Anterior cruciate ligament avulsion fracture

Clinical History

12-year-old boy who fell on his knee.

Companion Case: 21 F with limited knee extension 5 months after accident.

Imaging Findings

(Fig. 1-2) Coronal and sagittal MR images of the knee showing a recent, minimally displaced anterior tibial spine fracture fragment attached to an intact anterior cruciate ligament (ACL).

(Fig. 3-6) Companion case of a 21-year-old female patient with plain radiographs and MR images demonstrating a remote nonunited tibial fragment attached to an intact anterior cruciate ligament.

Discussion

The anterior cruciate ligament attaches proximally to the posterior medial surface of the lateral femoral condyle and distally to the anterior intercondylar area in the tibia. Its tibial insertion is broad and fanlike and is located slightly lateral and anterior to the anterior tibial spine [1]. Although tearing of the anterior cruciate ligament most commonly occurs at its midsubstance, an avulsion fracture of the ligament from its tibial insertion occurs in a minority of cases and is more common in children than in adults, particularly those between eight to thirteen years of age [1-2]. These injuries are more common in the skeletally immature population due to the relative increased strength of the ligament compared with the developing bone and growth plate [3]. In children, this injury occurs secondary to hyperflexion of the joint and internal tibial rotation and is uncommonly accompanied by other ligamentous injuries [1-2].

On conventional radiographs, these avulsion fractures may be difficult to recognize, although the presence of a small bone fragment in the region of the intercondylar notch with cortical irregularity of the adjacent tibial eminence suggestive of a donor site may be the usual findings. MR imaging is useful for confirmation of the tibial site of the fragment, evaluation of the rest of the ligament and assessment for additional injuries [2].

It is crucial for the radiologist to accurately identify this injury. Failure or delay in diagnosis may eventually result in nonunion and persistent clinical symptoms such as limitation or pain on knee extension and anterior instability [4]. In addition, classification of ACL avulsion fractures may dictate the management. The Meyers and McKeever classification system describes four subtypes of tibial spine fractures: presence of a minimally displaced fragment (type I), anterior elevation of the fragment (type II) complete separation of the fragment from the tibia (types III and IV) with rotational component or comminution (type IV). Type I injuries are managed conservatively, while arthroscopy is recommended for type II– IV lesions with types III and IV injuries requiring internal fixation [5].

In conclusion, it is imperative for the radiologist to recognize and correctly diagnose an ACL avulsion fracture to facilitate early and appropriate management and prevent future anterior knee instability.

Differential Diagnosis List

Final Diagnosis

Anterior cruciate ligament avulsion fracture

URL:	https://www.eurorad.org/case/13701
DOI:	10.1594/EURORAD/CASE.13701
ISSN:	1563-4086