Behind Shield Blunt Trauma: Characterizing the Back-Face Deformation of Shields with a Focus on Upper Limb Loading.

Shield back-face deformation (BFD) is the result of composite ballistic shields deflecting or absorbing a projectile's energy and deforming towards the user. BFD can result in localized loading to the upper extremity, where the shield is secured to the user. An augmented anthropomorphic test device upper extremity was used to quantify this applied load. Four locations along the upper extremity were tested-the hand, wrist, forearm, and elbow-for investigating differing boundary conditions and their effect on resultant load. Varying stand-off distances, the distance between the back of the shield and the force sensor, were investigated. Digital image correlation was also conducted to measure the dynamic displacement of the shield. The mean peak back-face velocity of the shield was 208.4 ± 38.8 m/s, while the average affected area was 1505 ± 158.3 mm2. Impulse was not significantly affected by anatomical location for the same stand-off distance; however, as stand-off distance decreased, the measured force significantly increased (p < 0.05). Notably, impact duration did not differ significantly for any of the impact scenarios. This is the first step in developing injury criteria for this region resulting from behind shield blunt trauma, and these data will be used for developing injury thresholds in post-mortem human surrogates.

Defects in articular cartilage metabolism and early arthritis in fibroblast growth factor receptor 3 deficient mice.

Fibroblast growth factor (FGF) receptor 3 has been identified as a key regulator of endochondral bone development and of post-natal bone metabolism through its action on growth plate chondrocytes and osteoblasts, respectively. It has also been shown to promote chondrogenesis and cartilage production by cultured pre-chondrogenic cells in response to FGF18. In the current studies, we show that the absence of signaling through Fgfr3 in the joints of Fgfr3(-/-) mice leads to premature cartilage degeneration and early arthritis. Degenerative changes in cartilage matrix included excessive proteolysis of aggrecan core protein and type II collagen, as measured by neo-epitope immunoreactivity. These changes were accompanied by increased expression of metalloproteinase MMP13, type X collagen, cellular hypertrophy and loss of proteoglycan at the articular surface. Using a novel micro-mechanical indentation protocol, it was shown that articular cartilage in the humeral head of 4-month-old Fgfr3(-/-) mice was less resistant to compressive force and less stiff than that of littermate controls. These results identify Fgfr3 signaling as a potential target for intervention in degenerative disorders of cartilage metabolism.

Tetrapolar measurement of electrical conductivity and thickness of articular cartilage.

A tetrapolar method to measure electrical conductivity of cartilage and bone, and to estimate the thickness of articular cartilage attached to bone, was developed. We determined the electrical conductivity of humeral head bovine articular cartilage and subchondral bone from a 1- to 2-year-old steer to be 1.14+/-0.11 S/m (mean+/-sd, n =11) and 0.306+/-0.034 S/m, (mean+/-sd, n =3), respectively. For a 4-year-old cow, articular cartilage and subchondral bone electrical conductivity were 0.88+/-0.08 S/m (mean+/-sd, n =9) and 0.179+/-0.046 S/m (mean+/-sd, n =3), respectively. Measurements on slices of cartilage taken from different distances from the articular surface of the steer did not reveal significant depth-dependence of electrical conductivity. We were able to estimate the thickness of articular cartilage with reasonable precision (<20% error) by injecting current from multiple electrode pairs with different inter-electrode distances. Requirements for the precision of this method to measure cartilage thickness include the presence of a distinct layer of calcified cartilage or bone with a much lower electrical conductivity than that of uncalcified articular cartilage, and the use of inter-electrode distances of the current injecting electrodes that are on the order of the cartilage thickness. These or similar methods present an attractive approach to the non-destructive determination of cartilage thickness, a parameter that is required in order to estimate functional properties of cartilage attached to bone, and evaluate the need for therapeutic interventions in arthritis.