Using embedded alginate microparticles to tune the properties of in situ forming poly(N-isopropylacrylamide)-graft-chondroitin sulfate bioadhesive hydrogels for replacement and repair of the nucleus pulposus of the intervertebral disc.

Low back pain is a major public health issue associated with degeneration of the intervertebral disc (IVD). The early stages of degeneration are characterized by the dehydration of the central, gelatinous portion of the IVD, the nucleus pulposus (NP). One possible treatment approach is to replace the NP in the early stages of IVD degeneration with a hydrogel that restores healthy biomechanics while supporting tissue regeneration. The present study evaluates a novel thermosensitive hydrogel based on poly(N-isopropylacrylamide-graft-chondroitin sulfate) (PNIPAAM-g-CS) for NP replacement. The hypothesis was tested that the addition of freeze-dried, calcium crosslinked alginate microparticles (MPs) to aqueous solutions of PNIPAAm-g-CS would enable tuning of the rheological properties of the injectable solution, as well as the bioadhesive and mechanical properties of the thermally precipitated composite gel. Further, we hypothesized that the composite would support encapsulated cell viability and differentiation. Structure-material property relationships were evaluated by varying MP concentration and diameter. The addition of high concentrations (50 mg/mL) of small MPs (20 ± 6 µm) resulted in the greatest improvement in injectability, compressive mechanical properties, and bioadhesive strength of PNIPAAm-g-CS. This combination of PNIPAAM-g-CS and alginate MPs supported the survival, proliferation, and differentiation of adipose derived mesenchymal stem cells toward an NP-like phenotype in the presence of soluble GDF-6. When implanted ex vivo into the intradiscal cavity of degenerated porcine IVDs, the formulation restored the compressive and neutral zone stiffnesses to intact values and resisted expulsion under lateral bending. Overall, results indicate the potential of the hydrogel composite to serve as a scaffold for supporting NP regeneration. This work uniquely demonstrates that encapsulation of re-hydrating polysaccharide-based MPs may be an effective method for improving key functional properties of in situ forming hydrogels for orthopedic tissue engineering applications.

The effectiveness of transverse abdominis training on balance, postural sway and core muscle recruitment patterns: a pilot study comparison across age groups.

[Purpose] This pilot study aims to determine whether improvements in postural sway, particularly among older adults, can be augmented immediately after training participants to activate and isolate the transverse abdominis (TrA) muscle. [Participants and Methods] Fifty six participants (in three age groups) took part in a single session TrA training intervention. Aspects of postural sway, balance and muscle activation patterns were measured before and after training and compared. [Results] There was significant improvement across four of six postural sway variables for the combined sample of all age groups. Older adults improved more than younger and middle-age participants in two important postural sway variables. No marked differences were evident in static reach distance across all age groups. There were no differences between groups with regard to surface electromyography (sEMG) amplitudes despite the emergence of different activation patterns among age groups. [Conclusion] Immediate effects were induced in postural sway measures after the single session training intervention. By improving neuromuscular control of the TrA and maximizing the efficiency of related proximal core muscles center of pressure (COP) sway velocities decreased during single limb standing (SLS).

Efficacy of eluted antibiotics through 3D printed femoral implants.

Costs associated with musculoskeletal diseases in the United States account for 5.7% of the Gross Domestic Product (GDP) (Weinstein et al. 2018). As such, there is a need to pursue new ideas in orthopaedic implants that can decrease cost and improve patient care. In the recent years, 3D printing of polymers using Fused Deposition Modeling (FDM) and metals using Direct Metal Laser Sintering (DMLS) has opened several exciting possibilities to create customized orthopaedic implants. Such implants can be engineered to release antibiotics in a controlled manner by infusing the drug into the material during manufacturing stage. However, the prevalence of high temperature could impact the anti-bacterial effectiveness of the eluted antibiotics in such implants. An alternative approach to circumvent this issue would be to modify the implant geometry to incorporate built-in design features such as micro-channels and reservoirs in which antibiotics can be introduced prior to the surgical procedure. Irrespective of the approach used, the ability of 3D printed orthopaedic implants to elute antibiotics, and the rate of elution are not well understood. The purpose of this article is to study the elution of doxycycline through 3D printed femoral implants using three different materials: Poly-Lactic Acid (PLA), Poly-Caprolactone (PCL) and Titanium grade Ti-6Al-4V. The PLA and Ti-6Al-4V implants were designed with built-in reservoirs and micro-channels in which doxycycline was introduced post the manufacturing stage. However, the PCL implants were printed from a PCL spool that was infused with doxycycline using an extruder. The PLA and Ti-6Al-4V experiments were run for a period of 31 days and the PCL experiment for one day. The antibacterial ability of eluted doxycycline from all implants were examined using Kirby-Bauer test on the bacteria E.coli k-12. The results show that most of doxycycline eluted through the three materials in the first 24 hours. After the initial spike, a steady release was achieved for the PLA and Ti-6Al-4V implants for 30 days. During this timeframe, Ti-6Al-4V implants released more doxycycline than the PLA implant. The eluted antibiotics through all the implants demonstrated the ability to kill bacteria in the subsequent Kirby-Bauer test. These outcomes show that irrespective of how the antibiotics were introduced, 3D printed polymeric and metallic implants have great potential in orthopaedic applications.

Characterization of injectable hydrogels based on poly(N-isopropylacrylamide)-g-chondroitin sulfate with adhesive properties for nucleus pulposus tissue engineering.

The goal of this work is to develop an injectable nucleus pulposus (NP) tissue engineering scaffold with the ability to form an adhesive interface with surrounding disc tissue. A family of in situ forming hydrogels based on poly(N-isopropylacrylamide)-graft-chondroitin sulfate (PNIPAAm-g-CS) were evaluated for their mechanical properties, bioadhesive strength, and cytocompatibility. It was shown experimentally and computationally with the Neo-hookean hyperelastic model that increasing the crosslink density and decreasing the CS concentration increased mechanical properties at 37 °C, generating several hydrogel formulations with unconfined compressive modulus values similar to what has been reported for the native NP. The adhesive tensile strength of PNIPAAm increased significantly with CS incorporation (p < 0.05), ranging from 0.4 to 1 kPa. Live/Dead and XTT assay results indicate that the copolymer is not cytotoxic to human embryonic kidney (HEK) 293 cells. Taken together, these data indicate the potential of PNIPAAm-g-CS to function as a scaffold for NP regeneration.

Biaxial tensile testing and constitutive modeling of human supraspinatus tendon.

The heterogeneous composition and mechanical properties of the supraspinatus tendon offer an opportunity for studying the structure-function relationships of fibrous musculoskeletal connective tissues. Previous uniaxial testing has demonstrated a correlation between the collagen fiber angle distribution and tendon mechanics in response to tensile loading both parallel and transverse to the tendon longitudinal axis. However, the planar mechanics of the supraspinatus tendon may be more appropriately characterized through biaxial tensile testing, which avoids the limitation of nonphysiologic traction-free boundary conditions present during uniaxial testing. Combined with a structural constitutive model, biaxial testing can help identify the specific structural mechanisms underlying the tendon's two-dimensional mechanical behavior. Therefore, the objective of this study was to evaluate the contribution of collagen fiber organization to the planar tensile mechanics of the human supraspinatus tendon by fitting biaxial tensile data with a structural constitutive model that incorporates a sample-specific angular distribution of nonlinear fibers. Regional samples were tested under several biaxial boundary conditions while simultaneously measuring the collagen fiber orientations via polarized light imaging. The histograms of fiber angles were fit with a von Mises probability distribution and input into a hyperelastic constitutive model incorporating the contributions of the uncrimped fibers. Samples with a wide fiber angle distribution produced greater transverse stresses than more highly aligned samples. The structural model fit the longitudinal stresses well (median R(2) ≥ 0.96) and was validated by successfully predicting the stress response to a mechanical protocol not used for parameter estimation. The transverse stresses were fit less well with greater errors observed for less aligned samples. Sensitivity analyses and relatively affine fiber kinematics suggest that these errors are not due to inaccuracies in measuring the collagen fiber organization. More likely, additional strain energy terms representing fiber-fiber interactions are necessary to provide a closer approximation of the transverse stresses. Nevertheless, this approach demonstrated that the longitudinal tensile mechanics of the supraspinatus tendon are primarily dependent on the moduli, crimp, and angular distribution of its collagen fibers. These results add to the existing knowledge of structure-function relationships in fibrous musculoskeletal tissue, which is valuable for understanding the etiology of degenerative disease, developing effective tissue engineering design strategies, and predicting outcomes of tissue repair.

Passive cervical spine flexion: the effect of age and gender.

BACKGROUND: Previous studies reported passive cervical range of motion under unknown loading conditions or with minimal detail of subject positioning. Additionally, such studies have not quantitatively ensured the absence of active muscle during passive measurements. For the purpose of validating biomechanical models the loading condition, initial position, and muscle activation must be clearly defined. A method is needed to quantify the passive range of motion properties of the cervical spine under controlled loading conditions, particularly in the pediatric population where normative clinical and model validation data is limited. METHODS: Healthy female pediatric (6-12years; n=10), male pediatric (6-12years; n=9), female adult (21-40years; n=10), and male adult (20-36years; n=9) volunteers were enrolled. Subjects with restrained torsos and lower extremities were exposed to a maximum 1g inertial load in the posterior-anterior direction, such that the head-neck complex flexed when subjects relaxed their neck musculature. Surface electromyography monitored the level of muscle relaxation. A multi-camera 3-D target tracking system captured passive neck flexion angle of the head relative to the thoracic spine. General estimating equations detected statistical differences across age and gender. FINDINGS: Passive cervical spine flexion equaled 111.0° (SD 8.0°) for pediatric females, 102.8° (SD 7.8°) for adult females, 103.8° (SD 12.7°) for pediatric males, and 93.7° (SD 9.9°) for adult males. Passive neck flexion significantly decreased with age in both genders (P<0.01). Females exhibited significantly greater flexion than males (P<0.01). INTERPRETATION: This study contributes normative data for clinical use, biomechanical modeling, and injury prevention tool development.

Evaluation of affine fiber kinematics in human supraspinatus tendon using quantitative projection plot analysis.

Structural constitutive modeling approaches are often based on the assumption of affine fiber kinematics, even though this assumption has rarely been evaluated experimentally. We are interested in applying mathematical models to understand the mechanisms responsible for the inhomogeneous, anisotropic, and non-linear properties of human supraspinatus tendon (SST); however, the relationship between macroscopic and fiber-level deformation in this tendon remains unknown and current methods for making this assessment are inadequate. Therefore, the purpose of this study was to develop an improved method for quantitatively assessing agreement between two distributions and to examine the affine assumption in SST by comparing experimental fiber alignment to affine model predictions using this analysis approach. Measured fiber angle values of SST samples in uniaxial tensile tests were compared with predictions of affine fiber deformation using modified projection plots, which provide a method for qualitative and quantitative comparisons of two distributions. The projection plot metrics of offset and range, which were developed in this study, are of particular benefit by providing a quantitative representation of agreement that can be subjected to statistical comparisons. For SST, offset and range values varied by tendon location and test orientation, with more affine deformation evidenced for tendon regions of higher alignment. Results suggest that non-affine fiber behavior is dependent on specific tissue, orientation of the applied stretch relative to the fiber organization, and length scale of the observation. In addition, this study has established a method for evaluating the affine assumption in other tissues.

Characterizing the mechanical contribution of fiber angular distribution in connective tissue: comparison of two modeling approaches.

Modeling of connective tissues often includes collagen fibers explicitly as one of the components. These fibers can be oriented in many directions; therefore, several studies have considered statistical distributions to describe the fiber arrangement. One approach to formulate a constitutive framework for distributed fibers is to express the mechanical parameters, such as strain energy and stresses, in terms of angular integrals. These integrals represent the addition of the contribution of infinitesimal fractions of fibers oriented in a given direction. This approach leads to accurate results; however, it requires lengthy calculations. Recently, the use of generalized structure tensors has been proposed to represent the angular distribution in the constitutive equations of the fibers. Although this formulation is much simpler and fewer calculations are required, such structure tensors can only be used when all the fibers are in tension and the angular distribution is small. However, the amount of error introduced in these cases of non-tensile fiber loading and large angular distributions have not been quantified. Therefore, the objective of this study is to determine the range of values of angular distribution for which acceptable differences (less than 10%) between these two formulations are obtained. It was found, analytically and numerically, that both formulations are equivalent for planar distributions under equal-biaxial stretch. The comparison also showed, for other loading conditions, that the differences decrease when the fiber distribution is very small. Differences of less than 10% were usually obtained when the fiber distribution was very low (κ ≈ 0.03; κ ranges between 0 and 1/3, for aligned and isotropic distributed fibers, respectively). This range of angular distribution greatly limits the types of tissue that can be accurately analyzed using generalized structure tensors. It is expected that the results from this study guide the selection of a proper approach to analyze a particular tissue under a particular loading condition.