Mechanical effects of continuous passive motion on the lumbar spine in seating.

The aim of this study was to develop a model which describes the mechanical spinal response to small alternating pelvic stimulation induced by an active rotational movement of a normal chair. The rotary continuous passive motion (RCPM) of the seat about a vertical axis of only 0.6 degrees resulted in an increased in spinal length as opposed to the normal daily shrinkage, and back patients experienced pain relief. Passive and active exercies have been broadly applied for treating and healing spinal disorders. A rigid body package (ADAMS Android) was used to translate the stimulation of the ischial tubersoity in caudo-cranial handing-over visualisation. The parameters of the model were set so that the values of the global stiffness and geometry of the intervertebral discs could be changed. In vivo validation of the model was based on force and moment measurements using an internal AO fixator. The predicitons of the model concerning natural frequency (4.5Hz) in vertical direction and the axial torsion response on small pelvic torsion are comparable with experimental data.

Functions of hip joint muscles.

The relative torque components (percentages of total hip torque) of 26 hip muscles about three joint axes were calculated using a human multibody model to determine muscle actions as a function of hip joint angle. It was demonstrated that the muscle actions obtained from this straight line model of hip musculature were in good agreement with published functions; the data on muscle function changes during hip joint motions exceed the data available in the literature and could extend the anatomical knowledge which is available today.

Development of a hybrid finite element model for individual simulation of intertrochanteric osteotomies.

Intertrochanteric osteotomies of the proximal femur are used to improve the anatomy and function of the hip joint in a number of orthopaedic diseases. To investigate the geometrical and biomechanical aspects of pre-operative planning we created a set of programs to automatically perform a simulation of intertrochanteric osteotomies on a three-dimensional finite element model of the human proximal femur based on computed tomography (CT) data and using uniform brick-shaped elements. To eliminate artefacts resulting from the rough surface of the brick elements, the femoral head was represented by a tetrahedron-based head that included a cartilage layer and a subchondral cortical zone. Applicability of the procedure was tested by performing a parametric study using a model created from CT scans taken in vivo, by applying individually calculated force conditions for the one-leg stance situation. We found a large influence of osteotomy angle on the observed stress in the femoral head cartilage, especially in a situation with insufficient containment of the femoral head. The model presented here is a biomechanical tool to simulate intertrochanteric osteotomies patient-specifically for a better understanding of the effects of such operations in the individual case. The open design of the described programs allows future interfacing with surgical navigation and robot systems.

Spatial measurement and transformation of muscular attachments at the lower extremity.

A large number of spatial coordinates were directly measured on two wet anatomical muscle specimens with a measuring device specifically designed to collect cylindrical coordinates. Three-dimensional coordinates of muscle attachment circumferences, joint centres and bone landmarks were collected. All the measured points of a segment, including the adjacent joint centres, were referred to a reference system defined by three marked screw heads at the femur and tibia. The points of the pelvic segment were referred to three marks on a lumbar mounting plate. This approach allows one to define and to transform the data to any reference frame.

Lumbar spine curvature during office chair sitting.

Prolonged sitting is generally accepted as a high risk factor in low back pain and it is frequently suggested that a lordotic posture of the lumbar spine should be maintained during sitting. We asked whether the sagittal curvature of the lumbar spine during sitting is affected by the seat tilt, backrest and the direction of the synchronised mechanism of the back and seat tilt (synchro tilt). Two office chairs were tested by multibody analysis interfacing a human model with a chair model. Results indicate that a synchronised mechanism of an office chair representing a posterior tilt of the seat while the backrest is reclined maintains an evenly distributed lumbar lordosis. The segmental angles are between 3.1 and 3.6 degrees at the lumbar vertebrae 1/2-4/5 (L1/2-L4/5). These lumbar spine segmental angles are not sensitive to the backrest height. In contrast, a synchro tilt concept with a reduction of the seat's posterior tilt while the backrest is reclined causes a strong reduction of the lumbar lordosis in backrest recline with a maximum reduction from 11.7 to 2.8 degrees in L4/5. As a consequence of these results, a synchro tilt concept with a posterior tilt of the seat while the backrest is reclined is preferable from the lumbar spine kinematics point of view.

Sensitivity of femoral strain pattern analyses to resultant and muscle forces at the hip joint.

An automated geometrical preprocessor was developed with the aim of creating three-dimensional finite element models (FEM) of the human femur. On the basis of postprocessed computed tomography data, this preprocessor makes possible rapid, flexible and regular meshing with 'brick' elements. Three different material properties were modelled at the present stage of development. Sensitivity analyses demonstrated that the strain energy density (SED) patterns of the different femoral parts were most sensitive to the implementation of an iliotibial tract force. The variation of the resultant hip force and abductor force direction within the sagittal plane demonstrated a SED minimum at an anterior inclination of 13 degrees; the variation of the resultant force direction within the frontal plane demonstrated a minimum SED at a medial inclination of 21 degrees relative to the mechanical axis of the lower limb. The orientation of the connecting line between the surface-SED-peaks in the horizontal view was found to be most sensitive to the variation of the resultant force within the sagittal plane.

Comparison of geometry-based and CT voxel-based finite element modelling and experimental validation.

The objectives of the paper presented here is the comparison of a geometry-based and voxel-based finite element (FE) method preprocessor of the human femur. The models were experimentally validated by strain gauge measurements (principal stress). The correlation coefficients (r) between the three methods (geometry-based FEM, voxel-based FEM, strain gauge measurements) were found to be in the range 0.91-0.94 (r2: 0.84-0.88). The relationships between the samples are highly significant (P = 0.001), where the strain gauge results are the independent variables. These results suggest that the validity with respect to the principal stress of a voxel-based modelling is similar to the validity of geometry-based modelling. In summary, therefore, we conclude that voxel-based meshing allows a straightforward interfacing with computerized tomography (CT) scans and might contribute to a clinically applicable FEM technology.

Spinal load changes during rotatory dynamic sitting.

OBJECTIVE: To measure load and moment changes acting on the lumbar spine during rotatory sitting. BACKGROUND: A new chair concept generating dynamic stimuli by alternating rotations in the horizontal plane of the chair's seat was recently developed. METHODS: Load and moment changes were measured telemetrically with a spinal fixator device in vivo. RESULTS: A rotatory frequency of 0.22 Hz with an amplitude of 1.8 degrees to the right and left side showed maximum axial force changes in the fixator of 23 N and maximum bending moment changes of 0.52 Nm. CONCLUSIONS: Lumbar force and moment changes during dynamic sitting occur, although only one patient was included in the study. Reasons could be temporary muscular activation in order to adapt the body's equilibrium conditions at the end-point rotation. RelevanceOur measurements suggest that a rotatory chair does have an effect on lumbar spine forces. It becomes more likely that this concept could improve the discs' nutrition and may prevent low back pain.

[Controlling and supporting apparatus of the deep finger flexor tendon and the long extensor tendon of the thumb. Clinico-anatomical analysis]. / Führungs- und Haltevorrichtungen der tiefen Fingerbeugesehne und der langen Daumenstrecksehne. Klinisch-anatomische Analysen.

Full function of finger flexion requires an intact pulley system. After destruction of the pulleys, reconstruction over the metacarpal head (A1) and over the center of the proximal and middle phalanges (A2, A4) should be carried out. Calculations on a computer model suggest that the reconstructed pulley must hold the tendon close to the underlying bone. Width of the pulley and distance from the next proximal or distal joint of the inserted pulley are of minor functional importance. The amount of maximum load on the pulley, exceeding the tension of the tendon, must be considered in operation and postoperative treatment. The extensor pollicis longus tendon lies at the dorsal tubercle (tubercle of Lister) within an osseous gliding surface, where it changes direction. The deviation angle depends on the radiocarpal position and causes, in addition to the usual tension, a pressure stress to that part of the tendon. A graphical analysis of statics, based on X-rays, demonstrates maximum loads on the gliding surface. They can increase in relation to the tension force of the tendon to about 50% in the antero-posterior and to 100% in the radioulnar projection. Densitography of the distal radius shows a maximum of density where the dorsal tubercle is most prominent. Nutritional problems of that tendon are predisposed by that particular mechanical stress. Therefore spontaneous rupture of the extensor pollicis longus tendon in patients with rheumatoid arthritis or after fracture of the distal radius occurs, though infrequently. In the case of immobilization, slight ulnar abduction within the radiocarpal joint remarkably reduces the tension and pressure stresses on the tendon.

[Functional importance of the m. extensor carpi ulnaris for distal radioulnar articulation]. / Funktionelle Bedeutung des M. extensor carpi ulnaris für das distale Radioulnargelenk.

Any analysis of the stability of the distal radio-ulnar joint should consider the extensor carpi ulnaris tendon. The static and kinematic function of this tendon depends on its relationship to the sigmoid notch of the radius. Morphological studies indicate the existence of a gliding surface in the sulcus of the tendon on the dorsal side of the ulnar head.

[Effects of shaft length of finger joint prostheses on tension distribution in the bone]. / Auswirkung der Schaftlänge von Fingergelenkprothesen auf die Spannungsverteilung im Knochen.

In order to investigate the influence of stem length in finger joint prostheses on stress in the surrounding bone area, finite element (FEM) calculations of finger bones before and after prosthetic replacement of metacarpophalangeal joints with cementless implants of different stem length were performed. CT scans of the metacarpal bone and proximal phalanx before and after implantation of a prototype of a noncemented semiconstrained implant for the MP joint, which has been developed to replace metacarpophalangeal joints destroyed by rheumatoid arthritis, were analysed. The FEM calculations showed comparatively decreased differences of the von-Mises stress after implantation of intramedullary stems reaching the middle of the diaphysis. At the metaphysis of the metacarpal head we found an increase of the von-Mises stress of 1.3 MPa (Mega Pascal = 10(6) Pa), an increase of 18.9 MPa around the shortest prosthesis and a decrease of 21.4 MPa around the prosthesis with the longest stem.

[Morphological and numerical studies of the stress compatibility of patella implants]. / Morphologische und numerische Untersuchungen zur Stresskompatibilität von Patellaimplantaten.

Failure of total knee arthroplasty is relatively often caused by problems of the patellofemoral replacement. The purpose of this study was to analyze the distribution of stresses within an anatomical patella and the changes in stress distribution after patellar resurfacing with a Miller-Galante I patellar implant using two- and three dimensional finite element models (FEM). To assess validity, FEM results were compared with morphological findings from contact radiographs and densitographs. Internal orientation of bone trabeculae is in good agreement with the orientation of theoretically calculated principal stresses. Almost unchanged principal tensile stresses after implantation, together with the lack of extreme stress peaks within the cancellous bone ensure stress compatibility of the implant. In the case of a firmly seated implant with good bone ingrowth, increased von Mises stresses are found near the fixation peg/plate junction. Their relevance for improved bone ingrowth near this part of the interface is emphasized. At the same time, material failure at the peg/plate junction can be better understood. An analysis of the early postoperative period assuming nonlinear interface conditions failed to demonstrate an uniform distribution of normal and tangential interface forces.

[Influence of cartilage cap modelling on FEM simulation of femoral head stress]. / Einfluss der Knorpelkappenmodellierung auf die FEM-Simulation der Femurkopfbeanspruchung.

The present study reports on the finite element analysis (FEA) of the femoral head in a process of preparation for a program for the realistic simulation of correctional osteotomies of the proximal femur. While the material properties have been studied extensively, only few publications consider the influence of the cartilage layer geometry on FE stimulation of the hip joint. Various models of the femoral head with and without the cartilage layer are generated and analysed. On looking at the maximum surface stresses, we found a strong influence of the cartilage layer and the subchondral osseous layer on the magnitude of the von Mises equivalent stress. The model with an anatomically realistic cartilage layer and compact bone shows stresses of between 4 and 5.5 MPa, depending on the position of the joint, while the model with a concentric cartilage layer has a maximum von Mises stress of 0.8 MPa. Only on simulation of a "realistic" cartilage layer, with a maximum thickness at the "pole" and minimum thickness at the "equator" do the changes in stress distribution--determined by changes in the position of the femoral head--become visible. Owing to major artefacts and the inability to create a realistic cartilage layer, voxel-based models of the femur are not suitable for the simulation of the femoral head surface.

[Systematic analysis of ground reaction forces before and after hip and knee arthroplasty]. / Systematische Analyse der Anderung der Bodenreaktionskräfte bei Hüft- und Kniearthrose prä- und postoperativ.

In this prospective study we employed a newly developed gait analysis system to compare the ground reaction force patterns in 15 patients before and after total hip or knee replacement. In this system, data are measured separately for each limb. Measured data were also obtained from 30 healthy adults and compared with those obtained from the patient group. We analysed the three-dimensional force patterns, impulse, frequency, stride and double stance, and compared changes in the postoperative gait patterns. The vertical force maxima Fy identify the peak forces obtaining during walking. The results showed significantly increased (p < 0.05) postoperative force maxima Fy2 and Fy3 for both knee replacement (Fy2: 82.48 to 86.17 and Fy3: 96.09 to 99.35% body weight, pre- and postoperatively, respectively) and hip replacement (Fy2: 84.44 to 88.08 and Fy3: 98.63 to 101.96% body weight, pre- and postoperatively, respectively). The ADAL system proved suitable for the easy performance of gait analysis, and thus may be of future value in the area of clinical quality assurance.

[Fully automated generation of hip prosthesis voxel models of the femur for finite element analysis. Comparison of direct and subsequent prosthesis implantation]. / Die vollautomatische Erstellung implantatbestückter Voxelmodelle des Femurs zur FE-Analyse, Vergleich von direkter und nachträglicher Prothesenimplantation.

Two means of generating implant-bearing FE voxel models of the human femur are described and compared. The first method consists in the fully automatic production of a femur model from CT data and subsequent implantation of a prosthesis stem on the basis of a redefinition of existing voxel elements using geometrical data control. The second method processes the CT data of a femur already bearing a prosthesis. Here, the prosthesis elements are generated separately from the surrounding bony elements on the basis of the differing CT density thresholds. Although the two implantation algorithms produce comparable results, they have different fields of application. The method of numerical stress analysis involving automatic generation of voxel models can be applied to implant-bearing femurs. Clinical follow-up observations of patients with prostheses made of titanium can be supported by FE calculations.

[Use of voxel-oriented femur models for stress analysis. Generation, calculation and validation of CT-based FEM models]. / Die Anwendung voxelorientierter Femurmodelle zur Spannungsanalyse. Generierung, Berechnung und Validierung CT-basierter FEM-Modelle.

A means of interfacing computed tomography with the finite element method for the analysis of stress distribution in human femurs is described. With the aid of data transfer and image processing programs, three-dimensional voxel models of four macerated and a fresh-frozen cadaveric femur were generated from CT data sets. The underlying basic principle is direct generation of model elements from one or more adjacent CT-voxels, which are characterised by their coordinates and local absorption coefficients. A new algorithm was used for smoothing the edges of the model. All the models were analysed with respect to their stress distribution under different load situations, and the results were compared with the data of a parallel study utilizing the strain-gauge technique. Using the method of linear regression, a high correlation (coefficient of determination r2: 0.80-0.91) was found between the calculated and experimentally measured principal stresses at the surface of the bones.

[Experimental determination of muscle coordination of the lower extremity]. / Experimentelle Bestimmung von Muskelkoordinaten an der unteren Extremität.

In order to describe the lines of action of muscles acting on joints, data on the geometrical position of insertions and origins are needed. These are of fundamental relevance for the biomechanical analysis, in particular for the calculation of joint loading. A measuring device with an accuracy of 0.1 mm was designed and constructed for the collection of three-dimensional experimental data from wet anatomical specimens. The main design features of the device include a vertically adjustable stand on wheels and a rotatable scale ring with a radial pointer. After setting the reading, the apparatus permits the measurement of cylindrical coordinates The margins of the attachments of muscles acting on the hip and knee joints were measured in two specimens. Two extensive sets of morphological data are now available and can be used to create complex biomechanical computer models.

[Automatic generation of 3-D finite element codes of the human femur]. / Automatisierte Generierung von 3-D Finite Elemente Codes des menschlichen Femurs.

The extremely irregular geometry of bones makes it very difficult to develop and preprocess three-dimensional finite element models (FEM). In the field of biomechanics, the finite element simulation method is a research tool of increasing importance for the prediction of stresses and the stress-compatibility testing of endoprostheses. With this in mind, a FEM preprocessor was developed with the aim of creating three dimensional models of human femurs. On the basis of postprocessed computed tomography data, this preprocessor makes possible rapid, flexible and regular cross-linking. "Brick" elements are used, which are of advantage in terms of computing time, accuracy and computer storage requirements. The software simplifies the generation of models and facilitates the investigation of individual variations, thus providing the basis for utilizing the method in the clinical setting.

[Predicting load bearing of the hip joint. Computerized analysis with a 3-D multibody model of the human]. / Belastungsvorhersagen am Hüftgelenk. Computeranalysen an einem 3-D-Mehrkörpermodell des Menschen.

Multibody analysis was applied to construct an advanced model of the human body, where the large joints and complete mass and inertial properties were implemented. The model represents the 50th-percentile rank of a male adult. The hip joint is controlled by three muscle forces. The muscle coordinates were taken from a data source, previously collected by our group. The model enables one to analyze 3D hip joint forces with respect to various joint angles and represents conceptually an improvement of the classical method of graphical statics, which was established by Pauwels [15]. A hip joint load of three times body weight was found in the single leg stance. A load of 3.7 times body weight was calculated when simulating a knee flexion angle of 90 degrees, and a ventral inclination of the resultant hip joint force was seen. A constant amount of gluteus medius muscle force was observed during flexion. An increasing flecting moment at the hip joint, however, had to be balanced by a significantly increased gluteus maximus muscle force. As a consequence, torsional forces can be studied by the system presented here and should also be considered when testing stems of hip prostheses. External muscle and joint forces are provided and can be used as input data for stress analyses.

[Testing dental implants with an in vivo finite element model]. / Zahnimplantattestung am In-vivo-Finite-Elemente-Modell.

The finite element method (FEM) makes it possible to simulate biomechanical situations on a computer. In the present study the so-called voxel method [9, 14, 17, 18, 19] was used for the construction of the mandible model. For this, the relationship between the biological tissue (e.g. bone) and the corresponding attenuation coefficient of CT data (Hounsfield units = HU) were utilized. The CT data were obtained from an edentulous patient provided with a prosthesis borne on two titanium implants. In a parallel study, the bite forces of the same individual were measured. These were recorded digitally in three dimensions (cranio-caudal, anterior-posterior and left-right) The forces determined by a special program were then transferred to the FEM model implants. We were able to show that a bar joining the implants had a far greater effect on maximum equivalent stress than was expected from the measuring data alone. The highest stress at maximum occlusive force was lowered by 704% on using the connecting bar. On chewing, a stress reduction of 59.9% was observed. The reduction in stress achieved by the bar could, we believe, prolong the life of the implant.