Apr. 16, 1999 A clinical study led by UC San Francisco researchers has 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 the 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, MD, UCSF professor of orthopaedic surgery, UCSF Stanford Health Care orthopaedic oncologist, 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 novel device, known as the compliant pre-stress system (CPS), is under investigation at UC San Francisco and has been in development for over ten years.
"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," said Richard O' Donnell, MD, UCSF assistant clinical professor of orthopaedic surgery and UCSF Stanford Health Care orthopaedic oncologist.
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, said the researchers. Since the early 1970's, 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," said Johnston.
When the implants become loose, said the researchers, patients experience pain, may start limping, or may not be able to walk.
The problem with the conventional system, said the researchers, 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, they said, the bone atrophies, resulting in loosening of the implant.
Another problem with the conventional device is that bone and metal are not compatible. The bone does not integrate or grow into the metal which plays a role in loosening of the implant.
Concerned with the high failure rate of conventional implants, the 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, UCSF researchers evaluated 25 clinical cases. A majority of the 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 12-62 with average age 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.
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