Biodegradable screws used in implants recently swept the marketplace, but as the screws were resorbed in the body, toxic materials were sometimes released. (Courtesy photo)

Biodegradable screws used in implants recently swept the marketplace, but as the screws were resorbed in the body, toxic materials were sometimes released. (Courtesy photo)

Resorbable Materials: Sometimes tradition is best

Metal has been used in patients since the time of Hippocrates (400 BCE) to repair broken bones and replace missing parts. The earliest metals included copper, tin or lead and often did not have the strength to last forever. Over the years, the metals we use have improved. They have become stronger and leach fewer ions into the body.

The vast majority of these implants did their job without causing complications, but since they remained inside the body, there was always a risk.

Over the past 20 years, the field of absorbable, or “resorbable,” implants has grown. It’s an attractive idea: Such an implant is often invisible on an X-ray. The body grows into it, or the device dissolves on its own as the body heals after the successful repair. Initially, resorbable synthetic materials (such as polylactides and collagen scaffolds) were the most commonly used to affix tissue. Sutures, which had originally been made of resorbable catgut, became permanent with the introduction of cotton and silk materials, then evolved to resorbable threads made of lactide polymers.

In one well-known application, the screws used to fix the tissue for ACL reconstruction were made of stainless steel, then titanium. More recently, biodegradable screws made of lactides (often mixed with calcium phosphate) swept the marketplace. It was believed that as the body healed, these screws would be replaced by bone in the bony tunnels, making any future surgery easier. But as these screws were resorbed in the body, toxic materials were sometimes released, causing cavities and cysts in the bone. Follow-up X-rays, instead of looking like a normal tibia, looked like Swiss cheese.

What caused this to happen?

Our bodies have evolved over millions of years to recognize and respond aggressively to foreign materials. Once materials are recognized as foreign, the body attacks with cells and enzymes that break down and chew up the invasive materials or wall them off with a protective layer of scar or bone. As the foreign materials are broken down by these systems, artificial components, which can have detrimental effects on the cells and surrounding tissues, are released.

Lactides, as mentioned earlier, are commonly used to make resorbable screws. But these screws release high concentrations of lactic acid in the tunnels where they press against the ligaments and bone. In this narrow space, the acid kills the responding cells, forming cysts in the bones. The acid also inhibits osteoblasts that build bone, preventing the body from filling in the defects.

As a solution, permanent crystalline plastic materials called PEEK were developed to replace the resorbing screws. Though they worked well and were invisible on X-rays, they were hard to find and difficult to remove if the joint was reinjured.

The end result?

Good old metal has returned as the material of choice in fixing bone to bone — and sometimes even soft tissue to bone, where sutures alone are not strong enough. The metal works without inciting the inflammation seen in resorbable materials. It’s also easy to see on X-rays and straightforward to remove at surgery.

The lessons for those of us who have seen enough pendulums swing in medicine is that newer is sometimes better — but not always.

Dr. Kevin R. Stone is an orthopedic surgeon at The Stone Clinic and chairman of the Stone Research Foundation in San Francisco.

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