Distal oblique bundle influence on distal radioulnar joint stability: a biomechanical study.

Chronic instability of the distal radioulnar joint (DRUJ) presents a highly disabling condition. Several surgical techniques have been reported for its treatment. These involve reconstruction of the distal oblique bundle (DOB) of the interosseous membrane (IOM) of the forearm. The aim of this study was to examine whether surgical reconstruction of the DOB is necessary to restore DRUJ stability following trauma with DOB disruption and to compare two restoration techniques utilizing a tendon or suture-button graft. Stability in supination and pronation was assessed by means of maximum torque and force in twenty forearms. Test cycles were performed with the DOB/IOM in an intact condition, with the DOB or distal IOM transected, and following surgical reconstruction of the DOB with either tendon graft or suture-button system. In pronation, the relative change in maximum axial force was significantly lower in samples with a transected DOB in comparison to samples without a preexisting DOB. No statistically significant differences were observed between forearms including DOB reconstruction and specimens in the intact and transected state. Neither were there statistically significant differences concerning the two surgical techniques. From a biomechanical perspective, surgical DOB reconstruction is hence not indicated in cases of isolated DOB rupture.

Dynamic load response of human dura mater at different velocities.

Despite of its assumed role to mitigate brain tissue response under dynamic loading conditions, the human dura mater is frequently neglected in computational and physical human head models. A reason for this is the lack of load-deformation data when the dura mater is loaded dynamically. To date, the biomechanical characterization of the human dura mater predominantly involved quasi-static testing setups. This study aimed to investigate the strain rate-dependent mechanical properties of the human dura mater comparing three different velocities of 0.3, 0.5 and 0.7 m/s. Samples were chosen in a perpendicular orientation to the visible main fiber direction on the samples' surface, which was mostly neglected in previous studies. The elastic modulus of dura mater significantly increased at higher velocities (5.16 [3.38; 7.27] MPa at 0.3 m/s versus 44.38 [35.30; 74.94] MPa at 0.7 m/s). Both the stretch at yield point λf (1.148 [1.137; 1.188] for 0.3 m/s, 1.062 [1.054; 1.066] for 0.5 m/s and 1.015 [1.012; 1.021] for 0.7 m/s) and stress at yield point σf of dura mater (519.14 [366.74; 707.99] kPa for 0.3 m/s versus 300.52 [245.31; 354.89] kPa at 0.7 m/s) significantly decreased with increasing velocities. Conclusively, increasing the load application velocity increases stiffness and decreases tensile strength as well as straining potential of human dura mater between 0.3 and 0.7 m/s. The elastic modulus of human dura mater should be adapted to the respective velocities in computational head impact simulations.

Micromechanically-motivated analysis of fibrous tissue.

Collagen fibers are the main load bearing component in fibrous tissues. Systematic analyses of their structure and orientation are thus crucial for the development of material models that enable to predict the mechanical tissue response. To this end, biaxial tests at different stretch ratios were performed on two tissue samples of the medial layer extracted from a human aorta. The tissues were loaded in the circumferential and axial directions simultaneously. We develop here a micromechanical model which is based on structural parameters of collagen fibers that were extracted from second-harmonic generation images of the two samples. The tissue is modeled as a periodic six-layered laminate in which the individual layers are treated as periodic fibrous structures with one family of fibers. We make use of the Hill-Mandel theory in the context of periodic homogenization to determine the overall mechanical tissue response. Both the analytical and numerical models are able to capture the overall mechanical response of the two tissue samples using a straightforward representation of the tissue structure together with a limited set of material parameters. Up to 10% of strains the model captures the almost linear response of both tissue samples. Beyond that stretch level the stiffening of the tissues becomes more evident, especially in the circumferential direction. In cases where the axial stretch is larger than the circumferential stretch the predictions are somewhat stiffer, while a very good agreement is obtained when the circumferential stretch is dominant. The stiffening of one tissue sample was substantially larger than the other, implying that higher-order stiffening mechanisms may kick in at larger strains. Our sensitivity analyses reveal that the parameters of the material model and the fiber dispersion have a minor effect on the tissue response. The novel modeling approach has the potential to reduce the need of time-consuming experimental data of the mechanical behavior of fibrous tissues.