INTRODUCTION
In 1972, President Nixon signed into law what is referred to today as the "FDA Impact regulation." This regulation, otherwise known as Title 21 CFR 801.410, has continues to influence our industry to this day. While the U.S. is the only country to have such a law, it has had global influence. It is still in effect today, and has not changed significantly since that time. The purpose of this article is to provide information on this topic that is useful to practicing Opticians today.

I am now approaching my 40th year of service with AO/SOLA/Zeiss. The topic of lens material impact resistance has been one that I have been involved with my entire career, and is almost certainly one that will continue to keep those in the industry busy for years to come.

In an earlier Opticians Handbook article on ANSI standards, I mentioned that my first industry Standards meeting was in 1990 in Washington, D.C. At that time the regulation had been in effect almost 20 years; and we met to provide guidance to the industry and the result was a booklet published by the OMA (now The Vision Council). While the regulation has not changed, the industry certainly has! Understanding how this applies to the manufacture and dispensing of spectacle lenses in the U.S. is important for us all.

PURPOSE
This article will provide insight and background on the topic of material strength and impact resistance testing, including how materials and coatings behave when subject to the "FDA Impact" Drop Ball. Whether you are aware of it or not, this regulation influences both the lens configuration (thickness) and makeup (coatings) for all finished lens product sold in the U.S.

For anyone reading this article who is looking to set policies and procedures for your business, I encourage you to study the FDA regulation itself, which should be the ultimate resource when studying this topic.

The "Impact" of Title 21 CFR 801.410—otherwise known as the FDA Test:
Pardon the pun in my title above, but when the FDA regulation went into effect in 1972, I doubt many realized the significance and "impact" or influence it would have on the industry some 42 years later.

At the time of its becoming law, the predominant material in use was glass. If supplied at thicknesses above 2mm and properly treated, glass can comply with the FDA regulation and pass drop ball (a fact that may not be obvious to some today). The main disadvantage with this test for glass is that each lens must be treated after edging, in order to provide impact protection. Plastic materials, on the other hand, can be batch tested by group. This is an important reason why glass has become less popular (from a processing /testing standpoint).

JUDGING WHEN A LENS "FAILS" THE TEST

The CFR regulation states the following with regard to failure criteria for individual lenses:
"To pass the test, the lens must not fracture; for the purpose of this section, a lens will be considered to have fractured if it cracks through its entire thickness, including a laminar layer, if any, and across a complete diameter into two or more separate pieces, or if any lens material visible to the naked eyes becomes detached from the ocular surface."

The image at right shows a typical failure pattern for glass during impact of a steel ball at energies above the minimum. It illustrates how this material may shatter when the impact energy imparted on the material exceeds the strength of the lens. Lens treatment, lens thickness, and the nature of the impact projectile all influence the fragment size and fracture pattern for this category of materials.

In the case of plastic materials (such as standard index resin), breakage patterns vary depending upon source the fracture origin. Like glass, plastic may still break in pieces (such as shown below at left) but they tend to be less in number than for glass. Plastic, may also fail as shown at right, where the center is "punched thru," a pattern seen when the coating on the back weakens the lens to the point of failure often in thin centered lens configurations.

Early in my career, I learned the hard way that there was a "new" plastic material that did not behave at all like glass or standard plastic, when subjected to the 5/8-inch drop ball test. In the late 1970s, I was testing a plastic lens with the 5/8-inch ball (shown at upper right in case of drop balls) in the AO Test lab. I vividly remember that when I tried to break the lens using larger ball sizes at greater heights to determine what energy was needed to break the lens, I got quite a surprise! I kept trying to break this lens (my first exposure to polycarbonate), and tried one of the larger ball sizes at a drop height of about 8 feet (standard test drop height is 50 inches). What I managed to do was destroy the test lens mount, but only deform and not break the lens. It turns out that polycarbonate, when properly manufactured and coated, is virtually unbreakable with such an impact. Rather than a larger mass/low velocity impact like the FDA Test, a small mass/high velocity projectile (bullet) can penetrate this material. The image below shows an AO test lab image of a 1/4 steel ball trapped in the material after being impacted at approximately 500 mph using a special gun designed for this purpose. The lens was thin enough and just the correct thickness to demonstrate this property.

The US ANSI Z87.1—2010 Industrial Safety standard now requires this test, using a 1/4 ball for lenses intended for safety lenses that are marked for impact protection. Lenses must pass this test at 150 feet per second, which precludes glass and most plastics other than Poly or Trivex.

Evaluation of lifestyle when advising the consumer:
The preceding information is interesting. However, more to the point is how does this knowledge influence material choice when advising a patient? Of course, an assessment of the patient's lifestyle and visual needs is an important aspect of the service one provides. For those patients who play sports or are children, the additional protection provided by Poly or Trivex may prove an important consideration when advising on material choice. It should be noted that depending upon the patient's needs and work habits, other materials (including glass) may be a suitable choice. Since we now know that all street wear thickness lenses sold in the U.S., irrespective of material, must be capable of meeting the FDA impact test, the U.S. consumer has protection not routinely afforded in other countries. The U.S. consumer is given a margin of protection that now has over four decades of experience and application in the U.S.

As written in the opening introduction of the OMA FDA Guidelines booklet published over two decades ago, the FDA test is referred to be "a quiet success." For more information on this topic, visit this link on the Optical Heritage Museum website.

Image Credits: All images in this article courtesy of the Optical Heritage Museum. The first image in this article is from a 1970s PPG brochure in the Museum archives; the remaining photos are from American Optical's files in the Optical Heritage Museum.