- CLINICAL DESCRIPTION:
- Penetrating keratoplasty (PK) is a surgical procedure in which the host cornea is replaced with donor cornea. Corneal graft sizes typically range from 7.5 to 8.5 mm. Sutures used to keep the graft in place can be radially interrupted sutures or a single continuous suture. Post-operative care is focused on controlling inflammation and preventing infection and includes the use of corticosteroids and prophylactic use of antibiotics.
- CONTACT LENS FITTTING:
- Contact lens fitting after penetrating keratoplasty is sometimes necessary for adequate visual acuity. Typically we begin fitting 6 to 12 months after surgery following removal of the sutures. The epithelium is intact 4 days post-operative, but the cornea as a whole may take 18 to 24 months for complete healing. The fitting process can begin as early as 3 months for some patients who require contact lenses for functional vision. However, these patients may require numerous lens changes as sutures are removed. Thus, it is best in most cases to wait at least 6 months before initiating contact lens treatment.
- The main concern of post-PK fitting is to minimize trauma to the corneal graft. Contact lenses can cause mechanical and physiological stress that can lead to infection or graft rejection. Typically, large diameter (9.5-12.0mm) RGP lenses are prescribed to minimize bearing on the graft-host interface and provide improved stability and centration. A large optic zone size will help to minimize glare. RGP lenses offer excellent oxygen transmission and have the ability to correct astigmatism and smooth out irregular corneal surfaces.
- When fitting the post-PK patient a careful evaluation of the central and peripheral cornea is warranted and best done with corneal topography. The corneal shape resulting from the graft procedure predicts which type of contact lens will be the most effective. Waring et al.4 and others have divided the topography into classifications with RGP fitting suggestions (Tripoli et al) 5. A prolate shape has a steeper central area and a flatter periphery, sometimes referred to as a proud graft. This shape is seen in 31% of post-PK corneas. An aspheric, biaspheric (Boston Envision) or in cases of a very steep graft a keratoconic lens design would be appropriate for a prolate shape. An oblate pattern is plateau shaped and is present in 31% of graphs. The donor cornea is flatter than the host cornea and can appear sunken. A "reverse geometry" (PK Bridge, Conforma Laboratories) lens with a flatter center and a steeper secondary curve would be suitable for this type of graft. Mixed prolate/oblate corneal shapes (18%) present with a flat side and a steep side with symmetrical astigmatism and can be corrected using a bitoric RGP lens. Asymmetrical astigmatism (9%) can be described as a combination of patterns with an irregular or possibly distorted cornea. Depending on the amount and location of irregularity a large standard tricurve, aspheric, or keratoconic design may be appropriate. One of the most challenging topographies to fit is the "steep-to-flat" pattern (13%). The steep meridian is 180* from the flat meridian; an extreme example is demonstrated in graft tilt. Lens centration is difficult, so large diameter lenses with large optic zone sizes and possibly aspheric curves are recommended.
- The initial trial lens base curve selection can be based on topographic maps, usually the average dioptric value 3mm from the center of the map, or on the average keratometry values. Use fluorescein to evaluate the base curve and peripheral curves. A goal of "divided support" with a balance of 1/3 touch and 2/3 clearance has been suggested to provide an even distribution of support. A tight lens will likely lead to lens adherence and compromise the cornea, while a lens that is too flat may cause mechanical injury and possibly corneal scarring. Lens centration and position are dependent on shape of the graft and lenses are not always centered. Lenses will move in the direction of least mechanical resistance and commonly demonstrate temporal and nasal displacement.
- In cases where sutures are present and a successful RGP fit cannot be obtained due to decentration or mechanical irritation of the sutures, a SoftpermTm (Ciba Vision Corp.) (central RGP surrounded by a soft contact lens skirt) or a piggyback system (RGP with a soft lens underneath) can be used to mask corneal astigmatism, improve lens centration and comfort. The Softperm lens must be carefully monitored, however, as it may become tight-fitting. In addition, the Dk/t values of both the Softperm and piggyback systems are significantly reduced compared to RGP lenses alone. Thus, neovascularization and corneal hypoxia are possible complications.
- Soft contact lenses for visual rehabilitation following PK can be used in cases with aphakia and anisometropia where vision could be theoretically corrected with spectacles, but would be cosmetically unattractive or cause aniseikonic symptoms. In patients with low or regular astigmatism, a soft toric contact lens can be used. Specialty soft lenses such as the Flexlens and Flexlens Toric (Paragon Vision Sciences) are available in extended base curves and powers and are helpful in fitting patients who are unable to be fit in standard lens parameters. Soft contact lenses with higher oxygen permeability are suggested to avoid complications of corneal hypoxia and neovascularization.
- BEST REFERENCES:
- The Eye Bank Association of America. October 2001. [www.restoresight.org] 1015 18th Street NW, Suite 1010 Washington, D.C. 20036 WWW site material copyright ©1996 -- The Eye Bank Association of America ©1996
- Maeno A, Naor J, Lee H, Hunter W, Rootman D. Three decades of corneal transplantation: indications and patient characteristics. Cornea 19(1):7-11, 2000.
- Price FW Jr, Whitson WE, Collins KS, Marks RG. Five-year corneal graft survival. A large, single-center patient cohort. Arch Ophthalmol. 111(6):799-805, 1993.
- 4. Waring G, Hannush S, Bogan S, Maloney R. Classification of corneal topography with videokeratography. In: Schanzlin DJ, Robin J (eds.) Corneal Topography: Measuring and Modifying the Cornea. New York; Springer Verlag 1992:70-71.
- Tripoli Nk, Ibrahim OS, Coggins JM, et al. Quantitative and qualitative topography classifications of clear penetrating keratoplasties. Invest Ophthal Vis Sci 30(suppl):480, 1990.
- OTHER REFERENCES:
- Collins R, Tate T. Managing sunken corneal grafts. Contact Lens Spectrum 16(1):39-40,42, 2001.
- Zadnik K. Fitting the postoperative corneal transplant patient. Contact Lens Spectrum 12(6):19, 1997.
- Caroline p, Zilge L. Postsurgical correction with contact lens fitting following penetrating keratoplasty. Bennett E, Weissman B (eds). Clinical Contact Lens Practice 1994:1-13.
- Cutler S. Post-penetrating keratoplasty. Homm M (ed). Manual of Contact Lens Prescribing and Fitting with CD-Rom 2nd Edition. Boston, Butterworth-Heinemann 2000:451-456.
Cornea and Contact Lens Living Library
Colleen Riley, O.D., M.S., F.A.A.O., Indiana University School of Optometry
Joel Silbert, O.D., F.A.A.O., New England College of Optometry