THERE ARE several good ways to murder your dentist: Sneak up behind him and strangle him with dental floss, stab him with the Water Pik, or perhaps drill him to death in the manner of Brian De Palma.

If you've given it that much thought, "Compromising Positions" is your movie, a droll whodunit for people who forget to floss. And more than that, it's a telling social commentary on the tragedy of wasted wives.

Susan Sarandon is the stalwart heroine, a mid-life wife whose complacent domesticity is destroyed when her periodontist, Dr. Fleckstein, is found murdered under his spit sink. Sarandon, a reporter before her marriage, begins to investigate the crime.

"Compromising Positions" opens with a patient's view of the periodontist. She is uh- huhing to inane questions through a jaw packed with cotton, utterly at the mercy of the smarmy Dr. Fleckstein, resplendent in peekaboo jacket and pinky ring.

Lonely Long Island housewives apparently find him charming. What can I say. And he finds them irresistible, groping in their mouths and under their skirts while they are temporarily distracted by the indelicate implements of dentistry.

The women, whose husbands are busy in the city, have lives as numbing as Novacaine. And they're desperately seeking somebody to be in Susan Isaacs' witty, if imperfect, screenplay based on her novel. Filmmaker Frank Perry, who also directed "Diary of a Mad Housewife," calls on his expertise as a veteran of disaffection.

Sarandon, with little make-up to hide her age, seems disturbingly real but very lovely in this metamorphic role. She's backed by a distaff supporting cast that includes charismatic Judith Ivey in a raunchy performance as her trusted friend. Looking at a lewd Polaroid of one of Dr. Fleckstein's flames decked out in vegetables, she remarks, "I must remember never to try her cole slaw."

Anne De Salvo perfects the role of Fleckstein's filthy rich, hardened widow. And fragile Deborah Rush gives a particularly touching performance as one of Fleckstein's used- up girlfriends.

"Compromising Positions" has its male stars and its youngsters, too, but they're almost peripheral. Raul Julia, badly cast as a hirsute suburban Kojak, is a major player as Sarandon's potential lover. Edward Herrmann has more success as her overstuffed, overworked husband. The director's men have mellowed, mercifully, since Richard Benjamin's whining in the "Mad Housewife" days -- except for Dr. Fleckstein, played with oily relish by Joe Mantegna till dead.

"Compromising Positions" has its problems, especially Julia's weak performance. But it's often on target, exposing the mechanics of the heroine's marriage, the woman herself and her languorous community where two patrol cars respond to a call about graffiti.

Of course, none of this ever would have happened if it weren't for plaque.