Introduction
Stable graft fixation is critical for a successful outcome after anterior cruciate ligament reconstruction (ACLR). The advantages of early joint motion and weight bearing after ACLR have been well documented. Anterior cruciate ligament (ACL) graft fixation must provide sufficient strength for rehabilitation and activities of daily living until biologic fixation takes place. The ultimate strength of all commonly used grafts in ACL reconstruction exceeds the strength of all current ACL fixation devices. Therefore the fixation device represents the weakest link in the early postoperative period. Fixation of the graft is especially important in the first 6 to 8 weeks to provide this initial stability while biologic healing begins. If the graft slips or loosens before biologic healing, the graft may heal in an elongated fashion, resulting in persistent knee instability. Animal studies have shown that fixation devices are the point of failure for ACL reconstructions for 6 weeks after bone-patellar tendon-bone (BPTB) autografts and 12 weeks after hamstring tendon (HT) grafts. , The ideal fixation device would facilitate but not interfere with healing of the ACL graft within the bone tunnel, be easily removed if necessary, and avoid an inflammatory response, irritation to the surrounding structures, and distortion on postoperative imaging.
The two most commonly used grafts in ACLR are BPTB and hamstring tendon (HT) grafts. BPTB and hamstring tendon (HT) grafts heal differently, which impacts selection fixation techniques. BPTB grafts require bone to bone healing, whereas hamstring and other soft tissue grafts require soft tissue to bone healing. As a result, the common fixation method for BPTB grafts and hamstring tendon (HT) grafts differs.
The bone quality of both the proximal tibia and distal femur has a substantial effect on ACL graft fixation. The bone mineral density of the proximal tibia is less than that of the distal femur, resulting in increased concern regarding fixation strength in the tibial tunnel. Decreased bone mineral density results in decreased strength of interference screw fixation. In addition, the force on the graft and the fixation device within the proximal tibia is oriented in line with the tunnel, resulting in greater forces being exerted on the tibial fixation device. For these reasons, tibial graft fixation is considered the weakest point in ACLR and requires special consideration.
Many different devices have been created in an attempt to accomplish stable ACL graft fixation. Broadly, ACL graft fixation devices can be categorized as intratunnel or suspensory fixation devices. Fixation devices exist as both absorbable and nonabsorbable devices and include different forms such as screws, staples, pins, and buttons. The majority of research has focused on the biomechanical properties of these devices, although fixation devices also have significant effects on graft healing within the tunnel, biologic reaction, appearance on postoperative imaging, and consequences for revision surgery. Although these implants often allow for successful ACLR, their use can be associated with numerous complications.
Suspensory Fixation
Suspensory cortical fixation devices are the gold standard for femoral fixation of soft tissues grafts. Cortical buttons can be used to achieve suspensory fixation on both the femoral and tibial sides. Cortical buttons may use a fixed length or an adjustable loop of nonabsorbable suture to fasten the graft to the button. Cortical buttons have demonstrated excellent strength, and clinical studies have shown excellent stability and functional outcomes. The ultimate load to failure of fixed-length and adjustable-loop cortical buttons has been shown to be greater than the forces seen during ACL rehabilitation. The use of cortical buttons also simplifies possible revision surgery given the lack of intratunnel hardware.
Fixed-length or continuous-loop buttons may present some technical challenges during ACL reconstruction. The predetermined length of the fixed loop does not allow for customization to a patient’s unique anatomy. As a result, care must be taken with accurate measurements of tunnel lengths. The surgeon must accurately determine the total length of the tunnel to the cortex, as well as accurately drill a certain depth to the tunnel. If an error occurs with measurement of the total length or depth of the tunnel, it will either lead to an inability to pass the graft or the insertion of only a small amount of graft within the tunnel. If the button is unable to be flipped, this is because of either the drilled tunnel or the continuous loop suture being too short. If this occurs, the graft must be removed from the knee and the femoral tunnel drilled deeper or the graft resecured with a longer loop. The use of intraoperative fluoroscopy can be helpful for assuring the button is flipped outside of the tunnel and is placed in the appropriate position. If it is determined postoperatively that the button is not flipped, the graft will lack adequate stability, and revision surgery is required ( Fig. 8.1 ). Furthermore, the surgeon should be aware if the cortical button is advanced beyond the cortical bone and through the iliotibial (IT) band. If the button is flipped, it can come to rest superficial to the iliotibial band and compromise the rigidity of the graft ( Fig. 8.2 ). A more subtle scenario can occur when soft tissue becomes interposed between the button and the cortex. Although there are data to suggest that 1 to 2 mm of soft tissue between the button and cortex may not compromise graft rigidity, every effort should be made to ensure the button is seated on cortical bone. This complication can also be recognized with the use of intraoperative fluoroscopy. If this occurs, pulling the graft back through the iliotibial (IT) band can be attempted, or a small lateral incision may be made to allow passage of the button back onto the cortical surface. If the continuous loop is too long, an adequate amount of graft may not be present in the tunnel. If the loop is visualized through the scope, it may be divided and the graft repassed with a shorter loop. If less then 15 mm of graft is in the tunnel, but the loop is not able to be visualized, a lateral incision may be required to remove the button and repass the graft with an appropriate length loop.
Adjustable loop devices allow more flexibility in terms of tunnel length and graft preparation. The adjustable loop obviates the need for calculating loop length and allows for a complete fill of the femoral tunnel. Furthermore, these devices allow for adjustment of the loop length after implantation and accommodate a more customizable length of the graft within the bone tunnel. Biomechanical studies have demonstrated adequate load to failure of adjustable loop fixation devices. However, concerns remain that the loops may lengthen with cyclic loading, leading to loosening of the graft and subsequent failure. This concern over loosening may potentially be mitigated by tying a knot over the button with the tensioning sutures, preventing loosening of the loop.
Other complications may also occur with the use of cortical buttons. If the far cortex of the tunnel is breached with the initial reamer, the button may not be able to achieve fixation on the cortex. If this occurs, a variety of techniques may be employed. Many companies make a larger button that can be used to bridge the length of the tunnel and achieve fixation ( Fig. 8.3 ). The graft and button may also be removed, and the graft fixed over a screw post.
Motion of the ACL graft within the tunnel has been shown to be detrimental to graft healing and can lead to tunnel widening. This motion can occur in two planes: the longitudinal and transverse plane. Motion in the longitudinal plane has been described as the “bungee effect,” whereas motion in the transverse plane has been referred to as the “windshield wiper effect.” Biomechanical studies have shown conflicting information on the effect of fixation on the amount of graft motion. Biomechanical studies have suggested that aperture fixation reduces translation of BTB and hamstring tendon (HT) grafts compared with suspensory fixation, whereas a cadaveric and clinical study found no difference between aperture and suspensory fixation. Clinical studies have not shown an advantage of one fixation device over another. In addition, a correlation has not been found between tunnel widening and functional outcome. A metaanalysis of stability after ACLR concluded no stability advantage of aperture fixation compared with suspensory fixation. Based on the current literature, the location of graft fixation does not correlate with clinical outcome.
Femoral transfixation devices achieve suspensory intratunnel fixation through the use of biodegradable or metallic pins which pass through the femoral portion of the graft. Transfixation pins may be used with soft tissue grafts and can simplify revision surgery. Biomechanically, transfixation pins have been found to have the highest load to failure and stiffness when compared with other fixation devices. However, the fixation pin can break, migrate out of the bone, or cause tunnel widening. Complications have also been reported with graft passage and fixation. The graft must be checked for initial stability, and if fixation is not achieved this must be addressed by repeating the steps for preparation of the transfixation pin. The majority of clinical studies have shown satisfactory results with use of transfixation pins. A recent registry study demonstrated the lowest failure rates with metal transfixation pins on the femur and metal interference screw on the tibia.
Suspensory fixation may also be achieved outside the tunnel through screws with spiked washers or screw post devices. The screw and spiked washer achieves fixation of hamstring tendon (HT) grafts by compressing the graft against the cortical bone. Biomechanical testing has demonstrated adequate load to failure; however, concerns remain regarding symptomatic hardware and the removal of cortical bone that may be attached to the device in the revision setting. There is a paucity of clinical studies on the use of screws and spiked washers. A screw post requires the free limbs of sutures attached to the graft to be tied around a screw which functions as the post. Given the length of the construct, concerns are present regarding the stiffness of the graft. Biomechanical studies have demonstrated lower load to failure with screw post fixation. However, a screw post offers an excellent salvage option when other fixation strategies have failed. It may also be used as a backup fixation device for the primary fixation device. The screw post construct offers the ability to troubleshoot and achieve fixation when any of the other fixation techniques go awry.
Aperture Fixation
Aperture fixation is most commonly achieved through the use of interference screws and stabilizes the graft near the joint line. Aperture fixation offers several advantages compared with suspensory fixation. It allows for direct fixation of the ACL graft to the tunnel wall, allows a decreased working length between femoral and tibial fixation devices, restores native footprint dynamics, results in less tunnel enlargement, and results in a decreased risk of symptomatic hardware. Although most commonly used for patellar tendon grafts, interference screws have a long track record of clinical success for both BTB and hamstring tendon (HT) grafts fixation. Interference screws, however, may compress the graft within the tunnel, which may affect its mechanical properties. In addition, in the setting of an all-soft tissue graft, the screw within the tunnel may limit the total volume of graft that is in contact with the tunnel, decreasing its healing potential.
Many different factors can contribute to the stability of aperture fixation, including the length and diameter of the screw, the position of the screw, the relationship of tunnel size to screw diameter and graft size, and bone quality. Fixation often relies on cancellous bone rather than stronger cortical bone. The angle of the screw is also important to consider. If the screw diverges from the trajectory of the tunnel, the strength of the fixation device may be decreased. If the screw diverges by more than 15 degrees, the pullout strength will be significantly reduced. To prevent screw divergence, the knee should be placed in the same position as the tunnel to allow the screw to follow a similar path. The use of a guidewire allows for improved placement of the hardware. However, the use of a guidewire does create the risk for guidewire breakage. The guidewire may be removed before complete insertion of the screw to reduce the risk of breaking. If the wire does break, an attempt may be made to remove the remnant; however, the integrity of the graft should not be compromised, and the tip may be left inside the bone.
Aperture fixation requires a tunnel that is competent on all sides to allow compression between the graft and the surrounding bone. During femoral tunnel creation, the posterior wall may become compromised, and as a result affect interference screw fixation strength. It is critical to recognize violation of the posterior wall and adjust the fixation method accordingly. Suspensory fixation may then be employed with the use of a cortical button or screw post in this situation.
Metal or bioabsorbable interference screws may be used as aperture fixation devices. Metal interference screws have a higher initial fixation strength and load to failure than bioabsorbable screws. Metal interference screws also mitigate the risk of screw breakage, as well as the possible biologic reaction to bioabsorbable screws. Metal screws, however, create a more difficult revision situation requiring hardware removal as well as artifact on postoperative magnetic resonance imaging. Metal interference screws may also lacerate the graft during insertion. A variety of commercially available protection devices are available to protect the graft if there is a concern regarding graft laceration.
Metal interference screws are generally avoided with the use of soft tissue grafts, given the concern over graft laceration and damage. However, the risk of graft laceration has been decreased attributed to dulling of the threads of metal interference screws. In this situation, a bioabsorbable interference screw may be more appropriate. However, there is less engagement with bioabsorbable screws into soft tissue grafts and potentially decreased initial stability. Given the poor engagement of the graft into the bone, increased screw size has less of an impact of initial stability. In these situations, a longer screw increases the total area of the soft tissue graft that is compressed into the bone and represents the best option for increasing stability. A recent systematic review evaluating the use of metal and bioabsorbable interference screws and soft tissue grafts demonstrated similar clinical and functional outcomes between bioabsorbable and metal screws for patellar tendon and hamstring tendon (HT) grafts. However, the authors did note prolonged knee effusions, femoral tunnel widening, and screw breakage with the use of bioabsorbable screws.
The most commonly used bioabsorbable screws on the market are those made with polylactic acid, its isomer poly-L lactic acid, or polyglycolic acid. Bioabsorbable screws offer some advantages compared with metal interference screws. They are less likely to damage the ACL graft during insertion, do not distort postoperative advanced imaging, and allow for simplified revision surgery. However, complications are associated with the use of bioabsorbable screws. The most common complication is screw breakage. This is reported to occur in 0.24% to 10% of cases. Screw breakage typically occurs during screw insertion. If the screw breaks, the stability of the graft fixation should be assessed. If only a few threads of the screw are broken and the remainder of the screw is intact, the screw may be left in place. If a large portion of the screw is broken or fixation is inadequate, the screw should be removed and a new fixation device should be used. A screw post or cortical button may be employed. If the screw cannot be removed, an attempt can be made to place a second screw in a separate location. The issue of screw breakage has been improved by changes to the screws and screwdrivers. The tunnel should be tapped or notched to decrease the risk of fracture of the screw.
Bioabsorbable screws are also associated with an inflammatory reaction that may be caused by the large number of acidic particles that are created during screw breakdown. Bioinert materials have been developed to decrease the local inflammatory reaction. Polyether ether ketone is a bioinert material with a modulus of elasticity close to cortical bone and does not resorb over time. Finally, bioabsorbable screws also have a higher cost and lower mechanical strength compared with metal interference screws. Overall, both metal and bioabsorbable interference screws have been used with no difference in clinical outcomes, failure rates, or radiographic tunnel dimensions.
Recent advances in bioabsorbable implants have led to the creation of biocomposite implants. A biocomposite implant combines a standard bioabsorbable screw with a bioceramic such as betatricalcium phosphate or hydroxyapatite. The addition of a ceramic material provides an osteoconductive scaffold to allow bone growth. Depending on the composition of biocomposite screws, they may be prone to similar reactions as the bioabsorbable screws. To date, few clinical data are available regarding the use of biocomposite screws.
Different techniques are also available for inserting interference screws on the tibia side. The graft should be sutured along the length of the planned fixation area to decrease graft slippage and increase load to failure. The screw may be placed concentrically or eccentrically within the tibial tunnel. Concentric screw placement involves placing the screw down the center of the graft, which theoretically increases the contract surface between the graft and bone tunnel. However, no differences have been seen in ultimate failure or graft slippage.
As stated previously, tibial-sided fixation of soft tissue grafts creates challenges caused by the line of pull on the graft, the decreased bone mineral density of the proximal tibia, the fixation of free tendon ends, and the longer healing time required for soft tissue grafts. As a result, screw and sheath devices have been created. These devices employ the use of an expansion screw inside a four-channeled sheath to separate and grip each tendon individually and compress the tendon into the bone. The use of these devices requires a tibial tunnel that is at least as long as the sheath. If the tibial tunnel is shorter than the sheath, this technique cannot be employed. The sheath device may also be inserted too deeply, which results in misplacement of the screw ( Fig. 8.4 ). Given the design of the sheath, it is often very difficult to remove the sheath from the tibial tunnel without significant damage to the graft. As a result, it is typically easier to remove the device by passing it up the tibial tunnel into the knee joint and removing it through one of the portals, with subsequent sheath reinsertion. Problems can also arise with lack of screw purchase or breakage. If this occurs, the sutures may be secured over a post or staple. If the graft is too short within the tunnel, the sheath device will become malpositioned with resulting loss of fixation. In this situation, a different fixation method should be employed, such as interference screw fixation alone or a fixation post.
