Cyclops lesion (knee)

Dr Henry Knipe and Assoc Prof Frank Gaillard et al.

The cyclops lesion, also known as localized anterior arthrofibrosis, is a painful anterior knee mass 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 pain during extension of the affected knee, with an eventual audible and palpable “clunk”, occurring typically 8 to 32 weeks (16 weeks on average) after ACL repair 3

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.

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 surrounded by dense fibrous tissue.

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

At MR imaging, 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.

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.

Inspired signs
Knee pathology

The knee is a complex synovial joint that can be affected by a range of pathologies:

Article information

rID: 1187
Section: Signs
Tag: knee
Synonyms or Alternate Spellings:
  • Localised anterior arthrofibrosis
  • Cyclops lesions

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Cases and figures

  • Figure 1: Polyphemus the cyclops
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  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4: without prior ACL reconstruction
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  • Case 5
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