Dr. Pomeranz's Ultimate Guide to :

MRI of the Knee

Cyclops Lesion

Cyclops lesions are typically the result of complications in the reconstruction of an anterior cruciate ligament (ACL). Also referred to as localized anterior arthrofibrosis, this fibrous anterior knee mass2 is a common cause of extension loss. Predisposing factors include “non-isomeric” or abnormal graft tunnel placement, small or dysplastic femoral notch, and unrecognized injuries of other ligaments.




Cyclops Lesion: Clinical Presentation

Extension of the repaired knee results in pain to the patient. Within 8 to 32 weeks of ACL repair, the patient may experience an audible and palpable “clunk”. Post ACL reconstruction, cyclops lesions are estimated to occur in approximately 5% of patients. 1



Cyclops Lesion: Pathology

Though the pathophysiology in not completely certain, the possible reasons could be:
  • Fraying of remnant ACL fibres
  • Excessive fibrosis
  • Uplifting of fibrocartilaginous tissue during ACL reconstruction 2



Cyclops Lesion: MRI

In all pulse sequences, the signal intensity of a cyclops lesion is intermediate to low. This is due to the fibrous structure of the lesion. Close to the tibial insertion of the reconstructed ACL, a mass such as this one is located anteriorly / anterolaterally in the intercondylar notch. 2



Cyclops Lesion: Treatment

Typically, arthroscopic excision is recommended. 2



Cyclops Lesion: References

  1. Runyan BR, Bancroft LW, Peterson JJ et-al. Cyclops lesions that occur in the absence of prior anterior ligament reconstruction. Radiographics. 27 (6): e26.
  2. Weerakkody Y., Gaillard F., et-al. Cyclops Lesion (Knee): Radiopaedia (sourced 10Jan2018): https://radiopaedia.org/articles/cyclops-lesion-knee




Cyclops Lesion: Example



Knee pain. No injury.
axial t2 fse    sagittal t2 fse
Image 1 - Axial T2 FSE                      Image 2 - Sagittal T2 FSE


  • No microtrabecular or macrotrabecular fracture.
  • Slightly laterally displaced patella. Massive induration of Hoffa fat pad.
  • No high-grade chondromalacia of the patellofemoral compartment.
  • Flexors and extensors are intact.
  • Partial-thickness chondral thinning of the weight bearing medial femoral condyle. Minimal marginal spurring medial femoral condyle. Stable healing complex vertical tear in the outer red-red zone posterior horn to posterior body of the medial meniscus likely from original pivot-shift injury.
  • PCL is intact. ACL graft is intact. Giant cyclops lesion (images 1 and 2, green arrows) anterior to the ACL graft favored over localized PVNS (also known as giant cell tumor of tendon sheath, intra-articular).
  • MCL and LCL are intact.
  • No high-grade chondromalacia of the lateral compartment. Minimal marginal spurring lateral femoral condyle. No lateral meniscus tear.
  • Posterolateral corner is intact.
  • 2+ proteinaceous effusion.
  • Swelling of the subcutaneous tissues anteriorly.


  • ACL graft is intact with giant cyclops lesion anterior to the ACL graft favored over localized PVNS.
  • Stable healing complex vertical tear outer healing red-red zone posterior horn to posterior body of the medial meniscus likely from original pivot-shift injury.