Laser Doppler velocimetry and flow visualization studies in the regurgitant leakage flow region of three mechanical mitral valves.

Streak line flow visualization and laser Doppler velocimetry (LDV) were conducted in the regurgitant leakage flow region of 3 mechanical heart valve types: CarboMedics, Medtronic Hall, and St. Jude Medical. Streak line flow visualization identified regions of high regurgitant flow, and LDV measurements were focused on those locations. Maximum regurgitant flow velocities after valve closure ranged from 0.7 to 2.6 m/s, and maximum Reynolds shear stress after valve closure ranged from 450 to 3,600 dyne/cm2. These data indicate that leakage flows can generate turbulent jets with elevated Reynolds stresses even in bileaflet valves.

Mean velocity and Reynolds stress measurements in the regurgitant jets of tilting disk heart valves in an artificial heart environment.

Laser Doppler velocimetry, with a high temporal resolution (1 ms time windows), was used to measure the flow field in two regions (major and minor orifices) near the aortic and mitral valves (Bjork Shiley monostrut Nos. 25 and 27, respectively) of the Penn State artificial heart. The motion of each valve was also investigated using a 1000 frame/s video camera in order to estimate the valve's closing velocity. Fluid velocities in excess of and opposite to valve closing velocity were detected near the valve, providing evidence of "squeeze flow." Maximum Reynolds shear stresses of approximately 20,000 dyn/cm2 and time-averaged Reynolds shear stresses of approximately 2000 dyn/cm2 were observed during the regurgitant flow phase. These elevated Reynolds shear stresses suggest that regurgitant jets play a role in the hemolysis and thrombosis associated with tilting disk heart valves in an artificial heart environment.

Three-component laser Doppler velocimetry measurements in the regurgitant flow region of a Björk-Shiley monostrut mitral valve.

Three-dimensional laser Doppler velocimetry measurements were acquired in a mock-circulatory loop proximal to a Björk-Shiley monostrut valve in the mitral position, and synchronous ensemble-averaging was applied to form an "average" beat. Two axial locations in the regurgitant flow region of the valve (in the minor orifice) were mapped, and maximum Reynolds shear stresses were calculated. A large spike in regurgitant flow was noted at the beginning of systole, which may be the squeeze flow phenomenon computed by other researchers. A region of sustained regurgitant flow 50 msec later was the focus of this study. Maximum velocities of approximately 3.7 mps were noted, and maximum Reynolds shear stresses of approximately 10,000 dyne/cm2 were calculated. Comparisons were made of two-dimensional (ignoring tangential component) versus three-dimensional shear stresses, and, in this case, in regions of high stress, the differences were insignificant. This suggests that the tangential component of velocity can probably be ignored in similar measurements where the tangential velocity is likely to be small.

Effects of tilting disk heart valve gap width on regurgitant flow through an artificial heart mitral valve.

While many investigators have measured the turbulent stresses associated with forward flow through tilting disk heart valves, only recently has attention been given to the regurgitant jets formed as fluid is squeezed through the gap between the occluder and housing of a closed valve. The objective of this investigation was to determine the effect of gap width on the turbulent stresses of the regurgitant jets through a Björk-Shiley monostrut tilting disk heart valve seated in the mitral position of a Penn State artificial heart. A 2 component laser-Doppler velocimetry system with a temporal resolution of 1 ms was used to measure the instantaneous velocities in the regurgitant jets in the major and minor orifices around the mitral valve. The gap width was controlled through temperature variation by taking advantage of the large difference between the thermal expansion coefficients of the Delrin occluder and the Stellite housing of Björk-Shiley monostrut valves. The turbulent shear stress and mean (ensemble averaged) velocity were incorporated into a model of red blood cell damage to assess the potential for hemolytic damage at each gap width investigated. The results revealed that the minor orifice tends to form stronger jets during regurgitant flow than the major orifice, indicating that the gap width is not uniform around the circumference of the valve. Based on the results of a red blood cell damage model, the hemolytic potential of the mitral valve decreases as the gap width increases. This investigation also established that the hemolytic potential of the regurgitant phase of valve operation is comparable to, if not greater than, the hemolytic potential of forward flow, consistent with experimental data on hemolysis.

Anterior release and fusion in pediatric spinal deformity. A comparison of early outcome and cost of thoracoscopic and open thoracotomy approaches.

STUDY DESIGN: A consecutive series of patients undergoing thoracoscopic anterior spinal release and fusion for scoliosis or kyphosis was compared with a consecutive series of patients treated with an open thoracotomy approach. OBJECTIVES: To compare the early clinical results, costs, and charges of performing an anterior thoracic spinal release and fusion with the two approaches. SUMMARY OF BACKGROUND DATA: The thoracoscopic approach to the spine is gaining acceptance, yet there are little data comparing the technique with standard open methods for the treatment of spinal deformity. METHODS: The first 14 thoracoscopic cases performed at the authors' hospital were compared with 18 open thoracotomy cases treated during the previous 12-month period. In each case the discs were excised and bone grafted before performing a posterior fusion. The early clinical outcomes and the hospital charges/costs were analyzed. RESULTS: The percent curve correction was similar between the thoracoscopic and open methods: scoliosis 56% and 60%, respectively; kyphosis, 88% and 94%, respectively. The blood loss and complication rates were similar between the two groups; however, the chest tube output was greater in the thoracoscopic group. The length of hospital stay was not reduced, and the cost of the open procedure is 29% less than the thoracoscopic approach. The minimally invasive thoracoscopic approach avoids cutting the chest/shoulder musculature, greatly decreasing the morbidity of anterior spinal surgery. CONCLUSIONS: The thoracoscopic technique is a safe and effective alternative to open thoracotomy in the approach to the anterior thoracic spine for the treatment of pediatric and adolescent spinal deformity.

Inaccurate projection of rat soleus motoneurons: a comparison of nerve repair techniques.

The objectives of this study were 1) to determine the degree to which soleus motoneurons find their appropriate target following crush and transection injuries to the sciatic nerve, and 2) to determine whether repair of a transected nerve with a silicone tube leads to greater specificity of reinnervation and recovery of muscle function than the standard epineurial suture repair method. Sixty adult female Sprague-Dawley rats were randomly assigned to one of three sciatic nerve injury groups: crush injury, transection with epineurial suture repair, or transection with a silicone tube repair. The degree to which soleus motoneurons were able to find their appropriate target following a sciatic nerve injury was examined using a double labeling dye technique in which the original soleus motor pool was labeled with fast blue and reinnervating motoneurons were labeled with Dil. Soleus motoneurons were able to find their appropriate target following a crush injury. The accuracy of reinnervation following a transection injury and repair, however, was relatively poor. Only 14% of the original soleus motoneurons found the correct target following a transection injury. Repair of a lesioned nerve with a silicone tube and a 5-mm gap as opposed to epineurial sutures did not increase the specificity of reinnervation or the degree of muscle recovery.

Functional recovery following neurorrhaphy of the rat sciatic nerve by epineurial repair compared with tubulization.

Recovery of motor function is often poor following transection injuries to peripheral nerves. The purpose of this study was to measure and compare functional recovery of the sciatic nerve in the rat following transection and neurorrhaphy with the use of a nerve guide tube and with traditional end-to-end epineurial repair. Muscle recovery was also evaluated following a crush injury, a model of an axonotmetic lesion. Recovery was assessed at 8, 16, and 32 weeks after injury by measuring the isometric contractile properties of the soleus muscle and at 8 and 16 weeks by measuring the conduction properties of the sciatic nerve. The mean conduction velocity of the sciatic nerve in the crush group and both transection groups was significantly slower than that of controls at both 8 and 16 weeks. Following a transection injury, the soleus became a significantly faster muscle as measured by time to peak twitch tension. By 32 weeks, the maximum isometric tension of the soleus muscle recovered to 90% that of the control group following a crush injury and to less than 70% following a transection injury and repair. Recovery was better in the epineurial repair group than in the tube repair group at 8 weeks, but no difference was found between the groups at 16 or 32 weeks. These results demonstrate that nerve guide tubes are a potential alternative to epineurial repair. The poor motor recovery following repair of transection injuries may be related to poor specificity of reinnervation.

[Clinically relevant biomechanics. 3. Critical reflections about the Burstone intrusion mechanics]. / Klinisch relevante Biomechanik--3. Mitteilung. Kritische Betrachtungen zur Intrusionsmechanik nach Burstone.

A segmented arch for intrusion of front teeth, as described by Burstone, is compared to a coil-and-stylus-model. The mechanics of constructive modifications are analyzed and their consequences for the movements of front teeth are discussed. It is shown, that in some cases extrusive forces occur, while intrusive forces are applied. In these cases it is necessary to reduce intrusive forces below 0.20 Newton per side. In order to avoid extrusive forces, it is recommended to apply intruding forces between the centers of resistance of the central and lateral incisors.

[Clinically relevant biomechanics. 2. Mechanics of non-symmetric front teeth-intrusion]. / Klinisch relevante Biomechanik--2. Mitteilung. Mechanische Grundlagen der asymmetrischen Intrusion von Frontzähnen mittels Segmentbogentechnik.

Front teeth can be intruded by the segmented arches described by Burstone. The article analyses the vertical forces under the influence of symmetric intrusion arches and examines the static situation caused by non-symmetric activation of symmetric intrusion mechanics. It shows that in some cases even extrusive forces can occur in the anterior segment, whereas intrusive forces are intended to be applied.

[Clinically relevant biomechanics--1. Moment/force ratio--terminologic clarification]. / Klinische relevante Biomechanik--1. Mitteilung. Der Moment/Kraft-Quotient--Eine terminologische Klarung.

The movement of a tooth under the influence of an orthodontic appliance is dependent upon the acting force and the resulting moment. This moment can be computed by adding the inherent moment of the force and the therapeutic moment of the appliance. The article points out the differences between these two kinds of moments. Moreover, it describes, that the resulting moment is the base for another kind of moment-to-force-ratio, Mi/F', which is the only parameter to predict the direction of orthodontic tooth movement.

[The computer simulation of orthodontic tooth movements]. / Computersimulation orthodontischer Zahnbewegungen.

A personal computer programme is presented which allows the prediction of orthodontic tooth movement under the influence of any given force and moment. The changes in position of the front teeth caused by the chosen forces appear on the monitor as a three-dimensional diagram and as a movie-like picture sequence.

Effects of pH and Water Activity on Microbiological Stability of Salad Dressing.

The water activity and pH of an experimental starch-based salad dressing were varied to evaluate inhibitory effects on microorganisms selected from groups known to be principal dressing spoilage agents. Dressing samples were inoculated with Lactobacillus fructivorans , Zygosaccharomyces bailii , or a yeast isolated from a spoiled commercial salad dressing. Both yeast and L. fructivorans displayed a minimum growth pH of approximately 3.55 to 3.60. The minimum aw observed was 0.89 for yeast growth and 0.91 for L. fructivorans . Combinations of aw and pH which imparted microbial stability without use of preservatives are described.