Keyoung Jin Chun

Mechanical Responses of Tendons to Repeated Extensions and Wait Periods

Tendon specimens were repeatedly extended to peak strains of either 2, 3, 4, or 6 percent. During the three 1800 s (30 min.) periods of cyclic extension, the peak loads relaxed with decreases in hysteresis and increases in slack strain. During the two 1800 s wait periods of no extension, the specimens recovered with increases in peak load and hysteresis and decreases in slack strain. However, the recovery during the wait periods was eradicated in the first few subsequent extensions and the relaxation continued as if there were no 1800 s wait periods. Stress-strain responses were well fit with power relations.

A tribological study of refrigeration oils under HFC-134a environment

HFC-134a is a potential alternative material for CFC-12, which depletes the ozone layer. However, problems arise when used together with conventional refrigeration oils due to the extreme polarities of HFC-134a. PAG (polyalkylene glycol) and esters are tested with HFC-134a. This investigation enhances the testing method by taking the compressors environment into account. Miscibility and material compatibility are proved with sealed glass tests. A testing environment charged with refrigerant gas more closely simulates the conditions of a compressor. The conventional refrigeration oils (mineral oils, alkylbenzene, PAO (polyalpha olefin)) tested here are immiscible with HFC-134a. However the PAG and ester oils are miscible with HFC-134a. The friction coefficient of the PAG/HFC-134a system is similar to that of the mineral oil/CFC-12 system at operating conditions. Ester oils are preferable at the start and stop condition in the lubricity aspect. PAG shows good lubricity in conditions of extreme contact pressure. Consequently, this test provides reliable results on compressor lubricity and material compatibility of refrigeration oils with HFC-134a. It suggests a methodological way for the proper selection of refrigeration oils that may improve the durability and performance of a compressor.

Low-intensity ultrasound stimulation prevents osteoporotic bone loss in young adult ovariectomized mice†

Osteoporosis is a disease characterized by low bone mass, increased bone fragility, and a greater risk for bone fracture. Currently, pharmacological intervention can generally aid in the prevention and treatment of osteoporosis, but these therapies are often accompanied by undesirable side effects. Therefore, alternative therapies that minimize side effects are necessary. Biophysical stimuli, especially low‐intensity ultrasound stimulation (LIUS), may be potential alternatives to drug‐based therapies for osteoporosis. Hence, we sought to address whether LIUS therapy can effectively prevent or treat osteoporotic bone loss induced by estrogen deficiency. LIUS (1.5 MHz frequency, 1.0 kHz pulse repetition on frequency, 30 mW/cm2 intensity, 200 µs pulse length) was applied to right tibiae of eight 14‐week‐old ovariectomized virgin ICR female mice for 20 min per day, 5 days per week, over a 6‐week period. Changes in 3D structural bone characteristics were detected using in vivo micro‐computed tomography. Left tibiae served as controls. Structural characteristics including bone volume/tissue volume, trabecular number, trabecular bone pattern factor, and mean polar moment inertia were significantly enhanced 6 weeks after LIUS compared to the control, nonstimulated group (p < 0.05). In particular, the bone volume/tissue volume in the region exposed directly to LIUS was significantly higher in the treated group (p < 0.05). These findings indicate that new bone formation may be activated or that bone structure may be maintained by LIUS, and that LIUS may be effective for preventing estrogen deficiency‐induced bone loss.

Use of the Microsoft Kinect system to characterize balance ability during balance training.

The risk of falling increases significantly in the elderly because of deterioration of the neural musculature regulatory mechanisms. Several studies have investigated methods of preventing falling using real-time systems to evaluate balance; however, it is difficult to monitor the results of such characterizations in real time. Herein, we describe the use of Microsoft’s Kinect depth sensor system to evaluate balance in real time. Six healthy male adults (25.5±1.8 years, 173.9±6.4 cm, 71.4±6.5 kg, and 23.6±2.4 kg/m2), with normal balance abilities and with no musculoskeletal disorders, were selected to participate in the experiment. Movements of the participants were induced by controlling the base plane of the balance training equipment in various directions. The dynamic motion of the subjects was measured using two Kinect depth sensor systems and a three-dimensional motion capture system with eight infrared cameras. The two systems yielded similar results for changes in the center of body mass (P>0.05) with a large Pearson’s correlation coefficient of γ>0.60. The results for the two systems showed similarity in the mean lower-limb joint angle with flexion–extension movements, and these values were highly correlated (hip joint: within approximately 4.6°; knee joint: within approximately 8.4°) (0.400.05). Large differences with a low correlation were, however, observed for the lower-limb joint angle in relation to abduction–adduction and internal–external rotation motion (γ<0.40) (P<0.05). These findings show that clinical and dynamic accuracy can be achieved using the Kinect system in balance training by measuring changes in the center of body mass and flexion–extension movements of the lower limbs, but not abduction–adduction and internal–external rotation.

A comparative study of histogram-based thresholding methods for the determination of cell-free layer width in small blood vessels

We have recently proposed a computer-based method utilizing a thresholding algorithm (the Otsu method) to provide a convenient way of measuring the cell-free layer width in vivo and in vitro. However, this method does not seem to be a universal method that can be applied to all microvascular studies. Thus, we examined four different histogram-based thresholding algorithms (Otsu, intermode, minimum and second peak) to provide a technical suggestion on the selection of a suitable thresholding algorithm for the cell-free layer measurement. All the measurements were taken in microvascular flows in the rat cremaster muscle recorded with a high-speed camera. The width of the cell-free layer manually measured was compared with that determined by the automated method utilizing the four thresholding algorithms. With our experimental system, results showed that the cell-free layer width determined by the minimum algorithm was in best accordance with the manual measurement. We concluded that the accuracy of the automated methods for determination of the cell-free layer width would depend on the image quality, in particular on the contrast between the red blood cell core and background, which might differ due to the different microscopic setup. Therefore, one may need to examine several appropriate thresholding methods when selecting the best suitable algorithm for the experimental conditions.

Sectional differences in tendon response

The objectives of this work here focus on the differences in responses to multiple cyclic tests of different sections along the length of the same tendon. Tendon specimens were obtained from the hindlimbs of canines and frozen to -70°C. After thawing, specimens were mounted in the immersion bath at room temperature (22°C). preloaded to 0.13 N and then subjected to 3% or 4% of the initial length at a strain rate of 5%/sec. It was found that different sections of the same long tendons had different resistances to deformation. In general, the bone end sections were stiffer and carried greater loads for a given strain than the muscle end sections, and the mid-portions were the least stiff and carried the smallest loads for a given strain. The results of this study offer new information about the mechanical responses of collagenous tissues. We know more about their responses to multiple cyclic extensions and how their responses are different from the positions along the length of the tendon specimen. The nature and causes of these differences in the stiffness are not fully known. However, it is clear that differences in the mechanical response of tendons and other connective tissues are significant to musculoskeletal performance.

Alteration patterns of trabecular bone microarchitectural characteristics induced by osteoarthritis over time

Information regarding the alteration of trabecular bone microarchitecture, which is one of the important criteria to estimate bone condition, induced by osteoarthritis (OA) is sparse. The current study therefore aimed to identify and quantify patterns of alterations in trabecular bone microarchitectural characteristics at tibial epiphysis induced by OA using in vivo microcomputed tomography. Fourteen 8-week-old female Sprague Dawley rats were randomly divided into control (n = 7) and OA (n = 7) groups. Rats in the OA group were administered monoiodoacetate into the knee-joint cavity. The tibial joints were scanned by in vivo microcomputed tomography at 0, 4, and 8 weeks after administration. Two-way analysis of variance with Tukey’s honestly significant difference post hoc test was carried out for statistical analyses. The results showed that patterns of alterations in the trabecular bone microarchitectural characteristics in the OA group were not different from those in the control group from 0 to 4 weeks (P > 0.05), but differed from 4 to 8 weeks (P < 0.05). In particular, both trabecular bone thickness and trabecular bone separation distributions over time (4–8 weeks) differed significantly (P < 0.05). These findings suggest that the patterns of bone microarchitecture changes brought about by OA should be periodically considered in the diagnosis and management of arthritic symptoms over time. Improved understanding of the alteration pattern on trabecular bone microarchitecture may assist in developing more targeted treatment interventions for OA.

Constitutive Model of Tendon Responses to Multiple Cyclic Demands ( I ) - Experimental Analysis -

The work reported here is an extensive study of tendon response to multiple cyclic tests including 3% constant peak strain level test (A-type test), 3% constant peak strain level test with two rest periods (B-type test), and 3–4% different peak strain level test (C-type test). A sufficient number of speciments were tested at each type of the test to statistically evaluate many changes in response during testing and differences in response between each type of the test. In cyclic tests, there were decreses (relaxations) in the peak stresses and hysteresis, increases in the slack strains, and during lower peak strain level (3%) cyclic block after higher peak strain level (4%) cyclic block in the C-type tests. Considering the results of this study and those of the other study of multiple cyclic tests with rest periods by Hubbard and Chun, 1985,recovery phenomena during the rest periods occurred predominantly at the beginning of the rest periods. Consistently in both studies, the effects of rest periods were small and transient compared to the effects of the cyclic extensions. The recovery with cycles at lower peak strain level (3%) after higher peak strain level (4%) in the C-type test has not been previously documented. This recovery seems to be a natural phenomena in tissue behavior so that collagenous structures recover during periods of decreased demand.

Constitutive Model of Tendon Responses to Multiple Cyclic Demands (II) - Theory and Comparison -

The hereditary integral form of a quasi-linear viscoelastic law has been employed. Four new concepts have been employed: 1. a reduced relaxation function with a non-linear exponential function of time, 2. an inverse method to determine the scale factor of the elastic response, 3. an instant elastic recovery strain during unloading, and 4. the results of a constitutive model for cyclic tests may be a function of the Heavyside class. These concepts have been supported by agreement between measured and predicted responses of soft connective tissue to three types of multiple cyclic tests which include rest periods of no extension and alternations between different strain levels. Such agreement has not been attained in the previous studies. Chun and Hubbard (2001) is our companion experimental analysis paper.

Spatial Variation in Material Properties in Fascicle-Bone Units from Human Patellar Tendon

An understanding of the mechanical responses of the patellar tendon (PT) subunits should aid in determining which portion of the tissue might best be used as a cruciate ligament replacement. Human cadaveric knees were obtained from young donors. Fascicle groups with patellar and tibial bone blocks (subunits) were cut to provide six equally-spaced test specimens for each PT. After potting each bone end in PMMA, specimens were mounted in a saline-filled chamber, preloaded to 0.26 N and then subjected to 40 cycles of preconditioning to 2.5 % of the initial length at a strain rate of 1.25 %/sec, and then preloaded to 0.26 N again and failed at a strain rate of 100 %/sec using an Instron. The moduli and maximum stresses were generally greater in the lateral and mid subunits than in the medial subunits. The strains to maximum stress were similar between the lateral and medial subunits, but mid subunits had larger strains. Most strain values were distributed between 10 % to 20 %. Mechanical responses of human PT do vary from location to location. In general, the mid and lateral subunits were stiffer and carried greater stresses than the medial subunits. The results of this research should eventually be important, e.g. in selecting which portion of the PT would be the most suitable for cruciate ligament replacements to use as an autograft. On the basis of strength and stiffness, the more lateral portion of the PT would seem to be more advantageous.