Technique

Creation of an appropriately located anteromedial portal is the most essential, primary step in ACLR surgery that uses the AMP technique (Fig. 47-1). Although some favor the use of an accessory AMP, we prefer instead to use a single AMP that is slightly more inferior than the standard portal in ACLR. The only exception to this approach is the need to perform a concomitant procedure that requires standard portal placement, such as meniscal repair, in which case two AMP portal incisions may be made. In this scenario, the first portal is established 1 to 2 mm inferior to the inferomedial pole of the patella and the second, femoral tunnel–creating AMP is 1 to 2 mm superior to the superior rim of the tibial plateau. Arthroscopic visualization of AMP creation from a standard anterolateral portal (ALP) is advised to avoid damage to the anterior horn of the medial meniscus, given the relatively inferior position of the AMP. In addition, some surgeons have recommended a more medial position of the portal compared with the AMP placement typically used in ACLR. However, we have found that damage to the cartilage of the medial femoral condyle can be a significant complication that is best avoided with AMP placement 2 to 3 mm medial to the medial edge of the patellar tendon.

Figure 47-1 A, In some knees, where the anatomic footprint for the femoral tunnel cannot be reached using transtibial (TT) technique, or in certain revision cases, use of the AMP for guide wire and reamer advancement may allow for optimal tunnel position. B, An anterior view of the knee demonstrates the angle of the femoral tunnel when the AMP is used to establish the tunnel, relative to the TT technique.

Following standard diagnostic arthroscopy and débridement of the torn ACL, a notchplasty may be performed, but we have found this necessary only in the minority of cases with abnormally narrow notches, less than 15 mm in width. Given the relatively inferior position of graft placement on the femoral condyle, compared with traditional transtibial technique, graft impingement is rarely encountered. The posterior aspect of the soft tissue at the ACL footprint on the tibial surface is used as a landmark for tibial tunnel creation, in conjunction with the posterior aspect of the anterior horn of the lateral meniscus. We prefer to completely débride the soft tissues and mark the center of the footprint with the electrocautery device or a small curette prior to insertion of a standard ACL guide. Following standard tibial tunnel reaming and use of the motorized shaver to eliminate bony debris, a reverse chamfer drill is used to smooth the posterior intra-articular edge of the tibial tunnel to prevent bony abrasion of the graft during cyclic knee flexion.

A similar approach as described for the tibial footprint is used to identify and mark the center of the femoral ACL footprint. The soft tissues are then completely débrided from the lateral wall of the intercondylar notch while preserving the mark for the center of the footprint. An arthroscopic probe is used to identify the back wall of the femoral condyle definitively to avoid back wall blowout. The AMP is used to introduce the offset femoral guide and the guide wire as a unit past the medial femoral condyle, just as Cain and colleagues⁸ initially described introduction of the guide wire and reamer as a unit. The knee must be hyperflexed 110 to 120 degrees to allow the trajectory of the guide wire directly into the center of the femoral footprint. Alternatively, flexible guide pins and reamers have been introduced in an effort to avoid the need for hyperflexion, minimize articular cartilage damage on the medial femoral condyle, and allow the length of the femoral tunnel to be maximized via a more proximally directed orientation. The guide wire is advanced to the level of the anterolateral femoral cortex and the offset guide is removed. A second guide wire is introduced through the AMP to the femoral footprint, just adjacent and parallel to the first, to allow for measurement of the approximate length from footprint to cortex to ensure adequate tunnel length. If insufficient tunnel length is anticipated, the angle of the guide wire can be altered to increase tunnel length or other techniques to address mismatch can be planned, such as slight shortening of one or both bone plugs, depending on the estimated length. The second guide wire is removed and the reamer is then introduced into the notch under arthroscopic visualization, taking care to avoid damage to the MFC cartilage by the edges of the reamer.

Provided the angle of knee flexion is not changed and the trajectory of the guide wire maintained, we have found the risk of damage to the cartilage or bending of the guide wire to be minimal. In addition, the 30-degree arthroscope may be replaced with a 70-degree arthroscope if adequate visualization of the femoral footprint cannot be achieved with instrumentation in the notch, although this is not necessary in most cases. The reamer is advanced 5 to 10 mm into the femoral ACL footprint and withdrawn slightly to allow for reassessment of the adequacy of the back wall, with a goal of 1 to 2 mm of intact posterior bone. The reamer is then advanced to the appropriate depth, which varies according to graft type and graft length. The guide wire–reamer unit is removed. A Beath pin with a looped passing suture is introduced through the AMP into the notch, and the knee is again hyperflexed, with direct assessment of avoidance of contact between the Beath pin and MFC before advancement into the femoral tunnel. The pin is passed through the skin of the anterolateral thigh. The loop of the passing suture is left in the notch, an arthroscopic grasper is introduced through the tibial tunnel, and the passing is suture brought out of the tibial tunnel.

Graft passage is performed in standard fashion, with free sutures on the femoral side of the graft having been fed through the looped passing suture. An arthroscopic probe or grasper is used to orient the femoral bone block of the graft in the proper trajectory for smooth advancement into the femoral tunnel. Graft fixation is performed in standard fashion, with a femoral interference screw passed through the AMP over a nitinol wire. Care must be taken to advance the screw into the tunnel with the knee in the same degree of hyperflexion that was used during femoral reaming. This avoids the complication of graft-screw divergence that has been reported for the AMP technique. Standard cycling of the graft and tibial interference screw fixation, with the knee in full extension and maximal manual traction on the graft, is then performed. A routine approach to wound closure is used.

Discussion

Use of the anteromedial portal in ACL reconstruction has the advantage of allowing for placement of a femoral tunnel in a more anatomic location than that seen with classic transtibial techniques. It can be particularly useful in revision surgery, in which the primary surgery may have involved placement of a more vertical femoral tunnel (e.g., at 11:00 or 1:00 o’clock, if not higher). Not only can a vertical primary position be responsible for graft failure through retear or persistent rotational instability, but the more anatomic placement may be performed without significant primary graft or tunnel débridement, interference screw removal, or bone grafting. In addition, use of the AMP has gained interest because of the growing popularity in double-bundle surgery, in which a more complex tibial tunnel configuration may warrant great flexibility in femoral tunnel placement, as is afforded by the AMP technique.

Despite its advantages in revision or double-bundle procedures, use of the AMP may have its greatest role as a new standard technique in primary ACL reconstruction, given the increasingly recognized importance of femoral tunnel position on restoration of native knee kinematics.^10,39,40 Despite the technical challenges associated with its use, complications can be avoided by a thorough understanding of the potential pitfalls and technical principles. Critical to success with AMP techniques are an understanding of native footprint anatomy, appropriate inferior AMP placement, introduction and advancement of instruments into the joint and notch under arthroscopic visualization, meticulous measurements of graft and tunnel length, and experience with appropriate flexion and hyperflexion angles of the knee for the different portions of the procedure. Although more clinical outcomes studies related to use of this technique are warranted, early, lower level evidence, cadaveric studies, and descriptions of its technique have been favorable.^4,5,12,14,19

It remains unclear how widespread AMP use will be in the future, but we believe that it should become a technique familiar to all surgeons performing ACLR, especially in the revision setting. One approach favored by many surgeons for primary ACLR is creation of the tibial tunnel and assessment of potential femoral tunnel positioning through the transtibial tunnel. Because even minute variations in knee anatomy and tibial tunnel position can influence the ability to achieve anatomic placement of the femoral tunnel, this step allows for use of the AMP technique at this time if the transtibial approach does not allow for optimal graft placement. In the senior author’s experience, an optimal femoral tunnel can often be achieved transtibially, and the transtibial approach can be used for the single-tunnel, single-bundle technique and the single-tunnel, double-bundle technique, as will be described.