Biodegradable plastic could aid bone healing Material would replace plates, clips, screws in fractures.

August 20, 1991|By Sue Miller | Sue Miller,Evening Sun Staff

Johns Hopkins scientists are developing a biodegradable material that eventually dissolves to promote stronger healing of broken bones.

The material could also reduce the number of refractures and make obsolete repeat surgeries to remove metal clips, screws and plates.

The biodegradable material comes from a new class of polymers -- a form of plastic -- which recently has become available through the laboratories of Kam Leong, a polymer scientist, located at both Johns Hopkins Medical Institutions and Johns Hopkins University's Homewood campus.

Dr. James D. Michelson said the "big advance" in Leong's work is that he has reached the point where he can custom-make the polymer to fit both the fracture and its expected healing time. Michelson is a Hopkins orthopedic surgeon who is collaborating with Leong on the development of the polymer.

"There are all kinds of tests one can do to characterize how quickly the polymer will dissolve," he said. "But, eventually you have to get to animals because there are all sort of enzymes floating around a fracture site that are not found in a beaker of saline. And, that's where we are now."

The refracture problem is one that has been around for quite a while and it has been suggested for some time that a plate that is less rigid than metal would provide benefits, Leong said.

A recent study, for example, has shown there is a one-third to one-half refracture rate in forearm fractures.

A plate is a long, slightly curved strip of metal -- eventually polymer -- that the orthopedic surgeon shapes, using special tools, to fit the bone better, much in the same way a dentist goes abut filling a cavity.

"Suppose you have a fracture in your foot," said Leong. "When you stand, the rigid metal [which is implanted at the site of the fracture] will carry all the load or weight. The bone will be healed, but it will be weak."

Using the polymer, the bone progressively assumes more weight as the biodegradable material melts away, until the bone is able to support all the weight on its own.

A "fair amount" of surgeons don't want to have to worry about refracturing or having to do another surgery, so a lot of people end up walking around for many years with metal plates and screws in them, said Michelson, who is also chief of orthopedics at the Veterans Administration Medical Center on Loch Raven Boulevard.

"The problem is that metal is a foreign body and theoretically that makes you a little bit more prone to get an infection," he explained. "And recently, there has been a lot of concern that metal left in the body for long periods of time could be a stimulus for cancerous tumors."

Any time a surgeon operates, there are always associated risks, such as bleeding, infection and nerve damage, according to Michelson. "So, even if you are just taking out pins, screws or a plate, bad things can happen," he said. "Besides, you have to make the same size incision as the first time, and, although it tends to be in the same place, it's painful and it takes time for the patient to recuperate."

Leong, who has been working on the promising polymer for three years, says it will take another five years before it is ready to be implanted, along with polymer pins and screws, in humans who suffer fractures or not-so-clean breaks involving bony fragments. The most common kind of fractures in both young and old patients affect the ankle and the hip.

Leong has done many tissue and cell studies, but is now moving on to animal studies with rabbits.

"We have already found that the polymer appears to be biocompatible with the bone and we have also shown that our polymer also interacts well with the bone," he said. "This polymer is called a polyphosphoester, which is from the new class of polymers."

In recent years, Leong also developed a biodegradable chemical wafer, first used by Dr. Henry Brem, a Hopkins neurosurgeon, as a treatment for brain cancer patients.

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