Extended push-out test to characterize the failure of bone-implant interface]

To study the mechanical behaviour of the implant-bone interface the push- or pull-out test was overtaken from material science. Most authors equate the maximum load (break point) with the failure of the implant integration. Extending the test procedure by acoustic emission analysis reveals the possibility to detect the failure of the interface more in detail and from its earliest beginning. The development of disconnection between host and implant was found to start long before the ultimate load is reached and can be monitored and quantified during this period. The active interface mechanisms are characterized by the distribution function of acoustic emissions and the number of hits per time defines the kinetics of the failure. From clinical studies a gradual subsidence of loaded implants is known starting long time before the definite implant failure. The presented extension of the push-out test with acoustic emission analysis allows the detection of a critical shear stress tc which demarks the onset of the gradual interface failure. We believe this value to represent the real critical load which should not be exceeded in the clinical application of intraosseous implants.     

Mammary implants, diving, and altitude exposure

Mammary implants were exposed to various simulated dive profiles followed by altitude exposures to stimulate aircraft travel and then were observed for bubble formation and volume changes. Minimal volume changes occurred after each dive. Numerous bubbles formed, however, reaching their maximum size in 3 hours. By comparison, when implants were exposed to high altitude following a dive exposure, significant volume changes occurred. This in vitro study showed that bubble formation and volume expansion occur after exposing implants to diving and altitude, but the circumstances required to produce these changes in vivo are extremely unlikely to occur normally.     

Finite element analysis of peri-implant bone volume affected by stresses around Morse taper implants: effects of implant positioning to the bone crest

Abstract

Objectives: The purpose of the present study was to evaluate the distribution and magnitude of stresses through the bone tissue surrounding Morse taper dental implants at different positioning relative to the bone crest. Materials and Methods: A mandibular bone model was obtained from a computed tomography scan. A three-dimensional (3D) model of Morse taper implant-abutment systems placed at the bone crest (equicrestal) and 2?mm bellow the bone crest (subcrestal) were assessed by finite element analysis (FEA). FEA was carried out on axial and oblique (45°) loading at 150 N relatively to the central axis of the implant. The von Mises stresses were analysed considering magnitude and volume of affected peri-implant bone. Results: On vertical loading, maximum von Mises stresses were recorded at 6-7?MPa for trabecular bone while values ranging from 73 up to 118?MPa were recorded for cortical bone. On oblique loading at the equiquestral or subcrestal positioning, the maximum von Mises stresses ranged from 15 to 21?MPa for trabecular bone while values at 150?MPa were recorded for the cortical bone. On vertical loading, >99.9vol.% cortical bone volume was subjected to a maximum of 2?MPa while von Mises stress values at 15?MPa were recorded for trabecular bone. On oblique loading, >99.9vol.% trabecular bone volume was subjected to maximum stress values at 5?MPa, while von Mises stress values at 35?MPa were recorded for >99.4vol.% cortical bone. Conclusions: Bone volume-based stress analysis revealed that most of the bone volume (>99% by vol) was subjected to significantly lower stress values around Morse taper implants placed at equicrestal or subcrestal positioning. Such analysis is commentary to the ordinary biomechanical assessment of dental implants concerning the stress distribution through peri-implant sites.     

A 1-year study of osteoinduction in hydroxyapatite-derived biomaterials in an adult sheep model: part I

The study presented here investigated hydroxyapatite biomaterials implanted in soft-tissue sites in adult sheep to determine whether these materials are osteoinductive and whether the rate of osteoinduction can be increased by manipulating the composition and porosity of the implants. For the study, 16.8-mm x 5-mm discs were prepared from mixtures of hydroxyapatite and beta-tricalcium phosphate. Five mixtures of hydroxyapatite-ceramic and hydroxyapatite-cement paste forms were studied: 100 percent hydroxyapatite-ceramic (Interpore), 60 percent hydroxyapatite-ceramic, 100 percent hydroxyapatite-cement paste, 60 percent hydroxyapatite-cement paste, and 20 percent hydroxyapatite-cement paste. Biomaterials were implanted in subcutaneous and intramuscular soft-tissue pockets in 10 adult sheep. Cranial bone grafts of equal dimension were implanted as controls. One year after implantation, the volume of all biomaterials and bone grafts was determined from a computed tomographic scan, and porosity and bone formation were determined using backscatter electron microscopy. Cranial bone and the 20 percent hydroxyapatite-cement paste implants demonstrated significant volume reduction in all sites after 1 year (p < 0.001). No significant difference in volume of the remaining four biomaterials was found. There was no significant change in pore size in the ceramic implants (range, 200 to 300 micro) and in the cement-paste implants containing 60 percent hydroxyapatite or more (range, 3 to 5 nm). Pore size in the cement-paste implants containing 20 percent hydroxyapatite increased significantly with resorption of the tricalcium-phosphate component, reaching a maximum of 200 to 300 micro in the periphery, where the greatest tricalcium-phosphate resorption had occurred. Both ceramic biomaterials demonstrated lamellar bone deposition within well-formed haversian systems through the entire depth of the implants, ranging from a mean of 6.6 percent to 11.7 percent. There was minimal bone formation in the cement-paste implants containing 60 percent hydroxyapatite or more. In contrast, cement-paste implants containing 20 percent hydroxyapatite demonstrated up to 10 percent bone replacement, which was greatest in the periphery of the implants where the greatest tricalcium-phosphate resorption had occurred. This study confirms the occurrence of true osteoinduction within hydroxyapatite-derived biomaterials, when examined using backscatter techniques. In this study, the rate of osteoinduction was greatest when a porous architecture was maintained, which was best achieved in ceramic rather than cement-paste forms of hydroxyapatite. Porosity and resultant bone formation in cement-paste implants can be improved by combining hydroxyapatite with a rapidly resorbing component, such as tricalcium phosphate.     

Biodurability of retrieved silicone gel breast implants

This study analyzed the shells of single-lumen silicone gel breast implants within the general context of device durability in vivo. The investigation included the major types of gel-filled implants that were manufactured in the United States in a 30-year period. The implants analyzed were Cronin seamed (two explants and one control), Silastic 0 and Silastic I (18 explants and seven controls), and Silastic II (22 explants and 43 controls). The biodurability of the explants was investigated with measurements of the mechanical and chemical properties of the various types of silicone gel control and explanted shells, with implantation times ranging from 3 months to 32 years. The shell properties measured for the controls and explants included the stress-strain relationships, tensile strength, elongation, tear resistance, moduli, cross-link density, and amount of extractable material in the shell. In addition, the mechanical properties of shells that had been extracted with hexane were analyzed for both explants and control implants. The silicone gel explants investigated in this study included some of the oldest explants of the various major types that have been tested to date. For assessment of long-term implantation effects, the data obtained in this study were combined with all known data from other institutions on the various major types of gel implants. The study also addressed the failure mechanisms associated with silicone gel breast implants. The results of the study demonstrated that silicone gel implants have remained intact for 32 years in vivo and that degradation of the shell mechanical and chemical properties is not a primary mechanism for silicone gel breast implant failure.     

Placement of endosteal implants in the zygoma after maxillectomy: a Cadaver study using surgical navigation

Endosteal implants facilitate obturator prosthesis fixation in tumor patients after maxillectomy. Previous clinical studies have shown, however, that the survival of implants placed into available bone after maxillectomy is generally poor. Nevertheless, implants positioned optimally in residual zygomatic bone provide superior stability from a biomechanical point of view. In a pilot study, the authors assessed the precision of VISIT, a computer-aided surgical navigation system dedicated to the placement of endosteal implants in the maxillofacial area. Five cadaver specimens underwent hemimaxillectomy. The cadaver head was matched to a preoperative high-resolution computed tomograph by using implanted surgical microscrews as fiducial markers. The position of a surgical drill relative to the cadaver head was determined with an optical tracking system. Implants were placed into the zygomatic arch, where maximum bone volume was available. The results were assessed using tests for localization accuracy and postoperative computed tomographic scans of the cadaver specimens. The localization accuracy of landmarks on the bony skull was 0.6 +/- 0.3 mm (average +/- SD), as determined with a 5-df pointer probe; the localization accuracy of the tip of the implant burr was 1.7 +/- 0.4 mm. The accuracy of the implant position compared with the planned position was 1.3 +/- 0.8 mm for the external perforation of the zygoma and 1.7 +/-1.3 mm for the internal perforation. Eight of 10 implants were inserted with maximal contact to surrounding bone, and two implants were located unfavorably. Reliable placement of implants in this region is difficult to achieve. The technique described in this article may be very helpful in the management of patients after maxillary resection with poor support for obturator prostheses.     

Significance of the surface of immobilized Saccharomyces cerevisiae cells in ethanolic fermentation

S. cerevisiae cells immobilized in alginate beads show in many cases an increase of mean single cell volume during long-time fermentations (successive batch cycles). The biomass loading capacity of the gel beads is characterized by a maximum volume but not by a maximum number of cells occupying the gel volume. In our system this loading capacity, i.e. the maximum volume fraction of cells per volume of beads, amounted to about 0.54. As a more important result it must be stated that the specific product formation rate in the case of fermentations negligibly influenced by diffusion hindrance is related to the total surface of the viable cells but not to their total number, total volume or total dry weight.     

Immediate breast reconstruction: reducing the risks

One-hundred and sixty-five consecutive immediate breast reconstructions in 157 patients were reviewed. Reconstructions were performed with tissue expanders (53 percent) or immediate gel prostheses (47 percent). Immediate reconstruction was associated with an 18 percent rate of implant loss. Certain risk factors were identified at the p less than 0.05 level using immediate gel implants: failure to achieve complete muscle coverage of the implant, smoking at the time of surgery, initial gel implants of 400 ml or more volume, and age. Expander loss was increased by detaching the pectoralis major (p less than 0.05) and probably by lack of complete muscle coverage in general. Chemotherapy, history of previous smoking, and clinical stage of the carcinoma did not seem to affect reconstructive success. Smoking and patient age should be considered during patient selection for immediate reconstruction. Muscle coverage of the prosthesis should always be attempted. Muscle coverage is mandatory in the smoker. Gel implants of 400 ml or more volume are to be avoided at the initial operation. This approach should enable all surgeons to achieve lower rates of implant loss.     

Importance of diameter-to-length ratio in selecting dental implants: a methodological finite element study

Implant dimensions greatly influence load transfer characteristics and the lifetime of a dental system. Excessive stresses at peri-implant area may result in bone failure. Finding the critical point at the implant–bone interface and evaluating the influence of implant diameter-to-length ratio on adjacent bone stresses makes it possible to select implant dimensions. For this, different cylindrical implants were numerically analysed using geometrical models generated from computed tomography images of mandible with osseointegrated implants. All materials were assumed to be linearly elastic and isotropic. Masticatory load was applied in its natural direction, oblique to occlusal plane. Maximum von Mises stresses were located around the implant neck at the critical point of its intersection with the plane of loading and were functions of implant diameter-to-length ratio. It was demonstrated that there exists a certain spectrum of diameter-to-length ratios, which will keep maximum bone stresses at a preset level chosen in accordance with patient's bone strength.     

Effects of time after ovariectomy, season and oestradiol on luteinizing hormone and follicle-stimulating hormone secretion in ovariectomized ewes.

During the breeding season, five groups of three ewes were implanted at ovariectomy with 0.36, 0.5, 1.0 and 6.0 cm oestradiol implants or implants containing no steroid. Eleven days after receiving implants, blood samples were taken every 10 min for 6 h; implants were then removed. Treatments were repeated three times during each of two consecutive breeding seasons and four times during the intervening anoestrus. In ovariectomized ewes without steroid treatment, luteinizing hormone (LH) pulse frequency increased from early to mid-breeding season, decreased to a minimum at mid-anoestrus and increased to reach a maximum at the mid-point of the second breeding season, subsequently declining. LH pulse amplitude was inversely related to frequency. Basal serum LH concentrations decreased gradually from the first breeding season to reach a minimum at mid-anoestrus and gradually increased to reach a maximum at the end of the second breeding season. Mean serum LH and follicle-stimulating hormone (FSH) concentrations were higher at the end of the second breeding season compared with the beginning of the first breeding season. All parameters of gonadotrophin secretion were decreased much more by oestradiol during the anoestrus than during the breeding season. LH pulse frequency was decreased during anoestrus and at high oestradiol concentrations during the first breeding season. Apart from LH pulse amplitude, the decreases in all parameters of gonadotrophin secretion were less during the second compared with the first breeding season. The minimum effective dose of oestradiol required to decrease mean and basal serum concentrations of LH during anoestrus was lower than in the breeding season. The minimum effective dose of oestradiol required to decrease mean serum concentrations of FSH was lower in the first compared with the second breeding season. Oestradiol depression of LH pulse amplitude and mean serum concentrations of LH and FSH showed a dose dependency during the breeding season. During anoestrus dose dependency was seen for basal concentrations of LH and mean serum concentrations of LH and FSH. We conclude that significant chronic changes in gonadotrophin secretion occur in the ewe with time after ovariectomy. Sensitivity to oestradiol also changes, and the effects of oestradiol are not always dose dependent. We suggest that the circannual pattern of LH pulse frequency and basal LH secretion are directly linked to the circannual cycle of photoperiod.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of a series of infections of Ostertagia circumcincta on gastric secretion of sheep.

Sheep which had been either previously infected with 3. circumcincta or maintained worm-free, were surgically prepared with separated fundic pouches and abomasal cannulae and subsequently infected with 20,000 O. circumcincta larvae three times weekly. A reduction in food intake and increases in total acid output from the pouches and plasma pepsinogen levels were evident in both groups of sheep 4 days after repeated infections commenced; effects which increased in severity after 12 or more days. Except for a transient period of slight failure, previously infected sheep retained the capacity to acidify their abomasal contents whereas previously worm-free sheep lost this capacity. These changes were reversed between 2 and 7 days after treatment with thiabendazole (88 mg.kg-1). Secretory capacity of the fundic pouches was tested with histamine (40 mug.kg-1), the histamine antagonist (burimamide 8 mg.kg-1) and atropine (100 mug.kg-1). Ostertagiasis reduced or abolished the stimulatory effects of histamine. An increase in secretion volume and acid output was obtained after food was freshly provided, even though as little as 25 gm was consumed. Atropine and burimamide both caused a profound decrease in pouch secretion and acid output. These data are consistent with the hypothesis previously stated that in ostertagiasis the hypersecretion from fundic pouches is due to increased levels of circulating gastrin.     

Long-term results of MISTI gold breast implants: a retrospective study

MISTI Gold breast implants (Bioplasty, St. Paul, Minn.) filled with polyvinylpyrrolidone-hydrogel were developed as a promising alternative to silicone-filled implants. Some studies have reported on the positive effects of the implant, such as improved radiolucency and biocompatibility of the gel; however, there are also reports that such implants increased in volume and were subject to capsular contracture in the human body, resulting in demands for their removal. The purpose of this retrospective study was to analyze the long-term results of a series of patients with MISTI Gold breast implants. Between 1991 and 1993, the authors inserted 83 MISTI Gold implants in 61 patients with an average age of 46 years (range, 16 to 69). The authors were able to follow up 48 patients with 71 MISTI Gold implants. The average follow-up was 68 months (range, 10 to 108 months). The retrospective study found that 59 percent of all MISTI Gold implants were removed after an average period of 4.14 years. The main reason for implant removal was an increase in volume of 38 percent, followed by capsular contracture in 14 percent of all 71 MISTI Gold implants. The average increase in volume of all removed MISTI Gold implants was 43 percent. Capsular contracture was graded as Baker I and II in 63 percent and as Baker III and IV in 37 percent. In conclusion, the authors believe that MISTI Gold implants do not fulfill the criteria of safe breast implants, and they agree with the December of 2000 opinion of the Medical Devices Agency of the Department of Health in London that the hydrogel-filled breast implants should not be used until more information about the filler material and its metabolic fate is available.     

A simplified procedure for mechanical testing of lumbar spinal fixation implants]

On the basis of the current ASTM and ISO standard proposals, a simplified test procedure for spinal fixation implants has been developed. It comprises static and dynamic tests aimed at evaluating the stiffness and strength of various different internal implants. Different methods of mounting the pedicle screws to the test device are shown to significantly affect the characteristic values and failure mechanisms of the implants. The feasibility of the procedure was investigated by comparing 7 different internal fixation implants. The reproducible results revealed general differences associated with the material, dimensions and design, which latter in particular correlated with the specific failure mechanisms. For longer-term in situ duration, testing of these implants should be expanded to include an analysis of wear and corrosion properties.     

A paradigm for the development and evaluation of novel implant topologies for bone fixation: Implant design and fabrication

The future development of bio-integrated devices will improve the functionality of robotic prosthetic limbs. A critical step in the advancement of bio-integrated prostheses will be establishing long-term, secure fixation to the remnant bone. To overcome limitations associated with contemporary bone-anchored prosthetic limbs, we established a paradigm for developing and fabricating novel orthopedic implants undergoing specified loading. A topology optimization scheme was utilized to generate optimal implant macrostructures that minimize deformations near the bone-implant interface. Variations in implant characteristics and interfacial connectivity were investigated to examine how these variables influence the layout of the optimized implant. For enhanced tissue integration, the optimally designed macroscopic geometry of a titanium (Ti)-alloy implant was further modified by introducing optimized microstructures. The complex geometries of selected implants were successfully fabricated using selective laser sintering (SLS) technology. Fabrication accuracy was assessed by comparing volumes and cross-sectional areas of fabricated implants to CAD data. The error of fabricated volume to CAD design volume was less than 8% and differences in cross sectional areas between SEM images of fabricated implants and corresponding cross sections from CAD design were on average less than 9%. We have demonstrated that this computational design method, combined with solid freeform fabrication techniques, provides a versatile way to develop novel orthopedic implants.     

Effect of PEEP and Tidal Volume on Ventilation Distribution and End-Expiratory Lung Volume: A Prospective Experimental Animal and Pilot Clinical Study

Introduction

Lung-protective ventilation aims at using low tidal volumes (V_T) at optimum positive end-expiratory pressures (PEEP). Optimum PEEP should recruit atelectatic lung regions and avoid tidal recruitment and end-inspiratory overinflation. We examined the effect of V_T and PEEP on ventilation distribution, regional respiratory system compliance (C_RS), and end-expiratory lung volume (EELV) in an animal model of acute lung injury (ALI) and patients with ARDS by using electrical impedance tomography (EIT) with the aim to assess tidal recruitment and overinflation.

Methods

EIT examinations were performed in 10 anaesthetized pigs with normal lungs ventilated at 5 and 10 ml/kg body weight V_T and 5 cmH₂O PEEP. After ALI induction, 10 ml/kg V_T and 10 cmH₂O PEEP were applied. Afterwards, PEEP was set according to the pressure-volume curve. Animals were randomized to either low or high V_T ventilation changed after 30 minutes in a crossover design. Ventilation distribution, regional C_RS and changes in EELV were analyzed. The same measures were determined in five ARDS patients examined during low and high V_T ventilation (6 and 10 (8) ml/kg) at three PEEP levels.

Results

In healthy animals, high compared to low V_T increased C_RS and ventilation in dependent lung regions implying tidal recruitment. ALI reduced C_RS and EELV in all regions without changing ventilation distribution. Pressure-volume curve-derived PEEP of 21±4 cmH₂O (mean±SD) resulted in comparable increase in C_RS in dependent and decrease in non-dependent regions at both V_T. This implied that tidal recruitment was avoided but end-inspiratory overinflation was present irrespective of V_T. In patients, regional C_RS differences between low and high V_T revealed high degree of tidal recruitment and low overinflation at 3±1 cmH₂O PEEP. Tidal recruitment decreased at 10±1 cmH₂O and was further reduced at 15±2 cmH₂O PEEP.

Conclusions

Tidal recruitment and end-inspiratory overinflation can be assessed by EIT-based analysis of regional C_RS.     

Tissue stresses and strain in trabeculae of a canine proximal femur can be quantified from computer reconstructions

A quantitative assessment of bone tissue stresses and strains is essential for the understanding of failure mechanisms associated with osteoporosis, osteoarthritis, loosening of implants and cell- mediated adaptive bone-remodeling processes. According to Wolff's trajectorial hypothesis, the trabecular architecture is such that minimal tissue stresses are paired with minimal weight. This paradigm at least suggests that, normally, stresses and strains should be distributed rather evenly over the trabecular architecture. Although bone stresses at the apparent level were determined with finite element analysis (FEA), by assuming it to be continuous, there is no data available on trabecular tissue stresses or strains of bones in situ under physiological loading conditions. The objectives of this project were to supply reasonable estimates of these quantities for the canine femur, to compare trabecular-tissue to apparent stresses, and to test Wolff's hypothesis in a quantitative sense. For that purpose, the newly developed method of large-scale micro-FEA was applied in conjunction with micro-CT structural measurements. A three-dimensional high-resolution computer reconstruction of a proximal canine femur was made using a micro-CT scanner. This was converted to a large-scale FE-model with 7.6 million elements, adequately refined to represent individual trabeculae. Using a special-purpose FE-solver, analyses were conducted for three different orthogonal hip-joint loading cases, one of which represented the stance-phase of walking. By superimposing the results, the tissue stress and strain distributions could also be calculated for other force directions. Further analyses of results were concentrated on a trabecular volume of interest (VOI) located in the center of the head. For the stance phase of walking an average tissue principal strain in the VOI of 279 strain was found, with a standard deviation of 212 microstrain. The standard deviation depended not only on the hip-force magnitude, but also on its direction. In more than 95% of the tissue volume the principal stresses and strains were in a range from zero to three times the averages, for all hip-force directions. This indicates that no single load creates even stress or strain distributions in the trabecular architecture. Nevertheless, excessive values occurred at few locations only, and the maximum tissue stress was approximately half the value reported for the tissue fatigue strength. These results thus indicate that trabecular bone tissue has a safety factor of approximately two for hip-joint loads that occur during normal activities.     

Tissue stresses and strain in trabeculae of a canine proximal femur can be quantified from computer reconstructions

A quantitative assessment of bone tissue stresses and strains is essential for the understanding of failure mechanisms associated with osteoporosis, osteoarthritis, loosening of implants and cell-mediated adaptive bone-remodeling processes. According to Wolff's trajectorial hypothesis, the trabecular architecture is such that minimal tissue stresses are paired with minimal weight. This paradigm at least suggests that, normally, stresses and strains should be distributed rather evenly over the trabecular architecture. Although bone stresses at the apparent level were determined with finite element analysis (FEA), by assuming it to be continuous, there is no data available on trabecular tissue stresses or strains of bones in situ under physiological loading conditions. The objectives of this project were to supply reasonable estimates of these quantities for the canine femur, to compare trabecular-tissue to apparent stresses, and to test Wolff's hypothesis in a quantitative sense. For that purpose, the newly developed method of large-scale micro-FEA was applied in conjunction with micro-CT structural measurements. A three-dimensional high-resolution computer reconstruction of a proximal canine femur was made using a micro-CT scanner. This was converted to a large-scale FE-model with 7.6 million elements, adequately refined to represent individual trabeculae. Using a special-purpose FE-solver, analyses were conducted for three different orthogonal hip-joint loading cases, one of which represented the stance-phase of walking. By superimposing the results, the tissue stress and strain distributions could also be calculated for other force directions. Further analyses of results were concentrated on a trabecular volume of interest (VOI) located in the center of the head. For the stance phase of walking an average tissue principal strain in the VOI of 279 strain was found, with a standard deviation of 212 microstrain. The standard deviation depended not only on the hip-force magnitude, but also on its direction. In more than 95% of the tissue volume the principal stresses and strains were in a range from zero to three times the averages, for all hip-force directions. This indicates that no single load creates even stress or strain distributions in the trabecular architecture. Nevertheless, excessive values occurred at few locations only, and the maximum tissue stress was approximately half the value reported for the tissue fatigue strength. These results thus indicate that trabecular bone tissue has a safety factor of approximately two for hip-joint loads that occur during normal activities.     

Numerical analysis of an osseointegrated prosthesis fixation with reduced bone failure risk and periprosthetic bone loss

Currently available implants for direct attachment of prosthesis to the skeletal system after transfemoral amputation (OPRA system, Integrum AB, Sweden and ISP Endo/Exo prosthesis, ESKA Implants AG, Germany) show many advantages over the conventional socket fixation. However, restraining biomechanical issues such as considerable bone loss around the stem and peri-prosthetic bone fractures are present. To overcome these limiting issues a new concept of the direct intramedullary fixation was developed. We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling by restoring the natural load transfer in the femur. Generic CT-based finite element models of an intact femur and amputated bones implanted with 3 analyzed implants were created and loaded with a normal walking and a forward fall load. The strain adaptive bone remodeling theory was used to predict long-term bone changes around the implants and the periprosthetic bone failure risk was evaluated by the von Mises stress criterion. The results show that the new design provides close to physiological distribution of stresses in the bone and lower bone failure risk for the normal walking as compared to the OPRA and the ISP implants. The bone remodeling simulations did not reveal any overall bone loss around the new design, as opposed to the OPRA and the ISP implants, which induce considerable bone loss in the distal end of the femur. This positive outcome shows that the presented concept has a potential to considerably improve safety of the rehabilitation with the direct fixation implants.     

Implantation of embryonic neocortex and spinal cord into injured peripheral nerve of adult rats

Spinal cord and cerebral cortex of 14-day-old embryos of Wistar rats were implanted into the sciatic nerve of mature rats in order to study dynamics of the development of neuronal and neuroglial elements in ectopic sites. By means of light and electron microscopy it has been stated that the implanted nerve cells of the cortex and spinal cord survive during 5 month and differentiate from neuroepithelial cells and neuroblasts up to young and mature neurons. It was found that thirty days after operation the spinal cord implants contained myelinated nerve fibers and numerous synapses. The data obtained suggest that the implants of fetal spinal cord are more favorable for regeneration of the injured nervous stems than the cerebral cortex.     

Relaxation dynamics of the gel to liquid-crystalline transition of phosphatidylcholine bilayers. Effects of chainlength and vesicle size.

The relaxation kinetics of the gel to liquid-crystalline transition of five phosphatidylcholine (DC14PC to DC18PC) bilayer dispersions have been investigated using volume perturbation calorimetry, a steady-state technique which subjects a sample to sinusoidal changes in volume. Temperature and pressure responses to the volume perturbation are measured to monitor the relaxation to a new equilibrium position. The amplitude demodulation and phase shift of these observables are analyzed with respect to the perturbation frequency to yield relaxation times and amplitudes. In the limit of low perturbation frequency, the temperature and pressure responses are proportional to the equilibrium excess heat capacity and bulk modulus, respectively. At all temperatures, the thermal response data are consistent with a single primary relaxation process of the lipid. The less accurate bulk modulus data exhibit two relaxation times, but it is not clear whether they reflect lipid processes or are characteristic of the instrument. The observed thermal relaxation behavior of all multilamellar vesicles are quantitatively similar. The relaxation times vary from approximately 50 ms to 4 s, with a pronounced maximum at a temperature just greater than Tm, the temperature of the excess heat capacity maximum. Large unilamellar vesicles also exhibit a single relaxation process, but without a pronounced maximum in the relaxation time. Their relaxation time is approximately 80 ms over most of the transition range.