Roentgen Stereophotogrammetric Analysis Methods for Determining Ten Causes of Lengthening of a Soft-Tissue Anterior Cruciate Ligament Graft Construct

There are many causes of lengthening of an anterior cruciate ligament soft-tissue graft construct (i.e., $graft+fixationdevices+bone$⁠), which can lead to an increase in anterior laxity. These causes can be due to plastic deformation and∕or an increase in elastic deformation. The purposes of this in vitro study were (1) to develop the methods to quantify eight causes (four elastic and four plastic) associated with the tibial and femoral fixations using Roentgen stereophotogrammetric analysis (RSA) and to demonstrate the usefulness of these methods, (2) to assess how well an empirical relationship between an increase in length of the graft construct and an increase in anterior laxity predicts two causes (one elastic and one plastic) associated with the graft midsubstance, and (3) to determine the increase in anterior tare laxity (i.e., laxity under the application of a $30N$ anterior tare force) before the graft force reaches zero. Markers were injected into the tibia, femur, and graft in six cadaveric legs whose knees were reconstructed with single-loop tibialis grafts. To satisfy the first objective, legs were subjected to $1500cycles$ at $14Hz$ of $150N$ anterior force transmitted at the knee. Based on marker 3D coordinates, equations were developed for determining eight causes associated with the fixations. After 1500 load cycles, plastic deformation between the graft and WasherLoc tibial fixation was the greatest cause with an average of $0.8±0.5mm$ followed by plastic deformation between the graft and cross-pin-type femoral fixation with an average of $0.5±0.1mm$⁠. The elastic deformations between the graft and tibial fixation and between the graft and femoral fixation decreased averages of $0.3±0.3mm$ and $0.2±0.1mm$⁠, respectively. The remaining four causes associated with the fixations were close to 0. To satisfy the remaining two objectives, after cyclic loading, the graft was lengthened incrementally while the $30N$ anterior tare laxity, $150N$ anterior laxity, and graft tension were measured. The one plastic cause and one elastic cause associated with the graft midsubstance were predicted by the empirical relationships with random errors (i.e., precision) of $0.9mm$ and $0.5mm$⁠, respectively. The minimum increase in $30N$ anterior tare laxity before the graft force reached zero was $5mm$⁠. Hence, each of the eight causes of an increase in the $150N$ anterior laxity associated with the fixations can be determined with RSA as long as the overall increase in the $30N$ anterior tare laxity does not exceed $5mm$⁠. However, predicting the two causes associated with the graft using empirical relationships is prone to large errors.