Introduction

Restoration of the native anatomy and performing individualized anterior cruciate ligament (ACL) surgery are critical to a successful outcome after ACL reconstruction. The surgeon needs to consider multiple anatomic factors, such as size and shape of the tibial and femoral insertion sites, notch characteristics, and the rupture pattern of the ligament. The ultimate goal is to restore the patient’s native anatomy to that prior to injury. In 10-40% of cases of ACL injuries, an isolated single-bundle tear of either the anteromedial (AM) or posterolateral bundle (PL) is present. As each bundle has its own unique biomechanical properties, reconstructing both bundles when only one is torn would not optimize the patient’s native knee kinematics. Depending on the degree of injury involvement of the remaining bundle, an isolated augmentation of the injured bundle may be considered. This chapter outlines techniques for isolated AM- or PL-bundle augmentation surgery for two scenarios: (1) traumatic single-bundle injury and (2) patholaxity after prior ACL reconstruction in the setting of a healed, well-placed ACL graft. The reader is referred to other chapters within this book for a more detailed description of the ACL anatomy and double-bundle reconstruction technique.

Background

Anatomic studies demonstrate that the AM bundle becomes tight in knee flexion while the PL bundle becomes tight with knee extension. Under a combined rotatory load of 10-Nm valgus and 5-Nm internal tibial torque, the in situ force of the PL bundle is higher at 15 degrees of flexion and lower at 30 degrees of flexion. The in situ force in the AM bundle is similar at 15 degrees and 30 degrees of knee flexion. These biomechanical properties correlate with varying contributions to knee stability depending on the degree of knee flexion. Furthermore, the likelihood of single-bundle ACL ruptures also varies depending on the position of the knee at time of injury.

Despite some early healing after injury, the majority of patients never return to their preinjury level of activity or sport. More than 30% of patients have fair or poor results at final follow-up. Seventy-two percent of partial ACL tears have activity-related symptoms at early follow-up. Noyes et al. followed 32 patients with partial ACL tears and found that 12 went on to complete rupture. Given these disappointing results, augmentation surgery may offer a more definitive solution.

Introduction

Restoration of the native anatomy and performing individualized anterior cruciate ligament (ACL) surgery are critical to a successful outcome after ACL reconstruction. The surgeon needs to consider multiple anatomic factors, such as size and shape of the tibial and femoral insertion sites, notch characteristics, and the rupture pattern of the ligament. The ultimate goal is to restore the patient’s native anatomy to that prior to injury. In 10-40% of cases of ACL injuries, an isolated single-bundle tear of either the anteromedial (AM) or posterolateral bundle (PL) is present. As each bundle has its own unique biomechanical properties, reconstructing both bundles when only one is torn would not optimize the patient’s native knee kinematics. Depending on the degree of injury involvement of the remaining bundle, an isolated augmentation of the injured bundle may be considered. This chapter outlines techniques for isolated AM- or PL-bundle augmentation surgery for two scenarios: (1) traumatic single-bundle injury and (2) patholaxity after prior ACL reconstruction in the setting of a healed, well-placed ACL graft. The reader is referred to other chapters within this book for a more detailed description of the ACL anatomy and double-bundle reconstruction technique.

Background

Anatomic studies demonstrate that the AM bundle becomes tight in knee flexion while the PL bundle becomes tight with knee extension. Under a combined rotatory load of 10-Nm valgus and 5-Nm internal tibial torque, the in situ force of the PL bundle is higher at 15 degrees of flexion and lower at 30 degrees of flexion. The in situ force in the AM bundle is similar at 15 degrees and 30 degrees of knee flexion. These biomechanical properties correlate with varying contributions to knee stability depending on the degree of knee flexion. Furthermore, the likelihood of single-bundle ACL ruptures also varies depending on the position of the knee at time of injury.

Despite some early healing after injury, the majority of patients never return to their preinjury level of activity or sport. More than 30% of patients have fair or poor results at final follow-up. Seventy-two percent of partial ACL tears have activity-related symptoms at early follow-up. Noyes et al. followed 32 patients with partial ACL tears and found that 12 went on to complete rupture. Given these disappointing results, augmentation surgery may offer a more definitive solution.

History

The patient history from a partial disruption of the ACL or from a patient who continues to have laxity despite prior reconstruction can be vague. In some instances of partial ACL disruption, a traumatic event may occur, however, in many cases, patients may report a minor event in which the knee may have felt as though it “shifted” or “rolled.” Patients may even return to play the same day or within a week of the initial injury. In rare cases, there may be a minor complaint of pain with certain activity, with no specific injury reported. Conversely, within the setting of prior ACL reconstruction, patients are usually able to describe their symptoms more concretely. The patient may state that the knee continues to feel unstable or has no strength. Oftentimes, coaches or the training staff may note the patient lacks the confidence in his or her knee due to subtle differences from the noninjured leg. Determining the exact degree of morbidity from these symptoms can be challenging but is very important in deciding whether surgical intervention is necessary.

Physical Examination

The physical examination should be similar to that performed after any knee injury. A soft endpoint during the Lachman test may indicate a total rupture of the ACL, while a firm endpoint may suggest an isolated AM- or PL-bundle rupture. Furthermore, a high-grade pivot shift is more often present in patients with total rupture and in patients with isolated PL-bundle rupture than in patients with isolated AM-bundle rupture. A positive anterior drawer test is present more often in patients with an isolated AM-bundle rupture than in patents with an isolated PL-bundle rupture. However, this is rarely seen clinically, as the secondary stabilizers are usually preserved within the injury pattern. Furthermore, if the patient is guarding or contracting the hamstrings, the clinician will find it difficult to detect a subtle difference.

Imaging

Besides plain radiographs, advanced imaging is important to determining the degree of ACL involvement. A magnetic resonance imaging (MRI) scan is obtained routinely in any patient suspected of having a partial ACL tear or continued laxity after prior single-bundle ACL reconstruction ( Fig. 97.1 ). Studies have shown that MRI diagnosis of partial ACL tears can be challenging. Disorganization or incomplete visualization of ACL fibers seen in conjunction with intact fibers, bowing of the ACL fibers, or increased intrasubstance signal on T2-weighted images can be suggestive of a partial ACL injury. One study demonstrated that only 12% of patients with a partial tear of the ACL had a bone contusion as comparison to 72% with complete tears. Oblique coronal, oblique axial, and oblique sagittal planes, which are aligned with the direction of the ACL fibers, can help assess the rupture pattern especially in ambiguous cases.

A, Sagittal T2 fat-saturated image shows an isolated tear of the posterolateral bundle (PL) of the anterior cruciate ligament. The anteromedial bundle (AM) appears intact. B, Isolated injury of the AM bundle. The PL bundle appears intact.

Indications

The indications for single-bundle augmentation surgery rely on patient symptoms and the preoperative evaluation. A patient who has undergone a prior single-bundle reconstruction with adequate graft placement and incorporation, but who continues to experience patholaxity is a good candidate for this procedure. Patients who have suspected partial tears of the ACL with a positive pivot-shift examination and an MRI that confirms the presence of a single injured bundle are also candidates for this procedure. However, these patients should demonstrate persistent laxity and instability after a proper rehabilitation program and functional bracing regimen.

Surgical Technique

A thorough examination under anesthesia is performed to confirm the findings from the office examination. Three arthroscopic portals are created in the same fashion as in the double-bundle technique. Diagnostic arthroscopy is undertaken. The notch is inspected for disruption of the fibers of the ACL. A probe is used to determine the extent of damage and involvement of the bundles. Placing the knee in the figure-four position can aid in viewing the posterior root of the lateral meniscus as it enters the tibia. Just anterior to this structure and often confluent with the lateral meniscus is the insertion of the PL bundle. These fibers are followed proximally to the lateral femoral condyle (LFC) origin and assessed for competence. The knee is then brought back to the neutral position, and the AM bundle is assessed. In the setting of prior single-bundle ACL reconstruction, the graft is inspected for proper tunnel placement and preparation is made for insertion of the unreconstructed bundle.

We prefer to use a hamstring autograft although a looped tibialis anterior allograft may be used, especially in an isolated AM bundle reconstruction owing to its larger size. The graft is trimmed so that after it is folded, it will fit snugly into a 7-mm tunnel.