Biomechanical Comparison of Hamstring Tendon Fixation Devices for Anterior Cruciate Ligament Reconstruction: Part 1. Five Femoral Devices
We conducted a study to biomechanically compare 5 femoral hamstring tendon fixation devices commonly used in anterior cruciate ligament reconstruction.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into porcine femurs using 5 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 (LTF) test. Failure mode, stiffness, ultimate load, and rigidity were recorded.
Two of 10 Delta screw (Arthrex), 10 of 10 Bio-TransFix (Arthrex), 10 of 10 Bone Mulch screw (Arthrotek), 10 of 10 EZLoc (Arthrotek), and 10 of 10 Zip Loop (Arthrotek) devices completed the 1500-cycle loading test. Residual displacement was lowest for Bio-TransFix (4.1 mm) followed by Bone Mulch (5.2 mm), EZLoc (6.4 mm), Zip Loop (6.8 mm), and Delta (8.2 mm). Mean stiffness was significantly (P < .001) higher for Bone Mulch (218 N/mm) than for Bio-TransFix (171 N/mm), EZLoc (122 N/mm), Zip Loop (105 N/mm), or Delta (84 N/mm). Mean LTF was significantly ( P < .001) higher for Bone Mulch (867 N) than for Zip Loop (615 N), Bio-TransFix (552 N), EZLoc (476 N), or Delta (410 N).
The Bone Mulch screw demonstrated superior strength in the fixation of hamstring grafts in the femur. Bio-TransFix was close behind. The Delta screw demonstrated poor displacement, stiffness, and LTF.
When used as the sole femoral fixation device, a device with low LTF, decreased stiffness, and high residual displacement should be used cautiously in patients undergoing aggressive rehabilitation.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices differed slightly (Table). Bone Mulch screw failed with a fracture through the femoral condyle extending to the bone tunnel. Zip Loop and EZLoc failed by pulling through their cortical attachment on the lateral femoral condyle. Bio-TransFix broke in the tunnel during LTF. Delta screw failed with slippage of the fixation device, and the tendons pulled out through the tunnel.
For the cyclic loading tests, only 2 of the 10 Delta screws completed the 1500-cycle loading test before failure. Of the 8 Delta screws that did not complete this testing, the majority failed after about 100 cycles. All 10 tests of Bone Mulch, Zip Loop, EZLoc, and Bio-TransFix completed the 1500-cycle loading test.
Residual displacement data were calculated from cyclic loading tests (Table). Mean (SD) residual displacement was lowest for Bio-TransFix at 4.1 (0.4) mm, followed by Bone Mulch at 5.2 (1.0) mm, EZLoc at 6.4 (1.1) mm, and Zip Loop at 6.8 (1.3) mm. Delta screws at 8.2 (1.4) mm had the highest residual displacement, though only 2 completed the cyclic tests. Bio-TransFix had significantly (P < .001) less residual displacement compared with EZLoc, Zip Loop, and Delta. Bone Mulch had significantly less residual displacement compared with Zip Loop (P < .05) and Delta (P < .01).
Stiffness data were calculated from LTF tests (Table). Mean (SD) stiffness was highest for Bone Mulch at 218 (25.9) N/mm, followed by Bio-TransFix at 171 (24.2) N/mm, EZLoc at 122 (24.1) N/mm, Zip Loop at 105 (18.9) N/mm, and Delta at 84 (16.4) N/mm. Bone Mulch had significantly (P < .001) higher stiffness compared with Bio-TransFix, EZLoc, Zip Loop, and Delta. Bio-TransFix had significantly (P < .001) higher stiffness compared with EZLoc, Zip Loop, and Delta.
Mean (SD) ultimate LTF was highest for Bone Mulch at 867 (164) N, followed by Zip Loop at 615 (72.3) N, Bio-TransFix at 552 (141) N, EZLoc at 476 (89.7) N, and Delta at 410 (65.3) N (Table). Bone Mulch failed at a statistically significantly (P < .001) higher load compared with Zip Loop, Bio-TransFix, EZLoc, and Delta. There were no significant differences in mean LTF among Zip Loop, Bio-TransFix, EZLoc, and Delta.
Discussion
In this biomechanical comparison of 5 different femoral fixation devices, the Bone Mulch screw had results superior to those of the other implants. Bone Mulch failed at higher LTF and higher stiffness. Bio-TransFix performed well and had residual displacement similar to that of Bone Mulch, but significantly lower LTF. Overall, EZLoc and Zip Loop were similar to each other in performance. The Delta (interference) screw performed poorly with respect to LTF, residual displacement, and stiffness; a large proportion of these screws failed early into cyclic loading.
Bone Mulch and Bio-TransFix overall outperformed the other fixation devices. These 2 devices are cortical-cancellous suspension devices, which provide transcondylar fixation and resist tensile forces perpendicular to the pullout force. Multiple biomechanical studies have found superior performance for these types of devices compared with various implants.10,13,15,16
Our results were similar to those of Kousa and colleagues,13 who found the Bone Mulch screw to provide highest LTF, highest stiffness, and lowest residual displacement. Another study found significantly higher stiffness for the Bone Mulch screw than for the Endobutton, a cortical suspensory fixation device.14 Bone Mulch failure modes differed, however. In the study by Kousa and colleagues,13 3 specimens failed with bending of the screw tip, and 7 failed with rupture of the tendon loop. All specimens in our study failed with fractures through the condyle. It is unclear why the failure modes differed, as we followed similar manufacturer protocols for inserting the device. It is possible the bone mass density of the porcine femurs differed between studies. This was not reported by Kousa and colleagues,13 and we did not perform testing either. However, all the porcine femurs were about the same age for testing of each device in this study.
Bio-TransFix has also been compared with various implants, but not in the same study. Brown and colleagues8 found the TransFix device significantly stiffer than the Endobutton CL. Shen and colleagues16 determined that TransFix had significantly lower residual displacement compared with Endobutton CL. Milano and colleagues15 compared multiple cortical suspensory fixation devices, including Endobutton CL, with TransFix and Bio-TransFix, and concluded the cortical-cancellous devices (TransFix, Bio-TransFix) offered the best and most predictable results in terms of elongation, fixation strength, and stiffness. TransFix has also been shown to be superior to interference screw fixation in biomechanical studies.10,15
