Tibial Origin Orientation

The orientation of the node that is coincident with the fixed tibial coordinate system is defined. This facilitates load application with respect to the tibia’s fixed coordinate system.

Transform

Three points are used to define the transform that is used to orient the origin of the fixed tibial coordinate system. One point is the origin of the fixed tibial coordinate system, and the other two are coincident with the \(x\) and \(y\) axis of the tibia’s fixed coordinate system. The point at the origin has already been defined when generating the joint coordinate system (see Model Development documentation for more information on the joint coordinate system’s connector elements). The other two points are defined with the equation below.

\[p_i = p_{origin} + n_i\]

Where \(p_i\) is the node along the \(i^{th}\) axis of the fixed tibial coordinate system, \(n_i\) is the corresponding unit vector that is parallel to the \(i^{th}\) axis of the fixed tibial coordinate system, and \(p_{origin}\) is the origin of the fixed tibial coordinate system.

In Abaqus, the *Transform option is used to define the transform, and this takes six values. The first three values are the difference between the coordinates of the origin and the point along the tibia’s x-axis (\(p_x - p_{origin}\)). Similarly, the second set of three values is the difference between the coordinates of the origin and the point along the tibia’s y-axis (\(p_y - p_{origin}\)).