Articular cartilage exhibits a poor intrinsic healing capacity when injured in trauma or by degenerative diseases such as osteoarthritis (OA). In cartilage tissue engineering, there are two prevailing points of view regarding implantation of engineered constructs in vivo. One approach places the cell-scaffold immediately into the defect site and relies on the in situ biological and loading environment to foster development of the fledgling construct. Another approach is to first cultivate constructs in vitro to permit some elaboration of material properties to minimize the propensity of construct cracking. Advocated by Guilak and co-workers (2000), the concept of Functional Tissue Engineering (FTE) has promoted the use of physiologic loading bioreactor systems to cultivate engineered cartilage tissues to better produce replacement tissues with functional (load- bearing) properties of articular cartilage. While an FTE approach has shown promise in guiding tissue development in culture, the clinical benefits of mechanical preconditioning of engineered constructs for cartilage repair is currently unknown. Tissue repair may be influenced by in vitro loading-induced composition and structure and/or by the act of preconditioning of chondrocytes to a dynamic loading environment prior to introduction in the joint loading milieu. In this competitive renewal two parallel but independent aims, one hypothesis-driven and the other model development-driven, are proposed and address the same fundamental effort to translate our basic tissue engineering studies to animal models and eventually to humans.
Specific Aim 1 (hypothesis-driven): To test the hypothesis that mechanical preconditioning improves engineered construct performance, compared to free-swelling (non-loaded) constructs, in the functional repair of full- thickness articular cartilage defects. Apply dynamic deformational loading (DL) or free-swelling (FS) to chondrocyte-seeded hydrogel constructs in serum-free culture. When constructs attain a Young's modulus of 25% and 100% of site-matched native canine articular cartilage, implant constructs into a defect in the canine femoral condyle (high load-bearing) or trochlear groove (moderate load-bearing) and monitor tissue repair (e.g., material properties, biochemical properties, arthroscopy, and lameness scores) over a 1-year period.
Specific Aim 2 (model development-driven): To develop a computational biphasic three-dimensional contact model of the canine knee that will provide insights to the engineered construct implantation environment. Perform validation of construct crack initiation in canine cadaver knee loading studies. This model will aid interpretation of the experimental defect repair results (Specific Aim 1) as well as provide general guidance for the requisite material properties of the engineered cartilage construct necessary to survive the in situ loading environment for a focal defect of a particular diameter and anatomical location, and initial material properties. To test our hypotheses and achieve our aims, a multidisciplinary team of investigators from Columbia University and the University of Missouri has been assembled. These studies have been designed with the anticipation that the outcomes of our specific aims will eventually allow us to design experiments to predict in vivo repair performance of engineered tissues.
As the growing promise of engineered cartilage grafts is realized in clinical practice, it will be important to understand the implications that mechanical preconditioning of engineered tissue grafts in culture has on tissue repair and clinical outcome. Moreover, orthopaedic surgeons will need specific guidelines to guide them in reparative strategies using engineered cartilage that may possess tissue properties inferior to the native tissue. A clinically relevant algorithm derived from translational evidence for guiding decisions regarding indications for current and future cartilage repair strategies would be invaluable to surgeons.
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