Beyond the Anterolateral Ligament: Response

Dear Editor: Among the most important topics in sports medicine and knee surgery today is the role of secondary restraints in rotatory knee instability and pivot-shift phenomenon in the anterior cruciate ligament (ACL)–deficient knee, as well as the related anatomic and biomechanical characteristics of the knee. In 2013, Claes and coauthors, with their controversial paper ‘‘Anatomy of the Anterolateral Ligament of the Knee,’’ rekindled interest in the anatomic features and function of the lateral capsule of the knee, whose role in tibial rotation and the pivot-shift phenomenon had been documented in several studies dating back to the 1960s. Even though recently published papers have challenged the prevalence and even the existence of the anterolateral ligament (ALL) as described by Claes et al, and although a precise and convincing anatomic description of the anterolateral capsule has not been provided, it is difficult to deny that secondary restraints act in conjunction with the ACL in controlling tibial internal rotation and the pivot shift. Is it possible to find one or more considerations (regarding the existence of the ALL, its function, and its lesion associated with ACL tears) on which all researchers could agree? I think that agreement can be reached on some points. In the lateral capsule, anterior to the lateral collateral ligament, a thickening exists, well described by Segond in the late 1800s as a ‘‘pearly fibrous band’’ and later described by others who termed it in different ways (eg, middle third capsular ligament, anterolateral femoral tibial ligament, anterolateral ligament). A tear of this structure, often associated with ACL tear, results in increased rotational instability and pivot-shift phenomenon. The avulsion fragment described by Segond on the lateral tibial plateau just below the articular cartilage (midway between the fibular head and Gerdy’s tubercle) is proof of the existence of tension-resistant collagenous fibers in the lateral capsule. But given the variable size of the ALL, its inconstant features, and its difficult anatomic characterization, how can the ALL alone bear a substantial load and play such an important biomechanical role? Two recently published papers have provided important contributions to our understanding of this complicated issue. Ferretti et al reported on the prevalence and pattern of injuries of secondary restraints occurring along with acute ACL tears. The authors reported that injuries of the lateral capsule occurred in about 90% of cases of apparently isolated ACL tears. However, the injury pattern did not involve a precise anatomic structure such as the ALL. Injuries actually involved a much wider area of the lateral capsule, often extending to the posterolateral capsule, deep in a constantly intact lateral collateral ligament. In a biomechanical study, Guenther et al determined the forces in the anterolateral structures in ACLintact and ACL-deficient knees in response to an anterior and internal rotation torque. The investigators concluded that forces transmitted between regions of the capsule were similar to the forces transmitted by other structures at the knee, suggesting that the ALL does not function as a traditional ligament but rather should be considered a sheet of tissue. The anterolateral capsule, even in the region of the ALL, is anatomically linked to other lateral structures including the lateral meniscus and the posterolateral capsule. It is therefore reasonable to argue that as a result of an abrupt internal rotation stress, injury is not confined to a limited area, even a thicker and possibly functionally predominant area, but extends posteriorly to the entire capsule like a sheet of fibrous tissue. We cannot deny the role of part or all of the lateral capsule as a secondary restraint to the ACL in controlling internal rotation of the knee. The ALL is a thicker portion of the capsule, and whether the ALL is a true ligament or not is of little if any importance. What matters is that ACL injuries often involve the lateral capsule (lateral complex), in part or in whole, and have a significant effect on the pivot-shift phenomenon. Further studies are needed to investigate functional anatomic features of the lateral capsule and to determine how its injury should be treated surgically in view of a more comprehensive approach to rotatory instability in the ACL-deficient knee. The paper by Guenther et al has made a remarkable contribution, and I congratulate the authors.