Lengthening of double-looped tendon graft constructs in three regions after cyclic loading: a study using Roentgen stereophotogrammetric analysis.

Lengthening of a double-looped tendon graft construct used to reconstruct the anterior cruciate ligament (ACL) can result in an increase in anterior knee laxity and affect the stability of the reconstructed knee. Three possible regions where lengthening of the construct can occur are (1) the region of the tibial fixation, (2) the region of the femoral fixation, and (3) the region of the graft between the fixations. One objective of this study was to demonstrate the feasibility of using Roentgen stereophotogrammetric analysis (RSA) to determine the lengthening in each region of a double-looped graft construct subjected to cyclic loading. A second objective was to determine which region(s) contributes most to an increase in length of this graft construct. Radio-opaque markers were attached to ten grafts to measure the lengthening in each of the three regions. Each graft was passed through a tibial tunnel in a bovine tibia, looped around a rigid cross-pin, and fixed to the tibia with a Washerloc fixation device. The grafts were cyclically loaded for 225,000 cycles from 20 to 170 N. Prior to and at intervals during the cyclic loading, simultaneous radiographs were taken of the tibia and graft. RSA was used to determine the 3-dimensional coordinates of the markers from which the lengthening in each region was computed at each interval. The regions of the tibial and femoral fixations were the largest contributors to the increase in length of the graft, with maximum average values of 0.91 and 0.76 mm respectively after 225,000 cycles. The region between the fixations contributed least to lengthening of the graft, with a maximum average value of 0.23 mm. More than 90% of the lengthening in each region occurred before 100,000 cycles of loading. RSA proved to be a useful method for measuring lengthening in all three regions of the graft construct. Lengthening of the graft construct in both regions of fixation is sufficiently large that the combined contributions may cause a recurrence of instability in some knees.