Of the procedures performed by surgeons specializing in sports medicine and by general orthopedists, anterior cruciate ligament (ACL) reconstruction remains one of the most common.1 Recent years have seen a trend toward replacing the “gold standard” of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.2 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, it is important to determine the strength of different methods of graft fixation.
Rigid fixation of hamstring grafts is recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand early rehabilitation forces as high as 500 N.2 There is therefore much concern about the strength of tibial fixation, given the lower bone density of the tibial metaphysis versus the femoral metaphysis. In addition, stability is more a concern in the tibia, as the forces are directly in line with the tibial tunnel.3,4
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed. There is much interest in determining which devices have the most fixation strength,4-9 but so far several products have not been compared with one another.
We conducted a study to determine if tibial hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Forty porcine tibias were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 4 different tibial fixation devices (Figure 1): Delta screw and Retroscrew (Arthrex, Naples, Florida), WasherLoc (Arthrotek, Warsaw, Indiana), and Intrafix (Depuy Mitek, Raynham, Massachusetts). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the tibias using the 4 tibial fixation devices. All fixations were done according to manufacturer specifications. All interference screws were placed eccentrically. The testing apparatus and procedure are described in an article by Kousa and colleagues.6 The specimens were mounted on the mechanical testing apparatus by threaded bars and custom clamps to secure fixation (Figure 2). Constant tension was maintained on all 4 strands of the hamstring grafts to equalize the tendons. After the looped end of the hamstring graft was secured by clamps, 25 mm of graft was left between the clamp and the intra-articular tunnel.
In the cyclic loading test, the load was applied parallel to the long axis of the tibial tunnel. A 50-N preload was initially applied to each specimen for 10 seconds. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 120 seconds were performed. Standard force-displacement curves were then generated. Each tibial fixation device underwent 10 cyclic loading tests. Specimens surviving the cyclic loading then underwent a single-cycle load-to-failure (LTF) test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF (yield load) data were generated from the single-cycle LTF test; ultimate LTF was defined as the load at the point where the slope of the load displacement curve initially decreases.
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 were similar. In all 10 tests, Intrafix was pulled through the tunnel with the hamstring allografts. WasherLoc failed in each test, with the tendons eventually being pulled through the washer and thus out through the tunnel. Delta screw and Retroscrew both failed with slippage of the fixation device and the tendons pulled out through the tunnel.