Anatomic Graft Placement in ACL Surgery: Plain Radiographs Are All We Need

The anterior cruciate ligament (ACL)

Intra- and postoperative plain radiographs are a reliable and effective way to assess anatomic ACL graft placement.

The restoration of native anatomy is one of the fundamental principles of orthopedics, pervasive throughout all of the various subspecialties. Surgeons have learned that the restoration of native anatomy is of paramount importance to any reconstructive procedure. One of the most commonly used instruments to assess bony anatomy is the standard radiograph. Traumatologists use radiographs pre-, intra-, and postoperatively to determine and evaluate fracture treatment because the success of a procedure depends on anatomical reduction. The outcome of a fractured acetabulum in a young person is determined by the ability of the surgeon to reproduce native bony anatomy within 2 mm of perfection. Should cruciate ligament surgeons not strive for the same?

Anterior cruciate ligament (ACL) injury and subsequent reconstructive surgery is common in the United States. For this reason, the anatomy, biomechanics, and reconstructive techniques have been, and continue to be, intensely researched and debated topics. Recent investigations have better defined gross, microscopic, and insertion site anatomy, which has improved radiologic and arthroscopic assessment of reconstructive techniques.1-7

It is a generally accepted principle that tunnel position/graft placement is the most critical component to a successful ACL reconstruction. Nonanatomic femoral and/or tibial tunnel placement can lead to poor clinical results.8-10 Pre-, intra-, and postoperative use of radiographic tunnel assessment to help the ACL surgeon reproduce normal anatomy have all been described.6,7,10-12 This article describes simple arthroscopic and radiographic landmarks for anatomic ACL reconstruction.

Femoral Tunnel

Coronal Placement

Coronal placement of the femoral tunnel is a topic that has come under recent scrutiny. It has been demonstrated that femoral tunnel placement at a more oblique angle vs a more vertical angle provides for greater rotational control (elimination of the pivot-shift).13,14 Furthermore, a more oblique location results in improved clinical outcome.15 The ideal location for the femoral tunnel as viewed on an anteroposterior radiograph is illustrated in Figure 1. Arthroscopically, bony landmarks can be referenced for proper tunnel placement. The tunnel should be drilled inferior to the intercondylar ridge (Figure 2).1,2The bifurcate ridge is another landmark that may be used as a reference point.3 Controversy remains about the specific technique that is best to achieve this tunnel placement (transtibial, medial portal, or 2-incision).

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Figure 1: AP radiograph showing anatomic locations for tunnel placement. Figure 2: Arthroscopic view of femoral tunnel position inferior to the intercondylar ridge (demarcated).

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Sagittal Placement

Sagittal placement of the femoral tunnel is the most important factor affecting postoperative graft isometry.16 Small variations in the anterior to posterior location of the femoral tunnel can result in significant differences in graft kinematics.16 It has also been shown to significantly affect International Knee Documentation Committee score.8 Excessive anterior placement of the tunnel is the most common technical mistake.17 This can commonly occur in the transtibial technique as the tibial bone tunnel may inhibit proper anatomic placement. Figure 3 demonstrates where the tunnel should appear on a lateral radiograph. Since the femoral footprint of the native ACL does not attach to the roof of the intercondylar notch,1-4,7 which is represented by Blumensaat’s line on a lateral radiograph, no part of the tunnel should intersect Blumensaat’s line.

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Figure 3: Lateral radiograph showing anatomic locations for tunnel placement.

Tibial Tunnel

Coronal Placement

Arthroscopic anatomic landmarks including the posterior cruciate ligament (PCL), the anterior horn of the lateral meniscus, the medial tibial eminence, and the ACL stump have all been used to determine intra-articular tunnel location.18,19 The center of the tibial tunnel should be directly medial to the anterior horn of the lateral meniscus. Extra-articularly, the tunnel should originate 1.5 cm medial to the medial margin of the tibial tubercle and 1 cm above the superior margin of the pes anserine.19 Radiographically, the extra-articular starting point, as described, and the intra-articular location, on the anterolateral slope of the medial tibial eminence, can be evaluated (Figure 1).20

Sagittal Placement

The sagittal location of the tibial tunnel is an important factor for successful reconstruction that is often not discussed. Excessive anterior placement of the tibial tunnel can lead to graft impingement and magnetic resonance imaging signal changes within the graft.10 Transtibial techniques frequently place the tunnel too posterior to avoid anterior and vertical femoral tunnel placement. The arthroscopic view of the anatomic location of a tibial tunnel is shown in Figure 4. Of note, the entire tunnel is approximately 1 cm anterior to the traditional nonanatomic placement popularized in the 1990s. The corresponding location on a lateral radiograph is demonstrated in Figure 3. The entire opening of the tibial tunnel at the joint line must be anterior to the medial tibial eminence.

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Figure 4: Arthroscopic view of tibial tunnel position with guide wire (A) and reamer (B).

Conclusion

Similar to the way a couple of millimeters in an articular reduction can make a significant difference in patient outcome, improper ACL graft placement may also have detrimental effects on patient outcome and cause recurrent instability. Intra- and postoperative plain radiographs provide a reliable and effective way to assess anatomic graft placement. As we strive to improve outcomes in ACL surgery by doing more precise anatomic reconstruction, similar to fracture surgery, intraoperative fluoroscopy or postoperative radiographs can be used to improve and evaluate their specific techniques for tunnel placement (Figures 5, 6). Whether the transtibial accessory portal or 2-incision outside-in technique for femoral drilling is used, the goal should be the same: restoration of native anatomy. The use of simple arthroscopic and radiologic landmarks can help surgeons be more precise during their personal learning curve.

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Figure 5: Postoperative AP radiograph after anatomic bone–patellar tendon–bone reconstruction. Figure 6: Postoperative lateral radiograph after anatomic bone–patellar tendon–bone reconstruction.