On his monthly trips to the orthodontist five years ago, 50-year-old Harry Heintzen was an oddity in the waiting room.

"It was mainly all kids, some 3 to 5 years old," he recalls. "Here I was in a business suit, six-foot-one sitting on a low stool wearing horn-rimmed glasses and braces," said the Voice of American division chief. "I was used as an example of someone who was cheerfully being straightened."

Today Heintzen would find a lot more middle-aged company in the waiting room. Adults have swollen the ranks of America's estimated four millions "tin-grins" just as declining birth rate wad depleting the traditional orthodontic market of young children and teen-agers.

Adults now make up between 10 to 15 per cent of all orthodontic patients, a jump of 6 per cent in the last five years, according to the 6,800-members American Association of Orthodontics, which itself has grown from 4,000 members 10 years ago.

More and more adults are requesting a mouthful of metal for several reasons - greater affluence, dental insurance programs that now pay for orthodontics treatment, increased awareness of dental health on the part of both the public and dentists a realization of what can be done to straghten adult teeth and improved techniques for doing so.

Orthodontists say many of their patients are referred to them by dentists who now realize that premature tooth loss, unhealthy gums and decay are accelarated by crooked, crowded teeth that catch food particles and bacteria that cannot be easily flossed away.

More concerned than ever about keeping their teeth longer, Americans are learning from their dentist that straight teeth and a "harmonious bite" are two of the basics - along with regular oral hygiene - in preventing the major cause of tooth loss today, periodontal (gum) disease.

Other orthodontic patients are sent by prosthodontists - the men who make artificial teeth - who are "just beginning to understand that adult teeth can be moved into positions where their work can be made easier," according to orthodontist Jeremy D. Orchin.

Some adults are bracing their teeth in order to correct bad bites and ailments like earaches that are related to poorly aligned jaws.

"The usual reaction is 'Why are you bothering?'" said a 61-year-old Mary Hoppe, a retired fiscal accountant, who has worn braces for 18 months. "But already it means that I can eat on a side of my jaw that I never used before."

For others, like 38-year-Old Gail Ferguson of Vienna, "Cosmetics are reason enough" to wear braces. Orchin agrees, "I think esthetics are just as important as health in today's society."

"I used to look in the mirror and say "If only that tooth was there; if only that one could be moved,'" said Heintzen. "No dentists had ever suggested (braces) tome before. Now I'm grateful to the orthodontist for what he has done."

Seeing the improvements also helps make more bearable the hefty orthdontist bills, which average $600 and can range to $2,000, according to the Orthodontic Association.

Barbara Thomas, 52, a retired television hostess, said that when she had her braces put on almost two years ago, "I just hoped the investment would be worth it." Now that she has seen the change in her own teeth, "It's worth every penny," she says.

The expense of wearing braces "will pay itself in the long run," said dentist-turned-orthodontist Michael A. McCombs, by the elimination of otherwise needed dental care and artificial teeth.

Many adults whose families could not afford braces for them are now making the trip to the orthodontist because of a newly acquired affluence or because the medical insurance offered by their employer now covers such treatments.

Adult fears about how they will look and how much pain theywill suffer soon give way to nonchalance and even pride in wearing braces - much like a "badge of courage" Orchin reports.

For many people straighter teeth mean a brighter personality. "They get so much positive social feedback. People admire them," McCombs observed. "Shy people who never smiled - maybe because they could not get lips over their teeth - start smiling." he said.

Braces are a good conversation piece especially when you see another adult wearing them. Immediately a rapport is established," said 30-year-old lawyer Meg Sandbridge.

Now that they have intruded onto the teenage domain of orthodontics in significant numbers some adult patients complain that orthodontists are "too oriented to children." Others lament that the chairs are too small.

As for the future, "research is geared to cosmetic orthodontics," McCombs said. Already patients with jobs where the metal look would be a major handicap - like actresses or FBI agents - can take advantage of a new type of plastic braces.

Instead of the stainless steel brace which is cemented around each tooth, a special type of resin is poured over the enamel of each tooth, filling up microscopic indentations. The plastic brackets that hold the wires are then set in this resin.

But these clear braces have not yet replaced the more popular metal ones because they tend to break more easily and absorb stains, turning yellowish, McCombs said.

In some families the metal look spans the generations gap and leads to dubious distinctions for the parent with braces. One child came home from a party given by his orthodontist and told his mother she should have gone to the party too because, "You would have gotten the prize for being the oldest."