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Research Papers

A Mechanism to Explain Physiological Lubrication

[+] Author and Article Information
David F. James1

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto M5S 3G8, Canadadavid.james@utoronto.ca

Garret M. Fick, W. Douglas Baines

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto M5S 3G8, Canada

1

Corresponding author.

J Biomech Eng 132(7), 071002 (May 14, 2010) (6 pages) doi:10.1115/1.4001422 History: Received October 30, 2009; Revised February 15, 2010; Posted March 15, 2010; Published May 14, 2010; Online May 14, 2010

A new mechanism of physiological lubrication is proposed to explain how low-viscosity synovial fluid prevents articular surfaces from contacting and wearing. The new mechanism is based on the hypothesis that the hyaluronic acid chains in synovial fluid bind to the cartilage surfaces through electrostatic charges, with the phospholipid layer on an articular surface supplying the necessary attractive charges. The stationary hyaluronic acid network causes a large hydrodynamic resistance to outward flow from the gap. To determine the effectiveness of the network in preventing contact, squeeze-film flow between two incompressible, permeable disks is analyzed when a constant load is suddenly applied, and the solvent—synovial fluid minus the hyaluronic acid—escapes through the network and through the permeable disks. The analysis yields the approximate time for the gap distance to decrease to asperity size. For realistic physiological parameters, the time for the surfaces to contact is a minimum of several minutes and likely much longer. The role of albumin in the synovial fluid is included because the large protein molecules are trapped by the small openings in the hyaluronic acid network, which increases the flow resistance of the network and thereby delays contact of the surfaces.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Bonding of the HA chains to cartilage through the phospholipid layer. The negative charges along the polyelectrolyte bond to the positively charged (hydrophilic) heads of the phospholipid chains, and the positively charged heads on the other side of the phospholipid layer bond to the negative charges on the surface of cartilage.

Grahic Jump Location
Figure 2

Schematic of the physical situation. Two permeable, incompressible disks, of radius R and thickness H, confine synovial fluid between them. The lower one is taken to be stationary and the upper one approaches it under the suddenly applied, constant load F. As indicated in Fig. 1, the HA chains of the fluid are assumed to be stationary, so that the remaining solvent flows outward, through the chains with velocity v and through the disks with velocity V. The permeabilities of the disks, representing cartilage, and the HA network are K and k, respectively. The fluid mechanics is analyzed to find the separation distance h as a function of time.

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