A 46-year-old woman presented with history of trauma seven months before.
Since then she had lost full extension; pain and an audible clunk with terminal extension.
Clinically the knee was stable and no meniscal tear was suspected.
X-ray and MRI were performed.
She has no history of arthroscopic knee surgery.
X-ray study was normal (Fig. 1).
MRI showed a 15 mm soft tissue nodule in the intercodylar notch near the tibial insertion of the anterior cruciate ligament (ACL). This lesion is isointense to muscle in PD, T1- and T2-weighted images (Fig. 2-4). No cruciate ligaments torn or meniscal tears were observed.
The cyclops lesion is a fibrous nodule in the intercondylar notch near the tibial insertion of ACL. It was first described in patients with ACL reconstruction [1] but recently it has been reported cases without this antecedent [2].
The exact aetiology is uncertain.
In patients with ACL reconstructions has been linked to a result of a fibroproliferative process after surgery (because of debris, residual tissue or broken graft fibers).
In patients without knee surgery it has been related with a history of trauma. Several authors have supposed that this lesion occurs as a result of microtrauma of subclinically torn ACL fibers even when there is clinically or radiologically intact ACL [1].
MRI shows a rounded mass with a similar signal intensity to the muscle in the intercondylar notch adjacent to the ACL [3].
Cyclops lesion when symptomatic is named cyclops syndrome. The symptoms are pain, loss full extension and a palpable and audible clunk with extension [4, 5].
Treatment is arthroscopic excision when symptomatic [6].
The soft tissue nodule is named cyclops lesion lesion because of its arthroscopic appearance like a rounded mass with vessels which resemble the eye of the cyclops of Greek mythology [5].
Cyclops lesion in absence of anterior ligament reconstruction
(1) From the anteroposterior and lateral X-ray views of the knee joint, the overall alignment appears normal. No obvious fracture line or significant osteophyte formation is observed, and there is no marked narrowing of the joint space. The cortical continuity of the distal femur and tibia is intact, and no apparent abnormal thickening of the soft tissue around the knee joint is noted.
(2) Knee MRI shows a round or round-like soft tissue nodule at the tibial attachment of the anterior cruciate ligament (ACL) (i.e., the anteromedial area between the femoral condyle and the tibial plateau). On T1- and T2-weighted images, the signal intensity is akin to or slightly lower than that of surrounding muscle; on fat-suppressed sequences, it also shows signal density close to muscle. The lesion has relatively clear margins, with local ligament fiber irregularity or mild thickening near the course of the ACL.
(3) No significant joint effusion is detected, and there is no clear radiological evidence of a meniscal tear. The tibiofemoral and patellofemoral joint surfaces remain well aligned. The overall course of the ACL is visible, but there may be minor fiber injury or partial fiber abnormality.
Based on the patient’s history of trauma, difficulty in fully extending the knee, and the MRI finding of a soft tissue nodule near the ACL, the following diagnoses are considered:
A. Cyclops lesion (fibrous nodule near the anterior cruciate ligament, also known in non-surgical cases)
• Commonly seen post-ACL reconstruction, but can also occur in patients with a history of trauma who have not undergone surgery;
• Radiologically, a circular or oval fibrotic growth near the ACL insertion site is typical;
• Typical symptoms include end-range extension pain or clicking, as well as limited extension.
B. A fibrous mass at the anterior horn of the meniscus or ligament attachments
• Although meniscal tears are often visible on MRI through abnormal signals or tear lines, not all lesions are clearly evident in routine sequences;
• In this case, there is no typical radiological sign of a meniscal tear;
• Localized fibrous tissue thickening or calcification may create the appearance of a mass.
C. Cartilage or synovial lesions (e.g., synovial tumors or loose bodies)
• Some synovial proliferations or localized chondral overgrowth can present as small nodules within the joint;
• However, their signal characteristics often differ from fibrous nodules, and patients typically show more obvious joint effusion or other signs of synovitis.
Considering the patient’s age, trauma history, clinical presentation (terminal knee extension pain, “catching” or “clunk” sensation), and an MRI finding of a fibrotic nodule with signal intensity similar to muscle near the ACL, the most likely diagnosis is:
Cyclops lesion (a fibrous nodule/proliferation near the ACL in a non-surgical context).
(1) Treatment Strategy
• If symptoms (especially end-range extension limitation, painful clicking) are pronounced, arthroscopic resection of the fibrous nodule can be considered to relieve mechanical impingement and pain;
• If symptoms are relatively mild, conservative treatment such as physical therapy (e.g., physiotherapy, muscle strengthening, and range of motion exercises) can be tried first. If functional impairment persists or pain worsens, surgical intervention should be considered.
(2) Rehabilitation and Exercise Prescription Recommendations
• Acute Phase/Preoperative Phase (if arthroscopic surgery is considered)
1. Main goals: alleviate pain and inflammation, maintain joint range of motion.
2. Perform gentle knee mobility exercises (flexion and extension in a non-weight-bearing position), avoiding excessive movement that triggers pain.
3. Use modalities such as ice or heat (depending on acute or chronic stage), 15–20 minutes each time, 2–3 times per day.
• Early Postoperative or Early Conservative Rehabilitation
1. Focus first on restoring knee extension and quadriceps strength. Exercises may include straight leg raises and isometric contractions of the affected leg, 10–15 repetitions per set, 2–3 sets per day.
2. Gradually increase range of motion within tolerable limits of pain.
3. Begin balance training: supported double-leg stance progressing to single-leg stance, to prevent loss of strength and coordination.
• Mid to Late Rehabilitation (weight-bearing permitted, progressive strengthening)
1. Progressively add assisted or lightly loaded squats and lunges, 10–15 repetitions per set, 2–3 sets per day, provided there is no significant joint pain.
2. Engage in low-impact aerobic activities such as stationary cycling or using an elliptical machine for 10–20 minutes, 3–4 times a week, keeping intensity moderate to low to avoid excessive joint stress.
3. Gradually include core and lower limb proprioception training (e.g., balance board drills) to enhance stability and prevent re-injury.
• Exercise Progression (FITT-VP Principle)
1. Frequency (F): 3–5 sessions per week, adjusted according to individual recovery;
2. Intensity (I): moderate to low intensity that does not cause severe pain, adjusted based on subjective feelings and weight-bearing capacity;
3. Time (T): approximately 20–30 minutes per session, extend as tolerated;
4. Type (T): focus on functional training for joint stability and muscle strengthening, complemented by aerobic exercises;
5. Progression (P): gradually increase load and exercise difficulty every 1–2 weeks, based on pain and functional status;
6. Volume (V) and Pattern (P): segment the training sessions or phases according to individual endurance and joint stability. If significant discomfort arises, adjust the plan promptly.
Note: For patients with weaker joint function or poor bone quality, follow professional medical and rehabilitation advice to avoid secondary injury from overtraining.
Disclaimer: This report is based solely on the provided clinical history and imaging data and offers a reference analysis. It does not replace a formal diagnosis or professional opinions of offline medical institutions. If further questions arise, the patient should seek medical attention for necessary examinations and treatments.
Cyclops lesion in absence of anterior ligament reconstruction
