Hybrid femoral fixation of soft-tissue grafts in anterior cruciate ligament reconstruction using the EndoButton CL and bioabsorbable interference screws: a biomechanical study.

PURPOSE: The purpose of this study was to evaluate the effect of hybrid femoral fixation with bioabsorbable interference screws (BioRCI; Smith & Nephew Endoscopy, Andover, MA) and EndoButton CL (Smith & Nephew Endoscopy) fixation. METHODS: Biomechanical testing of 3 different fixation techniques was performed by use of porcine hind-limb distal femurs and mature bovine extremity common extensor tendons. Two independent testing sessions were examined. The first testing session (group A) compared femoral fixation via the EndoButton CL device (n = 6) with femoral fixation via the EndoButton CL device with the addition of a BioRCI screw (n = 6). The second testing session (group B) compared femoral fixation via BioRCI screws alone (n = 6) with femoral fixation via the EndoButton CL device with the addition of a BioRCI screw (n = 6). The femur-graft complex was cyclically loaded between 50 and 250 N at 1 Hz for 1,000 cycles. After cycling, the amount of graft slippage was determined by measuring the change in grip-to-grip distance. The complex was then loaded to failure at 1 mm/s, and the ultimate tensile strength, stiffness, and mode of failure were determined. RESULTS: In group A the addition of an interference screw to the EndoButton CL fixation increased the ultimate tensile strength (1,364.7 +/- 102.4 N for EndoButton CL alone v 1,449.3 +/- 94.4 N for combined technique, P = .035) and stiffness (195.5 +/- 12.1 N/mm for EndoButton CL alone v 307.3 +/- 54.9 N/mm for combined technique, P = .004) and decreased the amount of graft slippage (2.6 +/- 0.5 mm for EndoButton CL alone v 2.0 +/- 0.3 mm for combined technique, P = .017). In group B the addition of the EndoButton CL device to interference screw fixation significantly increased the ultimate tensile strength (643.5 +/- 148.4 N for BioRCI screws alone v 1,290.3 +/- 254.4 N for combined technique, P = .004) but had no effect on stiffness (315.7 +/- 38.9 N/mm for BioRCI screws alone v 341.5 +/- 64.0 N/mm for combined technique, P = .267) or graft slippage (2.7 +/- 1.0 mm for BioRCI screws alone v 2.0 +/- 0.6 mm for combined technique, P = .087). CONCLUSIONS: Our study shows that hybrid femoral fixation of double-looped gracilis-semitendinosus grafts via the EndoButton CL device and a bioabsorbable interference screw is stronger than interference or EndoButton CL fixation alone with respect to ultimate tensile strength, stiffness, and slippage. The addition of an interference screw to suspensory fixation via the EndoButton CL device increased the ultimate tensile strength from 1,360 N to 1,450 N, improved reconstruction stiffness from 200 N/mm to 300 N/mm, and decreased the amount of graft slippage resulting from cyclic loading from 2.6 mm to 2.0 mm. CLINICAL RELEVANCE: The hybrid fixation of the EndoButton CL device and an interference screw is a stronger and stiffer construct than either device alone and allows for aperture fixation, which may translate into better clinical results.

Twisting and braiding reduces the tensile strength and stiffness of human hamstring tendon grafts used for anterior cruciate ligament reconstruction.

BACKGROUND: Twisting and braiding of four-strand hamstring tendon grafts used for anterior cruciate ligament reconstruction has been proposed, but not proven, as a method of improving tensile properties. HYPOTHESIS: Twisting and braiding four-strand human hamstring tendon grafts will have no significant effect on initial graft strength or stiffness. STUDY DESIGN: Paired in vitro biomechanical study. METHODS: In 12 matched cadaveric pairs, a doubled gracilis and semitendinosus tendon graft from one knee was twisted 180 degrees over a 30-mm length, while the doubled tendon graft from the contralateral knee was prepared for biomechanical testing with the graft strands in a parallel orientation. For an additional 12 matched pairs, a doubled graft was braided into a weave while the contralateral graft was prepared for testing in a parallel orientation. All four strands of each doubled tendon graft were equally tensioned with weights before being clamped in a tendon-freezing grip. Tensile testing was then performed. RESULTS: Twisting decreased graft strength by 26% (P < 0.01) and stiffness by 43% (P < 0.01), while braiding reduced strength by 46% (P < 0.01) and stiffness by 54% (P < 0.01), compared with parallel-oriented grafts. CONCLUSIONS: Equally tensioned, parallel four-strand human hamstring tendon grafts were significantly stronger and stiffer than twisted or braided four-strand hamstring tendon grafts. CLINICAL RELEVANCE: We caution against the use of twisted or braided four-strand hamstring tendon grafts for anterior cruciate ligament reconstruction.

Graft-bone motion and tensile properties of hamstring and patellar tendon anterior cruciate ligament femoral graft fixation under cyclic loading.

PURPOSE: To assess longitudinal graft-bone motion and tensile properties of the femur-anterior cruciate ligament (ACL) graft fixation-ACL graft complex based on the hypothesis that there is little difference in graft-bone motion between suspensory and aperture hamstring ACL femoral graft fixation techniques, and between hamstring and patellar tendon ACL femoral graft fixation techniques. TYPE OF STUDY: In vitro biomechanical study using human cadavers. METHODS: The distal femur-ACL graft fixation-ACL graft complex was cyclically loaded between 50 and 250 N at 1 Hz for 1,000 cycles with the direction of the load applied parallel to the axis of the femoral bone tunnel. Graft-bone motion was measured indirectly using retroreflective markers and a video motion-analysis system. Tensile testing to failure was performed at 1 mm/sec for fixation techniques completing 1,000 cycles without fixation failure. RESULTS: Among the hamstring fixation techniques, 4 of 13 Bio-Interference screws (Arthrex, Naples, FL), 2 of 12 LinX HT fasteners (DePuy Mitek, Norwood, MA), and 1 of 11 TransFix cross-pins (Arthrex) failed before completing 1,000 cycles. Five of 13 patellar tendon grafts fixed with metal interference screws, and 2 of 12 patellar tendon grafts fixed with a plastic button and No. 5 sutures failed before completing 1,000 cycles. Suture/button fixation of patellar tendon grafts resulted in significantly more graft-bone motion than hamstring tendon grafts fixed using the Bone Mulch Screw (Arthrotek, Warsaw, IN), or interference screw fixation of patellar tendon and hamstring grafts. Otherwise, there was no significant difference in graft-bone motion among the various hamstring fixation techniques or the various hamstring fixation techniques and interference screw fixation of patellar tendon grafts. Maximum graft-bone displacement after cyclic loading was significantly greater for hamstring grafts fixed with the EndoButton and EndoButton Tape (Smith & Nephew Endoscopy, Andover, MA) compared with the other hamstring fixation techniques and interference screw fixation of patellar tendon grafts. All fixation techniques except hamstring tendon grafts fixed with the Bio-Interference screw achieved at least 59% of maximum graft-bone displacement after 20 cycles. Hamstring tendon grafts fixed with the EndoButton CL were significantly stronger than all other hamstring and patellar tendon fixation methods. Patellar tendon grafts fixed with interference screws and hamstring tendon grafts fixed with interference screws and the Bone Mulch Screw and TransFix were significantly stiffer than hamstring tendon grafts fixed with the EndoButton CL. CONCLUSIONS: There is no significant difference in graft-bone motion between aperture and suspensory femoral fixation methods when the stiffness of the femur-ACL graft fixation-ACL graft complex is similar. CLINICAL RELEVANCE: The small differences in graft-bone motion reported in our study provide further evidence that graft-tunnel motion or the so-called bungee effect is unlikely to be the primary cause of radiographic bone tunnel enlargement following ACL reconstruction.

The effect of donor age and low-dose gamma irradiation on the initial biomechanical properties of human tibialis tendon allografts.

BACKGROUND: Most tissue banks recover and irradiate tibialis tendon allografts from donors aged up to 65 years. It is unknown whether donor age and low-dose gamma irradiation affect the initial biomechanical properties of tibialis allografts. HYPOTHESIS: Donor age up to 65 years and low-dose gamma irradiation do not significantly affect the initial biomechanical properties of tibialis allografts. STUDY DESIGN: Controlled laboratory study. METHODS: One hundred twenty-six tibialis tendon allografts (63 pairs, 37 human donors) were divided into 3 age groups: young (<45 years), middle (46-55 years), and old (56-65 years). Within each age group, half of the paired tendons underwent tensile testing as single-strand grafts and the other half as double-strand grafts. One tendon from each donor pair was randomly assigned to undergo terminal sterilization with an absorbed dose of 1.46 to 1.80 Mrad (14.6-18.0 kGy) gamma irradiation, whereas the other tendon received no irradiation. All tendon grafts were preconditioned with a cyclic load and tested to failure in tension. RESULTS: Irradiated single-strand tendons in the old age group had a longer displacement at failure compared to the middle but not the young age group. Nonirradiated double-strand tendons in the old age group had a lower failure stress. Single-strand irradiated old tendons had a lower stiffness, and all irradiated young tendons and old double-strand tendons had a higher failure stress compared to nonirradiated tendons. CONCLUSION: Donor age up to 65 years does not significantly affect the initial failure load, stiffness, or displacement at failure of tibialis allografts. An age-related decrease in failure stress was observed among nonirradiated tendons but not in tendons subjected to irradiation. CLINICAL RELEVANCE: The results provide biomechanical evidence for use of tibialis allografts from donors up to 65 years of age. Low-dose gamma irradiation does not negatively influence the initial biomechanical properties of tibialis allografts. Further studies examining age and irradiation effects after submaximal cyclic loading conditions are recommended.