Some bioengineering considerations for tissue engineering of articular cartilage

The mechanism(s) by which chondrocytes convert physical stimuli to intracel lular signals, which in turn direct cell activities, represents an area of intense current orthopaedic tissue engineering research. This report is aim ed at providing an overview of some biomechanical engineering factors that are required for pursuing this type of research. Two specific aspects of ca rtilage are addressed: (1) how does the tissue function biomechanically; an d (2) what is the nature of physical stimuli inside articular cartilage. By focusing on the effects of inhomogeneities of material properties, a descr iption of some of the mechanical and electrochemical events (the physical s timuli) that would occur in cartilage during loading is presented. Two simp le and common tests are considered: permeation and confined compression. Th eoretical analyses using appropriate constitutive laws (the biphasic and tr iphasic theories) reveal the details of how surface loadings are converted to mechanical and electrochemical signals by the extracellular matrix to hy draulic and osmotic pressures, fluid, solute and ion hows, matrix deformati ons, and electrical fields. The material inhomogeneities are shown to be ab le to significantly change the mechanical and electrochemical events within the extracellular matrix, and thus the environments around chondrocytes. M aterial inhomogeneities arising from the flow of interstitial fluid through the porous and permeable extracellular matrix also are discussed. In the a uthors' view, the charged extracellular matrix, together with the associate d interstitial fluid and ions, collectively can be thought of as a signal t ransducer, Knowledge of the nature of the mechanical and electrochemical ev ents in the extracellular matrix, and their variations with time and locati on during and after loading, is essential in the understanding of the mecha nical signal transduction mechanism(s) in chondrocytes and articular cartil age.