Biomechanical Comparison of Hamstring Tendon Fixation Devices for Anterior Cruciate Ligament Reconstruction: Part 1. Five Femoral Devices

Clinical outcomes of studies using TransFix for femoral fixation have been favorable, with improved Lysholm scores and improved laxity according to the KT-1000 test.¹⁷ However, multiple prospective studies have found no clinical difference in knee laxity between interference screw and Endobutton at 1- to 2-year follow-up^18-20 and no difference in clinical outcome scores, such as the International Knee Documentation Committee score.^11,18-20

Although these studies have shown no major clinical differences at short-term follow-up, the early aggressive rehabilitation period is the larger concern. Our study clearly demonstrated the biomechanical strength of transcondylar devices over other devices. The concern with transcondylar devices (vs other devices) is the increased difficulty that inexperienced surgeons have inserting them. In addition, when removed, transcondylar devices leave a large bone void.

In the present study, an important concern with femoral graft fixation is the poor performance of interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.⁷ In the present study, Delta screws consistently performed poorest with respect to ultimate LTF, residual displacement, and stiffness. Only 20% of these screws completed 1500 cycles. Poor performance of interference screws has also been seen in other studies in tibial graft fixation^21,22 and femoral graft fixation.^13-15 Given their poor biomechanical properties, as seen in our study and these other studies, we think use of an interference screw alone is a poor choice for fixation.

Combined fixation techniques—interference screw plus other device(s)—may be used in clinical practice, but the present study did not evaluate any. In a biomechanical study, Yoo and colleagues²³ compared an interference screw; an interference screw plus a cortical screw and a spiked washer; and a cortical screw and a spiked washer used alone in the tibia. Stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with the interference screw plus the cortical screw and the spiked washer. In a similar study involving femoral fixation, Oh and colleagues²⁴ demonstrated improved stiffness, residual displacement, and LTF in cyclic testing with the combination of interference screw and Endobutton CL, compared with Endobutton CL alone. Further studies may include direct comparisons of additional femoral fixation techniques using more than 1 device.

The Zip Loop, or Toggle Loc with Zip Loop technology, is a suspensory cortical fixation device. It was initially designed for use in ACL fixation but has also been used in other surgeries, including distal biceps repair²⁵ and ulnar collateral ligament reconstruction.²⁶ The device itself is easy to use; more important, it allows for adjustment of graft length within the bone tunnel after deployment of the cortical fixation. Few biomechanical studies have been conducted with Zip Loop.^9,12 The present study is the first to compare Zip Loop with devices other than suspensory cortical fixation devices. Zip Loop performed very well in LTF testing but had lower stiffness and higher residual displacement compared with the transcondylar fixation devices. Despite these findings, we have continued to use this device for femoral fixation in ACL reconstruction because of its ease of insertion, the ability to adjust graft tension within the bone tunnel, and the difficulties encountered inserting and removing transcondylar fixation.

We recognize the limitations in our study design with respect to how axial and cyclical loading compares with the physiologic orientation of the ACL during ambulation and running activities. This biomechanical study was not able to replicate these types of activities. However, it did provide good data supporting early rehabilitation with various fixation devices, though concern with use of interference screws remains.

Conclusion

Superior strength in fixation of hamstring grafts in the femur was demonstrated by Bone Mulch screws, followed closely by Bio-TransFix. Delta screws 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.