WasherLoc: Optimizing Tibial Fixation of Soft-Tissue Grafts Used for Anterior Cruciate Ligament Reconstruction

Introduction

The WasherLoc (Biomet, Warsaw, Indiana) is an extratunnel two-piece lag screw and spiked washer fixation device specifically designed for soft tissue graft fixation for Anterior Cruciate Ligament (ACL) reconstruction. The method of fixation of soft tissue grafts for ACL reconstruction is an important decision surgeons must make to optimize the superior properties of soft tissue graft sources and ACL reconstruction outcomes. The biomechanical properties of hamstring grafts are superior to patella tendon grafts; however, successful outcomes of ACL surgery can be achieved with various grafts sources, including hamstrings, bone–patellar tendon–bone grafts, quadriceps tendon bone grafts, and fresh frozen allografts. From a biomechanical standpoint, the role of fixation is more important when using soft tissue grafts, as it may take longer for biologic healing of a soft tissue graft to bone, and the early mechanical advantages occurring with the fixation of a bone plug in a bone tunnel are not realized with a soft tissue graft. In addition, biomechanical studies demonstrate tibial fixation to be the weak link with ACL reconstruction constructs. Therefore surgeons should use high strength, high stiffness, slippage resistant devices that maintain their fixation properties over time when securing a soft tissue graft on the tibial side in particular. The use of high stiffness fixation allows for lower graft tensioning needed to restore normal anterior-posterior laxity, compared with low stiffness fixation devices. The high graft tensioning required with the use of low stiffness fixation devices can be detrimental to graft remodeling, increase fixation failure risk, and lead to abnormal posterior tibial subluxation.

The decision whether to use intratunnel versus extratunnel fixation or both is important, as the biomechanical properties and the rate of healing of the soft tissue graft to bone is influenced by this decision. For example, the use of interference screws have comparable stiffness to the WasherLoc device and tandem screw fixation; however, the strength of fixation and graft slippage from interference screws are both inferior to WasherLoc and tandem screw fixation. Furthermore, the use of interference screw fixation appears to delay the rate of biologic healing of a soft tissue graft to bone when directly compared with the WasherLoc device in an ovine model. The effect of time on fixation strength of an extraarticular graft in an ovine model demonstrated maintenance of fixation strength with WasherLoc, but a statistical loss of fixation strength of an interference screw over the same 4-week period. Additional backup fixation is therefore recommended with tibial interference screw fixation, or a more conservative approach to postoperative rehabilitation should backup fixation not be utilized. Lastly, the use of an extratunnel device such as the WasherLoc for tibial fixation allows surgeons to use bone graft in the tibial tunnel, thereby decreasing tunnel expansion known to occur with soft tissue grafts while increasing the overall stiffness of the construct by 60 N/mm. In an in vivo study of 10 patients undergoing hamstring ACL reconstruction, bone dowels were harvested during surgery and then compacted into the tibial tunnel following WasherLoc fixation. The bone dowels averaged 23 mm in length and 7 mm in diameter. A cross-sectional area of the tibial tunnel was calculated on the day of surgery, 4 months and 1–2 years following surgery from CT scans. Ninety percent of the subjects had little to no tunnel expansion at their latest follow-up. Although tunnel expansion has not been associated with a diminished success rate of ACL surgery, limiting bone loss from tunnel expansion would be advantageous in the revision setting.

The cortical based fixation using a 6.0 mm cancellous lag screw as part of the WasherLoc device optimizes graft fixation, particularly in females and older patients with potentially decreased bone density. Interference screw fixation of soft tissue grafts in females may have inferior results compared with males undergoing similar fixation on the tibial side. The washer has four large tines to capture the graft to prevent extrusion from underneath the washer when the graft is compressed to bone. Thirteen small tines pierce the graft in addition to compression to achieve fixation. Two different washer sizes, 16 and 18 mm, are available based on graft and patient size ( Fig. 79.1 ). The WasherLoc hamstring construct has a strength of 905 N in human bone, with a stiffness of 248 N/mm comparable to the stiffness of the native ACL. Graft slippage is minimal with the WasherLoc. In an in vivo study using radiostereophotogrammetry to evaluate graft slippage and graft length changes of an ACL construct using anterior tibialis allograft fixated with WasherLoc on the tibia, the study demonstrated less than 1 mm of graft slippage from the tibial fixation in the setting of an aggressive postop brace-free rehabilitation protocol. The biomechanical properties of WasherLoc fixation optimize soft tissue graft tibial fixation, allow for tunnel bone grafting to minimize tunnel expansion, and improve construct stiffness, enhancing the success of ACL reconstruction surgery.