In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area |
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Authors: | E D Bonnevie V J Baro L Wang David L Burris |
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Affiliation: | (1) Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA;; |
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Abstract: | The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of
the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study
the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate
the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties
of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was
undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN.
Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements
of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid
normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at
high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold
difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed;
(3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid
load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of
friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced
the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering
tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact
areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective
pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions,
local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially
add important insights into the mechanical mechanisms of OA. |
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