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Biomedical subjects

K A Thomas

Publications and source records attributed to K A Thomas.

At least 37 records · Page 2Linked to original sources
PubMed1998

Elongation patterns of the collateral ligaments of the human knee.

OBJECTIVE: To determine the responses of the medial and lateral collateral ligaments (MCL, LCL) of the human knee to externally applied stresses. DESIGN: Differential variable reluctance transducers were used to measure length changes along the long posterior parallel fibers of the MCL and the middle third of the LCL through a flexion range of 15-120 degrees and a variety of external stresses. BACKGROUND: There is a lack of consensus regarding the collateral ligament response to internal and external tibial rotation. In addition, there are very few studies that have investigated the effects of quadriceps and hamstrings muscle group loads on the strain in the collateral ligaments. METHODS: Three series of tests were performed. First, the passive behavior of the ligaments was obtained as well as the ligament response to 3 degrees of varus and valgus rotation. The next series tested the ligaments' response to 0-10 degrees of internal and external tibial axial rotation. Finally, isolated and co-contracted quadriceps and hamstrings muscle group loads were applied. RESULTS: The instrumented portions of both ligaments were more strained in extension than flexion. Varus rotations stretched the LCL, whereas valgus rotations elongated the MCL. The strain in the MCL was shown to increase during external rotation and decrease during internal rotation. The LCL did not exhibit a uniform response across specimens to internal or external tibial axial rotation, but was consistent between left and right knees from a single cadaver. Highly consistent trends of interactions between muscle loads and the strain in both the MCL and LCL were noted. CONCLUSIONS: The responses of the MCL and LCL to applied stresses are dependent upon the flexion angle of the knee, the influence of muscle loading and, to a lesser extent, anatomic variation in the ligaments themselves. Guidelines for rehabilitation of the collateral ligaments following injury are suggested.

Journal Article↗
PubMed1998

Pressure-volume characteristics of the intact and disrupted pelvic retroperitoneum.

Hemorrhage is a major cause of mortality in pelvic fractures. Bleeding can be controlled in hypotensive patients by direct ligation, angiographic embolization, pelvic packing, and acute external fixation. Acute application of an external fixator can reduce pelvic volume and reduce bleeding fractures to effect tamponade. This therapy assumes that the pelvis represents a closed space, which clearly is not true anatomically. However, the premise may hold functionally. This study explored the relationship between pressure and volume in the intact and disrupted pelvic retroperitoneum. In cadaveric specimens, the external iliac vein was dissected, ruptured, and cannulated. This method allowed controlled flow of fluid, with simultaneous measurement of pressure, into the intact retroperitoneum. Open book pelvic fractures were created by applying external rotation to the pelvis through the femoral heads. The pressure-volume measurements, without and with external fixation applied, were repeated after the fracture, as well as after a laparotomy. In the intact retroperitoneum, pressures rapidly rose to an average of 30 mm Hg after infusion of 5 liters of fluid. After fracture, up to 20 liters of fluid could be infused at pressures not exceeding 35 mm Hg. External fixation increased pressures approximately 3 mm Hg at low fluid volumes, and approximately 11 mm Hg at the highest fluid volumes. Laparotomy decreased retroperitoneal pressure from approximately 35 mm Hg to approximately 15 mm Hg. The results of the study suggest that low-pressure venous hemorrhage may be tamponaded by an external fixator, given that enough fluid volume is present in the pelvic retroperitoneum. However, external fixation may not generate sufficient pressure to stop arterial bleeding. In any case, it seems that a large volume of fluid must be lost into the pelvis before an external fixator can have much effect on retroperitoneal pressures.

Aged↗
PubMed1998

Angiogenic potential of perivascularly delivered aFGF in a porcine model of chronic myocardial ischemia.

A number of heparin-binding growth factors, including basic (bFGF) and acidic (aFGF) fibroblast growth factors have been shown to promote angiogenesis in vivo. In this study, we employed a sustained-release polymer extravascular delivery system to evaluate the angiogenic efficacy of a novel form of genetically modified aFGF in the setting of chronic myocardial ischemia. Fifteen Yorkshire pigs subjected to Ameroid occluder placement on the left circumflex (LCX) artery were treated with perivascularly administered aFGF in ethylene vinyl acetate (EVAc) polymer (10 micrograms, n = 7) or EVAc alone (controls, n = 8). Seven to nine weeks later, after coronary angiography to document Ameroid-induced coronary occlusion, all animals underwent studies of coronary flow and global and regional left ventricular function. Microsphere-determined coronary flow in the Ameroid-compromised territory was significantly increased in aFGF-treated compared with control animals, and this improvement in perfusion was maintained during ventricular pacing. Left ventricular function studies demonstrated improved global and regional function in aFGF-treated animals. We conclude that local perivascular delivery of genetically modified aFGF results in significant improvement in myocardial flow and regional and global left ventricular function.

Animals↗
PubMed1998

Bone mineral density of the lumbar spine and proximal femur is decreased in children with sickle cell anemia.

Bone mineral density (BMD) was evaluated in the proximal femora (femoral neck, Ward's triangle, and greater trochanter) and lumbar spines of 25 black children and young adults with sickle cell anemia using dual-photon absorptiometry. Compared with normal subjects from the general population, the patients with sickle cell anemia exhibited lower bone mineral density values in all scan regions (approximately 6% to 21% lower than expected). These differences in the lumbar spine were significant for both girls and boys. When compared with normal black subjects from the general population, the girls with sickle cell anemia exhibited significantly lower lumbar spine bone mineral density, and the boys with sickle cell anemia exhibited significantly lower bone mineral density in the femoral neck and Ward's triangle. No consistent or significant correlations were found between the bone mineral density data and the patients' hematologic indices.

Absorptiometry, Photon↗
PubMed1998

Advanced practice nursing in Texas: the interface of accreditation, regulation, and certification.

The rapidity and degree of development of regulation by boards of nursing of advanced practice nurses (APNs) and programs for their preparation has varied from state to state over the past 20 years. In general, boards have authority only to regulate advanced practice through the recognition of APNs and the setting of standards and scope of their practice. In the early 1990s, lack of consistent APN educational program standards and experiences and criteria for recognition of APNs was problematic at the levels of accreditation, certification, and regulation. The interdependency of these concepts as well as the necessity and difficulty of linking them in a consistent, effective manner has become increasingly evident. The Texas Board of Nurse Examiners has been involved in deriving a model designed to ensure the education and recognition of APNs with emphasis on both professionalism and public safety. Because Texas is unique in size and geography, it mirrors many problems and issues of both large and small states.

Certification↗
PubMed1997

Evaluation of hip stability after simulated transverse acetabular fractures.

One of the major goals in managing acetabular fractures is the prevention of posttraumatic arthrosis. Unreduced fractures involving the weightbearing portion of the acetabulum may lead to posttraumatic arthrosis, whereas fractures outside this area portend a better prognosis. The purpose of this study was to help distinguish among fractures that require operative reduction, those that can be treated with traction, and those that require even less aggressive treatment. A model was developed to test hip stability after simulated transverse acetabular fractures. The results from this investigation suggest that transverse fractures with a roof arc angle of 90 degrees do not affect the weightbearing portion of the acetabulum. Fractures with a roof arc angle of 60 degrees begin to infringe on the weightbearing area, and those with roof arc angles of less than 60 degrees are clearly in the weightbearing region. Hip stability was significantly affected by the roof are angle and by the interaction of the roof arc angle and the angle of hip abduction or adduction. The data from the current study suggest that the area of the acetabulum considered to be weightbearing in transverse acetabular fractures may be more expansive than previously thought. The model developed may be used to investigate anterior and posterior column fractures.

Acetabulum↗
PubMed1997

Biomechanical analysis of nonreamed tibial intramedullary nailing after simulated transverse fracture and fibulectomy.

A tibial shaft fracture model was created to study the effects of an intramedullary nail and partial fibulectomy on fracture loading. Cadaveric lower extremities were instrumented with strain gages and subjected to biomechanical testing. A nonreamed nail was inserted into each tibia using only the proximal locking screws. Each specimen was tested under six conditions: intact tibia; intact tibia with nail; fractured tibia with nail removed and fibula intact; fractured tibia with nail and fibula intact; fractured tibia with nail and partial fibulectomy; and fractured tibia with partial fibulectomy and nail removed. In the intact tibia the anterior cortices were in relative tension compared with the posterior cortex. After transverse fracture this relative tension was increased. Inserting the nail after fracture significantly increased anteromedial and anterolateral compressive strains and decreased posterior strains. Performing a partial fibulectomy in the fractured tibia with a nail had no significant effect on the strain patterns. These results confirm the relative anterior tension present in the intact tibia and demonstrate an increase in this anterior tension following transverse fracture. Performing a partial fibulectomy or inserting an intramedullary nail increased anterior compressive loading. This loading alteration may be responsible for the clinical success seen using these treatment methods.

Analysis of Variance↗
PubMed1997

Effect of facial cooling on tympanic temperature.

BACKGROUND: In clinical practice, tympanic temperature is used as an estimate of body temperature. Theoretically, temperature recorded directly from the tympanum reflects the temperature of arterial blood circulating to the brain. However, some studies do not support this connection. Ear-based thermometers in clinical use, commonly called tympanic thermometers, detect heat emission from the aural canal and tympanum. Dissociation of core body temperature and tympanic temperature would suggest that factors other than arterial blood perfusion affect tympanic temperature. METHODS: In a controlled laboratory experiment with four adult volunteers, esophageal and tympanic temperatures were recorded repeatedly at 2-minute intervals during whole-body heating and cooling. Facial cooling, produced by a small electrical fan, was used in three subjects. RESULTS: The gradient between tympanic and esophageal temperature was inconsistent across subjects, with tympanic temperature both higher and lower than esophageal temperature. Correlations between esophageal and tympanic temperature varied widely across subjects. Fanning the face produced a decrease in tympanic temperature without an accompanying decline in esophageal temperature. CONCLUSIONS: Facial cooling in the form of fanning altered the relationship between tympanic and esophageal temperature. This result suggests the possible lowering of tympanic temperature by cooled facial venous blood flow. Use of tympanic temperature in circumstances in which facial temperature may be different from that of other regions of the body deserves further study.

Adult↗
PubMed1996

Identification of a natural soluble form of the vascular endothelial growth factor receptor, FLT-1, and its heterodimerization with KDR.

A soluble form of the vascular endothelial growth factor (VEGF) receptor FLT-1 was identified in conditioned culture medium of human umbilical vein endothelial cells. The endogenous soluble FLT-1 (sFLT-1) receptor is chromatographically and immunologically similar to recombinant human sFLT-1 and binds [125I]VEGF with a comparable high affinity. Human sFLT-1 is shown to form a VEGF-stabilized complex with the extracellular domain of KDR in vitro, suggesting that not only full-length receptors are capable of forming ligand-induced heterodimeric complexes but also sFLT-1 can form a dominant negative complex with the mitogenically competent full-length KDR receptor.

Amino Acid Sequence↗
PubMed1996

The effect of anterior thoracolumbar plate application on the compressive loading of the strut graft.

STUDY DESIGN: In vitro biomechanical testing was performed using a simulated vertebral body bone graft instrumented with strain gauges. Strains were recorded from various locations on the graft during axial compressive loading. Comparisons were made of the strain patterns recorded before and after application of two different anterior plates. OBJECTIVES: To quantify the changes in axial compression experienced by the strut graft in the presence of an anterior plate. SUMMARY OF BACKGROUND DATA: The use of anterior instrumentation to augment anterior thoracolumbar grafting offers the potential advantage of saving additional motion segments while being performed in a single-stage surgery. Several biomechanical studies have compared the anteriorly grafted and instrumented spine to the compressive axial stiffness and torsional rigidity of the intact spine. No previous study has addressed the loading patterns experienced by the graft before and after plating. METHODS: Anterior spinal plates of two designs (Amset ALPS Anterior Locking Plate System; AMS, Hayward, California; Anterior Thoracolumbar Locking Plate System; Synthes Spine, Paoli, Pennsylvania) were evaluated to determine the axial compressive forces experienced by the bone graft before and after application of the plates. Bovine spines harvested from 8- to 12-week-old calves were used for testing. All plates were tested in axial compression to 500 N. Simultaneous recordings were made of the axial strains on the simulated bone graft and the load applied to the construct. Comparisons were made of the strain through the graft without any instrumentation (graft alone, or baseline), with the plate applied, and after removal of the caudal screws (dynamization). RESULTS: With the application of the ALPS plate, the compressive strain through the graft adjacent to the plate averaged 77% of the graft alone construct (range, 39% to 158%). After application of the Synthes plate, the compressive strain through the graft adjacent to the plate averaged 34% of the graft alone construct (range, -14% to 97%). Once the caudal screws were removed, the dynamized construct allowed near-baseline compression through the graft. CONCLUSIONS: Although the literature indicates that the anteriorly instrumented spine may restore the overall spinal structure to near-normal resistance to rotation and flexion forces, the current study demonstrates that a significant amount of compression through the graft is lost by its application. This decreased axial loading further supports the concept of device-related osteopenia observed clinically with such devices.

Animals↗
PubMed1996

X-ray crystal structure of human acidic fibroblast growth factor.

Fibroblast growth factors (FGFs) are mitogenic and chemotactic agents for a wide variety of cell types and play a primary role in the regulation of angiogenesis. Angiogenesis is involved in a variety of critical physiological events including organogenesis, wound healing, ischemic collateral circulation, and solid tumor growth. High-resolution structural information is key to understanding the mechanism of action of these growth factors. We report here the X-ray crystal structure of human acidic FGF (aFGF), with data extending to 2.0 angstroms resolution. The crystal contains four independent molecules in the asymmetric unit. Each molecule contains a single bound sulfate ion, in similar juxtapositions. The bound sulfate is stabilized through hydrogen-bond interactions with residues Asn 18, Lys 113, and Lys 118 and defines a potential heparin binding site. The hydrogen bond with the N delta 2 moiety of Asn 18 appears to be the most conserved interaction, being similar to those observed for sulfate ion bound to human basic FGF (bFGF) and similar but not identical to interactions observed for bovine aFGF with heparin analogs. Of the added solvent groups, five ordered water molecules are conserved in each of the four independent structures of human aFGF. These water molecules, located at buried positions, provide hydrogen bonding partnerships with several buried polar groups in the core of the protein. A central interior cavity exists in each of the four structures, with sizes ranging from approximately 20 to 50 angstroms3. The cavity sizes appear to be significantly smaller than that observed in the related protein interleukin-1 beta. The region comprising the high affinity FGF receptor binding site is structurally very similar to the corresponding region from human bFGF, whereas the low affinity site is structurally quite different. The results provide a structural basis for the role of the low affinity binding site in FGF receptor discrimination.

Amino Acid Sequence↗
PubMed1996

Angiogenesis induced by acidic fibroblast growth factor as an alternative method of revascularization for chronic myocardial ischemia.

BACKGROUND: The effect of periadventitial administration of acidic fibroblast growth factor (FGF) on coronary microvascular reactivity and blood flow was examined in the collateral-dependent and normally perfused myocardium. METHODS: Ameroid constrictors were placed on the proximal left circumflex (LCx) coronary artery in 14 pigs. In seven pigs acidic FGF (10 micrograms) was administered into the perivascular space of the proximal LCx artery by using an ethylene vinyl acetate copolymer slow release device. After 7 to 9 weeks coronary arterial microvessels (70 to 150 microns) were studied in a pressurized (40 mm Hg) no-flow state with video microscopy. RESULTS: Relaxation mediated by beta-adrenoceptors and induced by isoproterenol (p < 0.05), and endothelium-dependent relaxation induced by adenosine 5' diphosphate (ADP) (p < 0.05) of isolated microvessels from the collateral-dependent LCx region were markedly reduced compared with the respective responses of vessels from the normally perfused left anterior descending (LAD) artery region. Relaxation induced by the adenylate cyclase activator forskolin and the guanylate cyclase activator sodium nitroprusside were unaltered. Chronic treatment with acidic FGF normalized responses to isoproterenol (p < 0.001 versus nontreated LCx) and ADP (p < 0.001 versus nontreated LCx) in the collateral-dependent LCx region, whereas responses to forskolin and sodium nitroprusside were not changed. Blood flow in the collateral-dependent LCx region (0.49 +/- 0.24 ml/min/gm) was less than that in the normally perfused LAD region (0.80 +/- 0.24 ml/min/gm, p < 0.05). Treatment with acidic FGF improved perfusion in the LCx region (0.80 +/- 0.06 ml/min/gm, p < 0.05) but did not significantly affect blood flow in the LAD territory (0.89 +/- 0.09 ml/min/gm). CONCLUSIONS: The periadventitial delivery of acidic FGF normalizes vasomotor regulation by beta-adrenergic and endothelium-dependent mechanisms and improves myocardial perfusion to the collateral-dependent myocardium. This may have implications regarding the treatment of patients with severe coronary artery disease who are not ameneable to conventional methods of revascularization.

Adrenergic beta-Agonists↗
PubMed1995

Purification and characterization of a naturally occurring vascular endothelial growth factor.placenta growth factor heterodimer.

Vascular endothelial growth factor (VEGF) is a potent and selective mitogen for endothelial cells that is angiogenic in vivo and induced by hypoxia. A homologous protein, placenta growth factor (PlGF), is also reported to be mitogenic for endothelial cells in culture. The rat GS-9L glioma cell line produces not only VEGF homodimers but also PlGF homodimers and a novel heterodimer composed of VEGF and PlGF subunits. All three dimeric forms were purified to apparent homogeneity, and their structures and mitogenic activities were compared. VEGF.PlGF heterodimers are vascular endothelial cell mitogens nearly as potent as VEGF homodimers. Therefore, some of the biological activities attributed to VEGF homodimers might be mediated by VEGF.PlGF heterodimers. In contrast, pure PlGF homodimers are mitogenic for endothelial cells only at high, possibly non-physiologic concentrations; thus the biological relevance of their mitogenic activity for these cells is not obvious. However, the existence of not only homodimers but also heterodimers clearly extends the similarity between the VEGF/PlGF and the homologous platelet-derived growth factor systems.

Amino Acid Sequence↗
PubMed1995

Acidic fibroblast growth factor stimulates motor and sensory axon regeneration after sciatic nerve crush in the rat.

A time course study in untreated male Sprague-Dawley rats showed that there was no significant difference in the rate of regeneration of motor and sensory axons after a crush injury. Acidic fibroblast growth factor, given topically directly to the site of the crush injury (osmotic minipump for three days) or systemically (i.v. once daily for three days), stimulated the regeneration of motor axons and myelinated sensory axons in the sciatic nerve of the rat. Dose-dependent increases in regeneration distance were seen after 3.6, 36 or 360 ng/day were applied locally and 3 or 10 micrograms/kg per day were given systematically. The greatest effects were achieved with 36 ng/day locally or 10 micrograms/kg per day systematically, when the increase in regeneration distance over three days compared to untreated rats was 47% and 48%, respectively. Administration of heparin vehicle had no significant effect on regeneration. We conclude that since a crush injury has little effect on the endoneurial tubes and supporting cells, the stimulatory effects of acidic fibroblast growth factor on peripheral nerve regeneration seen in this study are likely to be due to a direct acceleration of axonal extension. This is in contrast with the axonal regeneration that occurs across a gap after nerve transection, where axonal extension may be secondary to stimulatory effects on non-neuronal cells providing a supporting "bridge" across the gap. These results suggest that acidic fibroblast growth factor may be clinically useful in the treatment of peripheral neuropathy in man, particularly since systemic treatment for short periods may be effective.

Animals↗
PubMed1995

Effects of indomethacin on biologic fixation of porous-coated titanium implants.

The effect of perioperative administration of indomethacin on attachment strength and bone growth into porous-coated titanium implants was evaluated in the canine transcortical plug model. Various drug administration protocols simulating clinical use of indomethacin were studied. These included chronic treatment (starting 2 weeks prior to surgery), treatment immediately after surgery, and treatment 3, 6, 9, and 18 weeks following surgery. Indomethacin therapy was continued until sacrifice at 3, 6, 12, 18, or 24 postoperative weeks. Push-out testing was performed to determine the maximum bone-implant interface shear strength, and quantitative histologic analysis was used to determine percentage of bone ingrowth. When indomethacin was administered chronically or immediately after surgery, a statistically significant decrease in bone-implant interface attachment strength was seen at 3 postoperative weeks but not at later periods. No adverse effect was observed in any group after the 3-week period. Quantitative histologic analysis demonstrated no significant differences in percentage bone ingrowth among any of the treatment protocols at 3 or 6 weeks after surgery. No significant difference was observed between any of the groups at 18 or 24 weeks. The results of this study suggest that perioperative administration of indomethacin does not significantly affect attachment strength or bone ingrowth into porous-coated implants except at early periods, in which cases a transient decrease in attachment strength occurs.

Animals↗
PubMed1995

Biorhythms in infants and role of the care environment.

All physiologic functions demonstrate cyclic activity. Infant development includes maturation of biorhythms. Two basic changes in biorhythm include establishment of circadian rhythm and day-night pattern. Biorhythms are driven by internal pacemakers yet are highly susceptible to environmental influences. Sleep-wake pattern and body temperature rhythm are two examples of infant biorhythms that demonstrate significant change with development. Biorhythm is the scientific basis for interventions (such as cycled lighting, feeding schedule, and caregiving pattern) aimed at improving the fit between infant and the environment.

Circadian Rhythm↗