Dependence of nanoscale friction and adhesion properties of articular
cartilage on contact load
S.M.T. Chana, C.P. Neua,1, K. Komvopoulosb,n, A.H. Reddia
aCenter for Tissue Regeneration and Repair, University of California, Davis, Medical Center, Sacramento, CA 95817, USA
bDepartment of Mechanical Engineering, University of California, Berkeley, CA 94720, USA
a r t i c l e i n f o
Accepted 5 January 2011
a b s t r a c t
Boundary lubrication of articular cartilage by conformal, molecularly thin films reduces friction and
adhesion between asperities at the cartilage–cartilage contact interface when the contact conditions
are not conducive to fluid film lubrication. In this study, the nanoscale friction and adhesion properties
of articular cartilage from typical load-bearing and non-load-bearing joint regions were studied in the
boundary lubrication regime under a range of physiological contact pressures using an atomic force
microscope (AFM). Adhesion of load-bearing cartilage was found to be much lower than that of non-
load-bearing cartilage. In addition, load-bearing cartilage demonstrated steady and low friction
coefficient through the entire load range examined, whereas non-load-bearing cartilage showed higher
friction coefficient that decreased nonlinearly with increasing normal load. AFM imaging and roughness
calculations indicated that the above trends in the nanotribological properties of cartilage are not due
to topographical (roughness) differences. However, immunohistochemistry revealed consistently
higher surface concentration of boundary lubricant at load-bearing joint regions. The results of this
study suggest that under contact conditions leading to joint starvation from fluid lubrication, the higher
content of boundary lubricant at load-bearing cartilage sites preserves synovial joint function by
minimizing adhesion and wear at asperity microcontacts, which are precursors for tissue degeneration.
& 2011 Elsevier Ltd. All rights reserved.
The lubrication function of articular cartilage is essential for
maintaining tissue health and preventing disease and damage such
as osteoarthritis. While articular cartilage is lubricated by mixed
modes of lubrication (Lewis and McCutchen, 1959; Walker et al.,
1968; Unsworth, 1991; Gleghorn and Bonassar, 2008) in the pres-
ence of synovial fluid (Schmidt et al., 2007), boundary lubrication
occurs under contact conditions of high load, low sliding speed, and/
or reduced fluid viscosity. Under such interfacial conditions, the
asperities on the opposing surfaces are separated only by a molec-
ularly thin lubricant layer (Gleghorn and Bonassar, 2008), hereafter
referred to as a boundary film. In most sliding systems operating
under a range of loads and speeds, such as synovial joints where
mixed modes of lubrication are encountered during a walking cycle,
a sufficiently thick fluid film cannot be continuously maintained
between the articulating surfaces. As a consequence, contact occurs
at the higher summits of the countersurfaces, known as asperities. In
the absence of fluid film lubrication, for example, when the sliding
speed decreases to zero for motion reversal, the tribological behavior
of reciprocating systems is controlled by asperity contact interac-
tions (Bronzino, 2006; Neu et al., 2008; Winter, 2009). Thus,
boundary lubrication conditions are unavoidable in synovial joints.
The formation of a lubricious boundary film can be viewed as a last
line of surface protection against solid-solid contact, which is
precursor for high friction and excessive wear. Indeed, precocious
joint degeneration has been observed in the absence of an effective
boundary lubricant film (Marcelino et al., 1999; Jay et al., 2007).
The study of cartilage at the nano/microscale is important for
understanding the role of molecular boundary films in cartilage
tribology. However, the effect of the removal of boundary films and
other proteins during sliding on the friction properties of cartilage
has largely been ignored in macroscale friction studies. Although the
presence of boundary films is an important factor in measuring
friction at any scale (Jay et al., 2007; Gleghorn et al., 2009; Chan
et al., 2010), continuous sliding of cartilage in macroscale friction
tests removes not only molecularly thin boundary films, but also
other proteins including collagen (Stachowiak et al., 1994; Gleghorn
et al., 2010). Furthermore, in friction studies performed with
microprobe-based instruments such as the atomic force microscope
(AFM), the cartilage surface is not subjected continuously to normal
and shear loading, whereas in macroscale studies traditionally
performed with a tribometer, the cartilage surface is continuously
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nCorresponding author. Tel.: +1 510 642 2563; fax: +1 510 643 5599.
E-mail address: email@example.com (K. Komvopoulos).
1Present address: Weldon School of Biomedical Engineering, Purdue
University, IN 47907, USA.
Journal of Biomechanics 44 (2011) 1340–1345
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