Abstract
Introduction
The anterior cruciate ligament (ACL) is one of the most common ligament injuries. Several ACL reconstructions exist and are consequently performed. An accurate and comprehensive description of knee motion is essential for an adequate assessment of these surgeries, in terms of restoring knee motion.
Methods
We propose to compare these reconstructions thanks to an index of articular coherence. This index measures the instantaneous state surface configurations during a motion. More specifically, this refers to the position between two articular surfaces facing each other. First of all, the index has to refer to a position known to be physiological. This initial position of the bones, named reference, directly results from the segmentation of CT scans. First we compute all distances between the two surfaces and then we compute the Cumulative Distribution Function (CDF). We process this way for each iteration of the motion. Then we obtain a batch of CDF curves which provide us qualitative information relative to the motion such as potential collisions or dislocations. The graph of all CDF curves is called Figure of Articular Coherence (FoAC). A good articular coherence is characterised by CDF which are close to the reference. This qualitative method is coupled to a quantitative one named Index of Articular Coherence (IoAC) which computes the Haussdorff distance between the temporal distributions and the reference. This distance has to be as low as possible. The tools were tested on cadaveric experiments of ACL reconstruction provided by Hagemeister et al, (1999). They recorded the knee flexion/extension motion in following situations: the intact knee, after ACL resection, after three methods of ACL reconstruction on the same knee (‘over-the-top’ method (OTT), two different two tunnel reconstructions (2 tunnel). Our method was used, for the time being, for one specimen. We compare different post-surgery kinematics thanks to the FoAC and IoAC.
Results and discussion
Functions were well correlated to the reference, when considering flexion motion of the intact knee. Regarding the results of the FoAC after ACL resection, we observed that the functions evolve in the direction of decreasing distances more rapidly, compared to the intact knee. This means that both articular surfaces are close to collision when ACL is resected. Regarding the results of the IoAC for the different experimental situations, we observe that the highest index is obtained for the ACL resected knee. The OTT method has equivalent results to the intact knee and for both 2 tunnels reconstructions, the index values are higher than those obtained with the intact knee. This higher index can be explained by an over-constraint on the knee movement, the knee appears to be more stable but it can possibly rigidify the joint as well. Through the use of the FoAC and IoAC, we reached the same conclusions of Hagemeister's work in a way that is more intuitive than by using kinematic curves and angle values. Our tools can describe the instantaneous state of the joint and are able to compare kinematics corresponding to different types ACL reconstructions.
