Removal of the superficial zone of bovine articular cartilage does not increase its frictional coefficient

Objective: To investigate the role of the superficial zone in regulating the frictional response of articular cartilage. This zone contains the superficial protein (SZP), a proteoglycan synthesized exclusively by superficial zone chondrocytes and implicated in reducing the friction coefficient of cartilage. Design: Unconfined compression creep tests with sliding of cartilage against glass in saline were carried out on fresh bovine cylindrical plugs (circle divide6 mm, n = 35) obtained from 16 bovine shoulderjoints (ages 1-3 months). In the first two experiments, friction tests were carried out before and after removal of the superficial zone (similar to 100 mum), in a control and treatment group, using two different applied load magnitudes (4.4 N and 22.2 N). In the third experiment, friction tests were conducted on intact surfaces and the corresponding microtomed deep zone of the same specimen. Results: In all tests the friction coefficient exhibited a transient response, increasing from a minimum value (mu(min)) to a near-equilibrium final value (mu(eq)). No statistical change (P > 0.5) was found in Amin before and after removal of the superficial zone in both experiments 1 and 2. However, mu(eq) was observed to decrease significantly (P < 0.001) after removal of the surface zone. Results from the third experiment confirm that mu(eq) is even lower at the deep zone. Surface roughness measurements with atomic force microscopy (AFM) revealed an increase in surface roughness after microtoming. Immunohistochemical staining confirmed the presence of SZP in intact specimens and its removal in microtomed specimens. Conclusions: The topmost (similar to 100 mum) superficial zone of articular cartilage does not have special properties which enhances its frictional response. (C) 2004 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.