Angela Lin

Essential Role of Extracellular SOD in Reparative Neovascularization Induced by Hindlimb Ischemia

Neovascularization is an important physiological repair mechanism in response to ischemic injury, and its process is dependent on reactive oxygen species (ROS). Overproduction of superoxide anion (O2·−) rather contributes to various cardiovascular diseases. The extracellular superoxide dismutase (ecSOD) is one of the major antioxidant enzymes against O2·− in blood vessels; however, its role in neovascularization induced by tissue ischemia is unknown. Here we show that hindlimb ischemia of mice stimulates a significant increase in ecSOD activity in ischemic tissues where ecSOD protein is highly expressed at arterioles. In mice lacking ecSOD, ischemia-induced increase in blood flow recovery, collateral vessel formation, and capillary density are significantly inhibited. Impaired neovascularization in ecSOD−/− mice is associated with enhanced O2·− production, TUNEL-positive apoptotic cells and decreased levels of NO2−/NO3− and cGMP in ischemic tissues as compared with wild-type mice, and it is rescued by infusion of the SOD mimetic tempol. Recruitment of inflammatory cells into ischemic tissues as well as numbers of inflammatory cells and endothelial progenitor cells (c-kit+/CD31+ cells) in both peripheral blood and bone marrow (BM) are significantly reduced in these knockout mice. Of note, ecSOD expression is markedly increased in BM after ischemia. NO2−/NO3− and cGMP levels are decreased in ecSOD−/− BM. Transplantation of wild-type BM into ecSOD−/− mice rescues the defective neovascularization. Thus, ecSOD in BM and ischemic tissues induced by hindlimb ischemia may represent an important compensatory mechanism that blunts the overproduction of O2·−, which may contribute to reparative neovascularization in response to ischemic injury.

Pullout strength of suture anchors: Effect of mechanical properties of trabecular bone

This study investigated the relationships between trabecular microstructure and elastic modulus, compressive strength, and suture anchor pullout strength. Twelve fresh-frozen humeri underwent mechanical testing followed by micro-computed tomography (microCT). Either compression testing of cylindrical bone samples or pullout testing using an Arthrex 5mm Corkscrew was performed in synthetic sawbone or at specific locations in the humerus such as the greater tuberosity, lesser tuberosity, and humeral head. Synthetic sawbone underwent identical mechanical testing and microCT analysis. Bone volume fraction (BVF), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), and connectivity density were compared against modulus, compressive strength, and pullout strength in both materials. In cadaveric bone, modulus showed correlations to all of the microstructural properties, while compressive and pullout strength were only correlated to BVF, SMI, and TbSp. The microstructure of synthetic bone differed from cadaveric bone as SMI and TbTh showed little variation across the densities tested. Therefore, SMI and TbTh were the only microstructural properties that did not show correlations to the mechanical properties tested in synthetic bone. This study helps identify key microstructure-property relationships in cadaveric and synthetic bone as well as illustrate the similarities and differences between cadaveric and synthetic bone as biomechanical test materials.

The effect of the trabecular microstructure on the pullout strength of suture anchors

This study investigates how the microstructural properties of trabecular bone affect suture anchor performance. Seven fresh-frozen humeri were tested for pullout strength with a 5mm Arthrex Corkscrew in the greater tuberosity, lesser tuberosity, and humeral head. Micro-computed tomography analysis was performed in the three regions of interest directly adjacent to individual pullout experiments. The morphometric properties of bone mineral density (BMD), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbS), trabecular number (TbN), and connectivity density were compared against suture anchor pullout strength. BMD (r=0.64), SMI (r=-0.81), and TbTh (r=0.71) showed linear correlations to the pullout strength of the suture anchor with p-values<0.0001. A predictive model was developed to explain the variances in the individual BMD, SMI, and TbTh correlations. The multi-variant model of pullout strength showed a stronger relationship (r=0.86) compared to the individual experimental results. This study helps confirm BMD is a major influence on the pullout strength of suture anchors, but also illustrates the importance of local microstructure in pullout resistance of suture anchors.

Thermo-mechanical behavior and structure of melt blown shape-memory polyurethane nonwovens.

New processing methods for shape-memory polymers allow for tailoring material properties for numerous applications. Shape-memory nonwovens have been previously electrospun, but melt blow processing has yet to be evaluated. In order to determine the process parameters affecting shape-memory behavior, this study examined the effect of air pressure and collector speed on the mechanical behavior and shape-recovery of shape-memory polyurethane nonwovens. Mechanical behavior was measured by dynamic mechanical analysis and tensile testing, and shape-recovery was measured by unconstrained and constrained recovery. Microstructure changes throughout the shape-memory cycle were also investigated by micro-computed tomography. It was found that increasing collector speed increases elastic modulus, ultimate strength and recovery stress of the nonwoven, but collector speed does not affect the failure strain or unconstrained recovery. Increasing air pressure decreases the failure strain and increases rubbery modulus and unconstrained recovery, but air pressure does not influence recovery stress. It was also found that during the shape-memory cycle, the connectivity density of the fibers upon recovery does not fully return to the initial values, accounting for the incomplete shape-recovery seen in shape-memory nonwovens. With these parameter to property relationships identified, shape-memory nonwovens can be more easily manufactured and tailored for specific applications.

Characterization of a small animal growth plate injury model using microcomputed tomography.

Injuries to the growth plate remain a significant clinical challenge. The need to better understand mechanisms of growth disruption following transphyseal injuries and evaluate new therapeutic approaches to growth restoration motivates development of a well characterized model of growth plate injury. The goals of this study were to develop a growth plate defect model in the rat and to use microcomputed tomography (micro-CT) imaging to detect and quantify associated changes in growth plate morphology and mineralization over time following injury and in response to treatment. Three-dimensional images of the growth plate were created from micro-CT scans and used to quantify the volume of mineralized tissue within the defect site. Growth plate thickness and volume as well as the degree of growth plate fusion were also measured from the reconstructed 3D images. Growth deficiency was then quantified as a function of time post-injury from whole limb micro-CT scans. Finally, this model was used to determine the ability of an injectable in situ gelling hydrogel to prevent formation of a bony bridge within the defect and the subsequent effect on limb length deficiency and changes to growth plate morphology. Growth plate injury resulted in significant shortening of the defect limb by day 28 and significant thinning and fusion of the surrounding growth plate up to day 112. Limb length reduction was correlated with changes in the growth plate volume and average thickness at day 56. Injection of an in situ gelling agarose into the defect resulted in a reduction of limb length discrepancy as well as a thicker growth plate on average compared to empty defect controls. These results establish a novel method of characterizing changes in whole bone and growth plate morphology due to a growth plate injury and indicate that treatment with agarose hydrogel reduces limb length discrepancy but is not sufficient to regenerate growth plate tissue or fully restore growth function.

The Influence of Trabecular Microstructure on Mechanical Properties and the Pullout Strength of Suture Anchors

Suture anchors provide soft-tissue fixation, often tendons and ligaments, to bone. The most common type of surgery in which suture anchors are used is in rotator cuff repairs, where the anchor is implanted into the humerus to create a point of fixation for the supraspinatus.[1–2] Pullout strength, or the force necessary to pull the anchor from the bone, has been previously used as a metric to compare suture anchor performance. In investigating suture anchor performance, it has been suggested that pullout strength is positively correlated to bone mineral density (BMD).[2]Copyright

Passaging effects on mineralization and mRNA expression of rat calvarial cells seeded in monolayer and on 3D polymer constructs in vitro

In this study, primary rat calvarial cells were expanded and then seeded in monolayer and on porous 3D scaffolds to determine the effect of passaging on matrix mineralization and osteogenic gene mRNA production. Passage 1, 2 and 3 cells were seeded in collagen coated 6 well plates and on 5 mm /spl times/ 3 mm poly(L-lactide-co-D, L-lactide 70:30) (PLDL) discs (n=6 for each group). Monolayer culture was carried out to 3 weeks and plates were stained by Von Kossa to measure mineralization area. Constructs were cultured for 8 weeks and scanned by micro-computed tomography (/spl mu/CT) at 24 and 57 days to quantify mineralization. Both culture groups were analyzed with real-time quantitative RT-PCR to determine the amount of osteogenic gene mRNA produced. Von Kossa staining showed a statistically significant decrease in the amount of mineralized nodule formation as passage number increased. There was no significant difference in the amount of mineralization produced by each passage group in 3D culture as shown by /spl mu/CT analysis. There was no reduction in osteogenic mRNA detected by realtime quantitative RT-PCR for osteocalcin (OCN), osteopontin (OPN), osteonectin (ONN) or alkaline phosphatase (ALP) from passage 1 to 3 in 3D culture while 2D culture did show reductions in OCN and OPN with passaging.