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Future of hip bearings: more flexible designs
Research now points to the importance of avoiding stress shielding in the acetabulum.
In the future, total hip prostheses developers should concentrate on hemispherical shells with more geometrically flexible designs to address concerns over wear, noise and the long-term stress-related effects on the bony structures, according to an industry specialist.
"All contemporary hip bearings require a rigid or semi-rigid supporting structure, both in the femur and the acetabulum," said Michael T. Manley, PhD, chief scientific advisor to Stryker Orthopaedics, Mahwah, N.J.
He asked the audience at the American Association of Hip and Knee Surgeons Specialty Day, "What is a hip bearing? I suggest to you that it is the entire construct. The entire construct is simply there to hold up the articulation. And, therefore, to look at the articulation, you have to look at the entire construct of the hip."
He said that contemporary bearing technologies continue to decrease hip wear debris and the potential for osteolysis in young patients in the medium term. However, an overlooked consequence of current acetabular designs is that the abnormal stress distribution in the acetabular bony structures is caused by stiff, well-fixed acetabular shells.
To better understand the cause of wear-generated debris, Manley questioned whether particle integration occurs through overwhelming release, or by a slow release into compromised bone.
Manley presented laboratory data of his investigation on hip replacement stresses and whether stress shielding affects the replaced articulation. Using a hip from a 30-year-old donor, the investigator made a 3-D model of the joint. A total hip bearing was then added and it was exposed to 22 muscle forces and the forces associated with locomotion.
Manley then calculated the compressive forces on the model hip and compared them to the forces on a natural hip, which has most of the compressive forces on the posterior acetabular wall.
He found that a slight change in the femoral head position affected the joint stresses. Also, comparisons of the acetabular stress changes were not seen as the acetabular shell thickness was decreased.
"We went on to compare the natural hip to a cobalt chrome replacement or a polyethylene acetabular replacement," Manley said. "We saw huge differences between the compressive stresses seen in the acetabulum."
He said that this data indicate that it is possible to have stress shielding in acetabular structures and reducing the material thickness or wall thickness had no effect on it. Manley reviewed the literature of both spherical and hemispherical cups and found that the hemispherical cups produce less stress shielding. "Acetabular stress shielding with a press-fit design is inevitable," he said.
"Comparison of hemispherical acetabular shells with more geometrically flexible designs shows an improvement in acetabular stress state with the geometrically flexible devices," he reported. "It is likely that the future of acetabular bearings and acetabular reconstruction is a head/flexible socket bearing couple that can address current concerns, such as wear and bearing noise, while at the same time present long-term stress related deterioration of bony structures."
For more information:
- Manley MT. The future of hip bearings. Presented at the American Association of Hip and Knee Surgeons Specialty Day. March 25, 2006. Chicago.