Structural properties of a new device for graft fixation in cruciate ligament reconstruction: the shim technique.

Introduction: In ACL reconstruction using a soft tissue graft, aperture fixation with interference screws (IFS) can lacerate and rotate the graft and cause primary tunnel widening. To overcome these downsides, a new wedge-shaped implant (MegaShim, Karl Storz, Germany) was developed. We hypothesized that aperture fixation of hamstring ACL reconstruction using the MegaShim technique shows comparable structural properties when compared to interference screw fixation.
Methods: In a porcine knee model, ACL reconstructions with a tunnel diameter of 6, 7, 8 and 9 mm were performed and human hamstring grafts were fixed using the MegaShim technique (group I). In group 2, grafts were fixed in a hybrid fixation using a MegaShim and cortical flip button (FlippTack, Karl Storz, Germany). Interference screw graft fixation served as a control group (group III). Maximum load, yield load and stiffness were recorded using material testing machine. Grafts were cyclically preconditioned between 0 and 20 N for 10 cycles before the graft-bone complex was loaded to failure. Statistical evaluation was performed using SPSS Version 11.0.
Results: Mean maximum load to failure for the hybrid fixation was significantly higher than after interference screw or MegaShim fixation. The difference between MegaShim and interference screw fixation showed no significant difference for 6 and 7 mm sizes. An 8 and 9 mm MegaShim fixation resulted in significantly lower ultimate failure load compared to interference screw fixation. Stiffness of grafts fixed using 6 mm MegaShim was significantly lower than grafts fixed with hybrid or interference screw fixation, whereas no significant differences were found in the 7, 8, and 9 mm fixations.
Conclusion: Aperture fixation using the MegaShim technique provides comparable structural properties compared to interference screw fixation. Hybrid fixation using MegaShim and cortical flip button results in significantly higher ultimate failure loads than both aperture fixation approaches. Smaller grafts (6 mm) showed significantly lower ultimate failure load and stiffness than interference screw fixation. The "MegaShim technique" is an alternative to interference screw fixation concerning initial fixation strength.