title: The micromechanics of the superficial zone of articular cartilage.
abstract: OBJECTIVE
  To investigate the relationships between the unique mechanical and structural properties of the superficial zone of articular cartilage on the microscopic scale.
  
  
  METHODS
  Fresh unstained equine metacarpophalangeal cartilage samples were mounted on tensile and compressive loading rigs on the stage of a multiphoton microscope. Sequential image stacks were acquired under incremental loads together with simultaneous measurements of the applied stress and strain. Second harmonic generation was used to visualise the collagen fibre network, while two photon fluorescence was used to visualise elastin fibres and cells. The changes visualised by each modality were tracked between successive loads.
  
  
  RESULTS
  The deformation of the cartilage matrix was heterogeneous on the microscopic length scale. This was evident from local strain maps, which showed shearing between different regions of collagen under tensile strain, corrugations in the articular surface at higher tensile strains and a non-uniform distribution of compressive strain in the axial direction. Chondrocytes elongated and rotated under tensile strain and were compressed in the axial direction under compressive load. The magnitude of deformation varied between cells, indicating differences in either load transmission through the matrix or the mechanical properties of individual cells. Under tensile loading the reorganisation of the elastin network differed from a homogeneous elastic response, indicating that it forms a functional structure.
  
  
  CONCLUSIONS
  This study highlights the complexity of superficial zone mechanics and demonstrates that the response of the collagen matrix, elastin fibres and chondrocytes are all heterogeneous on the microscopic scale.
authors.affiliation: College of Engineering Mathematics and Physical Sciences, University of Exeter, UK. Electronic address: j.c.mansfield@exeter.ac.uk.
authors.forename: J C
authors.initials: JC
authors.lastname: Mansfield
authors.affiliation: College of Engineering Mathematics and Physical Sciences, University of Exeter, UK.
authors.forename: J S
authors.initials: JS
authors.lastname: Bell
authors.affiliation: College of Engineering Mathematics and Physical Sciences, University of Exeter, UK.
authors.forename: C P
authors.initials: CP
authors.lastname: Winlove
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002358:Cartilage, Articular / Q000502:physiology*; D019902:Chondrocytes / Q000502:physiology*; D003094:Collagen / Q000502:physiology; D019245:Compressive Strength; D004549:Elastin / Q000502:physiology; D005109:Extracellular Matrix / Q000502:physiology*; D006736:Horses; D008662:Metacarpophalangeal Joint; D036641:Microscopy, Fluorescence, Multiphoton; D013314:Stress, Mechanical; D013718:Tensile Strength; D016474:Weight-Bearing / Q000502:physiology*
chemical_list: D003094:Collagen; D004549:Elastin
keywords: Articular cartilage; Elastin; Micromechanics; Second harmonic generation; Two photon fluorescence