Biomechanics Data Processing.

Force and translation data were plotted against one another to create load-translation plots for the joint and ACL represented schematically in Fig. ​Fig.1.1. Anterior–posterior tibial translation, in situ forces in the joint, and in situ forces in the ACL were measured. APTT was calculated as the distance in the anterior–posterior plane between the point of maximum translation under applied anterior and posterior tibial loads (Fig. ​(Fig.1).1). In situ forces in the ACL and the joint were calculated using the principle of superposition to calculate the force components in three degrees-of-freedom under applied anterior tibial loads [10,19].

Schematic depicting the load-translation curve of the joint under anterior (positive) and posterior (negative) tibial translation and parameters determined from the curve

Biomechanical data at submaximum loads were assessed under applied anterior–posterior loads in a manner similar to that published by Imhauser et al. [16]. Force (either specific to individual tissues or the total joint) and displacement (measured at the joint level) data were collected at intermediate points defined at increments of 20% of the peak applied load for each specimen (Fig. ​(Fig.1).1). A custom matlab code was developed to fit data to biphasic curves in the anterior and posterior regions with an exponential fit (Eq. (1)) ranging from the passive path position to a transition point and a linear fit from that transition point to the maximum load position. The form of the exponential function is

where a and b are parameters to be fit. This process was iterated with each of the intermediate points defined as the transition point. The transition point resulting in the greatest combined r² value from the exponential and linear regions was selected, and a single curve combining the curve fits of the exponential and linear regions was generated. The process of selecting and generating a biphasic curve was repeated between the passive path position and the maximum applied posterior drawer. Parameters shown in Fig. ​Fig.11 were defined as follows based on previous literature [16,20]. A point of maximum curvature was determined for the new plot, and this was defined as the engagement point of the tissue. In situ slack was defined as the distance between the engagement point in the anterior and posterior directions. The stiffness of the tissue of interest, e.g., joint or ACL, was defined as the slope of the linear region of the plot under anterior load [16,20].