Vaulting to Success on Post-PKP Fit

 

Edited by Joseph P. Shovlin, O.D. 

 

Q When fitting RGPs on a post-op corneal transplant patient, should the lens be larger than the graft?Karlit Wong, O.D., Red Deer, Alberta

 

A  Yes. "The old literature suggested it might be better to fit within the borders of the graft, but my experience is totally the other way.  I always fit larger than the graft and seem to do well," says Barry Weissman, O.D., Ph.D., of the Jules Stein Eye Institute in Los Angeles.

 

   When rigid lenses only came in PMMA material, a larger lens often resulted in graft edema.¹  The recommendation now is to vault the graft whenever possible. "The graft-host interface is a very irregular area, so you want to rest the periphery of the contact on the host tissue, not the graft tissue," says Mary Jo Stiegemeier, O.D., of Beachwood, Ohio. " If you fit within the diameter of the graft, it's likely that you will bump or irritate the graft-host interface, which would cause neovascularization to the grafted tissue."

 

   Another advantage to using a larger lens: "It is easier to vault the graft than to stay small and centered in most irregular corneas," says Loretta Szczotka, O.D., of Cleveland's Case Western Reserve University.

 

Q  What lens designs or materials are appropriate for these patients?  Do you have any other fitting suggestions?

 

A  Dr. Szczotka recommends moderate- to high-DK materials such as Boston EO and Paragon HDS for patients with low amounts of astigmatism.  For those patients with higher astigmatism or who require a bitoric design, she suggested a lower DK material such as Boston ES for better lens stability. 

 

   Depending on corneal shape, patients may need a reverse geometry lens, an aspheric lens or a bitoric lens.  It's much easier to fit a with-the-rule cornea than one that's against the rule.

 

  A minimum 9.5mm diameter is necessary, but you're more likely to fit a 10-10.5mm diameter lens with a 9mm optic zone.  Ophthalmic technician Mark Andre, of the Casey Eye Institute in Portland, Ore., uses a post-pentrating keratoplasty lens he helped design for Lens Dynamics.  The lens has a 10.4mm diameter and 9mm optical zone, so it's big enough to vault all the irregularity within the graft-host interface.  The lens also offers a floating optic zone if the standard optic zone doesn't work.

 

   To fit this post-PKP lens, use topography and select an initial diagnostic lens closet to the temporal mid-peripheral radius.  Next, evaluate the fit with fluorescein.  "The patterns are not very pretty, but you want to look for the least amount of bearing on the steeper sections and the least amount of vaulting on the flatter areas," Dr. Stiegemeier says.

 

   Some additional pearls for fitting post-PKP patients:

  • Watch for broken sutures.  "Any loose or broken suture must be removed ASAP bt the surgeon to prevent a nidus for infection," Dr. Szczotka says.

  • Use topography to guide your fitting.  Use a temporal 180º meridian, about 4mm from the center, as the initial base curve.  Make changes to the base curve in 1.00D steps until you get a good fit.  Use fluorescein to help you, since you can't fit these patients empirically, Dr. Stiegemeier adds.  Next, over-refract the patient to determine the power.  If you only have keratometry, the steep K reading is a reasonable starting point.

  • Follow-up.  Dr. Szczotka recommends follow-up at least 2 weeks after dispensing, then usually 4 weeks later, then every 6 months.  During this time, you must monitor the patient for early signs of rejection.  Investigate promptly if the patient presents with redness, pain, light sensitivity, and decreased or cloudy vision.

  • Also look for microcystic edema, mechanical irritation and neovascularization. Be sure to differentiate graft rejection from graft failure, Dr. Stiegemeier says.  Graft failure can occur any time after penetrating keratoplasty.  In graft rejection, a clear graft succumbs to graft edema in conjunction with other infammatory signs.

E-mail questions to reviewofoptometry@jobson.com or mail them to Review of Optometry, 11Campus Blvd., Suite 100, Newton Square, PA 19073

 

1. Woodward EG. Postkeratoplasty. In: Phillips AJ, Speedwell L, eds. Contact Lenses. 4th ed. Oxford, England: Butterworth-Heinemann, 1997:713-20