Restoration of stability with return to activity is generally expected after anterior cruciate ligament (ACL) reconstruction; long-term success rates range from 75% to 95%.1 However, graft failure occurs most frequently with soft-tissue grafts fixated only with interference screws.2,3 Fixation failure also occurs more frequently at the tibial site.2 This failure has been attributed to extensive graft slippage in cases of soft-tissue fixation with interference screws.2 Interference screw fixation alone, with a double-looped hamstring tendon graft, fails at 350 N in young human tibiae.4,5 However, failure is limited with use of a bone–tendon–bone graft or with backup fixation, particularly at the tibial site.3 The superiority of bicortical fixation has also been proven.5-7
In addition, as shown in a goat model, ACL graft fixation is a major cause of failure in the immediate postoperative period, before biological incorporation of the graft.8 Fixation techniques for soft-tissue grafts must withstand stresses during the healing period (grafts may take up to 12 weeks to incorporate).9 Failures may result from forces exerted on the graft—forces that may be as high as 450 to 700 N during daily activities.10,11 Within the tibial tunnel, various fixation devices are used, including interference screws, staples, pins, buttons, and interference screw/sheath constructs.12,13 Primary fixation is commonly achieved with interference screws because of their ease of insertion and greater stiffness. However, fixation of the soft-tissue graft is influenced by several variables, including bone density, insertion torque, thread geometry, and interference screw material.14-16 Many of these variables, which are a source of inconsistency and concern during the immediate postoperative period, have led surgeons to seek alternative methods of backup fixation at the tibial site. Nevertheless, good clinical and subjective results have been found after ACL reconstruction with a 4-stranded semitendinosus tendon at 10-year follow-up.17
An anchor used in rotator cuff repair is the SwiveLock system (Arthrex). Major advantages of this system include ease and speed of insertion, good strength, and reduced need for later hardware removal.
We conducted a study to biomechanically evaluate 3 methods of tibial-sided fixation for ACL reconstruction: fully threaded interference screw only, interference screw backed with 4.75-mm SwiveLock anchor, and fully threaded bio-interference screw backed with 4.5-mm bicortical screw. We hypothesized that a fully threaded bio-interference screw backed with a 4.75-mm SwiveLock anchor would provide mechanical strength no different from that provided by backup fixation with a bicortical post at the tibial site. We further hypothesized that SwiveLock backup fixation would provide more strength than fixation with bio-interference screw alone.
Materials and Methods
The design of this study was adapted from one used by Walsh and colleagues,3 who compared 3 fixation methods: retrograde interference screw, suture button, and combined fixation. Tibiae inspected before selection showed no signs of injury or abnormality. Bovine extensor tendons, which lacked any defects along their entire length, were stored in saline 0.9% solution. Both the tibiae and the extensor tendons were stored at –20°C before completion of the tibial-sided ACL reconstruction. Thirty fresh-frozen, skeletally mature porcine proximal tibiae were selected and thawed at 4°C before preparation. Specimens were prepared by potting the diaphysis in fiberglass resin, and a tunnel 9 mm in diameter was drilled through the anteromedial aspect of the tibia.
For consistency, one author (CAV) prepared all 30 specimens. Both tails of all 30 bovine extensor tendons were whip-stitched with No. 2 FiberLoop (Arthrex) 9 mm in diameter. Grafts and tibiae were randomly divided into 3 sample groups. The first group was prepared by antegrade graft fixation within the tibial tunnel using a fully threaded 9×28-mm BioComposite interference screw (Arthrex). The second and third groups used the same primary fixation within the tibial tunnel along with 2 types of secondary fixation. These backup fixation groups included a 4.5-mm titanium bicortical post (Arthrex) and a 4.75-mm BioComposite SwiveLock C anchor (Arthrex) (Figure 1). The FiberLoop at the ends of the distal graft tails for backup groups were fixated 1 cm distal to the tibial tunnel and tapped before insertion of backup devices (Figures 2A, 2B). Insertion was completed after 4.5-mm bicortical and 4.75-mm unicortical drilling and tapping of the anteromedial cortices for the titanium posts and SwiveLocks, respectively. The free ends of the whip-stitched No. 2 FiberLoop were tied to the proximal end of the titanium post with a single surgical knot followed by 5 square knots.3 The free ends of the No. 2 FiberLoop were inserted into the eyelet of the 4.75-mm SwiveLock and 1 cm directly inferior to the tibial tunnel. Interference fit of FiberLoop with SwiveLock was achieved within the corticocancellous bone of the tibiae. All samples retained a 30-mm tendon loop superior to the tibial plateau to simulate intra-articular ACL length. Specimens were then stored at –20°C and thawed at 4°C before biomechanical testing.