In our patient, exploration revealed an avulsion of the ACL off the tibial footprint. The bulk of the disrupted portion of the ACL was in continuity with the anterior horn of the lateral meniscus, with minimal attachment to the tibia. To date, we are not aware of any previous case reports of this nature. Embryological studies have reported that the tibial insertion of the ACL develops from the same blastema as the menisci [9
]. Even in adults, a few fascicles of the anteromedial bundle of the adult ACL are known to blend with the anterior attachment of the lateral meniscus; however, this contribution is usually minimal [4
]. In contrast, it appeared that the main attachment of the ACL to the tibia was via the anterior horn of the lateral meniscus. These developmental observations may explain the injury pattern observed in this patient.
Pediatric ACL injuries typically manifest as avulsion fractures from the tibial eminence or the femur, since the weak point is the ligament–bone insertion. Tibial eminence fractures may heal without surgery if they are minimally displaced, or may require open or arthroscopically assisted reduction with internal fixation. This provides for bone–bone healing that results in a generally good outcome [25
], with occasional reports of objective knee laxity [12
], retropatellar knee pain [23
], and arthrofibrosis [21
In this patient, neither a mid-substance tear nor a tibial eminence fracture was noted. Instead, the patient avulsed the tibial insertion of the ACL from its small footprint, which included an extensive attachment to the lateral meniscus. Intraoperatively, the tissue was of good quality and was felt to be adequate for direct repair back to the tibial footprint with sutures that were brought through drill holes.
Several studies have investigated posterior meniscal root avulsions, an injury pattern commonly associated with signs of degenerative arthritis [6
]. The anterior and posterior root attachments maintain proper meniscal position and function, as well as prevent extrusion [5
]. Therefore, primary repair of the attachment site is felt to be the best option to restore function and potentially avoid subsequent degeneration [15
]. However, the majority of studies describing root avulsions have focused on the posterior root [2
]. To our knowledge, there are no studies that report an anterior lateral meniscal root avulsion as seen in this patient [2
]. Since anterior lateral meniscal pathology is uncommon following ACL injuries, and since the connection between the ACL and the anterior horn of the lateral meniscus was so robust during the surgery, we conclude that this represents a developmental variant for this patient. Alternatively, it is possible that the lateral meniscus and the ACL footprint were independently avulsed and later scarred together in continuity. Lastly, the connection of the ACL lateral meniscal variant to the very narrow intercondylar notch is unknown, but one could hypothesize it is part of the same dysplastic process. There were no other structural anomalies noted during arthroscopic evaluation of the knee.
In conclusion, we report a case of an ACL tibial avulsion with the anterior horn of the lateral meniscus attached to it in an adolescent patient. We feel this is a developmental anomaly given the dysplastic intercondylar notch and seamless transition from ligament to meniscus. It was treated by directly repairing the ACL and anterior meniscal root to the tibial footprint with sutures brought through two drill holes in the anterior tibia and tied over a bone bride. At 12-month follow-up, the patient is doing well clinically.