MedShape ExoShape Fixation

Introduction

Aperture fixation and graft compression have shown to be key aspects of a successful anterior cruciate ligament reconstruction (ACLR). In addition, applying graft compression potentially improves fixation strength, reduces bungee cord effect, and promotes direct bone-to-graft healing. The ExoShape Fixation System (MedShape, Inc., Atlanta, Georgia) accomplishes both soft tissue graft fixation and compression on both sides of the joint by utilizing unique shape memory polyetheretherketone (PEEK) technology. Shape memory materials can transform between different configurations when exposed to an activation trigger such as temperature or mechanical force. Both the femoral and tibial devices that comprise the ExoShape system utilize a mechanical force, via a second device component, to activate an instantaneous expansion within the body.

The femoral fastener ( Fig. 67.1 ) is composed of two components, the graft loop and wings that before deployment are compressed into a low-profile configuration. The soft tissue graft is loaded onto the loop component, with the graft bundles positioned on either side of the implant, and then inserted into the tunnel with the graft strands in the appropriate anatomic orientation. Advancing the wing component into the graft loop activates the shape memory, causing the wings to deploy and press directly into the surface of the bone tunnel without contacting the soft tissue graft. Simultaneously, the distal end of the loop component expands, thereby applying graft compression at aperture. The fastener for the tibial fixation (see Fig. 67.1 ) also consists of two components, a sheath and bullet. The sheath has an initial geometry such that its two sides are compressed, allowing the implant to be inserted between the graft bundles with minimal resistance. The sheath is then triggered to expand into its final shape by inserting the bullet. This expansion mechanism allows for circumferential compression of the graft without imparting any rotational force. A two- or four-stranded graft may be used and rotated prior to tibial fixation to replicate the posterior lateral and anterior medial bundles. Once the graft is placed in the desired orientation, it will not rotate or migrate longitudinally during deployment. Reported pullout strength for these fasteners exceeds 1200 N.

Surgical Technique

My preference is to harvest and prepare the soft tissue graft prior to the arthroscopic portion of the procedure. For the past decade, I have used a customized hybrid soft tissue graft based on the patient’s age, activity level, and size. The advantages I have noted are consistency of graft size, a stronger graft complex, very minimal donor site morbidity along with almost uniform tendon regeneration, a graft that more closely approximates the size of the patient’s native ACL, particularly in the larger individual, and a far lower failure rate than I was achieving with a gracilis/semitendinosus autograft. In general, if the patient is younger than 30 years of age and active, I harvest the semitendinosus tendon and augment it with a strip of semitendinosus allograft to produce a combined hybrid graft that is 9–11 mm in diameter and 25–28 cm in length, depending on the size of the patient. For patients older than the age of 30 who are not involved in ACL-dependent sports at a high level, I use an appropriately sized anterior tibialis tendon allograft. The grafts are prepared in the standard fashion with a #2 Ticron suture whipstitched along both ends, preferably using sutures of a different color. The two grafts are then doubled over on themselves to create a four-strand graft bundle. Ideally, the quadruple-stranded hybrid graft should measure a length of 12–14 cm and pass easily through the appropriate diameter cylindrical sizing guide based in general on the patient’s height and tibial width (9 mm for patients shorter than 5 feet, 10 mm for patients 5 feet to 5 feet 10 inches tall, and 11 mm for patients taller than 5 feet 10 inches). The grafts are then passed separately through the loop component of the femoral fastener and marked circumferentially with a skin marker at 25 mm measuring from the top of the device. The femoral fastener is available in millimeter size increments from 8 to 11 mm. We have found that using a device that is 1 mm smaller than the graft diameter makes passage through the bony tunnels easier without any change in the fixation strength.

Attention is then directed at the arthroscopic portion of the ACLR, where the standard approach is carried out with the femoral tunnel being prepared through either a transtibial or low medial portal approach. The tunnel needs to be drilled to a minimum length of 25 mm with a diameter that is 1 millimeter larger than the selected fastener size. When drilling the tibial tunnel, ensure that the length of the tunnel is at least 30 mm. An easy way this can be done is to insert a guide pin into the tibial guide and mark the pin 30 mm from the guide. When drilling the pin into the joint, observe that the mark has reached the guide prior to the pin penetrating the joint.

The fastener-graft complex is then pushed through the medial portal or the tibial tunnel (depending upon the drilling approach taken) and into the femoral tunnel, with the graft rotated in such a fashion as to replicate the position of the anterior medial and posterior lateral bundles ( Fig. 67.2 ). The retention rod on the femoral deployment gun is slightly flexible and can be assisted into the tunnel using a joker elevator. If necessary, the back of the deployment gun may be lightly tapped with a mallet until the marked portion of the graft is completely inserted in the tunnel. The trigger on the deployment gun is then released and squeezed while applying slight forward pressure, to advance the wings into the loop component, thus deploying the device. The device is fully deployed when there is no more resistance from the trigger handle, usually with two to three trigger pulls. Push the knob lock on the back of the gun forward, and rotate the knob on the handle counterclockwise until the gun and retention rod can be removed from the device. If using a medial portal, shuttle the sutured graft ends into the tibial tunnel.

Tension is then placed on the graft distally to ensure adequate fixation while the knee is taken through a range of motion of 0 to greater than 100 degrees and evaluated for any graft impingement or pistoning of the graft within the tibial tunnel. If there is inward pistoning of the graft in the tibial tunnel, note the position of the knee in which it occurs and secure the graft in this degree of flexion. If the graft is in the correct position this usually occurs with the knee in hyperextension. This avoids capturing the knee or stretching the graft trying to obtain full range of motion.

At this time the knee is placed in the desired degree of flexion to prepare for graft fixation in the tibia. Usually I position in full extension. The manufacturer’s instruction guide recommends the use of a graft bar to evenly tension the graft bundles. However, we have found it more effective to tension the graft manually.

At this point, the tunnel and graft are sequentially dilated to determine the appropriate fastener size. Insert the appropriate diameter trialator (1 mm less than the drilled tunnel) between the graft strands. Next, push or impact the trialator until the 30-mm laser mark is just below the cortical surface. Repeat dilation with a trialator 1 mm larger than the first. As tibial metaphyseal bone may compress beyond the drilled tunnel diameter, continue using sequentially larger trialators until the tip fits tightly between the graft bundles and moderate resistance is encountered. Next, select the tibial fastener diameter that matches the largest trialator diameter used. Pull the bullet back and insert the sheath component onto the tip of the inserter. Position the sheath such that the compressed sides are alongside the tensioned graft bundles and insert at the same angle as was used with the trialator. Tap the inserter until the sheath is 1–2 mm subflush with the cortical bone. The sheath position can be confirmed by removing the inserter. While still maintaining tension on the graft limbs, push the expansion bullet down to the sheath and load the deployment gun onto the fastener. Apply slight forward pressure to the gun, and squeeze the trigger to advance the bullet into the sheath component. Continue to advance the bullet until it is fully seated inside the sheath or when there is no further resistance on the gun trigger. Release the trigger and depress the release tab on the back of the handle. Flip the gold lock on the gun to the open position and detach the gun from the insertion rod. Toggle the insertion rod while it is still attached to the sleeve. There should be no movement of the fastener. In the event the graft is slack or the fastener appears loose, extract the device by simply applying tension to the insertion rod and replace with a larger fastener.

Next, reinsert the arthroscope and probe the graft to ensure that proper tension is maintained throughout the range of motion and that there is no penetration of the fixation device into the knee ( Fig. 67.3 ). If all looks good, remove the insertion rod from the fastener by rotating counterclockwise. Once the insertion rod is removed, suture the graft at the end of the sheath along with the overlying periosteum and cut the excess graft. An excellent method to test the fixation is to remove the knee from the knee holder (if one is used) and perform an aggressive pivot-shift test. This test alone applies significant strain on the graft, and failure here is easier to address than later.