Calibration¶
Introduction¶
This section explains how ligament properties are estimated.
Initial Guess/Bounds¶
Recruitment Length Approach¶
A rigid body kinematic model is used to define the initial guess and/or bounds for ligament lengths. The length of each ligament fiber is measured throughout the relevant loading cycles. The minimal recruitment length [BHD91] (\(R_j\)) is defined with the maximum measured length from the rigid body model, and the assumed maximum strain (\(e_j\)) eq ().
The minimum recruitment length is used to define the lower bounds of possible lengths for the fiber. Using this method to define the lower bounds would prevent over-recruitment of ligament bundles/fibers.
Passive Flexion - Literature based¶
There are multiple studies that report ligament recruitment throughout passive flexion, and passive flexion is likely a loading state that is available for the project. This approach uses a rigid body kinematic model to measure ligament fiber length throughout passive flexion. Results from the literature can be used to define the flexion angles where specific ligaments are recruited, and the lengths at these flexion angles can be used to define slack length.
The ligaments will likely not be maximally recruited during passive flexion, so the ligaments would have to be assumed to have an arbitrary strain (\(e_j\)) at these flexion angles. The initial guess for slack length (\(L_s\)) can be calculated,
Where \(L_f\) is the length at the desired flexion angle.
[BHD91] | (1, 2) L. Blankevoort, R. Huiskes, and A. Delange. Recruitment of Knee-Joint Ligaments. Journal of Biomechanical Engineering-Transactions of the Asme, 113(1):94–103, February 1991. WOS:A1991HQ77300013. doi:10.1115/1.2894090. |
[KSL+16] | Mohammad Kia, Kevin Schafer, Joseph Lipman, Michael Cross, David Mayman, Andrew Pearle, Thomas Wickiewicz, and Carl Imhauser. A Multibody Knee Model Corroborates Subject-Specific Experimental Measurements of Low Ligament Forces and Kinematic Coupling During Passive Flexion. Journal of Biomechanical Engineering, 138(5):051010–051010, March 2016. URL: http://dx.doi.org/10.1115/1.4032850, doi:10.1115/1.4032850. |