More than 50 body parts that age or injury or disease often destroy are now being replced by implants or soon will be, according to a report in Science magazine.

The report, written by Professor Larry L. Hench of the University of Florida, lists implants ranging from artificial heart valves, hips, elbows, knees, shoulders and wrists to substitute devices for the trachea (wind-pipe), tear ducts and the tiny delicate bones of the middle ear that are essential to normal hearing.

Hench also reports, however, that many of the 2 million to 3 million devices being implanted each year at hospitals in the United States leave much to be desired because they must frequently be replaced. The trouble is that some are eventually rejected by the body while others tend to wear out or break, he said.

At a news conference yesterday, he explained that the key factor is the development of material that the body will find more compatible than the 40 or so different metals, plastics, glues, special glasses and ceramics being used in substitutes for body parts.

Hench cited as a typical problem the mechanical damage hip and knee implants often inflict on surrounding tissue. The damage, he said, causes the implants to loosen and this can be followed by an inadequate blood supply to the area that, in turn, leads to infection and pain.

Referring to animal experiments already in progress, Hench said the answer to this problem may be provided by materials that continue to fit snugly because they are engineered to have bone grow into them to hold them in place.

In general, substitute body parts are expected to become more reliable as new materials are created with special properties and methods are developed to accurately predict how long they will last, Hench reported. He added that many of these advances are being made.