The plaster is virgin white but for the autograph of his 9-year-old son. A rubber band holds the cuff of his white buttondown shirt closed and the cast over his wrist really isn't very noticeable until you see him trying to use his black IBM Selectric typewriter.

He is a typical casualty of the Rollermania sweeping downtown Washington: a member of the over-30 crowd that has begun strapping on eight polyvinyl wheels in an attempt to recapture a vanishing childhood.

Physicians in emergency rooms throughout the Washington area and in other areas of the country report that adult roller skaters are fast replacing teen-age skateboarders as the orthopedists' delight.

"Six weeks or two months ago, a couple of roller skate (rental and sales) places opened up, one in Georgetown and one in Foggy Bottom. The very day they opened they sent us three or four injuries," said Dr. Tom Stair, assistant director of the Emergency Department of Georgetown University Medical Center.

"That sensitized us," said Stair, "and now we're noticing more roller skating injuries coming in."

Georgwtown is seeing the survivors of roller madness at the rate of about for to six a week, but nationally, the Consumer Product Safety Commission reports more than 70,000 skating injuries thus far this year.

Unlike the skateboard, which is used more frequently by children and teen-agers, roller skates are being used more and more by young adults in their 20s and 30s. Injuries caused by rollerskating accidents are as severe as those caused by the rolling boards, hospitals note.

"The skateboard injuries tended to be much more severe," said Stair. "Kids would get out in traffic, or flip over and crack their skulls."

Roller skating injuries, however, have been far more mundane in comparison -- skinned knees and elbows, sprained joints of every shape and form and cracked and broken wrists, elbows, ankles and, occasionally, knees.

Because adults are at times more cautious than children, some of the current crop of first-timers have been strapping on protective gear -- elbow and knee pads in particular -- and that may be cutting down on the injuries, say physicians, who for years have been advising skateboarding youngsters to wear just such armor.

Another thing which keeps the number of severe injuries down is that, when roller skaters fall, they are less likely than skateboarders to be totally out of control and can often ease into a fall, or at least protect the head and face.

Moreover, injured adult roller skaters can take solace in their numbers.

When the man with the white cast and shirt to match walked into George Washington University Medical Center's emergency room Saturday night cradling his broken wrist; he was asked how he did it.

"Skating," he told the doctor.

The doctor smiled. "We've been seeing a lot of skaters in here."