Prism repair

The Hardy Boys might have called what happened at the Boston Garden last Sunday ''The Backboard Mystery.'' Though the TV announcers had a ready explanation, Garden spectators were perplexed by the repair work performed on the rim during the Celtics-76ers basketball playoff game.

A small plastic prism that almost no one knew existed had come loose from the backboard and had to be replaced. The piece sits at the back of the mounting plate to the National Basketball Association's new collapsible Snap Back rims and prevents players from getting their fingers caught in the hinge. Since these rims release under 230 pounds of pressure, they lessen the possibility of bent rims and shattered backboards.

A variety of rims were tested before the Snap Back model was selected and employed in all NBA games this season. The prism, however, was developed only at the last minute at the NBA's request. Glued to the backboard, a few of them worked loose toward the end of the season. Ken Mahoney, president of the small Kansas firm that manufactures the rim, had delivered a spare prism to the Celtics only three days before Sunday's game.

Next year, the prisms won't be necessary because a shield will be built right into the basket bracket.