Construct Stability of C2 Replacement Strategies

Reconstruction of C2 after tumor destruction and resection remains a significant challenge. Most constructs utilize a strutgraft with plate or screw fixation. A novel C2 prosthesis combining a titanium mesh cage with bilateral C1 shelves and a T-plate has been used successfully in 18 patients. Supplemental posterior instrumentation includes C0-C3 or C1-C3. Biomechanical comparisons of this C2 prosthesis with traditional fixation options have not been reported. Five fresh-frozen human cadaveric cervical spines (C0-C5) were tested intact. Next, the C2 prosthesis, and strut graft and anterior plate constructs were tested with occiput-C3 and C1-C3 posterior fixation. Pure moment loads (up to 1.5 N-m) were applied in flexion and extension, lateral bending, and axial rotation. C1-C3 motion was evaluated using 3 camera motion analysis. Statistical significance was evaluated using one-way repeated measures ANOVA with Student-Newman-Keuls post hoc pairwise comparisons. All constructs provided a statistically significant decrease in motion in this C2 corpectomy model as compared to the intact condition. There was no significant difference in C1-C3 motion between the 4 constructs, regardless of whether the occiput was included in the fixation. Under these loading conditions, both the C2 prostheisis and strut-graft-plate constructs provided initial C1-C3 stability beyond that of the intact specimen. The occiput does not need to be included in the posterior instrumentation.

Pressure-Flow Characteristics During Peristaltic Transport of Non-Newtonian Fluid in Distensible Tube With Different Wave Forms

This study analyzes the pressure-flow characteristics during the peristaltic pumping of power law fluids in an axi-symmetric non-uniform distensible tube. The analyzed geometry is of a diverging shape that is common in several biological flow conduits, especially in mammals. Using the Fourier series, the dimensionless wall coordinates for sinusoidal, triangular, trapezoidal, and square wave forms are obtained to simulate wall movement. Equations expressing the pressure-flow rate relationship for different wall shapes are developed from the wave equation. Pressure-flow and velocity plots are obtained by solving the equations numerically. The results indicate that there is significant difference in pressure-flow relationship between Newtonian and non-Newtonian fluid. Also, the maximum flow rate can be achieved when the wall movement follows a square wave form.

Development of a Finite Element Model of the Inferior Glenohumeral Ligament of the Glenohumeral Joint

The shoulder is one of the most complex and often injured joints in the human body. The inferior glenohumeral ligament (IGHL), composed of the anterior band (AB), posterior band (PB) and the axillary pouch, has been shown to be an important contributor to anterior shoulder stability (Turkel, 1981). Injuries to the IGHL of the glenohumeral capsule are especially difficult to diagnose and treat effectively. The objective of this research was to develop a methodology for subject-specific finite element (FE) modeling of the ligamentous structures of the glenohumeral joint, specifically the IGHL, and to determine how changes in material properties affect predicted strains in the IGHL at 60° of external rotation. Using the techniques developed in this research, an improved understanding of the contribution of the IGHL to shoulder stability can be acquired.

Repeatable Dynamic Rollover Roof Test Fixture

Experimental rollover tests have been criticized for their poor emulation of actual rollovers and for their lack of repeatability. We have designed and built a test fixture that overcomes both of these criticisms. The fixture holds a passenger compartment, weighted to match the inertia characteristics of a complete vehicle, or a complete vehicle at the appropriate pitch and yaw. The compartment is then rotated about its principal (longitudinal) axis through an arc that mimics the rolling motion of an entire vehicle. At the appropriate roll angle and falling velocity, the roof strikes a moving patch of concrete. The compartment is controlled throughout the sequence and is suspended after the impact, so that a sequence of impacts can be individually studied in separate tests. Initial tests have shown that we can achieve repeatable impacts. Test variables include pitch, yaw, roll rate and vehicle center of gravity motion (both lateral and vertical velocity). This test device addresses the various shortcomings of previous rollover tests, fixtures and the various static and drop tests of vehicles conducted to determine rollover performance.

Inflation Testing as a Means of Measuring Failure Strength of Aortic Tissue

Abdominal Aortic Aneurysms (AAAs) are localized enlargements of the aorta. If untreated, AAAs will grow irreversibly until rupture occurs. Ruptured AAAs are usually fatal and are a leading cause of death in the United States, killing 15,000 per year (National Center for Health Statistics, 2001). Surgery to repair AAAs also carries mortality risks, so surgeons desire a reliable tool to evaluate the risk of rupture versus the risk of surgery.

A New Model for Biologic Constructs: Biotensegrity

Present biologic models envision organisms behave like the character ‘Topsy’ in Gone with the Wind; they “just grew.” Modeled of Lego©-like components, the individual structures are linked together as if they are automobile parts that are manufactured at different plants and assembled at some central factory. For the most part, hexahedral finite element meshes are used to model structures. When tetrahedral modeling is used, no account is made of the different mechanical properties that are inherent in triangulated structures, (trusses), that make the structures behave very differently than hexahedral-based models.

Large Deformation of Biological Cells by Optical Tweezers

The chemical and biological functions of living cells are known to be influenced strongly by mechanical forces and deformation, and the ability of cells to detect and support forces, in turn, is also affected by chemical and biological factors. Furthermore, the progression of a number of inherited and infectious diseases have also been identified to have a strong correlation with the mechanical deformation characteristics of biological cells. Consequently, the deformation characteristics of whole cells and cell membranes have long been investigated using a variety of experimental methods, such as the micropipette aspiration technique, and by computational modeling (see, for example, refs. [1, 2]). Recent advances in experimental techniques capable of probing mechanical forces and displacements to a resolution of picoNewton and nanometer, respectively, have facilitated use of mechanical test methods for living cells whereby precise measurements of response under different stress states could be investigated.

Effects of Blood Flow on Radiofrequency Ablation of Tumors: Finite Elements and In Vitro Models

The efficacy of radiofrequency ablation (RFA) treatments depends on the ability to ablate tumors completely while minimizing the damage to healthy tissue. Tissue cooling due to blood flow is an important factor affecting the size and shape of the ablation lesion. In this paper a new methodology for finite element modeling of the coupled electrical-thermal-flow process during RFA is presented. Our formulation treats heat losses due to blood flow explicitly rather than approximating the collective effects of blood vessles as a heat sink. Numerical models were compared to in vitro models using egg whites to simulate human tissue and a straight cylinder filled with a saline solution to simulate blood. Asymmetric burns were obtained close to the simulated blood vessels. Numerical results closely match the in vitro models.

Optical-Thermal Response and Dynamic Thermal Damage of Bio-Tissue During Laser Thermotherapy

Laser thermotherapy is a technique used for tumor treatment. It generates a local heating, causes thermal coagulation of living tissue and eliminates the tumor. Precise heating of tumor tissue with healthy minimum thermal injury to adjacent tissue is essential to thermotherapy. Understanding of heat transfer and optical-thermal interaction is important for control of temperature and design of thermotherapy. This study applies the Arrhenius damage model to describe the heat-induced change of optical properties. It calculates the distribution temperature, damage and optical-thermal response of bio-tissue during the laser treatment, and shows how these factors affect the effectiveness of laser thermotherapy. Similar research has been performed by Kim and coworkers [1996], Iizuka and coworkers [2000], and Whelan and coworkers [2000]. This study relaxes some conditions in previous investigations. It reveals the importance and the effect of size of the laser head.

Active Stability of the Glenohumeral Joint Decreases in the Apprehension Position

Muscle forces that compress the glenohumeral joint during midranges of motion may lead to increased translational forces in endrange positions, such as the apprehension position, where symptoms of anterior instability occur. The objective of this study was to quantify active stability provided by eight shoulder muscles in mid-range and end-range positions through muscle force vector analysis. Lines of action were derived from a standard geometric model and muscle force magnitudes were estimated with electromyography-based techniques. Resultant muscle force vectors were calculated by summing individual muscle force vectors. Compared to mid-range positions, lines of action of resultant force vectors were more anteriorly-directed in end-range positions. The deviation angle in the anterior direction was greatest (35°) and, consequently, stability was lowest in the apprehension position. Based on a sensitivity analysis, lines of action of resultant force vectors vary up to 6° within the population. In the apprehension position, muscle forces may promote anterior humeral head translation, predisposing the glenohumeral joint to anterior instability when other joint stabilizers are not functioning normally.