Smile physics: the latest braces employ basic science to put a smile on your face.

Mom, I look like Dracula," wailed 12-year-old Kaela of Denver. She was exaggerating, but like so many people young and old, Kaela was embarrassed by her smile. Her front teeth were crooked and her canines, or pointed teeth, protruded from her gums. Her parents were concerned too. Crooked teeth can cause chewing problems, and they are more susceptible to decay. Luckily, there was a solution to Kaela's problem: braces.

The word "braces" used to suggest a mouthful of metal. Today, braces are almost invisible, thanks to clear or tooth-colored brackets. More daring teens can coordinate brightly colored elastics and decorative retainers with their outfits or school colors.

Just as these fashionable features have made braces cooler, new technologies have made them more comfortable and efficient. In two years, for example, Kaela had a dazzling smile.

SPACE-AGE MATERIALS

Braces apply force on crooked or rotated teeth to straighten them. An arch wire connected to bands and brackets pulls or pushes the teeth into their proper positions (see miniposter, pp. 12-13).

Orthodontists, or dentists who specialize in straightening teeth, often use stainless steel for the arch wire. "One disadvantage of stainless-steel wires is that they put more force on the teeth initially, then that force dissipates very quickly," says orthodontist Christopher Carpenter of Denver.

But new nickel-titanium wires, developed through NASA's space program, keep the pressure on. That's because this alloy (mixture of metals) has an extraordinary characteristic that's called "shape memory" -- no matter how much an orthodontist bends the wire, it springs back to its original shape.

The key to nickel titanium's unusual property is temperature. When the alloy is cold (25 [degrees] - 35 [degrees] C or 77 [degrees] - 95 [degrees] F), the bonds between its atoms break. The wire becomes pliable, and an orthodontist can thread it through the brackets glued to a patient's teeth.

When the wire warms up in the patient's mouth (37 [degrees] C or 98.6 [degrees] F), the directional bonds between atoms reform, pulling the wire back to its original shape. As a result, nickel-titanium arch wires exert a continuous but gentle force.

Typically, patients who wear stainless-steel wires see their orthodontist every four weeks to have their braces tightened. A patient using nickel-titanium wires only visits every six to eight weeks. This means less discomfort for the brace wearer and shorter treatment time overall.

MAGNETIC SMILES

Magnets also offer alternative and more comfortable ways to fix teeth, especially buck teeth, or front teeth that project forward. Orthodontist Gene Dellinger of Fort Wayne, Ind., also uses magnets to close an open bite, a more serious problem.

Nine-year-old Ryan Spohn's molars (back teeth) stuck out of his gums too far. When he closed his mouth, his incisors (upper and lower front teeth) did not touch. This made it hard for Ryan to bite down on food.

Many orthodontists would have performed surgery to slice out part of Ryan's jawbone. But Dellinger bonded four magnets onto the surface of his molars. He positioned the magnets so that one faced the "like" pole of another. The magnets repelled each other, exerting about 240 grams of force, a little stronger than that of magnets clinging to your refrigerator door. In just 90 days, the magnets pushed Ryan's molars back into his jawbone and closed his bite (see photos, left,).

Magnets' attractive forces move teeth as well. Orthodontists put one magnet on a tooth that's impacted, or wedged in the upper jawbone, and another on the tooth below. The opposite poles of the magnets pull at each other, drawing the impacted tooth down.

TRAVELING TEETH

How can braces move teeth from one part of the jaw to another? When braces exert force, they cause cells known as osteoclasts to break down bone surrounding the tooth as it moves forward or back. At the same time, other cells called osteoblasts build up new bone that secures the tooth in its new position.

Ideally, braces should move teeth about one millimeter a month. "If we try to move the teeth too quickly, the bone cells won't work properly," says Carpenter. That's why most patients have braces for 12 to 36 months.

After braces come off, patients often wear retainers to keep teeth from traveling back to their original position. But the longer patients wear retainers, the less likely their teeth will shift in the future. And the brighter their smiles will be.