VivoSim can operate as a stand-alone product for biological and implant knee joints. It provides visualization and a quasi-static force balance solver, combined with a near-real-time solver for rigid-body surface contact kinematics and a multi-fiber ligament model. Features available to visualize and understand the ligament model include selectable visualization and enable/disable of individual fibers. Using input kinematics based on a virtual model of the VIVO™ machine actuators, the user can specify the joint flexion angle. The other five axes may be switched between user-specified position control or position driven by the solver and contact model to satisfy the force balance equation and contact constraint kinematics. External forces may be applied to the joint and will participate in the solver force balance. This capability can be used to visualize and understand how the multi-fiber ligament model interacts with kinematic constraints caused by the joint’s hard surfaces in response to external loads. By switching selected axes between solver-driven and user-controlled position, parametric behavior of the joint may be explored.

The rigid-body surface contact model is based on imported 3-D models provided by the user. External software is typically required in order to create a triangular-element surface mesh 3-D model of the joint. Submeshes are defined by the user to indicate regions where the contact search will operate. Unlike finite-element based approaches, the rigid-body contact model and quasi-static solver return results quickly, usually in a fraction of a second to a few seconds. Thus, interactive experimentation becomes possible, greatly enhancing the user’s ability to understand the behavior of the joint in normal and abnormal loading conditions. Ligament models can be evaluated based on how they load the contact surfaces, both under passive motion and in response to external loads.

The multi-fiber ligament model in VivoSim is upward-compatible with the multi-fiber ligament model in the VIVO™ joint simulator. In addition to a complete and exact replication of the straight-line fiber model used in VIVO™, VivoSim also supports one-point contact detection and wrapping of fibers when the straight line between the fiber endpoints (ligament insertion sites) intersects the surface mesh model of the joint. The contact detection algorithm in the initial release of VivoSim is designed to work with knee surfaces. When applied to other joints, it might not perform satisfactorily. This restriction is expected to be lifted in a future release. Note that since contact detection is not used when VivoSim operates in VIVO™-linked mode, this restriction does not apply in that mode.

VivoSim opens up several use cases for academic or commercial customers involved in biomechanics research and development. VivoSim is a powerful tool to explore quickly the joint poses and surface locations where the highest contact loads occur. This understanding can then be used to guide more intensive finite-element based analyses.

VivoSim can be used to develop and tune a multi-fiber ligament model. The built-in ligament verification function automatically produces plots of force (or moment) vs. displacement (or rotation) for AP (anterior-posterior), IE (internal-external), and VV (varus-valgus) motions, in addition to a passive motion FE (flexion-extension) plot. In each case, a family of curves is produced, where the different curves span a range of flexion angles.

Variations in implant design and possible surgical planning recommendations can be explored. Once a ligament model has been tuned to produce the desired passive-motion results with a model of a biological knee, the implant geometry can be introduced while holding the ligament model parameters constant. Also, the implant components can be moved relative to the bony geometry or the ligament insertion sites. The surfaces designated as targets for contact search can be altered as desired. Thus the contact kinematics can be switched to the implant surfaces while the original bones remain in the visible model for reference.

VivoSim can function as an adjunct to AMTI’s VIVO™ joint simulator. In this mode, the VIVO™ joint simulator provides a real-time physical solution of the physical joint contact mechanics and kinematics. Kinematic data from VIVO™ is passed over a LAN connection to VivoSim, where a virtual VIVO™ is re-created. This information drives an exact virtual replica of the actuators and mounting stages of the VIVO™. When a 3-D model of the joint is added and registered to the virtual actuators, VivoSim shows a visualization of the physical motion of the machine. VivoSim can also read the same .lig file used to implement a multi-fiber ligament model on the VIVO™ machine. The ligament model is then displayed in exact geometric relationship to the joint model, so that the interactions between joint, ligaments and machine kinematics may be visualized. Since VivoSim has a built-in duplicate of the ligament fiber calculations used in VIVO™’s real-time controller, the properties of individual ligament fibers – length, strain, tension, force and moment components – can be displayed and analyzed. Resultant force and moment on the joint due to the ligament model are also displayed in exact agreement with the computation on the VIVO™ machine.

Multi-fiber ligament models can be edited and viewed in VivoSim, using its facilities for visualizing and evaluating ligament models. Any multi-fiber ligament model in VivoSim can be downloaded to VivoControl and the VIVO™ machine when the LAN connection between VivoSim and VivoControl is online. Multi-fiber ligament models saved in VivoSim can also be loaded into VivoControl from its own user interface because both programs share a common format for ligament files ( .lig file type).

VivoSim can record motion for playback later and for archiving. The archived motion results are stored in an ASCII-readable format and can readily be imported to the customer’s other analysis software. Also, the files may be played back in VivoSim. VivoSim’s playback mode supports variable-speed playback, frame-by-frame advance, pause, rewind and fast-forward.

In VIVO™-linked mode, VivoSim may be used with any joint that can be mounted on the VIVO™ machine. As noted above, the contact detection algorithm in the initial release of VivoSim is designed and tested for knee joints and might not be satisfactory for other joints. In VIVO™-linked mode, the contact detection algorithm is not used, because all kinematics – including the contact constraint – are determined by data from the machine. The multi-fiber ligament code in VivoSim duplicates that in the VIVO™ and therefore is not affected by the type of joint being tested.

To operate in real-time display mode for a VIVO™ system, both the VIVO™ host computer (running the VivoControl user interface) and the VivoSim computer must be connected on the same LAN subnet. The VIVO™ host computer is equipped with a second LAN port, which may be used for this purpose. The LAN connection between the computers is managed in two phases. First, the connection is established. This needs to be done only once per session. Once the LAN connection established, VivoSim may be switched between “local” and “tracking” mode whenever desired. In “local” mode, VivoSim operates exactly as in standalone mode, with the added capability to download the multi-fiber ligament model to VivoControl at any time. “Tracking” mode changes the source for kinematics to the data coming from the VIVO™ machine.

In VIVO™-linked mode, the primary use for VivoSim is to visualize and explore the multi-fiber ligament model. Ligaments are drawn as colored chains. The color may be changed. Ligaments can be selected individually or multiply. Selected ligaments are simultaneously highlighted in the graphics screen and in the ligament editor, if it is active. Selected ligaments may be disabled and enabled.

VivoSim’s ligament wrap mode can be enabled when operating in linked mode. Ligament wrapping is not supported on the VIVO™ simulator. By comparing the difference between ligament forces and moments with wrapping enabled and disabled, the effect of wrapping may be quantified.