Mechanical Characterization of Costal Cartilage

Rib fractures are a common morbidity following automotive crashes. While the rib bones are wellcharacterized, and articular cartilage from joints such as the knee and shoulder has been extensively mechanically characterized, almost no mechanical examination has been performed of the rib-cage costal cartilage. Costal cartilage is a hyaline cartilage, like articular cartilage, but it differs from articular cartilage in both structure and composition. In addition, the costal cartilage undergoes a transformation with aging, the "amianthoid change", in which the tissue becomes more tendon-like: the collagen fibrils become larger and more aligned, while the proteoglycan content is depleted. The costal cartilage can also calcify, leading to dramatic mechanical stiffening. A decrease in the mechanical compliance of aging costal cartilage, in conjunction with a decrease in bone stiffness, represents an important factor potentially associated with the increased incidence of rib fractures in elderly persons. The current study incorporates in vitro examination of the mechanical behavior of costal cartilage in conjunction with a finite element modeling study. Experimental data will be presented for young porcine costal cartilage and for tissue from adult human cadavers of different ages. Mechanical testing is performed using an indentation methodology to allow for localized probing of tissue properties, including elastic and time-dependent behavior. Examination of costal cartilage includes characterization of both the perichondrium surface layer and of the bulk cartilage properties. Mechanical property results from costal cartilage tissue are used as input for finite element simulations examining changes in the thorax response to automotive injury with aging. The information obtained in the current study has direct relevance to the development of both physical and computational surrogates for the aging thorax. Injury Biomechanics Research