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

Abstract

Tibial fixation for anterior cruciate ligament reconstruction continues to remain the weak link biomechanically with any anterior cruciate ligament (ACL) reconstruction. Unlike the early mechanical fit that can be achieved with bone plug grafts, hamstring grafts must depend solely on the performance of the fixation devices to maintain graft tension and avoid graft slippage. Intratunnel bone grafting improves construct stiffness and can lessen the tibial tunnel expansion seen with soft tissue grafts used for ACL reconstruction. The low profile WasherLoc tibial fixation device provides high strength, high stiffness and slippage resistance fixation of a soft tissue graft and allows for intratunnel bone grafting thus optimizing the biomechanical properties of ACL reconstruction and clinical outcomes.

Keywords

WasherLoc, fixation, tibia, ACL, hamstring graft

Keywords

WasherLoc, fixation, tibia, ACL, hamstring graft

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.

Washerloc Fixation Technique

Once the tibial tunnel has been established, the distal end of the tunnel is prepared to countersink the WasherLoc. The WasherLoc preparation must be performed before graft passage. The authors typically harvest a core of cancellous bone to be used to bone graft the tibial tunnel once the graft is fixated using a 7 mm bone harvest tube during tibial tunnel preparation. An aimer, counterbore awl, and counterbore reamer are used to prepare the bone and insert the WasherLoc washer (see Fig. 79.5 ). Use the aiming guide with the counterbore awl to ensure proper placement of pilot hole for counterbore reamer. The aiming guide is placed into the tibial tunnel and fully seated as proximally as possible. The aimer is angled toward the fibular head or slightly anterior ( Fig. 79.2 ). With the aimer in place, an awl is impacted into the cancellous bone of the distal tunnel edge. The counterbore reamer is then seated into the created pilot hole ( Fig. 79.3 ). Prior to the reamer making contact with the bone, the reamer is put in forward motion and carefully advanced into the bone. The goal is to simply ream the anterior wall of the distal tibial tunnel until the reamer is flush with the posterolateral tunnel wall. When performed correctly, the reamer creates a flat surface for seating of the WasherLoc washer on corticocancellous bone ( Fig. 79.4 ). Once the bone has been prepared for the WasherLoc, the graft can be passed and fixed on the femoral side. Once fixed on the femoral side and cycled, the graft is fixed on the tibial side. Because of the high stiffness of the WasherLoc, the graft should be tensioned and fixed near full extension, particularly if a stiff rigid femoral fixation device is also used. The washer is assembled onto the inserter and drill guide, prior to placement ( Fig. 79.5 ). The graft strands are first captured inside of the four long tines prior to impaction, while an assistant pulls unmeasured tension equally on all graft bundles ( Fig. 79.6 ). The WasherLoc washer is impacted along the same path taken by the awl and reamer until fully seated against the graft. Forceful impaction is not required. Once the washer is seated, compressing the graft to bone, the inner awl is removed and a 3.2-mm drill is placed down the inserter and drilled through the far cortex, taking care not to plunge into the soft tissues ( Fig. 79.7 ). The inserter is removed and a depth gauge placed to determine screw length. The appropriate length 6.0 mm screw is placed again, following the same path as the drill. Bone wax is placed over the tip of the screw to protect the graft as the screw is inserted ( Fig. 79.8 ). Tighten the screw with two-finger tightness to prevent crushing or overcompressing the graft. Remove any residual bone wax ( Fig. 79.9 ). The completed fixation should be low profile and countersunk, flush with the cortex of the tibial shaft. Bone harvested with a bone harvest tube or a collection of bone reamings can be packed into the tibial tunnel. A dilator is advanced into the end of the tibial tunnel by hand. Bone is then gently impacted from the harvest tube into the distal end of the tunnel, filling any voids and sealing off the distal tunnel and completing tibial fixation ( Fig. 79.10 ). The excess graft can be trimmed and removed, or the distal graft strands can be flipped over the washer to provide additional soft tissue coverage ( Fig. 79.11 ).