Open up and say ha!

A lady walked into a dentist operatory and exclaimed, ''I don't know which is worse, having a baby or having a root canal." The dentist responded, ''Well, make up your mind so I know how to tilt the chair."

-- ''The Oldest Dental Joke," www.dentistry.com

Heard the one about the stand-up comedian and the ''anti-dentite" gag embedded in one episode of his TV sitcom?

''Seinfeld" fans have -- and can quote liberally from the show in which Jerry is convinced his dentist has converted to Judaism solely so he can crack Jewish jokes. A peeved Jerry makes up an anti-dentist joke of his own, it gets around, and, well, yada yada yada.

Would Seinfeld dare go anti-dentite on a roomful of real dentists? Attendees at the 31st annual conference hosted by the Massachusetts Dental Society are about to find out.

The five-day Yankee Dental Congress, which kicks off today, is expected to bring more than 28,000 dental health professionals to Boston, with the bulk of the program devoted to topics such as posterior composite resin restoration, pain management, forensic dentistry, and socket grafting. On the lighter side, conferencegoers can also dabble in knitting, yoga, golf tips, and tai chi, make side trips to local museums, discuss retirement goals and investments, or even attend a workshop in conflict resolution through the use of humor.

But the main draw, and toughest ticket, is Friday night's headline entertainer: the toothy Seinfeld himself. He'll perform before 3,200 fans at the Boston Convention & Exhibition Center. Brookline dentist Dr. Helen Foo will not be among them, unfortunately -- the show, open only to conventioneers, sold out too quickly, Foo says -- yet she's curious how Seinfeld might mold his humor to an audience that actually drills and fills for a living.

''Jerry is very into his mouth, I hear, very anal about flossing," Foo says in a slightly snarky tone of voice. According to several of her patients, that's not out of character for a dentist who's funnier than a whiff of nitrous oxide when she puts on the white coat.

That the loss may be Seinfeld's and not just Foo's is evident once Foo starts describing a patient who wanted to show off her new breast implants. In all their splendor. While seated in the exam chair.

''So then she says, 'Go ahead, feel free to rest on them while you work,' " recalls Foo, who says she regularly regales patients with stories like this one in hopes they'll keep smiling through all the rinsing and spitting.

Marblehead dentist Dr. Steven Baratz goes Foo one better, offering to fill in for Seinfeld if the comedian fails to show up this weekend. Not that there's anything wrong with his day job, Baratz says. However, keeping things light around the office is pretty much second nature for Baratz, from the flashing ''OPEN" sign suspended over his exam chair to the set of rubber eyeballs he keeps on hand as props.

Indeed, deep down below the gumline, Baratz sounds like a man who'd just as soon tickle a funny bone as scrape an incisor.

''Are dentists a humorous group in general? I'd have to say, no," Baratz says when asked how dentists are perceived by the public. Most dentists are more organized and buttoned-up than he is, he continues, which may explain why they're thought of as all inlay and no byplay, so to speak.

''I have an ADD personality myself -- I'm not organized at all," Baratz says. ''I love puns and verbal wordplay, though. And the more I can be myself, the more relaxed I am and the better I work."

OK, but what's with the rubber eyeballs?

''As I tell patients," he deadpans, ''I never know when I'll need another set of eyes."

My dental hygienist is cute. Every time I visit, I eat a whole package of Oreo cookies while waiting in the lobby. Sometimes she has to cancel the rest of the afternoon's appointments.

-- comedian Steven Wright

How exactly did dentists get pegged as dour, humorless practitioners of the cap and crown? Do they laugh at themselves? Do they trade Stupid Patient Tricks by e-mail? See the world as somberly as they'd study an abscessed root?

As with most walks of life, pop culture creates its own impression of the profession -- and one not always as sweetly benign, either, as a Bill Cosby monologue about numbed lips and burning tooth enamel. For every W.C. Fields bumbling his way through ''The Dentist" or Tim Conway injecting himself with Novocain in a hilarious ''Carol Burnett Show" skit, there's a darker, more discomfiting portrayal in circulation, full of sharp instruments and screeching drills.

Think ''Marathon Man," the 1976 film starring Laurence Olivier as a sadistic Nazi tooth doctor. Or the more recent HBO film ''The Dentist," in which Corbin Bernsen plays a homicidal maniac with a DDS degree. Humorist Joe Queenan has written an entire essay on the treatment -- and mistreatment -- of dentists by Hollywood, drolly titled ''The Drilling Fields."

Real dentists like Dr. John Herzog can relate to Queenan's complaint, however tongue-in-cheek, that some serious ''cinematic abuse" is going on here.

''We're often portrayed as wack jobs," agrees Herzog, a general practitioner in Beverly. ''It goes with the territory."

By the same token, he says, dentists have long used humor to ease patients' anxiety. Not by developing a repertoire of one-liners, necessarily, but by sharing funny stories that humanize the visit. And that's easier for dentists to do than for specialists like endodontists, according to Herzog and others in the profession, because specialists might see a patient only once as opposed to over a long period of time.

''Look, there are few nice ways to say, 'You need a root canal,' " Herzog says. ''You have to acknowledge things that might not be so desirable. So you might as well break the ice with a little humor."

Dr. Corine Barone Cognata, a pediatric dentist in Marblehead, doesn't so much break the ice with her patients and their anxious parents as she does smash it to pieces with the business end of a magic wand. At least a couple of times a year, Barone puts on a Tooth Fairy costume to lecture at professional conferences. On Halloween, she hands out toothbrushes instead of candy. If life is a play, she says, why not dress the part? Fighting tooth decay is hard enough.

''I'm a little Gilda Radner-ish that way," she confesses. Having started out in general practice, Barone later switched to pediatrics because, as she puts it, ''it's a legal license to act like a child and get away with it."

Are dentists generally too serious about their work? ''Some do take themselves pretty seriously," she says. ''The work is one thing, but to me, anyone who takes himself too seriously hasn't lived enough."

Why did the guru refuse Novocain when he went to the dentist?

He wanted to transcend dental medication. -- www.dentistry.com

At the top of the dental-humor pyramid stand a handful of practicing dentists who moonlight as stand-up comics.

One of them is Dr. Daniel Greenstein, a Coral Springs, Fla., practitioner who bills himself as Dr. Dan, America's Funniest Dentist. Greenstein has performed at comedy clubs around the country, as well as dental conferences and corporate outings. His two CDs, ''Spit Happens" and ''Will Drill for Food," feature bits like ''Acute Necrotizing Ulcerative Gingivitis," ''Imagine (No Insurance)," and ''The Cancellation Blues."

Greenstein will not be coming to Boston this week. Not for lack of trying, though.

''Every year I send them my stuff," he complains in a phone interview from Florida. ''And every year, I don't hear back from them."

Bring up Seinfeld's name, and the subject is touchier than a sore molar.

''He doesn't play the piano, and he can't even do cleanings," Greenstein says with a dismissive snort. ''And Seinfeld is making, what, a hundred grand for this gig? Hey, I'd do it for 95,000."

Somewhat more seriously, Greenstein acknowledges he was never much of a cutup in dental school.

''That whole side of me was buried," he says. Then, about 20 years ago, he entered a contest sponsored by Colgate to crown America's Funniest Dentist. Greenstein made it to the finals and never looked back, or even sideways much, polishing a routine that pokes fun at what dentists most fear and abhor, from deadbeat patients to coldhearted insurance companies.

''My whole thing is, the dentist strikes back," Greenstein says. ''We know patients don't want to be there, so I turn the tables on them."

During her own years in dental school, Helen Foo says she was admonished for talking to patients too much.

''So I'd say, 'But who else is in the room?' " she says.

Do dentists deserve their solemn reputations, or has the public been missing something?

''I want to say yes, they deserve that reputation. But I also think it's changing," replies Foo, adding with a laugh: ''I try to get to know my own patients very well. I want them to be more than just a mouth."

Joseph P. Kahn can be reached at jkahn@globe.com.