Tissue differentiation

Tissue differentiation - understanding the implant/bone interface

Gary Beaupré, PhD; Nick Giori, PhD; MD Dennis Carter, PhD; Stuart Goodman, MD

One of the most challenging problems limiting the long-term success of hip and knee replacements is implant loosening. A loose and painful implant typically requires revision surgery for removal of the prosthesis and reimplantation with a new prosthesis. For more than 30 years, bone cement has been used to secure the implant to the supporting bone. However, the unavoidable accumulation of fatigue cracks in the cement inevitably leads to implant loosening and the need for revision.

During the 1980's researchers and implant manufactures developed a new generation of implants that no longer relied on the use of cement for implant fixation. With these cementless designs it was thought that fixation would occur via the generation or differentiation of new bone between the implant and the adjacent bony bed (Fig. 1). The process of bony fixation, however, has proven to be both unpredictable and unreliable. Frequently, a layer of fibrous tissue rather than bone develops at the implant-bone interface.

Figure 1. Close-up of the implant-bone interface surrounding a joint replacement.

We are working on the development of theoretical models (Fig. 2) that will enable us to predict the type of interface tissue that will develop or differentiate around an implant in response to the local stress and strain environment. Using a novel implantable chamber made out of pure titanium (Fig. 3) we are studying the influence of shear strain on tissue differentiation in an animal model. These types of studies are essential if we are to understand the mechano-biological processes controlling the biological fixation of cementless implants. This knowledge, in turn, will lead to the development of new implant designs that will function better and last longer than current joint replacements.

Figure 2. Palo Alto VA/Stanford tissue differentiation hypothesis.

Figure 3. Exploded view of titanium tissue differentiation chamber.

Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: Gary Beaupré.