GP Lens Case Grand Rounds Troubleshooting Guide – 44

Post-LASIK: Mary Jo Stiegemeier, OD

Background

Penetrating keratoplasty (PKP), also known as a corneal transplantation or grafting, is an operation in which abnormal corneal host tissue is replaced by healthy donor cornea. This procedure is one of the most successful transplants in the United States with a success rate of 90 percent or greater with approximately a quarter of these patients needing post-operative contact lenses. The most common indications for a PKP are keratoconus, Fuchs’ endothelial dystrophy, and pseudophakic bullous keratopathy.

Significant visual distortion from the surgery results in large and varying amounts astigmatism throughout the cornea with most grafts maintaining a prolate shape (like the normal human cornea) and others exhibiting an oblate shape. Furthermore, the donor cornea may also be sunken, proud, or tilted with respect to the graft-host interface. Due to these corneal irregularities, soft contact lenses and/or spectacles rarely give satisfactory vision. Many of these patients can however achieve optimal visual acuity with rigid contact lenses that correct for significant astigmatism and distortion secondary to the PKP.

If the patient requires early visual rehabilitation, post-PKP contact lens fittings may be performed as early as 3 months with sutures intact and as late as 12 months with all sutures removed. In early-fit cases, the lenses will need to be changed as the sutures are cut. Lens modifications can however be kept to a minimum if the post-PKP eye is fit 12 months later with all of a sutures removed where stable refractions and keratometry readings can be obtained.

Due to the size and shape of the graft and the transition area between the graft and host tissue, it is best to use a large diameter lens to maintain lens centration. Since the graft-host interface is very irregular, the periphery of the contact lens should rest on the host tissue and not the graft tissue. Fitting a lens within the diameter of the graft will most likely cause neovascularization to the grafted tissue secondary to lens irritation to the graft-host interface. My experience has shown that a lens with a diameter of 10.0mm to 11.00mm whose optic zone (approximately 9.0 mm) vaults the graft host interface and has minimal bearing have the greatest success rates.

Four Measurements Needed to Fit Post-PKP Patients:

  • Manifest Refraction: Refractions can be very difficult, often with high degrees of astigmatism. Keep in mind that astigmatism can be irregular and that refractive errors may be from one end of the spectrum to the other.
  • Keratometry: Use this measurement as a guide, remembering that it is only a measure of the central 3 mm of the cornea. The compromised apical and peri-apical areas of the cornea can make these readings on the graft challenging so several keratometry readings may be necessary to obtain more reliable measurements. Remember to assess the mires for distortion and irregularity.
  • Topography: This is the single most informative measure of corneal regularity and irregularity. Maps easily show astigmatism, corneal apex, graft-host interface, size of the recipient bed, neovascularization, and scarring. They may also display steep areas that progress to flatter areas. This is one of the most difficult resulting topographies to fit. Look at the axial topographical map for quantitative data and identify whether it is a prolate, oblate, mixed, asymmetric, or steep to flat topography. Serial topography can also assist in assessing the stability of the graft over time and contact lens fittings should only be commenced when topographies are stable.
  • Radius of curvature: Determine the radius of curvature 4.0 mm from the visual axis along the temporal 180o meridian from the topography. Select the lens that is closest to that radius and use that lens as your initial diagnostic lens. If you only have keratometry however, the steep K reading is a reasonable starting point.

As mentioned previously, the initial lens should have a diameter larger than the graft to include the graft-host interface. Larger diameters offer better centration and stability and avoid insult to this junction. My initial lens of choice is the large-diameter post-graft design lens by Lens Dynamics. This lens is specifically designed for the post-PKP eye and is large enough to vault the irregularity within the graft-host interface.

This post-graft design is a 10.4 mm diameter lens with a standard 9.0 mm optic zone and multiple peripheral curves. The multiple peripheral curves are a proprietary design and can be ordered as standard (which is available on the fitting set) or steeper than standard by 0.5 mm to 1.0 mm or flatter than standard by 1.0 mm. The large diameter is used to vault the central graft area and the peripheral curve structure is used to fit the host peripheral cornea. The 9.0 mm optical zone is larger than the usual 7 mm to 8mm graft diameter and is the standard. This aids in fitting the lens because it limits the variables of change when adjusting the base curve.

Changing the optical zone diameter affects the base curve-cornea relationship while changing the diameter affects centration. Modifying one variable independently of the other affects the fitting relationship in a more subtle way. If the standard 9.0 mm optical zone does not obtain an acceptable fluorescein pattern, it is possible to use the floating optical zone (FOZ) instead. This optical zone is variable and is inversely proportional to the steepness of the base curve.

Once an initial lens has been selected using topography, use 2% sodium fluorescein to evaluate the fit and change the base curve steeper or flatter in 1.00 D steps to achieve the desired fluorescein pattern. Look for centration and good tear exchange with adequate movement. The fluorescein pattern should have the least amount of bearing on the steeper sections and the least amount of vaulting on the flatter areas. Keep in mind that the pattern is almost always irregular and the ideal fit is one that shows neither excessive lift-off nor excessive bind in any one particular area. Next, over-refract the patient to determine the final lens power.

If spherical lenses fail to center as needed, try an aspheric lens. These designs work well especially when the resultant topography is prolate in shape. You may also need to try a back toric or bitoric design if the level of astigmatism gives a poor fit with the aspheric lens. These designs are highly effective especially if the astigmatism is mixed or moderate to severe and regular. Irregular corneas (where the center is flatter than the periphery) may require a reverse geometry lens, such as plateau lenses, where the mid-peripheral curve is steeper than the base curve. These lenses are also very successful especially when the resultant topography is oblate in nature. SoftPerm lenses are also an alternative for these patients as they are very comfortable and give good acuity. However, they should be used as a last resort if possible due to the increased risk of neovascularization to the graft.

When ordering a final lens for post-PKP patients with low amounts of astigmatism, consider a moderate to high Dk material such as Boston EO and Optimum Extra. For those with higher amounts of astigmatism patients or those who require a bitoric design, a lower Dk material such as Boston ES may allow for better lens stability.

M.L., 49 year-old Caucasian male, presented for a contact lens fitting. He had undergone bilateral PKP in September 2002 left eye and January 2003 right eye for keratoconus. All stitches were removed in the right eye 3 days prior to our office visit.

Test Procedures, Fitting/Refitting, Design & Ordering

Manifest Refraction:

OD: +3.50-6.00×040 20/200
OS: +3.50-5.00×116 20/200

Topography:

OD: central K readings 58.00 D/40.87 D @ 100
4 mm temporal to visual axis: approximately 55.50 D

OS: 53.87 D/40.12 D @ 140
4 mm temporal to visual axis: approximately 44.50 D

Biomicroscopy: graft clear and compact OU

Looking at the radius of curvature 4.0 mm from center along the temporal 180o meridian, the central K readings, and lenses available in my diagnostic fitting set, I selected a 7.67 mm radius/-2.75 D lens for the right eye for the initial trial lens.

For the left eye, the topography showed a very steep radius of curvature 4.0 mm temporally. Due to this corneal irregularity, I determined that selecting this steep trial lens would most likely not fit the patient well. Going about one third of the way from flat to steep K, I selected a 7.85 mm radius/-4.00D diagnostic lens design as the initial trial lens.

Lens Selection:

OD
Lens Dynamics Post-Graft Design
7.67 mm radius, 10.4 diameter, 9.0 mm optical zone
Standard peripheral curve
-2.75 D power
Boston ES material

OS
Lens Dynamics Post-Graft Design
7.85 mm radius, 10.4 diameter, 9.0 mm optical zone
Standard peripheral curve
-4.00 D power
Boston ES Material

With fluorescein, I determined base curve-cornea relationship in the right eye to be too flat and the left eye to be even but too flat in the periphery.

Subsequently, my next diagnostic lens selection for the right eye was a 7.50 mm radius/-4.87 D in the right eye. With fluorescein, I determined that the lens was good but needed a steeper peripheral curve to better fit the host peripheral cornea.

I attempted to fit the left eye with the 7.67 mm radius lens used previously on the right eye but I determined that this lens was too flat. I modified the previous 7.85 mm radius lens by decreasing the base curve by 0.05 mm, which allowed me to steepen the periphery of the lens while maintaining the size of the optic zone. Applying the SAMFAP rule, I ordered a 7.90 mm radius/-3.75 D lens for the left eye.

Lens Ordered:

OD
Lens Dynamics Post-Graft Design
7.50 mm radius
10.4 diameter
9.0 mm optical zone
Peripheral curve 1.00 mm steeper than the standard
-2.75 D power
Boston ES material

OS
Lens Dynamics Post-Graft Design
7.90 mm radius
10.4 diameter
9.0 mm optical zone
Peripheral curve 0.5 mm steeper than the standard
-3.75 D power
Boston ES material

Follow-Up Care/Final Outcome

The final visual achieved with an over refraction was 20/30+ OD and 20/20 OS. On follow-up I noted a patch of neovascularization to the host-graft interface but not across the graft and restarted that eye on steroid therapy. Consequently, I re-fitted this patient with a more gas-permeable lens material and steepened the base curves by 0.1 mm with the lens powers adjusted accordingly.

Discussion/Alternative Management Options

The cornea usually has a prolate configuration that is normally steep centrally and progressively flattens toward the periphery. This allows rigid lenses to fit near the corneal apex and visual axis of a normal eye. In irregular corneas, however, rigid lenses will decenter to the steepest portion of the cornea, which can be located anywhere on the cornea. In spite of this, with large diameter lenses and patience from these patients, the decentered lens will not be an issue if part of the optic zone covers a portion of the pupil and allows for sufficient tear film exchange.

Topography is a helpful tool because it can guide optometrists to a particular type of lens. It allows clinicians to choose the initial diagnostic lens and displays the corneal irregularity, the graft-host interface, and the shape of the peripheral host cornea. Nevertheless, final lens specifications cannot be determined without diagnostic fitting and the use of 2% sodium fluorescein to obtain the best base curve-cornea relationship.

Knowledge of the many proprietary designs and understanding fluorescein patterns is essential to being able to fit these patients with a lens that can:

  • correct their refractive needs,
  • be able to be worn comfortably for the majority of their day, and
  • not further compromise their corneal physiology, either from mechanical or hypoxic changes.

Since these corneas are already compromised, attention to the corneal graft is vital to properly assess and monitor its viability. If present, assess the suture integrity and any mechanical irritation from harsh areas of bearing. Complications to look for include chronic punctate epithelial erosions, microcystic edema, and neovascularization. Keep in mind that these complications are possible in the presence of an adequate fitting lens.

Furthermore, be sure to differentiate graft failure from graft rejection. Graft failure can occur at any time after the penetrating keratoplasty. Graft rejection however, presents with graft edema in combination with other signs of inflammation.

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