In order to obtain a visual and quantitative verification of the appropriateness of one, two, and three degree of freedom
models for motion, we have developed an interactive system for the independent adjustment and definition of multiple degree-of-freedom
linkage systems representative of human leg and arm motion. The system is built so that, once the kinematic structure is defined, control
points for interactive definition of muscle-tendon and ligament paths may be manually adjusted and refined, providing a tool for interactive
musculoskeletal modeling and simulation.
All kinematic transformation nodes are built as linkages within an openGL hierarchical structure. The structure for independent adjustment
of each axis of motion required tracking the inverse of all transformations applied to the axis during visualization and adjustment.
The inverse is applied to all structures below the axis of interest so that only the axis is effected during 3D adjustment.
The system allows for the visual adjustment and verification of the placement of an axis or axes. This interactive task is carried out
through control of point of view of the observer, the position and orientation of the view, and the position and orientation of each axis.
These dynamic view and control commands are carried out simultaneously with rotational control of distal joint segments about their
defined axis or axes. With such immediate and interactive flexibility, the user is able to rapidly iterate upon appropriate axis placement
based upon a 3D visual verification of joint congruence throughout joint range-of-motion.
The current development environment is a dual processor 700 Mhz Pentium III Windows 2000 system using Visual C++ v6.0, and OpenGL with the
GLUT Library. The graphics driver is the Evans & Sutherland Tornado using Realimage technology. In addition to mouse and keyboard interactive
methods, this system utilizes pop-up menus with control widgets and 6 DOF control using a Spaceball (Spaceball model 3003, Spacetec IMC Corp., Lowell, MA).
Structures for this kinematic model are derived from axial computerized tomography (CT) slices of fresh-frozen cadaver specimens. One mm thick
slices spaced at 1 mm are used for the joint areas which require the greatest resolution. One mm thick slices spaced 5 mm apart are used for
the mid shaft areas of bones. This approach helps to maintain highest detail in critical areas and save on structure size where such detail is not needed.
The limbs used in the model were scanned on a General Electric Computerized Tomography scanner (GE Model 9800). The images are processed with
Mimics software to yield standard stereolithography files describing each individual bone as a triangulated surface.
The simulation software is then used to read in each stereolithography according to a kinematic hierarchy. Axes (up to three per diarthroidal joint)
positions and orientations must initially be manually adjusted per literature references when available, or visual approximation when they are
not. Various display modes (shading, wireframe, 3D stereo) and control methods ranging from keyboard to GUI to spaceball are available.
Muscle-tendon and ligament paths may be described in terms of a sequence of manually positioned control points, some in a fixed position relative
to a particular bone, others designed to slide smoothly over the surface of the bone. Once a path has been defined, it may be modeled as either a
line segment or one of a variety of spline curves, and the moment arm as a function of joint angle may be viewed in real-time or recorded.
This project was supported by a grant provided by the Texas Advanced Technology Program (Project Number: 004952-0011-1999), with additional support from Sulzer Orthopedics, Inc., Austin, TX.