Use of TunneLoc for Anterior Cruciate Ligament Reconstruction

Introduction

Similar outcomes have been reported after anterior cruciate ligament (ACL) reconstruction with either a bone–patellar tendon–bone graft or hamstring tendon graft, with no difference in graft rupture rates between the two groups at 10 years in one cohort study. Although the outcomes are similar for the two graft choices, there are unique characteristics of each graft that must be considered when obtaining fixation. Intuitively, bone–tendon–bone grafts have bone available on the ends of the graft, which can be utilized to improve fixation with use of an interference screw. The screw will obtain purchase in both the bone of the graft as well as the bone of the tunnel, providing secure fixation. Also, those grafts have some inherent stability within the tunnel because of the friction between the graft bone and walls of the tunnel.

Fixation of hamstring tendon grafts, on the other hand, can be more challenging. One biomechanical study has shown that hamstring tendon grafts fixed with an interference screw failed at lower loads (536 N) than patellar tendon grafts fixed with an interference screw (658 N) on the tibial side, and irreversible graft motion was increased in the hamstring group (0.38 mm) compared with the patellar tendon group (0.03 mm), indicating that there was some graft slippage despite the interference screw. They also noted continuous slippage with increasing loads in the soft tissue grafts fixed with an interference screw. In addition, fixation on the tibial side is often considered to be more problematic than the femoral side, because the bone mineral density of the femoral bone is greater than the tibial metaphyseal bone, and ultimate failure of hamstring tendon grafts fixed with an interference screw has been shown to be directly related to metaphyseal bone mineral density. Therefore numerous tibial fixation devices have been designed with these challenges to fixation in mind.

There are varieties of ways to fix the graft distally in the tibial tunnel, and they can be divided into cortical versus metaphyseal fixation. Cortical fixation involves using implants to directly compress the graft onto cortical bone or simply placing a screw distally to serve as a post to tie the graft sutures over. Broad washers with spikes can be impacted into the cortical bone to provide strong immediate fixation. Screws with or without washers serve as simple posts where the suture limbs of the graft can be tied and provide a simple fixation point. However, hardware prominence can be a problem with these cortical fixation devices, due to the superficial nature of the anteromedial tibia. Metaphyseal fixation devices attempt to eliminate prominent hardware and include the use of interference screws inserted within the tunnel to fix the graft against the wall. The screws differ in the use of sharp threads, round threads, and additional sheaths.

Screw and sheath devices were created as an attempt to decrease the graft cutout and slippage seen in some studies of interference screws. Interference screws can potentially weaken the graft and the tunnel with its threads as it is being inserted, especially in the case of allografts. One study showed that traditional interference screws can cause tunnel widening and prevent circumferential tendon-tunnel healing, resulting in inferior strength and stiffness at 4 weeks, compared with cortical fixation. Some studies have compared different techniques of drilling the tibial tunnel and using serial dilators to decrease the incidence of tunnel widening and slippage, but found no significant difference compared with traditional extraction drilling in initial fixation strength of a hamstring tendon graft.

In an attempt to improve these issues with tibial fixation, we have been using the Biomet TunneLoc ( Fig. 76.1 ). This device is both a soft tissue tibial fixation device and a graft-tensioning device. We believe it maximizes intratunnel fixation and bony contact of the hamstring graft. With this device, each of the four strands of the hamstring graft are spread out by the four quadrants of the implant and held in that quadrant by longitudinal ridges. This increases the contact surface area of each limb with the bone of the tibial tunnel. In addition, the implant differs from a screw in that the wide smooth outer surface of the TunneLoc is designed to uniformly impact the graft onto bone. Since the total radial force generated during interference fixation affects the pullout strength, this combination of graft distribution and wide contact surface area increases the pullout strength. This is similar to some screw and sheath devices, which have been shown to provide the highest ultimate failure loads and the least amounts of cyclic displacement in biomechanical studies. In an unpublished biomechanical study, there was no statistically significant difference in yield load and cyclic displacement between the TunneLoc and the Intrafix, a screw and sheath device. Unlike the screw and sheath devices or an interference screw, TunneLoc also achieves improved cortical fixation by having a beveled end that matches the tibial cortex, eliminating protrusion of the implant outside of the tunnel ( Fig. 76.2 ). This design feature allows maximal cortical contact between the graft and cortical bone, and cortical fixation of the graft limbs has been shown to decrease slippage in both in vivo and in vitro studies.

Additionally, TunneLoc is a tensioning device, and it eliminates viscoelastic creep present in other tibial fixation systems that do not use an in situ graft tensioner. Nurmi et al. have shown that even grafts preconditioned on a graft tensioner on the back table suffer a steady decline of 60% within 60 minutes after initial tensioning. TunneLoc allows the graft to be tensioned and conditioned with cyclic loading in situ, until final fixation while maintaining optimal graft tension. We found that even if surgeons use manual traction in situ to tension the graft during fixation, there was 13%–27% error in reproducing a desired tension in our unpublished study of graft tensioning among 12 practicing surgeons. The self-locking tension gauge on the TunneLoc allows accurate and reproducible tension to be maintained, with ability to readjust in situ to ideal tension until the final implant is inserted.