Experimental determination of stress distributions in articular cartilage before and after sustained loading

Abstract

Objective. To test the hypotheses that stress concentrations exist within articular cartilage, and are intensified by sustained ‘creep’ loading. Design. Matched-pair comparison of stress distributions in cartilage, in vitro, before and after creep. Background. The ability of cartilage to equalise 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 × 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.