Biodegradable implant for broken bones.

Biodegradable implant for broken bones 

A newly patented polymer undergoing animal tests shows promise as a biodegradable "bone plate"--an internal splintlike brace--to hold broken bones together until they heal. At present, many compound fractures and limb breaks at other than a bone's midshaft have to be supported with a screwed-on internal stainless steel plate for 12 weeks, or until a bone mends, explains Deger C. Tunc, principal scientist in the orthopedic division of Johnson & Johnson Products Inc.'s research center in North Brunswick, N.J. Since steel plates don't degrade, follow-up surgery is eventually required to remove the plate and screws. But according to Tunc, the new polymeric material remains strong and rigid for at least 12 weeks, then slowly decomposes into lactic acid, a chemical that occurs naturally in the body.

Polymers are molecules made by linking a chain of identical subunits. The substance in Tunc's study is made by linking lactide monomers into a much longer "polylactide" polymer than had ever been achieved before. To do this required calculating, through computer modeling, the precise combination of processing characteristics--such as polymerizing temperature and ratio of catalyst to monomer--that would minimize unused monomer, Tunc says.

The new molecule's added length and "molecular weight" contribute not only to its greater strength but also to its slow dissolution. Previous polylactide implants degraded so rapidly that within four weeks or so they had totally lost any value as a support, Tunc says. Implants of the new material, tested in dogs, lasted far longer: 20 percent of the polymer's breakdown products still remained two-and-a-half years after implant, Tunc says. Tests in both dogs and rabbits failed to indicate any toxicity or irritation associated with the body's breakdown and elimination of the implants, he reports. Pending Food and Drug Administration approval, Tunc says, Johnson & Johnson may initiate human tests of the implant material later this year.