For the cyclic loading tests, 8 of the 10 Delta screws and only 2 of the 10 Retroscrews completed the 1500-cycle loading test before failure. The 2 Delta screws that did not complete the testing failed after about 500 cycles, and the 8 Retroscrews that did not complete the testing failed after about 250 cycles. All 10 WasherLoc and Intrafix devices completed the testing.
Residual displacement data were calculated from the cyclic loading tests (Table). Mean (SS) residual displacement was lowest for Intrafix at 2.9 (1.2) mm, followed by WasherLoc at 5.6 (2.2) mm and Delta at 6.4 (3.3) mm. Retroscrew at 25.5 (11.0) mm had the highest residual displacement, though only 2 completed the cyclic tests. Intrafix, WasherLoc, and Delta were not statistically different, but there was a statistical difference between Retroscrew and the other devices (P < .001).
Stiffness data were calculated from the LTF tests (Table). Mean (SD) stiffness was highest for Intrafix at 129 (32.7) N/mm, followed by WasherLoc at 97 (11.6) N/mm, Delta at 93 (9.5) N/mm, and Retroscrew at 80.2 (8.8) N/mm. Intrafix had statistically higher stiffness compared with WasherLoc (P < .05), Delta (P < .01), and Retroscrew (P < .05). There were no significant differences in stiffness among WasherLoc, Delta, and Retroscrew.
Mean (SD) ultimate LTF was highest for Intrafix at 656 (182.6) N, followed by WasherLoc at 630 (129.3) N, Delta at 430 (90.0) N, and Retroscrew at 285 (33.8) N (Table). There were significant differences between Intrafix and Delta (P < .05) and Retroscrew (P < .05). WasherLoc failed at a significantly higher load compared with Delta (P < .05) and Retroscrew (P < .05). There were no significant differences in mean LTF between Intrafix and WasherLoc.
Discussion
In this biomechanical comparison of 4 different tibial fixation devices, Intrafix had results superior to those of the other implants. Intrafix failed at higher LTF and lower residual displacement and had higher stiffness. WasherLoc performed well and had LTF similar to that of Intrafix. The interference screws performed poorly with respect to LTF, residual displacement, and stiffness, and a large proportion of them failed early into cyclic loading.
Intrafix is a central fixation device that uses a 4-quadrant sleeve and a screw to establish tensioning across all 4 hamstring graft strands. The theory is this configuration increases the contact area between graft and bone for proper integration of graft into bone. Intrafix has performed well in other biomechanical studies. Using a study design similar to ours, Kousa and colleagues7 found the performance of Intrafix to be superior to that of other devices, including interference screws and WasherLoc. Starch and colleagues10 reported that, compared with a standard interference screw, Intrafix required significantly higher load to cause a millimeter of graft laxity. They concluded that this demonstrates superior fixation strength and reduced laxity of the graft after cyclic loading. Coleridge and Amis4 found that, compared with WasherLoc and various interference screws, Intrafix had the lower residual displacement. However, they also found that, compared with Intrafix and interference screws, WasherLoc had the highest ultimate tensile strength. Their findings may be difficult to compare with ours, as they tested fixation of calf extensor tendons, and we tested human hamstring grafts.
An important concern in the present study was the poor performance of the 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.11 Delta screws and Retroscrews have not been specifically evaluated, and their fixation strengths have not been directly compared with those of other devices. In the present study, Delta screws and Retroscrews consistently performed the poorest with respect to ultimate LTF, residual displacement, and stiffness. Twenty percent of the Delta screws and 80% of the Retroscrews did not complete 1500 cycles. The poor performance of the interference screws was echoed in studies by Magen and colleagues12 and Kousa and colleagues,7 in which the only complete failures were in the cyclic loading of the interference screws.
Three possible confounding factors may have affected the performance of the interference screws: bone density of porcine tibia, length of interference screw, and location of screw placement. In addition, in clinical practice these screws may be used with other modes of graft fixation. Combined fixation (interference screws, other devices) was not evaluated in this study.
Porcine models have been used in many biomechanical graft fixation studies.4,6,7,12,13 Some authors have found porcine tibia to be a poor substitute for human cadaver tibia because the volumetric density of porcine bone is higher than that of human bone.14,15 Other authors have demonstrated fairly similar bone density between human and porcine tibia.16 The concern is that interference screw fixation strength correlates with the density of the bone in which screws are fixed.17 Therefore, one limitation of our study is that we did not determine the bone density of the porcine tibias for comparison with that of young human tibias.