A substance dentists have long used to strengthen teeth is a key part of a new treatment that appears to safely curb spinal osteoporosis, a progressive bone disorder afflicting some 5 million Americans, researchers say.

University of Texas Southwestern Medical School researchers on Thursday announced results from a seven-year trial of the new treatment.Doctors say spinal osteoporosis results from a gradual loss of trabecular bone, found primarily in the spine. It most commonly affects elderly women and results in shortened stature, curvature of the back and a protruding abdomen that can cause back pain and digestive discomfort.

The treatment employs a slow-release form of sodium fluoride, a compound used by dentists to strengthen teeth which - when given with calcium citrate - can reverse the effects of osteoporosis by adding bone mass, said Dr. Charles Pak, mineral metabolism chief at Southwestern Medical Center.

Pak said fluoride has been available as an osteoporosis treatment for decades but caused serious side effects, including corrosion of the stomach lining, painful swelling of the joints and stress fractures.

"These complications have precluded the long-range acceptance of use of fluoride in the United States," said Pak. "Our treatments have been designed to overcome these problems."

In the new treatment, fluoride is encapsulated in a waxy pill that allows it to bypass the stomach before much of the compound is released. That helps alleviate gastrointestinal discomfort and allows the fluoride to be absorbed slowly into the blood, at an effective but not toxic rate, Pak said.

Each year, an estimated 500,000 people suffer osteoporosis spine fractures, and a third of women over age 65 will have spinal fractures during their lives, the Southwestern researchers said.

"Treatments such as estrogen and calcium supplements, which are directed at preventing bone loss, are useful in averting the development of osteoporosis when they are applied early, such as during the early postmenopausal period," said Pak. "But they have limited value in patients with established osteoporosis, who have already sustained a substantial bone loss. Our treatment is directed toward the latter group."

Pak said the treatment, which has been conducted on experimental basis at 10 hospitals around the country, does not reverse spinal curvature or other damage but does increase bone mass and forestall further breakage.

He said it cannot be used by people who have sustained hip fractures because of differences in the makeup of hip bones.

Pak said he plans to apply to the federal Food and Drug Administration in April for approval of the treatment. An FDA review usually takes nine months to 21/2 years, he said.

The new drugs promise to increase the availability and usability of osteoporosis treatments, said Dr. Lawrence Raisz, director of endocrinology and metabolism at the University of Connecticut.

"Everybody is anxious to have a treatment approved by the FDA," Raisz said.

The new drugs were developed by Pak and a team of researchers with the help of the Mission Pharmacal Co. of San Antonio.

Since 1982, the group has tested the treatment on 251 patients in Dallas, Fort Worth, Temple and Tyler, Texas; Charleston, S.C.; Cleveland and Indianapolis.

Less than 5 percent of patients at Southwestern experienced gastrointestinal problems when treated with fluoride for an average of 3.5 years, and about 6 percent complained of joint pain.