Construction of a Realistic, Whole-Body, Three-Dimensional Equine Skeletal Model using Computed Tomography Data

A realistic skeletal model of a horse is created from equine computed tomography data that can be manipulated to understand the functional, anatomical, and biomechanical modeling of the whole equine body. The technique allows for rapid and easy positioning of the model to help characterize varying equine postures. The model can help characterize equine postures associated with degenerative tissue diseases and identify postures that reduce mechanical stresses.

The method can be adapted and modified to assist in modeling of other species for clinical purposes, such as dogs or other academic pursuits like modeling evolutionary transitions. Demonstrating the procedure will be Alex Lee, a graduate student from the Osborn laboratory, shown here with Dr.Osborn. Begin by placing graphic joints inside the forelimb in all areas of movement.

Press the F3 key to enable the rigging menu set. In the menus, click on Skeleton and Create Joints, to select the create joints tool. In the View panel of the software, click in the approximate areas of the joints in the order of one to 10 and press the enter key.

Adjust the position of the joints by clicking on the desired joint and using the move tool, performed by pressing the W key to translate the joint into the desired position. Alternatively, adjust a joint by clicking on the desired joint and altering the Translate X, Translate Y, and Translate Z values found in the Channel box or Layer editor panel. Next, create five separate inverse kinematic handles.

In the menus, click on Skeleton and Create IK Handle to select the Create IK Handle tool. Using the Create IK Handle tool, select joint one, then joint three, and name this IK handle, Front Leg IK"in the Outliner panel. Then use the tool and select joint three and joint seven.

Name this IK handle, Front Lower IK.Use the tool and select joints seven and eight and name this IK handle, Front Toe 1 IK.Repeat the step with joints eight and nine to create the Front Toe 2 IK.Then with joints nine and 10 to create the Front Toe 3 IK.To create forelimb controls, create a non-uniform rational B-splines, or NURBS, circle by selecting Create NURBS Primitives, and Circle. Create two NURBS circles encircling joint 3 and joint 10, and name them Front Control"and Front Lower Control, respectively in the Outliner panel. Create another NURBS circle and select it.

In the Channel box or Layer editor panel, change the Rotate Z value to 90. Using the move tool, place it at the tip of joint 10 and name it Front Flick Control"in the Outliner panel. Group Front Toe 1 IK, Front Toe 2 IK, and Front Toe 3 IK by Control selecting all three and pressing the G key.

Name this group, Front Toe Group"in the Outliner panel. Parent the IK Handles and Front Toe Group to the controls, by first Control selecting the Front Leg IK then the Front Control and press the P key. Control select the Front Lower Control, then the Front Control, and press the P key.

Control select the Front Lower IK, then the Front Lower Control, and press the P key. Control select the Front Flick Control, then the Front Lower Control, and press the P key. Control select the Front Toe Group, then the Front Flick Control, and press the P key.

Next use the Bind Skin tool to bind the bone meshes to the joint rig. Select the bone mesh, and shift select the most proximal joint, and use the Bind Skin tool found under Skin and Bind Skin. Using the Create Joints tool, create a joint inside the sesamoid bone mesh.

Using the Bind Skin tool, bind the sesamoid bone mesh to the joint. Select the joint inside the sesamoid bone and shift select the nearest forelimb joint and press the P key to parent it. Create a NURBS plane with the length roughly equal to the length of the spine.

Select 1 for U patch and the number of thoracic and lumbar vertebrae for V patch. Select the square found next to the Create Plane tool under Create NURBS Primitives, and Plane. To rebuild the plane with altered options, press the F2 key to enter the Modeling menu set.

Select the Plane in the View panel and select the Rebuild Tool settings by selecting the square next to the Rebuild tool under Surfaces and Rebuild. Select the Number of Spans U as 1, the Number of Spans V as the number for V patch, and 1 Linear for both the Degree U and Degree V options. Keep the other settings at default and press the Rebuild button.

To create nHairs, press the F5 key to enter the FX menu set. Use the Create Hair tool with altered options by selecting the square next to nHair and Create Hairs. Set the output to NURBS curves, U count to 1, V count to the number for V patch, then press the Create Hairs button.

Delete nucleus1, hairSystem1OutputCurves group, and hairSystem1 in the Outliner panel. Fully expand the group labeled hairSystem1Follicles and delete all items labeled with curve. Select the plane then move and orient it so that it is roughly overlapping with the spine by using the Move and Rotate tools.

Select the plane, hold the right mouse button, and select Control Vertex to make all the vertices of the plane visible. Move the vertices to orient the follicles between the vertebrae at the height where the spinal cord would be. Create a number of separate joints at any place in the View panel.

The position of these joints will be corrected in later steps. In the Outliner panel, select a created joint, then Control select a nurbsPlaneFollicle and press the P key. Repeat with the other joints created in the previous step and the other nurbsPlaneFollicle objects.

In the Outliner panel, Control select all the joints. In the Channel box or Layer box panel, set the Translate X, Y, and Z to zero. Control select all joints in the Outliner panel, and press the Control and D keys to duplicate them.

Control select all the duplicate joints in the Outliner panel and press the Shift and P keys to unparent them. Bind the joints under nurbsPlaneFollicle with their respective vertebra mesh by pressing the F3 key to enter the Rigging menu set. Click on the original joint under nurbsPlaneFollicle, Shift select the respective vertebra mesh.

Then use the Bind Skin tool under Skin and Bind Skin. Repeat these actions for each joint and vertebrae mesh. Control select all duplicate joints and the plane and use the Bind Skin tool to bind all duplicate joints to the plane.

In the 3D animation and modeling software, the graphic ribbon spine enables the natural movement of the bony spine. The software displays the model with the bone meshes attached to the graphic rigging system making it possible to control the skeletons position. The rigging of each limb with joints allows for positioning and the creation of movement using graphic joints with numbers 1 to 10 for forelimbs and graphic joints with numbers 11 to 17 for hindlimbs.

The 3D equine model was matched to classic Muybridge photos as proof of concept, and to create the first animations. The model can be moved from a normal posture into various postures like a transversal rotation of the spine, making it possible to understand the relationship of postures to pathomechanical force regimes and the resulting degeneration of the affected skeletal elements, joints, and soft tissues. The movements of the 4D model have been compared to videos from the side, back, and front to more accurately depict the motion of the spine and to videos of horses at the walk, canter, and trot to create animations of those gaits.

Studying whole-body mechanics is necessary for understanding the effects of degenerative joint diseases. This model provides a way to study the relationship between aberrant movement and postures and resulting tissue degeneration. This unique whole-body equine model based on computed topography data enables the study of this research area and others because of its flexibility and ability to be modified by users to answer their specific questions.