Despite decades of clinical research in artificial joints and underlying failure mechanisms, systematical and reproducible identification of reasons for complications in total knee replacements (TKR) remains difficult. Due to the complex dynamic interaction of implant system and biological situs, malfunction eventually leading to failure is multifactorial and remains not fully understood. The aim of present study was to evaluate different TKR designs and positions with regard to joint kinematics and stability under dynamic conditions by using a robot-based hardware-in-the-loop (HiL) setup. An industrial 6-axis robot with 6-axis force-torque sensor mounted into its end-effector moved and loaded real, commercially available TKR (bicondylar, cruciate-retaining) that were in virtual interaction with a subject-specific computational multibody model representing the anatomical situs of the knee joint while performing passive seated deep knee flexion. The subject-specific musculoskeletal multibody model (MMB) included rigid bones of the lower right extremity. Bone and cartilage geometries were reconstructed from MRT/ CT data sets preserving anatomical landmarks and allowing for the calculation of inertial properties. M. quadriceps femoris was modeled as single passive tensile force elements. Knee ligaments were modelled as elastic spring elements with a nonlinear force-displacement characteristic. Providing the flexion angle, the robot moved and loaded the mounted femoral implant component with respect to the tibial component while being in continuous interaction with the MMB. Several influencing parameters like implant position (internal/external rotation, varus/valgus alignment) and design (fixed vs. mobile bearing, tibia-insert height) as well as ligament insufficiency and joint loading on joint kinematics and stability was systematically analysed.Introduction
Material & methods
The purpose of this study was to experimentally evaluate impingement and dislocation of total hip replacements while performing dynamic movements under physiological-like conditions. Therefore, a hardware-in-the-loop setup has been developed, in which a physical hip prosthesis actuated by an industrial robot interacts with an in situ-like environment mimicked by a musculoskeletal multibody simulation-model of the lower extremity. The multibody model of the musculoskeletal system comprised rigid bone segments of the lower right extremity, which were mutually linked by ideal joints, and a trunk. All bone geometries were reconstructed from a computed tomography set preserving anatomical landmarks. Inertia properties were identified based on anthropometric data and by correlating bone density to Hounsfield units. Relevant muscles were modeled as Hill-type elements, passive forces due to capsular tissue have been neglected. Motion data were captured from a healthy subject performing dislocation-associated movements and were fed to the musculoskeletal multibody model. Subsequently, the robot moved and loaded a commercially available total hip prosthesis and closed the loop by feeding the physical contact information back to the simulation model. In this manner, a comprehensive parameter study analyzing the impact of implant position and design, joint loading, soft tissue damage and bone resection was implemented.Introduction
Methods
The higher resisting torque against dislocation and the large range of motion due to the enlarged effective head diameter substantiate the use of eccentric dual-mobility cups in case of total hip joint instability [1,2]. As a result of force-dependent self-centering mechanism, an increased movement of the intermediate-component can be expected whose effect on wear propagation is unknown so far. Currently available hip joint simulators are only able to vary the load by the absolute value and not by the direction of resulting force. Therefore, the uniaxial force transmission may lead to a unique and stable alignment of the intermediate-component during testing. The purpose of this numerical study was to evaluate relative movements of the intermediate-component during daily life activities with respect to wear propagation. The numerical analysis was based on a standard dual-mobility system consisting of a polished metallic cup, a UHMWPE intermediate-component (40 mm outer diameter) with an eccentric offset of 2 mm and a 28 mm ceramic femoral head [Fig. 1]. The relative motion of the intermediate-component was affected by the geometrically generated self-centering torque (TC) and the friction torque for inner (TFi) and outer (TFo) articulation around the centre of rotation Z1[Fig. 2]. In order to consider lubrication conditions the lambda ratio was estimated for different daily life activities [3], including the calculation of composite roughness and minimum film thickness for a ball-on-plate configuration. The friction torque was related to the product of load (Introduction:
Method:
Dislocation of total hip replacements (THRs) remains a severe complication after total hip arthroplasty. However, the contribution of influencing factors, such as implant positioning and soft tissue tension, is still not well understood due to the multi-factorial nature of the dislocation process. In order to systematically evaluate influencing factors on THR stability, our novel approach is to extract the anatomical environment of the implant into a musculoskeletal model. Within a hardware-in-the-loop (HiL) simulation the model provides hip joint angles and forces for a physical setup consisting of a compliant support and a robot which accordingly moves and loads the real implant components [2]. The purpose of this work was to validate the HiL test system against experimental data derived from one patient. The musculoskeletal model includes all segments of the right leg with a simplified trunk. Bone segments were reconstructed from a human computed tomography dataset. The segments were mutually linked in the multibody software SIMPACK (v8.9, Simpack AG, Gilching, Germany) by ideal joints starting from the ground-fixed foot. Furthermore, inertia properties were incorporated based on anthropometric data. Inverse dynamics was used to obtain muscle forces. Thus, optimization techniques were implemented to resolve the distribution problem of muscle forces whereas muscles were assumed to act along straight lines. For validation purposes the model was scaled to one patient with an instrumented THR [1]. Averaged kinematic measurements were used to obtain joint angles for a knee-bending motion. Then, the model was exported into real-time capable machine code and embedded into the HiL environment. Real implant components of a standard THR were attached to the endeffector of the robot and the compliant support. Finally, the HiL simulation was carried out simulating knee-bending. Experimentally measured hip joint forces from the patient [1] were used to validate the HiL simulation.Introduction
Methods
At present, wear investigations of total hip replacement (THR) are performed in accordance with the ISO standard 14242, which is based on empirically determined relative motion data and exclusively describes the gait cycle. However, besides continuous walking, a number of additional activities characterize the movement sequences in everyday life and influence the wear rates as well as the size and shape of wear debris. Disagreements of in vitro and in vivo wear mechanisms seemed to be a result of differences between in vitro and in vivo kinematics and dynamics. This requires an optimization of the current test procedures and parameters. Hence, the aim of the present study was to evaluate most frequent activities of daily living, based on available in vivo data, in order to generate parameter sets according to loading and rotational movements close to the physiological situation. For the generation of angular patterns, time-dependent three-dimensional trajectories of reference points were used from the HIP98 database of Bergmann. The data set was evaluated and interpolated using analytical techniques to simulate consecutive smooth motion cycles in hip wear simulators or further test devices. The calculated relative joint movement was expressed by an ordered set of three elementary rotations and was complemented with three force components of the joint contact force to generate kinematically and dynamically consistent parameter sets. The obtained sets included the activities walking, knee bending, stair climbing and a combined load case of sitting down and standing up for an averaged patient. Generated slide tracks, created by the use of the angular patterns, demonstrated differences according to the kinematics between selected daily life activities and those established for the ISO standard 14242. In particular, for the relative flexion-extension rotational movement, routine activities showed significant higher ranges of motion. Additionally, the depicted force pattern underlined that the prevailing force component varied considerably between different activities. These deviations in range of motion and joint forces could be attributed to disagreements between in vitro and in vivo results of THR wear testing. The Integration of frequent activities of daily living in the in vivo test protocol could be realized by means of the sequential arrangement of the four investigated activities.
