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Biomedical subjects

A Rohlmann

Publications and source records attributed to A Rohlmann.

At least 91 records · Page 5Linked to original sources
PubMed1984

A comparison of hip joint forces in sheep, dog and man.

The hip joint forces of sheep and dogs were measured with instrumented endoprostheses and the results were compared with reported data concerning these forces in man. In all animals load directions with 0 to 30 degrees inclinations relative to the femoral axis predominated. The transverse components mostly acted from medio-ventral directions. While the force orientations varied little during each single stance phase, they changed rapidly during the swing phase. Strong inter- and intra-individual differences of load directions were found in all animals. Irregular forces, acting upwards or transverse to the femur, were frequently observed. Maximum joint forces were up to 110% of body weight and depended more on the postoperative time than on the walking speed. Load orientations in the animals were similar to those reported for man. In this regard sheep and dogs appear equally well suited for tests of hip endoprostheses for man.

Animals↗
PubMed1983

Finite-element-analysis and experimental investigation in a femur with hip endoprosthesis.

The stress distribution in a human femur with an endoprosthesis was determined. The finite element method (FEM) was used for a three-dimensional model with more than 15000 degrees of freedom. Geometrical and material data had been taken for this model from a left femur with endoprosthesis. On the contralateral bone a strain gauge investigation was performed to validate the calculations. Reasonable agreement was achieved. Various modes of loading were investigated. A perfect bond at the interface between materials of different elastic moduli was assumed. The results are valid for endoprosthesis with such structured stem surfaces as allow transfer of tensile and shear stresses.

Biomechanical Phenomena↗
PubMed1982

Finite-element-analysis and experimental investigation of stresses in a femur.

The state of stress of an intact femur was analysed using a three-dimensional finite element model. One of a pair of femora was used for determination of data for the 3-D model. The other was instrumented with 34 rosette strain gauges for experimental measurements. Good agreement of analytical and experimental data was achieved. For six modes of loading, the deflections and the principal and comparison stresses were determined and compared. The upper one third and the diaphysis of the femur are differently affected in their state of stress, by different modes of loading including simulation of the abductor muscles and the iliotibial tract. For qualitative stress studies of the diaphysis, loading by a single force on the femoral head parallel to the shaft axis may be adequate.

Femur↗
PubMed1981

[The stresses in femur (author's transl)].

The influence of pull in the iliotibial tract on the stresses in the human femur was investigated using the finite element method. Stresses and strains were computed for a model, assuming different forces in the iliotibial tract. The results were experimentally confirmed using strain gauges. The pull in the iliotibial tract mainly reduces the high tensile strains at the lateral side.

Biomechanical Phenomena↗
PubMed1980

Material properties of femoral cancellous bone in axial loading. Part II: Time dependent properties.

In part I of this communication we reported on some time independent material properties of cancellous bone specimens from different regions of human femora. In part II we will report on our investigations of the time dependent behaviour, i.e. stress relaxation and creep. Cylindrical specimens were obtained from the head and condyles of pairs of cadaveric femora and subjected to axial loading. The data were evaluated statistically. The medianL values for relaxation of cancellous bone were greater in the femoral head than in the condyles, greater proximally than distally and greater medially than laterally in the condyles. The distribution of creep was found to be the reverse. The correlation analysis showed that a linear correlation between compressive strength, apparent density and the time dependent properties cannot be assumed. The time dependent properties reported here would appear to demonstrate the visco-elastic behaviour of cancellous bone. An experimental foundation and explanation is presented for the clinical practice of re-tightening cancellous bone screws one time only.

Biomechanical Phenomena↗
PubMed1980

Material properties of femoral cancellous bone in axial loading. Part I: Time independent properties.

The time independent material behavior of cylindrical specimens obtained from the cancelous bone of 20 cadaveric human femora were determined. In this part of the publication, the nominal values for compressive strength, limits of elasticity (yield point), strain, elastic modulus and apparent density are being reported for the cancellous bone of the femoral head and condyle. The correlations between the various parameters are analysed. A positive linear correlation between the four parameters compressive stength, limit of elasticity, modulus of elasticity and apparent density could not be excluded. The material properties vary considerably both within one single bone and between individuals. Compressive strength, modulus of elasticity and apparent density found for cancellous bone of the femoral head are greater than those found in the condyles. Within the condyles, compressive strength, elastic modulus and apparent density increase from the proximal parts to the parts closer to the joint. The medial femoral condyle showed higher compressive strength than the lateral one. Relating each of the three other parameters to the apparent density of the individual specimen did not result in equalizing the data for the material properties. This indicates that the mechanical properties of cancellous bone are strongly related to the direction of loading.

Aged↗
PubMed1980

[Finite element analysis, its limitations and the relevance of its results to orthopaedic surgery as shown by simultaneous experimental measurements (author's transl)].

For a stress analysis of the femur, intact or with an implant, the Finite Element Method appears to be the only practicable one. In order to evaluate the limitation of this method and the relevance of its results for orthopaedic problems, bones were modelled for 2-dimensional and 3-dimensional analysis. Strain gauges were attached to the actual bones. Strains were calculated and measured exerimentally. The results led to these conclusions: The Finite Element Method alone is an appropriate means for qualitative and comparative studies. Complicated geometry of bones and uncertainties about their mechanical properties make it mandatory that calculated values by corroborated by experimental measurements at least in certain points. This ought to be considered particularly if practical consequences are to be drawn from Finite Element calculations in orthopaedic surgery.

Epoxy Resins↗
PubMed1980

[The stresses in femur. I. Intact femur without consideration of the iliotibial tract (author's transl)].

Stresses in femur under different load conditions were computed with the finite element method. The computer model was composed of 1950 spatial elements. The calculated results were confirmed using 34 strain gauge rosettes, attached on the femur surface. The load conditions were chosen as at the stance phase of walking and as standing on two legs. Additionally, the effect of joint forces under different directions were investigated. It is the main conclusion of this work that the computed quantitative results are only relevant when the geometry and the material properties of the metaphysical bone are copied exactly for the computer model.

Biomechanical Phenomena↗
PubMed1979

[A technique for reproducible roentgenograms of the intercondylar sulcus for the study of the femoropatellar (author's transl)].

Roentgenographic documentation of certain features of FP-joint geometry and orientation may serve as guideline in deciding on the form of treatment for chondromalacia or recurrent patellar dislocation. Reproducible conditions for taking roentgen films are equally important for this purpose as well as for quantitative measurements and possible statistical work. A new positioning device for the patient's legs has been designed utilizing a parallellogram frame. The roentgenographic technique for skyline views at 30 degree, 60 degree and 90 degree inclination of the central beam relative to the femoral axis is described. The advantages over previous techniques are the ease of handling the positioning frame, the need for only vertical and horizontal adjustment of the roentgen tube, independence of the type of tube or table, reproducibility of cassette and patient positioning.

Adult↗
PubMed1978

[Walking with canes and forearm-crutches (author's transl)].

Partial weight bearing is frequently prescribed but cannot be controlled adequately. In a previous paper the change of forces at the hip joint as effected by a one sided cane was determined by instrumentation of the cane and a mechanical analysis of gait on a walkway. In the present study we looked at the conditions for control of partial weightbearing when two forearm crutches are used. Instrumented crutches and a forceplate were used. In walking with two forearm crutches the total of the ground reaction forces and the force pattern differ from those in free walking. The total of two crutch forces plus the force at the leg with partial weightbearing exceeds that caused by body weight alone. This is due to mass accelerations in a changed gait pattern. When the maximal leg force is reduced from 100% body weight to zero, the additional dynamic forces exceed those caused by body weight alone by 4%-19%. Only 2% of the additional dynamic forces act on the controlateral crutch while the rest is transmitted through the ipsilateral crutch. The crutch force pattern on the ipsilateral side depends more on individual gait characteristics than does that on the controlateral side. Load reduction is more pronounced in the late stages of the stand phase than in the early ones.

Adult↗