Objective. Osteoarthritis (OA) is characterized by progressive degeneration of articular cartilage and remodeling of the subchondral bone plate, comprising calcified cartilage and underlying subchondral bone. Calcified cartilage remodeling due to upward invasion by vascular canals or to calcified cartilage erosion may contribute to biomechanical alteration of the osteochondral tissue and its subchondral bone plate component. The study hypothesis was that hydraulic conductance of osteochondral tissue and subchondral bone plate increases with structural changes indicative of increasing stages of OA.Methods. Osteochondral cores were harvested from the knees of cadaveric tissue donors and from discarded fragments from patients with OA undergoing knee surgery. The osteochondral cores from tissue donors were macroscopically normal, and the cores from patients with OA had partial-thickness or full-thickness erosion to bone. The cores were perfusion-tested to determine the hydraulic conductance, or ease of fluid flow, in their native state and after enzymatic removal of cartilage. Adjacent portions were analyzed by 3-dimensional histology for calcified cartilage, subchondral bone, and subchondral bone plate thickness and vascular canal density.Results.

Limited proteolysis of the tubulin dimer (αβ) by subtilisin occurs more rapidly with β than with α tubulin. This leads to the formation of an intermediate hybrid dimer, αβs, before both C termini are cleaved to form tubulin S(αsβs). The three forms of tubulin usually coexist in subtilisin‐treated preparations and such cross‐contamination can be reliably detected only by running SDS‐polyacrylamide gels well beyond expulsion of the dye front. Previously published preparations have not ruled out such contamination or have formed poorly reversible polymers. Because ion exchange separation incurred substantial protein losses, we have developed a new protocol for rapid preparation of tubulin S (αsβs, free of αβ or αβs) that is based on proteolysis at low ionic strength. This increases the relative rate of C terminal cleavage of β tubulin. The product forms sheets, bundles, or rings that are depolymerized by cold, salt, and podophyllotoxin, partially depolymerized by Ca2+, and has a decreased critical concentration for polymerization that can be further decreased by taxol. We have also found a method for forming nearly pure αβs dimers by using methods that retard proteolysis of the C terminus of α tubulin. Cell Motil. Cytoskeleton 43:63–71, 1999. Published 1999 Wiley‐Liss, Inc.

These results indicate that mild wear (i.e., wear-line formation) at the articular surface has deleterious functional effects on articular cartilage and represent an early aging-associated degenerative change. The identification and recognition of functional biomechanical consequences of wear-lines are useful for planning and interpreting tensile biomechanical tests in human articular cartilage.

Introduction:The zone of calcified cartilage (ZCC) anchors articular cartilage (AC) to subchondral bone through a layer of intermediate stiffness. The regulation and functional consequences of cartilage calcification may vary with depth from the articular surface. The hypothesis of this study was that the in vitro calcification of immature AC occurs selectively in the deep region and is associated with a local increase in stiffness.Methods:AC and growth plate cartilage (GPC) from calves were incubated in DMEM, 1% fetal bovine serum, 100 µg/mL ascorbate, and ±10 mM β-glycerophosphate (βGP) for up to 3 weeks. To assess the time course and effects of cell viability and βGP, full-depth strips of AC and GPC were analyzed by histology, indentation, and 45Ca++ uptake. To assess the effect of tissue zone, disks harvested from surface and deep zone AC and from reserve and hypertrophic zone of GPC were incubated independently and analyzed by compression and for 45Ca++ uptake and biochemical components.Results:The deep ~20% of immature AC calcified within 3 weeks, with calcification dependent on cell viability and βGP. Mineral was deposited continuously around cells in AC but only between cell columns in GPC. The deep zone of AC exhibited a compressive modulus of 0.53 MPa after βGP-induced calcification, ~4-fold stiffer than AC incubated without βGP.Conclusions:Cartilage explants exhibit inherent zone-specific calcification processes, resulting in an increase in stiffness associated with cartilage calcification. Such properties may be useful for engineering a biomimetic ZCC tissue to integrate cartilaginous tissue to bone, thereby forming a mechanically functional osteochondral unit.