Today's News

Friday, March 2, 2001

Gene therapy: graft substitutes for fracture care

By Laird Harrison

The job of fixing broken bones is rapidly changing thanks to computerized imaging, less invasive tools, gene therapy and graft substitutes, said panelists at Thursday morning's symposium on Emerging Technologies for Fracture Care.

Less invasive system

Philip J. Kregor, MD, of Charlotte, N.C., introduced a new procedure called the Less Invasive Stabilization System (L.I.S.S.) in which surgeons insert a fixator through an incision only a couple of inches in diameter, then slide it under the muscle along a fractured tibia or femur. The screws used to attach the fixator to the bone each need their own small incision.

The technique is promising because incisions are much smaller than in conventional techniques and the muscle is less disrupted. Also, the screws can be locked to the plate, so that there is less movement over time. The procedure is most useful for difficult fractures, such as very osteoporotic bone, Dr. Kregor said.

Michael J. Bosse, MD, of Charlotte, N.C., who served as a "provocateur" on the panel questioned whether the evidence is strong enough for physicians to begin using L.I.S.S. "Minimally invasive and tissue-sparing surgery is the right thing to do," he said. "But we have no evidence of better outcomes than with conventional procedures such as double plating and retrograde nails."

Dr. Kregor answered that he got good results in an unpublished study comparing L.I.S.S. to the historical data on conventional therapies, but he admitted that no one has yet completed a randomized, prospective trial.

Dr. Kregor acknowledged that, at $1,800, L.I.S.S. is more expensive than conventional techniques. While he has used L.I.S.S. on his own, he said, it's best to have two assistants.

Computer-assisted surgery

Computers, too, can help surgeons do less invasive surgery, said David M. Kahler, MD of Charlottesville, Va.

Two types of computer-assisted orthopaedic surgery are now in vogue, Dr. Kahler said. In the first, surgeons produce three-dimensional images of bones to be treated with CT scanners. During surgery, the computer uses a camera over the operating table and light-emitting diodes attached to the surgical instruments to estimate where the instruments are in the image. With this guidance, said Dr. Kahler, "you don't have to make a big incision and horse around through the soft tissue."

The average hospital stay using the three-dimensional CT-based system is only two days after surgery. The operating time is also reduced. And less radiation is needed than in conventional fluoroscopy.

But the CT-based system has drawbacks as well. Because CT-scanners are so big, the images can't be updated after the patient leaves the X-ray room. If the bone moves in that time, the image may no longer be accurate.

So surgeons have come up with a solution: virtual fluoroscopy. As in conventional fluoroscopy, surgeons take pictures in the operating room. The advantage is that fewer pictures are needed because the computer can substitute some of them with accurate models. As a result, both the patient and the physician are exposed to less radiation. That's important, Dr. Kahler said, because there is evidence that orthopaedic surgeons may be exposed to enough radiation from fluoroscopy to increase their risk of cancer.

The equipment is expensive: $250,000 for virtual fluoroscopy; $400,000 for a three-dimensional CT system. "When people ask me if I can do this faster, better and cheaper, I say two out of three isn't bad," concluded Dr. Kahler.

Growth factors

Cell biologists are also adding to the tools of the orthopaedic surgeon, said Thomas A. Einhorn, MD, of Boston. Several growth factors have been shown to speed bone healing when injected into the site of a fracture. One, bone morphogenetic protein 2 may be available commercially in the United States within 24 months.

In animal experiments, surgeons have also been able to introduce genetically modified cells which produce their own growth factor and speed healing.

None of these will take the place of surgery, however. "These techniques are not intended to make bad surgeons good, they will only make good surgeons better," Dr. Einhorn said.

Bone graft substitutes

Chemistry, too, offers promise for surgeons, said Robert W. Bucholz, MD, of Charlotte, N.C. Medical companies are offering a wide variety of materials to surgeons to replace or substitute grafts. Currently, 51 percent of the 600,000 grafts done annually in the United States are autografts, 39 percent are allografts and 10 percent are synthetic.

The graft substitutes offer the key advantage that they are available in unlimited supply. There are mostly biocompatible and reabsorbable, are osteoconductive, produce excellent surface bone growth and have adequate mechanical properties in compression.

On the other hand, they are not osteoinductive, they are mechanically weak in tension and shear, they have brittle handling properties, they may inadvertently be displaced into adjacent joints and they are hard to test. Also crystalline ceramics are poorly reabsorbed and may have adverse effects on bone remodeling as a result.

The most promising application for substitute grafts, said Dr. Bucholz, is for volume augmentation and as a medium to carry antibiotics. The best results have been shown in filling metaphyseal defects.

Measuring outcomes

Mark F. Swiontkowski, MD, of Minneapolis, brought optimistic news about the way fracture research is conducted. Ten years ago, he said, orthopaedic surgeons used too many fracture classifications and unvalidated outcomes scales. They misunderstood severity measures and confused clinical and functional outcomes.

Since then, fracture classifications have been standardized and can be reproduced, outcomes scales have been validated and useful computer programs have been developed. Today, orthopaedic surgeons better understand the need to demonstrate efficacy and the principles of clinical research.

Now the surgeons must move forward in developing multicenter, prospective, controlled trials.

Finally, Dr. Swiontkowski said, reprising a theme touched on by nearly every speaker, "we must look at the impact as it relates to cost."

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Last modified 02/March/2001 by IS