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].
Bilateral cyclops lesion (localized anterior knee arthrofibrosis)
Based on the provided knee MRI images, a fibrous nodular lesion can be observed in the anterior intercondylar notch region (near the anterior margin of the ACL). The T2-weighted sequence shows moderate to slightly mixed signal intensity. The lesion is relatively small (approximately 1–1.5 cm), with mild involvement of the surrounding soft tissue. There is no obvious bony abnormality or other significant cartilage damage. Combined with the patient’s clinical manifestations (extension limitation, a popping or “click” sound at terminal knee extension), this indicates that the local soft tissue nodule exerts a mechanical effect on joint movement.
Based on the patient’s clinical data (short stature, no significant history of trauma, progressively limited knee extension, and popping sound) and imaging findings, a “Cyclops” lesion (also known as a Cyclops lesion) is most likely.
Considering the imaging findings, clinical symptoms, and patient history, the most probable diagnosis is: an anterior knee “Cyclops” lesion, causing extension limitation and terminal popping.
Treatment Strategy:
When both imaging and clinical symptoms are clear, the first-choice treatment is typically arthroscopic debridement and removal of the fibrous nodule to restore full knee extension. If the lesion is small and symptoms are mild, conservative treatment (physical therapy, anti-inflammatory medication, joint mobilization exercises, etc.) may be tried first; however, in most cases, surgical intervention is necessary to completely resolve symptoms.
Rehabilitation and Exercise Prescription:
1) Early Stage (0–2 weeks post-op): Passive range of motion (ROM) training is recommended, focusing on safe-range knee extension exercises and isometric quadriceps contractions.
2) Mid Stage (2–6 weeks): Gradually increase the range of motion, perform low-load closed kinetic chain training for the lower limbs (such as seated or partial squats), and continually monitor for joint swelling and pain.
3) Late Stage (6 weeks onwards): Once knee function is largely restored, incorporate low-impact aerobic activities (e.g., cycling, brisk walking). Progress to running or jumping (higher-impact exercises) only when pain-free, aiming to improve lower-limb strength and coordination.
4) Individual Considerations: Since the patient is short in stature and has a history of growth hormone deficiency, attention should be paid to bone and cartilage health. Increase the load gradually to avoid re-injury of the joint.
This report provides a reference analysis based on the supplied information and cannot replace an in-person diagnosis or the advice of a professional physician. If you have additional questions or if your symptoms worsen, please seek medical attention promptly for personalized care.
Bilateral cyclops lesion (localized anterior knee arthrofibrosis)
