Evidence against tailstrings increasing the rate of pelvic inflammatory disease among IUD users.

A meta-analysis was conducted to determine whether the presence of an IUD tailstring is associated with increased rates of pelvic inflammatory disease among IUD users. Included were seven clinical studies that compared the same IUD with and without a tail, and fifteen clinical studies that compared two similar copper IUDs that differed in the polymer used for the tailstring (polypropylene versus polyethylene). No increase in PID rates was found among users of IUDs with versus without a tail (RR = 1.2, 95% confidence interval 0.6-1.7), or among users of copper IUDs with different tailstring materials (polypropylene vs. polyethylene, RR = 0.9, 95% confidence interval 0.4-1.5). These findings support the hypothesis that the presence of a tailstring does not increase the rate of PID among users of IUDs.

Finite element evaluation of the AIA shear specimen for bone.

An elastic-plastic finite element analysis is performed on the AIA shear specimen to evaluate its effectiveness to yield ultimate shear strength values. The effect of geometry, material properties, and yield criteria are discussed in the light of applications to human femoral cortical bone. Specimen dimensions are noted as follows: W, width, D, hole diameter and H, distance between holes. As the H/D ratio increases the stress distribution tends more toward pure shear at the same time the overshoot in the shear distribution increases. An H/D ratio equal to 1.2-1.5 is optimal. The H/W parameter does not affect the overshoot noticeably but it does slightly affect the purity of shear. The material parameters do affect the performance of the shear specimen. However, the effect of the material parameters are far more pronounced in the anisotropic case than it is in the isotropic case. In the isotropic case, the Young modulus does not affect the overshoot. The increase in Poisson's ratio does slightly decrease the overshoot. For the anisotropic case, the increase in the ratio of shear modulus to Young modulus in the transverse direction (G/E2) results in an increase in the overshoot (in the shear distribution). The increase in the ratio of the Young modulus in the transverse direction to that of the axial direction (E2/E1) also results in an increase in the overshoot. Creating a notch at the top of the hole is shown to have the effect of decreasing the overshoot. Its effect on the purity of the shear is rather slight. It is found that plasticity is initiated at the sides of the two holes where the tensile normal stresses are maximum. The plastic region first expands around the perimeter of the hole then radially outward; and finally, it expands into the significant region. If the W/H parameter is less than 5, a sizable portion of the width of the specimen around the hole can go plastic with the significant region still being in the elastic state. Such a situation can cause tearing of the specimen across the width. A W/H ratio of 6 or more can prevent that danger. It is also found that the onset of plasticity brings about higher overshoot and higher purity of shear. The notched shear specimen performs better in actual tests and is more reliable in producing shear failures. The shear strength results obtained from AIA shear tests tend to confirm those shear strength results obtained from torsion tests.

Pressure control to accommodate patient breathing efforts during volume ventilation.

Intermittent positive-pressure ventilation is used to support patients whose unassisted breathing is inadequate. Mechanical ventilators deliver pressurized gas to the patient's lungs by using a pattern of volume and timing that is preset by the clinician. A weakness of existing control methods is their emphasis on maintaining adequate gas exchange while poorly accommodating the patient's efforts to reassume control of the delivery pattern. A method is proposed to control airway pressure within a breath by making it respond to measurements of volume. This method using pressure as a function of volume, or P(V) method, permits the patient to have transient control over flow rate and delivered volume. In addition, an adaptive controller is included that modifies the applied pressure during subsequent breaths; it assures an average flow rate and delivered volume at the levels prescribed by the clinician, when sustained changes occur in airway resistance, lung-thorax compliance, or breathing efforts. Analyses and computer simulations suggest that the P(V) method will be better than conventional volume ventilation in accommodating, within a breath, transient breathing efforts without long-term degradation of the prescribed delivery pattern. The P(V) method can restore the delivery pattern, using the adaptive controller, within a few breaths after changes occur in the patient's lung mechanics. We conclude that the P(V) method is feasible, that it may represent an improved method of patient ventilation, particularly during fighting or weaning from the ventilator, and that it warrants further investigation.

Finite element temperature analysis of a total hip replacement and measurement of PMMA curing temperatures.

Analytical and experimental studies were performed to examine some characteristics of poly(methyl methacrylate) bone cement in the context of total hip replacement. Proceeding from fundamental principles of balance of energy and heat conduction, three simulation models of total hip replacement, using the finite-element method of numerical analysis, were used to predict the temperature response in the femoral prosthesis, cement, cancellous bone, and bone-cement interface. When cement collars are a centimeter or less in thickness, the models predict little likelihood of permanent bone thermal necrosis. In addition, laboratory experimental results indicate that peak temperatures attained in Simplex-P and Palacos-R cement specimens are nearly identical and that adding 0.5 and 1.0 g of the antibiotic gentamicin to Palacos-R has negligible influence on peak temperature. Experimental results also indicate that cooling cement specimens to 8 degrees C extends the set time, which in turn could significantly increase the likelihood of stem-cement loosening during surgery.

Stability of the upper lumbar spine following progressive disruptions and the application of individual internal and external fixation devices.

UNLABELLED: Five fresh human cadavera were tested to determine range-of-motion measurements at the interspace of the first and second lumbar vertebrae after progressive disruption of the joint followed by internal and external stabilization. The disruption progressed from posterior to anterior, leaving the anterior longitudinal ligament and anterior part of the annulus fibrosus intact. Flexion-extension range of motion was most sensitive to progressive disruptions and was significant following disruption of the facets. The Taylor-Knight brace was effective for limiting lateral motion, fair for limiting flexion-extension, and not effective for rotation. The three-point hyperextension brace was fairly effective for flexion-extension only. The body cast was effective in limiting all motions. Wire loops partially cut through the spinous processes in all cases with extreme flexion. Harrington distraction rods were effective in limiting motion if under proper tension, but they dislodged in three of the five specimens. CLINICAL RELEVANCE: Data from this study show that flexion of the second lumbar vertebra on the first of 20 degrees or a lateral bend of 10 degrees seen on a routine roentgenogram without vertebral fracture indicates that all posterior ligaments and at least part of the annulus fibrosus must be disrupted. Because internal fixation failed on occasion, we strongly urge the use of external fixation and careful mobilization of the patient to prevent flexion and rotation if internal stabilization is used for disruptions of the upper lumbar spine. The body cast was the most effective in limiting motion of the external fixation devices tested.

The contribution of the cruciate ligaments to the load-displacement characteristics of the human knee joint.

Human knee specimens were subjected to anterior-posterior, medial-lateral, varus-valgus, and torsional displacement tests. Loads were recorded for the intact joint and for the joint with all soft tissues cut except for the cruciate ligaments. The effect of condylar interference was determined for anterior-posterior, medial-lateral, and torsional displacements. The variation in load with flexion angle was considerable for medial-lateral (0-90-deg flexion) displacements, and less for varus-valgus (0-45-deg flexion) displacements. The cruciates were found to carry almost the entire anterior-posterior load; they carried a significant percentage of the medial-lateral load which varied considerably with flexion angle. A small, but not insignificant percentage of the varus-valgus load was carried by the cruciates and the variations with flexion angle were small. In torsion, the cruciates resisted only internal rotation. In the tested displacement ranges, condylar interference had a small effect on the medial-lateral load but did not affect anterior-posterior or torsional loads.

Control of occupational exposure to nitrous oxide in the oral surgery office.

Occupational exposure to inhalation anesthetics can be substantially reduced by control measures that have been recently developed. The incentive for their use should be the suspected relationship between chronic exposure to trace concentrations of agents, such as nitrous oxide, and health hazards. Control measures include the use of a newly developed scavenging nasal mask, relatively gastight anesthetic equipment, and vented suction machine, supported by an air monitoring program. With these measures, the concentration of nitrous oxide inhaled by the oral surgeons studied was reduced 97%.

Control of occupational exposure to nitrous oxide in the dental operatory.

Methods were developed for controlling the dental team's occupational exposure to nitrous oxide. The most applicable and effective use of these methods included the use of properly maintained gas delivery equipment, a double-walled scavenging nosepiece and vented suction machine, and minimizing speech by the patients. These methods were evaluated by measuring concentrations of nitrous oxide present in the air inspired by dental personnel. Before their use, the dentist inhaled 900 ppm nitrous oxide; their application reduced his inhaled concentration to 31 ppm, representing a 97% reduction. These methods were well accepted during 157 procedures completed by a group of eight dentists engaged in private practice (four general practitioners, two pedodontists, and two oral surgeons).

Distribution of waste anesthetic gases in the operating room air.

Epidemiologic and animal studies identify a strong relationship between chronic exposure to anesthetic gases and health hazards. Efforts to reduce exposure of personnel require an understanding of the distribution of anesthetic waste gases in the operating room air. Concentrations of nitrous oxide and halothane were measured at numerous stations throughout an operating room and a delivery room in the absence of personnel. Air conditioning flow rates and flow patterns were varied, as was the height of the anesthetic gas source. Air flow patterns were found to dominate the anesthetic gas distribution, while buoyancy effects were negligible. Venting waste gases at the floor does not significantly reduce exposure of personnel. Areas of high concentration were observed; their occurrences and locations varied strongly with air flow patterns. The exhaust grille is the best location for a single measurement of the average room concentration. Recirculating air-conditioning systems reduce energy costs; however, only the non-recirculating portion of the air exchanges reduces waste gas concentrations.

Modeling of the human leg in ski injuries.

This study establishes that the human lower extremity-ski system has a fundamental frequency of approximately 1 Hz. The fundamental mode dominates the system response when loading times are short compared to the first natural frequency. Consequently, the use of a single degree of freedom approximation and the use of impulse criteria can provide important insights into the problem of skiing injuries. Specifically, the sport of snow skiing develops forces with frequency content in the static, transition, and impulse regions of the leg-ski response spectrum. Therefore, ski release bindings must be designed to safely and effectively couple the leg to the ski throughout this loading environment. An important means of evaluating possible binding designs is by simulating their characteristics in a computer model. The model will apply realistic loading histories to the ski or binding, will evaluate the strain in the bones and ligaments of the lower extremity, and will determine the release point of the binding. The program presented in this discussion is one stage in the development of this total system capability.