Cyclops lesion (knee)

Last revised by Yvana Toh on 9 May 2024

The cyclops lesion, also known as localized anterior arthrofibrosis, is a painful anterior knee mass associated with loss of extension that arises as a complication of anterior cruciate ligament (ACL) reconstruction, although has rarely been reported in patients with ACL injuries that have not been reconstructed. 

Cyclops lesions occur with an estimated frequency of ~5% (range 1-9.8%) of patients following ACL reconstruction. They are rarely encountered in patients who have not had ACL reconstruction but have nonetheless sustained ACL injuries 3

Patients present with anterior knee pain along with loss of extension of the affected knee 5, with an eventual audible and palpable “clunk”, occurring typically 8 to 32 weeks (16 weeks on average) after ACL repair 3. The term cyclops syndrome is used in patients with a post ACL reconstruction decrease in knee extension 4.

The exact etiology is uncertain and may be related to gradual fraying and bunching up of remnant ACL or graft fibers, excessive fibrosis, or alternatively due to the uplifting of fibrocartilaginous tissue during drilling of the tibia for anterior cruciate ligament reconstruction, which serves as a nidus for fibrous tissue deposition. Impingement of the graft in knee extension is also postulated 4.

The end result, regardless of cause, is a rounded fibrous mass sitting in the anterior intercondylar notch. Pathologically, the lesion consists of central granulation tissue lined by synovium and surrounded by dense fibrous tissue.

As with other internal derangements of the knee, MRI is the modality of choice for assessing the postoperative knee. 

On MRI, a soft-tissue mass is seen anteriorly or anterolaterally in the intercondylar notch near the tibial insertion of the reconstructed anterior cruciate ligament. Because of its fibrous content, a cyclops lesion typically has intermediate to low signal intensity with all pulse sequences. Lesion signal is usually heterogeneous and there is variable contrast enhancement. Cyclops lesions can be associated with anterior interval lesions.

There are no differentiating features on MRI between symptomatic and asymptomatic lesions.

Treatment is arthroscopic excision.

The lesion was so named because of its bulbous appearance and characteristic focal areas of reddish-blue discolouration (from venous channels) that resemble an eye at arthroscopy.

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