Rigid Contact Lens Fitting1



Rigid Contact Lens Fitting1


Peter R. Kastl

Corey Dickson



In recent years, rigid contact lens fitting has been believed by some to be superfluous, as cheap, disposable soft lenses became available, and refractive surgery began making inroads into the patient base previously reserved for contact lens fitting. However, not only has there remained a place for rigid lenses, but also refractive surgery has created a need for specialty fitting of a new type of patient, the postrefractive surgery patient.

Corneal physiology or tear physiology is not explained in this chapter because these topics are covered elsewhere in this volume. In this chapter, an attempt is made to explain the nomenclature of rigid lenses and the fitting of rigid lenses of all types. When the reader has finished studying this chapter, he or she should be able to fit rigid contact lenses.


TERMINOLOGY

At the outset, it is important to differentiate between a “hard” lens and a “rigid” lens. Although the terms actually are synonymous, the patient hears the term hard lens and associates it with polymethylmethacrylate (PMMA) lenses, the original hard lenses. Nearly all new contact lens patients have heard of hard lenses and equate these lenses with pain.1,2,3,4,5 Therefore, instead of using the term hard lenses, the practitioner should refer to rigid lenses, meaning rigid gas-permeable (RGP) lenses, to diminish any patient misgivings. Because patients often are not familiar with RGP lenses, the practitioner has an opportunity to educate them. Table 54.1 lists RGP materials arranged by the amount of oxygen that can permeate through the material.


BASIC CONCEPTS


Cornea

To fit contact lenses well, one must understand the contour of the cornea. The corneal surface is not a sphere; rather, it is aspheric, with an apical zone6,7,8,9 (Fig. 54.1). This zone is the area of the cornea over which the corneal curvature is regular or constant. Several topography devices have been invented to enable the practitioner to understand the contour of a particular patient’s cornea.


Keratometry

Although topography devices are available, rigid lenses still are fitted based on keratometry. We perform keratometry to measure the curvature of the apical zone. Never forget that the keratometer actually measures the distance between two points which are 3 mm apart on the cornea. The keratometer produces patterns that are reflected back from the cornea; these are viewed as mires. Mires first must be aligned by rotating the barrel of the keratometer (Fig. 54.2).

Once the barrel of the keratometer is rotated, then the mires are partially aligned (Fig. 54.3). The next step is to rotate each drum (horizontal and vertical) until the plus and minus mires are superimposed (Fig. 54.4).

After the mires are aligned, each of the two drums on the keratometer yields a meridional reading in both millimeters and diopters (D). In this chapter, diopters are used. An example of such a reading is 42.00 D @ 180° and 44.00 D @ 90°. The final keratometry readings (K readings) are written in a shorthand form as the flatter (smaller) reading in diopters, followed by the steeper (larger) reading in diopters, followed by the meridian of the steeper reading, for example, 42.00/44.00 @ 90°. The flatter meridian is called K, and rigid lens fitting always is based on this number. In this example, K is 42.00 D.








TABLE 54-1 Rigid Gas-Permeable Lens Materials



























































































































































































































































































































































































































































Name

Material

Dk

Wetting Angle

Specific Gravity

Color

EW

UV

Refractive Index


Accucon

Pemufocon A

25

<25

1.16

Blue, dark blue, brown, gray, green


x

1.458


Boston equalens

Itafluorofocon A

47

30

1.19

Blue

x

x

1.439


Boston equalens II

Oprifocon A

85

30

1.24

Blue, green

x

x

1.423


Boston EO

Enflufocon B

58

49

1.23

Blue, brown, gray, green, ice blue, electric blue


x

1.429


Boston ES

Enflufocon A

18

52

1.22

Blue, brown, gray, green, ice blue, clear


x

1.443


Boston II

Itafocon A

12

20

1.13

Blue



1.471


Boston IV

Itafocon B

19

17

1.10

Blue



1.469


Boston XO

Hexafocon A

100

49

1.27

Blue, green, violet, ice blue


x

1.415


Boston XO2

Hexafocon B

141

38

1.19

Blue, green, violet, ice blue


x

1.424


Optimum classic

Roflufocon A

26

12

1.189

Blue, glacier blue, gray, green


x

1.453


Optimum comfort

Roflufocon C

65

6

1.178

Blue, glacier blue, gray, green, forest green, brown


x

1.441


Optimum extra

Roflufocon D

100

2

1.166

Blue, glacier blue, clear, gray, green


x

1.433


Optimum extreme

Roflufocon E

125

6

1.155

Blue, glacier blue, gray, green


x

1.433


Fluorex 300

Flusilfocon A

30

12.6

1.113

Aqua, blue, clear, gray, green, rose brown



1.465


Fluorex 500

Flusilfocon B

50

13.3

1.105

Aqua, blue, clear, gray, green, rose brown



1.460


Fluorex 700

Flusilfocon C

70

15.3

1.097

Aqua, blue, clear, gray, green, rose brown



1.457


Fluoroperm 30

Paflufocon C

30

12.8

1.14

Blue, crystal blue, clear, gray, green, majestic Blue


x

1.466


Fluoroperm 60

Paflufocon B

60

14.7

1.15

Blue, crystal blue, brown, clear, green

x

x

1.453


Fluoroperm 92

Paflufocon A

92

16

1.10

Blue, clear, gray, green

x

x

1.453


Fluoroperm 151

Paflufocon D

151

42

1.10

Blue, clear, green

x

x

1.442


Optacryl 60

Kolfocon A

18

25

1.13

Blue, green, violet



1.467


Paragon HDS

Paflufocon B

58

14.7

1.16

Blue, crystal blue, green, forest green


x

1.449


Paragon HDS 100

Paflufocon D

100

42

1.10

Sapphire blue, emerald green

x

x

1.442


Paragon thin

Paflufocon C

29

12.8

1.14

Clear, sapphire blue, emerald green


x

1.463


Paraperm 02

Pasifocon A

15.6

23.1

1.12

Blue, clear, green, cool green, electric blue



1.480


Paraperm EW

Pasifocon A

56

26

1.07

Blue, clear, green

x


1.467


Hybrid FS

Hybufocon A

31

0

1.183

Blue, clear, gray, green


x

1.447


Hydro 2

Filofocon A

50

<5

1.146

Soft blue, soft green, ocean blue


x

1.463


Onsi-56

Onsifocon A

56

7.2

1.206

Blue, gray, green, blue uv, onsure (blue-violet)



1.452





Clear, dark blue, dark green, dark brown



Tyro-97

Hofocon A

97

25

1.187

Blue, gray, green, blue uv, clear, onsure (blue-violet)



1.440


Flosi

Wilofocon A

26

23.5

1.270

Blue, dark blue, brown, clear, gray, green, dark green, violet



1.455


PMMA

Polymethylmethacrylate

<0.02

25

1.190

Blue, brown, clear, gray, green, pink



1.495


SA-18

Kolfocon A

18

<25

1.126

Light/mild/dark blue, light/mild/dark green, brown, gray, dark gray, violet, clear



1.469


SA-32

Kolfocon B

32

< 25

1.101

Light/mild/dark blue, light/mild/dark green, brown, gray, dark gray, violet, clear



1.467





Trans-air color match: forest green, violet, red, dark blue



OP-2

Lotifocon B

15.9

17

1.115

Blue, brown, clear, gray, green


x

1.467


OP-3

Lotifocon A

30

21

1.115

Blue, dark blue, brown, clear, gray, green, dark green


x

1.457


OP-6

Lotifocon C

60

26

1.113

Blue, brown, clear, gray, green


x

1.447


SGP I

Telefocon A

22

<30

1.126

Blue, brown, clear, gray, green



1.475


SGP II

Telefocon B

43.5

<30

1.126

Blue, brown, clear, gray, green



1.471


SGP III

Unifocon A

43.5

<20

1.126

Blue, clear, green



1.481


Menicon Z

Tisilfocon A

163

24

1.20

Blue


x

1.436


Rigid gas-permeable lens materials are shown with their physical characteristics. Note that some block UV light and some are approved for extended wear.







FIG. 54.1 Sagittal drawing of a cornea, demonstrating that the cornea is an aspheric, rather than a spherical object.


Astigmatism

Keratometry can show whether corneal astigmatism is with the rule or against the rule. With-the-rule astigmatism means that the corneal “egg” is lying on its side; against-the-rule astigmatism means that the egg is standing on end (Fig. 54.5). Here are two keratometric examples: with-the-rule, 42.00/44.00 @ 90°; against-the-rule, 42.00/44.00 @ 180°.

Rigid contact lenses have a tendency to move along the steeper meridian. Thus, with-the-rule astigmatism is good for rigid contact lens fitting because the lens moves up and down with the blinking of the upper lid; against-the-rule astigmatism is not as good because the lens tends to move side to side with each blink. Usually, corneal astigmatism is with-the-rule.






FIG. 54.2 Keratometric mires before any alignment has taken place. The arrow shows the direction in which the keratometer’s barrel must be rotated to begin alignment.






FIG. 54.3 Partial alignment of keratometric mires. The arrows show that the horizontal and vertical drums of the keratometer must be adjusted to align the mires.

Thus far, we have considered only corneal astigmatism, which is measured with a keratometer. In clinical practice, we also measure refractive astigmatism. Whereas keratometry measures only corneal astigmatism, refraction measures a combination of both corneal and residual astigmatism. Residual astigmatism is also called lenticular astigmatism because it is believed to originate from a tilted crystalline lens. We decide which contact lens to fit based on how well the refractive astigmatism and the keratometric astigmatism match.






FIG. 54.4 Mires aligned.






FIG. 54.5 Hypothetical “eggs” showing with-the-rule and against-the-rule astigmatism. Arrows indicate lens movement with blinking.

Following is an example of matching astigmatism, where the corneal shape is responsible for the entire refractive astigmatism; a simple rigid lens would work well because a rigid lens cancels out corneal astigmatism:









Keratometry:

42.00/44.00 @ 90°


Refraction:

-3.00 + 2.00 × 90°


Following is an example of residual astigmatism. Note that there is only 0.50 D of corneal astigmatism but 1.50 D of refractive astigmatism. The difference between the two, 1.00 D, constitutes the residual astigmatism. This difference requires toric lens fitting because a spherical rigid lens cancels out only corneal astigmatism:









Keratometry:

42.00/42.50 @ 90°


Refraction:

-3.00 + 1.50 × 90°



Lens Types and Parameters

Lenses are either soft or hard. Soft contact lenses (SCLs) are “all alike” because they are made from hydrogels, water-containing soft material. Rigid lenses currently are made from RGP materials; the “old” hard lenses were made of PMMA, a plastic that was not oxygen permeable.

All contact lenses share the same important parameters: diameter, central posterior curve (CPC, also known as base curve, or BC), and power (Fig. 54.6). Diameter is measured in millimeters, CPC is measured in diopters (for rigid lenses) and millimeters (for SCLs), and power is measured in diopters. Diameter and CPC determine the sagittal depth of a lens (Fig. 54.7). No one has proven a numerical relationship between sagittal depth and lens fit, but most practitioners believe that an increase in sagittal depth for either a soft or rigid lens results in a more tightly fitting lens.10,11,12,13 Thus, the lens itself tightens as the sagittal depth increases. Increasing the diameter of a lens increases the sagittal depth to a great degree and also results in tight fitting.






FIG. 54.6 Line drawing of a contact lens, demonstrating the diameter and central posterior curve (CPC).


Tear Lens

Lens power constitutes most, but not all, of the effective power of a rigid lens. Depending on its relationship to the cornea, the CPC also contributes to the effective power because it creates a “tear lens.” If a lens is “steeper than K” it produces a tear lens that functions as a plus lens. This plus tear lens must be corrected in the final or prescribed lens power by adding minus power (Fig. 54.8). Conversely, if the rigid lens is fitted “flatter than K,” then a minus tear lens is created, which needs to be corrected in the final or prescribed lens power by adding plus power (Fig. 54.9). The actual correction for the tear lens can be remembered as a mnemonic: SAM FAP—steeper, add minus; flatter, add plus.






FIG. 54.7 Line drawing of a contact lens, demonstrating sagittal depth.






FIG. 54.8 Contact lens fitted “steeper than K.” Note the “plus” tear lens beneath the contact lens.


Minus Cylinder Thinking

When we work with rigid lenses, we always convert to minus cylinder, then drop the cylinder. Why? We can answer this question in two steps:



  • Because the cornea is a convex surface, any corneal astigmatism is a plus cylinder. Such a refractive error is neutralized by an equal and opposite power, that is, minus cylinder. Thus, we start by writing the refraction in minus cylinder form.


  • When we place a rigid contact lens onto the eye, the resulting concave tear film is a minus cylinder, which automatically corrects the plus cylinder of the astigmatic cornea. Thus, we can “drop the minus cylinder” and consider only the resulting sphere power for the contact lens.

Thus the rule, “Convert to minus cylinder and drop the cylinder.” An example of this idea is to take a refraction of -3.00 + 1.00 × 90°. We convert to minus cylinder form: -2.00 – 1.00 × 180°. By dropping the cylinder, we end up with a rigid contact lens power of -2.00 D. Remember that this calculation assumes that all refractive astigmatism is corneal in origin.


Vertex Distance Correction

Any spectacle correction intended to be used for contact lens fitting needs to be corrected for vertex distance if the refraction is greater than +4.00 or -4.00 D. This concept is easier to understand with a couple of examples. We shall consider first a +10-D and then a -10-D lens, both located 13 mm (vertex distance) from the eye.






FIG. 54.9 Contact lens fitted “flatter than K.” Note the “minus” tear lens beneath the contact lens.

In the first example, the focal length of a +10 lens is (1,000 mm/10) = 100 mm (Fig. 54.10). By placing a contact lens onto the eye, the focal length becomes shorter: 100 – 13 = 87 mm. Thus, the power of the contact lens is (1,000/87) = +11.50 D (rounded to nearest eighth of a diopter).

In the second example, the -10 lens has the same focal length (100 mm), but in the opposite direction (Fig. 54.11). When it is placed onto the eye as a contact lens, the focal length increases: 100 + 13 = 113 mm. Thus, the contact lens has a power of (1,000/113) – 8.87 D (rounded to nearest eighth of a diopter).

In both cases, for either plus or minus lenses, always add plus power when correcting for vertex distance.


Lens Fitting Example

This is the type of problem seen on written examinations. We want to fit a rigid lens on a patient with the following readings:









Keratometry:

42.00/43.50 @ 90°


Refraction:

-9.00 + 1.50 × 90°


Suppose the refraction was done with a vertex distance of 13 mm and that we want to fit 0.50 D steeper than K. What is the power of the final lens? This type of problem is solved in three steps:

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Jul 10, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Rigid Contact Lens Fitting1

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