Load sharing within a human lumbar vertebral body using the finite element method.

STUDY DESIGN: A finite element parametric study was performed to investigate the structural roles of the vertebral cortical shell and the trabecular centrum. OBJECTIVES: To address the debated issue of the relative load-carrying role of the vertebral cortical shell. SUMMARY OF BACKGROUND DATA: Several experimental and computational studies have been aimed at quantifying the load-carrying roles of the human vertebral cortical shell and trabecular centrum. These studies, however, have supported no consensus. METHODS: A finite element model of three lumbar vertebral bodies was developed to predict the fraction of the total compressive load acting on the lumbar vertebral body, under two different loading conditions, that was supported by the cortical shell. Parametric variations in vertebral material and geometric properties were examined to determine how this fraction was influenced by such changes. RESULTS: The fraction of the compressive load supported by the cortical shell was found to be strongly dependent on the distance from the endplate, increasing from about 34% at either endplate to approximately 63% at the midtransverse plane. This fraction was independent of the loading characteristics, proportional to the properties of the cortex, and inversely proportional to the modulus of the centrum. Additionally, the cortical shell force fraction was affected significantly by changes in the overall vertebral geometry. CONCLUSIONS: Our findings indicate that the structural dominance of the cortical shell and centrum alternate depending on the location within the vertebral body. However, as age-related bone loss progresses, the load-carrying role of the cortical shell could increase significantly.

Detection of surgical glove integrity.

Surgical glove integrity is essential for universal precautions; glove safety is verified by the water load test (WLT). Concerns regarding glove injury have prompted newer testing methodologies, including electrical conductance testing (ECT); however, the sensitivities of these tests are not known. We compared the sensitivity of WLT and ECT in detecting glove needle-stick injury in two commonly used brands of surgical gloves. Punctures were made with hollow-bore and solid surgical needles of various configurations. The WLT failed to detect glove holes from the smallest-caliber needles and only detected the injury in 60 per cent for the largest caliber. The ECT provided a graded index of glove injury in all holes made by both solid surgical needles and hollow-bore needles. The WLT is a poor test for clinical defects in latex surgical gloves; the ECT is significantly more sensitive and provides a gauge of the cross-sectional area of the defect. Interbrand differences in self-sealing properties of surgical gloves were evidenced and may be clinically relevant after glove perforation.

Biomechanical analysis of supracondylar femoral fractures fixed with modern retrograde intramedullary nails.

OBJECTIVES: Several new retrograde supracondylar intramedullary nails have been developed to specifically address fractures of the distal femur. The nails appear clinically effective, but there are few biomechanical data documenting the stability of the fixation or the mechanical stiffness of the different designs. The goal of this study was to assess the torsional and bending stiffness of four designs of intramedullary nails developed for this application. METHODS: Four nail designs were tested in torsion and bending to determine system stiffness: Ace supracondylar, Richards "five hole" and "multi-hole" supracondylar, and Biomet retrograde. The nails were inserted into cadaveric femurs in which a one-centimeter distraction osteotomy had been created seven centimeters proximal to the condyles. The constructs were then tested on an Instron biaxial testing system. RESULTS: There were no statistically significant differences in bending stiffness among the groups of nails (range 0.79 to 1.18 newtons/meter; p > 0.1). However, the Ace nails (1.10 newtonmeters/degree) did exhibit a statistically lower torsional stiffness compared with the other nails (2.20 to 2.21 newton-meters/ degree; p < 0. 1). No differences were noted as a function of the number of locking holes. CONCLUSIONS: The bending stiffness of four currently available designs of retrograde intramedullary nails does not appear to be dependent on design variations. The torsional stiffness did vary among the four designs, but this was not determined by the number of fixation holes provided. It appears that a well-placed retrograde supracondylar nail of modern design should have sufficient stiffness to support the femur and provide stability during fracture healing.

Measurements of permeability in human calcaneal trabecular bone.

2.36 cm diameter cores of trabecular bone (n = 22), oriented in the mediolateral direction, were obtained from fresh-frozen calcanei of 16 cadavers ranging in age from 32 to 89 yr. The cores were defatted and tested to determine values for permeability along the cylinder axis. Permeability values (0.4-11 x 10(-9)m2) were found to be strongly correlated with specimen porosity (78-92%) through a linear relationship (r2 = 0.91). These results provide essential information for the biphasic or poroelastic modeling of fluid-filled trabecular bone at this location.

Potential role of nuclear magnetic resonance for the evaluation of trabecular bone quality.

This paper discusses two novel applications of nuclear magnetic resonance (NMR) as an investigational tool for the assessment of cancellous bone microarchitecture. It further outlines extensions of the method for in vivo clinical evaluation of bone strength in patients with skeletal disorders such as osteoporosis. The first method relies on the hypothesis that the presence of two phases of different magnetic permeability, i.e., bone and bone marrow, causes a spatial nonuniformity of the magnetic field across the measurement volume. The resulting spread in resonance frequency shortens the decay time constant (T2*) of the time domain proton signal in bone marrow or its substitute (water). Increased trabecular spacing, such as it occurs in osteoporosis, reduces the spatial field inhomogeneity and thus prolongs T2*, which has been shown both in vitro and in vivo. Subjects with osteoporosis, characterized by either low bone mineral density and/or spine compression fractures, have T2* values that are significantly prolonged. The second method focuses on a direct measurement of micromorphometric parameters of cancellous bone, using the principles of proton NMR microscopy in conjunction with computer processing of the resulting digital images. Image contrast between the trabeculae and the intertrabecular space is based on the marrow protons providing a signal, as opposed to bone, which appears with background intensity. Once tissues have been classified (into bone and marrow), for example, by means of a histogram-based segmentation algorithm, bone area fraction, mean trabecular plate density (MTPD), and mean trabecular plate thickness (MTPT) can be computed without the need for further operator intervention.(ABSTRACT TRUNCATED AT 250 WORDS)

A wide-band high-resolution spectrum analyzer.

This paper describes a two-million-channel 40-MHz-bandwidth, digital spectrum analyzer under development at the Jet Propulsion Laboratory. The analyzer system will serve as a prototype processor for the sky survey portion of NASA's Search for Extraterrestrial Intelligence program and for other applications in the Deep Space Network. The analyzer digitizes an analog input, performs a 2(21)-point, Discrete Fourier Transform, accumulates the output power, normalizes the output to remove frequency-dependent gain, and automates simple signal detection algorithms. Due to its built-in frequency-domain processing functions and configuration flexibility, the analyzer is a very powerful tool for real-time signal analysis and detection.