Experimental determination of stress distributions in articular cartilage before and after sustained loading.
OBJECTIVE
To test the hypotheses that stress concentrations exist within articular cartilage, and are intensified by sustained 'creep' loading.


METHODS
Matched-pair comparison of stress distributions in cartilage, in vitro, before and after creep.


BACKGROUND
The ability of cartilage to equalize contact stresses between articulating surfaces may be compromised by undulations in the subchondral bone, and by variations in chemical composition. Furthermore, any stress concentrations within cartilage may be affected by creep loading, which-reduces its water content.


METHODS
Sixteen specimens of apparently normal cartilage-on-bone, 12 mm x 15 mm, were removed from the femoral condyles and tibial plateaux of patients undergoing total knee replacement. The cartilage was subjected to a nominal compressive stress of 2 MPa by means of a 10 mm-diameter flat impermeable indentor. During the 20 s loading period, a miniature pressure transducer, side-mounted in a 0.9 mm-diameter needle, was pulled through the cartilage in a direction parallel to the surface, while transducer output and position were sampled at 25 Hz. 'Stress profiles' were obtained with the transducer pointing in the 12 o'clock and 3 o'clock directions, and were repeated after creep loading at 1.6 MPa for 2 h.


RESULTS
Validation tests indicated that transducer output was proportional to the average compressive stress, but overestimated it by 9-15%. Stresses were greatest under the centre of the indentor, and showed local variations ('concentrations') which were increased in number and size after creep loading.


CONCLUSIONS
Measured values of compressive stress incorporate small systematic errors. Nevertheless, the results presented clearly support both hypotheses.
Adams
M A
MA
Department of Anatomy, University of Bristol, UK. M.A.Adams@bris.ac.uk
Kerin
A J
AJ
Bhatia
L S
LS
Chakrabarty
G
G
Dolan
P
P
D000368:Aged; D001696:Biomechanical Phenomena; D002358:Cartilage, Articular / Q000502:physiology*; D004548:Elasticity; D005260:Female; D005269:Femur / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D008592:Menisci, Tibial / Q000502:physiology; D008875:Middle Aged; D008962:Models, Theoretical; D015203:Reproducibility of Results; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology*