Introduction
Tibial side fixation of soft tissue grafts has been challenging because the line of pull on the graft in the tibia is parallel to the axis of the tunnel and because the bone in the tibia has a lower bone mineral density (BMD). Free tendon ends are more difficult to fix well than looped ends or a bone block. Finally, tendon grafts heal more slowly than bone–tendon–bone grafts, requiring that soft tissue fixation devices withstand more cyclical loading before biological fixation takes place.
The INTRAFIX and Bio-INTRAFIX systems were the first intratunnel, sheath, and screw anchors for tibial fixation of soft tissue grafts. These anchors employ an expansion screw and a four-channel sheath to separate and grip each tendon strand, thus ensuring that each strand is pressed into bone. Mechanically they were designed to improve upon the fixation strength of interference screws, which showed a relative lack of strength and stiffness in many tests in human and animal bone. In response, some surgeons would not use interference screws alone, and they recommended routine backup fixation. When compared with interference screws alone in human bone, INTRAFIX and Bio-INTRAFIX demonstrated a significantly higher failure load, greater stiffness, and lower slippage in cyclical tests. These results were confirmed by others in human and animal bone when manufacturer’s recommendations were followed. Other screw and sheath systems have similarly shown superior tensile properties.
From a biological standpoint, INTRAFIX compresses each tendon into bone in contrast to interference screws, which leave some tendons without direct bony contact. This arrangement theoretically improves osteointegration of the graft. Histology from an animal study employing the INTRAFIX demonstrated early developing, direct bone-to-tendon healing, typically seen with the use of interference screws on all sides of the device ( Figs. 80.1 and 80.2 ).
In addition to highly desirable mechanical and biological qualities, there are several other advantages to a screw and sheath design: First, the device is low profile and rarely requires later removal, a potential problem encountered with most extracortical devices that fix onto the tibial cortex. Second, tunnel widening is infrequently encountered. Third, the graft strands are protected from laceration and twisting during screw insertion, either of which can compromise the strength of the graft construct as a whole. Finally, it is possible to identify and separate the bundles to create a single tunnel, double-bundle construct when used with a similar device—the Femoral INTRAFIX system—on the femur.
Surgical Technique
Graft Preparation
The two autograft hamstring tendons are cut to a total length of 20–22 cm, and the opposite ends of the tendons are whipstitched for a distance of 4–5 cm using a #2 nonabsorbable suture. The doubled graft is then folded at its midsection over a passing suture, creating a four-strand 10–11-cm graft. The INTRAFIX and Bio-INTRAFIX can be used with five- and six-strand graft constructs but not with shorter grafts. This length of tendon graft allows for 25 mm of the doubled gracilis semitendinosus tendon (DGST) graft to be inserted into the femoral tunnel and typically results in a significant length of suture-reinforced tendon within the tibial tunnel and a short (1 cm) length of the tendons extending outside the tibial tunnel. Planning the length of the graft so that some sutured tendon lies within the tunnel is important because suture-reinforced tendon constructs fixed with interference screws have a 30%–40% increased pullout strength.
Use with Allografts
Doubled allograft semitendinosis, gracilis, or peroneus longus tendons are prepared in the same fashion described previously for the DGST autograft. However, if a large single soft tissue allograft such as a tibialis anterior tendon is used, we prefer to divide each end of the allograft in two for a distance of 5 cm and then whipstitch each strand so that a four-stranded construct comparable to a DGST is created at its distal end. If preferred, the graft can also be left in its two-stranded form, placing the limbs 180 degrees apart in the tibial tunnel as the sheath is inserted.
The graft construct is then placed on a tensioning board to remove creep from the graft-suture construct. This is particularly important if supplemental distal cortical fixation is deemed necessary, as described in the Troubleshooting section.
Tibial Tunnel
Because our preferred method for performing endoscopic anterior cruciate ligament (ACL) reconstruction is the three portal technique in which the femoral tunnel is drilled through an accessory anteromedial portal, the only constraint is that the start point of the tibial tunnel on the tibial crest allows the creation of a 35–45-mm tunnel. A tibial tunnel length of 35–40 mm is optimal because it will prevent the 30-mm INTRAFIX or Bio-INTRAFIX from protruding into the joint. If the surgeon chooses a transtibial approach, the tunnels are typically shorter than 30 mm and cannot be fixed with the current INTRAFIX and Bio-INTRAFIX. Positioning the tibial tunnel should be done in a manner that avoids graft impingement in the notch, which is associated with effusions, loss of extension, anterior knee pain, quadriceps weakness, and increased anterior laxity.
Tunnel Sizing
Whether using the plastic INTRAFIX or the Bio-INTRAFIX, the diameter of the tibial tunnel should equal the diameter of the suture-reinforced end of the graft ( Table 80.1 ). The tibial tunnel should be drilled with a fluted drill to prevent anterior drift of the tunnel as the dense proximal cortex at the ACL tibial attachment is breeched.
| Graft | Tunnel | Trial | Sheath | Screw |
|---|---|---|---|---|
| 7 | 7 | Small (7–8) | Small | 6–7 |
| 7.5 | 7.5 | Small (7–8) | Small | 6–7 |
| 8 | 8 | Small (7–8) | Small | 6–8 |
| 8.5 | 8.5 | Small (7–8) | Small | 6–8 |
| 9 | 9 | Large (9) | Large | 7–9 |
| 9.5 | 9.5 | Large (9) | Large | 7–9 |
| 10 | 10 | Large (9) | Large | 8–10 |
| 10.5 | 10.5 | Large (9) | Large | 8–10 |
| INTRAFIX Sizing Guidelines (mm) | ||||
|---|---|---|---|---|
| Graft | Tunnel | Screw ∗∗ | ||
| 7 | 7 | 6–8 | ||
| 8 | 8 | 7–9 | ||
| 9 | 9 | 8–10 | ||
| 10 | 10 | 8–10 | ||
∗ The sizing guideline above is recommended for a four-stranded graft.
After drilling the tibial tunnel, it is important to clear soft tissue from around the entry point of the tibial tunnel using an electrocautery pencil and a Cobb periosteal elevator. Direct visualization of this area helps ensure that the sheath and screw are neither over- nor underinserted into the tunnel, and it improves the ability to see and trim excess tendon and sheath at the end of the case so that there is no prominence that might later irritate the patient.
Femoral Graft Fixation
The INTRAFIX tibial fastener can be used with any femoral fixation device, provided that the technique permits equal tensioning of all four graft strands. This is an important goal because, as shown by Hamner et al., equally tensioned DGST grafts are stronger and stiffer than a 10-mm, central-third patellar tendon autograft. However, when no attempt was made to equally tension all four graft strands, the ultimate failure load and stiffness of the DGST graft were not statistically different from that of a doubled semitendinosus tendon graft alone.
A single tunnel, double-bundle reconstruction can be performed when the Tibial INTRAFIX or Bio-INTRAFIX device is used with the Femoral INTRAFIX system (see Chapter 73 ).
Graft Passage, Graft Tensioning, and Tibial Fixation
In order for the the INTRAFIX device to function properly, the strands of the graft need to be parallel and untangled within the tibial tunnel. This can be accomplished easily if the surgeon arranges the strands in this way by holding them separate from one another as the graft is drawn into the knee.
After the femoral side of the DGST (or soft tissue allograft) has been securely fixed in the lateral femoral condyle, the whipstitches from the matched tendon ends are tied together to create a loop approximately 10–12 cm from the end of the tibial tunnel. This step is repeated for the semitendinosus tendon or the corresponding opposite ends of the tibialis/soft tissue tendon allograft ( Fig. 80.3A and B ).
The two suture loops are placed around the Tie Tensioner (DePuy Synthes Mitek Sports Medicine, Raynham, Massachusetts) but can also be held by hand. The Tie Tensioner equally tensions and spreads the four strands apart, allowing for central access to the bundle, and frees one hand for the surgeon. Because it contains a calibrated spring, it allows for quantification of the tension applied to the graft at the time of fixation. Prior to inserting the INTRAFIX, cycle the knee 0–90 degrees approximately 10–15 times, with a tension of 60–80N maintained on the graft limbs to achieve stress relaxation of the femoral fixation device, and to remove creep from the DGST or allograft whipstitches.
The usual excursion pattern detected with our bone tunnel placements results in the DGST graft pulling into the tibial tunnel by a few millimeters during the last 20 degrees of terminal extension. In this case, we fix the graft with the knee at 20 degrees of flexion because it is easier to do so at this position. When a larger excursion is detected, we fix the tibial side near full extension. Because of the high fixation strength and stiffness and the resistance to slippage of the INTRAFIX and Bio-INTRAFIX, we caution against applying excessive tension (greater than 80N) to the graft at the time of fixation, and against fixing the knee at a flexion angle greater than 20 degrees. High graft tension results in the graft construct being under tension through a greater range of motion, subjects the graft to higher abrasion forces at the femoral tunnel edge (killer angle) during knee motion, and can overconstrain or capture the knee.
Device Insertion
Concentric device placement within the tibial tunnel is critical to the success of the technique. To achieve this, the central axis of the tibial tunnel is identified by passing a stout guidewire, blunt arthroscopic sheath trocar, or a Trailblazer (Smith & Nephew Endoscopy, Andover, Massachusetts) through the center of the Tie Tensioner and down the center of the four graft strands into the knee joint. Once the central axis of the tibial tunnel is identified, the Tie Tensioner should be held in this orientation during all the subsequent steps to avoid divergent placement of the INTRAFIX sheath and screw. The surgeon can improve his or her ability to maintain this orientation by placing several fingers or the entire side of the hand, holding the tensioner on the tibia during the next steps.
Next, the four-quadrant Trial (dilator) is inserted down the center of the Tie Tensioner, at the center of the four graft strands, and oriented so that each graft strand sits in its own channel ( Fig. 80.4A and B ). Maintaining the desired tension on the graft, the four-quadrant Trial is tapped into the tibial tunnel for a distance of 35 mm. This step compresses and separates the four tendon strands, and in the case of smaller tunnels (7–8 mm), notches the bone tunnel wall to accept the sheath. It is important to keep the Trial oriented along the axis of the tibial tunnel as it is impacted, because the tip of the Trial has a tendency to diverge proximally while being driven into the tunnel. There are two sheath sizes for the Bio-INTRAFIX system, small and large, and a corresponding smaller and larger Trial appropriate to each. The smaller sheath is used for 7- and 8-mm tunnels and the larger for 9- and 10-mm tunnels.
After dilating the tibial tunnel, the 30-mm INTRAFIX sheath is placed on the sheath inserter with the derotation tab on the sheath oriented to match the tab on the sheath inserter. The knee is positioned at the chosen flexion angle, and a final tension of 60–80N is applied to the soft tissue graft using the Tie Tensioner.
The INTRAFIX sheath is inserted at the center of the four graft strands, taking care that each graft strand is positioned into a separate channel of the INTRAFIX sheath in the same orientation that was used in the Trial step. The derotational tab on the sheath is typically oriented at the 12-o’clock position ( Fig. 80.5 ). Orienting the derotational tab at this position prevents overinsertion. The inserter is tapped into the tunnel until the derotational tab is flush with the cortex. The sheath inserter is removed, and the 0.042-inch guidewire for the INTRAFIX tapered screw is inserted through the center of the sheath until a loss of resistance is felt as the tip of the guidewire enters the knee joint.
As stated earlier, clearing the soft tissue from the bone tunnel opening enables the surgeon to have a much better appreciation of the depth of insertion of the sheath, and later of the screw, and of the possible need to trim any protruding tendon or sheath after the screw has been inserted.
For the plastic INTRAFIX, a tapered screw size 1 mm larger than the tibial tunnel diameter is used. For example, an 8-mm tapered screw is used for a 7-mm tibial tunnel. Given the typical size of DGST grafts, the 7- to 9-mm tapered screw is most commonly used. While maintaining tension on the graft strands, the screw is inserted into the plastic sheath until its superior/proximal aspect is 1 mm above the tibial cortex ( Fig. 80.6 ). This is important because the fixation strength of any intratunnel device, such as an interference screw, is a function of BMD and is significantly improved when the base of the device engages the hard cortical bone of the distal tunnel. BMD rapidly decreases from the cortex to the cancellous bone of the interior tunnel.
The tension on the graft strands from the Tie Tensioner should prevent the sheath from rotating during screw insertion in hard bone, but some rotation of the outer sheath outside the tunnel is acceptable because the sheath within the tunnel does not move in concert. Protruding or prominent areas of the polyethylene sheath are trimmed flush with the tibial cortex using a #15 blade and a small bone rongeur.
The technique for insertion of the Bio-INTRAFIX device is identical, but the sizing scheme differs from that just described. Because the polylactide co-glycolide/beta-tricalcium phosphate (PLGA/TCP) sheath is incompressible and because the insertion torque is higher than with the plastic version, the screw size closely matches the tunnel diameter. The Bio-INTRAFIX sheath adds more than 1 mm to the diameter of the Bio-INTRAFIX screw, however, so the effective diameter of the fixation device is oversized to the tunnel diameter by this amount, as is the usual practice with interference screws and with the plastic INTRAFIX.
The stability and range of motion of the knee are checked. It is important to verify that the patient has full range of motion before leaving the operating room. The arthroscope is inserted into the knee, and graft tension and impingement are assessed. Our usual graft placement and tensioning technique results in the four strands of the DGST being maximally tight between 0 and 20 degrees, with the graft tension decreasing slightly as the knee is flexed to 90 degrees.
Troubleshooting
Sheath Overinsertion
Overinsertion of the sheath is a rare problem because both the sheath and sheath insertor have a tab that stops the assembly from advancing too deeply. Nevertheless, if sheath overinsertion occurs and the opening to the sheath cannot be seen, then central placement of the screw cannot be ensured and screw insertion should be delayed until the sheath is pulled back into position or replaced by another sheath. Because the ridges on the sheath are biased to resist slippage of the graft proximally, attempts to grasp the sheath and pull it out of the tibial tunnel may be unsuccessful. Cutting the sheath while in the tunnel or blindly grabbing at it with an instrument such as a pituitary rongeur can damage the graft strands and the sutures holding them, risking rupture during tensioning. A safer method involves pushing the sheath further up the tunnel, together with pulling the graft proximally with a probe inside the joint until the sheath can be seen entering the knee joint. At this point, the sheath can be grasped and removed through one of the portals. The graft is then retensioned using the tensioner, and the standard steps noted previously are repeated.
Screw Breakage
With the introduction of the PLGA/TCP Bio-INTRAFIX, screw breakage is a rare event because of the very low coefficient of friction with this material. Breakage can occur if the tunnel is undersized, if the bone is particularly hard, or if a less compressible graft, such a tibialis anterior, is used. It can also occur because of failure to insert the screw along the central axis of the sheath and tunnel, or because of failure to seat the screwdriver fully within the screw.
If screw breakage happens early during insertion, and if the tip can be withdrawn, then a smaller screw can be inserted so long as the sheath is not damaged. More often, it is difficult to withdraw the screw tip with the driver due to a lack of purchase, and because the tip of the screw can bind within the sheath. In this case, use an easy-out device, such as those marketed to remove stripped or damaged bioabsorbable, interference screws, and core the screw out from within the sheath. Replace the sheath, and insert a smaller screw. If the smallest of the screws (6–7 mm) was used initially, then another screw of the same size is likely to suffer the same fate. In this case, remove the sheath with a grasper and then insert the larger 9-mm dilator into the tunnel among the graft strands, which will enlarge the tunnel a bit further. Then insert a new screw and sheath of the same size as originally used.
If screw breakage occurs later during insertion but a substantial portion of the screw is in the tunnel, use a rongeur or saw to trim the prominent portion of the screw, and tie the sutures from the tendon ends onto a staple or screw and washer distal to the tunnel.
Failure to Advance
Changes in thread configuration and material composition (see above) have greatly improved ease of insertion of the screw into the sheath. Failure to advance is caused by a mismatch between the tunnel size and the graft-anchor construct, or when the screw strips, which can occur if the screwdriver was not seated fully. In these cases, a strategy similar to the case of screw breakage is applied. If failure to advance happens early in the process of insertion, first try to back the screw out and insert a smaller screw. If that fails, resect the prominent portion of the screw, core out the tip, and replace it with a smaller screw if the sheath is not damaged. If failure to advance happens later in the process of screw insertion, then resect the prominent portion and tie the sutures onto a post or staple.
Low Bone Density
The fixation strength of any intratunnel fixation device is greatly dependent on the local bone density. If during the insertion of the tapered screw the surgeon subjectively feels that there was low insertion torque, or if the patient has soft bone as assessed during drilling and dilation of the tunnel, then we recommend that supplemental tibial fixation be used. Depending on the graft length, the whipstitches from the tendon ends can be secured to a screw-post, or the tendons can be secured with a spiked washer and screw, or with staples below the tibial tunnel opening using one or two small barbed staples (Smith & Nephew Orthopaedics, Memphis, Tennessee).
Too Short a Graft
Finally, the surgeon may be faced with a graft that is not long enough and with ends that are recessed in the tunnel. If the graft is recessed to the degree that identification of the individual strands is not possible, then concentric placement of the sheath becomes much more difficult. One could try to separate the strands blindly, but then insertion of the dilator runs the risk of rupturing the sutures, with loss of ability to tension. In this case, therefore, it is probably best to tie the sutures onto a fixation post or use an interference screw as the sole means of fixation, or to use a hybrid of the two methods.
Closure and Postoperative Dressings
The sartorius fascia that was preserved during the hamstring tendon graft harvest is closed over the tibial hardware and repaired back to the tibia with a 0 absorbable suture. The subcutaneous tissue and skin are closed in layers with fine absorbable sutures. A running 3-0 Prolene (Ethicon, Sommerville, New Jersey) subcuticular pullout suture or 4-0 Monocryl produces a very cosmetic closure. A light dressing is applied over the wound, followed by a thigh-length antiembolism stocking (Cryocuff, Aircast, Summit, New Jersey) and a full-length, hinged knee brace.
Related posts:
Risk of Anterior Cruciate Ligament Injury as a Function of Type of Playing Surface
Hamstring Harvest Technique for Anterior Cruciate Ligament Reconstruction
Femoral Tunnel Creation Using the Zimmer Biomet SwitchCut Femoral Aimer
Cortical Screw Post Femoral Fixation Using Whipstitches, Fabric Loop, or Endobutton: The Universal Salvage
Sonographically Guided Anterior Cruciate Ligament Injection: Technique and Potential Use for the Treatment of Partial Anterior Cruciate Ligament Tear
Factors Associated with Increased Allograft Failure Rate in Anterior Cruciate Ligament Reconstruction
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