Radiographic Comparison of Forearm Symmetry in Healthy Individuals and its Importance in the Diagnosis of Longitudinal Radioulnar Dissociation.

PURPOSE: Longitudinal radioulnar dissociation (LRD) is an injury often missed upon initial presentation. A recent study examined a radiographic screening test in cadavers that showed increased interosseous distance when the interosseous ligament (IOL) was divided. For this test to be clinically useful, it is necessary for uninjured forearms to have similar interosseous spaces. The purpose of this study was to determine the typical differences between right and left interosseous spaces of healthy individuals. METHODS: Anterior-posterior x-rays of bilateral forearms in maximum supination of 28 surgical residents with no history of injury were obtained. These images were uploaded into a picture archiving and communication system and then digitized. The length of the radius was measured (Xr). The maximum interosseous distance (Dmax) between the radius and ulna as well as the interosseous distance at a location 0.3 Xr from the distal radioulnar joint was measured. The right and left arm distances were compared. Also, an outlier analysis was used to evaluate forearm rotational asymmetry between right and left arms. RESULTS: The outlier analysis revealed two sets of forearm x-rays were rotationally different compared to the rest of the group due to asymmetric arm positioning; these data were excluded from the analysis. The average difference in Dmax was 1.7 mm (standard deviation [SD] 1.5) between right and left arms, and this was found at a position of 0.28 Xr on average. The difference in interosseous distance measured at a fixed location 0.3 Xr was 1.6 mm (SD 1.5). No significant difference was found between the paired right and left arms for Dmax or at 0.3 Xr. CONCLUSIONS: There does not appear to be any significant difference between the maximum interosseous distance of right and left arms in healthy individuals. Therefore, analyzing bilateral forearm x-rays may be a simple LRD screening test. CLINICAL RELEVANCE: Understanding the degree of normal variation in the forearm bone spacing might inform evaluation of abnormal forearm bone alignment resulting from LRD.

Design and usability of a system for the study of head orientation.

The ability to control head orientation relative to the body is a multisensory process that mainly depends on proprioceptive, vestibular, and visual sensory systems. A system to study the sensory integration of head orientation was developed and tested. A test seat with a five-point harness was assembled to provide passive postural support. A lightweight head-mounted display was designed for mounting multiaxis accelerometers and a mini-CCD camera to provide the visual input to virtual reality goggles with a 39° horizontal field of view. A digitally generated sinusoidal signal was delivered to a motor-driven computer-controlled sled on a 6-m linear railing system. A data acquisition system was designed to collect acceleration data. A pilot study was conducted to test the system. Four young, healthy subjects were seated with their trunks fixed to the seat. The subjects received a sinusoidal anterior-posterior translation with peak accelerations of 0.06g at 0.1 Hz and 0.12g at 0.2, 0.5, and 1.1 Hz. Four sets of visual conditions were randomly presented along with the translation. These conditions included eyes open, looking forward, backward, and sideways, and also eyes closed. Linear acceleration data were collected from linear accelerometers placed on the head, trunk, and seat and were processed using MATLAB. The head motion was analyzed using fast Fourier transform to derive the gain and phase of head pitch acceleration relative to seat linear acceleration. A randomization test for two independent variables tested the significance of visual and inertial effects on response gain and phase shifts. Results show that the gain was close to one, with no significant difference among visual conditions across frequencies. The phase was shown to be dependent on the head strategy each subject used.

Non-Destructive Spectroscopic Assessment of High and Low Weight Bearing Articular Cartilage Correlates with Mechanical Properties.

OBJECTIVE: Autologous articular cartilage (AC) harvested for repair procedures of high weight bearing (HWB) regions of the femoral condyles is typically obtained from low weight bearing (LWB) regions, in part due to the lack of non-destructive techniques for cartilage composition assessment. Here, we demonstrate that infrared fiber optic spectroscopy can be used to non-destructively evaluate variations in compositional and mechanical properties of AC across LWB and HWB regions. DESIGN: AC plugs (N = 72) were harvested from the patellofemoral groove of juvenile bovine stifle joints, a LWB region, and femoral condyles, a HWB region. Near-infrared (NIR) and mid-infrared (MIR) fiber optic spectra were collected from plugs, and indentation tests were performed to determine the short-term and equilibrium moduli, followed by gravimetric water and biochemical analysis. RESULTS: LWB tissues had a significantly greater amount of water determined by NIR and gravimetric assay. The moduli generally increased in tissues from the patellofemoral groove to the condyles, with HWB condyle cartilage having significantly higher moduli. A greater amount of proteoglycan content was also found in HWB tissues, but no differences in collagen content. In addition, NIR-determined water correlated with short-term modulus and proteoglycan content (R = -0.40 and -0.31, respectively), and a multivariate model with NIR data was able to predict short-term modulus within 15% error. CONCLUSIONS: The properties of tissues from LWB regions differ from HWB tissues and can be determined non-destructively by infrared fiber optic spectroscopy. Clinicians may be able to use this modality to assess AC prior to harvesting osteochondral grafts for focal defect repair.

Mechanical behavior of porcine thoracic aorta in physiological and supra-physiological intraluminal pressures.

Understanding the mechanical behavior of aorta under supra-physiological loadings is an important aspect of modeling tissue behavior in various applications that involve large deformations. Utilizing inflation-extension experiments, the mechanical behavior of porcine descending thoracic aortic segments under physiological and supra-physiological intraluminal pressures was investigated. The pressure was changed in the range of 0-70 kPa and the deformation of the segment was determined in three dimensions using a custom-made motion capture system. An orthotropic Fung-type constitutive model was characterized by implementing a novel computationally efficient framework that ensured material stability for numerical simulations. The nonlinear rising trend of circumferential stretch ratio [Formula: see text] from outer toward inner wall was significantly increased at higher pressures. The increase in [Formula: see text] from physiological pressure (13 kPa) to 70 kPa was 13% at the outer wall and 22% at the inner wall; in this pressure range, the longitudinal stretch ratio [Formula: see text] increased 20%. A significant nonlinearity in the material behavior was observed as in the same pressure range, and the circumferential and longitudinal Cauchy stresses at the inner wall were increased 16 and 18 times, respectively. The overall constitutive model was verified in several loading paths in the [Formula: see text] space to confirm its applicability in multi-axial loading conditions.

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

BACKGROUND: Frequent mild head injuries or concussion along with the presence of headache may contribute to the persistence of concussion symptoms. METHODS: In 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.  RESULTS: A 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. CONCLUSIONS: Headache 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.

Broach Handle Design Changes Force Distribution in the Femur During Total Hip Arthroplasty.

BACKGROUND: Curved broach handles were developed to overcome limited surgical exposures during total hip arthroplasty. Some authors report increased intraoperative fracture rates during limited exposures. This study evaluates mechanical force ratios transmitted to the bone while broaching with curved vs straight handles. METHODS: An experimental model utilized a 6-axis load cell to measure force distributions produced by 4 different broach handles, each with increasing offset and curvature. Handles were separately impacted and dynamic variables assessed. Handles were then digitized using a high-resolution optical system and a finite element analysis (FEA) was performed to account for trabecular bone and vary the location of mallet impact. Off-axis forces, broaching construct moments, and stress within surrounding bone were computed. RESULTS: Using the experimental model, high-offset handles lost on average 4% more hammering force to the horizontal axis. When the FEA utilized moduli of elasticity to estimate broaching through osteoporotic trabecular bone, horizontally displaced forces (toward cortical bone) were magnified from 4% to a maximum value of 52%. Both the experimental construct and FEA confirmed that larger offset handles increase moment-to-force ratios up to 163%-235%, thus rotating the proximal and distal ends of the broach toward cortical bone. CONCLUSION: Broach handle design is an important determinant of resultant forces transmitted to the broach (and ultimately the bone) during total hip arthroplasty. Unwanted off-axis forces and enhanced rotational dynamics may play a role in intraoperative fractures during femoral canal preparation.

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.

Simulation and experimental studies in needle-tissue interactions.

This work aims to introduce a new needle insertion simulation to predict the deflection of a bevel-tip needle inside soft tissue. The development of such a model, which predicts the steering behavior of the needle during needle-tissue interactions, could improve the performance of many percutaneous needle-based procedures such as brachytherapy and thermal ablation, by means of the virtual path planning and training systems of the needle toward the target and thus reducing possible incidents of complications in clinical practices. The Arbitrary-Lagrangian-Eulerian (ALE) formulation in LS-DYNA software was used to model the solid-fluid interactions between the needle and tissue. Since both large deformation and fracture of the continuum need to be considered in this model, applying ALE method for fluid analysis was considered a suitable approach. A 150 mm long needle was used to bend within the tissue due to the interacting forces on its asymmetric bevel tip. Three experimental cases of needle steering in a soft phantom were performed to validate the simulation. An error measurement of less than 10 % was found between the predicted deflection by the simulations and the one observed in experiments, validating our approach with reasonable accuracy. The effect of the needle diameter and its bevel tip angle on the final shape of the needle was investigated using this model. To maneuver around the anatomical obstacles of the human body and reach the target location, thin sharp needles are recommended, as they would create a smaller radius of curvature. The insertion model presented in this work is intended to be used as a base structure for path planning and training purposes for future studies.

Interosseous Ligament and Transverse Forearm Stability: A Biomechanical Cadaver Study.

PURPOSE: The interosseous ligament (IOL) is known to be an important longitudinal stabilizer of the forearm. We hypothesize that it may also contribute to transverse stability, with pronosupination tensioning of the radius relative to the ulna. Therefore, when injured, we predict the interosseous space should widen in the transverse plane, enough to be appreciable on plain radiographs. A measurable difference in interosseous space, comparing an injured with an uninjured forearm, can potentially be of diagnostic and clinical value. METHODS: Ten fresh-frozen cadaver arms (from 5 individuals) were radiographed in 6 different positions of forearm supination, first in an uninjured state and then with the IOL sectioned, both partially (central band only) and completely. The transverse interosseous distance was measured on radiographs using edge detection software and compared using analysis of variance and contrast analysis. The maximum range of pronosupination was also compared before and after injury, using a paired t test. RESULTS: Average maximum supination increased from 84° to 106°, and pronation from 69° to 84°, after the IOL was sectioned completely. Sectioning of the IOL led to a statistically significant increase in the interosseous distance, a minimum of 2 mm, in all but one forearm position. CONCLUSIONS: The IOL of the forearm plays an important role in providing transverse stability to the radius and ulna. When the IOL is sectioned, the forearm exhibits increased pronosupination range of motion. Radiographs of bilateral forearms taken in identical rotational position can reliably differentiate between an intact and torn IOL in cadavers. CLINICAL RELEVANCE: The IOL's stabilizing role during forearm rotation suggests a novel strategy for diagnosing forearm IOL injury using comparative radiographic measurements.

Characterization of the viscoelastic behavior of a simplified collagen micro-fibril based on molecular dynamics simulations.

Collagen fibril is a major component of connective tissues such as bone, tendon, blood vessels, and skin. The mechanical properties of this highly hierarchical structure are greatly influenced by the presence of covalent cross-links between individual collagen molecules. This study investigates the viscoelastic behavior of a collagen lysine-lysine cross-link based on creep simulations with applied forces in the range or 10 to 2000pN using steered molecular dynamics (SMD). The viscoelastic model of the cross-link was combined with a system composed by two segments of adjacent collagen molecules hence representing a reduced viscoelastic model for a simplified micro-fibril. It was found that the collagen micro-fibril assembly had a steady-state Young׳s modulus ranging from 2.24 to 3.27GPa, which is in agreement with reported experimental measurements. The propagation of longitudinal force wave along the molecule was implemented by adding a delay element to the model. The force wave speed was found to be correlated with the speed of one-dimensional elastic waves in rods. The presented reduced model with three degrees of freedom can serve as a building block for developing models of the next level of hierarchy, i.e., a collagen fibril.

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.

Use of High-Speed X ray and Video to Analyze Distal Radius Fracture Pathomechanics.

The purpose of this study is to investigate the failure sequence of the distal radius during a simulated fall onto an outstretched hand using cadaver forearms and high-speed X ray and video systems. This apparatus records the beginning and propagation of bony failure, ultimately resulting in distal radius or forearm fracture. The effects of 3 different wrist guard designs are investigated using this system. Serving as a proof-of-concept analysis, this study supports this imaging technique to be used in larger studies of orthopedic trauma and protective devices and specifically for distal radius fractures.

Investigation of mechanisms of viscoelastic behavior of collagen molecule.

Unique mechanical properties of collagen molecule make it one of the most important and abundant proteins in animals. Many tissues such as connective tissues rely on these properties to function properly. In the past decade, molecular dynamics (MD) simulations have been used extensively to study the mechanical behavior of molecules. For collagen, MD simulations were primarily used to determine its elastic properties. In this study, constant force steered MD simulations were used to perform creep tests on collagen molecule segments. The mechanical behavior of the segments, with lengths of approximately 20 (1X), 38 (2X), 74 (4X), and 290 nm (16X), was characterized using a quasi-linear model to describe the observed viscoelastic responses. To investigate the mechanisms of the viscoelastic behavior, hydrogen bonds (H-bonds) rupture/formation time history of the segments were analyzed and it was shown that the formation growth rate of H-bonds in the system is correlated with the creep growth rate of the segment (ß=2.41ßH). In addition, a linear relationship between H-bonds formation growth rate and the length of the segment was quantified. Based on these findings, a general viscoelastic model was developed and verified here, using the smallest segment as a building block, the viscoelastic properties of larger segments could be predicted. In addition, the effect of temperature control methods on the mechanical properties were studied, and it was shown that application of Langevin Dynamics had adverse effect on these properties while the Lowe-Anderson method was shown to be more appropriate for this application. This study provides information that is essential for multi-scale modeling of collagen fibrils using a bottom-up approach.

Comparison of crossed screw versus plate fixation for radial neck fractures.

BACKGROUND: Fixation of radial neck fractures can be achieved with a plate and screw construct or, in absence of comminution, with two obliquely-oriented screws. This study investigated the mechanical properties, specifically the stiffness and load to failure, of these two fixation strategies in a cadaver model. METHODS: Ten matched-pair radii were removed from fresh cadaver arms. A transverse osteotomy was created at the neck of each radius. Right-sided radii were fixed with two oblique headless compression screws; left-sided radii were fixed with a radial neck plate. The distal aspect of each radius was potted in urethane casting resin. The radial head was loaded in shear in 4 different planes (medial to lateral, lateral to medial, posterior to anterior, and anterior to posterior) using an Instron machine. Stiffness and load to failure were recorded. FINDINGS: The stiffness of both constructs was similar in all planes except for loading from medial to lateral where the screw construct was 1.8 times stiffer. Average ultimate failure occurred at 229N for the screws and 206N for the plate. Failure strength was not statistically different. However, mode of failure differed for both fixation constructs, the plate failed in bending while the screws failed by pullout and fracture. INTERPRETATION: The two strategies provide similar strength and stiffness for the fixation of transverse, non-comminuted radial neck fractures. While plate and screw constructs are more appropriate for axially unstable or comminuted fractures, two oblique screws might be preferred for simple transverse neck fractures since this strategy requires less exposure and the implant is buried.

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.

Visual conflict and cognitive load modify postural responses to vibrotactile noise.

BACKGROUND: Underlying the increased incidence of falls during multitasking is a reduced ability to detect or attend to the sensory information signaling postural instability. Adding noise to a biological system has been shown to enhance the detection and transmission of weakened or sub-threshold cutaneous signals. If stochastic resonance is to become an effective adjunct to rehabilitation, we need to determine whether vibrotactile noise can be effective when added to an environment presenting with other sensory noise. METHODS: Sub-threshold vibration noise was applied for 30 sec at the soles of the feet in 21 healthy adults (20-29 yrs) between two 30-sec periods of no vibration. During the trials, subjects stood quietly with eyes closed or while viewing a visual scene that rotated in continuous upward pitch at 30 deg/sec. Subjects were also tested with these two visual conditions while performing a mental calculation task. It was hypothesized that sub-threshold vibration would increase regularity of postural sway, thereby improving postural stabilization during an attention demanding task but exerting less effect with multiple sensory demands. An ellipse fit to the covariance matrix revealed excursion of center of pressure (COP) and center of mass (COM) responses in the anterior-posterior and lateral planes. RMS values and approximate entropy of the COP and COM were calculated and statistically compared. RESULTS: The addition of vibrotactile noise to the plantar surface during quiet stance with eyes closed reduced the area of the COM and COP responses, which then returned to pre-vibration levels after vibration was removed. Postural sway was generally increased with both visual field rotations and mental calculation compared to the eyes closed condition. The effect of sub-threshold vibratory noise on postural behavior was modified when visual field rotations and mental calculation was combined. It was shown that the measure of approximate entropy reflected increased task complexity. CONCLUSIONS: Our results suggest that the impact of destabilizing signals is modulated when combined with vibrotactile stimulation. The strong aftereffects of the vibration stimulus suggest that the system has adapted to the sensory array even in the short time period tested here. The results imply that application of vibrotactile stimulation has the potential for diminishing sway magnitudes while increasing the potential for response variability, thereby presenting a non-invasive method of reducing the potential for falls.

Mechanical response of brain tissue under blast loading.

In this study, a framework for understanding the propagation of stress waves in brain tissue under blast loading has been developed. It was shown that tissue nonlinearity and rate dependence are the key parameters in predicting the mechanical behavior under such loadings, as they determine whether traveling waves could become steeper and eventually evolve into shock discontinuities. To investigate this phenomenon, in the present study, brain tissue has been characterized as a quasi-linear viscoelastic (QLV) material and a nonlinear constitutive model has been developed for the tissue that spans from medium loading rates up to blast rates. It was shown that development of shock waves is possible inside the head in response to high rate compressive pressure waves. Finally, it was argued that injury to the nervous tissue at the microstructural level could be partly attributed to the high stress gradients with high rates generated at the shock front and this was proposed as a mechanism of injury in brain tissue.

Polyacrylamide phantom for self-actuating needle-tissue interaction studies.

This study presents a polyacrylamide gel as a phantom material for needle insertion studies specifically developed for self-actuating needles to enhance the precise placement of needles in prostate. Bending of these self-actuating needles within tissue is achieved by Nitinol actuators attached to the needle body; however these actuators usually involve heating that can thermally damage the tissue surrounding the needles. Therefore, to develop and access feasibility of these needles, a polyacrylamide gel has been developed that mimics the thermal damage and mechanical properties of prostate tissue. Mechanical properties of the polyacrylamide gel was controlled by varying the concentrations of acrylamide monomer and N,N-methylene-bisacrylamide (BIS) cross-linker, and thermal sensitivity was achieved by adding bovine serum albumin (BSA) protein. Two polyacrylamide gels with different concentrations were developed to mimic the elastic modulus of the tissue. The two phantoms showed different rupture toughness and different deflection of bevel-tip needle. To study the thermal damage, a Nitinol wire was embedded in the phantom and resistively heated. The measured opaque zone (0.40mm) formed around the wire was close to the estimated damage zone (0.43mm) determined using the cumulative equivalent minutes at 43°C.

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 construct's 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.