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Biomedical subjects

D J Schaefer

Publications and source records attributed to D J Schaefer.

At least 19 recordsLinked to original sources
PubMed2005

IEEE Committee on Man and Radiation (COMAR) Technical Information Statement "exposure of medical personnel to electromagnetic fields from open magnetic resonance imaging systems".

Open magnetic resonance imaging (MRI) systems enable performing image-guided medical procedures for long periods of time very close to, or inside, the patient imaging area. Medical personnel can be exposed to relatively high static, gradient, and radiofrequency fields compared to most other MRI systems. The Committee on Man and Radiation of the Institute of Electrical and Electronics Engineers calculated or used existing data on magnetic flux densities and field strengths in or near the patient area to assess occupational exposure levels. Potential exposures to each field type were analyzed and compared to relevant values specified in international exposure limits including those of the Institute of Electrical and Electronics Engineers and the International Commission on Nonionizing Radiation Protection. Exposures of the head or torso of a worker to gradient fields near the center of the patient-imaging area can exceed most exposure limits even for times less than a second. Exposures to radiofrequency fields can exceed limits if sustained exposures (minutes or more) occur to parts of the body. Static magnetic fields used by present Open MRI systems are below exposure limits of all of the standards that address these fields. Overall results of this study suggest that manufacturers and others who program or operate Open MRI systems should take care to ensure that operating parameters produce exposures that comply with the relevant exposure limits. Also, since field levels fall off rapidly with increasing distance, user practices may be implemented that reduce exposures significantly.

Electromagnetic Fields↗
PubMed2005

Fibrin gel-immobilized primary osteoblasts in calcium phosphate bone cement: in vivo evaluation with regard to application as injectable biological bone substitute.

Osteogenic injectable bone substitutes may be useful for many applications. We developed a novel injectable bone substitute based on osteoblast-fibrin glue suspension and calcium phosphate bone cement (BC). Human osteoblasts were isolated from trabecular bone samples and cultured under standard conditions. Osteoblasts were suspended in fibrinogen solution (FS). BC was cured with thrombin solution. 8 x 4 mm injectable bone discs were prepared using silicon molds and a custom-made applicator device. Discs containing BC, BC/FS, or BC/FS/osteoblasts were implanted subcutaneously into athymic nude mice. After 3, 9 and 24 weeks, specimens were explanted and subjected to morphologic and biomechanical evaluation. In vitro fibrin gel-embedded osteoblasts displayed a differentiated phenotype as evidenced by alkaline phosphatase, collagen type 1 and von Kossa stains. A proportion of osteoblasts appeared morphologically intact over a 3-day in vitro period following application into the BC. BC/FS and BC/FS/osteoblast discs were sparsely infiltrated with vascularized connective tissue. There was no bone formation in implants from all groups. However, positive von Kossa staining only in BC/FS/osteoblast groups suggests engraftment of at least some of the transplanted cells. Biomechanical evaluation demonstrated initial stability of the composites. Young's modulus and maximal load did not differ significantly in the BC/FS and BC/FS/osteoblast groups. The practicability of osteoblast-containing injectable bone could be demonstrated. The dense microstructure and the suboptimal initial vascularization of the composites may explain the lack of bone formation. Modifications with regard to enhanced osteoblast survival are mandatory for a possible application as injectable osteogenic bone replacement system.

Adult↗
PubMed2004

Computer aided designed neo-clavicle out of osteotomized free fibula: case report.

Total clavicle reconstruction is a challenging task. We performed a reconstruction of the ventral shoulder girdle by calculating a 3D DICOM representation of the left clavicle to create a right neo-clavicle. Two cuts in correct position and angle leads to a natural 3D shape of the new clavicle. The data were used with a thermo-jet procedure to form model slices of thermoplastic wax. Subsequently, the double titanium osteotomy template with correct cut-angulation was constructed. A 40-year old patient presented with symptoms of progressive pain and instability in the shoulder girdle resulting from complete right clavicle resection due to desmoid tumour 23 years earlier. During the operative procedure, dissection, guided double osteotomy, microvascular anastomoses and acromioclavicular-sternoclavicular fixation were performed. The computer-assisted planning resulted in the exact calculation of the two osteotomy cuts, hence, the 3D appearance of the neo-clavicle. Two years postoperatively, patient showed slightly improved elevation and complete recovery from pain. Our operative procedure demonstrates that the computer-assisted planning with construction of a wax model and an osteotomy template is a useful approach to plan the two precise cuts leading to a predictable shape of the clavicle.

Adult↗
PubMed2002

Human recombinant EGF protein delivered by a biodegradable cell transplantation system.

We have previously shown a new approach to expand cultured human keratinocytes and reconstitute the epidermis in full-thickness wounds using a new microsperical transport system. This was a new approach to increase the cell yield for seeding without altering the anchoring proteins by enzymatic steps. That time we used Cytodex 3 which failed to be degraded and induced an inflammatory reaction in a t-cell-deficient organism. Therefore, we have investigated another microcarrier consisting of PLGA, which is a well-known carrier material for cell culture and transplantation. After coating the PLGA carrier with gelatine the seeding time of viable cells reached 4 h and the cell gain after 7 days of spinner culture was 16-fold. At 14 days after transplantation, we could detect a new stratified epithelium in our full-thickness wound healing model. Because cytokines play a major role in wound healing, we loaded this carrier material with different concentrations of rhEGF, showing a dose dependent release of the protein in vitro and in vivo. This result might lead to a different approach in the treatment of wounds.

Absorbable Implants↗
PubMed2001

Tissue engineering in plastic reconstructive surgery.

Tissue engineering (TE) is a new interdisciplinary field of applied research combining engineering and biosciences together with clinical application, mainly in surgical specialities, to develop living substitutes for tissues and organs. Tissue engineering approaches can be categorized into substitutive approaches, where the aim is the ex vivo construction of a living tissue or organ similar to a transplant, vs. histioconductive or histioinductive concepts in vivo. The main successful approaches in developing tissue substitutes to date have been progresses in the understanding of cell-cell interactions, the selection of appropriate matrices (cell-matrix interaction) and chemical signalling (cytokines, growth factors) for stimulation of cell proliferation and migration within a tissue-engineered construct. So far virtually all mammalian cells can be cultured under specific culture conditions and in tissue specific matrices. Future progress in cell biology may permit the use of pluripotent stem cells for TE. The blueprint for tissue differentiation is the genome: for this it is reasonable to combine tissue engineering with gene therapy. The key to the progress of tissue engineering is an understanding between basic scientists, biochemical engineers, clinicians, and industry.

Animals↗
PubMed2000

[Cell transplantation in surgery--reality and prospects for tissue engineering].

Traditionally surgical repair of tissue defects and loss or failure of function has relied on mechanical means, medical (drug) treatment, autologous and allogenic transplantation, and alloplastic/synthetic devices. Tissue engineering represents a new interdisciplinary field of applied research combining engineering and biosciences together with clinical application (mainly in surgical specialities) to develop living substitutes for tissues and organs. The understanding of cell-cell interactions and chemical signalling (growth factors) and the selection of appropriate matrices (cell-matrix interaction) is the key for success. Gene therapy represents the logical combination with tissue engineering on the molecular biology level. Application of cultivated skin and cartilage has already become reality, engineering of vascularized, more complex organs remains a challenge for this century.

Cells, Cultured↗
PubMed2000

Spiral scan peripheral nerve stimulation.

Time-varying magnetic fields induce electric fields that can cause physiological stimulation. Stimulation has been empirically characterized as a function of dB/dt and duration based on experiments using trapezoidal and sinusoidal gradient waveforms with constant ramp time, amplitude, and direction. For two-dimensional (2D) spiral scans, the readout gradient waveforms are frequency- and amplitude-modulated sinusoids on two orthogonal axes in quadrature. The readout gradient waveform therefore rotates with amplitude and angular velocity that are generally not constant. It does not automatically follow that spiral stimulation thresholds can be predicted using available stimulation models. We scanned 18 normal volunteers with a 2D spiral scan and measured global thresholds for axial, sagittal, and coronal planes. We concluded that the stimulation model evaluated accurately predicts slew rate-limited spiral mean stimulation thresholds, if the effective ramp time is chosen to be the half-period at the end of the spiral readout.

Electric Stimulation↗
PubMed2000

Review of patient safety in time-varying gradient fields.

In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (i.e., tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.

Animals↗
PubMed2000

[Tissue engineering with mesenchymal stem cells for cartilage and bone regeneration].

Tissue engineering offers the possibility to fabricate living substitutes for tissues and organs by combining histogenic cells and biocompatible carrier materials. Pluripotent mesenchymal stem cells are isolated and subcultured ex vivo and then their histogenic differentiation is induced by external factors. The fabrication of bone and cartilage constructs, their combinations and gene therapeutic approaches are demonstrated. Advantages and disadvantages of these methods are described by in vitro and in vitro testing. The proof of histotypical function after implantation in vivo is essential. The use of autologous cells and tissue engineering methods offers the possibility to overcome the disadvantages of classical tissue reconstruction--donor site morbidity of autologous grafts, immunogenicity of allogenic grafts and loosening of alloplastic implants. Furthermore, tissue engineering widens the spectrum of surgical indications in bone and cartilage reconstruction.

Bone Regeneration↗
PubMed2000

[Tissue engineering: possibilities and perspectives].

Successful tissue engineering requires intensive co-oporation between clinicians, biologists (cell culture, gene therapy), chemical engineers (biomaterials) and industrial partners. In case of wound healing tissue engineered constructs have already been applied successfully in burns and chronic wounds. In order to improve carrier and matrix function biomaterials still have to be optimized. The potential of such constructs might even be enhanced by gene therapeutical methods. The complex mammalian organism has to be considered as the gold standard and the model for perfect tissue engineering. The problem of vascularization of complex organs yet has to be solved. In general it seems to be more promising to substitute deficient components in vivo and to rely on modulating influences within the host organism rather than to create complex organs ex vivo.

Animals↗
PubMed1999

Cultured epidermal keratinocytes on a microspherical transport system are feasible to reconstitute the epidermis in full-thickness wounds.

Research efforts to modify cultured autologous skin transplants for large full-thickness burn wounds and in chronic ulcers have shifted from multilayered differentiated grafts ("sheet" grafts) toward smaller units of basal undifferentiated single cell suspensions in a transport medium and subconfluently covered static carriers. It has been shown that wounds transplanted with single cell suspensions reconstitute the epidermis. However, this technique requires the detachment of the keratinocytes from the culture flasks by enzymatic digestion-digestion that might alter the anchoring proteins of the cells. A new approach might be to circumvent the enzymatic digestion to harvest the keratinocytes. This study reports a technique to culture epidermal cells on spherical microcarriers as a suspension culture and transport vehicle. The spherical microcarrier consists of a 100-microm-diameter collagen-coated dextran carrier (Cytodex 3 Pharmacia) and has been used previously for enzyme production commercially. With this new approach, we seeded the human keratinocytes in a spinner-like system onto microspheres and transplanted these micrografts onto full-thickness wounds on the back of nude mice. After 14 days, we showed a reconstituted epithelium that was multilayered and keratinized compared to control wounds. We believe that this is the first step of a new approach to increase the cell yield for seeding without altering the anchoring proteins by enzymatic steps, leading to a superior transplantation method for keratinocytes.

Adult↗
PubMed1999

Physiologic effects of intense MR imaging gradient fields.

The strength duration relationship for peripheral nerve stimulation by MR imaging pulsed gradient magnetic fields was measured in 84 human subjects. The data were fitted to the hyperbolic strength-duration relationship: dB/dt=b(1 + c/d), where b is rheobase, c is chronaxie, and d is duration, and dB/dt is reported as the maximal value on the axis of the bore. For sensation threshold, average (b,c) (15 T/s, 0.37 ms) for the y-gradient and (26 T/s, 0.38 ms) for the z-gradient coil. The dB/dt intensity to induce a sensation which the subject described as uncomfortable was about 50% above the sensation threshold. Experiments with dogs showed that the cardiac stimulation by pulsed magnetic gradient fields is exceedingly unlikely.

Abdominal Muscles↗
PubMed1998

[Using autoclaved spongiosa].

Since transmission of HIV through allogenic bone grafts has been established, the concept of cryopreservation of allogenic bone had to be reconsidered. The strict guidelines of the scientific board of the Bundesärztekammer of 1990 are very labour-, time- and money-intensive. We have therefore moved to autoclaving allogenic cancellous bone. This is harvested from femoral heads during THR in slices of 2 to 3 mm, then cleaned under non-sterile conditions with a hard water jet followed by an ultrasonic bath for approx. 20 minutes. The slices are then double sealed individually and autoclaved at a temperature of 134 degrees C and a pressure of 2.5 atmospheres. Storage is in sealable containers at room temperature, so the material available at any time. Since the bone tissue has been freed of most organic matter and therefore lost its bone-specific antigenic structure, all that is left is the anorganic component with its inimitable architecture. This treatment results in a cancellous bone graft which is sterile, biocompatible and osteoconductive. Biologically it is inferior to autogenic and cryopreserved bone. Experimental and clinical studies show, however, that autoclaved cancellous bone can be a suitable substitute in well-selected indications. To elucidate the ultrastructural changes of the spongiosa and proteins induced by autoclaving further investigations are necessary. For example, the specific proteins involved need to be determined.

Bone Transplantation↗
PubMed1998

Hemarthrosis and hip joint pressure in femoral neck fractures.

In a prospective clinical study the intraarticular pressure of 55 patients with intracapsular femoral neck fractures was measured intraoperatively with the hip in different positions. Intraarticular hemarthrosis was quantified by a preoperative sonography examination. In 75% of the patients, increased intraarticular pressure caused by the hemarthrosis was found. The spontaneous median pressure increased significantly from 22 mm Hg with extension (28 mm Hg) and internal rotation of the hip joint (56 mm Hg). The lowest pressure was found in 70 degrees flexion (15 mm Hg). The median pressures increased within the first 24 hours after injury from 26 mm Hg in the first 6 hours to 46 mm Hg from 7 to 24 hours. Even in the first and second weeks after trauma, increased median pressures were detected (8.5 mm Hg and 13 mm Hg, respectively). No significant difference was found between undisplaced and displaced fracture types. Because increased joint pressure in other studies correlates with reduced perfusion of the femoral head, it can be deduced that reduction maneuvers without capsulotomy can compromise the circulation of the femoral head. Capsulotomy and osteosynthesis of the femoral neck at the earliest time possible is the best prophylaxis of tamponade. If the osteosynthesis is delayed, a preoperative sonography after admission and a control sonogram after 6 hours is recommended. In the event of relevant hemarthrosis, immediate therapeutic drainage is suggested for patients who will receive joint conserving osteosynthesis.

Adult↗
PubMed1998

Safety aspects of switched gradient fields.

At sufficient amplitude, gradient-induced electric fields may stimulate nerves and muscles in patients during MR examinations. This article reviews the physics, physiology, and safety aspects of nerve and muscle (including respiratory and cardiac) stimulation. This article also reviews experimental data on gradient-induced stimulation. While threshold stimulations are just perceptible, patients may experience pain at high electric field amplitudes. Safety margins required to prevent cardiac stimulation and to limit painful stimulations are investigated.

Animals↗
PubMed1998

Safety aspects of radiofrequency power deposition in magnetic resonance.

Radio frequency fields used in MR imaging may result in patient heating. This article reviews the physics, physiology, and safety considerations of radio frequency heating of patients during MR examinations. Frequently, monitoring equipment, implants, braces, surface coils, or other devices are introduced into the patient vicinity. A section investigates safety aspects of such devices. Appropriate limits for radio frequency power deposition during MR examinations are explored.

Animals↗