Thin Lenses and Bifocals for High Myopia*

THIN LENSES AND BIFOCALS FOR HIGH MYOPIA* SIDNEY

L.

OLSHO,

M.D.

Philadelphia

Highly myopic patients are often more more apt to be reflections due to prismatic concerned about the increasing thickness effect and an unsightliness due to the apand weight of their lenses than about the pearance from the front of a deep circular advance of their myopia. band around the disc (fig. 1, A). For many years the two objections, Obrig (Arch. of Ophth., 1934, v. 12, p. thick edges and lens weight, have been 2) described improved, lenticular-type, met by the grinding away of the pe- strong concave lenses. A 25- to 30-mm. ripheral lens surfaces, restricting the cor- diameter, circular, spherical, concave surrection to a central circle. Lenses of this face of the desired power is generated on design are known to opticians as "Lentic- a lens blank previously made thin and flat. ular." Any desired cylinder can be ground on Concave Lenticular lenses are made in the front surface. The result is an ultratwo forms: thin lens known as a Myodisc lens (fig. 1. With a convex periphery, as in fig- 2,A,B). ure 1, A, B. One of my corrections (-10. D. sph. 2. With a flat periphery, as in figure 1, =c= -0.75 D. cyl. ax. 1800 ) so made with C, D. a 29-mm. circle, has an edge thickness of The convex periphery leaves a larger only 20 mm. The margin of the concave central circle but creates a heavier lens. disc of this lens, shown in figure 2, B, is This type can be put into a rim, but the extremely abrupt or sharp. When the eye sharp edge does not fit well into a rim- reaches this margin there is no blur; the less mounting. When lenses of any con- eye passes directly to the plano cylinder siderable edge thickness are fitted into as if it had reached a lens margin. rims it is an advantage to require that all It is desirable to set these lenses as of the bevel be to the rear of the rim. close to the eyes as possible and well up. The flat Lenticular lens has a square In rimless styles, the drop oval perimetric edge which a good workman can fit well or orbal shape is advantageous. The field into rimless frames (fig. 1, E). of vision afforded by the disc is ample, It is not necessary to order toric curves provided that the lenses are placed close for highly myopic eyes. A cylinder if to the eyes. Clarity of vision is greater needed can be ground on the anterior sur- than that afforded by a full-sized lens face. which, because of its weight, has dropped In ordinary Lenticulars the margin of too far away from the eyes or slipped too the concave disc (fig. 1, B, D) is not low. perfectly sharp and well defined. In the An ultrathin lenticular one-piece rnyopolishing process it becomes slightly disc bifocal is also described in Obrig's rounded, producing an aberration blur if article. Two concave discs are ground on the eye reaches it. This marginal blur is a flat lens blank. A small arc of the lower more manifest in the Lenticulars with the disc is sacrificed, as shown in figure 2, convex periphery. In these there are also C, D. The lens has two widely separated * Read before the Eye Section of the Phila- optical centers. I find this lens unwearable. A prism, base-front (fig. 2, C) is delphia County Medical Society, Nov. 1, 1934. 281

282

SIDNEY L. OLSHO

o

.~ 0 A

a

CONVEX PERIPHERY

LENTICULAR CX PERIPHERY

C.

FLA T PERIPHERY

D

LENTICULAR FLAT PERIPHERY

E

SQUARE EDGES

Fig. 1 (Olsho). A Note prismatic effect at margin of circle. D. The convex periphery affords a larger circle but a heavier lens. CD. The flat periphery creates a smaller circle and a thinner lens. The square edges of the flat lens fit better into rimless mountings.

o A

MYODISC

B

MYODISC

C

CROSS SECTION

D

MYODISC BIFOCAL

Fig. 2 (Olsho). AB. Ultrathin edges on high-minus corrections. The margin of the circle is sharp. The eye passes into the plano without encountering an intervening blurred zone. CD. Myodisc bifocal. Independent optical centers are widely separated, leading to confusion in the transition to the reading zone. Base-front prism. Double images. Broad blind zone.

i--c7 I, "

":. \ .

~ '\

I', A

ULTEX

A

B

ULTEX AONE PIECE

C

KRYPTOK FLINT FUSED

Fig. 3 (Olsho). A Ultex "A" cross section. In the reading zone of a high-minus lens the eye encounters a strong prism, base down. Nasally there is a favorable prism, base in. D. Ultcx "A" one-piece bifocal. Heavy base-down prism also in the segment. This is the least suitable bifocal for high-minus corrections. C Kryptok bifocal. Fused flint segment. The thick edges of the segments are down, intensifying the prism, base down, present in all minus corrections.

created and a wide blind zone exists between the upper and lower lens portions. BIFOCALS FOR MYOPIC EYES

It is apparent by referring to figure 3, A that when the eye is directed to the

lower part, or reading zone, of any strong concave lens it encounters a heavy prism, base down. By the selection of the correct segment in a bifocal for a myopic eye this strong, base-down prism can be partially neutralized or at least not intensified.

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LENSES FOR HIGH MYOPIA

In the commonly used Ultex one-piece bifocal, for example, with the semilunar segment "A," the construction is such that the lower margin of the lens becomes greatly thickened, and this generates additional prism base-down (see fig. 3, A

o A

PANOPTIK. BARIUM CROWN 'USED

Desiring to obtain a thin bifocal -10 D. sph. =c= -0.75 D. cy1. ax. 1800 , add +3.00, I had bifocals constructed of each of the types shown in figure 4, A, B, and C, and had them made flat lenticular, specifying that a circle 28 to 30 mm.

o B

C

'ULVUE. BARIUM CROWN 'USED

UNIVIS. 'LINT SEGMENT ,USED C TYPE

D

GENERAL CROSS SECTION

Fig. 4 (Olsho ). A. Panoptik, Barium Crown modern, fused segment. B. Fulvue, Widesite "A," Barium Crown modern, fused segment. C. Univis "C," Flint modern, fused segment. D. The segments are fused into the anterior surface. There is no angle for reading. The thicker edges of the segments are up; thus it is more favorable to minus corrections. Ultrathin lenticular lenses are impossible except with a very small circle and segment.

and B). Moreover, in the commonly used Kryptok fused bifocals the horseshoeshaped button becomes thicker toward its lower margin. This means added prismatic effect base-down in a concave correction (see fig. 3, C). There is already a heavy base-down prism in the lower zone of a high-minus lens. This base-down prism becomes excessive if either of the two types of bifocals previously mentioned is ordered for a highly myopic eye. On the other hand, bifocals whose design is similar to those shown in figure 4, A, B, C, D, have segments of which the thick edges are up. These tend to diminish-or, at least, they do not intensifythe prism, base down, that is present in the reading zone of a strong concave lens. It seems good practice to use segments of this design only when the distance correction or its horizontal meridian is concave, and to use segments whose thin edge is up when the distance correction or its horizontal meridian is convex.

in diameter should be used and that the top of the segments should be 3 mm. below the optical centers of the distance

() A

CROSS SECTION

B.

MONOCENTRIC CEMENTED MYODISC BIFOCAL

Fig. 5 (Olsho). A. The segment is properly angled. It does not add to lens thickness. The Opif ex segment is extremely thin. B. The thin segment presents a very fine line anteriorly. Segment and lens are monocentric and homogeneous.

lenses. The results were similar. The thickness of the lens edge in the thinnest pair was 5 mm. The lenses were quite heavy. In each instance the segment was found to be fused into the anterior almost flat surface, as shown in figure 4, D. The

284

SIDNEY L. OLSHO

inclination for the reading segment would To obtain the best optical results the be more nearly correct were it mechani- bifocal must be constructed in conformity cally possible to have the segment fused with the specifications to be enumerated, into the concave posterior surface. and these require a high order of workThis last thought gave rise to the idea manship: of reverting to a cemented bifocal but rThe segment or wafer is of the type utilizing modern bifocal knowledge in its 1 known as Opifex. It is ultra thin, with design. keen, knifelike edges. These segments give the bifocal an almost one-piece efTHE MYODISC WITH MONOCENTRIC fect. The wafer is fragile and requires CEMENTED SEGMENTS support or a "backing" in the grinding The concave myodics single lens, previ- and handling. The thinness makes for ously referred to, is used in the construe- lightness and invisibility. The surface that

Fig. 6 Fig. 7 Fig. 6 (Olsho). Contrasting the edges of a modern fused bifocal and the Myodisc with monocentric cemented segments. Both pairs are of the same focus (-10.00 D. sph ~ -0.75 D. cyl. ax. 180 add 3.00). 0

•

+

Fig. 7 (Olsho). Myodisc with monocentric cemented bifocal segments. The visibility is intensified here by the background and lighting.

tion of the Myodisc Cemented Bifocal. This cemented bifocal possesses a degree of optical perfection impossible of attainment, up to the present time, in any onepiece or any fused bifocal. It has certain disadvantages not completely separable from bifocals of the cemented type. The well-known disadvantages of ordinary cemented wafers include visibility and the possibility of loosening of the wafer or the chipping of its edges. These disadvantages have been mitigated in the Myodisc Cemented Bifocal, nevertheless, these bifocals are suitable mainly for careful and fastidious patients. Mine have been satisfactory in every respect for two years and give promise of continuing so indefinitely.

is to make contact is absolutely true to the concave surface of the major lens, tending to make it adhere with a minimum of the usual cement. The segment is ground with its thick edge up (fig. 5, A). The upper edge is straight across and has a very fine ground surface. This edge takes a slant of 10 degrees, downward and backward, much of which, however, is lost, as the segment rests in its concave bed. Though this upper edge is thicker than its rounded lower margin, it presents from the anterior aspect of the lens only as an exceedingly fine line. The top edge of the segment is placed 2 to 3 mm. below the center of the concave-disc circle. It is not so necessary optically to have an inset,

LENSES FOR HIGH MYOPIA

for the nasal halves of a pair of strongly concave lenses act as a pair of prisms, base in. Therefore the eyes may not have to converge to receive the rays emanating from some near object. The rounded lower margin of the cemented segment shown is an arc that is concentric to the circumference of the concave disc major circle (fig. 5, B). It will be observed that the segment is amply large. Optically the wafer is made monocentric with the distance lens. There is, therefore, no blind area, or "jump," in the eye's transition to the reading zone. Furthermore, there is a remarkably slight loss of clarity when looking at the floor through the reading zone, even with the plus 3 add, in my case, for the residual correction is still a minus 7 diopter. As the wafer lies within the concavity of the disc it naturally takes an adequately tilted position for reading. This is not true of the segments fused into the anterior surfaces of the several modern fused bifocals (fig. 4, D). The cemented segment does not necessitate adding thickness to the lens, as is the case with fused segments. The edges of the cemented segment are not exposed, for it lies well recessed

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within the protecting margins of the major circle. Since both the cemented segment and the major lens are of crown glass, the combination is homogeneous and the problem of combining unlike types of glass, as in fused bifocals, does not arise. It is best to clean the lenses with soap and cold water. Subsequent to the description of the Monocentric Cemented Myodisc Bifocal, an Ultra Thin Lenticular Barium Fused Panoptik Bifocal for highly myopic patients has been made. The first pair was made on my prescription. The circle was kept round in the presence of the cylinder by the generation of a cylinder on the flat periphery. The lens has great beauty. The segment is quite small and less visible. It is permanent but more expensive than the cemented myodisc bifocal. The button is fused on the front surface (not the better position). In order to attain the thin edge (2 mm.) on the same prescription (-10. D. sph. =()= -0.75 D. cyl. ax. 1800 ) the myodisc circle had to be reduced to a diameter of 25 mm. The distance field is therefore correspondingly diminished. The wearer has found this lens to be somewhat less efficient than the cemented lens. ~ 235 South Fifteenth Street.