Teaching Case

A 25-year-old male patient with growth hormone deficiency (body height 139 cm), presented with mild anterior knee pain, loss of extension (10 o right, 14 o left) and an audible clunk at the terminal extension of the knee. There was neither a history of ACL reconstruction nor a previous knee injury.

MRI was performed to investigate the cause of limited extension and to evaluate the meniscus's and chondral integrity. An ovoid-shaped soft tissue lesion was found in the anterior intercondylar notch, which showed intermediate signal intensity on both T2-w and fat suppressed PD-w MR images. The imaging findings were consistent with a cyclops lesion. On arthroscopy, the lesion was confirmed and removed.

A. Background
Disease description
The cyclops lesion is a fibrous nodule in the anterior intercondylar notch anterior to the tibial insertion of ACL. Cyclops lesions were initially reported to occur in patients after ACL reconstruction, with an average interval time of 16 weeks between repair and arthroscopy [1]. Recent reports refer cases after minor knee injury as a result of sub-clinically torn ACL fibres [1, 2, 3] as long as 23 years after initial trauma [4].

Pathophysiology- pathogenesis
M. Tonin et al. proposed that osseous particles may trigger the development of the cyclops lesion. Moreover they suggested that the normal reparative inflammatory process can by its own cause the formation of the localized anterior arthrofibrosis [5].
Another potential factor in the pathogenesis is the size of the intercondylar notch. M. Fujii et al. suggested that a smaller intercondylar notch size may contribute in cyclops lesion formation [6].
Our case also suggests that the narrowed intercondylar notch predisposes to ACL impingement and subsequent cyclops lesion formation.

Arthroscopic
The arthroscopic appearance of the cyclops lesion reveals a soft tissue nodule with prominent surface vessels resembling the eye of the Cyclops in Greek mythology [7].

B. Clinical Perspective
The Cyclops lesion-related syndrome includes progressive loss of extension of the knee joint, pain and audible clunk at terminal extension [7]. Not all patients will develop loss of extension. The type of the nodule (true cyclops vs cyclopoid scars) and its size seem to be the most decisive factors in the final development of clinical syndrome or a sub-clinical course [8]. True cyclops and bigger size remnants are associated with progressive loss of knee extension.

C. Imaging perspective
MR signal characteristics

Cyclops lesions are typically small, with a mean size of 13 × 12 × 12 mm in a review of 20 lesions [9]. The MR imaging signal characteristics are predominantly heterogeneous or intermediate signal intensity on T2-w and PD-w MR images [1].

D. Outcome
Arthroscopic excision is the treatment of choice with an excellent prognosis.

E. Take Home Message, Teaching points
Radiologists should be aware of cyclops syndrome in every patient presenting with progressive loss of knee extension. It has been reported as soon as within 8 weeks after ACL reconstruction [1] and as late as 23 years after a minor knee injury without reconstruction [4].