The effect of pressure loading on the blood flow rate in human skin.

The effect of pressure on the blood flow in skin is of considerable clinical interest. Methods are described for the estimation of skin blood flow from the disappearance rate of an injection of 133Xe in saline. The flow rate may be monitored for a period long enough to establish the normal flow and the reduced flow resulting from a constant pressure load. Initial results indicate that the flow is reduced greatly by pressures up to 10 mmHg. This result is interpreted as a demonstration of an auto regulatory mechanism of skin blood flow. Above 30 mmHg the flow continues to decrease essentially to zero as systolic pressure is approached.

Biomechanical properties of dermis.

Data obtained from a wide variety of test methods both in vitro and in vivo and histological studies of stressed skin have led to an understanding of the mechanical properties of the dermis and the relation of these properties to the structure of the collagen and elastin fiber networks of the dermis. The mechanical properties are found to be well adapted to the mechanical function of the dermis. The viscoelastic nature of the skin shows that a simple structural model based only on collagen and elastin is not adequate for a full understanding of this tissue. The role of proteoglycans in bonding collagen fibrils into large fibers and in connecting these fibers into the fibrous net of the dermis is not well understood. Mechanical testing can yield information on the nature of bonds at this level of structure.

Age-related changes in the mechanical properties of human skin.

The mechanical properties of skin have been studied both in vivo and in vitro by a variety of test methods. These properties are well matched to the function of the skin, and they depend on the geometry of the collagen and elastin networks of the dermis. The time dependence of these properties is thought to be related to the "ground substance" components of the dermis. Age-related changes in the mechanical properties are a function of the degradation of the elastin network and of some as yet undefined changes in the viscoelastic properties of the "ground substance."

Comparison of residual stability in thoracolumbar spine fractures using neutral zone measurements.

Because treatment algorithms for spinal injuries depend largely on the clinical assessment of stability after injury, this study both quantified and compared the mechanical stability after three different patterns of injury in the thoracolumbar spine. We created compression fractures, burst fractures, and flexion-distraction injuries in 26 thoracolumbar specimens from human cadavers in order to compare residual stability as a function of type of injury. Spinal stability was evaluated using measurements of the boundaries of the neutral zone, which provide a measure of spinal laxity in various directions of motion. An increase after injury was indicative of greater spinal laxity and hence reduced residual stability. Geometric characteristics (or parameters) of the neutral zone boundaries were used for statistical comparison between the types of injury. Of the three groups, burst fractures retained the least residual stability and compression fractures, the greatest. The angular ranges of motion in the neutral zone for burst fractures demonstrated increases (compared with average values for intact specimens) of 154% in flexion-extension, 134% in lateral bending, and 108% in torsion after injury. The results for flexion-distraction injuries were similar to those for burst fractures in flexion-extension (126%) and torsion (62%); however, more residual stability was retained in lateral bending than was seen for burst fractures (48%). Compression fractures retained the most residual stability, with increases in motion of 40% in flexion-extension, 56% in lateral bending, and 3% in torsion. These findings may be useful in determining the necessity for surgical stabilization of the spine and selection of the appropriate system of fixation.

Load fatigue performance of two implant-abutment combinations.

The load fatigue performance of the CeraOne and EsthetiCone implant systems was investigated. Three torques (standard torque, standard torque + 20%, and standard torque -20%) were used to tighten the screws. A rotating-beam fatigue model was used. A 70-N load was applied to the specimens at a 30-degree angle to the long axis. Two-way analysis of variance revealed (1) no statistically significant difference between the three torques for each system, and (2) a statistically significant difference between the implant systems. Implant failure occurred in 10 of 15 CeraOne specimens. In the EsthetiCone system, only abutment screw failure occurred.

Normal and shear stresses on a residual limb in a prosthetic socket during ambulation: comparison of finite element results with experimental measurements.

Interface stresses on a below-knee amputee residual limb during the stance phase of gait calculated using an analytical finite element model were compared with experimental interface stress measurements. The model was quasi-static and linear. Qualitatively, shapes of analytical and experimental interface stress waveforms were similar in that they were double-peaked with some distinct features apparent. However, quantitatively analytical resultant shear stress magnitudes were less than experimental values at all transducer measurement sites. Analytical normal stresses were less than experimental values at postero-proximal, postero-distal, and anteromedial proximal sites, but were greater than experimental values at antero-lateral distal and antero-lateral proximal sites. Anterior resultant shear angles were directed more distally in the model than in clinical data, an expected result since there was no relief for the tibial crest in the model. Model sensitivity analyses to shank loads showed interface normal and resultant shear stresses were most sensitive to axial force, sagittal bending moment, or sagittal shear force. The finite element model presented in this paper is significant because it contributes toward development of an analytical modeling technique to predict interface stress distributions for proposed prosthetic designs, provides insight into physical explanations of features apparent in interface stress waveforms (thereby enhancing understanding of interface mechanics), and provides insight into nonlinear characteristics that need to be added to improve the model.

Interface shear stresses during ambulation with a below-knee prosthetic limb.

Shear stresses on a residual limb in a prosthetic socket are considered clinically to contribute to tissue breakdown in below-knee amputees. When applied simultaneously with normal stresses, they can cause injury within the skin or can generate an abrasion on the surface. To gain insight into shear stresses and parameters that affect them, interface stresses were recorded on below-knee amputee subjects during walking trials. On the tibial flares, resultant shear ranged from 5.6 kPa to 39.0 kPa, while on the posterior surface it ranged from 5.0 kPa to 40.7 kPa. During stance phase, anterior resultant shears on a socket were directed toward the apex while posterior resultant shears were directed downward approximately perpendicular to the ground. Waveform shapes were usually double-peaked, with the first peak at 25% to 40% into stance phase and the second peak at 65% to 85% into stance. Application of these results to residual limb tissue mechanics and prosthetic design is discussed.

How does vacuum forming affect Pelite mechanical properties?

Pelite is a polyethylene closed cell foam commonly used as an interface material in prosthetics. Both normal and vacuum-formed Pelite were tested under compression and under shear loading. For shear testing, the results were not significantly different for normal and vacuum-formed Pelite. For normal Pelite, the slope of the stress-strain curve was 1.17MPa (+/- 0.14) while for vacuum-formed Pelite it was 1.24MPa (+/- 0.22). Compressive results, however, were significantly different. Below 80kPa of applied compression, the slope of the stress-strain curve for normal Pelite was 0.99MPa (+/- 0.11) while for vacuum formed Pelite it was 0.72MPa (+/- 0.12). Between 80kPa and 200kPa of applied compression, the slope of the stress-strain curve for normal Pelite was 0.45MPa (+/- 0.03) while for vacuum formed Pelite it was 0.55MPa (+/- 0.05). Reasons for the differences and their significance to interface mechanics and computer-aided prosthesis design are discussed.

Interface pressures and shear stresses: sagittal plane angular alignment effects in three trans-tibial amputee case studies.

Interface pressures and shear stresses at different sagittal plane angular alignment settings were measured on 3 trans-tibial amputee subjects ambulating with patellar-tendon-bearing total contact prostheses. Substantial socket-shank angular alignment modifications in the sagittal plane had minimal effect on stance phase peak interface pressures, though more substantial effects on stance phase peak resultant shear stresses. No consistent trend of a greater stress at misaligned vs nominally aligned settings was identified. Changes in interface stresses from session to session tended to be greater than those for different alignment settings, suggesting that subjects compensated well for misalignments but less well for session differences.

Clinical measurement of normal and shear stresses on a trans-tibial stump: characteristics of wave-form shapes during walking.

Stresses on the surface of a stump within a prosthetic socket during walking can potentially traumatize stump tissues. To gain insight into stresses and design parameters that affect them, normal and shear interface stresses were measured on three unilateral trans-tibial amputee subjects during walking trials. During stance phase repeated characteristics in wave-form shapes from different subjects were apparent. They included "loading delays", "high frequency events (HFE's)", "first peaks", "valleys", "second peaks", and "push-off". Characteristics did not necessarily occur at the same time from one step to the next but their timings matched well with events in shank force and moment data which were collected simultaneously. For "plantarflexion" and "dorsiflexion" alignment changes, the above wave-form characteristics were still present but their timings within the stance phase changed. The physical meaning and relevance of the characteristics to stump tissue mechanics are discussed.

An angular alignment measurement device for prosthetic fitting.

A device to measure socket/shank angular alignment in a prosthesis equipped with a Berkeley Adjustable Leg is described. Angular alignment in the sagittal plane can be measured over the entire 20-degree range with a repeatability of 1 degree. This device can be a useful prosthetics fitting, teaching, and research tool.

A sensitive and rapid mass spectrometric method for the simultaneous measurement of eight steroid hormones and CALIPER pediatric reference intervals.

OBJECTIVES: To develop an accurate assay and establish the normal reference intervals for serum cortisol, corticosterone, 11-deoxycortisol, androstenedione, 21-hydroxyprogesterone, testosterone, 17-hydroxyprogesterone, and progesterone. These steroids are commonly used as biomarkers for the diagnosis and monitoring of endocrine diseases such as congenital adrenal hyperplasia. Appropriate age- and gender-stratified reference intervals are essential in accurate interpretation of steroid hormone levels. DESIGN AND METHODS: The samples analyzed in this study were collected from healthy, ethnically diverse children in the Greater Toronto Area as part of the CALIPER program. A total of 337 serum samples from children between the ages of 0 and 18years were analyzed. The concentrations were measured by using an LC-MS/MS method. The data were analyzed for outliers and age- and gender-specific partitions were established prior to establishing the 2.5th and 97.5th percentiles for the reference intervals. RESULTS: Reference intervals for all hormones required significant age-dependent stratification while testosterone and progesterone required additional sex-dependent stratification. CONCLUSIONS: We report a sensitive, accurate and relatively fast LC-MS/MS method for the simultaneous measurement of eight steroid hormones. Detailed reference intervals partitioned based on both age and gender were also established for all eight steroid hormones.

Bone-titanium implant response to mechanical stress.

The purpose of this study was to investigate the nature and strength of any bond which may develop between alveolar bone and titanium allow implants. Implants were placed in a previously prepared edentulous region in the mandibles of dogs. The implants were conical in shape with circumferential grooves to provide axial retention. Five months later the implants were rotated. No indication was found of an adhesive bond being broken.

Indicator pastes: their behavior and use.

Indicator pastes were subjected to a laboratory investigation and a clinical trail. The tests showed: 1. An increase in temperature increased the rate of flow of the pastes. 2. An increase in applied pressure increased the rate of the flow of the pastes. 3. Pastes containing a small percentage of filler were compressed to a smaller thickness than those containing a greater percentage of filler. 4. The zinc oxide-hydrogenated oil mixture demonstrated a biphasic flow pattern. Pressure Indicator Paste demonstrated a simple flow pattern. 5. Similar tests did not give identical results. However, the characteristics of flow curves for similar tests were consistent. 6. A clinical trial using eight subjects showed no marked differences between three indicator pastes but did show significant differences between the interpretations of paste patterns by four dentists. This evidence suggests that the nature of the suspension medium affects both the type of flow and the rate of flow of an indicator paste. The percentage composition of filler affects both the final film thickness and the rate of flow. Improved criteria for the interpretation of paste patterns must be developed before clinical differences in paste behavior are noted.