The anterior cruciate ligament (ACL) consists of dense connective tissue enveloped in a synovial membrane, which places the ligament in an intra-articular but extrasynovial position.^2,⁵ It attaches proximally on the posterior aspect of the lateral femoral condyle (LFC) and runs in an oblique course distally through the intercondylar notch to insert between the medial and the lateral tibial spines. Many authors have studied the ACL bundle anatomy and reached a general consensus that the ACL consists of two bundles: an anteromedial (AM) bundle and a posterolateral (PL) bundle (Fig. 10-1A). The AM bundle is slightly larger in diameter than the PL bundle. The bundles are named for their relative positions on their tibial insertion sites⁴ (see Fig. 10-1B). Recently, the two-bundle anatomy was also verified in a fetal study by Ferretti and coworkers.⁶

FIGURE 10-1 The anterior cruciate ligament (ACL) consists of two functional bundles, the anteromedial (AM) bundle and the posterolateral (PL) bundle. A, Arthroscopic view of the AM and PL bundle. B, Cadaveric dissection of tibial AM and PL insertion site.

The authors have studied the bony topography of the femoral attachment of ACL extensively. Using fetal specimens, cadavers, and in vivo arthroscopic observation, we have identified two osseous ridges that define the origins of the AM and PL bundles. The lateral intercondylar ridge runs proximal to distal through the entire ACL femoral attachment. With the knee in extension, no fibers of the ACL are attached anterior to this ridge. A second osseous ridge, the lateral bifurcate ridge, divides the femoral attachments of the AM and the PL bundles. It is important to note that when the knee is in full extension (anatomic position), the femoral origin of the AM bundle is located at the posterior and proximal portion of the lateral intercondylar wall, whereas the origin of the PL bundle is located slightly distally. The two bundles are parallel in extension. As the knee is flexed to 90°, which is the typical position during ACL reconstruction, the origins of the two bundles change from a vertical alignment to a horizontal alignment and the bundles cross.⁴ The lateral intercondylar ridge delineates the superior border of ACL femoral attachment (Fig. 10-2), whereas the lateral bifurcate ridge runs from superior to inferior and separates the femoral AM and PL attachments.

FIGURE 10-2 The lateral intercondylar ridge (arrows) marks the superior aspect of the femoral insertion sites of the AM and PL bundles when the knee is in 90° of flexion. The lateral bifurcate ridge (arrowheads) separates the insertions between the two bundles.

Biomechanically, the two bundles are not isometric throughout the range of knee motion. Generally, the AM bundle maintains at a constant level of tension throughout the range of motion, with some increase when the knee is flexed that reaches a maximum at 60°.⁷ The tension of the PL bundle is more variable because it tightens in knee extension and slackens in flexion past 30°. Thus, the AM and PL bundles have varying contributions to knee stability at different flexion angles. The AM bundle limits anteroposterior (AP) translation throughout knee motion, whereas the PL bundle plays an important role in limiting not only anterior tibial translation but also rotation.^3,^7,¹⁰

Biomechanical studies have emphasized the importance of both bundles in knee stability. Yagi and associates ¹³ showed that double- bundle (DB) ACL reconstruction better restores knee biomechanics than single-bundle (SB) ACL reconstruction. The addition of a PL bundle produces in situ forces within each bundle that closely match the in situ forces found in a native ACL ligament. Tashman and colleagues¹¹ studied the in vivo kinematics after ACL reconstruction with the use of high-speed stereoradiography. This study demonstrated that SB ACL reconstruction sufficiently restored AP tibial stability. An unexplained increase of 3° to 4° in adduction and external tibial rotation was reported. Zantop and coworkers¹⁵ showed that isolated transection of the AM bundle increased anterior tibial translation at 60° and 90° of knee flexion significantly, whereas isolated transection of the PL bundle significantly increased anterior tibial translation at 30° of flexion. In addition, PL bundle transection led to significantly increased rotation at 0° and 30° in response to a combined rotatory load when compared with the intact knee and the AM bundle–deficient knees. This study supports the concept that SB (AM bundle) reconstruction cannot restore native knee stability, particularly rotatory stability.

Primary Knees

DB ACL reconstruction is indicated for acute or chronic ACL ruptures that cause symptomatic instability. The criteria for DB ACL reconstruction are (1) a history of instability, shifting, or giving-way with activities; (2) positive Lachman and pivot shift tests; (3) increased KT-1000 measurement compared with the contralateral knee; and (4) patient desires to undergo ACL reconstruction. Whereas some surgeons reserve the DB procedure for patients with global ligamentous laxity or very large pivot shifts, the authors currently advocate the procedure for most patients with clinical instability from an ACL tear.

Revision Knees

DB ACL reconstruction as a revision procedure may be indicated when an SB ACL reconstruction has failed owing to graft rupture or when an SB graft is intact but does not provide adequate stability. In the latter case, depending on the position of the original tunnels, the surgeon may either augment the intact SB graft with an additional graft or implant two new grafts. In the authors’ practice, patients frequently present with an intact SB graft but complain of clinical symptoms of instability and demonstrate a positive pivot shift on examination. In these cases, there is often a mismatch in the femoral and tibial tunnel placements. For example, the tibial tunnel occupies the position of the PL bundle and the femoral tunnel is located in a high position in the notch above the AM bundle attachment (Fig. 10-3). This pattern of mismatch is often a result of the transtibial technique for femoral tunnel preparation, in which the femoral tunnel position is dictated by the tibial tunnel position. The graft in this case may provide stability in the AP plane but does not provide adequate rotational stability and results in symptomatic instability.

FIGURE 10-3 This sagittal magnetic resonance imaging (MRI) scan shows an example of tunnel mismatch. The tibial tunnel occupies the position of the PL bundle. The femoral tunnel is closer to the attachment of the AM bundle.

A nonanatomic tunnel placement, or tunnel mismatch, may lead to graft laxity and ultimate failure. In the laboratory, increased osteoclastic activity has been observed at the graft-bone interface in such mismatched tunnels (Fig. 10-4). When a graft is placed at a nonanatomic position relative to the native ACL, it experiences large tensile forces secondary to this abnormal position. These abnormal forces compromise biologic healing and ultimately may contribute to graft laxity and failure.

FIGURE 10-4 Osteoclasts are shown at the graft–bone tunnel interface. We have observed increased osteoclastic activity at the graft-bone interface in cases of tunnel mismatch.

CONTRAINDICATIONS

Primary Knees

There are certain contraindications to DB ACL reconstruction. Patients with ACL rupture who do not have symptomatic instability and do not participate in cutting and pivoting sports may choose not to undergo ACL surgery, because they can function well without an intact ligament. Patients with a limitation of knee motion and flexion contractures must first be rehabilitated appropriately with aggressive physical therapy before consideration of DB ACL reconstruction. Patients with active infection are not candidates for reconstructive surgery. Skeletally immature patients are not candidates for DB ACL reconstruction owing to the risk of growth arrest from transphyseal tunnel drilling.

Revision Knees

Contraindications to DB ACL reconstruction in the revision setting include active infection and skeletal immaturity. Excessive tunnel widening and bone loss from prior ACL reconstruction surgery necessitates staging of the revision procedure. In these cases, bone-grafting of the femoral and tibial tunnels is first performed. After the bone graft has healed, revision DB ACL surgery is accomplished.

CLINICAL EVALUATION

Primary Knees

The first portion of the clinical evaluation consists of obtaining a thorough history from the patient. The timing and description of the injury mechanism should be reviewed carefully because this information may raise suspicion of injury to particular structures depending on the direction of force to the knee. The patient should be asked if he or she felt or heard a pop at the time of the injury. In addition, the development of an effusion and difficulty with weight-bearing should be noted. Episodes of recurrent shifting or instability are important to document, including the situations in which they occurred. Any previous treatment of the injury such as physical therapy or bracing should be documented. The patient’s preinjury activity level should be determined to assess if he or she engages in activities, such as soccer or basketball, that require cutting and pivoting motions. Any previous history of knee injury or knee surgery should be explored.

The second portion of the clinical evaluation is a comprehensive physical examination. Weight-bearing alignment is evaluated for any valgus or varus deformity. Before the patient sits or lies on the examining table, it is important to assess her or his gait for the presence of a limp and note whether any assistive devices are required for ambulation. With the patient supine, the presence and size of an effusion is noted. Quadriceps function and atrophy are assessed, including the patient’s ability to perform a straight leg raise without an extensor lag. Joint range of motion is measured and compared with that of the contralateral knee. The patellofemoral joint is examined for apprehension, crepitation, and facet tenderness. The ligament examination includes the Lachman, pivot shift, anterior drawer, posterior drawer, and valgus and varus laxity tests. The ligament examination is performed bilaterally for comparison. Joint line tenderness and the McMurray flexion test are done to detect meniscal pathology. KT-1000 testing of both knees is performed and the difference in millimeters recorded.

When an ACL injury is suspected, diagnostic studies are ordered. Radiographs include a weight-bearing posteroanterior flexion view, a lateral view of the injured knee, and a Merchant view of both knees. In patients with abnormal alignment or previous surgery or who are older, long-leg cassette films are obtained to quantify malalignment. Radiographs are scrutinized for joint space narrowing, osteochondral lesions, fractures, and alignment abnormalities.

The other diagnostic study of importance is magnetic resonance imaging (MRI). At the authors’ center, a special protocol to study the ACL consists of a series of images in the plane of the two bundles of the ACL (Fig. 10-5A) in addition to the usual coronal, axial, and sagittal images. This provides improved visualization of the individual AM and PL bundles, allowing identification of individual bundle ruptures (see Fig. 10-5B) and the location of the rupture as either off the femur or the tibia or in mid-substance. In addition, assessment is performed of the condition of the other knee ligaments, menisci, cartilage, and bony structures.

FIGURE 10-5 A, An example of the special MRI scan in the oblique plane of the ACL clearly demonstrates the presence of intact AM and PL bundles. B, MRI scans in the plane of the ACL allow identification of individual bundle rupture. In this case, the PL bundle is intact and the AM bundle is torn.

Revision Knees

Clinical evaluation of a patient with a failed previous ACL reconstruction is identical to that of a patient with a primary ACL rupture, including a complete history and comprehensive physical examination. It is important to ascertain the mechanism of graft failure as well as the timing in relation to the primary procedure. Some patients will recall a specific episode in which the knee was reinjured. Others will not recall a reinjury but will state that the knee never felt stable even after the initial ACL reconstruction. Another important component of the history includes the operative report from the primary surgeon to identify the type of graft used, fixation methods, and tunnel widths.

A complete physical examination is performed, as previously described. The surgeon must pay specific attention to previous incisions and sites of graft harvest in order to plan the revision appropriately. In some cases, the Lachman and KT-1000 measurements are not as impressive as the pivot shift, indicating the presence of a nonanatomic graft that provides AP stability but not rotational stability.

Radiographs are required to determine fixation methods used in the primary procedure (Fig. 10-6A). Joint space narrowing, osteochondral lesions, and tunnel width are identified. MRI is performed to determine whether the original graft is intact or ruptured, assess tunnel location and width (see Fig. 10-6B and C), and identify additional pathology involving the cartilage or menisci.