Corneal surgical techniques: Conjunctival pedicle grafts and beyond (Proceedings)
Corneal surgical techniques available for use in veterinary medicine are reviewed. Surgeries discussed include linear grid keratotomy, multiple superficial punctate keratotomy, diamond burr superficial keratectomy, thermokeratoplasty, keratectomy, conjunctival graft placement (pedicle, island, bridge, advancement, etc.), corneoconjunctival transposition flaps, penetrating and lamellar keratoplasties (corneal transplants), biosynthetic graft placement (A-cell, BioSist), and amnion graft placement.
Indolent corneal ulcers are poorly healing non-infected superficial corneal ulcers with a characteristic clinical appearance involving nonadherent peripheral epithelial "lipping." This occurs commonly as a primary condition in middle-aged dogs of certain breeds like the Boxer and Corgi. It appears to occur due to a combination of age and heredity. Histopathologically, a hyaline membrane is identified in the exposed anterior stroma, and breaking through this acellular membrane appears critical to promoting healing by exposing the Type I collagen. Linear Grid Keratotomy (LGK) and its close relative Multiple Superficial Punctate Keratotomy (MSPK) have been utilized successfully by veterinary ophthalmologists for years for this purpose. The techniques are described. Although they can be performed awake, the procedures are more safely and probably more effectively performed with sedation or anesthesia. Recently a battery-operated hand-held burr covered in fine diamond grit has been utilized by ophthalmologists to gently debride the ulcer beds of people and small animal patients with indolent ulcers following epithelial debridement. According to preliminary data, Diamond Burr Superficial Keratectomy (DBSK) appears to result in faster healing times than LGK, with few complications, and perhaps less scarring.
Thermokeratoplasty is another procedure which may be used to treat indolent ulcerations. Although it has been reported for use in treating primary indolent ulcers, this procedure is probably most commonly used to treat indolent ulcers occurring secondary to generalized corneal edema. Corneal endothelial dystrophy or degeneration results in progressive poorly medication-responsive edema in dogs. In its advanced stages, bullous keratopathy and ulcers occur. The procedure utilizes a handheld cautery unit (or less commonly a CO2 laser) to place multiple evenly spaced anterior stromal scars over the corneal surface, resulting in localized contraction at each site. Substantial fibrosis results from this procedure so it is reserved for end-stage eyes to promote comfort.Partial-thickness corneal opacities may be removed by keratectomy. Cats may develop darkly pigmented and necrotic corneal sequestra, particularly within chronically ulcerated corneas. Brachycephalic cats like Persians are more commonly affected and may have contributing conformational factors. Depth varies and may be difficult to evaluate with with accuracy preoperatively. Superficial sequestra may be carefully surgically excised in lamellar fashion under the operating microscope until no visibly abnormal tissue remains. If the resultant surgical corneal defect is shallow, it may be left to epithelialize on its own. Deeper defects require adjunctive tissue placement over the keratectomy bed. These procedures are the same as those described under stromal ulcerations below.
Corneal ulcerations may initially be treated medically, but rapidly progressive ulcers and those exceeding 50% stromal depth should be treated surgically in most cases. Conjunctival pedicle flaps have the advantage of supplying epithelium, physical support, and an immediate blood supply into the defect within the normally avascular cornea. It does not fill in the defect, however, and results in a fairly substantial scar. Variations of this conjunctival grafts may be performed and include island grafts and advancement flaps. Corneoconjunctival transposition flap is a form of autogenous corneal transplant in which clear corneal tissue attached to adjacent bulbar conjunctiva is elevated and slid into a central corneal defect, resulting in a clearer visual axis and better visual outcome. It also has the advantage of physically filling the defect and providing rapid wound strength. This procedure is excellent for filling a deep keratectomy defect and may even result in decreased rate of corneal sequestrum recurrence. It may not be the best choice for corneal ulcers with uncontrolled infection. Biosynthetic materials, such as BioSist and A-cell, are collagen substrates harvested from pigs and may be used in the eyes. They have the advantage of ready availability and relative ease of use (when compared to corneal transplants). They may be used alone or placed under conjunctival flaps as a stromal substitute. The materials may be used to overly the defect, but are more commonly sewn in place. Their tensile strength is probably insufficient alone to appropriately treat the deepest defects.
Corneal transplantation may be successfully performed in veterinary medicine. Fresh corneal transplant placement is limited, however, for practical reasons. In human ophthalmology, full-thickness fresh transplants may be used to treat corneal edema, but in veterinary medicine, partial thickness frozen transplants are the norm and are usually performed to provide tectonic support to compromised corneas. When frozen tissue is used, the corneal endothelium will not function. Some opacity is nearly inevitable with frozen transplants, whether full- or partial thickness, but the degree of opacity varies. "Buttons" of frozen corneal tissue may be used to replaced diseased tissue excised as part of other surgeries, such as posterior lamellar keratoplasty for deep stromal abscess in the horse.
Collagenolytic or "melting" ulcers are most commonly treated medically or by conjunctival graft placement. Placement of corneal tissue or collagen substitutes is unlikely to be useful. An alternative surgery is placement of sterile frozen amnion tissue over the defect. It may be used as a graft material and sewn in place, or used as an overlay, or both may be used simultaneously. With amnion overlay, the tissue appears to absorb infectious organisms as well as factors involved in perpetuating the meting process, and the graft material gradually sloughs off rather than becoming incorporated. Outcome varies, but the procedure appears highly promising for this frustrating disease.