Original Research

Evaluation of 3 Fixation Devices for Tibial-Sided Anterior Cruciate Ligament Graft Backup Fixation

Am J Orthop. 2015 July;44(7):E225-E230

References

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32. De Wall M, Scholes CJ, Patel S, Coolican MR, Parker DA. Tibial fixation in anterior cruciate ligament reconstruction: a prospective randomized study comparing metal interference screw and staples with a centrally placed polyethylene screw and sheath. Am J Sports Med. 2011;39(9):1858-1864.

In this study, interference screw with SwiveLock backup demonstrated a mean (SD) ultimate load to failure of 1007 (176) N, comparable to that found by Walsh and colleagues³ for retrograde bio-interference screw with suture button, 1027 (157.11) N. In a study comparing quadrupled hamstring tibial graft fixation, Intrafix (DePuy Mitek) and an 8×25-mm Bioscrew (Linvatec) demonstrated mean (SD) single-cycle yield loads of 1332 (304) N and 612 (176) N, respectively.¹³ These results are similar to the ultimate yield loads in the present study: bicortical-post group, 1148 (186) N; SwiveLock group, 1007 (176) N; screw-only group, 778 (139) N. Differences may be attributed to hamstring tendons used in a quadrupled manner in the aforementioned study.^12,13 Last, mean (SD) ultimate load to failure in a study that used only a retrograde bio-interference screw (9×20 mm) was 679.00 (109.44) N,³ similar to the 778 (139) N found for interference-screw-only in the present study. The difference is likely attributable to the longer screw (9×28 mm) in our study. Using SwiveLock C in cortical bone, Barber and colleagues¹⁸ found mean (SD) loads to failure up to 711.9 (89.1) N.

Clinically, it has been shown that a statistically significant increase in anterior laxity occurred between 4 months and 2 years in 10.7% of patients who underwent hamstring ACL reconstruction.²⁰ The knees were clinically categorized as unstable or severely abnormal. The authors concluded that the clinical outcome was more likely influenced by the methods used to fix the free ends of the graft, specifically with 2 staples or a washer. To simulate early postoperative rehabilitation in the present study, cyclic loading of the graft was performed. Ultimate load to failure was then determined in order to evaluate catastrophic failure strength of the backup fixation devices in comparison with the interference-screw-only group without supplementary fixation.

It has been shown in autologous hamstring ACL reconstruction that a centrally placed polyethylene screw with sheath (Intrafix) performed as well as a standard, eccentrically placed metal interference screw with staple.¹⁰ It is therefore logical that backup fixation with use of a similar device (eg, SwiveLock, bicortical post) is necessary to ensure comparable clinical outcomes in relation to a screw/sheath device that has been shown to endure the highest yield loads.^{2,9,12,13,21-23} Potential benefits of using SwiveLock anchors for backup fixation include a statistically significant increased mean (SD) ultimate yield load of 229 (176) N over interference screw only. These results are similar to those in comparable studies: 218.3 (59.7) N²⁴ and 165 (24.15) N²⁵ in healthy bone with a reported bone mineral density (BMD) of 1.39 g/cm², similar to that of skeletally mature porcine tibia (1.220-1.675 g/cm²).³ In addition, ease of insertion of this device over a bicortical post was demonstrated. The titanium post required bicortical drilling as well as measurement with a depth gauge to ensure adequate screw length. This process appeared to require more time during specimen preparation and theoretically could prove to be more dangerous clinically.⁷ However, caution in using a SwiveLock anchor in osteoporotic bone is advised because of reduced pullout strength.²⁶ In this case, bicortical-post backup fixation may be more suitable. Moreover, although not demonstrated in this study, hardware prominence and irritation with a post may cause postoperative morbidity necessitating future removal.²⁰ Hardware removal was the most common reason for additional surgery using hamstring tendons as graft.²⁰ A second surgery for hardware removal was required in 21% of these patients.²⁰ This is unlikely to occur with a SwiveLock, as the anchor is buried within cortical bone.

Limitations

Regarding use of nonhuman tissues in a biomechanical model, porcine tibiae and bovine extensor tendons were used because of availability, consistency among specimens, and cost-effectiveness. However, bovine extensor tendons have been shown to exhibit stiffness and viscoelastic properties similar to those of a human hamstring graft.²⁷ In addition, the BMD of the porcine tibiae used in this study was not tested because of time involved and cost-efficiency. However, it has been shown that average BMD of porcine tibiae, 1.220-1.675 g/cm², is similar to that in a young athletic population, 1.24-1.62 g/cm².^3,28-31 We therefore assumed similarity to a young athletic population and uniformity of BMD of the porcine tibiae used in this study.

In addition, the biomechanical testing protocol did not simulate physiologic loading within the tibial tunnel. Moreover, the testing protocol used loads of only 250 N during cyclic testing for 500 cycles. This simulates only the early rehabilitation period and not the healing period, which may last up to 12 weeks.⁹ In addition, as previously mentioned, forces on the graft may be as high as 450 to 700 N.^11,32 Pullout testing in line with the long axis of the tibia was performed in order to compare mechanical testing results with those of similar studies.^3,12,13 Last, the P of .07 for the comparison of ultimate load to failure between the 2 backup fixation groups suggests that this study may have been underpowered.