Prosthetic Device Stimulates Bone Growth in Cancerous Legs

April 14, 1999
News Office: Alice Trinkl; Lordelyn P. del Rosario (415) 502-6397

A clinical study led by UCSF Medical Center researchers found that a new prosthetic device primarily used to save legs with cancer stimulates bone development. The device directs forces or stress on the leg bone rather than the metal implant and this, says researchers, is necessary to prevent loosening and failure often seen in conventional implants.

"With this new device, we've changed the whole picture of how forces go through the bone," said James Johnston, an orthopedic oncologist at UCSF Medical Center and principal investigator of the study. "The fixation device produces an environment where bone appears to heal to a metal surface in a pattern similar to fracture healing."

The device, known as the compliant pre-stress system (CPS), is under investigation at UCSF and has been in development for over 10 years.

Richard O' Donnell, a UCSF orthopedic oncologist, said, "For patients with cancer of the leg, the findings suggest this device will not only prevent loosening of the prosthetic device but will also allow patients to function long term."

UCSF researchers presented preliminary findings April 14 at the International Society of Limb Salvage Surgeons meeting in Cairns, Australia.

Prior to the use of prosthetic devices, patients with cancer of the leg required amputation. Since the early 1970s, prosthetic devices have been used to replace cancerous bone and knee joints, allowing the leg to function normally.

The conventional system involves a titanium implant, a six-inch stem that is cemented in the canal of the femur or the thighbone.

"Previous research has shown that the conventional implants are becoming loose and failing in approximately 50 percent of the cases after 10 years and 75 percent of the cases after 20 years," Johnston said.

When the implants become loose, patients experience pain, may start limping, or may not be able to walk.

The problem with the conventional system is that bone surrounding the metal stem disappears over time because the conventional device is stress shielding -- it prevents force or stress to the bone. Without stress, the bone atrophies, resulting in loosening of the implant, researchers said.

Another problem with the conventional device is that bone and metal are not compatible. The bone doesn't integrate or grow into the metal, which plays a role in loosening of the implant.

Concerned with the high failure rate of conventional implants, UCSF researchers designed the new CPS system and have been studying the effectiveness on patients for the past seven years.

The compliant-pre-stress system (CPS) uses a shorter metal three-inch titanium stem fixed to the femur with five pins. Inside the implant is a series of spring washers. When the surgeon tightens the implant to the bone, the washers act like a spring and generate a stress to the bone. This provides the stability necessary for a person to walk with the implant. The spring washers direct the forces on the leg from walking to the leg bone rather than the prosthetic device.

In the study, researchers evaluated 25 clinical cases. A majority of patients either had tumors in the femur or experienced loosening of the conventional system. In other patients, the device was used on the arm. Age range of patients was 12 to 62 with an average age of 31 years. Follow-up of cases ranged from two years to seven years, with average follow-up of three-and-a-half-years.

Clinical findings showed no signs of stress-shielding or late loosening -- loosening of the implant after one year with the implant. X-ray examinations in these cases showed bone growth and development around the implant as early as five months, said the researchers. In addition, X-rays also show integration of bone into the device.

The study was funded by Biomet Inc.