Comparison of natural crosslinking agents for the stabilization of xenogenic articular cartilage.

Osteochondral xenografts are potentially inexpensive, widely available alternatives to fresh allografts. However, antigen removal from xenogenic cartilage may damage the extracellular matrix and reduce compressive stiffness. Non-crosslinked xenogenic cartilage may also undergo rapid enzymatic degradation in vivo. We hypothesized that natural crosslinking agents could be used in place of glutaraldehyde to improve the mechanical properties and enzymatic resistance of decellularized cartilage. This study compared the effects of genipin (GNP), proanthocyanidin (PA), and epigallocatechin gallate (EGCG), on the physical and mechanical properties of decellularized porcine cartilage. Glutaraldehyde (GA) served as a positive control. Porcine articular cartilage discs were decellularized in 2% sodium dodecyl sulfate and DNase I followed by fixation in 0.25% GNP, 0.25% PA, 0.25% EGCG, or 2.5% GA. Decellularization decreased DNA by 15% and GAG by 35%. For natural crosslinkers, the average degree of crosslinking ranged from approximately 50% (EGCG) to 78% (GNP), as compared to 83% for the GA control. Among the natural crosslinkers, only GNP significantly affected the disc diameter, and shrinkage was under 2%. GA fixation had no significant effect on disc diameter. Decellularization decreased aggregate modulus; GA and GNP, but not EGCG and PA, were able to restore it to its original level. GNP, PA, and GA conferred a similar, almost complete resistance to collagenase degradation. EGCG also conferred substantial resistance but to a lesser degree. Overall, the data support our hypothesis and suggest that natural crosslinkers may be suitable alternatives to glutaraldehyde for stabilization of decellularized cartilage. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1037-1046, 2016.

A Coupled Experiment-finite Element Modeling Methodology for Assessing High Strain Rate Mechanical Response of Soft Biomaterials.

This study offers a combined experimental and finite element (FE) simulation approach for examining the mechanical behavior of soft biomaterials (e.g. brain, liver, tendon, fat, etc.) when exposed to high strain rates. This study utilized a Split-Hopkinson Pressure Bar (SHPB) to generate strain rates of 100-1,500 sec(-1). The SHPB employed a striker bar consisting of a viscoelastic material (polycarbonate). A sample of the biomaterial was obtained shortly postmortem and prepared for SHPB testing. The specimen was interposed between the incident and transmitted bars, and the pneumatic components of the SHPB were activated to drive the striker bar toward the incident bar. The resulting impact generated a compressive stress wave (i.e. incident wave) that traveled through the incident bar. When the compressive stress wave reached the end of the incident bar, a portion continued forward through the sample and transmitted bar (i.e. transmitted wave) while another portion reversed through the incident bar as a tensile wave (i.e. reflected wave). These waves were measured using strain gages mounted on the incident and transmitted bars. The true stress-strain behavior of the sample was determined from equations based on wave propagation and dynamic force equilibrium. The experimental stress-strain response was three dimensional in nature because the specimen bulged. As such, the hydrostatic stress (first invariant) was used to generate the stress-strain response. In order to extract the uniaxial (one-dimensional) mechanical response of the tissue, an iterative coupled optimization was performed using experimental results and Finite Element Analysis (FEA), which contained an Internal State Variable (ISV) material model used for the tissue. The ISV material model used in the FE simulations of the experimental setup was iteratively calibrated (i.e. optimized) to the experimental data such that the experiment and FEA strain gage values and first invariant of stresses were in good agreement.

Anisotropic compressive properties of passive porcine muscle tissue.

The body has approximately 434 muscles, which makes up 40-50% of the body by weight. Muscle is hierarchical in nature and organized in progressively larger units encased in connective tissue. Like many soft tissues, muscle has nonlinear visco-elastic behavior, but muscle also has unique characteristics of excitability and contractibility. Mechanical testing of muscle has been done for crash models, pressure sore models, back pain, and other disease models. The majority of previous biomechanical studies on muscle have been associated with tensile properties in the longitudinal direction as this is muscle's primary mode of operation under normal physiological conditions. Injury conditions, particularly high rate injuries, can expose muscle to multiple stress states. Compressive stresses can lead to tissue damage, which may not be reversible. In this study, we evaluate the structure-property relationships of porcine muscle tissue under compression, in both the transverse and longitudinal orientations at 0.1 s-1, 0.01 s-1, or 0.001 s-1. Our results show an initial toe region followed by an increase in stress for muscle in both the longitudinal and transverse directions tested to 50% strain. Strain rate dependency was also observed with the higher strain rates showing significantly more stress at 50% strain. Muscle in the transverse orientation was significantly stiffer than in the longitudinal orientation indicating anisotropy. The mean area of fibers in the longitudinal orientation shows an increasing mean fiber area and a decreasing mean fiber area in the transverse orientation. Data obtained in this study can help provide insight on how muscle injuries are caused, ranging from low energy strains to high rate blast events, and can also be used in developing computational injury models.

Evaluation of facial nerve function following surgical approaches for maxillofacial trauma.

PURPOSE: The aim of this study was to report facial nerve injury following extraoral surgical approaches for the treatment of maxillofacial trauma, using the House-Brackmann facial nerve grading system (HBFNGS) as a means of classifying and measuring the degree and type of injury. MATERIALS AND METHODS: The sample comprised 100 consecutive cases of various maxillofacial trauma in which extraoral surgical approaches were used. Variety of surgical approaches such as coronal, preauricular, endaural, retromandibular, and submandibular approach and its modifications were used based on the anatomic location of the fracture and the accessibility required for its reduction and fixation. Facial nerve function of all patients was evaluated preoperatively and 24 hours after surgery. Patients who presented postoperative facial nerve injury were likewise examined using the HBFNGS at 24 hours, 1 week, 1 month, 3 months, and 6 months. RESULTS: Of the 100 patients, temporofacial branch involvement was seen in 11 cases, whereas cervicofacial branch involvement was seen in 6 cases. Complete recovery of the temporofacial branches was seen in a period of 3-4 months; whereas cervicofacial branches recovered in 5-6 months postoperatively. CONCLUSION: The frequency of facial nerve injury was related to various surgical approaches in maxillofacial trauma. Facial nerve impairment was found to be temporary in all cases, although the recovery of cervicofacial branches took a longer time. Moreover, there is a need to standardize the reporting of facial nerve recovery.