Otto Müller

Substrate dependent differences in morphology and elasticity of living osteoblasts investigated by atomic force microscopy

We have used the atomic force microscope (AFM) as a tool for testing the biocompatibility of implant materials by investigating the adhesion behavior of osteoblast cells in vitro. This technique allowed the investigation of cytomorphology and cytomechanical properties of living cells on a submicrometer scale. Cell adhesion was investigated on Cobalt-Chromium (CoCr), Titanium (Ti) and Titanium-Vanadium (TiV) substrates, which are of great interest in the field of implant research. The elastic properties and the morphology of living osteoblasts on the metallic substrates were compared with those of osteoblasts cultured on glass and tissue culture polystyrene (PS). Furthermore, a characterization of the surface roughness of the substrates was performed and the surface coverage of proteins after incubation with cell culture medium on the substrates was observed with the AFM.

The effect of different quadriceps loading patterns on tibiofemoral joint kinematics and patellofemoral contact pressure during simulated partial weight-bearing knee flexion

PurposeThe purpose of this in vitro study was to investigate the influence of different quadriceps loading patterns on tibiofemoral joint kinematics and patellofemoral pressure.MethodsA dynamic muscle-loaded knee squat was simulated on eight knee specimens with an upright knee simulator while measuring tibiofemoral joint kinematics and patellofemoral pressure distribution. The quadriceps muscle was attached to three actuators simulating the three main extensor muscles, and five different quadriceps loading patterns were tested.ResultsTibial axial and varus-valgus-rotation are affected most while changing quadriceps loading patterns from lateral to medial. Higher internal tibial rotation is associated with higher medial muscle load compared to the symmetrical loading condition. Contact force, contact area and maximum peak pressure rise with increasing flexion angles. Accentuating the vastus lateralis muscle induces a significant reduction in patellofemoral contact force and a 30% diminished contact area at 90° of flexion.ConclusionStrengthening the vastus medialis muscle leads to increased internal tibial rotation, thus optimizing patella tracking by lowering the Q-angle. In contrast, weakness of the vastus medialis muscle causes decreased tibial internal rotation and is associated with lower patellofemoral contact pressure and contact area. Vastus medialis exercise is advisable to improve patella tracking but may not be recommended in patients with disorders due to increased patellofemoral contact pressure.

Simulation of force loaded knee movement in a newly developed in vitro knee simulator / Simulation von belastungsabhängigen Kniebewegungen in einem neuartigen Knie-Simulator für In-vitro-Studien

Abstract Simulating knee movement under physiological muscle loading is a prerequisite in order to improve surgical treatment and rehabilitation techniques. An apparatus is presented which can simulate five knee muscles to control a definite amount of body weight using the ankle force as the target value for the control mechanism. The influence of different amounts of simulated ankle forces upon the knee movement was investigated. The apparatus was constructed in a closed kinetic chain design similar to the so-called Oxford rig. Three quadriceps muscles and two hamstring muscles were controlled by electrical servo motors via tendon clamps in order to adjust a target value for the simulated body weight. Three fresh frozen cadaveric specimens were used to validate the apparatus and to examine the difference between loaded and unloaded knee flexion from 10° to 90°. In one specimen, up to 250 N simulated ankle force could be achieved for a single leg knee flexion. Among the kinematic variables, tibial rotation was influenced the most when varying the amount of simulated body weight. Although the knee kinematics changed considerably with increasing simulated bodyweight, the shapes of the kinematic profiles remained similar, indicating that qualitative clinical insights can still be elucidated with partially (but reasonably) loaded knees. Zusammenfassung Die Simulation der Kniebewegung unter physiologischer Muskelbeanspruchung ist eine Grundvoraussetzung zur Verbesserung von Operationsverfahren und Rehabilitationstechniken am Knie. Es wird eine Apparatur vorgestellt, welche mittels fünf simulierter Muskelzüge die Fußgelenkskraft als Regelgröße zur Simulation des Körpergewichts benutzt. Der Einfluss von unterschiedlichen Gewichtswerten auf die Bewegung des Kniegelenks wurde untersucht. Die Apparatur ist als geschlossene kinematische Kette, ähnlich dem sogenannten Oxford Rig, konstruiert. Drei Kniestrecker und zwei -beuger werden durch Servomotoren gesteuert, um einen definierten Wert des Körpergewichtes zu simulieren. Tiefgefrorene Kadaverpräparate wurden verwendet, um die Apparatur zu validieren und um den Unterschied zwischen einer belasteten und unbelasteten Kniebeuge zu untersuchen. Es konnte eine Körpergewichtskraft bis zu 250 N simuliert werden. Von allen kinematischen Parametern der Kniebewegung ist die Tibiarotation am meisten durch unterschiedliche Gewichtswerte bestimmt. Obwohl sich die Bewegung des Kniegelenks mit zunehmendem Körpergewicht ändert, verhalten sich die kinematischen Messgrößen ähnlich. Um klinische Fragestellungen der Kniebewegung im Kadavermodel zu simulieren, genügt es, nur Teile des Körpergewichtes zu simulieren.

Forces in anterior cruciate ligament during simulated weight-bearing flexion with anterior and internal rotational tibial load.

This study determined in-vitro anterior cruciate ligament (ACL) force patterns and investigated the effect of external tibial loads on the ACL force patterns during simulated weight-bearing knee flexions. Nine human cadaveric knee specimens were mounted on a dynamic knee simulator, and weight-bearing knee flexions with a 100N of ground reaction force were simulated; while a robotic/universal force sensor (UFS) system was used to provide external tibial loads during the movement. Three external tibial loading conditions were simulated, including no external tibial load (termed BW only), a 50N anterior tibial force (ATF), and a 5Nm internal rotation tibial torque (ITT). The tibial and femoral kinematics was measured with an ultrasonic motion capture system. These movement paths were then accurately reproduced on a robotic testing system, and the in-situ force in the ACL was determined via the principle of superposition. The results showed that the ATF significantly increased the in-situ ACL force by up to 60% during 0-55 degrees of flexion, while the ITT did not. The magnitude of ACL forces decreased with increasing flexion angle for all loading conditions. The tibial anterior translation was not affected by the application of ATF, whereas the tibial internal rotation was significantly increased by the application of ITT. These data indicate that, in a weight-bearing knee flexion, ACL provides substantial resistance to the externally applied ATF but not to the ITT.

Differences in knee joint kinematics and forces after posterior cruciate retaining and stabilized total knee arthroplasty

BACKGROUND Posterior cruciate ligament (PCL) retaining (CR) and -sacrificing (PS) total knee arthroplasties (TKA) are widely-used to treat osteoarthritis of the knee joint. The PS design substitutes the function of the PCL with a cam-spine mechanism which may produce adverse changes to joint kinematics and kinetics. METHODS CR- and PS-TKA were performed on 11 human knee specimens. Joint kinematics were measured with a dynamic knee simulator and motion tracking equipment. In-situ loads of the PCL and cam-spine were measured with a robotic force sensor system. Partial weight bearing flexions were simulated and external forces were applied. RESULTS The PS-TKA rotated significantly less throughout the whole flexion range compared to the CR-TKA. Femoral roll back was greater in the PS-TKA; however, this was not correlated with lower quadriceps forces. Application of external loads produced significantly different in-situ force profiles between the TKA systems. CONCLUSIONS Our data demonstrate that the PS-design significantly alters kinematics of the knee joint. Our data also suggest the cam-spine mechanism may have little influence on high flexion kinematics (such as femoral rollback) with most of the load burden shared by supporting implant and soft-tissue structures.

Influence of bi- and tri-compartmental knee arthroplasty on the kinematics of the knee joint.

BackgroundThe cruciate ligaments are important stabilizers of the knee joint and determine joint kinematics in the natural knee and after cruciate retaining arthroplasty.No in vitro data is available to biomechanically evaluate the ability of the anterior cruciate ligament (ACL) to maintain knee joint kinematics after bicruciate-retaining bi-compartmental knee arthroplasty (BKA).Therefore, the objective of the current study was to investigate the kinematics of the natural knee joint, before and after installing bicruciate-retaining BKA and posterior cruciate retaining total knee arthroplasty. Specifically, we incorporated a dynamic knee simulator to simulate weight-bearing flexions on cadaveric knee specimen before and after surgical manipulations.MethodsIn this cadaveric study we investigated rotational and translational tibiofemoral kinematics during simulated weight-bearing flexions of the intact knee, after bi-compartmental knee arthroplasty (BKA+), after resecting the ACL in BKA (BKA-), and after posterior cruciate retaining total knee arthroplasty (TKA).ResultsRotation of BKA+ is closest to the intact knee joint, whereas TKA shows significant differences from 30 to 90 degree of flexion. Within the tested flexion range (15 to 90 degree of flexion), there was no significant difference in the anterior-posterior translation among intact, BKA+, and TKA knees. Resecting the ACL in BKA leads to a significant anterior tibial translation.ConclusionsBKA with intact cruciate ligaments resembles rotation and translation of the natural knee during a simulated weight-bearing flexion. It is a suitable treatment option for medial and patellofemoral osteoarthritis with advantages in rotational characteristics compared to TKA.

The Anterior Cruciate Ligament Provides Resistance to Externally Applied Anterior Tibial Force But Not to Internal Rotational Torque During Simulated Weight-Bearing Flexion

PURPOSE We investigated knee kinematics during simulated weight-bearing flexion and determined the effect of 3 different parameters of external tibial loading on the kinematics of the anterior cruciate ligament (ACL)-intact and ACL-deficient knee. METHODS Ten human knee specimens were mounted on a dynamic knee simulator, and weight-bearing muscle-loaded knee flexions were simulated while a robotic/universal force sensor system was used to provide external tibial loads during the motion. Three different loading conditions were simulated: partial body weight only, an additional 50 N of anterior tibial force (ATD), or an additional 5 Nm of internal rotational tibial torque (IRT). After arthroscopic transection of the ACL, these 3 trials were repeated. The kinematics were measured with an ultrasonic measuring system for 3-dimensional motion analysis, and different loading and knee conditions were examined. RESULTS When the ACL was intact, ATD and IRT barely changed the anterior tibial translation. However, in the absence of the ACL, ATD significantly increased the anterior tibial translation by 5 mm whereas IRT did not. The application of IRT increased the internal tibial rotation of ACL-intact knees, but there was no difference in the internal rotation before and after transection of the ACL. Regardless of ACL status, the difference in the anterior tibial translation and the internal tibial rotation across different external tibial loadings was greater at lower flexion angles and gradually diminished with increasing flexion angles. CONCLUSIONS We established an experimental protocol, incorporating a dynamic knee simulator and a robotic/universal force sensor system, to successfully measure the kinematics of the knee joint while applying external forces in weight-bearing flexion. Our findings suggest that, in muscle-loaded knee flexion, the ACL provides substantial resistance to externally applied ATD but not to IRT. CLINICAL RELEVANCE Information from this study allows us to better understand the function of the ACL and, hence, treatment of injuries to this important stabilizing ligament.

Talonavicular arthrodesis or triple arthrodesis: Peak pressure in the adjacent joints measured in 8 cadaver specimens

Background Talonavicular arthrodesis is a differential indication for triple arthrodesis. Differences regarding intraarticular pressure loads on the adjacent joints have not been investigated to date, but they are of clinical relevance when considering long-term joint degeneration. Methods We used a dynamic foot model to measure intraarticular peak pressures with electronic sensors in 8 anatomical specimens in different areas of the ankle joint and in the naviculocuneiform joint. Force was applied to extrinsic tendons via cables attached to computer- regulated hydraulic cylinders. A ground reaction force was simulated in a tilting angle- and force-controlled translation stage. Results We measured significantly higher peak pressures in the ankle joint after triple arthrodesis (5.7 Mpa) than after talonavicular arthrodesis (5.2 Mpa), with differences especially in the anterior section (5.2 Mpa as compared to 4.6 Mpa). Centrally, the peak pressure was similar, at 4.6 MPa and 4.5 Mpa, respectively. In the posterior area, the peak pressure after triple arthrodesis was lower (4.1 MPa as opposed to 4.4 Mpa). After triple arthrodeses, we measured higher values laterally/medially in the ankle joint (5.5 MPa/4.6 Mpa) than after talonavicular arthrodesis (5.1 MPa/4.4 Mpa). In the naviculocuneiform joint, we again saw higher peak pressures after triple arthrodesis than after talonavicular arthrodesis. Interpretation Our findings from this cadaver study indicate a lower and more evenly distributed peak pressure load in the ankle joint after talonavicular arthrodesis than after triple arthrodesis; thus, mechanically, a selective arthrodesis appears to be more favorable. In contrast, triple arthrodesis leads to an increase in peak pressure in the ankle joint, which may in turn lead to joint degeneration.

Changes in Chopart joint load following tibiotalar arthrodesis: in vitro analysis of 8 cadaver specimen in a dynamic model

BackgroundIn the current discussion of surgical treatment of arthroses in the ankle joint, arthrodesis is in competition with artificial joint replacement. Up until now, no valid biomechanical findings have existed on the changes in intraarticular loads following arthrodesis. One argument against tibiotalar arthrodesis is the frequently associated, long-term degeneration of the talonavicular joint, which can be attributed to changes in biomechanical stresses.MethodsWe used a dynamic model to determine the changes in intraarticular forces and peak-pressure in the talonavicular joint and in the calcaneocuboid joint on 8 cadaver feet under stress in a simulated stance phase following tibiotalar arthrodesis.ResultsThe change seen after arthrodesis was a tendency of relocation of average force and maximum pressure from the lateral onto the medial column of the foot. The average force increased from native 92 N to 100 N upon arthrodesis in the talonavicular joint and decreased in the calcaneocuboid joint from 54 N to 48 N. The peak pressure increased from native 3.9 MPa to 4.4 MPa in the talonavicular joint and in the calcaneocuboid joint from 3.3 MPa to 3.4 MPa. The increase of force and peak pressure on the talonavicular joint and decrease of force on the calcaneocuboid joint is statistically significant.ConclusionThe increase in imparted force and peak pressure on the medial column of the foot following tibiotalar arthrodesis, as was demonstrated in a dynamic model, biomechanically explains the clinically observed phenomenon of cartilage degeneration on the medial dorsum of the foot in the long term. As a clinical conclusion from the measurements, it would be desirable to reduce the force imparted on the medial column with displacement onto the lateral forefoot, say by suitable shoe adjustment, in order to achieve a more favourable long-term clinical result.

Translational and rotational knee joint stability in anterior and posterior cruciate-retaining knee arthroplasty.

This study investigated passive translational and rotational stability properties of the intact knee joint, after bicruciate-retaining bi-compartmental knee arthroplasty (BKA) and after posterior cruciate retaining total knee arthroplasty (TKA). Fourteen human cadaveric knee specimens were used in this study, and a robotic manipulator with six-axis force/torque sensor was used to test the joint laxity in anterior-posterior translation, valgus-varus, and internal-external rotation. The results show the knee joint stability after bicruciate-retaining BKA is similar to that of the native knee. On the other hand, the PCL-retaining TKA results in inferior joint stability in valgus, varus, external rotation, anterior and, surprisingly, posterior directions. Our findings suggest that, provided functional ligamentous structures, bicruciate-retaining BKA is a biomechanically attractive treatment for joint degenerative disease.