title: Atomic force microscope investigation of the boundary-lubricant layer in articular cartilage.
abstract: OBJECTIVE
  To determine the roles of superficial zone protein (SZP), hyaluronan (HA), and surface-active phospholipids (SAPL) in boundary lubrication of articular cartilage through systematic enzyme digestion using trypsin, hyaluronidase, and phospolipase-C (PLC) surface treatments.
  
  
  METHODS
  The friction coefficient of articular cartilage surfaces was measured with an atomic force microscope (AFM) before and after enzyme digestion. Surface roughness, adhesion, and stiffness of the articular surface were also measured to determine the mechanism of friction in the boundary lubrication regime. Histology and transmission electron microscopy were used to visualize the surface changes of treatment groups that showed significant friction changes after enzyme digestion.
  
  
  RESULTS
  A significant increase in the friction coefficient of both load-bearing and non load-bearing regions of the joint was observed after proteolysis by trypsin. Treatment with trypsin, hyaluronidase, or PLC did not affect the surface roughness. However, trypsin treatment decreased the adhesion significantly. Results indicate that the protein component at the articular cartilage surface is the main boundary lubricant, with SZP being a primary candidate. The prevailing nanoscale deformation processes are likely plastic and/or viscoelastic in nature, suggesting that plowing is the dominant friction mechanism.
  
  
  CONCLUSIONS
  The findings of this study indicate that SZP plays an intrinsic and critical role in boundary lubrication at the articular surface of cartilage, whereas the effects of HA and SAPL on the tribological behavior are marginal.
authors.affiliation: Center for Tissue Regeneration and Repair, Department of Orthopaedic Surgery, University of California, Davis, Sacramento, CA 95817, USA. smtchan@ucdavis.edu
authors.forename: S M T
authors.initials: SM
authors.lastname: Chan
authors.forename: C P
authors.initials: CP
authors.lastname: Neu
authors.forename: G
authors.initials: G
authors.lastname: Duraine
authors.forename: K
authors.initials: K
authors.lastname: Komvopoulos
authors.forename: A H
authors.initials: AH
authors.lastname: Reddi
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001696:Biomechanical Phenomena; D002358:Cartilage, Articular / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure*; D002417:Cattle; D017276:Friction; D006133:Growth Substances / Q000378:metabolism*; D008153:Lubrication; D018625:Microscopy, Atomic Force; D011509:Proteoglycans / Q000378:metabolism*; D013582:Synovial Fluid / Q000378:metabolism / Q000502:physiology*; D000099158:Proteoglycan 4
chemical_list: D006133:Growth Substances; D011509:Proteoglycans; C498836:PRG4 protein, Bos taurus; D000099158:Proteoglycan 4