Problems with rimless glasses

Eyeglasses can prove fashionable as well as functional and are available in three basic looks -- fully rimmed, semi-rimless and rimless. Fully-rimmed glasses have the frame encircling both lenses. Semi-rimless spectacles are framed only at the top or -- rarely -- at the bottom. Rimless eyewear has no frames around the lenses.

In 2006, 15 per cent of all eyeglasses sold in the United States were rimless. Rimless eyewear is lightweight and blends with the wearer's face. Some disadvantages are associated with this type of eyewear, however.

Most pairs of glasses consist of two lenses, a bridge or centre piece, which is the short piece connecting the two lenses, and two temples, or end-pieces, which rest atop the wearer's ears. The lenses in rimmed eyewear styles are anchored by a supportive frame. This support cushions and helps protect the lenses.

The exposed lens edges in rimless glasses can chip if you drop your eyeglasses on a hard surface. The Trivex lens product is ideal for rimless eyewear due to its inherent strength, according to Certified Optician Renee Bentley. Polycarbonate and high-index lenses are secondary choices.

You can request a special hard coat treatment for your eyeglass lenses. This coating is applied to the entire lens surface and provides additional protection against damage.

The end pieces of rimless glasses are attached directly to the lenses and centre piece via one of three basic mounting system.

The optical lab personnel drills holes in each lens with a screw system. Then a screw and hex nut is used to attach the lenses to the centre piece and to the temples. The drilled holes can create stress points and render the lenses susceptible to chipping and flaking.

The optical staff can use a small, cone-shaped, chamfer tool to sand the holes to present a smooth surface. The lab may apply a metal or plastic collar or plastic bushing under the screw head to lessen stress.

Glasses manufactured with the screws and hex system are more likely to crack than those made using a drill-less or compression system. The screws can loosen and displace the eyewear. Bentley cautions her clients to remove their glasses with both hands. "When you use only one hand in removing and positioning your eyewear, you're applying unequal force on the frame and this may help loosen the lenses and misalign the frame."

Another fabrication method uses compression technology. Bridge and temple prongs with a bushing, or small, round, plastic cover, are pressed through the drilled holes. This technique creates less lens stress than screws, as well as cushioning the side-to-side movement that can contribute to lens cracking and displacement.

A "drill-less" tension system uses a thin, coated stainless steel wire or nylon monofilament line fitted into a thin channel in the edge of each lens to secure it. This wire encircles the lens and passes through the bridge to a connector hidden inside a hollow end-piece. Lab personnel use a wire tensioning screw to tighten the cable and secure the lenses.