Indentation stiffness of aging human costal cartilage

Costal cartilage, connecting the ribs and sternum, serves a mechanical function in the body. It undergoes structural changes with aging but it is unclear if its material properties are affected by these changes. To investigate this question, experimental indentation load-relaxation tests were performed on human costal cartilage as a function of specimen age and sex. The experimental data were fit to spherical indentation ramp-relaxation solutions generated previously by elastic-viscoelastic correspondence [Mattice JM, Lau AG, Oyen ML and Kent RW. Spherical indentation load-relaxation of soft biological tissues. J Mater Res 2006;8:2003-10]. Numerical values of short- and long-time shear modulus and of material time-constants were examined as a function of age. Costal cartilage calcification was assessed with blinded scoring of computed tomography reconstructions of the ribcage and mechanical properties were correlated with calcification score. Overall, the costal cartilage midsubstance was slightly stiffer than articular cartilage, and did not show significant variation in stiffness with age or specimen calcification. Increased age did result in increased local variability of the indentation stiffness results. Future studies will be required to address the findings of the current study that although calcification did increase with age, the calcification was primarily found on the costal cartilage periphery, thus insignificantly affecting the midsubstance stiffness.