Smokers' broken bones take a lot longer to heal. But scientists now are studying whether giving up cigarettes for even a week or two after a fracture might make the difference between a speedy recovery and months of easy-to-reinjure mushy bones.

"There's a window," predicts orthopedic specialist Michael Zuscik of the University of Rochester.

If he's right, it could dramatically change orthopedic practice for the nation's 48 million smokers.

Bone damage is arguably the least publicized of tobacco's harms.

The first time many smokers ever hear of the problem is if they need spinal fusion, a back operation that surgeons often won't perform unless patients kick the habit — with a urine test to prove they quit. That's because the surgery is far more likely to fail in smokers than nonsmokers.

Smokers who break a leg require 62 percent more time to heal.

Then there's the silent toll smoking can wreak by contributing to bone-thinning osteoporosis.

Yet tobacco's nicotine provokes a powerful addiction; it can take repeated attempts to succeed in quitting. Those who do often use nicotine patches or gum to wean themselves.

Here's the rub: Zuscik's early research suggests nicotine may be a key bone-damaging culprit — and that it does its dirty work almost immediately by affecting stem cells stored in the bone marrow, called mesenchymal stem cells, that move in to begin healing an injured bone.

"The most important steps that occur involving these mesenchymal stem cells happen during the first days and weeks of the healing process," Zuscik explains. "The whole thing is kind of derailed."

Now, armed with a new $1.4 million grant from the Defense Department, Zuscik is out to prove that theory, and whether going cold-turkey for a short time after breaking a bone or undergoing bone surgery might help smokers heal faster.

It's of interest to the military because surveys show up to 34 percent of troops smoke, compared with about 22 percent of the general population, and bone damage, particularly to the arms and legs, is common among soldiers injured in combat.

While the link between smoking and bone harm is clear, no one knows why it occurs, says Dr. Thomas Einhorn, chairman of orthopedic surgery at Boston University. Nicotine may not play the only role; there are lots of toxins in cigarette smoke.

But the Rochester team's theory is plausible, he says. And it's crucial to pursue because if they're right, using nicotine patches or gum immediately after a bone injury would likely be as bad as continuing to smoke.

Stem cells are building blocks for tissue, and the first step toward bone healing is for mesenchymal stem cells to transform into cartilage-forming cells. They build a scaffolding over the fracture, which gradually fills in and hardens into bone. It takes about three months. Stress a healing fracture before then and the still soft cartilage can break again easily, causing lasting pain.

Put nicotine onto those stem cells and they go into overdrive, making an enormous amount of cartilage, Zuscik discovered in tests with mice.

"Too much of a good thing is a bad thing," he explains. "What you end up with, we hypothesize, is a situation where the healing process ends up taking longer."

Nicotine seems to do that by parking in receptors on the stem cells' surface that are intended for acetylcholine, a chemical that helps nerve cells communicate. If the stem cells turn into nerve cells, they'll need those receptors. If they turn into cartilage-forming chondrocytes, the receptors quickly disappear. Zuscik's preliminary data suggests they're gone in a week.

So the nicotine has only a short time to jump into those cellular docking sites. Hence Zuscik's theory that this is a window during which smokers should heal more like nonsmokers if only they could abstain.

It will take a few years of additional animal research before that theory can be tested in smokers, he says, although there's no down side to people trying to kick the habit in hopes it will help heal their bones.

Indeed, there's some indirect evidence that quitting helps: In 2000, Kentucky researchers reviewed the medical records of 357 spinal fusion patients. About three-quarters of both nonsmokers and those who kicked the habit while healing recovered well enough to return to work, compared with just half of the smokers.