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Biomedical subjects

U Witzel

Publications and source records attributed to U Witzel.

31 records · Page 2Linked to original sources
PubMed1992

[Alloplastic replacement of the anterior cruciate ligament. Material technical and biomechanical principles, surgical technique].

The Trevira-ligament of Polyaethylenter-ephthalat (= Trevira-hochfest 730) seems to be the best qualified replacement of the anterior cruciate ligament at the present. It meets all material and technical requirements and makes allowances for all biomechanical knowledges. The modified over-the-top-technique by arthroscopy or mini-arthrotomy (minimal operative trauma) makes it possible to conserve still existing ligamentous and soft tissue and guarantees the post-operative immediate functional therapy. A potential reinstability--independent of each cause--doesn't imply worse starting conditions and enables corrections including the substitute of a ruptured synthetic ligament.

Anterior Cruciate Ligament↗
PubMed1991

[Strut fracture of a convex-concave 60 degree Björk-Shiley mitral valve prosthesis 5 years after implantation--metallurgic analysis of the prosthesis strut].

Acute mechanical failure of prosthetic heart valves is rare, but associated with high mortality when occurring. For convexo-concave Björk-Shiley prostheses only fractures of the outlet strut are reported. We present a case of lethal mechanical complication 5 years after implantation. By additional metallurgic analysis we were able to identify a sequential course of the outlet strut fracture. This could lead to new approaches for early detection of this complication.

Adult↗
PubMed1981

[The fixateur-externe-osteosynthesis without osseous support (external distance-osteosynthesis) in the lower limbs. Biomechanic, surgical and osteoplastic basic principles (author's transl)].

The stability of external distance osteosynthesis for complete tibia and femur defects rests solely on the rigidity of the external support. The rigidity of the external support is an function of the mechanical data of its individual elements, of the hold between the bone and the other system, as well as of the individually optimized construction form. The response of an osseous contact area which is not resistant to compression when external osteosynthesis is applied is mechanically similar to the response to distance osteosynthesis in the presence of a defect. The basic application rules for distance osteosynthesis of the lower limbs are given. The symmetric frame fixator applied in a three-dimensional arrangement is indicated for tibia defects. External distance osteosynthesis of femur defects can only be recommended when plate osteosynthesis alone or plate osteosynthesis in conjunction with a bracket fixator are out of the question. The various application forms for external osteosynthesis of femur defects are described and their indication with respect to knee function given. Although none of the construction forms for defects in the tibia and especially in the femur can eliminate mechanical interfragmental movement of the main fragments, their clinical use has been successfully established; however, surgical and osteoplastic measures must ensure more and more biological stability with time. The necessary surgical techniques and procedures are described. Whereas even extensive defects of the femur can be bridged exclusively with autologous cancellous bone, varied measures are required for the lower leg, depending on the various individual conditions. Should the direct bridging of the main tibia fragments not be appropriate, fibular-tibial synostosis is necessary. The various options for such fibular-tibial osteoplasty which may be required, depending on the individual case, are described.

Biomechanical Phenomena↗
PubMedDate not supplied

[Thread design of screw-in acetabulum prostheses].

INTRODUCTION AND AIM OF STUDY: The screw-in behaviour and correct positioning of threaded cups is largely determined by the design of the thread. Up to now the various thread designs have not been systematically classified. METHODS: The thread designs of 10 first generation and 27 second generation threaded cups were analyzed using a tool setter and a no-touch light section technique. The following parameters were evaluated: thread shape, pitch, number of turns, rows of teeth, tooth length and shape. RESULTS: Threads of the first generation were V-cut or saw shaped. Thread depth was 3.1 mm on average; the mean width was 0.9-3 mm. Single-thread patterns predominated. The number of turns ranged from 1-7; the pitch was from 2.5-5 mm (single thread), or up to 20 mm (triple and quadruple threads). Cups had 3-16 rows of teeth. Threads of the second generation are V-cut, saw or flat shaped. Thread depth is 3 mm on average; mean width ranges from 0.3-2.2 mm. 72% of the threads have single-thread patterns. The number of turns ranges from 2-5; the pitch is from 2.5-6.2 mm (single thread). Cups have 4-24 rows of teeth. CONCLUSIONS: There are many different thread patterns with widely varying parameters. Contemporary threaded cups have a narrow V-cut and saw shaped threads or flat thread with depths up to 3 mm, no more than 5 turns, and pitch values of approx. 4.5 mm.

Acetabulum↗
PubMedDate not supplied

The role of the zygomatic arch in the statics of the skull and its adaptive shape.

The zygomatic arch of mammals is usually considered a phylogenetic relic of the fenestrations of the skull roof which may be observed in morphological sequences of primitive vertebrate skulls. If this concept is correct, the element is comparable (though not homologous) to the jugal arches of diapsid reptiles. Two major questions then remain unanswered: why different elements are maintained in reptiles and mammals during evolution, and why the arches are maintained as relics of ancestral forms. It is tempting to respond to the latter question with a very simple answer, namely that the elements function in order to sustain mechanical stresses. In this paper, we raise the questions which quality of stresses occurs in a primate skull within the zygomatic arches and what relationship these stresses hold to the morphology of these bony elements. An answer has been sought by means of finite element stress analysis. We found that the zygomatic arch in primate skulls represents a structure which carries, under all biologically relevant conditions, either compressive or tensile stresses. In a very simple model of the human skull under bite forces, a strip of stresses occurs lateral to the orbits, which seems roughly comparable to the zygomatic arch. Once such a structure exists and is used as an insertion of adductor muscles, it will be exposed to bending stress in side view and in frontal view. Morphological details of the zygomatic arch (curvature, profile, suture) are well suited to sustain the evoked stresses by a minimum of material.

Adaptation, Biological↗
PubMedDate not supplied

Functional structure of the skull in hominoidea.

Finite elements stress analysis (FESA) was used to investigate the flow of compressive forces which occur if a homogenous, three-dimensional body representing the skull is loaded by simulated bite forces against the tooth row. Model 1 represents the snout alone. Bite forces are applied simultaneously, but increase rearward. Stresses in the model concentrate along the anterior contour and the lower surface of the model, leaving unstressed a nasal opening and a wide naso-oral connection. Model 2 represents the facial region, as far as the temporomandibular joint. The orbits and the nasal cavity are assumed to be present a priori. Model 3 applies reactions to the bite forces in the temporal fossa, corresponding to the origins of the masticatory muscles. Regions of the model under compressive stress correspond closely to the arrangement of bony material in a hominoid skull. If only the stress-bearing finite elements on each section are combined, and the stress-free parts neglected, the resulting three-dimensional shape is surprisingly similar to a hominoid skull. If bite forces are applied to parts of the tooth row only, the stress patterns are lower, asymmetrical and do not spread into all regions that are stress-bearing in simultaneous biting on all teeth. In model 2, the highest stresses occur at the tooth roots and along the forehead on top of the nasal roof. There are no marked stress concentrations on top of the orbits. The resulting shape resembles that of an orang-utan. In model 3, the highest stresses also occur at the tooth roots, but the circles of force mostly close below the brain case, so that the stress concentration in the forehead region remains much less marked. In this model, however, the stress concentrations are very similar to hollow brow ridges. The entire resulting shape resembles that of gorilla or chimpanzee skulls. A typical gracile australopithecine skull (STS-5) also shows clear similarities to the patterns of stress flow in our models. Compared to our earlier study of the modern human skull, differences relate to: the relative length and width of the dental arcade, the relative size of the brain case and the position of the arcade relative to the brain case. It seems that these traits are the points of attack of selective pressures, while all other morphological details are simply consequences of stress flow.

Animals↗