91色情片

Dental braces may soon be less painful to wear. Braces are kept tight using
rubber bands, but this can injure the gums and means the braces have to be
tightened every four weeks. Now the Temple University School of Dentistry in
Philadelphia has designed braces that replace the rubber with a hatch that
swings up like a tiny garage door. Wires secured beneath the hatch give a
gentler, steadier pull and need be tightened only every 10 weeks.