Biomechanics
Laboratory
Simulation Development
Introduction
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.
Methods
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. nd model. 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.
Recent progress
Here is a
summary
of progress over the past year in MS Word format.
Acknowledgments
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.
