Aberrant Insertion of the Anterior Cruciate Ligament on the Lateral Meniscus

Abstract

Case:

A 16-year-old woman presented with acute on chronic knee pain and instability following a twisting injury. The tibial insertion of the anterior cruciate ligament (ACL) was nonvisualized on magnetic resonance imaging. A cord-like ACL, originating from the lateral intercondylar notch and inserting smoothly into the anterior horn of the intact lateral meniscus, was found on arthroscopy. Her ACL was reconstructed with quadriceps tendon autograft.

Conclusion:

We report a rare case of dysplastic ACL inserting into the lateral meniscus in a patient without obvious skeletal deformities. This pathology could be congenital or traumatic. Surgeons and radiologists should be aware of this rare variant.

Congenital aplasia of the anterior cruciate ligament (ACL) is an exceedingly rare condition, estimated to occur in only 0.017 of 1,000 live births^1,2. ACL anomalies, such as aplasia or dysplasia, are frequently associated with concurrent congenital skeletal deformities such as fibular hemimelia and congenital femoral deficiency^3,4. At the level of the knee, ACL dysplasia is often associated with narrowing of the femoral notch and hypoplasia of the tibial spines^1,3.

There have been 5 reported cases of a ligamentous structure following the normal course of the ACL, but inserting into the anterior horn of the lateral meniscus rather than the tibial spines, sometimes referred to as an anterolateral meniscofemoral ligament^5-9. In 4 of these cases, there was concurrent aplasia of the ACL, while in 1 case the native ACL was present along with the anomalous ligament⁹. Four of these cases were suspected to be congenital variants, while one, in which the avulsed ACL inserted smoothly into the avulsed lateral meniscus on an avulsion fracture, was questionably anomalous healing response in response to trauma vs. congenital hypoplasia⁸. Here, we describe the case of a patient with no apparent skeletal deformities presenting with knee pain and instability, bucket handle tear of the medial meniscus with suspected ACL tear, subsequently found on arthroscopy to have absent ACL footprint at the normal tibial site of insertion, and instead a cord-like structure inserting directly into the anterior horn of the lateral meniscus.

The patient and his parents were informed that data concerning the case would be submitted for publication, and they provided consent.

Case Report

A 16-year-old female patient presented to an outpatient orthopaedic clinic with acute on chronic left knee pain and instability. The patient was previously seen for similar ipsilateral knee pain at which time she was diagnosed with patellofemoral subluxation/instability. She then had an acute exacerbation of symptoms while playing lacrosse; she described twisting of her knee as she went to shoot and felt a pop. She had mild subsequent swelling but denied significant effusion. There was immediate onset of pain and difficulty fully extending and flexing the knee. Less than 2 weeks after injury, the patient presented for evaluation. On physical examination, there was trace effusion, and active range of motion was 15 to 130 with anterior knee pain at both terminal flexion and extension. Grade 1A Lachman, negative anterior and posterior drawer, both were limited by guarding. McMurray was also not possible to be accurately assessed due to guarding. Radiography results of the left knee showed closed no acute fracture or dislocation, a small joint effusion, and no obvious patellar displacement (Fig. 1). Magnetic resonance imaging (MRI) radiographs of the left knee at 1.5 T showed closed physes, a bucket-handle tear of the medial meniscus, and the tibial attachment of the ACL was “not visualized, consistent with a tear” per Radiology (Fig. 2). Based on this initial diagnosis, the patient was consented for bucket handle meniscus repair with possible ACL reconstruction.

Fig. 1.

Anterior-Posterior and lateral radiographs of the patient's left knee showing no acute fracture or dislocation, a small joint effusion, and no patellar displacement.

Fig. 2.

Figs. 2-A through 2-F MRI radiographs of the patient’s left knee. Fig. 2-A Double posterior cruciate ligament (PCL) sign indicative of bucket handle tear of the medial meniscus. Figs. 2-B through 2-D Absent ACL insertion onto the tibial eminence in sagittal sections. Figs. 2-E through 2-F Displaced bucket handle segment within the intercondylar notch [arrow(s)]. ACL = anterior cruciate ligament, MRI = magnetic resonance imaging.

Intraoperative knee examination revealed evidence of a clear grade IIB Lachman. Owing to the presence of the bucket-handle meniscus tear, pivot shift test was not performed. Standard anterolateral and anteromedial portals were made for arthroscopy. On arthroscopic examination of the intercondylar notch, the bucket handle tear of the medial meniscus was confirmed and reduced with a probe (Fig. 3). The ACL had an atypical appearance of an attenuated cord-like structure, originating from the lateral side of the intercondylar notch, with a smooth appearing insertion into the anterior horn of the lateral meniscus (Fig. 3). Neither an ACL remnant nor stump was observed or evident in the usual tibial insertion site (Figs. 3-F and 4). The medial and lateral tibial spines appeared significantly hypoplastic. The lateral meniscus, including the anterior root, and posterior cruciate ligament (PCL) were intact without evidence of tear.

Fig. 3.

Intraoperative arthroscopic examination of the intercondylar notch. Fig. 3-A bucket handle tear of the medial meniscus (asterisk). Figs. 3-B through 2-F The ACL had an atypical appearance of a cord-like structure (star), originating from the lateral side of the intercondylar notch, with a smooth appearing insertion into the anterior horn of the lateral meniscus. ACL = anterior cruciate ligament.

Fig. 4.

Intraoperative arthroscopic examination of the intercondylar notch. Neither an ACL remnant nor stump was observed or evident in the usual tibial insertion site.

After the inside-out bucket handle repair of the medial meniscus, we turned our attention to the band of tissue that seemed to be ACL; given this cord-like tissue was attenuated and incompetent, both ends were detached and debrided. There was no further ACL tissue visualized, and thus, the decision was made to reconstruct the ACL using a quadriceps tendon autograft.

At the 1-week follow-up, her pain was well-controlled, and she followed standard protocol for an ACL reconstruction with concurrent bucket-handle repair. At the 8-month follow-up, she reported no pain, no instability events, and she was progressing back to dynamic unrestricted activities.

Discussion

This study reports a rare case of dysplastic ACL insertion into the anterior horn of the lateral meniscus. The definitive etiology of the observed insertion is unclear; it is possible that this patient had a chronic ACL tear, with a cord-like insertion onto the anterior horn of the lateral meniscus as a result of aberrant healing. Given the ACL appeared perfectly fused to the anterior horn of the lateral meniscus and that the tibial spines appeared hypoplastic arthroscopically, it is also possible that this was congenital. Furthermore, on review of the preoperative MRI, the coronal view does not in fact show the ACL, as was initially interpreted, but rather shows a cord-like structure stretching from the anterior horn of the lateral meniscus to the femoral intercondylar notch (Fig. 5). Unfortunately, tunnel views of the knee and plain radiographs of the tibia/fibula were not obtained preoperatively as there was no suspicion for congenital musculoskeletal aberrancy.

Fig. 5.

Coronal MRI shows a cord-like low density structure stretching from the lateral femoral intercondylar notch to the anterior horn of the lateral meniscus (arrows). MRI = magnetic resonance imaging.

Manner et al. describes a classification system for ACL aplasia: Type I (aplastic or hypoplastic ACL, normal PCL), Type II (aplastic ACL, hypoplastic PCL), and Type III (aplastic ACL and PCL), with higher grade classifications correlating more significant bony changes of the tibial spines and femoral condyles¹. Aplasia of the ACL can also occur traumatically, as noted by Dohle et al¹⁰. The authors posit that changes in the tibial eminence are essential for the absence of the ACL to be congenital rather than traumatic¹⁰. There is controversy in the existing literature regarding the optimal management of congenital aplasia of the ACL, and whether reconstruction is beneficial in asymptomatic patients ³.

In the literature, there are accounts of 5 similar cases of dysplastic ACLs, in which a cord-like structure inserted on the anterior horn of the lateral meniscus; however, each of these 5 studies had notable differences from the patient presented here (Table I). Lee et al. describe a similar patient, presenting with acute on chronic knee pain and instability but who denied trauma, was found to have an anterolateral meniscofemoral ligament and aplasia of the ACL. The medial tibial spine and lateral femoral condyle were hypoplastic, suggesting congenital abnormality⁶. Silva and Sampaio describe another case of anterolateral meniscofemoral ligament with ACL aplasia and tibial spine abnormality, although their patient also had a concurrent ring-like discoid meniscus⁵. Toy et al. describe an instance in which the ACL and anterior horn lateral meniscus were avulsed and connected, raising the same question of developmental vs. traumatic etiology ⁸. Razi et al. reported a case of anterolateral meniscofemoral ligament, but with a “ring-like lateral meniscus” and more prominent medial tibial eminence⁷. Finally, Kim et al. describes a case in which there is an insertion of 2 anomalous “ligamentous structures” on the medial and lateral menisci, which came together and ran parallel to a normal appearing ACL⁹.

TABLE I.

Summary of Currently Published Cases Regarding Dysplastic ACL

Report	Patient Age and Sex	Symptoms	Injury Event Before Presentation	Initial MRI Findings	Concomitant Lesions	Morphological Characteristics	Treatment
Silva and Simpao6	13, F	Intermittent knee instability and pain	Yes	Intact ACL, bucket handle tear of lateral meniscus	Untorn ring-like discoid meniscus, lateral femoral condyle hypoplasia, tibial eminence hypoplasia	ACL aplasia, no tibial insertion of anterior horn lateral meniscus, ligament from medial border of lateral meniscus to lateral femoral condyle	Diagnostic arthroscopy only
Lee et al.7	34, M	Acute on chronic knee pain and instability	No	ACL aplasia, bucket handle tear or lateral meniscus	Hypoplastic medial tibial spine and lateral femoral condyle, intact menisci	Cord-like structure from anterior horn lateral meniscus to medial border lateral femoral condyle	ACL reconstruction
Razi et al.8	24, M	Knee instability	Yes	Complete ACL tear	Ring-like lateral meniscus, prominent medial tibial eminence	ACL agenesis with thick band-like structure from anterior horn lateral meniscus to intercondylar notch of lateral femoral condyle	ACL reconstruction
Toy et al.9	14, M	Knee pain, intermittent swelling, instability	Yes	Wavy patterned ACL, small anterior horn lateral meniscus	Narrow femoral intercondylar notch	Anterior horn of lateral meniscus connected to ACL and avulsed from tibial insertion	Direct repair of ACL and anterior horn lateral meniscus
Kim et al.10	18, M	Knee pain and catching sensation	No	Posterior horn lateral meniscus tear	Radial tear of posterior horn lateral meniscus	2 distinct ligamentous structures inserting on anterior medial and lateral menisci, combining and running parallel to an intact ACL	Partial meniscectomy for radial tear of the posterior horn lateral meniscus
Norman et al., 2024	16, F	Acute on chronic knee pain and instability	Yes	Nonvisualized tibial ACL insertion	Bucket handle tear of medial meniscus, ACL aplasia, hypoplastic tibial spines	Cord-like structure inserting smoothly on lateral meniscus	Bucket handle meniscus repair and ACL reconstruction

ACL = anterior cruciate ligament.

A weakness of this case is that preoperative tunnel views of the knee were not obtained, so we are unable to ascertain if the patient’s native knee had morphologic changes of the femoral notch or tibial eminence hypoplasia. If these views were to be obtained postoperatively, and there were morphologic changes, it would be difficult to ascertain whether these changes were simply postoperative.

This case describes apparent ACL dysplasia in an individual without obvious skeletal abnormality or lateral meniscus derangement. As ACL dysplasia is typically associated with conditions such as fibular hemimelia, few prior cases have reported dysplasia in skeletally normal individuals, and most of these cases have noted concurrent abnormality of the lateral meniscus. It is important for sports medicine surgeons and musculoskeletal radiologists to be aware of this potentially rare variant when interpreting imaging and treating patients.