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Lecture Topic: APPLICATION OF TISSUE ALLOGRAFT IN OPHTHALMOLOGY
Many kinds of allograft are used in Ophthalmology. These include cornea, limbus, sclera, fascia lata, dura mater, amnion, and hydroxy apatite etc. Autografts such as conjunctival graft, mucosal graft, dermis fat graft and split thickness skin graft are also used.
Two major types of corneal transplantation (keratoplasty): therapeutic keratoplasty and refractive keratoplasty.
Guillaume Pellier (1789) first suggested to use a glass disc. Then Samuel Bigger (1835) suggested to use pig corneas. But the first successful keratoplasty was done by Stocker (1952) for corneal edema. Two major types of therapeutic corneal transplantation: lamellar keratoplasty and penetrating keratoplasty.
Lamellar keratoplasty or split thickness transplantation:
Removal and replacement of less than the total thickness of the cornea is done. The host descemet’s membrane and endothelium are left intact as the base onto which donor tissue is laid. This technique is used for anterior stromal or surface irregularities, but the endothelium is intact. The disadvantages include limiting of visual function due to opacification and vascularization of the interface between host and donor tissue, diffculty of the technique, limiting of indications only in superficial diseases.
Penetrating Keratoplasty or full thickness transplantation
This technique is for the defect in corneal endothelium or if the stroma is deeply ulcerated or perforated. The goal of penetrating keratoplasty is to remove opaque or diseased cornea that occludes the visual axis or threatens the integrity of the eye. Donor cornea in penetrating keratoplasty, in contrast to lamellar keratoplasty, is essential to have a viable endothelial layer, to keep the graft clear.
Preservation of donor cornea:
Moist chamber at 40C. This is a universal method of eye bank storage. The upper time limit is approximate 48 hours. The major advantage of this method are the efficacy, simplicity and uniform availability.
Tissue Culture medium: Several kinds of culture media are available. (I) McCarrey-Kaufman or M-K medium, which was introduced in 1974, consists of TC 199 with 5% dextran and antibiotics, can storage donor cornea upto 4 days at 40C, (II) new solutions that contain chondroitin sulphate in tissue culture medium. Ex: K-sol, CSM, Dexsol, Optisol which can storage cornea upto 14 days at 40C.
Organ Culture: Cornea can be stored at 370C for 35 days in this medium. Disadvantages are the degeneration of central stromal keratinocytes, easy to contamination of the medium, complexity and high cost of the system.
Cryopreservation: Expensive and complex technique. Can store for years.
Glycerin: Long term preservation but not preserve viable endothelial cell. Used for lamellar keratoplasty or emergency patch graft.
Sclera has been used as a tectonic material for the globe and adnexa for many years. The advantages are: (I) The vascularity: So it lack hypersensitivity reactions to cause rejection of the graft, (II) the strength and the flexibility of the tissue due to triple coat of interwoven fiber, (III) The elimination of surgery for autogenous grafts, (IV) the availability of tissue through eye banks, (V) ease in manipulation and preservation, (VI) unchangeable over a long period of time.
Nowadays sclera has been used in many procedures including the treatment of lid retraction or lid malpositions, enucleation surgery, repair of exposed orbital implants, retinal detachment surgery, scleral patch in scleromalacia or corneal perforation, reinforcement in myopia, sclera patch in trabeculectomy, repair of periodontal defect and myringoplasty and tympanic membrane reconstruction.
Banked scleral graft can be preserved by several methods including: frozen, lyophilized, -300C stored in neosporin solution, 70-95% ethanol sloution, glycerol, and formaldehyde.
The use of sclera in Ophthalmology:
Correction of cicatricial entropion and trichiasis
Correction of lid retraction in Grave’s ophthalmopathy and over correction in ptosis surgery.
Treatment of exposed or extruded orbital implants
Treatment of enophthalmos after enucleation
Tectonic material for scleral and corneal perforation.
Scleral reinforcement in myopia
The use of fascia lata in Ophthalmology:
In the cases of ptosis, demonstrating poor or no elevator function such as congenital ptosis, blepharophimosis syndrome, mechanical ptosis because of the lid tumor, need to correct the ptosis by frontalis suspension.
Banked human fascia lata obviates the difficulties of autograft but has an inferiority because of its lower success rate. In short-term follow-up reports found that sucsess rates of preserved fascia lata have been 90 to 97% and the failures have usually occurred early in the postoperative course, within 3 to 6 months after implantation. A success rate from frontalis suspension surgery using lyophilized human fascia lata was 50% at 8 to 9 years. Fascia lata can be used mainly for following situations:
Correction of lid malpositions
Orbital floor fracture
Extruding orbital implant
Retinal detachment surgery
Sclero malacia perforans
Scleral reinforcement in myopia
After being cut into 5mm strips from donors, it was soaked in balanced saline solution and placed under sterile conditions into polyethylene envelops with 1- 2 ml of the same solution. The package was heat sealed and placed in a second package that was similarly sealed. The fascia was then subjected to 3-4 million rads of gamma radiation from Cobalt source.
Fascia lata is stored in moist packages and sterilized by 3 to 4 million rads of gamma irradiation. It can store at room temperature safe upto one year.
Tissue specimen were frozen at –1960C and lyophilized in individual glass container. Lyophilization over a period of five days dried the specimen to a residual moisture of 5%.
Lyophilization offers the distinct advantage of markedly decreasing the antigenic potential of the graft while preserving its tensile strength, besides. It is easy to store at room temperature and unlimited shelf life.
Dura mater can be used in correction of lid, fracture floor of orbit, retinal detachment, tectonic graft in scleromalacia, scleral reinforcement and graft in pterygium.
Hydroxyapatite is a new orbital implant material used for enucleation, evisceration or secondary implant to replace the volume of orbit. Hydroxyapatite is completely biocompatible, nontoxic and non allergenic. Besides in enucleation and evisceration, hydroxyapatite implant have been used in many procedures including repairing orbital floor fractures, maxillofacial onlay grafting, alveolar ridge augmentation, cranial reconstruction, middle year reconstruction and laryngeal framework support.
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