treatment of dry eyes.61 Amniotic membrane has been used as a cover for orbital prostheses and successfully used for the closure of a conjunctival defect following hydroxyapatite orbital implant exposure.62 Stewart et al. successfully used AM for reconstruction of the upper lid and the fornix in cryptropthalmos.63 Due to its beneficial effect in facilitating rapid epithelialization it appears to be a promising substitute to conventional grafts like mucous membrane grafts.
A broad-spectrum topical antibiotic is used for one to two weeks initially, until the epithelium heals. Topical steroids are used for six to eight weeks in tapering doses to reduce surface inflammation. Systemic immunosuppression is not required.64
Complications of AMT
In the immediate postoperative period one may come across hematoma formation under the membrane.65 The blood usually absorbs or may need drainage, by making a small opening in the graft, if excessive. Premature degradation of the membrane and cheese wiring may need frequent repeat transplantations. Occasionally, a residual subepithelial membrane may persist in some cases and inadvertently opacify the visual axis.
The incidence of post -AMT microbial infections is as low as 1.6%.66 This value is much lower than the 8% reported with use of fresh AM. Gram-positive organisms are the most frequent isolates.67 Gabler et al. reported a case of sterile hypopyon after repeated AMT.68
Calcification occurs in about 12.8% of cases.69 White plaques have been attributed to ciprofloxacin therapy.25 The key to reducing postoperative complications is meticulous selection of both donor and recipient and maintaining high standards of quality assurance.
Ex vivo expansion of epithelial cells on the AM
One of the newer and exciting applications of AM has been its use as a carrier for cultured limbal cells for ocular surface reconstruction procedures.3 It is now accepted that the surface epithelium regenerates from progenitor stem cells located at the limbus (cornea) or fornices (conjunctiva), from which new cells migrate and differentiate into daughter cells.70
An alternative therapeutic strategy that has been in vogue recently is the concept of expanding limbal epithelial cells in vitro using AM as a carrier or substrate. Schwab et al. earlier showed with similar technique of cultured limbal transplantation in 19 eyes (18 patients) 75% success with ocular surface destruction with no complications.71 Expanding corneal epithelial cells from limbal biopsies ex vivo for use in ocular surface damage offers many significant advantages. One of them being only a small amount of limbal tissue is harvested from the uninvolved eye. Conventional limbal allografts require up to 12-clock hrs of limbal tissue and have the potential risk for limbal deficiency developing in the donor eye. Several studies reported excellent outcome of transplantation of cultivated limbal stem cell on denuded AM for LSCD.727374
Amniotic membrane as a carrier in this instance has several distinct advantages over the other substrates that have been used. The basement membrane of AM contains Type IV collagen and laminin which plays an important role in cell adhesion.75 Further, it acts as a natural substrate for the cell growth and when transplanted gets integrated onto the corneal surface. It also enables easier handling during transplantation. Tseng et al. proposed that culturing the explants on an intact AM with devitalized epithelium favors expansion of an epithelial phenotype that closely resembles limbal stem cells.
The success of AMT is dependent on the underlying condition and given the sub-optimal results in some indications, stringent case selection is recommended. The spectrum of clinical indications continues to expand and encompass a varying range of ocular surface pathology.76
It is now evident that AMT has certainly gained an acceptable position in the surgical armamentarium of the ocular surface surgeon. The relative ease of the procedure, repeatability and freedom from intraocular intervention makes it an attractive surgical option. The low rate of intraoperative and postoperative complications and the avoidance of immunosuppression are other advantageous features of this procedure.
The future of AMT looks promising and seems like it is here to stay in the management of ocular surface disorders. With continued technological advancements in tissue processing, newer preserved forms such as the low-heat dehydrated AM are being made commercially available. Sutureless applications with fibrin glue have been aimed at making the procedure easier and more comfortable for the patient.77
Further non-surgical innovations such as AMX and Prokera have made access to amnion easier than ever before. AMX is a topical application of AM extracts, currently available for use in Europe. Tseng et al. have devised Prokera, which comprises AM attached to a soft contact lens-sized conformer for easy insertion. The utility of AM in healing ocular surface defects is unquestionable. However, there is still a lack of evidence based on randomized controlled studies to prove the benefits of AMT compared to other alternative modalities of treatment.78
Source of Support:
Conflict of Interest:
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