Advanced techniques in soft and rigid contact lens fitting

Many ophthalmic assistants play an expanded role in contact lens delivery and thus require detailed know-ledge of contact lenses. Although the changing and ordering of lenses may be beyond the scope of the ophthalmic assistant, an understanding of how to modify lenses and a review of abnormal symptoms and signs are not. This chapter highlights only some of the problems encountered; further information may be obtained from our textbook, Fitting Guide for Rigid and Soft Contact Lenses: A Practical Approach . 4th ed. St Louis: Mosby; 2002.

Abnormal symptoms and signs

It is important to recognize purely adaptive symptoms and differentiate them from pathologic symptoms that could result in corneal damage. The ophthalmic assistant may be the first to hear of these symptoms and should alert the ophthalmologist to patients’ complaints that might lead to serious corneal damage.

Flare or streaming of lights or glare from oncoming headlights may occur when the lens or the optical portion of the lens (optic zone) is too small. This symptom occurs because the pupil of the eye dilates at night or in a darkened room and the patient begins to have interference in his or her vision from the edge of the optic zone or the edge of the lens. It can be corrected by making the lens larger or by increasing the size of the optic zone.

Blurring of vision in daytime through the normal pupil may be a result of the lens riding too high or gravitating too low after each blink. This can be observed directly by watching the patient view a vision chart or distant object. The patient may respond by producing streaming opposite to the displacement of the lens ( Fig. 16.1 ). Lenses may ride high because of a high minus power and edge thickness that cause the lens to be lifted up by the upper lid. A small lens under a tight lid will also ride high. A lens that rides low may be too heavy because of thickness or because of high plus power or there may be insufficient edge thickness that will not permit the upper lid to grasp the lens and lift it. Redesigning the edge to provide proper compensation will permit the lens to center better. If this fails, smaller lenses may be required. Occasionally, the lens will slide nasally or temporally because of an abnormally centered cornea. These lenses should be replaced with lenses of larger diameter or a larger optic zone.

Fig. 16.1

Flare. The zone of streaming is always opposite to the displacement of the lens. Such a lens requires recentering. Uniform flare indicates that the optic zone of the lens is too small.

(From Stein HA, Slatt BJ, Stein RM. Fitting Guide for Rigid and Soft Contact Lenses: A Practical Approach . 4th ed. St Louis: Mosby; 2002.)

Central corneal edema ( Fig. 16.2 ) with consequent blurring of vision occurs when there is insufficient oxygenation of the cornea brought about by poor tear exchange. The edematous area appears hazy on the slit-lamp microscope, particularly if the light is shone off to the side of the area to be viewed so the area is illuminated from behind rather than directly. When the epithelial edema advances, some cells may die, causing central stippling, which will stain with fluorescein dye. The stippling may represent only a few small, discrete spots at the beginning, but the spots may increase in number as the condition progresses. If the condition progresses, punctate staining is said to occur. Patients who develop these objective signs will often complain of spectacle blur for some time after the lenses are removed. To correct this situation, the tear exchange must be improved. This can be done in the original rigid lens by:

  • Blending the junctions of the curve better

  • Flattening the peripheral curve

  • Reducing the total diameter

  • Reducing the diameter of the optic zone by increasing the width of the peripheral curve

  • Fenestrating the lens

Fig. 16.2

Corneal edema created by a contact lens.

(From Rosenthal P. Corneal contact lenses and corneal edema. Int Ophthalmol Clin . 1968;8:611.)

Corneal abrasion ( Fig. 16.3 ) may follow corneal edema caused by lack of oxygen, but may also result from too flat a lens rubbing on a portion of the cornea. Evaluation of the fit of the lens to indicate that the abraded area lies just under the touch area of the contact lens will determine whether the lens is rubbing the cornea and is too flat, too loose, or both. The excessive movement adds to the friction of the cornea and can be corrected with a new lens by:

  • Increasing the rigid lens diameter

  • Increasing the optic zone diameter

  • Reducing the edge thickness

  • Steepening the lens case curve

  • Steepening the peripheral curve

Fig. 16.3

Abnormal staining patterns.

Three o’clock and nine o’clock staining (see Fig. 16.3 ) of the cornea with fluorescein refers to erosions at the 3 and 9 o’clock positions in the exposed portion of the palpebral fissure. It is usually believed to be a result of dryness because of inadequate blinking while wearing the contact lens so that the small exposed portion of the cornea on each side of the lens becomes dry. This symptom is not usually seen with smaller and thinner lenses. Various methods, such as a smaller, thinner lens; blinking exercises; or artificial tears may help eliminate this problem.

Insertion abrasions may result from improper or clumsy insertion. Abrasion causes either immediate pain or pain after removal of the lens. Further practice in insertion should be undertaken under observation of the instructor.

Foreign bodies trapped under the lens will show varying types of zigzag scratch marks on the cornea, which will stain with fluorescein. Common substances, such as mascara and cosmetics may be the offending agents and should be used with caution.

Arc staining may occur either from poor insertion technique or most commonly from a sharp junction line between the central posterior curve and the intermediate or peripheral posterior curve. A proper blend is required.

Bubbles with staining occur when the lens is steep and there is too much sagittal depth to the lens so that air is trapped under the central curve. A flatter lens is required.

Blurring after reading may occur in the myope nearing the age of 40 years. The introduction of contact lenses requires a further accommodative effort and convergence that the patient cannot compensate for. Reading glasses may be required.

If the blur in the nonpresbyope occurs soon after insertion, it may be caused by the lower lid pushing the lens upward, thus causing poor centering of the lens in the reading position. This may be corrected by making the lens smaller.

If blurring occurs after prolonged reading, it may be the result of inadequate tear exchange and corneal anoxia. Concentrated reading reduces the blink reflex, induces staring, and consequently permits less tear exchange. A smaller lens, a flatter peripheral curve, or a material with a higher DK may alleviate this problem. For individuals who require prolonged reading activity, blinking exercises can be recommended.

Corneal warpage or induced astigmatism, both regular and irregular, can occur, resulting either from poorly fitting lenses that alter the corneal curvature or from chronic hypoxia of the cornea from overwearing the contact lenses. A complete reassessment of the fit is called for and possibly refitting in a more permeable material.

Follow-up keratometry

By performing keratometry on repeat visits, one can detect any molding or distortion of the cornea or induced astigmatism. One should record the difference in the K readings at subsequent visits. A notation, such as K + 1.25 + 0.50, indicates that the cornea has steepened by this amount in each meridian. Any changes over 1.00 diopter indicate that the wearing time should be reduced or the lens adjusted. Distortion of the mires also indicates that a change is needed.

Special lenses

The methods of contact lens fitting for correction of the most common defects of the eye were described in Chapter 14 . The techniques and devices available have proved highly successful for the majority of cases. However, as with other natural systems, variations in the anatomy and physiology of the eye are broad. Thus for those defects associated with more extreme variations, corrective devices and methods must be custom-built to a highly sophisticated level. For example, patients with extremely high myopia (−6.00 diopters or greater) or aphakic patients (+8.00 diopters or greater) require modifications of the normal lens design because of an extrathick (myopic) or extrathin (hyperopic) peripheral edge of the corneal contact lens. Similarly, extreme cases of keratoconus require bicurvature lenses for effective apposition to the eye, whereas highly astigmatic cases require an asymmetric (nonspherical), nonrotating lens design. Special rigid lenses have been developed for a steepened apical characteristic of the cone in keratoconus. Many eye patients older than 40 years require bifocal lenses, and special lens systems with bifocal lens characteristics can be prepared for their accommodation. Certain pathologic cases requiring telescopic lenses also present a need for specially designed lenses.

Contact lenses for high myopia

Myopia is the most common reason why a patient seeks contact lenses. Contact lenses for high myopia have not only the added feature of cosmetic enhancement by replacing glasses but also increased optical benefit because the contact lens rests on the eye, and thus the retinal image is larger and more normal than it would be with spectacles. In addition, the high myope no longer has a visual field restricted by the edges of glasses and frames.

However, as the minus power of a contact lens increases, so does the edge thickness. This increase in edge thickness creates a base-up wedge effect, which causes the lens to be pulled up by the upper lid and consequently to ride high so that the patient fails to look through the center of the lens. To reduce the thickness of the edge, it must be shaved off to prevent the upper lid from tugging upward on the lens. This in effect results in a lenticular-designed lens for high minus powers. The higher the minus power, the more the anterior edge has to be reduced. Aspheric lens designs have thinner peripheral profiles.

Aphakic lenses

With the more common use of intraocular lenses, aphakic lenses have declined in use. Aphakic contact lenses are primarily used when an intraocular lens is not appropriate, or for an older-generation patient when intracapsular cataract procedure was the surgery of choice. It is obvious why intraocular lenses are preferred: they generally offer better vision and freedom from the daily handling of contact lenses. For an older adult who may have a hand tremor, lax eyelids, or a tear deficiency, this freedom from the hazards of contact lens wear is important. Yet not all cataract extractions are treated with intraocular lenses. Implants may not be inserted if any of the following occurs:

  • Angle-closure glaucoma

  • Recurrent iridocyclitis

  • Any surgical contingency that makes the insertion of an intraocular lens hazardous

Thus knowledge of aphakic contact lenses should be acquired despite a definite downgrading of their importance in the management of a cataract patient.

Aphakic individuals require strong plus lenses. As the power of a plus lens increases, so too does its central thickness. This increase in central thickness creates a base-down wedge effect at the upper edge of the lens, thus the upper lid forces the lens downward. The high-plus lens is also heavy, which causes it to gravitate downward. One way of reducing the thickness and thus the weight of a high-plus lens is to keep the overall diameter of the lens very small. Unfortunately, this does not solve the problem if the patient has large pupils or keyhole iridectomies, in which case the patient may be looking through the lens edge. A more practical way of fitting a high-plus lens with a reduction in the thickness and weight is to add a lenticular design on the lens ( Fig. 16.4 ). It is important that the lenticular optic portion of the lens completely covers the pupil, otherwise the patient will complain of blurry vision and glare. Another solution is to put a myopic edge finish on any high-plus lens, which will help the upper eyelid elevate the lens.

Fig. 16.4

Single-cut and lenticular-cut lenses.

Contact lenses for astigmatism

Residual astigmatism occurs when a contact lens is placed on an eye and an astigmatic refractive error still results. Several conditions may contribute to this residual astigmatism, but it is most commonly induced by the crystalline lens of the eye (so-called lenticular astigmatism ). If the amount is small, it will not significantly interfere with vision. If the amount of astigmatism is large, however, vision will be substandard unless one can compensate for this with the contact lens. Although this requires a complex type of lens, essentially a toric surface is ground on the front or back of the lens to prevent the lens from rotating, so that the toric surface is lined up with the axis of astigmatism. One must stop rotation of the lens by introducing a weight, such as a prism ballast to the lens at the correct position.

Nonrotating lenses

Patients with a moderate or high degree of astigmatism will experience difficulties in wearing spherical contact lenses. Symptoms of blurred vision, excessive awareness of lens edge, and slipping of the lens off the cornea or even completely out of the eye will be encountered because of the rocking effect of the lens over the flatter meridian. Residual astigmatism, in which the cornea is spherical but the patient has a cylindric spectacle prescription, is another indication for nonrotating lenses. In these cases, the astigmatism comes from the lens of the eye and it is necessary to add prism to the lens to stop lens rotation and to properly orient the cylinder over the correct optical axis.

Several types of nonrotating lenses have been designed.

Noncircular shapes

A truncated lens is really a circular lens in which the bottom or top portion, or both (double-truncated lens), has been cut off. The corners at the edge of the truncation are smoothed off. A double-truncated lens, although infrequently used, will tend to stabilize so that the flat portion lies adjacent to the upper and lower lid edges. If the edges are rounded off more, the lens assumes an oval shape, but this type of lens rotates frequently and thus negates the use for which it was intended. Rectangular and triangular lenses have been designed, but they have shown little practical value.

Toric curve lens

A lens may be cut so that it is not spherical on its back surface. It is called a toric back curve lens when it has two meridians of curvature on its back surface that are designed to conform somewhat to the two meridians of curvature of the front surface of the cornea. This lens is used to correct a high degree of astigmatism. When the lens is placed in the eye, it tends to align its curvature with that of the cornea. However, an optical problem of astigmatism may exist that may require the grinding of a toric surface on the front of the lens, the so-called front surface toric lens . These are used primarily to treat residual astigmatism in patients with spherical corneas. In some instances, toric surfaces may be ground on both the front and back surfaces, the so-called bitoric contact lens .

Prism ballast lenses

When a prism is ground into a contact lens, the heavier base of the prism swings the lens around so that the heavier base rides low, attracted by gravity, and thus further rotation of the lens is eliminated ( Fig. 16.5 ). One can incorporate up to 3.00 diopters of prism in a lens to provide sufficient weight. In addition to preventing rotation, a prism may be used to create weight in a lens that tends to ride high or to reduce excessive lens movements. The weight of prism ballast lenses and the thinner superior edge of the lens, which fits under the upper lid, provide stability and prevent rotation of the lens.

Fig. 16.5

Prism ballast to provide weight and stop rotation of a lens.

Correction of high astigmatism

Astigmatism may result from corneal surfaces of different radii or from changes in the lens of the eye. The latter is less common, but nevertheless does occur and accounts for residual astigmatism when corneal astigmatism has been fully corrected. It may also account for a very irregular corneal surface. Corneal topography is an excellent way of analyzing the corneal surface.

Keratometer readings provide a good index of the amount of corneal astigmatism present. Most spherical-based lenses are the first choice for fitting eyes with corneal astigmatism. Tear fluid readily fills in the interface and provides a good optical result in most cases. However, in some cases, these lenses will not provide adequate tear interchange, rocking occurs, or poor staining is found. A back surface toric lens will be required that will conform to the corneal toricity. Diagnostic trial lenses may be a valuable adjunct. Frequently, changing the back surface to a toric surface causes induced or residual astigmatism; this must be corrected by grinding a toric surface on the front of the lens. This constitutes the so-called bitoric lens .

Toric soft contact lenses

A wide variety of toric soft contact lenses are on the market, and fitters must familiarize themselves with what is available on a lens-by-lens basis. Specific lenses are not discussed here because there are excellent fitting guides available from the manufacturers. Before choosing a specific lens, the fitter must make sure that the lens is available in parameters that match the patient’s refractive error.

Some “off-the-shelf” toric lenses are available in a limited range of cylinders and axes. Manufacturers make lenses in the most commonly requested power ranges and these are available immediately. Most soft toric lenses are available in powers from − 8.00 to + 4.00 diopters, with cylinder powers of − 0.75 to − 2.50 diopters. Daily disposable toric lenses are available in limited cylinder power and axis. A diagnostic lens of the selected toric design must be evaluated on the patient’s eye for fit and position of axis.

For “custom” work, a fitting set must also be used. The fitting lenses are spherical designs with the diameters and orientation systems of the toric lenses that the patient will eventually wear. Toric lenses that can be fit in this manner are available in powers from −20.00 to +20.00 diopters, with axes of rotation available in 5-degree increments. A few manufacturers provide toric lenses made to order with even greater power and cylinder and with axes of rotation in 1-degree increments.

Several design alternatives are used to maintain the orientation of toric lenses. Double slab-off , the creation of thin zones on the inferior and superior parts of the lens, allows lids to hold the lens in position ( Fig. 16.6 ). As with bifocal soft contact lenses, prism ballasting ( Fig. 16.7 ), periballasting, and truncation are also used to maintain orientation.

Fig. 16.6

Double slab-off soft lens used to correct astigmatism. The lens is made thinner superiorly and inferiorly so that thinner portions tend to rotate and come to rest under the upper and lower eyelids.

(From Stein HA, Slatt BJ, Stein RM. Fitting Guide for Rigid and Soft Contact Lenses: A Practical Approach . 4th ed. St Louis: Mosby; 2002.)

Fig. 16.7

(A) Lens on immediate insertion. (B) The weighted portion, combined with the torsional effect of the eyelid muscles, rotates the lens to a stable position.

(From Stein HA, Slatt BJ, Stein RM. Fitting Guide for Rigid and Soft Contact Lenses: A Practical Approach . 4th ed. St Louis: Mosby; 2002.)

Toric lenses have orientation (or scribe) marks near the edge, some at the 3 o’clock and 9 o’clock positions, and others at the 6 o’clock position. The manufacturer can sometimes be verified by the marks or by the laser identification marks on the lens.

Contact lenses for keratoconus

Keratoconus is a forward bulging of the central cornea with irregular astigmatism ( Fig. 16.8 ). It usually begins in adolescence and progresses over the next several years. It is often bilateral, although one eye advances more than the other. Its cause is still unknown, but there is a strong hereditary feature.

Fig. 16.8

(A) Normal eye. (B) Keratoconus. Note thinning of the cornea as well as the forward protrusion.

This irregular astigmatism cannot be corrected by normal spectacles, but in the majority of cases vision can be satisfactorily corrected by contact lenses. In addition to vision correction, contact lenses tend to flatten and give symmetry to the cornea, although they do not change the progress of the disease. The apex of the cornea is very sensitive in early stages, giving rise to photophobia and lens-fitting problems, but in later stages the cornea becomes relatively insensitive ( Fig. 16.9 ).

Fig. 16.9

Keratoconus. Note the forward protrusion and thinning of the cornea.

(From Soper JW, Girard LJ. Special designs and fitting techniques. In: Girard LJ, ed. Corneal Contact Lenses . 2nd ed. St Louis: Mosby; 1970.)

The thinness of the cornea, ruptures in Descemet’s membrane, and small apical tears may be detected in advanced cases by slit-lamp biomicroscopy, the hand keratoscope, or Placido’s disc. Keratoconus is often associated with patients who have hay fever, atopic dermatitis, eczema, or asthma. Topography is often helpful ( Fig. 16.10 ).

Fig. 16.10

Topography indicating steepening inferiorly.

A slit-lamp diagnosis is difficult to make in the early phase, whereas in the late stages, the diagnosis becomes obvious with apical thinning, Fleischer’s keratoconus ring, increased endothelial reflex, increased visibility of the nerve fibers, and scarring of Bowman’s membrane. Keratometry is very helpful in the early phase of this disease because the mire images appear distorted and irregular. The two principal meridians are not at right angles to each other and the dioptric value of the readings is much higher than normal (≥48.00 diopters). The range of the keratometer may have to be extended to accurately record the full corneal curvature. The addition of a 1.25-diopter lens over the front of the keratometer either by hand or by the special retention ring is most useful (conversion by means of a table is required for true K readings; Table 16.1 ).

Table 16.1

Dioptric curves for extended range of keratometer

Courtesy Bausch & Lomb.

High power (with +1.25 lens over aperture) Low power (with −1.00 lens over aperture)
Drum reading True dioptric curvature Drum reading True dioptric curvature Drum reading True dioptric curvature Drum reading True dioptric curvature
52.00 61.00 46.87 55.87 42.00 36.00 36.87 30.87
51.87 60.87 46.75 55.75 41.87 35.87 36.75 30.75
51.75 60.75 46.62 55.62 41.75 35.75 36.62 30.62
51.62 60.62 46.50 55.50 41.62 35.62 36.50 30.50
51.50 60.50 46.37 55.37 41.50 35.50 36.37 30.37
51.37 60.37 46.25 55.25 41.37 35.37 36.25 30.25
51.25 60.25 46.12 55.12 41.25 35.25 36.12 30.12
51.12 60.12 46.00 55.00 41.12 35.12 36.00 30.00
51.00 60.00 45.87 54.87 41.00 35.00
45.87 54.87
50.87 59.87 45.75 54.75 40.87 34.87
50.75 59.75 45.62 54.62 40.75 34.75
50.62 59.62 45.50 54.50 40.62 34.62
50.50 59.50 45.37 54.37 40.50 34.50
50.37 59.37 45.25 54.25 40.37 34.37
50.25 59.25 45.12 54.12 40.25 34.25
50.12 59.12 45.00 54.00 40.12 34.12
50.00 59.00 44.87 54.00 40.00 34.00
44.87 53.87
49.87 58.87 44.75 53.75 39.87 33.87
49.75 58.75 44.62 53.62 39.75 33.75
49.62 58.62 44.50 53.50 39.62 33.62
49.50 58.50 44.37 53.37 39.50 33.50
49.37 58.37 44.25 53.25 39.37 33.37
49.25 58.25 44.12 53.12 39.25 33.25
49.12 58.12 44.00 53.00 39.12 33.12
49.00 58.00 43.87 52.87 39.00 33.00
43.87 52.87
48.75 57.75 43.75 52.75 38.87 32.87
48.62 57.62 43.62 52.62 38.75 32.75
48.50 57.50 43.50 52.50 38.62 32.62
48.37 57.37 43.37 52.37 38.50 32.50
48.25 57.25 43.25 52.25 38.37 32.37
48.12 57.12 43.12 52.12 38.25 32.25
48.00 57.00 43.00 52.00 38.12 32.12
38.00 32.00
47.87 56.87 38.00 32.00
47.75 56.75 37.87 31.87
47.62 56.62 37.75 31.75
47.50 56.50 37.62 31.62
47.37 58.37 37.50 31.50
47.25 56.25 37.37 31.37
47.12 56.12 37.25 31.25
47.00 56.00 37.12 31.12
37.00 31.00

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Jun 26, 2022 | Posted by in OPHTHALMOLOGY | Comments Off on Advanced techniques in soft and rigid contact lens fitting

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