A clinical study led by UC San Francisco researchers hasfound that a new prosthetic device primarily used to savelegs with cancer stimulates bone development. The devicedirects forces or stress on the leg bone rather than themetal implant and this, says the researchers, is necessaryto prevent loosening and failure often seen in conventionalimplants.
"With this new device, we've changed the whole picture ofhow forces go through the bone," said James Johnston, MD,UCSF professor of orthopaedic surgery, UCSF Stanford HealthCare orthopaedic oncologist, and principal investigator ofthe study. "The fixation device produces an environmentwhere bone appears to heal to a metal surface in a patternsimilar to fracture healing." The novel device, known as thecompliant pre-stress system (CPS), is under investigation atUC San Francisco and has been in development for over tenyears.
"For patients with cancer of the leg, the findings suggestthis device will not only prevent loosening of theprosthetic device but will also allow patients to functionlong term," said Richard O' Donnell, MD, UCSF assistant clinical professor oforthopaedic surgery and UCSF Stanford Health Careorthopaedic oncologist.
UCSF researchers presented preliminary findings April 14 atthe International Society of Limb Salvage Surgeons meetingin Cairns, Australia.
Prior to the use of prosthetic devices, patients with cancerof the leg required amputation, said the researchers. Sincethe early 1970's, prosthetic devices have been used toreplace cancerous bone and knee joints, allowing the leg tofunction normally.
The conventional system involves a titanium implant, a sixinch stem that is cemented in the canal of the femur or thethighbone.
"Previous research has shown that the conventional implantsare becoming loose and failing in approximately 50 percentof the cases after 10 years and 75 percent of the casesafter 20 years," said Johnston.
When the implants become loose, said the researchers,patients experience pain, may start limping, or may not beable to walk.
The problem with the conventional system, said theresearchers, is that bone surrounding the metal stemdisappears over time because the conventional device isstress shielding - it prevents force or stress to the bone. Without stress, they said, the bone atrophies, resulting inloosening of the implant.
Another problem with the conventional device is that boneand metal are not compatible. The bone does not integrateor grow into the metal which plays a role in loosening ofthe implant.
Concerned with the high failure rate of conventionalimplants, the UCSF researchers designed the new CPS systemand have been studying the effectiveness on patients for thepast seven years. The compliant-pre-stress system (CPS) usesa shorter metal three inch titanium stem fixed to the femurwith five pins. Inside the implant is a series of springwashers. When the surgeon tightens the implant to the bone,the washers act like a spring and generate a stress to thebone. This provides the stability necessary for a person towalk with the implant. The spring washers direct the forceson the leg from walking to the leg bone rather than theprosthetic device.
In the study, UCSF researchers evaluated 25 clinical cases. A majority of the patients either had tumors in the femur orexperienced loosening of the conventional system. In otherpatients, the device was used on the arm. Age range ofpatients was 12-62 with average age 31 years. Follow-up ofcases ranged from two years to seven years, with averagefollow-up of three and a half years. Clinical findingsshowed 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 anddevelopment around the implant as early as five months, saidthe researchers. In addition, X-rays also show integrationof bone into the device. The study was funded by Biomet, Inc.
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