Biomechanical Comparison of Hamstring Tendon Fixation Devices for Anterior Cruciate Ligament Reconstruction: Part 2. Four Tibial Devices
We conducted a study to biomechanically compare 4 tibial hamstring tendon fixation devices commonly used in anterior cruciate ligament reconstruction.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into porcine tibias using 4 separate fixation devices. For each device, 10 specimens were tested (1500-cycle loading test at 50-200 N). Specimens surviving the cyclic loading then underwent a single load-to-failure test. Failure mode, stiffness, ultimate load, and residual displacement were recorded.
Eight of 10 Delta screw (Arthrex), 2 of 10 Retroscrew (Arthrex), 10 of 10 WasherLoc (Arthrotek), and 10 of 10 Intrafix (Depuy Mitek) devices completed the 1500-cycle loading test. Residual displacement was significantly (P < .001) lower for Intrafix (2.9 mm), WasherLoc (5.6 mm), and Delta (6.4 mm) than for Retroscrew (25.5 mm). Mean stiffness was significantly (P < .05) higher for Intrafix (129 N/mm) than for the other devices. Mean load to failure was highest for Intrafix (656 N), then WasherLoc (630 N), Delta (430 N), and Retroscrew (285 N).
The Intrafix device demonstrated superior strength in the fixation of hamstring grafts in the tibia. WasherLoc was close behind.
Another important variable that could have affected the performance of the interference screws is screw length. One study found no significant difference in screw strength between various lengths, and longer screws failed to protect against graft slippage.18 However, Selby and colleagues19 found that, compared with 28-mm screws, 35-mm bioabsorbable interference screws failed at higher LTF. This is in part why we selected 35-mm Delta screws for our study. Both 35-mm Delta screws and 20-mm Retroscrews performed poorly. However, we could not determine if the poorer performance of Retroscrews was related to their length.
We used an eccentric placement for our interference screws. Although some studies have suggested concentric placement might improve fixation strength by increasing bone–tendon contact,20 Simonian and colleagues21 found no difference in graft slippage or ultimate LTF between eccentrically and concentrically placed screws. Although they were not biomechanically tested in our study, a few grafts were fixed with concentrically placed screws, and these tendons appeared to be more clinically damaged than the eccentrically placed screws.
Combined tibial fixation techniques may be used in clinical practice, but we did not evaluate them in our study. Yoo and colleagues9 compared interference screw, interference screw plus cortical screw and spiked washer, and cortical screw and spiked washer alone. They found that stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with interference screw plus cortical screw and spiked washer. In a similar study, Walsh and colleagues13 demonstrated improved stiffness and LTF in cyclic testing with the combination of retrograde interference screw and suture button over interference screw alone. Further study may include direct comparisons of additional tibial fixation techniques using more than one device. Cost analysis of use of additional fixation devices would be beneficial as well.
Study results have clearly demonstrated that tibial fixation is the weak point in ACL reconstruction3,17 and that early aggressive rehabilitation can help restore range of motion, strength, and function.22,23 Implants that can withstand early loads during rehabilitation periods are therefore of utmost importance.
Conclusion
Intrafix demonstrated superior strength in the fixation of hamstring grafts in the tibia, followed closely by WasherLoc. When used as the sole tibial fixation device, interference screws had low LTF, decreased stiffness, and high residual displacement, which may have clinical implications for early rehabilitation after ACL reconstruction.
