Soroush Assari

Biomechanical analysis of second-generation headless compression screws

INTRODUCTION Headless Compression Screws (HCS) are commonly utilized for the fixation of small bone and articular fractures. Recently several new second generation HCS (SG-HCS) have been introduced with the purported benefits of improved biomechanical characteristics. We sought to determine and compare the biomechanical efficiencies of these screws. MATERIAL AND METHODS Five HCS including four second generation (Mini-Acutrak 2 (Acumed), Twinfix (Stryker), Kompressor Mini (Integra), HCS 3.0 (Synthes)) and one first generation (Herbert-Whipple) were studied. Polyurethane foam blocks that represented osteoporotic cancellous bone (0.16 g/cc) with a simulated transverse fracture at the waist were utilized and five screws of each brand were tested for the generated compression force and fastening torque during insertion with and without pre-drilling. RESULTS The generated compression force was highest for Mini-Acutrak 2 (45.41 ± 0.88 N) and lowest for Herbert-Whipple (13.44 ± 2.35 N) and forces of Twinfix, Kompressor Mini, HCS 3.0 were in between in descending order. The compression force of SG-HCS increased slightly without pre-drilling but it was not statistically significant while the fastening torque increased significantly. Slight over-fastening beyond the recommended stage significantly reduced the compression force in Twinfix and Kompressor and had no or moderate effect in other screws. CONCLUSION All SG-HCS demonstrated greater biomechanical characteristics than the first generation Herbert-Whipple screw. The Mini-Acutrak 2 with a variable pitch design generated the maximum compression force and showed the most reliability and sustainability. Screws with independently rotating trailing heads (Twinfix and Kompressor Mini) demonstrated loss of compression with extra turns. The increase of fastening torque due to over-fastening and loss of compression at the same time in some screw designs, demonstrated how the fastening torque (applied by the surgeon) can be a misleading measure of the compression force. Application of SG-HCS in osteoporotic bone without pre-drilling can slightly increase the compression force.

Association of Football Subconcussive Head Impacts With Ocular Near Point of Convergence

IMPORTANCE An increased understanding of the relationship between subconcussive head impacts and near point of convergence (NPC) ocular-motor function may be useful in delineating traumatic brain injury. OBJECTIVE To investigate whether repetitive subconcussive head impacts during preseason football practice cause changes in NPC. DESIGN, SETTING, AND PARTICIPANTS This prospective, observational study of 29 National Collegiate Athletic Association Division I football players included baseline and preseason practices (1 noncontact and 4 contact), and postseason follow-up and outcome measures were obtained for each time. An accelerometer-embedded mouthguard measured head impact kinematics. Based on the sum of head impacts from all 5 practices, players were categorized into lower (n = 7) or higher (n = 22) impact groups. EXPOSURES Players participated in regular practices, and all head impacts greater than 10g from the 5 practices were recorded using the i1Biometerics Vector mouthguard (i1 Biometrics Inc). MAIN OUTCOMES AND MEASURES Near point of convergence measures and symptom scores. RESULTS A total of 1193 head impacts were recorded from 5 training camp practices in the 29 collegiate football players; 22 were categorized into the higher-impact group and 7 into the lower-impact group. There were significant differences in head impact kinematics between lower- and higher-impact groups (number of impacts, 6 vs 41 [lower impact minus higher impact = 35; 95% CI, 21-51; P < .001]; linear acceleration, 99g vs 1112g [lower impact minus higher impact= 1013; 95% CI, 621 - 1578; P < .001]; angular acceleration, 7589 radian/s2 vs 65 016 radian/s2 [lower impact minus higher impact= 57 427; 95% CI , 31 123-80 498; P < .001], respectively). The trajectory and cumulative burden of subconcussive impacts on NPC differed by group (F for group × linear trend1, 238 = 12.14, P < .001 and F for group × quadratic trend1, 238 = 12.97, P < .001). In the higher-impact group, there was a linear increase in NPC over time (B for linear trend, unstandardized coefficient [SE]:  0.76 [0.12], P < .001) that plateaued and resolved by postseason follow-up (B for quadratic trend [SE]: -0.06 [0.008], P < .001). In the lower-impact group, there was no change in NPC over time. Group differences were first observed after the first contact practice and remained until the final full-gear practice. No group differences were observed postseason follow-up. There were no differences in symptom scores between groups over time. CONCLUSIONS AND RELEVANCE Although asymptomatic, these data suggest that repetitive subconcussive head impacts were associated with changes in NPC. The increase in NPC highlights the vulnerability and slow recovery of the ocular-motor system following subconcussive head impacts. Changes in NPC may become a useful clinical tool in deciphering brain injury severity.

Frequent mild head injury promotes trigeminal sensitivity concomitant with microglial proliferation, astrocytosis, and increased neuropeptide levels in the trigeminal pain system.

BackgroundFrequent mild head injuries or concussion along with the presence of headache may contribute to the persistence of concussion symptoms.MethodsIn this study, the acute effects of recovery between mild head injuries and the frequency of injuries on a headache behavior, trigeminal allodynia, was assessed using von Frey testing up to one week after injury, while histopathological changes in the trigeminal pain pathway were evaluated using western blot, ELISA and immunohistochemistry. ResultsA decreased recovery time combined with an increased mild closed head injury (CHI) frequency results in reduced trigeminal allodynia thresholds compared to controls. The repetitive CHI group with the highest injury frequency showed the greatest reduction in trigeminal thresholds along with greatest increased levels of calcitonin gene-related peptide (CGRP) in the trigeminal nucleus caudalis. Repetitive CHI resulted in astrogliosis in the central trigeminal system, increased GFAP protein levels in the sensory barrel cortex, and an increased number of microglia cells in the trigeminal nucleus caudalis.ConclusionsHeadache behavior in rats is dependent on the injury frequency and recovery interval between mild head injuries. A worsening of headache behavior after repetitive mild head injuries was concomitant with increases in CGRP levels, the presence of astrocytosis, and microglia proliferation in the central trigeminal pathway. Signaling between neurons and proliferating microglia in the trigeminal pain system may contribute to the initiation of acute headache after concussion or other traumatic brain injuries.

Biomechanical comparison of locked plating and spiral blade retrograde nailing of supracondylar femur fractures

OBJECTIVE Biomechanical comparison between locked plating and retrograde nailing of supracondylar femur fractures with simulated postoperative weight-bearing. METHODS The Locking Condylar Plate (LCP) and Retrograde/Antegrade EX Femoral Nail (RAFN) were tested using 10 paired elderly cadaveric femurs, divided into Normal and Low Bone Mineral Density (BMD) groups, with a simulated AO/OTA type 33-A3 supracondylar femur fracture. Each specimen was subjected to 200,000 loading cycles in an attempt to simulate six weeks of postoperative recovery with full weight-bearing for an average individual. The constructs subsidence due to cyclic loading, and axial stiffness before and after the cyclic loading were measured and their correlation with BMD was studied. The two implants were compared in a paired study within each BMD group. RESULTS LCP constructs showed higher axial stiffness compared to RAFN for both Normal and Low BMD groups (80% and 57%, respectively). After cyclic loading, axial stiffness of both constructs decreased by 20% and RAFN constructs resulted in twice as much subsidence (1.9 ± 0.6mm). Two RAFN constructs with Low BMD failed after a few cycles whereas the matched pairs fixed with LCP failed after 70,000 cycles. CONCLUSIONS The RAFN constructs experienced greater subsidence and reduced axial stiffness compared to the LCP constructs. In Low BMD specimens, the RAFN constructs had a higher risk of failure.

Correlations between transmural mechanical and morphological properties in porcine thoracic descending aorta.

Determination of correlations between transmural mechanical and morphological properties of aorta would provide a quantitative baseline for assessment of preventive and therapeutic strategies for aortic injuries and diseases. A multimodal and multidisciplinary approach was adopted to characterize the transmural morphological properties of descending porcine aorta. Histology and multi-photon microscopy were used for describing the media layer micro-architecture in the circumferential-radial plane, and Fourier Transform infrared imaging spectroscopy was utilized for determining structural protein, and total protein content. The distributions of these quantified properties across the media thickness were characterized and their relationship with the mechanical properties from a previous study was determined. Our findings indicate that there is an increasing trend in the instantaneous Young׳s modulus (E), elastic lamella density (ELD), structural protein (SPR), total protein (TPR), and elastin and collagen circumferential percentage (ECP and CCP) from the inner towards the outer layers. Two regions with equal thickness (inner and outer halves) were determined with significantly different morphological and material properties. The results of this study represent a substantial step toward anatomical characterization of the aortic wall building blocks and establishment of a foundation for quantifying the role of microstructural components on the functionality of aorta.

Computational simulation of the mechanical response of brain tissue under blast loading

In the present study, numerical simulations of nonlinear wave propagation and shock formation in brain tissue have been presented and a new mechanism of injury for blast-induced neurotrauma (BINT) is proposed. A quasilinear viscoelastic (QLV) constitutive material model was used that encompasses the nonlinearity as well as the rate dependence of the tissue relevant to BINT modeling. A one-dimensional model was implemented using the discontinuous Galerkin finite element method and studied with displacement- and pressure-input boundary conditions. The model was validated against LS-DYNA finite element code and theoretical results for specific conditions that resulted in shock wave formation. It was shown that a continuous wave can become a shock wave as it propagates in the QLV brain tissue when the initial changes in acceleration are beyond a certain limit. The high spatial gradient of stress and strain at the shock front cause large relative motions at the cellular scale at high temporal rates even when the maximum stresses and strains are relatively low. This gradient-induced local deformation may occur away from the boundary and is proposed as a contributing factor to the diffuse nature of BINT.

CEREBRAL BLOOD PRESSURE RISE DURING BLAST EXPOSURE IN A RAT MODEL OF BLAST-INDUCED TRAUMATIC BRAIN INJURY

Blast-induced traumatic brain injury (bTBI) has been called the signature wound of war in the past decade. The mechanisms of such injuries are not yet completely understood. One of the proposed hypotheses is the transfer of pressure wave from large torso blood vessels to the cerebrovasculature as a major contributing factor to bTBI. The aim of this study was to investigate this hypothesis by measuring cerebral blood pressure rise during blast exposure and comparing two scenarios of head-only or chest-only exposures to the blast wave. The results showed that the cerebral blood pressure rise was significantly higher in chest-only exposure, and caused infiltration of blood-borne macrophages into the brain. It is concluded that a significantly high pressure wave transfers from torso to cerebrovasculature during exposure of the chest to a blast wave. This wave may lead to blood-brain barrier disruption and consequently trigger secondary neuronal damage.Copyright

Computational Simulation of Shock Tube and the Effect of Shock Thickness on Strain-Rates

Blast-induced neurotrauma has become an increasing concern with the advancement of explosive devices and high rates of loading. Recent experiments show that under blast loading conditions, brain tissue undergoes small displacements that are much lower than the threshold of traumatic brain injury. Based on the nonlinear viscoelastic nature of brain tissue, stress waves generated in the tissue due to blast loading can evolve into shock waves, which create high spatial and temporal pressure gradients at the shock front. In this study, the effect and importance of shock front thickness in simulating the response of tissues in shock tube scenarios has been investigated. It is shown that such measures can have a significant effect on prediction on injury in computational models.

Investigation of inhomogeneous and anisotropic material behavior of porcine thoracic aorta using nano-indentation tests.

This study investigates the inhomogeneity and anisotropy of porcine descending thoracic aorta in three dimensions using a custom-made nano-indentation technique and a quasi-linear viscoelastic modeling approach. The indentation tests were conducted in axial, circumferential, and radial orientations with about 100 μm spatial resolution. The ratio of the elastic moduli obtained in different orientations was used to quantify the tissue local anisotropy. The distal sections were generally stiffer than the proximal ones in both axial and circumferential indentations. Four distinct layers were identified across the thickness with significantly different mechanical properties. The stiffness of the medial quadrant was significantly lower than all other quadrants in axial indentation. The anisotropic behavior of the tissue was more pronounced in the lateral quadrant of the distal sections. The results of this study can be used to better understand the mechanisms of aorta deformation and improve the spatial accuracy of computational models of aorta.

Mechanical Instability of Aorta due to Intraluminal Pressure

Dynamic mechanical instability in aorta due to intraluminal pressure may result in a buckling-type deformation and an increase in the pressure-induced tissue stresses and strains. The stability behavior of thoracic aorta was investigated with two boundary conditions that represented two extreme cases of in vivo constraints. The pinned–pinned boundary condition (PPBC) resulted in a decoupled system of equations while the equations for the clamped–clamped boundary condition (CCBC) were coupled. The stability regions around a physiological reference point were generated and the effects of variations in loading and geometric parameters were studied. In CCBC, the critical intraluminal pressures were higher by a factor of two to four compared to PPBC. The highest critical pressures remained below the peak aortic pressures that occur in motor vehicle accidents, which confirmed that mechanical instability can be a mechanism contributing to traumatic injury and rupture of aorta.