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Preclinical testing of total knee replacements (TKR) is crucial for evaluating new implant designs. Dynamic experimental testing focus mostly on level walking and squats, failing to represent a full range of daily activities. Moreover, the contribution of the ligament apparatus is often simplified. Therefore, this study transferred five daily activity load cases—level walking, downhill walking, stair descent, squat, and sit-to-stand—onto a six-degree-of-freedom (6-DOF) joint simulator with a cruciate-retaining bicondylar TKR and a virtual ligament apparatus. Forces and kinematics were based on telemetric data from an ultra-congruent TKR. The resulting kinematics, kinetics, and tibiofemoral contact surfaces were evaluated. Additionally, variations of the virtual ligament apparatus on the joint simulator, e.g. resection of the posterior cruciate ligament (PCL), have been used to assess its influence on the resulting joint dynamics. Results showed that tibiofemoral contact area was more influenced by dynamics than kinematics. Virtual PCL resection shifted the tibia posteriorly (up to 3 mm) and increased abduction (up to 0.5°). Different results were seen across all load cases. The exceptions are the squat and sit-to-stand load cases with similar patterns. Thus, cruciate-retaining TKR can be tested using telemetric data from ultra-congruent TKR, aiding in comprehensive evaluations.

In biomechanical research, advanced joint simulators such as VIVOTM offer the ability to test artificial joints under realistic kinematics and load conditions. Furthermore, it promises to simplify testing with advanced control approaches and the ability to include virtual ligaments. However, the overall functionality concerning specific test setup conditions, such as the joint lubrication or control algorithm, has not been investigated in-depth so far. Therefore, the aim of this study was to analyse the basic functionality of the VIVOTM joint simulator with six degrees of freedom in order to highlight its capabilities and limitations when testing a total knee endoprostheses using a passive flexion–extension movement. For this, different test setup conditions were investigated, e.g., the control method, repeatability and kinematic reproducibility, waveform frequency, lubrication, and implant embedding. The features offered by the VIVOTM joint simulator are useful for testing joint endoprostheses under realistic loading scenarios. It was found that the results were highly influenced by the varying test setup conditions, although the same mechanical load case was analysed. This study highlights the difficulties encountered when using six degrees of freedom joint simulators, contributes to their understanding, and supports users of advanced joint simulators through functional and tribological analysis of joint endoprostheses.

The implantation of total knee replacements is an effective treatment for advanced degenerative knee joint diseases. Implant positioning relative to the bones affects the loads occurring in the artificial joint, joint stability, and postoperative functionality. Variance in implant positioning during the surgical implantation of a total knee replacement cannot be entirely ruled out. By simulating implant malpositioning in an experimental setting, particularly critical cases of malalignment can be identified, from which guidelines for orthopedic surgeons can be derived. The AMTI VIVO™ six degree of freedom joint simulator allows reproducible preclinical testing of joint endoprostheses under specific kinematic and loading conditions. It features a virtual ligament model that defines up to 100 ligament fibers between the articulating components. This paper presents a method to investigate the effect of different implant positions on the biomechanics of the knee after total knee arthroplasty. For this, the VIVO joint simulator requires no modification of the physical setup by moving virtual ligament insertion points relative to the bone. As a proof of concept, exemplary shifts and rotations of the femoral and tibial implant components are performed, and dynamic results are compared to a musculoskeletal multibody digital twin and findings from the literature. Video Abstract.

The AMTI VIVO™ six degree of freedom joint simulator allows reproducible preclinical testing of joint endoprostheses under specific kinematic and loading conditions. When testing total knee endoprosthesis, the articulating femoral and tibial components are each mounted on an actuator with two and four degrees of freedom, respectively. To approximate realistic physiological conditions with respect to soft tissues, the joint simulator features an integrated virtual ligament model that calculates the restoring forces of the ligament apparatus to be applied by the actuators. During joint motion, the locations of the ligament insertion points are calculated depending on both actuators’ coordinates. In the present study, we demonstrate that unintended elastic deformations of the actuators due to the specifically high contact forces in the artificial knee joint have a considerable impact on the calculated ligament forces. This study aims to investigate the effect of this structural compliance on experimental results. While the built-in algorithm for calculating the ligament forces cannot be altered by the user, a reduction of the ligament force deviations due to the elastic deformations could be achieved by preloading the articulating implant components in the reference configuration. As a proof of concept, a knee flexion motion with varying ligament conditions was simulated on the VIVO simulator and compared to data derived from a musculoskeletal multibody model of a total knee endoprosthesis.

Background Despite advances in total knee arthroplasty, many patients are still unsatisfied with the functional outcome. Multibody simulations enable a more efficient exploration of independent variables compared to experimental studies. However, to what extent numerical models can fully reproduce knee joint kinematics is still unclear. Hence, models must be validated with different test scenarios before being applied to biomechanical questions. Methods In our feasibility study, we analyzed a human knee specimen on a six degree of freedom joint simulator, applying a passive flexion and different laxity tests with sequential states of ligament resection while recording the joint kinematics. Simultaneously, we generated a subject-specific multibody model of the native tibiofemoral joint considering ligaments and contact between articulating cartilage surfaces. Results Our experimental data on the sequential states of ligament resection aligned well with the literature. The model-based knee joint kinematics during passive flexion showed good agreement with the experiment, with root-mean-square errors of less than 1.61 mm for translations and 2.1° for knee joint rotations. During laxity tests, the experiment measured up to 8 mm of anteroposterior laxity, while the numerical model allowed less than 3 mm. Conclusion Although the multibody model showed good agreement to the experimental kinematics during passive flexion, the validation showed that ligament parameters used in this feasibility study are too stiff to replicate experimental laxity tests correctly. Hence, more precise subject-specific ligament parameters have to be identified in the future through model optimization.

In biomechanical research, advanced joint simulators such as VIVO[sup.TM] offer the ability to test artificial joints under realistic kinematics and load conditions. Furthermore, it promises to simplify testing with advanced control approaches and the ability to include virtual ligaments. However, the overall functionality concerning specific test setup conditions, such as the joint lubrication or control algorithm, has not been investigated in-depth so far. Therefore, the aim of this study was to analyse the basic functionality of the VIVO[sup.TM] joint simulator with six degrees of freedom in order to highlight its capabilities and limitations when testing a total knee endoprostheses using a passive flexion–extension movement. For this, different test setup conditions were investigated, e.g., the control method, repeatability and kinematic reproducibility, waveform frequency, lubrication, and implant embedding. The features offered by the VIVO[sup.TM] joint simulator are useful for testing joint endoprostheses under realistic loading scenarios. It was found that the results were highly influenced by the varying test setup conditions, although the same mechanical load case was analysed. This study highlights the difficulties encountered when using six degrees of freedom joint simulators, contributes to their understanding, and supports users of advanced joint simulators through functional and tribological analysis of joint endoprostheses.

In biomechanical research, advanced joint simulators such as VIVO offer the ability to test artificial joints under realistic kinematics and load conditions. Furthermore, it promises to simplify testing with advanced control approaches and the ability to include virtual ligaments. However, the overall functionality concerning specific test setup conditions, such as the joint lubrication or control algorithm, has not been investigated in-depth so far. Therefore, the aim of this study was to analyse the basic functionality of the VIVO joint simulator with six degrees of freedom in order to highlight its capabilities and limitations when testing a total knee endoprostheses using a passive flexion-extension movement. For this, different test setup conditions were investigated, e.g., the control method, repeatability and kinematic reproducibility, waveform frequency, lubrication, and implant embedding. The features offered by the VIVO joint simulator are useful for testing joint endoprostheses under realistic loading scenarios. It was found that the results were highly influenced by the varying test setup conditions, although the same mechanical load case was analysed. This study highlights the difficulties encountered when using six degrees of freedom joint simulators, contributes to their understanding, and supports users of advanced joint simulators through functional and tribological analysis of joint endoprostheses.

Aseptic implant loosening after a total joint replacement is partially influenced by material-specific factors when cobalt–chromium alloys are used, including osteolysis induced by wear and corrosion products and stress shielding. Here, we aim to characterize a hybrid material consisting of alumina-toughened zirconia (ATZ) ceramics and additively manufactured Ti-35Nb-6Ta (TiNbTa) alloys, which are joined by a glass solder. The structure of the joint, the static and fatigue shear strength, the influence of accelerated aging, and the cytotoxicity with human osteoblasts are characterized. Furthermore, the biomechanical properties of the functional demonstrators of a femoral component for total knee replacements are evaluated. The TiNbTa-ATZ specimens showed a homogenous joint with statistically distributed micro-pores and a slight accumulation of Al-rich compounds at the glass solder–TiNbTa interface. Shear strengths of 26.4 ± 4.2 MPa and 38.2 ± 14.4 MPa were achieved for the TiNbTa-ATZ and Ti-ATZ specimens, respectively, and they were not significantly affected by the titanium material used, nor by accelerated aging (p = 0.07). All of the specimens survived 107 cycles of shear loading to 10 MPa. Furthermore, the TiNbTa-ATZ did not impair the proliferation and metabolic activity of the human osteoblasts. Functional demonstrators made of TiNbTa-ATZ provided a maximum bearable extension–flexion moment of 40.7 ± 2.2 Nm. The biomechanical and biological properties of TiNbTa-ATZ demonstrate potential applications for endoprosthetic implants.

Ribonucleic Acid (RNA) is a ubiquitous, highly functional biomolecule found throughout life. As a polyelectrolyte, RNA is negatively charged and requires the presence of positive counterions to condense into the specific, folded structures that are necessary for the chemistry of life. RNA molecules are especially influenced by divalent metal ions such as Mg 2+, the presence of which not only helps to neutralize the RNA, but creates an overall attractive free energy—an important effect not found under simple monovalent salt conditions. Methods of modeling ionic and polyionic systems date back to the early 20th century, when Peter Debye and Erich Hückel developed their purely analytical, Poisson-Boltzmann formalism of electrolytes that persists as the primary model used for most theoretical applications. However, the Debye-Hückel model breaks down when considering highly correlated polyions such as nucleic acids, as well as divalent and other high-valence ionic solutions in concentrations similar to those found in vivo. The theoretical limits of the Debye-Hückel model in tandem with the rise of simulation and numerical methods for studying biological systems has led to a need for alternative models of polyelectrolyte systems that will correctly account for complex ionic conditions while simultaneously allowing for calculations fast enough to be incorporated into simulation. In this thesis, we study the interaction between RNA and positive counterions, focusing specifically on divalent cations. We begin with a discussion the structure and function of RNA, and we cover new research showing that RNA has more functionality and biochemically relevant applications than ever previously thought. This is followed by a recap of prior theoretical studies and models of electrolyte and polyelectrolyte systems—both for RNAs specifically as well as general models. We then present our own model for RNA-ion interactions, which allows us to calculate the counterion-mediated free energy of RNA, but only through an NP-complete optimization problem. A heuristic approach to this NP-complete problem is developed, which is shown to work very well for all functional RNAs smaller than the ribosome, and is computationally fast and effective enough to be incorporated into full simulation. Monte Carlo simulations of RNA are run with our implicit divalent ion model incorporated, and we show that our model matches closely with experiment. Most biomolecular simulation is done via molecular dynamics methodology, and to account for this, we further develop our numerical approach so that ensemble forces may be calculated and incorporated into molecular dynamics simulations. Finally, we develop a full coarse-grained model of RNA that accurately handles the attractive electrostatic forces generated by the presence of divalent counterions, and use this model to simulate and study RNA 2-way junctions.

First-Generation Detectors Early electronic edge detectors could be influenced by a variety of environmental and process factors that degrade accuracy, including temperature, humidity, air turbulence, vapors and gases, dust, ink, coating overspray, web flutter, curl and web path changes. Dust or other contaminants accumulating on the optical detector's lenses will absorb some of the bean's energy and prevent it from reaching the receiver, resulting in incorrect redirection of the web. The two beams are located less than an inch apart, but oriented so the web passes only through the sensing beam. Because the web never blocks the reference beam, it provides a direct, real-time indication of how environmental and other changes are affecting the performance of the detector.