Amniotic Membrane Transplant
Amniotic membrane, or amnion, is the innermost layer of the placenta and consists of a thick basement membrane and an avascular stromal matrix. Amniotic membrane transplantation has been used as a graft or as a dressing in different surgical subspecialties. In the field of ophthalmology, it is used broadly 1) to reconstruct the ocular surface after various procedures; 2) as a graft for ocular surface melts; and 3) as a bandage to promote healing in cases of persistent epithelial defects or ocular surface inflammation. All of these indications make use of amniotic membrane's ability to promote healing.
Live fetal membrane, which included both amnion and chorion, was first documented by De Rotth in 1940 for use in conjunctival reconstruction. A very low success rate was reported. During the same year, Brown used rabbit peritoneum as a temporary patch for ocular surface burns which Sorsby later modified with human amniotic membrane in 1946 and 1947. In 1995 Kim and Tseng reintroduced the idea of amniotic membrane for ophthalmic use which has since become increasingly popular.
Mechanism of Action
The basement membrane component of amniotic membrane is similar in composition to the conjunctiva. For this reason, current theory suggests that amniotic membrane augments support for epithelial cells, limbal stem cells, and corneal transient amplifying cells. Clonogenicity is maintained which promotes both goblet and non-goblet cell differentiation while excluding inflammatory cells and their protease activities. Furthermore, the stromal face of amniotic membrane suppresses myofibroblast differentiation of normal fibroblasts to reduce scar and vascular formation. This action assists in healing for conjunctival reconstruction, epithelial defects, and stromal ulceration.
After surgical removal of a pterygium, a conjunctival defect remains. This defect could be left alone to heal by secondary intention, sutured directly via primary closure, grafted with a conjunctival autograft, or grafted with amniotic membrane. With intraoperative steroid injection into the surrounding tissue defect, amniotic membrane transplantation has a comparable recurrence rate to conjunctival autograft. The application of mitomycin-C does not further reduce the recurrence rate according to some studies. There are many conflicting reports regarding the rate of postoperative pterygium recurrence after the use of amniotic membrane. Amniotic membrane could be considered when the size of the operative defect would be problematic for direct closure, or when a conjunctival autograft cannot be obtained due to scarring or presence of glaucoma devices.
Reconstruction of the Conjunctiva Surface
In addition to pterygium surgery, amniotic membrane transplantation (AMT) has been used for other conjunctival reconstructive purposes. Removal of ocular surface tumors results in a defect which is often revised with amniotic membrane. Surgical use of AMT for the revision of scars and symblepharon has also been well-documented. In cases of conjunctivochalasis that fail medical therapy, AMT has been used to reconstruct the ocular surface. Scleral melts have been treated in conjunction with cadaveric sclera and overlying AMT with good success. One report indicates that a leaking trabeculectomy bleb can be revised with amniotic membrane.
Limbal Stem Cell Deficiency or Persistent Epithelial Defects
Amniotic membrane can be used in cases of partial and total limbal stem cell deficiency. In cases of total limbal stem cell loss, AMT alone will not suffice and needs to be used in conjunction with allogeneic stem cell transplantation. For cases of partial limbal stem cell loss, and also in the case of persistent epithelial defects, amniotic membrane has shown to enhance epithelialization and improve vision with and without allogeneic limbal cell transplantation.
Newer techniques include using autologous and allogeneic stem cells cultivated in the laboratory on amniotic membrane and then transplanting this combined tissue onto severely damaged corneas with no endogenous stem cells.
Amniotic membrane has been used for both infectious and sterile ulcers which involve thinning and perforation. For ulcers with significant tissue loss, amniotic membrane may be applied in layers to build thickness to the defect. Such an intervention is intended to retard protease activity and to provide bulk for the defect in the hopes of promoting faster healing and avoiding cornea transplantation. It can provide a temporizing measure before cornea transplantation or possibly suffice as a permanent treatment. A thickened form of amniotic membrane can also be used as a patch graft in cases of infectious scleritis.
Ocular Surface Burns
Ocular surface burns may be secondary to direct heat (e.g. burns to the face) or be caused by chemicals (alkalis and acids). AMT performed immediately after an ocular surface burn has been reported to reduce the patient's pain, accelerate healing, and reduce fibrosis and neovascularization. The surgery involves securing a sheet of amniotic membrane over the entire ocular surface up to the eyelid margins.
Many reports in the literature illustrate the use of human amniotic membrane harvested from placenta at the time of caesarean section and preserved until use on the ocular surface. Cryopreserved amniotic membrane is available and commonly used, and it retains the histological and morphological properties of fresh tissue. Amniotic membrane can be surgically attached to the ocular surface by absorbable or non-absorbable sutures. Biological tissue adhesives have also been used to attach amniotic membrane to the ocular surface.
Allogeneic tissue has an implicit risk of infectious disease transmission. In general, amniotic membrane is procured from potential donors undergoing caesarean section who have been screened for communicable diseases such as HIV, hepatitis, and syphilis. The placenta is cleaned with a mixture of balanced salt solution, penicillin, streptomycin, neomycin, and amphotericin B. The amnion is separated from the chorion by blunt dissection under sterile conditions, attached to nitrocellulose paper strips, and stored in glycerol solution. The tissue is either stored in that solution for fresh use or cryopreserved at a temperature of -80 degrees Celsius. Currently, there are no published reports of communicable disease transmission from AMT. One report exists of a sterile hypopyon after repeated transplantation of human amniotic membrane onto the corneal surface.
- Amniograft & Prokera: http://www.biotissue.com/
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