Biomedical subjects

A Rohlmann

Publications and source records attributed to A Rohlmann.

At least 19 recordsLinked to original sources

PubMed2006

In vivo glenohumeral contact forces--measurements in the first patient 7 months postoperatively.

Knowledge of forces in the glenohumeral joint is essential for understanding normal and pathologic shoulder function. It forms the basis for performing fracture treatment or joint replacement surgery, for optimizing implant design and fixation and for improving and verifying analytical biomechanical models of the shoulder. An instrumented shoulder implant with telemetric data transmission was developed to measure six components of joint contact forces and moments. A patient with humeral head arthrosis achieved good joint function after its implantation. During the first 7 postoperative months, the contact force remained below 100% BW (percent body weight) for most activities of daily living. It ranged up to 130% BW for arm positions close to the limits of motion or when acting against external resistance. When the patient tried to turn a blocked steering wheel with maximum effort, the force rose to about 150% BW, the highest level observed thus far. Of great interest were the force directions relative to the humerus, especially those in the sagittal plane, which were not greatly influenced by the type of exercise, the arm position or the external resistance. The moments due to friction in the joint reached 5.2 Nm. The friction-induced shift of contact forces relative to the implant head centre ranged up to 6.3mm. These first worldwide in vivo measurements of glenohumeral contact forces are being continued in more patients and for longer postoperative times.

Follow-Up Studies↗

PubMed2005

[Effect of vertebral body stiffness before and after vertebroplasty on intradiscal pressure].

Fractures of osteoporotic vertebral bodies are increasingly stabilized with bone cement. The effects of vertebral-body stiffness before and after augmentation with bone cement and of wedge-shaped vertebral body fractures on intradiscal pressure are insufficiently known. In a finite element model of the lumbar spine the elastic modulus of cancellous bone as well as the amount and the elastic modulus of bone cement were varied and the dependency of intradiscal pressure on these parameters was calculated. In addition, a wedge-shaped vertebral-body fracture was simulated. The bulge of the vertebral-body endplate and thus the intradiscal pressure depends strongly on the grade of osteoporosis in the vertebral body. The influence of amount and elastic modulus of bone cement on intradiscal pressure is small. A wedge-shaped vertebral-body fracture causes an anterior shift of upper-body centre of gravity. If this shift is not compensated, it leads to an increased flexion moment that has to be balanced by muscle forces. In addition, this shift leads to a stronger increase of intradiscal pressure than the augmentation of the vertebral body with bone cement.

Bone Cements↗

PubMed2005

Effect of an interspinous implant on loads in the lumbar spine.

Interspinous process implants are increasingly used to treat canal stenoses. Little information exists about the effects of implant height and stiffness on the biomechanical behavior of the lumbar spine. Therefore, a three-dimensional nonlinear finite element model of the osseoligamentous lumbar spine (L1 to L5) was created with a slightly degenerated disc at L3/L4. An interspinous implant was inserted at that segment. Implants with different heights and stiffnesses were studied. The model was loaded with the upper body weight and muscle forces to simulate walking and 25 degrees extension. Implant forces are influenced strongly by the height and negligibly by the elastic modulus of the implant. Intersegmental rotation at implant level is markedly reduced, while intradiscal pressure is slightly increased. Implant size and stiffness have only a minor effect on intradiscal pressure. The maximum von Mises stress in the vertebral arch is strongly increased by the implant.

Elasticity↗

PubMed2005

Effects of the rib cage on thoracic spine flexibility.

Besides protecting the internal organs of the thorax, the rib cage is the site of numerous muscle attachments. It also decreases the overall flexibility of the thoracic spine. This study developed finite element (FE) models of the thoracic spine with and without the rib cage, and the effects of the rib cage on thoracic spine flexibility were determined. The numerical models were validated by comparing the maximum rotation of the models for several loading cases with experimental data in the literature. After adapting the material properties for the discs and ligaments, the calculated maximum rotations differed from the measured median values by less than 1 degrees without the rib cage and by less than 2.5 degrees with it. The rib cage decreased the mean flexibility of the thoracic spine by 23% to 47%, depending on the loading plane. Assuming the ribs to be rigid beams required a corresponding reduction of ligament stiffnesses in order to achieve the same agreement of the maximum rotations with the measured median values. Interconnecting the FE thoracic spine model plus rib cage with the existing detailed FE lumbar spine model improves the simulation of force directions of muscles attached to the rib cage or thoracolumbar spine. In addition, such a model is suitable for determining the effects of lumbar spine implants on spinal balance.

Computer Simulation↗

PubMed2003

ISSLS prize winner: A novel approach to determine trunk muscle forces during flexion and extension: a comparison of data from an in vitro experiment and in vivo measurements.

STUDY DESIGN: Disc pressure and fixator load were measured in an in vitro setup and compared to in vivo measurements with the identical transducers from the two groups participating in this study. OBJECTIVES: The goal of this in vitro study was to determine the magnitude of trunk muscle forces during flexion and extension. The loading conditions in this study accounted for body weight, local and global muscles, and forces resulting from the support of the abdominal soft tissue in different postures. Resulting intersegmental motions and intradiscal pressure in each segment and the six load components in both rods of an internal fixator were determined. SUMMARY OF BACKGROUND DATA: The spine is primarily stabilized by muscle forces, which greatly influence spinal loads. However, little information exists on the magnitudes of trunk muscle forces during postures like flexion and extension of the upper body. METHODS: Seven human cadaveric lumbar spines were mounted in a spine tester and adjusted to different degrees of flexion and extension of the upper body with different hip flexions. For each specimen, a total of 124 load cases were studied. They included combinations of a vertical compressive load, a follower load and forces pulling with cables at a plate fixed at the cranial end of the specimen to simulate rectus abdominis, erector spinae, and a supporting force of the abdomen. The muscle forces were varied until the external moment, necessary to keep the lumbar spine specimen in the examined posture, was zero. This was achieved with different muscle force combinations. Loads on internal fixators as well as intradiscal pressure and intersegmental rotation at all levels were measured. The muscle force combination that caused intradiscal pressures and loads in the internal fixator closest to those measured in vivo were assumed to be the muscle forces which can be expected in vivo. RESULTS: Generally, intradiscal pressure was closer to in vivo measurements than the fixator loads. The force in the m. erector spinae increased with the flexion angle but was only slightly influenced by extension. The estimated forces in the erector spinae were 100 N for standing, 130 N for 15 degrees extension, and 520 N for 30 degrees flexion of the upper body. Little influence was found on the intersegmental motion. CONCLUSION: In vitro loading conditions can be approximated closely to in vivo conditions with the simulation of an axial preload, local, and global muscles. This novel approach can help to estimate muscle forces, which can usually not be measured. The results from this study provide important input for FEM models, which may then allow the investigation of different load cases.

Adolescent↗

PubMed2003

Hip joint contact forces during stumbling.

AIM: To determine whether load directions for stumbling are similar to those for common activities and whether stumbling can be realistically simulated under laboratory conditions without endangering the patients. METHOD: The magnitudes and directions of hip contact forces were measured during real and simulated stumbling and compared with those found during various other everyday activities. Measurements were obtained by use of hip implants with built-in load sensors and telemetry. RESULTS: Peak forces are approximately twice as high during real stumbling as during any other activity and may range higher than eight-times the body weight. Simulated stumbling leads to much lower contact forces, especially if this happens after a warning. Accidental stumbling in everyday situations should, therefore, be avoided, especially in patients with hip replacements or arthrosis. CONCLUSIONS: The directions of peak hip contact forces relative to the femoral bone are nearly constant for any activity, including real stumbling. This observation supports the assumption that muscle and bone anatomy plus muscle function are optimized in order to minimize stresses in bone and muscles. Any impairment of such a mechanically balanced system will increase the musculoskeletal loads. Malposition of total hip implants or muscle deficits caused by the surgical approach must, therefore, be avoided or minimized.

Accidental Falls↗

PubMed2003

Influence of a screw with a hinge on the pull-out force at the cranial end vertebra in ventral derotation spondylodesis (VDS).

INTRODUCTION: Pull-out of the cranial end-vertebra screw following a correction of a scoliosis with the VDS implant is a common complication, especially in the thoracic region. It requires an extension of the fusion length or at least reduces the outcome of the correction. MATERIALS AND METHODS: We therefore looked for a new approach to solve the problem of end-vertebra screw pull-out. It was postulated that the axial pull-out force at the end-vertebra screw is reduced by a screw with a hinge between the head and threaded part. The aim of our study was to calculate theoretically and then determine experimentally the screw pull-out force of regular and modified VDS screws with a hinge during lateral shifting of the screw heads in the frontal plane. RESULTS: The use of a modified end-vertebra screw with a hinge on the cranial end vertebra of the correction region changes the shape of the threaded rod between the last two screws. Theoretically estimated, this leads to a reduction of the pull-out force to one-fourth of the value for the original VDS screw. Experimentally, the mean pull-out force for the modified end-vertebra screw was 110+/-8.6 N in comparison with the original screw, where the pull-out force was 214+/-7.2 N. CONCLUSION: A reduction of screw pull-out can be expected when a screw with a hinge is used for the cranial end vertebra.

Bone Screws↗

PubMed2003

Influence of graded facetectomy and laminectomy on spinal biomechanics.

Facetectomy and laminectomy are techniques for decompressing lumbosacral spinal stenosis. Resections of posterior bony or ligamentous parts normally lead to a decrease in stability. The degree of instability depends on the extent of resection, the loading situation and the condition of the intervertebral discs. The correlation between these parameters is not well understood. In order to investigate how these parameters relate to one another, a three-dimensional, non-linear finite element model of the lumbosacral spine was created. Intersegmental rotations, intradiscal pressures, stresses, strains and forces in the facet joints were calculated while simulating an intact spine as well as different extents of resection (left and bilateral hemifacetectomy, hemilaminectomy and bilateral laminectomy, two-level laminectomy), disc conditions (intact and degenerated) and loading situations (pure moment loads, standing and forward bending). The results of the modelling showed that a unilateral hemifacetectomy increases intersegmental rotation for the loading situation of axial rotation. Expanding the resection to bilateral hemifacetectomy increases intersegmental rotation even more, while further resection up to a bilateral laminectomy has only a minor additional effect. Hemilaminectomy and laminectomy only differ in their effect for ventriflexion and muscle-supported forward bending. Two-level laminectomy increases the intersegmental rotation only for standing. Degenerated discs result in smaller intersegmental rotations and higher disc stresses at the respective levels. Decompression procedures affect the examined biomechanical parameters less markedly in degenerated than in intact discs. Resection of posterior bony or ligamentous elements has a stronger influence on the amount than on the distribution of stresses and deformations in a disc. It has only a minor effect on the biomechanical behaviour of the adjacent region. Spinal stability is decreased after a laminectomy for forward bending, and after a two-level laminectomy for standing. For axial rotation, spinal stability is decreased even after a hemifacetectomy. Patients should therefore avoid excessive axial rotation after such a treatment.

Biomechanical Phenomena↗

PubMed2003

Instrumented forceps for measuring tensile forces in the rod of the VDS implant during correction of scoliosis.

Ventral derotation spondylodesis (VDS) is the standard in ventral scoliosis surgery. Especially in the thoracic spine, there are no alternatives to VDS with compression and derotation as its correction forces. However, pull-out of the end-vertebra screw during correction of scoliosis with the VDS implant is a common complication involving particularly the cranial end-vertebra screw in the thoracic region. This complication requires an extension of the fusion length or reduces at least the outcome of the correction. There are no in vivo data on correction forces in ventral scoliosis surgery. Thus the correction depends on the skill and experience of the surgeon. An instrumented forceps developed and built to measure forces in the longitudinal rod allows axial tensile forces to be determined in the longitudinal rod during surgery. The instrumented forceps has the advantage of reducing the risk of screw pull-out. Furthermore, viscoelastic behavior of the spine can be measured during ventral correction. In addition, knowledge of the correction forces improves our biomechanical understanding of the spine, especially during correction of scoliosis. Intraoperative force measurement is in no way detrimental to the patient.

Bone Screws↗

PubMed2002

[Effect of implants for replacement of a vertebral body on the mechanical function of the lumbar spine].

A three-dimensional, nonlinear finite element model of the lumbar spine was created. A bisegmental internal spinal fixation device and successively two kinds of vertebral body replacements were integrated into the computer model. The model was loaded with pure moments in the three anatomical main planes as well as with forces that are expected during standing. A vertebral body replacement drastically reduces the mobility in the implant region. An implant has only a minor influence on the mobility and stresses in the adjacent regions for the loading cases chosen. A preload, for example, caused by additional distraction from the precisely fitted ventral implant, exerts a very strong effect on the stresses in the end plates that are in contact with the implant. Different sizes of the contact area between implant and vertebral body have only a local effect on stress distribution.

Computer Simulation↗

PubMed2002

[Spinal load bearing during sitting in an office chair with a tilting back].

Long periods of quiet sitting is considered a cause of low back pain. It is often assumed that spinal loads are high, especially when sitting erect. Modern office chairs with a tiltable back permit changes in the seated posture. In the most reclined position, some new chairs even match a kyphotic form of the lumbar spine. It is assumed that sitting on such a chair reduces low back pain. With the aim of determining spinal loading in different sitting positions, the loads acting on implanted fixation devices were measured telemetrically in two patients. Loads were measured in patients sitting on six different chairs with tiltable backs. In modern chairs, implant loading was always lower than while walking. In the end-tilt position of the chairback, loads were always lower than when the chairback was upright. Even when the lordotic curvature of the lumbar spine was "corrected", loads on the fixator were lower than when the subject was seated in the upright position. In a modern chair, spinal loading is no higher than with non-adjustable office chairs.

Biomechanical Phenomena↗

PubMed2002

[Effect of a double intervertebral cage on the mechanical behavior of the lumbar spine].

A three-dimensional nonlinear finite element model of the lumbar spine was developed. Paired threaded cages and a monosegmental internal spinal fixation device were integrated into the computer model. The model was loaded with such forces as apply during standing, as well as with pure moments in the three main anatomical planes, plus an additional preload. The latter was generated by shortening the distance between the pedicle screws on the longitudinal rod of the fixator. With the exception of torsional loading, an implant appreciably reduces the mobility in the segment concerned. At the loads studied, cages had only a minor impact on the movements and stresses in the adjacent regions, but a strong influence on the stresses in the endplates in contact with them. A preload increases these stresses dramatically. Contact conditions between vertebral body and cages also have a marked effect on the stress distribution in the corresponding vertebral endplate, especially in the case of extension loading. Owing to the preload, maximum stresses were higher for the rigid bond than when contact elements were used.

Biomechanical Phenomena↗

PubMed2001

Influence of a follower load on intradiscal pressure and intersegmental rotation of the lumbar spine.

STUDY DESIGN: Intradiscal pressure and intersegmental rotation of human lumbar spines were measured in vitro. OBJECTIVES: To determine the effect of a follower load on mechanical behavior at all levels of the lumbar spine. SUMMARY OF BACKGROUND DATA: Different loads have been proposed for studying the mechanical behavior of the lumbar spine. The influence of a follower load on intradiscal pressure at the different levels is unknown. METHODS: Ten human cadaveric lumbar spines were loaded in the three main anatomic planes with pure moments of 3.75, 7.5, and 7.5 Nm plus a follower load of 280 N. Intradiscal pressure and intersegmental rotation were measured at all levels. RESULTS: An additional follower load increased the intradiscal pressure, slightly reduced the intersegmental rotation for axial rotation, and hardly affected intersegmental rotation for lateral bending and flexion-extension. CONCLUSIONS: A superimposed follower load renders spinal loading with pure moments more physiologic.

Adult↗

PubMed2001

Is it possible to simulate physiologic loading conditions by applying pure moments? A comparison of in vivo and in vitro load components in an internal fixator.

STUDY DESIGN: Loads acting in an internal fixator measured in vitro under the application of pure moments such as those commonly used for implant testing and basic research were compared with loads measured in 10 patients in vivo. OBJECTIVES: To investigate whether these recommended loading conditions are valid by comparing in vivo measurements and those obtained in an in vitro experiment. SUMMARY OF BACKGROUND DATA: Pure bending moments are often preferred as loading conditions for spinal in vitro testing, either for implant testing or basic research. The advantage of this loading pattern is that the bending moment is uniform along the multisegmental specimen. However, functional loading of the spine by muscles or external loads subjects the spine to a combination of forces and moments. METHODS: In an in vivo experiment, loads acting on an internal spinal fixator in 10 patients were determined before and after anterior interbody fusion during flexion, extension, left and right lateral bending, and left and right axial twisting of the upper body with the patient standing. For comparison, an equivalent in vitro data set was created with 7 human lumbar specimens in which the same type of fixator was used. All specimens were tested under the application of pure bending moments in the three main motion planes in the intact state with fixator, after corpectomy, and with bone graft. RESULTS: Consistent qualitative agreement between in vivo and in vitro measurements for the loads acting in the internal spinal fixator were found for axial rotation and lateral bending. For flexion and extension, reasonable agreement was found only for the intact spines with fixators. After corpectomy and after inserting a bone graft, the median values for axial force and bending moment in the sagittal plane in vitro did not agree with in vivo measurements. An axial preload in the in vitro experiment slightly increased the axial compression force and flexion bending moment in the fixators. CONCLUSIONS: The application of pure moments to intact lumbar spinal specimens in vitro produces forces and moments in implants comparable with loads observed in vivo. During basic research on intact specimens or implant testing involving a removed disc or corpectomy, muscle forces are necessary to simulate realistic conditions.

Humans↗

PubMed2001

[The biomechanics of kyphosis].

Kyphosis is a dorsally convex curvature of the spine in the sagittal plane. A pathological kyphosis of the thoracic spine has a Cobb angle greater than 50 degrees. It occurs when the anterior and/or posterior load-transferring elements of the spine are overloaded or damaged. Wedge-shaped vertebral bodies may be found at one or several levels. A wedge-shaped vertebral body increases the curvature of the spine and moves the centre of gravity of the body parts above of the corresponding vertebral body in a ventral direction. This increases the flexion-bending moment acting on the spine. In the case of a wedge-shaped deformation of a vertebral body, the relative increase of the flexion moment is a function of the wedge angle and of the original position of the centre of gravity of the upper body.

Biomechanical Phenomena↗

PubMed2001

Effect of an internal fixator and a bone graft on intersegmental spinal motion and intradiscal pressure in the adjacent regions.

Stabilizing a lumbar spine with an implant alters the mechanical properties of the bridged region. In order to determine whether this procedure is associated with higher loads in the adjacent segments, seven lumbar cadaver spines were mounted in a spine tester and loaded with pure moments of flexion/extension, left and right lateral bending, and left and right axial rotation. The material studied comprised intact lumbar spines, intact spines with bisegmental internal spinal fixators, and postcorpectomy spines both with a graft and fixators and with fixators alone. Intradiscal pressures and intersegmental motion were measured at all levels. In the bridged region, these parameters were strongly affected by an internal fixator. In most cases, the effect was small in the regions above and below the fixators. Highly significant differences in these regions (P<0.01) were far below the interspecimen range. We did not find any case where both intradiscal pressure changes and intersegmental motion showed highly significantly differences in the regions adjacent to the bridged one. Our results suggest that disc degeneration, which is sometimes found at the level directly above and below the fixators, is not caused by mechanical factors.

Adult↗

PubMed2001

Frictional heating of total hip implants. Part 1: measurements in patients.

Hip implants heat up due to friction during long lasting, high loading activities like walking. Thermal damage in the surrounding soft and hard tissues and deteriorated lubrication of synovial fluid could contribute to implant loosening. The goal of this study was to determine the implant temperatures in vivo under varying conditions. Temperatures and contact forces in the joints were measured in seven joints of five patients using instrumented prostheses with alumina ceramic heads and telemetry data transmission. The peak temperature in implants with polyethylene cups rose up to 43.1 degrees C after an hour of walking but varied considerably individually. Even higher temperatures at the joints are probable for patients with higher body weight or while jogging. The peak temperature was lower with a ceramic cup, showing the influence of friction in the joint. During cycling the peak temperatures were lower than during walking, proving the effect of force magnitudes on the produced heat. However, no positive correlation was found between force magnitude and maximum temperature during walking. Other individual parameters than just the joint force influence the implant temperatures. Based on the obtained data and the available literature about thermal damage of biological tissues a detrimental effect of friction induced heat on the stability of hip implants cannot be excluded. Because the potential risk for an individual patient cannot be foreseen, the use and improvement of low friction implant materials is important.

Aged↗

PubMed2001

Frictional heating of total hip implants. Part 2: finite element study.

Due to higher friction artificial hip joints warm up more than natural joints during walking and other continuous activities. This could lead to thermal damage in the surrounding tissues and be a reason for long-term implant loosening, an effect which has not yet been investigated. In vivo measurements with instrumented implants showed temperatures inside the prosthetic head up to 43.1 degrees C (Part 1 of this work). Based on the experimental data a finite element model was developed to calculate the temperatures in the tissues surrounding the hip implant to determine whether these tissues can heat up to critical levels. Various parameters were investigated which could account for the variations in the measured temperatures in the patients, including the perfusion rate in tissues, the volume of synovial fluid, and different implant materials. We found that the synovial fluid is most endangered by thermal damage and consequent deterioration of lubricating properties. Implants with a cobalt-chromium head and a polyethylene cup are unfavourable as they can elevate the temperature in the synovia to more than 46 degrees C. With regard to thermal properties stems made from cobalt-chromium alloys are superior to titanium stems, by better conducting heat to the femur and minimizing the synovial fluid temperature. Factors determining the temperatures during walking are insufficiently known or cannot be determined in the individual patient. Therefore, the risk of a thermally induced implant loosening cannot currently be estimated. Under unfavourable conditions such a risk exists, however. General improvements of implant materials and clinical studies on the possibility of implant loosening due to high temperatures are therefore required.

Alloys↗