Validation of a multi-body optimization with knee kinematic models including ligament constraints.

Motion analysis aims at evaluating the joint kinematics but the relative movement between the bones and the skin markers, known as soft tissue artifact (STA), introduces large errors. Multi-body optimization (MBO) methods were proposed to compensate for the STA. However, the validation of the MBO methods using no or simple kinematic constraints (e.g., spherical joint) demonstrated inaccurate in vivo kinematics. Anatomical constraints were introduced in MBO methods and various ligament constraints were proposed in the literature. The validation of these methods has not been performed yet. The objective of this study was to validate, against in vivo knee joint kinematics measured by intra-cortical pins on three subjects, the model-based kinematics obtained by MBO methods using three different types of ligament constraints. The MBO method introducing minimized or prescribed ligament length variations showed some improvements in the estimation of knee kinematics when compared to no kinematic constraints, to degree-of-freedom (DoF) coupling curves, and to null ligament length variations. However, the improvements were marginal when compared to spherical constraints. The errors obtained by minimized and prescribed ligament length variations were below 2.5° and 4.1mm for the joint angles and displacements while the errors obtained with spherical joint constraints were below 2.2° and 3.1mm. These errors are generally lower than the errors previously reported in the literature. As a conclusion, this study presented encouraging results for the compensation of the STA by MBO and for the introduction of anatomical constraints in MBO. Personalization of the geometry should be considered for further improvements.