Introduction

The fixation of a soft-tissue anterior cruciate ligament (ACL) graft is more challenging than a bone–patellar tendon–bone (BPTB) graft because the tendon heals slower to a bone tunnel and the fixation is stressed earlier and more vigorously. Slower healing coupled with early and more vigorous stress at the site of fixation can cause slippage and loss of stability with a soft-tissue ACL graft that might otherwise not occur with a BPTB graft. This chapter focuses on the use of the WasherLoc and bone dowel to fix a soft-tissue ACL graft to the tibia. The WasherLoc and bone dowel is a simple, low-profile fixation technique that promotes early tendon tunnel healing, resists slippage under cyclical load and exercise, and prevents tunnel widening, which simplifies revision surgery.

WasherLoc and Bone Dowel Surgical Technique

The superior clinical and biomechanical performance of the WasherLoc and bone dowel during aggressive rehabilitation and testing in the laboratory has been extensively documented since the technique was introduced in 1997.^1–13 The WasherLoc is a multi-spiked washer with four long peripheral spikes that engage cortical bone and multiple shorter spikes that purchase the soft-tissue graft (Fig. 43-1). The WasherLoc comes in two lengths (long and standard) and three diameters (14, 16, and 18 mm). The preferred fixation for a 9- to 10-mm diameter soft-tissue graft is the 18-mm-long spike WasherLoc; for a 7- to 8-mm diameter graft, the 16-mm long spike WasherLoc.

Fig. 43-1 The WasherLoc is a low-profile washer with four long peripheral spikes and multiple short central spikes. The long peripheral spikes engage cortical bone and contain the soft-tissue anterior cruciate ligament (ACL) graft under the washer. The multiple shorter spikes purchase the soft-tissue graft.

The WasherLoc is seated in a counterbore inside the distal end of the tibial tunnel. The counterbore recesses the WasherLoc below the cortical surface, which eliminates hardware irritation of the overlying skin (Fig. 43-2).⁹ A self-tapping, cancellous screw compresses the WasherLoc and soft-tissue graft against the back wall of the tibial tunnel. The tip of the screw engages the lateral tibial cortex, which avoids any damage to the more posterior neurovascular structures. The portion of the tunnel anterior to the soft-tissue graft is dilated, and the bone dowel is compacted into the tunnel. The following is a detailed description of the surgical technique with pertinent illustrations.

Fig. 43-2 The correct orientation of the WasherLoc is perpendicular to the back wall of the tibial tunnel. The correct orientation of the self-tapping, cancellous compression screw is toward the fibula, which avoids damage to the neurovascular structures that are more posterior. The tip of the screw purchases the lateral tibial cortex. The bone dowel fills the anterior and medial tibial tunnel, causing the narrowing seen in the distal half of the tibial tunnel on the anteroposterior and lateral view.

Harvest a Bone Dowel from the Tibial Tunnel

Remove the cortex overlying the distal end of the tibial tunnel. Choose a cannulated reamer that matches the diameter of the soft-tissue ACL graft, and ream over the tibial tunnel guidewire. Slide the calibrated plunger over the tibial guidewire. Impact an 8-mm bone dowel harvester over the plunger and guidewire to the subchondral bone (Fig. 43-3). Rotate the bone dowel harvester several times clockwise and counterclockwise to break off the cylindrical bone. Remove the bone dowel and harvester. If the guidewire is removed with the bone dowel, then insert an 8-mm reamer into the tibial tunnel and rethread the guidewire through the cannulation in the reamer. Finish reaming the tibial tunnel.

Fig. 43-3 An 8-mm diameter bone dowel harvester is driven over a tibial guidewire to the level of the subchondral bone. The harvester is rotated several times to break the bone dowel away from the subchondral bone. The typical length of the bone dowel is 20 to 30 mm.

Drill the Counter Bore

Use electrocautery and a ronguer to remove a small section of the superficial layer of the medial collateral ligament (MCL) that overlies the cortical opening of the tibial tunnel. Insert the counterbore guide into the tibial tunnel until the vertical sleeve abuts against the distal end of the anterior edge of the tibial tunnel. Point the vertical sleeve at the fibula, and impact the awl. Insert the counterbore into the awl hole, and orient the counterbore so that it is parallel to the posterior wall of the tibial tunnel and oblique to the anterior tibial cortex. Remove a small amount of bone from the anterior tibial cortex until the counterbore is flush with the back wall of the tibial tunnel (Fig. 43-4).

Fig. 43-4 The counterbore is oriented parallel to the back wall of the tibial tunnel and is aimed toward the fibula. The purpose of the counterbore is to recess the WasherLoc inside the tibial tunnel to avoid irritating the overlying soft tissues.

Impact the WasherLoc

Position the knee in full extension. Thread the awl into the drill sleeve and the drill sleeve into the WasherLoc. Tension the soft-tissue graft. Rotate the flat edge of the WasherLoc distal, place half of the soft-tissue ACL graft on each side of the awl, insert the awl in the hole, and direct the tip of the awl toward the fibula. Impact the WasherLoc into the back wall of the tibial tunnel until it is fully seated (Fig. 43-5).

Fig. 43-5 The awl (A) is threaded into the drill sleeve (B), which is threaded into the WasherLoc. Half of the graft is placed on either side of the awl. The WasherLoc is oriented parallel to the back wall of the tibial tunnel and is aimed toward the fibula.

Dilate the Tibial Tunnel

Confirm that stability has been restored to the knee and the tension in the ACL graft is correct. Place the tip of the tapered dilator between the anterior surface of the soft-tissue graft and tibial tunnel (Fig. 43-6). Gently impact the dilator to the level of the joint line, which is typically 25 mm.

Fig. 43-6 The cone-shaped dilator is inserted anterior to the soft tissue anterior cruciate ligament (ACL) graft and is impacted 25 mm into the tibial tunnel. The cone-shaped space prevents the cylindrical-shaped bone dowel from being driven into the joint.

Compact the Bone Dowel

Stuff any loose bone reamings and wallplasty fragments into the tibial tunnel, and compact the bone with a 7 or 8 impingement rod. Place the plastic cover on the sharp tip of the bone dowel harvester, and drive the bone dowel into the tibial tunnel (Fig. 43-7).

Fig. 43-7 The plastic sleeve is placed over the sharp tip of the bone dowel harvester and inserted into the dilated opening. The bone dowel is compacted into the tibial tunnel and anterior to the soft tissue anterior cruciate ligament (ACL) graft.

Promoting Tendon–Tunnel Healing of a Soft-Tissue Anterior Cruciate Ligament Graft

Healing of a soft-tissue ACL graft¹⁴ is a greater concern than that of a BPTB graft because a tendon graft heals slower than a bone plug during the first 6 weeks of implantation.¹⁵ Healing is more of a problem in the tibia than in the femur because the marrow is filled with more fat¹⁶ and the bone is softer.¹⁷ Therefore a soft-tissue graft requires better fixation technique than a BPTB graft,¹⁵ especially in the tibia.¹⁶ The consequence of not addressing the slow tendon–tunnel healing is slippage during early rehabilitation.¹⁸ Slippage is more likely with a soft tissue ACL graft than with a BPTB graft because there is less pain early on.¹⁹

Strategies That Promote Tendon–Tunnel Healing

One strategy for promoting tendon–tunnel healing is the use of a long, snug tunnel (Fig. 43-8). The healing of a tendon graft is stronger and stiffer when the tunnel is lengthened and the fit between the tendon and tunnel wall is snug.²⁰ Lengthening the tunnel requires placement of the fixation device at the end of the tunnel and not inside (intratunnel device).¹² Compaction of a bone dowel into the tibial tunnel along side a soft-tendon graft increases the snugness of fit by filling gaps between the tendon and tunnel wall.^21,22

Fig. 43-8 Fixation of a soft tissue anterior cruciate ligament (ACL) graft in the tibia is problematic because the cancellous bone is softer and more filled with fat than the femur. The use of a WasherLoc and bone dowel promotes tendon–tunnel healing by providing a long, snug tunnel; circumferential healing (arrows); and the addition of a biologically active substance (A). The use of an intratunnel device such as an interference screw or IntraFix retards tendon–tunnel healing by allowing only one-sided healing between the tendon and tunnel wall (B).

A second strategy for promoting healing is to allow circumferential and avoid one-sided healing between the tendon and tunnel wall (see Fig. 43-8). The healing of a tendon graft is stronger and stiffer when the tendon heals to the tunnel circumferentially and is not one-sided. Circumferential healing requires placement of the fixation device at the end of the tunnel so that the entire surface area of the tunnel can heal to the graft.¹² One-sided healing occurs with intratunnel devices such as the interference screw.²³ The interference screw “interferes” and slows tendon–tunnel healing because the screw prevents one side of the tendon from healing to the graft.¹²

A third strategy is to surround the tendon graft with a biologically active substance. Healing of a tendon is accelerated and stronger when a biologically active substance is inserted in the tunnel with the graft. Wrapping periosteum around the graft accelerates the healing process of a tendon in a bone tunnel and leads to better biomechanical fixation in a shorter period of time.²⁴ Adding bone morphogenetic protein accelerates the healing process when a tendon graft is transplanted into a bone tunnel.²⁵ The acceleration of healing by periosteum and bone morphogenetic protein suggests that compaction of autogenous bone into the tunnel might also benefit healing. Autogenous cancellous bone has viable osteoblasts that may initiate, regulate, and accelerate the ingrowth of bone into the tendon. The use of the WasherLoc at the end of the tibial tunnel with compaction of bone dowel fulfills these criteria for promoting tendon–tunnel healing by providing a long, snug tunnel; circumferential healing; and the addition of a biologically active substance.

WasherLoc Resists Slippage Under Cyclical Load

Several biomechanical studies have shown that the WasherLoc has superior resistance to slippage, higher stiffness, and higher strength than other soft tissue tibial fixations.^{2,3,4,6–9,11–13} When studies are performed using human bone, the WasherLoc slips less, is stiffer, and stronger than interference screw, double staples, soft tissue washer and screw, and sutures tied to a post.⁹ Although there has been no head-to-head comparison of the IntraFix versus the WasherLoc in human bone, the IntraFix has been shown to slip more under cyclical load than the interference screw,²⁶ which indicates that the IntraFix slips substantially more than the WasherLoc.

Cyclical testing simulating 6 weeks of normal walking showed that the slippage of a two-strand soft tissue graft fixed with a WasherLoc was less than 0.6 mm, which is clinically imperceptible (Fig. 43-9). The fixation site was loaded 225,000 times from 0N to 170N to simulate the predicted number of steps in 6 weeks of normal walking and the load in the ACL during normal gait.¹³ The slippage resistance of the WasherLoc is consistent with clinical results that showed excellent anterior stability with use of brace-free, aggressive rehabilitation and early return to sport at 4 to 6 months.^1,5

Fig. 43-9 Slippage of a two-strand soft tissue anterior cruciate ligament (ACL) graft fixed to the tibia with a WasherLoc was measured using roentgen stereophotogrammetric analysis. The site of fixation was loaded from 0N to 170N for 225,000 cycles, which simulated the loading of the ACL in 6 weeks of normal walking. The slippage was less than 0.6 mm, which is clinically imperceptible and justifies the use of the WasherLoc with brace-free, aggressive rehabilitation and early return to sport at 4 months.

Bone Dowel Limits Tunnel Widening at 1 to 2 Years

Tunnel expansion in ACL reconstruction is greater with a hamstring autograft than with a BPTB autograft^27–29 and occurs with a variety of hamstring fixation devices.^28–32 The clinical consequences of the common phenomenon of tunnel expansion are being defined; however, tunnel expansion can complicate revision surgery.^33,34 Therefore a technique for fixing a hamstring graft to the tibia that limits tunnel expansion to the cross-sectional area of the reamer might have a clinical benefit by simplifying revision surgery.

In an in vivo study, a bone dowel averaging 23 mm in length and 7 mm in diameter was harvested from the tibial tunnel in 10 subjects undergoing hamstring ACL reconstruction. The cross-sectional area of the tibial tunnel was calculated on the day of surgery, 4 months, and 1 to 2 years postoperatively from computed tomography scans. The bone dowel reduced the cross-sectional area of the tunnel on the day of surgery and limited tunnel expansion to that of the cross-sectional area of the reamer at 4 months and 1 to 2 years (Fig. 43-10). Ninety percent of the subjects treated with a bone dowel had little to no tunnel expansion at 1 to 2 years. The limitation of tunnel expansion to that of the cross-sectional area of the reamer has not been shown with other tibial fixation techniques. Limiting tunnel expansion to that of the cross-sectional area of the reamer should simplify revision surgery.¹⁰

Fig. 43-10 A computed tomography scan of the bone dowel in the tibial tunnel alongside a double-looped hamstring graft (blue) is shown postoperatively and at 2 years. The bone dowel indents the tibial tunnel, reducing and changing the shape of the space occupied by the anterior cruciate ligament (ACL) graft from round to a smaller crescent shape. Although the tunnel widens somewhat after 2 years, the cross-sectional area of the tunnel at 2 years was no larger than the cross-sectional area of the reamer used to drill the tunnel. The bone dowel limits tunnel widening to that of the reamer at 2 years by shrinking the tunnel cross-sectional area on the day of surgery.

(From Matsumoto A, Howell SM. Time related changes in the cross-sectional area of the tibial tunnel after compaction of an autograft bone dowel alongside a hamstring graft. Arthroscopy. In Press.)

Conclusion

This chapter provides scientific documentation that the WasherLoc and bone dowel meet all the challenges of fixing a soft tissue ACL graft to the tibia. Fixation at the end of the tunnel and the compaction of the bone dowel allow circumferential tendon–tunnel healing and increase the snugness of fit, which solves the problem of a soft tissue ACL graft healing slower than a BPTB graft. Mechanical fixation of the WasherLoc in cortical bone and gripping of the soft tissue graft with multiple spikes solve the problem of the site of fixation being more vigorously stressed than with a BPTB graft. The compaction of the bone dowel prevents tunnel widening, which simplifies revision surgery. The use of the WasherLoc and bone dowel is a simple, low-profile fixation technique that promotes early tendon–tunnel healing, resists slippage under cyclical load and exercise, and is well suited for brace-free, aggressive rehabilitation of patients reconstructed with a soft tissue ACL graft.