Postpenetrating Keratoplasty

546SPECIAL TOPICS

II.Topographic maps can maximize efficiency and minimize time when fitting post-penetrating keratoplasty (PK) corneas. Once a shape factor is decided, an initial lens design and lens can be selected. Several surface patterns can be found following a transplant. Tripoli et all has designated different topographies after the sutures are removed. Regular astigmatism can be seen as a prolate shape (steeper centrally and flattening peripherally), or oblate pattern (flatter centrally and steepening peripherally). There can be a combination butterfly pattern of a mixed prolate/oblate within the donor cornea. Asymmetric astigmatism may be seen in which the two steep semimeridians are not aligned along a single meridian. A sloped pattern, resulting from uneven suturing or healing, can be seen as a steep red area on one side and becoming progressively flatter toward the other side (steep to flat). Finally, every once in a while a cornea gives a similar appearance to that of a keratoconic pattern. There is inferior steepening with a more normal appearance superior.

III.Lenses of several designs need to be considered when fitting grafts, owing to the variety of postsurgery topography. Larger diameters usually work better than smaller ones. Standard lens designs as well as aspheric, biaspheric lenses (the Boston Envision), multiaspheric and bitoric lenses could all be considered. Several companies make lens designs specifically to address the cornea that has undergone a penetrating keratoplasty.

A.The POST PK by Lens Dynamics consists of two basic designs. The first is a 10.4 diameter with a 9.0-mm fixed optic zone. The second is a 10.4 mm diameter with a floating optic zone. The optic zone decreases as base curve steepens. There are proprietary sets of peripheral curves that flatten from the base curve to fit the outer portion of the cornea (the host) prior to surgery.

B.The PSC (Post Surgical Cornea) is a lens by Infinity Optical. It is fit on the sagittal value principle and has the same basic concept as their keratoconic lenses. The difference is that these lenses are larger in diameter and flatter in base curve. The periphery of the lens should rest on the peripheral area of the cornea as far away from the surgery as practical. The optic zone is spherical and 1.5 mm smaller than the diameter of the lens. The peripheral curve system is aspheric with three edge lift values available. The trial lenses will be in the standard lift.

C.Lexington PK (Post Graft) is a bridge design by Aero labs. It is meant to vault the donor button and graft–host junction and fit on to the host cornea. There are two diagnostic sets. The diameter/optic zones are 9.7/8.1 and 10.2/8.4. The peripheral curvatures are of a proprietary nature.

D.Reverse geometry designs can address some of the idiosyncrasies of the post-PK cornea, especially those with a resultant oblate configuration. The reverse geometry’s secondary curve is steeper than the base curve of a lens, creating a plateau shape. Any reverse geometry lenses designed for corneal reshaping can be considered (CRT, The Dreim Lens, among others). Some of the specific proprietary reverse geometry designs are:

1.The Dyna Intralimbal (DIL) Lens by Lens Dynamics is a large lens to be fit within the limbus (as opposed to some designs that extend onto the sclera) and to vault the graft. The standard DIL diameter is

11.2mm (optic zone 9.4 mm), but is available from 10.0 to 11.5 mm. The lens design is custom made so most parameters are available: most any powers, toric designs, most any base curves, reverse geometry secondary curves, different optical zone sizes, and a proprietary peripheral curve system that starts with a standard edge lift but has flatter or steeper edge lifts available.

2.The PK Bridge by Conforma Labs. This lens has a secondary curve that is a reverse aspheric surface. It progresses from the center toward the paraperipheral region with no hard zone. The targeted amount of steepening is about 3.00 D. The peripheral curve is an aspheric flattening. The downside of this lens is that it induces some cylinder when verified on radiuscope and lensometer.

3.The NRK by Lancaster Contact Lens is a patented design, which follows scientific formulas to produce a contact lens that is steeper in the periphery than the central base curve. Diameters are usually

10.0mm or larger with an 8.0-mm optic zone, but these can be altered by the fitter as needed. Final design parameters are suggested after a trial fitting

4.The SurgiLens by ConCise is a reverse geometry design that is fit from trial lenses of known parameters. The diagnostic lenses are larger diameters (10.4, 10.6)/8.0 optic zone with the reverse curve

2.00D steeper than the base curve. All parameters are known and the fitter will alter them as needed.

5.The OK series by Contex offers a lot of variety as far as reverse geometry designs are concerned. The second steeper curve is designated by the name on the lens: OK-3 (3.00 D steeper than the base curve); OK-4 (4.00 D steeper than the base curve); OK-6 (6.00 D steeper than the base curve); etc. The secondary steeper curve can be spherical or aspheric. The peripheral curve is aspheric. Larger diameters are recommended. A good starting point is a 10.2-mm diameter, an 8.0-mm optic zone, with the OK-3 design.

6.The Plateau Lens by Menicon and ConCise Contact Lens (San Leandro, CA) is a proprietary design consisting of a spherical base curve, a secondary curve steeper than the base curve (usually around

3.00D but as high as 7.00 D) and an aspheric peripheral curve system. All parameters can be designated by the fitter, however, the standard lens has a 10.0 mm/8.0 mm(diameter/optic zone) and a secondary curve that will be 3.00 D steeper than the designated base curve.

7.Many other labs now have their versions of reverse geometry designs

that are not intended for Ortho Keratology: ABB Optical (S Corneal—biaspheric design and Reverse Adaptive), ABBA Optical (Surgical C4), Blanchard (CentraGraft), C&E GP Specialists (ALIGNMENT SERIES), Correctech, Inc. (XLT—a bridge design).

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a.Note on the CentraGraft by Blanchard. The reverse geometry is not the secondary curve but the wide peripheral curve. It is a larger version (10.1 D/7.0 optic zone) of their CentraCone. The base curve is spherical with an aspheric paracentral zone that applanates the cornea tangentially.

IV. Scleral Lenses may be a successful choice. The idea of a scleral lens is to vault the cornea and limbus and rest totally on the sclera. Tear exchange will not occur with larger diameters, as it is a closed system. Oxygen and gaseous byproducts will disperse through the lens plastic, which is why a good oxygen transmitting material is needed. Depending on the needs of the patient, they may not obtain all day contact lens wear due corneal changes resulting from metabolic byproducts.

A.Boston Scleral lens prosthetic device is available through the Boston Foundation for Sight. It is a fluid-ventilated GP contact lens that rests entirely on the sclera creating a fluid-filled space over the diseased cornea. It is made of the Equalens II material.

1.Jupiter Mini Scleral from Innovations In Sight and Correctech, Inc., is a lens of smaller diameter that rests on the sclera, vaulting the cornea and limbus. The large 18.0 design, made from Equalens II material, is a closed system. This lens should not rest on the cornea itself or limbus. Fit is by diagnostic lenses. There are three peripheral designs, A (flat), B (average), and C (steep). There is a smaller 15.0 D lens, made from Boston XO material. This design will have some tear exchange. Light corneal bearing is acceptable in areas of corneal protrusion.

2.Macro/Macro Elite by C and H Contact Lens is a semi scleral design as it not meant to vault the entire cornea. The lenses diameters are in the 13.8 to 14.5 mm range.

V.Initial lens selection will vary depending on the surface patterned revealed from corneal topography. Tripoli et al. have designated different topographies after the sutures are removed.1

A.Prolate pattern has a steeper central area and a flatter periphery. This is seen in 31% of the corneas. The donor cornea is almost protruding. Sometimes, a donor button that is too large will cause this type of topography after the graft is in place. By using corneal topography as a guide, several lens designs could be trialed initially. A large diameter sphere, aspheric, or biaspheric (the Boston Envision) lens would be a good initial choice. Depending on the amount of toricity, a toric base or bitoric lens would be a reasonable choice. On occasion, depending on the location and the size of the toric portion of the graft, a keratoconic design might be the optimum choice (CD-ROM Fig. 96 Penetrating keratoplasty: Prolate Graft: 2 of 7).

B.Oblate pattern is plateau shaped. This is seen in 31% of the corneas. The flatter donor cornea is surrounded by steeper host cornea. A reverse geometry can be ideal for this type of pattern, but it is not the absolute rule. Depending on the amount of astigmatism present, a toric basic

curve or bitoric reverse geometry design might allow for better weight distribution. On occasion, a standard design (be it spherical, aspheric, or toric) might be successful if there is good lens/eyelid interaction, but usually there is too much edge lift and central bearing (CD-ROM Fig. 97 Penetrating keratoplasty: Oblate Graft: 3 of 7).

C.Mixed Prolate/Oblate Pattern has a flat meridian and a steep meridian of the cornea. There is a roughly symmetrical astigmatism present. This is seen in 18% of the corneas. A toric base curve or bitoric design proves to be the most successful in these cases (CD-ROM Fig. 98 Penetrating keratoplasty: Mixed Graft: 4 of 7).

D.Asymmetric Astigmatism is a combination of all patterns and can be seen in this pattern. The cornea looks irregular and distorted. This is seen in 9% of the corneas. Larger lenses, (biaspheric [Boston Envision]), Dyna IntraLimbal, standard or bridge design works well for these patterns. An attempt to fit the peripheral host cornea as best as possible is a good goal to strive for. On occasion a toric base curve (or bitoric design) will distribute the weight of the lens more appropriately. The post-PK or PSC designs might be a good choice in these situations (CD-ROM Fig. 99 Penetrating keratoplasty: Asymmetric Graft: 5 of 7).

E.Steep to Flat is steep on one side, flat on the other side. It is like a mixed prolate oblate pattern without the symmetrical astigmatism. This is seen in 13% of the corneas. This by far is the most difficult pattern to fit. The lens wants to center over the steepest part of the cornea. This often leads to a decentered lens onto the sclera. Large diameters are needed so the optic zone will cross the visual axis. One could start with a Dyna Intralimbal, large sphere, aspheric, or biaspheric (Boston Envision) design. Good lid interaction is needed for stabilization. The PSC or post-PK might be an optimal choice as an initial lens selection (CD-ROM Fig. 100 Penetrating keratoplasty: Steep Graft: 6 of 7).

F.Keratoconic pattern will appear when the cornea protrudes. If this pattern results, a keratoconic design could be a good starting point. An attempt to fit a large, high-riding lens would also be a reasonable option. The Centra-Graft. With its Bi-Symm design might be a good choice if inferior edge standoff is present (CD-ROM Fig. 101 Penetrating keratoplasty: Keratoconic Graft: 7 of 7).

VI. Evaluation of the fit begins with insertion of the selected lens on the eye and allowing the patient to adapt. Instill fluorescein and evaluate the lens position and pattern. Thoughts vary as to whether the optic zone should be within the donor button or larger and vaulting it. One needs to be flexible when fitting the grafted cornea. What is an optimal fit for one graft will be totally inappropriate for another. The important point to remember is to try to distribute the weight bearing forces of the contact lens as evenly as possible.

A.Observe the center of the graft. Is there too much bearing? Then by using standard methods, attempt to vault the area more. Is there an obvious toricity? Would toric base curves be more stable or is the reverse

looks very aligned, centrally, midperipherally, with acceptable edge lift. However, the lens would not stay attached to the lid once the patient adapted. Note in this inferior position, the heavy bearing as the lens presses against the inferior quadrant of the host cornea. With a heavy blink, an air bubble is induced (CD-ROM Fig. 108 Penetrating keratoplasty cases: Patient A: Video: 10 of 15).

6.Video

SPE Bitoric

BC 8.23 mm/7.62 mm D/OZ 9.6 mm/9.8 mm ( K = 3.25 D)

BVP PL – 3.25 D X flat meridian

A flatter base curve was attempted to improve the fit. This lens does maintain a longer lid attached position with the blink, but only initially. When held up by the lid, note the overall flat pattern (central bearing with midperipheral and peripheral pooling) and excessive edge lift inferiorly. The lens has a tendency to release from the eyelid, preferring the inferior position. In this position, the central bearing and midperipheral pooling is still present as well as the inferior arcuate bearing as the lens presses against the inferior quadrant of the host cornea (CD-ROM Fig. 109 Penetrating keratoplasty cases: Patient A: Video: 11 of 15).

A minus edge carrier could be placed on the lens to help maintain a superior position, but the reverse geometry design offered a better fit. The NRK looked like a better fit over the bitoric.

The following lens was ordered:

NRK #1

BC 7.75 mm D/OZ 10.3 mm/8.0 mm BVP +0.75 D

7.Video

NRK #1

BC 7.75 mm D/OZ 10.3 mm/8.0 mm BVP +0.75 D

This is the ordered lens on a 2-week progress visit. The patient has limited wearing time to 8 hours, as instructed. He is using the Boston System, no enzyme at this point. Visual acuity with a −1.00 D overrefraction was 20/40.

The lens centers fairly well. It stays in place on lateral movement. Most of the time it is tucked under the upper eyelid, but it will release on occasion. There is heavy central bearing, midperipheral, and peripheral pooling. This a flat fit (CD-ROM Fig. 110 Penetrating keratoplasty cases: Patient A: Video: 12 of 15).

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8.Video

In grafts, flat fits are acceptable if the eye remains healthy. Although hard to see in this video, there was some superficial punctate staining that developed under the area of bearing. It was followed for a few weeks and was not acceptable (CD-ROM Fig. 111 Penetrating keratoplasty cases: Patient A: Video: 13 of 15).

9.Video

NRK

BC 7.80 mm D/OZ 9.5 mm/7.8 mm BVP –1.00 D

A marginally flatter but considerably smaller-in-diameter lens was tried in an attempt to reduce the staining. Decreasing the diameter of a lens, as well as flattening the base curve, should produce an overall flatter relationship by reduction of sag. However, there was an overall decrease in lens mass to allow a looser, lighter fit. The position is under the eyelid in a central temporal location. This is only in primary gaze. As the patient looks laterally, the lens moves freely and does not dislocate. There is a central area of bearing; however, the weight seems to be distributed more than the previous 7.75-mm base curve.

There is midperipheral and peripheral pooling, indicative of the flat fit; however, as the patient looks laterally there is light contact of the peripheral curves with the host cornea. The lens is comfortable and the patient doesn’t really notice any difference in acuity than the previous one. Overrefraction is +0.50 D 20/40. The patient was released with this lens (CD-ROM Fig. 112 Penetrating keratoplasty cases: Patient A: Video: 14 of 15).

When the patient returned 2 weeks later, the lens position and movement remained the same. The superficial punctate keratitis (SPK) was no longer present. The same parameters were ordered with a −0.50 power. He is presently appreciating 20/40 vision, wearing the lens all waking hours, and very happy.

This is an example where the fluorescein pattern may not be pretty or perfect, but it satisfies criteria. In the future, toric base curves with the reverse geometry configuration may be considered.

IX. Penetrating Keratoplasty Cases: Patient B

A.History

A 79-year-old white male presented for a contact lens fitting of his left eye. He had a penetrating keratoplasty secondary to bullous keratopathy 9 years previously.Visual acuity with a manifest refraction of + 0.50 −6.00

× 90 was 20/50.

B.Topography

This is an interesting surface pattern. The pattern appears to be a mixed prolate/oblate pattern. Centrally there are approximately 6 diopters of

against-the-rule astigmatism, which is present in the refraction. There is an obvious steepening in the inferior nasal quadrant (from about 5 o’clock to 10 o’clock) that will be the challenge of this fit (CD-ROM Fig. 113 Penetrating keratoplasty cases: Patient B: 2 of 6).

1.Fitting Considerations

With the against-the-rule astigmatism over the visual axis, a bitoric lens was first attempted. The approach was treating the eye as a routine toric cornea. If relatively high position of the lens is maintained, inferior lens–cornea interaction may be avoided where the cornea is relatively steep.

C.Trial Fittings

1.Video

SPE Bitoric

BVP PL –3.00 D × flat meridian

A flat K, slightly steeper than the flat Sim K, was chosen. Looking carefully at the topography, it can be noted that the Sim K value is a very small area. The dioptric value is surrounded by an area that is slightly steeper in value. This was the flat meridian on the chosen lens.

The lens centers and has a central, superior, central type of movement. When the lens rises under the eyelid, the fluorescein pattern reveals a more even distribution of bearing forces. The light bearing centrally over the flat area of the cornea is evident, which is at the lower position of the lens. The paracentral area in the superior cornea shows light clearance. The edge lift is adequate.

When the lens releases from the lid and falls, the bearing now appears in the superior quadrant of the lens. The light paracentral pooling is displayed in the midperipheral region. Light bearing areas can be seen anterior to the peripheral curve. Overall there is a good weight distribution of lens, but a more constant superior position is preferred (CD-ROM Fig. 114 Penetrating keratoplasty cases: Patient

B:Video: 3 of 6).

To maximize lid grab, a minus-edge lenticular carrier will be

ordered. The diameter was made slightly larger as well. In an attempt to contour the cornea a little better, a steeper base curve in both meridians will be the trialed. Overrefraction yielded 20/50 acuity.

2.Video

SPE Bitoric

Boston ES

BC 7.42 mm/6.96 mm (45.50/48.50) D/OZ 9.8 mm/7.5 mm

BVP +0.50 −3.00 × flat meridian

556 SPECIAL TOPICS

The steeper lens centers and the larger diameter help keep it in a more superior position, but it continues to release from the eyelid. In the superior position a more aligned pattern can be seen; however, as the lens drops the flat meridian becomes more evident. The bearing is not heavy, so this is acceptable. There is a little more bearing in the midperipheral area just anterior to the peripheral curve than preferred, but only when the lens is in the lower position. There is excellent tear exchange. The patient appreciates 20/50 vision and is very happy (CD-ROM Fig. 115 Penetrating keratoplasty cases: Patient B: Video: 5 of 6).

The patient built up wearing time conservatively and is now wearing the lens for all waking hours. The Boston care system was dispensed. Postwear biomicroscopy remains unremarkable. This lens satisfies the criteria for a postpenetrating keratoplasty fit. Decreased lens diameter to the original trial size may be considered in the future to minimize some of the bearing anterior to the peripheral curve.

X.Penetrating Keratoplasy Cases: Patient C

A.History

An 87-year-old white female was referred for a contact lens postpenetrating keratoplasty OS secondary to pseudophakic bullous keratopathy. Refraction OS was −6.50 −1.00 × 35 yielding a visual acuity of 20/60+. She had a great positive attitude about her ability to handle a contact lens, even though she had never worn lenses before.

B.Topography

This topography demonstrates what would be considered a flat to steep shape. If one looks at the simulated keratometric readings and the refraction, it does not appear as if there is a lot of astigmatism. This is the beauty of corneal topography. One can see on closer inspection that indeed there is more present, as evidence of the minimum K reading. Note that the X and Y axes are not aligned at zero. The real cylinder is probably just off the visual axis. The central findings are only a minimal part of this picture (CD-ROM Fig. 116 Penetrating keratoplasty cases: Patient C: Topography: 2 of 3).

1.Comments

Both a reverse geometry lens and regular design of the same base curve were attempted at approximately the same diameter. There was no appreciable difference. There was significant bearing in the central region of the lens with pooling in the midperipheral and peripheral region. The fluorescein pattern of the standard design seemed to bear more evenly across the center than the reverse geometry lens. A 3 D toric base curve was trialed but this decentered inferiorly. A large diameter aspheric lens was tried in the hope that a lens can be found to fit the eye.

C.Video

Achievement lens from Art Optical

Boston 7 material

BC 7.75 mm D 9.4 mm BVP PL

With a steep to flat topographic configuration, one never knows how a given design is going to respond on an individual’s eye until it is tried. Lenses will center over the steepest area of the cornea. Many times it is not at a desirable position. With a large and flat lens, the lid interaction can significantly change an unsuccessful attempt to a successful one. The patient has an extremely ptotic eyelid. Other designs were attempted, but this standard design seemed to stay on her eye and give her the most consistent vision of all those tried. The lens fulfilled two important criteria: the lens stayed on the eye and it remained centered. Although the pattern appeared significantly flat in the center with a large amount of edge lift, the fit was acceptable. Final acuity was 20/40 (CD-ROM Fig. 117 Penetrating keratoplasty cases: Patient C: Video: 3 of 3).

REFERENCE

1.Tripoli NK, Ibrahim OS, Coggins, JM, et al. Quantitative and qualitative topography classification of clear penetrating keratoplasties. Invest Ophthamol Vis Sci 1990;30(Suppl):480.