Computational analysis of viscoelastic properties of crosslinked actin networks

Mechanical force plays an important role in the physiology of eukaryotic cells whose dominant structural constituent is the actin cytoskeleton composed mainly of actin and actin crosslinking proteins (ACPs). Thus, knowledge of rheological properties of actin networks is crucial for understanding the mechanics and processes of cells. We used Brownian dynamics simulations to study the viscoelasticity of crosslinked actin networks. Two methods were employed, bulk rheology and segment-tracking rheology, where the former measures the stress in response to an applied shear strain, and the latter analyzes thermal fluctuations of individual actin segments of the network. It was demonstrated that the storage shear modulus (G′) increases more by the addition of ACPs that form orthogonal crosslinks than by those that form parallel bundles. In networks with orthogonal crosslinks, as crosslink density increases, the power law exponent of G′ as a function of the oscillation frequency decreases from 0.75, which reflects the transverse thermal motion of actin filaments, to near zero at low frequency. Under increasing prestrain, the network becomes more elastic, and three regimes of behavior are observed, each dominated by different mechanisms: bending of actin filaments, bending of ACPs, and at the highest prestrain tested (55%), stretching of actin filaments and ACPs. In the last case, only a small portion of actin filaments connected via highly stressed ACPs support the strain. We thus introduce the concept of a ‘supportive framework,’ as a subset of the full network, which is responsible for high elasticity. Notably, entropic effects due to thermal fluctuations appear to be important only at relatively low prestrains and when the average crosslinking distance is comparable to or greater than the persistence length of the filament. Taken together, our results suggest that viscoelasticity of the actin network is attributable to different mechanisms depending on the amount of prestrain.National Institutes of Health (GM076689

Is radial shortening useful for Litchman stage 3B Kienbock’s disease?

Treatment of Litchman stage 3 Kienböck’s disease is still controversial. In this study our aim was to evaluate the effectiveness of radial shortening on stage 3B Kienböck’s disease in comparison with stage 3A cases. Radial shortening was performed for 23 patients who had stage 3A (group I, n = 13) and 3B (group II, n = 10) Kienböck’s disease between 1994 and 2004. The radial osteotomy was performed 4.5 cm proximal to the distal articular surface. The mean shortening was 2.6 mm (range 2 to 4.5). The average follow-up period was 85 months (range 26–147). Based on the modified Nakamura system, the mean clinical points were 14.3 in group I and 13.3 in group II. There was no statistical difference between both groups with regard to clinical points (P = 0.483). The extension-flexion arc showed significant improvement in both groups. Based on the results of this long-term follow-up study, we concluded that radial shortening osteotomy can be performed in the treatment of type 3B Kienböck’s disease as reliably as type 3A, despite the lack of evident radiological improvement