Evolving Concepts in Tunnel Placement for Acl Reconstruction

Chlodwig Kirchhoff

Peter U. Brucker

Andreas B. Imhoff

Rupture of the anterior cruciate ligament (ACL) is one of the most frequent injuries of the knee joint in nowadays field of orthopedic surgery (1, 2). The ACL consists of at least two functional bundles (3, 4 and 5) with its unique stabilizing effect represented by the anteromedial (AM) bundle for predominantly anterior stability and the posterolateral (PL) bundle for predominantly rotational stability. A complete rupture of the ACL usually results in a decreased stability in both the anterior-posterior (a-p) and the rotational axis, which can be tested during the clinical examination performing the two main examination maneuvers, the Lachman test (6, 7 and 8) and the pivot-shift test (9, 10),

The standard operation technique for ACL reconstruction continues to evolve over the last decades. Advances in the treatment of ACL injuries include extensive investigations of injury, technical improvements providing more anatomic reconstructions, considerations of the relative success rate of the variety of graft options, and effects of different rehabilitation and training methods.

Single-bundle ACL reconstruction techniques were routinely performed over the last decades and progressively modified in an anatomical manner performing more centrally placed tunnels within the widely spread anatomical footprint of both bundles at the femur as well as the tibia to maintain adequate anterior and rotational stability. Recent attempts have focused on recapitulating the AM and PL bundles using a variety of double-bundle techniques to improve the results of surgical reconstruction of the ACL. These double-bundle techniques were introduced to respect the complex anatomy of the native ACL more closely.

Independently of single- or double-bundle ACL reconstruction techniques, the most critical factor for successful ACL reconstruction is the correct placement of the femoral and tibial tunnel(s). Indeed, incorrect placement of the tunnels may result in loss of extension and/or flexion causing improper graft tension resulting in premature graft failure by elongation or rerupture. Due to relevant alterations in knee kinematics, long-term consequences of incorrect tunnel placement may induce early increased cartilage degeneration and osteoarthritis. The essential factor for a successful ACL reconstruction is graft fixation within the tunnels (11). Various anatomic aperture and extra-anatomic fixation techniques have been described. Combinations of anatomic and extra-anatomic fixation techniques were introduced as hybrid techniques. All fixation techniques include advantages and disadvantages.

Typical potential disadvantages such as graft-tunnel motion (12), windshield wiper effect (13), and/or suture stretch-out (14) might result from indirect extra-anatomical and extra-articular fixation techniques (11).

Since various rupture patterns of the ACL have been recognized (15), these different rupture types might need differentiated reconstruction techniques especially when autologous hamstring tendon graft techniques were considered. This seems of importance since the hamstring muscles are known for protecting the ACL graft by their synergistic function (16). Thus, the logic consequence is an implementation of a concept of differentiated tunnel placement with hamstring autografts.

CLINICAL EVALUATION

Physical Examination

The clinical evaluation of the joint is important in the diagnostic phase of pathology and in the assessment of surgical technique. The appraisal of ACL reconstruction is usually based on objective clinical examinations; many tests are dedicated to objectively determining an ACL lesion, each with its own specificity and sensitivity. In this context, the Lachman test and the pivot-shift test are the most important clinical tests for detection of an ACL rupture. For performing the Lachman test, one hand fixes the distal femur of the patient laying in a supine position, whereas the knee is flexed at approximately 20°. The other hand pulls the proximal tibia in an anterior direction. If the tibia reacts like a drawer anteriorly with no abrupt stopping, a rupture of the ACL is presumable (6). In some cases, 4 to 6 weeks following an ACL injury, the Lachman test may be more or less negative, caused by scar tissue among the stumps of the ACL (17).

The pivot-shift test is another manual test to identify ACL ruptures or relevant underlying laxity of the knee joint. The test’s name comes from the pivot (axis), shift (dislocation) and was first described by the group of Macintosh (10) in 1972. To perform this test, the patient lies in a relaxed supine position. The examiner takes the heel with one hand by slightly rotating the shank internally, whereas the other hand is used to conduct a valgus stress to the tibial head. In case of an ACL rupture, the tibial head slides anteriorly in a subluxation position. At 30° to 40° knee flexion, the tibial head jumps back into the physiologic position. Normally, this appears with a snapping sensation or a visible bounce emitted by the iliotibial band sliding over the lateral femoral epicondyle to its posterior location. If the examiner feels this effect, the test is positive with a high specificity of a ruptured ACL (18). The test can be falsified, if a medial-capsule-ligament instability or a ruptured iliotibial band is present (9, 10).

The drawer test is also used to examine the ACL’s integrity. The patient is placed in a supine position on the table with a 90° flexed knee and 45° flexed hip joint. The examiner places the hands around the proximal tibia with the thumbs crossing the anterior joint line. The patient’s foot is anchored in a neutral position by the examiner’s thigh. The examiner tells the patient to relax the hamstrings. Once the patient is relaxed, the examiner attempts to pull the tibia anteriorly. Instability is determined by examining both sides and comparing the amount of present excursion. Overall, however, this test is not as sensitive as the aforementioned tests.

Imaging

Preoperative Examination

The magnetic resonance imaging (MRI) examination of the injured knee has been established as the gold standard imaging examination to identify musculoskeletal pathologies in acute or chronic knee damage. The cruciate ligaments are located intracapsularly and extrasynovially. Acute ACL tears are mostly associated with hemarthros (19). For the routine MRI of the knee, the positioning of the knee is in 10° to 15° external rotation, performing 3-mm-slice-thick images. On MRI, the collateral damage of the knee in terms of meniscal lesions, additional ligamentous injuries, and bone bruise can be detected as well and may lead to different therapy. In this context, the detection of a bone bruise can be helpful and is most commonly cumulated, deeper, and more intense in the lateral compartment after ACL rupture and persists at least for 4 months (20, 21). The appearance of a deep sulcus in the lateral femur condyle on MRI in patients with torn ACLs is deeper (>1.5 mm) compared with the sulcus of ACL sufficient knees (1.2 mm). The deep sulcus sign of the lateral condyle is used as an indirect sign of a torn ACL (22) in cases where the MRI do not allow for a detailed view of the ACL.

Postoperative Examination

Misplaced tunnels cause unfavorable results, therefore, precise postoperative analysis of tunnel placement is essential. Conventional radiographs in two planes represent the standard method for assessing the position of both the femoral and the tibial tunnels in a-p and lateral projections. If a bone-tendon-bone graft is used, the bone blocks within the tunnels and metal implants such as interference screws or crosspins are visible. However, in the case of exclusive soft-tissue tendon grafts, such as the hamstrings, tunnel, and graft positions are more difficult and often impossible to assess.

MRI in contrast allows for a precise assessment of tendon grafts and their position to anatomical landmarks. Determination of the femoral and tibial tunnels is hardly susceptible to mistakes. The relationship of the tendon grafts to the surrounding bone and the posterior cruciate ligament (PCL) can be evaluated three dimensionally. Impingement of the graft against the roof (notch impingement), the lateral femoral condyle (lateral impingement), or the PCL can be diagnosed directly on the sagittal, oblique coronal, or horizontal scans, respectively. This means that those patients may be reoperated early (e.g., by a notchplasty) before graft failure occurs. In addition, the position of bioabsorbable fixation devices can be detected, which is limited using conventional radiographs.

TIMING AND TECHNIQUE

The treatment of ACL ruptures depends on different patients’ factors such as age, profession, sporting activity, comorbidities, and compliance. The single-bundle ACL reconstruction techniques were performed in the beginning and also modified performing more centrally placed tunnels within the widely spread anatomical footprint of both bundles at the femur as well as the tibia.

Recent attempts to improve the results of surgical reconstruction of the ACL have focused on recapitulating the AM and PL bundles using a variety of double-bundle techniques. These double-bundles techniques were introduced to be able to respect the complex anatomy of the native ACL more closely. The advantages of the double-bundle technique include an improved stability of the knee joint, especially for internal rotation of the tibia, less bone loss due to smaller tunnels, but also aperture fixation to prevent tunnel enlargement. However, this rather novel technique incorporates some disadvantages such as higher surgical and technical demands on the surgeon, higher implant costs due to the insertion of four bioresorbable interference screws and the possibility of more complicated revision surgery.

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