Abstract
Studies continue to demonstrate the importance of the anterolateral ligament (ALL) as a secondary restraint in rotational stability of the knee. No clinical exam exists to reliably test the ALL. Advanced imaging allows the surgeon to reliably identify the ALL as an independent structure of the lateral knee. This technique paper provides a reproducible method for identification of the ALL on 3T magnetic resonance imaging based on previously conducted cadaveric dissections of the ligament.
Multiple studies have investigated the anatomic and biomechanical characteristics of the anterolateral ligament (ALL). Previous biomechanical studies have demonstrated the importance of the ALL in rotational stability of the knee.1, 2, 3 Cadaveric studies have demonstrated residual internal rotation and pivot shift instability when treating knees with combined anterior cruciate ligament (ACL) and ALL injuries with ACL reconstruction alone.4 Although the current indications for ALL ligament reconstruction are not well defined, studies continue to demonstrate its importance as a secondary restraint in rotational stability of the knee.
The ALL was first described by Segond in 1879 as an avulsion of the lateral capsule.5 Hughston followed by calling it the third midcapsular ligament, but more recently there has been a renewed focus on its importance to knee kinematics.6 A cadaveric biomechanical study noted a load to failure of the ALL at 175 N and stiffness at 20 N/mm.7 The ALL lengthens and tightens with knee flexion and internal rotation.5, 6 This supports the concept that the ALL is a secondary restraint that stabilizes internal tibial rotation with increasing flexion. In cadaveric studies, sectioning of the ALL increased internal rotation laxity with concomitant ACL or iliotibial band (ITB) injuries, leading to a higher grade pivot shift.8
Several cadaveric studies describe the anatomy of the ALL, with an emphasis on its femoral, tibial, and meniscal attachments and their relationship to other structures of the knee. The most accepted origin of the ALL is slightly anterior and distal to the fibular collateral ligament (FCL) on the lateral femoral epicondyle.9, 10, 11 However, some studies have demonstrated variability in its femoral origin, specifically noting that the ALL can originate posterior and proximal to the FCL.11, 12, 13 More distally, the ALL can be found superficial to the popliteus tendon before bifurcating just proximal to the lateral inferior geniculate artery. After the bifurcation, the ALL differentiates into meniscal and tibial insertions.9 Specifically, the tibial insertion of the ALL is on the anterolateral tibia, approximately 5.4 mm beneath the plane of the lateral tibial plateau, midway between the lateral border of Gerdy's tubercle and the anterior margin of the fibular head.7, 9 Previous literature described the ALL as a capsular structure; however, histologic analysis confirms that the ALL's morphology is ligamentous, with a dense network of organized type I collagen.11 Cadaveric dissections have been correlated with magnetic resonance imaging (MRI) to determine that the ALL is a structure that can be clearly delineated on advanced imaging.9 On MRI, the ALL was found to be an average of 36 mm long, 1.5 mm thick, and 5.5 mm wide and was a radiographically distinct structure from the FCL, ITB, and capsule.9 See Table 1 for a brief overview of the anatomy of the ALL.
Table 1.
The Origin, Insertions, and Dimensions of the Anterolateral Ligament Based on Anatomical Studies
| Anatomy of the anterolateral ligament |
| • Femoral origin: Lateral femoral epicondyle, anterior and distal to fibular collateral ligament |
| • Meniscal insertion: Lateral meniscus |
| • Tibial insertion: Anterolateral tibia, approximately 5 mm distal to lateral tibial plateau, halfway between Gerdy's tubercle and anterior fibular head |
| • Ligament dimensions: 36 mm long, 1.5 mm thick, 5.5 mm wide |
The purpose of this technique paper is to provide a reproducible method for identification of the ALL on 3T MRI based on previously conducted cadaveric dissections of the ligament.
Technique
Radiographic assessment of the ALL should consist of the use of a 1.5T, or preferably 3.0T, MRI, with slice thickness set to 0.5 mm to 1.0 mm using a dedicated knee coil if possible (Siemens Medical Solutions, Malvern, PA). The knee should be held in approximately 15° of flexion with neutral rotation. Due to the small size of the ligament and its lateral location on the knee, coronal and axial images using proton density with fat saturation sequences are the most helpful sequences in identifying the ALL.
Coronal imaging should start with identification of the origin of the ALL. Due to the ligament's proximity to the FCL, it can be difficult to differentiate the unique origins of these structures. Despite variations, the origin of the ALL can typically be isolated just anterior and distal to the FCL.8, 9, 10, 11, 12 The ALL can then be followed distally as it moves superficial to the traversing popliteus tendon until its bifurcation to its tibial and meniscal insertions. A reproducible landmark used for this bifurcation is the lateral inferior geniculate artery, as the meniscal insertion branches just proximal to this vessel (Figs 1 and 2). The tibial insertion is approximately 5 mm distal to the lateral tibial plateau.
Fig 1.
Right knee 3T magnetic resonance imaging, coronal plane, T2 with fat saturation, supine with 15° knee flexion, demonstrating the anterolateral ligament and its relationship to surrounding structures. (ALL, anterolateral ligament; FCL, fibular collateral ligament.)
Fig 2.
Left knee 3T magnetic resonance imaging, coronal plane, T2 with fat saturation, supine with 15° knee flexion, demonstrating the anterolateral ligament and its relationship to surrounding structures. (ALL, anterolateral ligament; FCL, fibular collateral ligament.)
Axial imaging is also helpful with identification of the ALL. The first step is to identify the lateral femoral epicondyle proximally. At the epicondyle, the ALL insertion is just distal and anterior to the FCL insertion, but often it is very difficult to differentiate between the 2 structures. Examining more distal axial cuts should help to distinguish between the FCL and ALL. On the axial images, the following structures from posterior to anterior can help to locate the ALL: the biceps femoris tendon, FCL, and ITB (Figs 3 and 4). The ALL is located halfway between the ITB and the FCL. More distally, the ALL will continue in an anteroinferior direction until its bifurcation to its meniscal insertion just proximal to the lateral inferior geniculate artery, which is difficult to visualize on axial imaging. The tibial insertion occurs approximately halfway between the FCL (lateral) and the ITB (medial), approximately 5 mm distal to the articular surface of the lateral tibial plateau. Please refer to Table 2 for tips on identification of the ALL.
Fig 3.
(A-C) Right knee 3T magnetic resonance imaging, axial plane, T2 with fat saturation, supine with 15° knee flexion, demonstrating the anterolateral ligament and its relationship to surrounding structures when viewed from proximal to distal (A→B→C). (ALL, anterolateral ligament; FCL, fibular collateral ligament; IT, iliotibial.)
Fig 4.
(A-C) Left knee 3T magnetic resonance imaging, axial plane, T2 with fat saturation, supine with 15° knee flexion, demonstrating the anterolateral ligament and its relationship to surrounding structures when viewed from proximal to distal (A→B→C). (ALL, anterolateral ligament; FCL, fibular collateral ligament; IT, iliotibial.)
Table 2.
Tips and Tricks for Identification of the Anterolateral Ligament on Coronal and Axial Imaging
| Coronal | Axial |
|---|---|
| • Identify fibular collateral ligament | • Identify biceps femoris tendon |
| • Identify popliteus tendon | • Identify fibular collateral ligament at medial epicondyle |
| • Identify anterolateral ligament origin between above 2 structures | • Identify iliotibial band |
| • Identify lateral inferior genicular artery | • Identify anterolateral ligament anterior to fibular collateral ligament and posterior to iliotibial band |
| • Identify meniscal and femoral branches at level of lateral inferior genicular artery | • Follow down to tibial attachment |
For an audio and visual step-by-step guide for identification of the ALL, please view the Video 1 associated with this manuscript.
Discussion
The ALL is a distinct structure in the knee that acts as a secondary restraint to internal rotation of the knee with increasing degrees of flexion. Its clinical significance and role in pathology continue to be a topic of discussion. As noted previously, there is progressive lengthening of the ALL in internal rotation with increasing flexion, especially in the ACL-deficient knee. ACL tears with concomitant ALL injuries can compromise ACL reconstruction and lead to persistent rotational instability. This rotational instability could increase rotational strain on the ACL reconstruction and potentially increase the graft's risk of failure. There is currently no clinical exam to identify an ALL injury, therefore, MRI plays a crucial role in diagnosis and treatment of this pathology. Table 3 notes advantages and disadvantages to using this technique for identification of the ALL.
Table 3.
The Various Advantages and Limitations of Magnetic Resonance Imaging (MRI) to Visualize the Anterolateral Ligament (ALL)
| Advantages | Limitations |
|---|---|
| • Reproducible method to locate the intact ALL using coronal and axial MRI | • Not yet validated to see damage or tears to the ligament |
| • Thin cuts and 3T MRI allow for better visualization of the small ALL | • Does not assess functionality of the ligament and need for surgical reconstruction |
| • Consistent methodology for evaluation of the ligament allows for more consistent visualization |
Multiple studies have demonstrated the ability to identify the ALL as a distinct structure on MRI. Cadaveric studies have also shown correlation of findings (size of ALL, insertion, origin, and relationship to surrounding structures) between anatomic dissection and MRI with good interobserver reliability.9 The ALL has also been identified using ultrasonography, a tool that could be crucial to clinical evaluation of this structure.14
Diagnosis of ALL injuries in ACL-deficient knees has been described in 2 studies. Claes et al.15 noted in a study with 206 patients that when the ALL was identifiable, over 75% had a concomitant ALL injury and that a majority of these injuries occurred in the ligament's tibial attachment. Hartigan et al.16 noted that while the ALL was identified in 100% of acutely injured ACL knees, radiologists were unable to reliably classify the ligament as intact or injured when the study was performed on a 1.5T MRI with slice thickness of 4.0 mm. By using a 3.0T magnet with a more standardized approach to image interpretation, and by applying the identification techniques outlined in this article, the surgeon can more readily identify the ALL. The authors believe that proper identification of this ligament and potential pathologies will allow researchers to better determine its clinical significance.
In conclusion, the ALL is a distinct structure of the lateral knee that plays a role in rotational stability of the knee and that can be identified using MRI with the techniques described in this study.
Footnotes
The authors report that they have no conflicts of interest in the authorship and publication of this article.
Supplementary Data
Step-by-step guide to identification of the anterolateral ligament using right knee 3T magnetic resonance imaging, axial and coronal plane T2 with fat saturation imaging with the patient supine and the knee in 15° of flexion.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Step-by-step guide to identification of the anterolateral ligament using right knee 3T magnetic resonance imaging, axial and coronal plane T2 with fat saturation imaging with the patient supine and the knee in 15° of flexion.