Plenary Speaker: Van Mow, Ph.D.
Stanley Dicker Professor of Biomedical Engineering and Orthopaedic Bioengineering, Director, Liu Ping Laboratory for Functional Tissue Engineering Research, Chair, Department of Biomedical Engineering

3.45 PM - 4.30 PM, August 31, 2006

Abstract:
Articular cartilage is the load-bearing tissue within all freely moving joints of mammals, i.e., the diarthrodial joints such as hips, knees, shoulders, etc.  All diarthrodial joints must support loads of high magnitude, and function with a remarkably low coefficient friction even with the generally slow reciprocating motions.  For example, in the knee or hip, the magnitude of loading may reach higher than 15x body weight, with a normal stress up to 20 MPa acting on its articulating surfaces.  Even the shoulder, generally considered as a non-weight bearing joint, but it is actually not a non-load bearing joint.  Due to the lever law effect, there is a 20 to 1 disadvantage; thus a 10N load carried by an outstretched arm may be magnified to 200N acting across the glenohumeral joint of the shoulder.  Similarly, in the patello-femoral joint (PFJ) of the knee, again with an approximate 20 to 1 disadvantage, the force and stress levels acting across the PFJ may reach similar magnitudes.  In addition, these loads are applied, in a normal young vigorous individual, about one million times a year, with a cyclic frequency usually less than 1Hz.  For athletes, these operational mechanical requirements are increased many times.  It is no wonder that for some unlucky individuals, they develop arthritis in the hip and knee (most frequently); this is a form of failure in these natural bearings.  For any artificial material (whether it is plastic or stainless steel) used in joint prostheses to replace these failed biological bearings, prostheses failure often occur.  Tissue engineered constructs planned for replacing of damaged cartilage or resurfacing the joint surfaces have not met these demanding functional requirements; indeed, all tissue engineered cartilage have much inferior properties when compared to normal healthy cartilage, and indeed even inferior when compared with cartilage obtained from necropies of osteoarthritic joints.  Thus, there appears to be something special of natural articular cartilage that renders it to function for many decades with no signs of impairment.  What that something is has been the focus of worldwide attention for many years.

In brief, articular cartilage (and most biological tissues) is a hierarchical material with specific micro- and ultra-structural architectural feature variations that spans 8 decades of dimensional scale.  Over the years, it has been established from much basic biochemistry studies that the nano-scale structures of glycosaminoglycan and tropo-collagen molecular form at the 10^-9 to 10^-8m important interactions in determining the physical properties of these fundamental building blocks of the solid organic matrix of the tissue. At two orders of magnitude up, from 10^-7 to 10^-6m, i.e., at the ultra-scale level, the physical interactions resulting from the complex organizations of the proteoglycans and collagen network are important in determining the cohesiveness and strength of the porous-permeable matrix. At the micro- and meso- scale, 10^-5 to 10^-3m interactions between cells and their extracellular matrix are important in the mechano-transduction of mechanical and physical signals that modulates biosyntheses of all the constituents that comprise the tissue; these constituents form the tissues that must function within our bodies at the macro-scale, e.g., hips, knees, shoulders, etc. These elemental components form the structural anisotropies and compositional inhomogenieties that afford the tissue with a wide variety of complex mechano-electrochemical phenomena, which in turn endow this tissue not only with intriguing material properties, but also make possible their function in the strenuous mechanical environments normally found in all diarthrodial joints, as the superb bearing materials that we all know as articular cartilage.  This lecture will provide a summary of our current knowledge of some of these burgeoning fields under the rubric of biomechanics, and looks to new and challenging problems of study in functional tissue engineering  toward finding an answer(s) to the etiology of osteoarthritis and repair of damaged joint surfaces.

References
The reader may wish to consult the following texts for more information:

Brandt KD, Doherty M, Lomander LS (eds): Osteoarthritis, Oxford University Press, 2000; pp511

Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, American Academy of Orthopaedic Surgeons, Rosemont, IL 2000, pp872

Guilak F, Butler DL, Goldstein SA, Mooney DJ (eds): Functional Tissue Engineering, Springer-Verlag, Inc, New York, 2003, pp426

Mow VC, Huiskes R (eds):  Basic Orthopaedic Biomechanics and Mechano-biology, 3rd Edition, Lippincott Williams & Wilkins, Philadelphia, 2005, pp720