Variations in the mechanical properties of cartilage from the canine scapulohumeral joint

Objective - To measure the intrinsic material properties of scapulohumeral joint cartilage in adult dogs and determine whether regional differences exist within or between the humeral and glenoid cartilages. Samples - Paired shoulder joints from 7 clinically normal adult dogs. Procedure - An automated indentation apparatus was used to obtain the intrinsic mechanical properties of the cartilage at 7 sites on each joint surface. Results - Topographic variations in mechanical properties of the glenoid and humeral cartilages were observed. The largest aggregate modulus (H_A) for the humerus was seen at the caudocentral site (0.92 MPa) and for the scapula was seen at the centrocenter site (0.84 MPa). The mean shear modulus (μ) of humeral cartilage (0.23 MPa) was significantly greater than that of the glenoid cartilage (0.19 MPa). The mean Poisson's ratio (ν) of humeral cartilage (0.24) was significantly smaller than that for the glenoid cartilage (0.29). Mean humeral cartilage aggregate modulus (0.71 MPa) was larger than the value for glenoid cartilage (0.67 MPa), but these differences were not significant. There were no significant differences in the compressive stiffness of the opposing cartilage in the canine scapulohumeral joint. Conclusions - Differences in mechanical properties between opposing humeral and glenoid cartilages are not a cause of cartilage injury in the scapulohumeral joint of adult dogs. The mechanical properties of cartilage from young dogs with open physes and incomplete subchondral bone plates may be different from those of adult dogs.