CHI, K.-J.; Duke Univ., Durham, NC: Functional Implications of the Mechanical Inhomogeneity in Mammalian Paw Pads

Mammalian paw pads have well-developed subdermal adipose tissue partitioned into chambers by fibrous membranes; hence they can be viewed as a hydrostatic support. Morphological studies show that pad tissue is inhomogeneous with the general trend that chamber size increases and wall thickness decreases the closer they are in location to the metacarpal- (or metatarsal-) phalangeal (MCP or MTP) joint. Structural analysis suggests that the compressive stiffness (E) of a hydrostatic support is proportional to the ratio of wall thickness to chamber diameter. In this study, I hypothesized that E of pad tissue shows inhomogeneity dictated by morphology; more specifically, I predicted that E decreases toward the MCP (or MTP) joint. This hypothesis was tested on 5 paw pads: left fore and hind pads from 2 dogs and one left hind pad from 1 captive spotted hyena. Each paw pad was first cut in the frontal plane into three layers, each 4 mm thick. Tissue cores 5 mm in diameter were sampled from each layer and kept individually for quasi-static tests with a maximal compressive strain of 0.5 at 0.01mm/s. Stress-strain data were curve-fitted, from which E was calculated as the slope at strain= 0.5. Results suggest E has the lowest value near the MCP (or MTP) joint, and increases eccentrically (2x-16x within layer) and ventrally (6-16x, between layers). For samples from equivalent locations, hind pads have greater E than fore pads (4.6x), and the hyena pad has greater E than dog pads (6x). These results confirm the mechanical inhomogeneity within a paw pad, which may relate to pad function during locomotion: location with lower E is for braking while that with greater E is for force transmission (propulsion). The greater E found in pads of the hyena and the larger dog gives insights into how pad mechanics scale with body size.