Frame Materials & Fitting: Complete Guide for ABO Exam

Why Frame Materials & Fitting Matter for Your ABO Exam

Frame selection and fitting are core optician skills. The ABO dedicates 12-18 questions to frames, covering materials (metal, plastic, special alloys), construction types (full-rim, semi-rimless, rimless), bridge styles (saddle, keyhole, adjustable), temple designs (skull, library, cable), frame measurements, and proper fitting techniques. You need to know which materials suit which prescriptions, how to measure frames correctly, and how to adjust frames for comfort and optical centering.

Frame materials have dramatically evolved beyond basic metal and plastic. Titanium offers strength and hypoallergenicity. Memory metal (Flexon, beta-titanium) returns to shape after bending. Acetate provides vibrant colors and patterns. TR-90 nylon combines flexibility with durability. Each material has advantages and limitations—titanium is expensive, acetate requires frequent adjustment, TR-90 can't be heat-adjusted. Understanding these trade-offs helps you recommend the right frame for each patient's needs and budget.

Fitting isn't just about comfort—it's about optical performance. If the frame sits too low, the patient looks through the wrong power zone. If pantoscopic tilt is excessive, it induces unwanted prism. If the frame is too loose, it slides down and vertex distance changes. The ABO tests whether you understand proper frame positioning (optical center at pupil, vertex distance 12-14mm, pantoscopic tilt 8-12 degrees) and adjustment techniques (temple spread, nose pad adjustment, pantoscopic tilt adjustment).

In this guide, you'll learn all major frame materials and their properties, frame construction types and when to use each, bridge styles and their fitting characteristics, temple designs and when each is appropriate, how to measure frames using the boxing system, proper frame fitting principles, and common adjustment techniques. By the end, you'll confidently answer frame questions and fit glasses that are both comfortable and optically correct.

Frame Materials: Metals

Metal frames offer durability, adjustability, and a professional look. The ABO tests the properties of each metal type and when to recommend them.

Stainless Steel

Stainless steel is strong, corrosion-resistant, and affordable. It's the workhorse of metal frames—durable enough for everyday wear, adjustable for proper fit, and available in thin wire designs for minimal weight. Stainless steel holds adjustments well and resists rusting from sweat or humidity. Downside: heavier than titanium or aluminum, and some people develop nickel allergies (stainless steel contains 8-12% nickel).

Titanium

Titanium is the premium metal choice. It's extremely strong (comparable to steel), corrosion-proof, hypoallergenic (no nickel), and 40% lighter than stainless steel. Titanium frames are comfortable for all-day wear and ideal for patients with metal sensitivities. Titanium welds don't corrode, making it perfect for rimless frames (drill mounts don't rust). Downside: expensive (2-3x the cost of stainless steel), and harder to adjust (requires more force and specialized tools).

Beta-Titanium (Memory Metal)

Beta-titanium is a titanium alloy with "memory"—it flexes and returns to original shape without permanent deformation. Frames made from beta-titanium (often branded as Flexon) are nearly indestructible. You can bend the temples repeatedly and they spring back. This is ideal for active lifestyles, children who are hard on frames, and anyone who wants durability. Beta-titanium is hypoallergenic like pure titanium. Downside: expensive, and the flexibility makes precise adjustments difficult (frame doesn't "hold" adjustments like steel does).

Monel

Monel is a nickel-copper alloy that was historically the standard for metal frames. It's strong, corrosion-resistant, and easy to adjust. Monel holds adjustments well and is affordable. Downside: contains high nickel content (60-70%), causing allergic reactions in sensitive patients. Monel has been largely replaced by stainless steel and titanium in modern frames, but older frames and some budget frames still use it.

Aluminum

Aluminum is extremely lightweight and corrosion-resistant. It's used in high-end frames where weight reduction is the priority. Aluminum frames are comfortable for large sizes or high prescriptions where weight is a concern. Downside: softer than steel or titanium, so adjustments may not hold as well. Aluminum also oxidizes over time, creating a dull surface finish.

Frame Materials: Plastics

Plastic frames are popular for their style versatility, color options, and comfort. The ABO tests the properties of different plastic materials.

Cellulose Acetate (Zyl)

Acetate is the gold standard for plastic frames. It's derived from plant cellulose (cotton or wood pulp), hypoallergenic, and accepts vibrant colors, patterns, and laminations better than any other material. Acetate frames can have tortoiseshell patterns, transparent colors, or multi-layer designs. Acetate is easily heat-adjustable—you can reshape it with a frame warmer. Downside: requires frequent adjustment (doesn't hold shape as well as metal), and can become brittle with age or exposure to heat/chemicals.

Propionate

Propionate is a nylon-based plastic that's lighter and more flexible than acetate. It's hypoallergenic, holds color well, and resists chemical degradation better than acetate. Propionate is used in lightweight sport frames and children's frames because it flexes without breaking. Downside: doesn't accept deep colors or patterns as well as acetate, and can feel less "premium" than acetate.

TR-90 Nylon

TR-90 is a thermoplastic nylon known for extreme flexibility and impact resistance. It's used in sports frames, children's frames, and active lifestyle eyewear. TR-90 frames can be twisted, bent, and stepped on without breaking. They're lightweight, hypoallergenic, and resistant to chemicals and temperature changes. Downside: cannot be heat-adjusted like acetate (it's thermoset after molding), and limited color/pattern options compared to acetate. TR-90 frames must fit correctly from the start—you can't reshape them.

Optyl (Epoxy Resin)

Optyl is a thermoset plastic (epoxy resin) known for being lightweight, hypoallergenic, and retaining its shape better than acetate. Optyl frames don't stretch or deform with wear. They're durable and hold adjustments well. Optyl was very popular in the 1980s-1990s but has been largely replaced by acetate and modern nylons. Downside: limited color options compared to acetate, and can become brittle with age.

Polyamide (Grilamid)

Polyamide (often branded as Grilamid) is a high-performance nylon used in premium sport and safety frames. It's extremely lightweight, flexible, and impact-resistant. Polyamide maintains its properties across wide temperature ranges, making it ideal for outdoor sports. Downside: expensive and limited to specialized applications (most patients don't need this level of performance).

Frame Construction Types

Frames are categorized by how they hold the lenses. The ABO tests when each construction type is appropriate and which prescriptions work with each.

Full-Rim (Full-Frame)

Full-rim frames encircle the entire lens with a groove (plastic frames) or metal rim. The lens sits in the groove or behind a metal rim held by a screw-in cover. Full-rim frames provide maximum lens support and protection. They work with all prescriptions, including high minus (thick edges) and high plus (thick centers). Full-rim frames are the most versatile and durable construction. Downside: heavier than rimless or semi-rimless, and more visible (less "invisible" aesthetic).

Semi-Rimless (Half-Frame, Supra)

Semi-rimless frames have a rim on the top and a nylon cord (supra) or metal wire holding the bottom of the lens. The top portion is fully enclosed for support; the bottom is minimally visible for aesthetic appeal. Semi-rimless frames provide a balance between support and invisibility. They're suitable for low to moderate prescriptions where the lens bottom isn't too thick. Downside: nylon cord eventually stretches or breaks (needs replacement every 1-2 years), and not suitable for high minus prescriptions (thick bottom edges put stress on the cord).

Rimless (Three-Piece Mounting)

Rimless frames have no frame around the lens—just temples and a bridge attached directly to the lens via drill mounts (small screws through holes drilled in the lens). Rimless frames are the most invisible and lightweight construction. They provide excellent peripheral vision (no frame blocking side view). Rimless frames require impact-resistant lens materials (polycarbonate or Trivex) to withstand the stress at drill mount points. Downside: lenses are exposed and vulnerable to scratches/chips, drill mounts can loosen over time (require periodic tightening), and not suitable for high prescriptions (thick lenses look poor in rimless design).

When to Recommend Each Type

Full-rim: All prescriptions, especially high Rx, durability priority, maximum protection. Semi-rimless: Low to moderate Rx, aesthetic preference, balance of support and invisibility. Rimless: Low Rx only, minimize frame visibility, peripheral vision priority. Always recommend polycarbonate or Trivex for rimless.

Bridge Styles and Fitting

The bridge connects the two lenses and rests on the nose. Bridge style affects comfort, fit, and optical centering. The ABO tests when each bridge type is appropriate.

Saddle Bridge (Plastic Frames)

Saddle bridges rest directly on the nose without nose pads. The bridge spreads weight across a large area, providing comfort for most nose shapes. Saddle bridges are standard in plastic frames. They're simple, durable, and require no adjustment hardware. Downside: limited adjustability—if the fit isn't right, you can't easily change the bridge width or height. Works best for average nose shapes.

Keyhole Bridge

Keyhole bridges have a distinctive keyhole-shaped cutout that rests on the sides of the nose rather than the top. This vintage style distributes weight differently than saddle bridges and works well for narrow or low-bridge noses. Keyhole bridges are primarily aesthetic—they create a retro look. Functionality is similar to saddle bridges with slightly less contact area.

Adjustable Nose Pads (Metal Frames)

Metal frames typically have adjustable nose pads attached to pad arms. The pads can be moved closer/farther, higher/lower, and angled to fit various nose shapes. This adjustability makes metal frames suitable for wide range of face shapes and allows precise optical centering. Nose pads are either silicone (soft, hypoallergenic) or acetate (harder, more durable). Pads should be replaced periodically when they turn yellow or hard. The ABO expects you to know that adjustable nose pads provide the most fitting flexibility.

Fixed Bridge Pads

Some metal frames have fixed pads molded into the bridge (no adjustment). These are used in low-cost frames or specific designs where adjustment isn't needed. Limited fitting flexibility—if it doesn't fit out of the box, it won't work.

Temple Designs

Temples (earpieces) hold the frame on the face. Different temple styles suit different activities and preferences. The ABO tests when to recommend each style.

Skull Temples (Standard)

Skull temples extend straight back from the front, then curve down behind the ear. They're the standard temple design—comfortable for most people and suitable for general wear. Skull temples provide good retention without being overly tight. They work for all face shapes and most activities. This is the default temple style unless there's a specific reason to choose something else.

Library Temples (Straight Back)

Library temples extend straight back without curving down. They rest on top of the ear rather than hooking behind. Library temples are used for occasional wear (reading glasses left on a desk) or for patients who frequently put glasses on/off. They're easier to slide on and off than skull temples but provide less retention—not suitable for active wear or sports. Also called "paddle temples" when the ends are widened.

Cable Temples (Riding Bows)

Cable temples curve around the back of the ear in a full loop, like a hook. They provide maximum retention—the frame doesn't slide off even during head movement or bending over. Cable temples are used for safety glasses, industrial eyewear, and patients who need secure fit (surgeons, chefs, mechanics). Children's glasses often have cable temples to prevent loss. Downside: harder to put on/off, and can feel tight if not properly adjusted.

Convertible (Comfort) Cable Temples

Convertible cable temples have a hinged end that can flip up (acts like a skull temple) or down (acts like a cable temple). This provides the convenience of skull temples with the security of cables when needed. Used in sports eyewear and safety glasses where versatility matters.

Spring Hinges

Spring hinges allow temples to flex outward beyond 90 degrees without breaking. They're not a temple style per se but a hinge feature that increases durability and comfort. Spring hinges accommodate larger heads, absorb impacts, and allow the frame to flex during wear. They're standard in children's frames and sports frames. The ABO may ask about spring hinges as a durability feature.

Frame Measurements: The Boxing System

Frames are measured using the boxing system, which defines a rectangular "box" around each lens opening. The ABO tests your ability to identify and use these measurements.

Eye Size (A Measurement)

The horizontal width of the lens opening at its widest point. Measured in millimeters. Typical range: 40-60mm. Larger A = wider lens, more peripheral coverage. The ABO expects you to know this is the first number in frame sizing (e.g., 52-18-140 means A=52mm).

Bridge Size (DBL - Distance Between Lenses)

The distance between the two lens boxes at their closest point. Measured in millimeters. Typical range: 14-24mm. Smaller DBL = closer-set eyes, larger DBL = wider-set eyes. This is the second number in frame sizing (52-18-140 means DBL=18mm). The ABO tests DBL frequently.

Temple Length

The length of the temple from the hinge to the end, measured along the temple bend. Typical range: 135-150mm. This is the third number in frame sizing (52-18-140 means temple=140mm). Temple length affects how far the frame extends back—too short and it won't reach the ear, too long and it extends past the ear uncomfortably.

B Measurement (Vertical Height)

The vertical height of the lens opening at its tallest point. Not typically stamped on frames but critical for progressive and bifocal fitting. Minimum B measurement for progressives is usually 28-30mm. The ABO tests B measurement in progressive lens questions.

Effective Diameter (ED)

The diameter of the smallest circle that completely encloses the lens shape. ED determines the minimum lens blank size needed to cut the frame shape. ED = 2 × longest radius from the geometric center to the farthest edge point. The ABO tests ED calculations.

Proper Frame Fitting Principles

A properly fitted frame ensures comfort and correct optical performance. The ABO tests the standards for proper fit.

Optical Center Alignment

The lens optical centers must align with the patient's pupils when looking straight ahead in primary gaze. Measure PD (pupillary distance) and ensure the frame centers are close to that measurement. For high prescriptions, precise optical centering prevents unwanted prism. The ABO expects you to know that improper centering induces prism via Prentice's rule.

Vertex Distance

The distance from the back of the lens to the corneal apex should be 12-14mm. Too close and lashes touch the lens. Too far and effective power changes (especially in high Rx). Adjust nose pads or bridge to achieve proper vertex distance. This ties into vertex distance compensation for high prescriptions.

Pantoscopic Tilt

The frame front should tilt slightly downward (top of frame farther from face than bottom). Standard pantoscopic tilt is 8-12 degrees. This aligns the lens optical axis with the patient's downward reading gaze and reduces reflections. Too much tilt induces prism. Too little tilt causes reflections from overhead lights. Adjust pantoscopic tilt by bending temples at the hinge or adjusting nose pad angle.

Face-Form (Wrap Angle)

The frame front should curve slightly to follow the face contour. Standard face-form is 5-10 degrees of wrap on each side. This improves peripheral vision and reduces glare from side light. Too much wrap (sports frames with 20+ degrees) can induce oblique astigmatism in prescription lenses. The ABO tests face-form in the context of lens aberrations.

Temple Spread and Pressure

Temples should spread to match the patient's head width without excessive pressure. The frame should feel secure but not tight. Pressure points (behind ears, on nose, at temples) indicate poor fit—adjust temple spread, nose pads, or temple curve to redistribute pressure evenly.

How the ABO Exam Tests Frames

The ABO includes 12-18 questions on frames, covering materials, construction, measurements, and fitting. Here's what to expect.

Question Types

Material Properties: "Which frame material is hypoallergenic and lightest?" Answer: Titanium. "Which plastic frame material can be heat-adjusted?" Answer: Acetate. These test material knowledge.

Frame Construction: "What frame construction requires polycarbonate or Trivex lenses?" Answer: Rimless (drill mounts need impact resistance). "Which construction provides maximum lens support?" Answer: Full-rim.

Bridge Types: "Which bridge style offers the most adjustability?" Answer: Adjustable nose pads (metal frames). These test fitting knowledge.

Measurements: "What does the A measurement represent?" Answer: Horizontal lens width (eye size). "What is DBL?" Answer: Distance between lenses (bridge size). Memorize the boxing system.

Study Tips

Create a materials comparison chart: metal types, plastic types, properties, advantages, disadvantages. Memorize key differences (titanium = hypoallergenic/light, acetate = heat-adjustable/colorful, TR-90 = flexible/can't adjust).

Understand frame sizing: A-DBL-Temple (52-18-140). Know what each number means and typical ranges. Practice identifying measurements on actual frames. The ABO loves giving you a frame size and asking what each measurement represents.

ABO Practice Questions

Test your frame knowledge with these ABO-style questions. Try to answer before revealing the solutions.

Related ABO Topics

Frame materials and fitting connect to several other ABO concepts. Review these topics to strengthen your understanding: