OCT. 21 -- By simulating the natural load on human thigh bones, a new artificial hip design might someday help prevent post-surgical atrophy, a common problem among younger, more active patients, University of Delaware scientists reported today.
"Conventional prosthetic designs remove so much stress from the patient's leg, they can allow thigh bones to weaken, especially among those younger than about age 60," UD researcher Michael H. Santare explained during the American Society of Biomechanics meeting. "The UD design significantly reduces this stress shielding because it's based on an analysis of real loading conditions within the human hip region."
A computer model of the UD hip design demonstrated up to 20 percent less shielding of normal stress to key hip areas, compared to the best-performing prosthesis--a 1995 model available only in Europe, says Santare, an associate professor of mechanical engineering.
In computer simulations, UD's artificial hip "consistently performs better than the conventional design or the European design," according to Santare and his colleague, Mechanical Engineering Prof. Suresh G. Advani.
An orthopedic surgeon collaborating with the UD scientists, Freeman Miller, M.D., of the Alfred I. DuPont Institute, says the new prosthetic design could potentially be a promising alternative for younger patients.
The research remains preliminary, Santare emphasizes. The team's findings so far are based on computer simulations and mechanical tests, he says. But, animal studies are planned in the near future.
A common problem
Some 134,000 patients received total hip replacement in 1995, according to the American Academy of Orthopaedic Surgeons. Nearly a third of those surgeries--32.6 percent--were performed on patients younger than 65. Women represented 59.7 percent of all hip-replacement patients.
Osteoarthritis, the most common reason for hip replacement therapy, gradually damages the cartilage in the hip, allowing the thigh and hip bones to grind together.
Though total hip-replacement techniques were being investigated in the 18th century, viable prosthetics weren't available until the late 1950s, Santare explains.
That's when researcher J. Charnley demonstrated the "intramedullary" method, which involves removing some portion of the thigh bone, or femur, and inserting a rigid rod into the medullary canal--the bone's hollow interior. A knob is fixed to the top of the rod, replacing the femural "head" that normally rests within the hip, like a ball in a socket.
The technique has traditionally been "very successful in older patients," Santare says. Because the Charnley technique dramatically changes the normal stress and load patterns on the patient's leg, however, "it can cause problems in younger people, or those who are very active," he explains.
"The bone atrophies in regions where it's unloaded," Santare notes. "It's natural for the bone to take back calcium where it's not needed anymore, and then you can see a reduction in bone mass. Atrophy and reabsorption lead to a loosening and failure of the joint or the bone."
An alternate approach, pioneered in 1995 by E. Munting and M. Verhelpen, has more closely approximated normal loading conditions among patients in Europe. The design requires a much shorter rod, which is attached to the top of the thigh by several small bolts, inserted through the exterior side of the bone, the lateral cortex region.
This newer strategy has reportedly been successful so far, Santare says. But, Miller says, inserting bolts into the femur could increase the risk of cracks.
Taking a tip from nature
To reduce stress shielding and prevent bone atrophy, Santare and Advani first modeled the function of a healthy hip--thanks to the National Center for Supercomputing Applications-to simulate natural conditions. Then, they compared those findings with an analysis of the stress patterns produced by a conventional artificial hip, and by the Munting and Verhelpen design.
Their resulting design greatly reduces the prosthetic rod, which seems to be responsible for much of the rigidity and altered loading that occurs following hip replacement. In its place, they use a system of cables to fix the implant to the proximal femur. A plate-like base is used to transfer the bending loads from the prosthesis to the bone. Further improvement can be achieved by using advanced composite materials to tailor the stiffness of the implant--the subject of ongoing research at UD, Santare says.
The design is still in prototype form. But, mechanical tests using bone from a cadaver suggest a viable new technology, Santare says, and the team is seeking patent support. They also hope to complete animal studies with sheep at the New Bolton Veterinary Hospital of Pennsylvania, he adds.
Support for the UD research was provided by the Whitaker Foundation. Along with Santare and Advani, the team included postdoctoral researcher Makarand G. Joshi and graduate student Shawn P. Riley.
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