Soft-tissue interface

In-Vivo Percutaneous Implant Experiment. The principle of percutaneous attachment has extensive application in many biomedical areas, including the attachment of dental and orthopedic prostheses directly to skeletal structures, external attachment for cardiac pacer leads, neuromuscular electrodes, energy transmission to artificial heart and for hemodialysis. Several attempts to solve the problem of fixation and stabilization of percutaneous implants(19) have been made. Failures were also attributed to the inability of the soft tissue interface to form an anatomic seal and a barrier to bacteria. In the current studies, the effect of pore size on soft tissue ingrowth and attachment to porous polyurethane (PU) surface and the effect of the flange to stem ratio and biomechanical compliance on the fixation and stabilization of the percutaneous devices have been investigated.(20)... 

Attachment to and differentiation of mesenchymal cells at the hydroxyap-atite/soft tissue interface of the concavities. Expression of TGF-/1 and BMPs/OPs family member genes in differentiating osteoblast-like cells resident within the concavities of the smart biomimetic matrices as shown by Northern blot analyses of tissue harvested from the concavities of the substratum [65]. 

Figure 5.3(a) Optical micrograph of alumina and soft tissue interface. 

Transducers contain a piezo-electric crystal that vibrates when an electric current is passed through it, producing an ultrasound beam which propagates through the soft tissues. Reflection or refraction of the ultrasound beam may occur when it reaches a soft tissue interface of different acoustic impedance. The reflected ultrasound beams then return to the transducer where they are converted to electrical currents and subsequently to form an image. When there is a large difference in impedance, for example at a hone-soft tissue interface, a bright echo is produced. 

Fig. 5.6a-d. Bone surface abnormalities that are detectable with US. a Normal bone a straight regular interface separates the bone from the soft-tissues, b Outgrowths or plus images a focal projection of bone (arrows) is observed in the soft tissues, c Irregularities of the cortical outline the bone-soft tissue interface is rough (arrowheads) focal breaks (white arrow) or step-off deformities (black arrow) can be seen, d Defects or minus images a focal loss of bone (arrows) is observed. Soft tissues intervene within the defect... 

Vascular access ports typically consist of a self-sealing siUcone septum within a rigid housing which is attached to a radiopaque catheter (see Radiopaques). The catheter must be fabricated from a low modulus elastomeric polymer capable of interfacing with both soft tissue and the cardiovascular environment. A low modulus polyurethane-based elastomer is preferred to ensure minimal trauma to the fragile vein. 

In regard to Equation (5.4), we have to note that without the above mentioned assumptions the nonlinearities will contain weighting factors that are proportional to the corresponding wave-vector mismatch and inversely proportional to refractive index, thus suggesting that the THG signal is sensitive to the refractive index interface(s) as well. In order to differentiate between contrast mechanisms in THG imaging of soft tissue materials it would be important to know the relationships of the corresponding linear and nonlinear optical parameters. A nonlinear optical... 

Promotes bone ingrowth Does not induce soft tissue growth at bone/implant interface... 

Ultrasound imaging is based on reflection ofthe sound waves (1-3 MHz) at the borders between areas with different acoustic impedance, which is determined by the speed of propagation of sound and the density of the tissue. Because the interfaces between blood and other soft tissues, for example the heart or liver, do not... 

Porous interface with large porosity to provide vigorous ingrowth of soft tissue to form an anatomic seal and a barrier to bacteria. 

Ultrasound imaging is a non-invasive, portable and relatively inexpensive imaging modality, which is used extensively in the clinic. An ultrasound transducer (also called scanhead) sends short pulses of a high-frequency sound wave (1-10 MHz) into the body. At interfaces between two types of tissue, the wave will be refracted and part of the sound wave is reflected back due to Snells law. How much is reflected depends on the densities of the respective tissues, and thus the speed of the sound wave within the different tissues. In addition, parts of the sound wave are also backscattered from small structures at tissue boundaries or within the tissue. High-frequency sound waves propagate weU through soft tissue and fluids, but they are more or less stopped by air or bone. In clinical practice, this limitation is referred to as an acoustic window . The transducer not only sends the wave into the body but also receives part of the reflected and/or backscattered wave, also named echo . In clinical practice, ultrasound is used in a... 

The foreign body reaction occurring around soft tissue implants and thrombosis on surfaces in contact with blood are the major reactions encountered with implants. Both reactions involve the interaction of cells with the implant, especially in the later stages, and much previous study has therefore emphasized cellular events in the biocompatibility process. However, cells encounter foreign polymer implants under conditions that ensure the prior adsorption of a layer of protein to the polymer interface. The properties of the adsorbed layer are therefore important in mediating cellular response to the material. 

At implantation of PHB with contact to bone, the overall tissue response was favorable with a high rate of early healing and new bone formation with some indication of an osteogenic characteristic for PHB compared with other thermoplastics, such as polyethylene. Initially, there was a mixture of soft tissue, containing active fibroblasts, and rather loosely woven osteonal bone seen within 100 pm of the interface. There was no evidence of a giant cell response within the soft tissue in the early stages of implantation. With time this tissue became more orientated in the direction parallel to the implant interface. 

FIGURE 44.6 Finite-element analysis employed to determine the sensitivity of interface pressure to socket-shaped rectification (a) limb and socket (b) elements in layers representing idealized geometry of bone, soft tissue, and socket liner (c) rectification map of radial differences between the external free shape of the limb and the internal dimensions of socket and (d) FE predictions of direct pressure. (Courtesy of Zhang Ming, King s College, London.)... 

Two primary limitations of these modeling efforts involve the representation of tissue properties across the entire limb and the interface condition between the residual limb and prosthesis. The ability of current finite-element models to estimate prosthetic interface stresses, while performing reasonably well in some cases, has not been highly accurate. Nevertheless, the methodology has potential. Advances in finite-element software enabling nonlinear elastomeric formulations of bulk soft tissue, contact analysis, and dynamic analysis may help address some of the current model limitations. Corresponding advances in pressure-transducer technology will help validate the computer models and facilitate interpretation of the analyses. 

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