Abstract
Summary Statement
We evaluated the mechanical strength of two cortical suspension devices by reproducing clinical situation for ACL reconstruction. A most important factor affecting the displacement during cyclic load was the length of the tendon rather than the length of the device.
Introduction
A definite consensus for the optimal graft fixation technique to the femur in an anterior cruciate ligament (ACL) reconstruction has not been reached, although there have been several fixation techniques such as cortical suspension devices, transfixation devices, and interference screws. The purpose of this study was to evaluate the mechanical strength of two cortical suspension devices by reproducing actual clinical situation for ACL reconstruction in order to compare the TightRopeTM as a new adjustable-length loop device and the EndoButtonTM as a well-known fixed-length loop device under the consistent conditions.
Methods
Two cortical suspension devices were tested under cyclic and pull-to-failure loading conditions in both an isolated device setup and a specimen setup to make a complete bone-device-tendon construct in porcine femurs and bovine flexor tendons using a tensile testing machine. Especially to examine the influence of the length of the tendon and the device, a total length of the bone tunnel was fixed to 35 mm, and an effective length of tendon in the bone tunnel was adjusted to 15 mm for the EndoButton group (EB), the TightRope 15 group (TR15) and 21 mm for the TightRope 21 group (TR21).
Results
In the isolated device testing, the ultimate tensile strength of the EB (1430 ± 148 N) had significantly higher than that of the TR (866 ± 53 N), and also had significant difference in the specimen testing. The displacement in the isolated device testing after preloading for the EB (1.09 ± 0.29 mm) showed statistically lower than that for the TR (2.57 ± 1.19 mm), and had a significant difference after the cyclic load. In the specimen testing, on the other hand, the displacement after preloading showed no statistical difference between the EB (1.06 ± 0.30 mm), the TR21 (1.76 ± 2.28 mm) and the TR15 (1.51 ± 1.78 mm). But limiting only to the displacement from 1000 to 2000 cycles, the TR21 (0.92 ± 0.44 mm) showed statistically higher than the TR15 (0.49 ± 0.22 mm).
Discussion
Although current results indicated that the EB had greater mechanical strength than the TR, both devices are presumed to provide sufficient fixation strength under clinical conditions. An important new finding from the current study was the measurement of initial displacement from the state of initial fixation until loading began and 50 N of tension was applied. In isolated device testing, the TR had significantly more displacement than the EB during pre-loading. This may reflect the TR's loops stretch until a certain amount of tension is applied. In the comparison of the TR21 and the TR15, the TR21 had a significantly larger displacement with a cyclic load from 1000 to 2000 cycles. This result indicated that a most important factor affecting the displacement during cyclic load was the length of the tendon rather than the length of the device. Thus, we should decide the length of the tendon in the bone tunnel to avoid the displacement of the graft.
