Kids surfing the sidewalks this summer on the Ripstik Caster Board, a two-wheeled variation on the standard four-wheeled skateboard, don’t have to put a foot down, but they may want to keep their guard up.

The Ripstik, named 2008’s outdoor toy of the year by the Toy Industry Association, lets riders accelerate and even ride uphill without pushing off the ground by shifting their body weight atop its two rotating platforms and wheels that turn 360 degrees.

But the wheeled fun comes with a risk of injury, especially for beginners. We asked a an emergency room physician about that after hearing about kids who’d been injured while riding.

Since the toy’s fall 2006 debut, Julian Orenstein of Shady Grove (Md.) Adventist Hospital says, he has treated Ripstik-related scrapes, fractures and head injuries, but the damage has been no more and no worse than what he sees from traditional skateboards.

Orenstein, who bought a Ripstik for his 11-year-old daughter, says that the toy improves balance and fine motor control, and that most injuries should keep kids out of action for only a few days.

His advice to parents and caregivers? Make sure riders wear a helmet and ride on smooth surfaces, just as with bicycles, skateboards and rollerblades. “Kids will fall,” Orenstein says, so they need to be protected.

A “How to Ride” video posted by Ripstik’s manufacturer, Razor USA, on its Web site urges riders to wear protective gear such as elbow and knee pads, adhere to age and weight restrictions (the board is not recommended for kids under age 8 or over 220 pounds) and avoid riding on wet pavement.

Orenstein advises parents to be cautious but not overprotective: “Don’t let the risk of potential injury stop them from doing something that’s good for them, great exercise and lots of fun.”