Functional tissue engineering parameters toward designing repair and replacement strategies.

Abnormal joint kinematics and loads induced after soft tissue injuries are assumed to contribute to long-term degenerative joint disease and osteoarthritis. Controlling abnormal kinematics after repair and reconstruction of these injured structures would seem to be important for limiting wear of the articular cartilage surfaces. In this paper, we propose to expand the paradigm of functional tissue engineering to more fully characterize normal joint function and to establish design parameters for soft tissue repair and reconstruction to ultimately protect joint surfaces after surgery. Structure-function relationships are examined for tissues of increasing complexity, from tendons to menisci. Emphasis is placed on understanding normal in vivo function of tissues by conducting biomechanical experiments in vitro that better mimic in vivo conditions. This process yields nine classes of functional tissue engineering parameters: differential fiber length, in vivo force and displacement, variations in relative attachment site locations, loading from adjacent structures, fiber interactions, types of insertion, regional variations in material properties, nonparallel fiber orientations, and complex loading within the structure. These functional tissue engineering parameters are useful not only for understanding the function of normal tissues but for more effectively designing their repair and replacement. This paper concludes with a discussion of research directions that investigators might take to establish tissue-specific functional tissue engineering parameters for improving joint function and reducing articular surface degradation and osteoarthritis.