Polymers near metal surfaces: selective adsorption and global conformations.

We study the properties of a polycarbonate melt near a nickel surface as a model system for the interaction of polymers with metal surfaces by employing a multiscale modeling approach. For bulk properties, a suitably coarse-grained bead-spring model is simulated by molecular dynamics methods with model parameters directly derived from quantum chemical calculations. The surface interactions are parametrized and incorporated by extensive quantum mechanical density functional calculations using the Car-Parrinello method. We find strong chemisorption of chain ends, resulting in significant modifications of the melt composition when compared to an inert wall.

Assessment of the directional elastic moduli of ewe vertebral cancellous bone by vibrational testing.

The ovariectomized ewe is being used as an animal model for postmenopausal osteoporosis. Data on the mechanical properties of ewe vertebral cancellous bone is needed to assess its effectiveness as a model for vertebral osteoporosis. This study utilized traditional compression testing and a novel nondestructive vibrational testing method to assess the directional mechanical properties of ewe vertebral cancellous bone. Composition and density properties were also assessed. It was hypothesized that vibrational testing would have utility in that it would allow for the anisotropic stiffness of cancellous bone to be assessed nondestructively. The present study has found that ewe vertebral cancellous bone has similar physical and mechanical properties to humans. The vibrational testing method described was able to nondestructively provide a valid measure of stiffness that was correlated with stiffness estimates from traditional compression testing. Furthermore, the stiffness measure from the vibration test was found to be sensitive to the architecture of cancellous bone. These results suggest the promise of this testing method for the nondestructive mechanical assessment of skeletal tissue.

Placement of screws in the sustentaculum tali. A calcaneal fracture model.

Current methods of treating calcaneus fractures vary greatly, and many techniques of internal fixation have been described. The study of these fractures has been limited in part by the lack of a suitable laboratory model. In this study, a new cadaveric model of calcaneus fractures was developed, using a combination of osteotomies and impaction. The model allows a pattern of intraarticular injury to be reproduced consistently. The model was used to examine one aspect of internal fixation. It was hypothesized that fixation would be more stable if the screws supporting the posterior facet were incorporated into the lateral plate, as opposed to being separate from the plate. Six pairs of anatomic specimen legs were used, and each pair was divided randomly between two experimental groups. In Group A (screws out), the posterior facet screws were outside the plate, and in Group B (screws in), the screws were incorporated into the plate. The strength of the reconstructed calcanei were evaluated by axial loading of the limb through the tibia. Stiffness and energy to failure were significantly greater and Bohler's angle significantly less compromised in Group B. It was concluded that the position of the articular fragment of comminuted calcaneal fractures will be maintained at higher loads when the screws in the posterior facet are incorporated into the lateral plate. The model of calcaneal fractures described in this study may be suitable for examining other aspects of fixation.

Holding power of different pin designs and pin insertion methods in avian cortical bone.

OBJECTIVE: To measure pullout strength of four pin types in avian humeri and tibiotarsi bones and to compare slow-speed power and hand insertion methods. STUDY DESIGN: Axial pin extraction was measured in vitro in avian bones. ANIMAL POPULATION: Four cadaver red-tailed hawks and 12 live red-tailed hawks. METHODS: The pullout strength of four fixator pin designs was measured: smooth, negative profile threaded pins engaging one or two cortices and positive profile threaded pins. Part 1: Pins were placed in humeri and tibiotarsi after soft tissue removal. Part 2: Pins were placed in tibiotarsi in anesthetized hawks using slow-speed power or hand insertion. RESULTS: All threaded pins, regardless of pin design, had greater pullout strength than smooth pins in all parts of the study (P < .0001). The cortices of tibiotarsi were thicker than the cortices of humeri (P < .0001). There were few differences in pin pullout strengths between threaded pin types within or between bone groups. There were no differences between the pullout strength of pins placed by slow-speed power or by hand. CONCLUSIONS: There is little advantage of one threaded pin type over another in avian humeri and tibiotarsi using currently available pin designs. There were few differences in pin pullout strengths between humeri and tibiotarsi bones. It is possible that the case of hand insertion in thin cortices minimizes the potential for wobbling and therefore minimizes the difference between slow-speed drill and hand insertion methods. CLINICAL RELEVANCE: Threaded pins have superior bone holding strength in avian cortices and may be beneficial for use with external fixation devices in birds.

Axial vibration of threaded external fixation pins: detection of pin loosening.

The hypothesis of this study was that a nondestructive vibrational method could detect bone lysis at the external fixation pin-bone interface prior to current clinical and radiographic methods. In vitro models were used to simulate changes observed during pin loosening in vivo. Fixation pin axial natural frequency decreased with decreasing tensile modulus of the material into which it was implanted. In a live animal study the right tibia of 12 dogs was fractured and stabilized with a four-pin unilateral external fixation frame. The axial natural frequency of each pin was measured and radiographs were taken at 0, 2, 4, 6, 8, and 10 weeks after surgery. The natural frequency did not change when the first radiographic changes around the interface were observed. Pins were palpably stable at this point. As loosening progressed, the natural frequency did decrease. Frequency and quasistatic tests of dissected pin-bone structures revealed a good correlation between natural frequency and pin-bone interface stiffness. In addition, the measurement of natural frequency was more sensitive to bone structure changes at the pin-bone interface than low-load quasi-static stiffness. Therefore, a nondestructive vibration technique could be used instead of low-load quasistatic tests for assessing the pin-bone interface ex. vivo.

Biomechanical evaluation of a toggle pin technique for management of coxofemoral luxation.

Toggle pin stabilization is an accepted technique for the management of coxofemoral (CF) luxation in dogs. The purpose of this study was to determine, in vitro, the respective contributions of several aspects of toggle pin repair to the overall stability of fixation. Factors evaluated were the manner and frequency with which toggle pins oriented on insertion, effect of orientation on toggle pin strength, effect of suture type on ligament prosthesis strength and load sustained by the fixation, and comparison of repair using a modified toggle design to that of capsulorrhaphy. When placed in cadavers using standard technique, conventional toggle pins were found to orient significantly more frequently in one of two possible positions. Mechanical testing of fixations performed in experimentally luxated cadaver hips demonstrated a high (12/20) incidence of toggle pin failure using the conventional implant in the most common orientation. When tested alone, toggle pins were weakest mechanically in this orientation. Rotating the implant 180 degrees increased mean load to failure by 249%. There was no significant difference in load sustained by conventional toggle fixations using No. 2 braided polyester versus 50 lb test monofilament nylon as the suture ligament prosthesis. However, the higher stiffness of the polyester suture may be more favorable for use in this application. Fixation using a toggle rod designed to allow evaluation of construct stability when failure of the toggle is eliminated resulted in an increase in maximum load sustained before luxation (47% of the intact control hips). This load was not significantly different than the resistance to luxation afforded by capsulorrhaphy. This study suggests that when implanting conventional toggle pins, consideration should be given to ensuring placement in the strongest orientation.

Holding power of orthopedic screws in the large metacarpal and metatarsal bones of calves.

Holding power was determined for various orthopedic screws in bones of calves. Holding power was defined as maximal tensile force required to remove a screw divided by thickness of bone engaged by the screw (kN/mm). Comparative pull-out tests were performed, using pairs of large metacarpal or metatarsal bones from calves aged 3 to 14 days. Comparisons were made of the holding power of 6.5-mm fully threaded cancellous screws and 5.5-mm cortical screws in the proximal and distal metaphyses, and of 4.5-mm and 5.5-mm cortical screws in the diaphysis. Sixteen repetitions of each comparative trial were performed. There was no statistically significant difference in the holding power of 4.5- and 5.5-mm cortical screws in the diaphysis. There was no significant difference in the holding power of 5.5-mm cortical and 6.5-mm fully threaded cancellous screws in the proximal metaphysis. In the distal metaphysis, 6.5-mm fully threaded cancellous screws had significantly (P < 0.001) greater holding power than did 5.5-mm cortical screws. There was no significant difference between the mean holding power of 5.5-mm cortical screws in the proximal metaphysis and 5.5-mm cortical screws in the distal metaphysis. There was significantly (P < 0.01) greater mean holding power of 6.5-mm cortical, fully threaded cancellous screws in the distal metaphysis, compared with the proximal metaphysis.

Characterization of the non-linear loading curve of rat skin.

A phenomenological model was developed to characterize the non-linear portion of stress-strain curves for skin tissue removed from laboratory rats and tangent modulus calculations were used to characterize the linear portion of the curves. Modified coefficient of determination calculations obtained from non-linear regression analysis showed that the model was able to fit the actual data above the 90% level. The model coefficients and tangent modulus values were used to statistical comparisons. Combination treatments of radiation, hyperthermia and surgical incision were used to analyze changes in the loading response of rat skin. Least significant difference statistical analysis (P < 0.10) of the model coefficients showed that radiation treatment affected skin stiffness only in tissue when surgical wounding was applied and that radiation-hyperthermia treatment affected skin stiffness only in tissue when surgical wounding was not applied. Hyperthermia applied with no radiation treatment did not affect skin stiffness. Surgical wounding resulted in decreasing skin stiffness.

Subsidence of an uncemented canine femoral stem.

Factors contributing to subsidence were analyzed by radiographic evaluation and mechanical testing of 36 canine cadaver femora during and after insertion of an uncemented porous-coated femoral stem and by radiographic evaluation of 35 canine total hip arthroplasties. Mean percentage of canal fill in immediate postoperative radiographs, and percentage of canal fill at midimplant and distal implant locations, were accurate predictors of subsidence. Force required to implant the femoral stem was strongly correlated with force required for implant subsidence. Femoral morphology and percentage of canal fill at the middle and distal sites were accurate predictors of subsidence. Implants in femora with a stovepipe morphology (canal flare index less than or equal to 1.8) were six times more likely to subside than implants in femora that had a normal appearance (canal flare index 1.8 to 2.5), and 72 times more likely to subside than implants in champagne-fluted femora (canal flare index greater than or equal to 2.5). Femora with more than 85% mean, middle, or distal canal fill were less likely to subside.

Effects of freezing on mechanical properties of rat skin.

Two test specimens of skin were cut from the lateral aspect of each hind limb of 9 rats. Specimens were contiguous, thereby providing matched pairs. One specimen was immediately placed in liquid nitrogen for 5 minutes, then stored at -70 C and tested within 3 to 4 weeks. Within 5 minutes of harvest, the second specimen was used for immediate material testing. Basic engineering material tests were used to measure strength, loading response, and elastic and viscous properties. Each matched pair of tissues was used for the same procedure. Quasistatic uniaxial tensile tests were used to apply deformations to the test specimens, and resulting loads were recorded. Stress and strain were calculated from the recorded data, providing information on yield strength, ultimate strength, fracture strength, and loading response. Each matched pair of specimens represented 1 repetition; 6 repetitions were made of each observation. Statistical analysis indicated that tissue freezing significantly (P less than 0.05) increased fracture strength, but did not affect strength, ultimate strength, or loading response. Dynamic vibration response tests were used to find mechanical mobility of the specimens, thereby providing information on elastic and viscous behaviors, which were quantified by calculation of spring and damping coefficients, respectively. As before, 6 repetitions were used. Statistical analysis indicated that tissue freezing did not affect these coefficients.

Development of a simulator for use in the measurement of chain saw vibration.

The measurement of vibration production of light, hand held power tools is important in the assessment of the potential for elicitation of vibration injury to the operators of these tools. The manner in which these measurements are made can greatly affect the results. The measured vibration production of the tool becomes a function of the operator holding the saw during the measurement and his physical characteristics. The objective of the work reported here was to develop a simulator of the operator which might be used in the vibration production analysis of chain saws. The operator was modelled in terms of the driving point mechanical impedance characteristics of humans for input to the hands. A simulator was developed based on the driving point impedance characterization of the operator and evaluated with chain saw vibration measurements.