Age-dependence of the Pattern of Cartilage Electromechanical Properties in the Ovine Stifle Joint

Methods: All articular surfaces were obtained from closed-joints from animal with no indications of joint pathology. Electromechanical properties were measured manually using the Arthro-BSTTM, in vitro, on visually normal tibial plateau, femoral condyle, trochlea and patellar cartilage surfaces of ovine (n=4) stifle joints with visually normal cartilage. Two sheep were mature (older than 2 years old) and two lamb were immature (younger than 1 year old). Average electromechanical mappings were done with interpolation of theraw measurement and a moving average of four nearest neighbor points.

Results: The distribution pattern of electromechanical properties on mature articular surfaces had large spatial variation while it was more homogenous on immature surfaces. The medial and lateral compartments were becoming heterogeneous at a young age (Fig.1) whereas the patellofemoral compartment was homogenous (Fig.2). Visually, the color of the young articular cartilage was blue white while the older surfaces were slightly yellow. The immature articular cartilage was also considerably thicker than the mature. A one-way ANOVA was conducted to assess the effect of age (Fig.3). Sidak’s multiple comparisons test showed two regions of the femoral condyles to be significantly different with age (p<0.05 and p<0.0001 respectively), the lateral central central (position 2) and the medial central central (position 3) regions. On the tibial plateau, only one region is significantly different (p<0.001) with age - the lateral central central region (position 7). On the trochlea, the central proximal (position 9) region is significantly different (p<0.01) and on the patella, the central central (position 13) region is significantly different with age as well (p<0.0001). One-way ANOVA showed a significant difference in the QP value of articular surfaces between animals of different age (p<0.0001).

Discussion: There is a marked age-dependence in the electromechanical properties of articular cartilage. This difference arises from changes in biochemical composition and structural arrangement of extracellular matrix constituents of articular cartilage. Articular cartilage is also thicker and more cellular in younger specimens than in adult tissue. Mature articular cartilage is also characterized by a depth-dependent structure and composition of the extracellular matrix [3]. This age-related increase in spatial patterning of composition and architecture of articular cartilage is well known in terms of depth-dependence; however our data suggest an increased patterning orthogonally across the articular surface, as well. In addition, the articular cartilage in the medial and lateral knee compartment of the tibial plateau starts to be topographically defined earlier than the articular cartilage in the patellofemoral compartment. This may be due to larger weight-bearing forces on these compartments than on the patellofemoral compartment. In summary, the pattern of electromechanical properties is homogeneous in immature articular cartilage and becomes more topographically defined with age.

Significance: Distinguishing articular cartilage aging versus disease and assessment of the quality of cartilage repair requires detailed knowledge of spatial variations of electromechanical properties, as this information can provide diagnostic and structure-function relationships. Acknowledgements: Funding provided by the National Sciences and Engineering Research Council (NSERC) and the Fonds québécois de la recherche sur la nature et les technologies (FQRNT).