Virtually every patient who enters an ophthalmologist’s or other eye care practitioner’s office receives a refraction and most receive a prescription for spectacles. Even if patients plan to use contact lenses as their main corrective device, they should have glasses for backup. Thus the ophthalmic assistant needs some information about the construction and types of spectacle frames and the types of lenses currently used.
Despite an accurate prescription, many patients are unhappy with their glasses because of the design, fit, or prescription. The goal of correcting a refractive error is not only to achieve the best possible vision for each eye, but also to do so with a pair of glasses that match the patient’s aesthetics and lifestyle as well. Glasses that make the wearer feel good and look good are more likely to be worn regularly. The purpose of this chapter is to provide a brief résumé of the types of eyeglasses available to the patient, reviewing the advantages and disadvantages of each.
The use of spectacles has its origin in ancient history ( Fig. 13.1 ). In the early periods, optical glass was of poor quality and was made from scarce pebbles of quartz or semiprecious stones. The first primitive spectacles were balanced precariously on the nose, tied to the ears by means of thread or string or held in the hand as one holds a present-day lorgnette. Their use was confined solely to monks and other learned men of the day. By the 17th century, spectacles were in common use and were elaborately fashioned of gold or silver for members of the aristocracy, whereas tortoiseshell frames were used by members of the upper middle class.
Despite the improvements and refinements in the manufacture and dispensing of frames, the final choice of the right set of spectacles is a personal one, derived not by any scientific formula but by the whims, fancies, and needs of the individual.
Today’s fashion industry has influenced frame design tremendously. It is common to see fashionable frames with international designer names (e.g., Prada, Tom Ford, and Salvador Ferragamo). The stigma of wearing glasses has transcended into a fashion statement and has exploded into the sunglass arena. Frame selection has evolved from a simple visual necessity to a method of communicating a personal statement, a fashionable tool for displaying an image one wishes to present.
A frame for every face
As a rule, frames look best when they complement a patient’s facial form. Current fashion trends should be taken into account; if the style of the moment is small round frames, public demand will lean in that direction. A knowledgeable assistant will be able to tell whether the trendy look suits a particular patient, and if not, what different styles to offer. Here it is best to refer to classic rules for assistance. The first step is to determine which of the seven basic face shapes the patient has: oval, diamond, round, square, base-down triangle, oblong, and inverted triangle ( Fig. 13.2 ).
The most flattering eyeglass frames are those that are the right depth and width for the face. In general, a flattering frame shape is opposite to the face shape. A round face, for example, looks best with a squared, angular frame, not a rounded shape.
The Vision Council’s website ( www.eyecessorize.com ) devotes an entire section to selecting the correct frame shape. Oval: The chin is slightly narrower than the forehead and the cheekbones are high. There appears to be a natural facial balance. Best options: Choose frames that are as wide as or wider than the broadest part of the face. Select frames in proportion to the face size. Most frame shapes are good on an oval face.
Diamond: A small forehead and wide temple area gradually narrow to a small chin. Often, cheekbones are high and dramatic. Best options: To widen the forehead and jaw and to minimize the cheekbones and wide temple area, select frames that are top-heavy with sides that are straight or angled outward toward the bottom. Frames should be square, rimless, or shaped with a straight top and curved bottom. Avoid frames with decorative temples that make the middle of the face look wider.
Round: Full face with very few angles that appears equal in height and width. Best options: To make the face seem longer and thinner, select oval, slightly curved angular frames that feature high or mid-height temples. Clear bridges and color on the temple area flatter this face.
Square: This face has a broad jawline and forehead, with wide angular cheekbones and chin. Best options: To lengthen the face, try subtly curved frames, no wider than the widest part of the face. For a high-style look, experiment with stark, geometric shapes that have color concentrated on the outside corners. Square faces can also wear top-heavy frames, oval shapes with temples in the center, and frames with decorative temples hinged above the eye level.
Inverted triangle: Below a wide forehead, this face shape narrows into high cheekbones and a narrow chin. Best options: To add width to the chin and cheeks, select frames that angle outward at the bottom but are no wider than the forehead. Low temples, light colors, and rimless styles balance the face. Frames with rounded tops and square bottoms, aviators, and butterfly shapes are other options.
Base-down triangle: A narrow forehead becomes fuller at the cheeks and chin. Best options: Frames should add width to the forehead but soften and narrow the jaw, chin, and cheeks. Flattering frames angle outward at the top corners. They should be as wide as or slightly wider than the broadest part of the jaw. Square, aviator, and metal frames with rimless bottoms are flattering selections, or try top-heavy frames with angled bottoms.
Oblong: This face shape is longer than it is wide and the forehead, cheek, and jawline are comparable in width. Best options: To shorten and widen the face, select styles that extend beyond the widest part of the face. Choose frames with decorative temples, strong top bars, and round bottom lines. Round, deep, or square shapes shorten and soften an oblong face.
Although fashion is key to frame design, so too are comfort, durability, and thinness. A wide range of frame materials is available; each has its own characteristics.
Spectacles can be defined as an optical appliance composed of lenses and a frame with sides, called temples , extending over the ears. The front, main part of the frame holds the actual lenses in front of the eyes, the pads give support on the nose, and the temples hold the front part in the correct position before the eyes. Frames may be made of metal, rubber, wood, plastic, or a combination of metal and plastic.
Fig. 13.3 illustrates the anatomy of frames. Bridge size may be noted as distance between lenses (DBL), which should not be confused with the distance between the (optical) centers (DBC). Temples often are marked with the overall temple length expressed in either inches or millimeters. When two numbers are found on the temple, both overall length and length to the bend are given.
Originally, frames were designed and handmade of silver or solid gold for the aristocracy. These metals have gradually been replaced by other metals, such as gold-filled, nickel, aluminum, stainless steel, memory metal such as Flexon, and titanium ( Table 13.1 ).
|Material||Composition (%)||Features||Adjustment considerations|
|Titanium||A silver-gray element alloyed with other elements||Cannot solder with existing techniques|
|Aluminum||Alloyed with copper, silicone, iron, bronze, manganese, zinc, chromium|
|Cobalt||Difficult to adjust|
|Memory metals||Titanium and nickel alloy|
Plastic and composite frames
There are two types of plastic frames: those molded to shape from plastic materials in an injection-molding machine and higher-quality frames cut to shape from a flat piece of plastic, which is then machined and polished to form the finished frame.
Ophthalmic frames made from different types of plastic and composite materials include the following:
Propionate is a spun cast, easy to manufacture lightweight plastic with translucent colors to rival Optyl. It is easily recognizable and needs only a hot-air blower for adjustments. Propionate holds its shape well but, like all plastic, this feature depends on the manufacturing quality.
SPX is stronger and more flexible than propionate, enabling the production of very thin plastic frames. If SPX is overheated, it will shrink and not return to its original size. Thus lenses are mounted while the frame is warm or at room temperature.
Cellulose acetate is a common plastic used today that comes in various colors and patterns. It will burn if a flame is held in contact with the material but is self-extinguishing; when the flame is removed, the plastic will cease to burn. Clear acetate will not change color or yellow with age, and cellulose acetate does not become brittle.
Lucite (Plexiglas, Perspex) is a much tougher plastic and is available in solid colors only (or two-tone or fade-away patterns, produced by laminating two colors). Once the frame is fitted, it retains its shape better than cellulose acetate or nitrate. Lucite does not change color, but manipulation requires much more heat than do other materials.
Nylon (Grilamid, Trogamid) frames are lightweight, hypoallergenic, and relatively unbreakable. They are injection molded and thus of one solid color. Some types are dyed after completion to give the appearance of stock sheet materials also found in cellulose acetate. Nylon requires a high temperature to glaze. Lenses should be cut as close as possible to final size and shape. Because nylon can become dried out, patients should soak the frames in water monthly.
Polycarbonate frames are a good choice for safety and sports use because they have high impact resistance. The frames are clear so they do not block the field of vision. They are not adjustable, but elastic straps and rubber bridge pieces offer flexibility and comfort.
Polyamide/copolymide is a blend of nylons that is durable, light, and flexible. It holds translucent colors well and is scratch-resistant. Lenses should be edged to exact size and inserted cold into the frame, then the frame can be shrunk by carefully applying heat. Polyamide will shrink if placed in a salt pan.
Optyl is an epoxy resin, weighs 30% less than cellulose-based frames, and is considered hypoallergenic. To insert lenses, edge them slightly oversized and then heat the frame. Do not cool in water. Adjustments should be made by heating and adjusting one part at a time, preferably using a hot-air blower with a narrow opening. The frame will hold its shape until heated again. The temples can be lengthened by heating until soft, then gently stretching.
Thermoplastic polyester elastomer (Hytrel) frames are a polyether–ester block copolymer combining the characteristics of high-performance elastomers and flexible plastic. The frame offers mechanical strength and durability in a flexible component. The material is not affected by a wide range of temperatures and it maintains flexibility and lens retention.
Memory plastic is tough and virtually unbreakable. It is extremely flexible and can be bent or twisted and still maintain its original fit. Its shape is not affected by heat and the color resists scratching, chipping, or wearing off.
Kevlar is made of strong fibers mixed with a hybrid nylon. It is available in limited colors. Lenses should be cut to the exact size, because although Kevlar will not shrink or expand when exposed to heat, it can be softened by heat for lens insertion.
Composite combines Kevlar, polyamide, and ceramic fiber and is used for nonprescription sun lenses, as well as prescription lenses. Lenses to be inserted should be cut to exact size with more bevel on the front side. Plastic lenses are recommended and should be inserted cold. Glass lenses require a slight preheating of the frame. The frame should never be heated with lenses inserted, and never to temperatures greater than 160 degrees.
The plastic frame is basically rigid and it keeps its adjustment well, once fitted. The colors, patterns, and styles available in plastic are almost limitless. Most frames have fixed plastic pads that rest on the side of the nose. It should be noted, however, that plastic does become brittle with age.
There are two basic types of bridges: the saddle ( Fig. 13.4 ) and the keyhole ( Fig. 13.5 ). The saddle bridge rests lower on the nose and creates the illusion of shortening a long nose. A keyhole bridge is more flattering to a round, short nose. Most plastic temples have a metal core for rigidity and strength. Some fronts are braced with metal, buried in the plastic, running across the top of the front from hinge to hinge. These frames are desirable for children and athletes.
A combination frame consists of a metal chassis with a decorative metal or plastic top and two fully adjustable pads ( Fig. 13.6 ). Combination frames with movable pads are ideal for patients with high hyperopic or high myopic corrections, inasmuch as the distance of the lens from the eye can be accurately placed, as can the height of the bifocal or trifocal segment. The advantage of movable pads is that they can be moved in the up-and-down adjustment and backward and forward to fit the patient’s face. In prescriptions for a heavy lens, these frames should be ordered with jumbo pads (oversized plastic pads), which give a larger weight-bearing surface on the nose and have less tendency to leave marks.
Rimless frames look much like combination frames in that there is a decorative top but no apparent metal holding the lens shape. Rimless frames may be of metal or plastic construction. In the case of plastic, there is actually a nylon cord fitted into a groove cut in the edge of the lens that holds the lens to the top. This nylon cord is almost invisible and gives the frame a rimless appearance.
In some metal rimless frames, the lenses are held in position by hidden notches in the lenses. The notches are engaged by tabs protruding from the top, but when the frame is viewed from the front the tabs are out of sight behind the top.
Older-model rimless spectacles actually had holes drilled through the lenses and small nuts and bolts placed through these holes to hold the lenses in position ( Fig. 13.7 ). This type of rimless frame was easily shattered when dropped and therefore is now practical only when high-index, polycarbonate, or Trivex lenses are used. Modern-day semirimless spectacles have a pressure mount with a post shim to hold the lenses in place.
Most frame measurements are based on the box system, whereby the lens is enclosed in a rectangle and the distances between opposite sides are taken as the eye size . An imaginary line running through the center of the lenses is called the datum line . This is a very important line, because all measurements are taken at this point ( Fig. 13.8 ).
On the back of a frame are a few figures, such as 46 × 22, which represent the lens size (46) and the bridge size (22). All measurements are in millimeters.
The other measurements are for the temples; they vary according to the use for which the glasses are intended and the consequent shape of the temples.
The temple is the long strut that extends from the lateral aspect of the eyepiece and rests on the ear. The temple length (e.g., 140 mm) is expressed as the overall length from the hinge end to the end that rests behind the ears. Most plastic temples have a metal core for rigidity and strength.
Several varieties of temples are available. The more commonly used types are:
Cable temples ( Fig. 13.9A ). Cable temples sometimes are known as riding bow temples or curl side temples. The cable temple is either metal or plastic, with the ear portion made of a flexible metal that can be shaped to fit the contour behind the ear. Another type of riding bow temple has a much stiffer metal core covered with plastic. A comfortable fit is possible only when this temple is contoured to the back of the ear. This type of temple is ideal for children and active people, for those who are constantly looking down, or for positions in which the spectacles might otherwise slide off the face.
Straight temples ( Fig. 13.9B ). Straight temples sometimes are called library temples. These are ideal for people who constantly take their glasses off and for those in religious orders whose ears are concealed under a habit, making it difficult to put on or take off glasses that have riding bow or other paddle temples.
Paddle temples ( Fig. 13.9C–E ). Paddle temples are sometimes called skull temples or hockey-end temples. These temples are ideal for general use and are the most common type used today.
Frame manufacturers produce the majority of frames with standard eye sizes and bridge sizes. This leaves a small portion of the population with unusual facial measurements unable to obtain a correct fit with the commercially produced frames. Fortunately, some optical companies will make frames for this group by hand and will produce frames for special vocational or medical requirements. A few of these are mentioned here.
Frames for individuals with low, flat bridges
Because infants and some people have practically no bridge to their noses, the ordinary plastic frame fits too low on the face and is so close to the eyes that the lenses interfere with the eyelashes. A handmade frame with special nose construction can be made to look like a standard frame yet have the necessary low-set, thickened portion to the bridge so that the spectacles can be properly adjusted in front of the patient’s eyes. Before this development, such patients had to be fitted with frames that had adjustable pads to raise the frame and move it away from the face. Today, some manufacturers offer frames which feature a bridge specifically designed to fit this facial shape. These frames are designed with wider pads to move the frame away from the face.
There are conditions, such as an anesthetic cornea or a “dry eye,” for which it is necessary to enclose the eye between the frame and the face. There are many ways of doing this, but most side-shield constructions are not attractive and usually are bulky, hard, and poor fitting. With the use of a soft, transparent plastic, a shield can be produced and trimmed with a pair of scissors to exactly fit the individual patient. An added bonus is that it is almost invisible and, being soft and pliable, it does not interfere with the glasses being folded up in the standard manner.
Although the ideal solution in the case of ptosis is an operation, there may be contraindications to surgery. Frames can be fitted with a ptosis crutch, which is a small piece of wire or plastic affixed to the inside of a spectacle frame. This wire can be adjusted to raise the eyelids of the patient so afflicted.
Dispensing spectacle frames
It is important that spectacles are produced so that the patient, when viewing distance objects, looks through the optical centers of both lenses. Consequently, it is essential to know the distance between the visual axes through the pupils of the patient, so that the lenses may be correctly mounted in the spectacle frame at the same interpupillary distance. The term pupillary distance is abbreviated PD.
Unfortunately, the visual axis through the human eye does not pass through the center of the pupil, as one might expect, but varies from patient to patient and is on the nasal side of the pupil. Therefore any mechanical device that measures from the center of the pupil of one eye to the center of the pupil of the second eye will give a measurement greater than the actual distance between the visual axes of the two eyes.
This error is of little significance if the prescription is a weak one. However, a few millimeters of error will produce a decidedly uncomfortable pair of glasses for those patients requiring unusually strong (greater than ± 4.00 diopters) prescriptions: binocular aphakes, myopes, hyperopes, or those with large amounts of astigmatism. Because it is difficult to assess the visual axis or the center of the pupil, the practice is to measure (if both pupils are the same size) from the nasal edge of the pupil on the patient’s right eye to the temporal side of the pupil on the patient’s left eye ( Figs. 13.10 and 13.11 ).
Two PDs are taken, one for distance, where the visual axes are parallel, and one for near, which is the close working distance of the patient.
There is only one accurate method of measuring PD—the light reflex method —the result of which gives the distance between the visual axes of the two eyes, rather than the distance between the center of one pupil and the center of the other. The difference between the two methods of measurement may be 2 to 5 mm. There are many optical interpupillary gauges that can give the measurement from pupil to pupil very accurately for distance vision; the PD can then be converted for near vision by means of tables. One gauge on the market today uses the accurate reflex method ( Fig. 13.12 ).
Special considerations in measuring PD include the following:
If the patient has pupils of different size and the standard method with a PD rule is to be used, the measurements should be made from the nasal side of the limbus of the patient’s right eye to the temporal side of the limbus on the other, ignoring the measurements from the pupil.
If the patient can see from only one eye, the measurement can be taken for the good eye from the center of the bridge of the nose to the center of the pupil, because an inaccuracy of a few millimeters one way or the other has no significance.
If the patient has a squint, the measurement can be taken from the inner canthus of one eye to the outer canthus of the opposite eye, giving a reasonably accurate PD. A better way is to occlude the turning eye and measure from the center of the bridge of the nose to the center of the uncovered eye and then repeat with the other eye covered. The sum of the two measurements is the PD.
The PD is taken so that the optical centers of the lenses will be directly in front of the visual axes through the pupils ( Fig. 13.13 ). If the optical centers are not so placed, an unwanted prism is incorporated in the glasses.
The optical center can be defined as the thinnest part in the center of any minus lens or the thickest part of the center of a plus lens. Only at the optical center do rays of light go through the lens without bending. To make this clearer, a minus lens can be represented by two prisms, bases out, and a plus lens, bases in. When these lenses have their centers in line with the visual axes, they are correctly positioned in front of the patient’s eyes ( Fig. 13.14 ).
If the PD is wrong and the lenses are off-center, the vision is bent by the lenses toward the base of the prism in each case and the object would appear to be displaced laterally. The eyes would have to turn in or out to try to correct this, causing discomfort to the patient. If the lenses are high power, plus or minus, so much prism can be introduced that double vision will result. In the case of a lens of +10.00 diopters, if the PD is out 1 mm then 1.00 diopter of prism that has not been prescribed is introduced into the prescription.
Measuring pupillary distance with a ruler and the reflex method
Most times, the PD is measured with a device called a pupillometer. When checking the PD manually, the following equipment is required:
Small PD rule, graduated in millimeters
Pinpoint of light, such as a bare bulb of an ophthalmoscope battery handle or a penlight
To take a distance PD , the procedure is as follows:
Sit approximately 16 to 18 inches from the patient to be examined.
Hold the light source immediately under your left eye. Place the PD rule across the bridge of the patient’s nose so that it will extend to cover the lower half of both pupils.
Make sure the patient looks directly at the light bulb.
Place the zero mark of the ruler on the pinpoint reflection of the light on the cornea. Use your left eye for this purpose.
Without disturbing the PD ruler and without the patient moving the head, move the light to a position underneath your right eye.
Make sure the patient is still looking at the light.
Note the measurement on the PD rule of the reflection of the light on the cornea of the patient’s left eye. (In doing this you will observe that the pinpoint of light is not in the center of the pupil but at some point nasal of center. The measurement is an accurate one of the distance between the visual axes of the two eyes [incorrectly termed the distance PD for want of a better term].)
Note the measurement at the center of the nose (this may be useful if the patient’s face is grossly asymmetric).
Obtaining the PD for near is done in a similar manner, but the distance between the patient’s eyes and the observer’s eyes should be adjusted to the appropriate near distance. (It varies from person to person and from occupation to occupation.) The observer proceeds as before. Then the light is placed under one eye and the patient looks at the light. A measurement is taken with the PD rule from the reflection on the cornea of one eye to the reflection on the cornea of the other eye, without moving the light. This will be the “near” PD.
Spectacle frames often have to be chosen for their physical advantages in supporting the lenses that are required. The heavier the lenses, the greater the distribution of weight on the nose. The new technology that has made possible a larger selection of eye fashion may limit the choice of frames for the high myope, high hyperope, and the rare aphake without an intraocular lens.
Spectacle frames, as previously noted, should also provide cosmetic enhancement to the wearer. The design of the frame should take into consideration the contours of the face. In persons with unusually long faces, frames that are noticeably longer horizontally than vertically are very effective in reducing the long appearance of the face. An unusually long nose can be diminished optically by a low-fitting bridge bar. Dark colors accentuate this shortening effect. A small nose can be made to look longer by a slender bridge bar set high up or a keyhole bridge. Frames that are greater in depth at their outer ends than at their middle help to make eyes that are too closely set together appear to be more widely separated; the opposite type of frame helps to mitigate the effect of eyes that are set unusually far apart. Thus an illusion of wide-set eyes can be created by having the frame color fade away at the bridge.
Brightly colored frames complement the natural tones of a light-haired individual, whereas darkly colored frames blend better with a dark complexion. Dark-colored frames draw attention to nose width and should be avoided by people with very narrow or very wide noses.
The actual choice of frames or style is highly personal. A variety of frames are available for both indoor and outdoor sports, for motoring, and for business and social occasions.
The frames should hold both lenses firmly in a direction perpendicular to the visual axes because tilting can introduce optical errors that may be considerable in lenses of high power. Glasses for distance should sit vertically, whereas glasses for reading should be slightly lowered and tilted downward. The frames should be comfortable at all times and cause no irritation of the skin on which they rest, and the lenses should be as close to the eyes as possible without touching the eyelashes.
Spectacle lenses are made from plastic, high-index plastic, polycarbonate, Trivex, or glass.
Most single-vision lenses are made up in a curved form rather than the flat form ( Fig. 13.15 ).
Optical laboratory statistics for the latter part of 2015 reveal that glass lenses were dispensed for only 3% of patients, polycarbonate lenses for 37% of patients, and plastic lenses for 60% of patients, with high-index plastic lenses accounting for one-third of the plastic lenses dispensed.
Historically, many types of glass were used for optical purposes. The primary glass used for ophthalmic lenses was crown glass of 1.523 refractive index. Also used was flint glass with a refractive index of 1.62 when a higher refractive index was desired, as in the making of bifocal or achromatic lenses. Flint glass contained lead, which is absent from crown glass. Glass with an index of 1.6 was produced by adding more titanium dioxide to the glass mix. The main advantages of crown glass in optical spectacles were its excellent optical properties and its resistance to scratching.
A higher-index glass was Lantal (1.9) from Carl Zeiss, Germany ( Fig. 13.16 ). These lenses were used for individuals with high myopia or hyperopia and were remarkably thin.