A 39-year-old female with a history of ACL reconstruction 10 years ago presented with restriction of movement and occasional pain in the right knee.
MRI demonstrated a 10mm ovoid soft tissue lesion was found in the anterior intercondylar notch, which showed intermediate signal intensity T2, PD and PDFS MR images. (Figures 1, 2 and 3). The imaging findings were consistent with localized anterior arthrofibrosis.
A complete tear of the ACL graft was noted with associated buckling of the PCL (Figure 4). A vertical tear involving the body of the medial meniscus was also noted. (Figure 5)
Mild knee joint effusion was present.
Anterior arthrofibrosis, also known as cyclops lesion, is a fibrous nodule in the intercondylar notch near the tibial insertion of the ACL. The arthroscopic appearance of the soft tissue mass with its surface vessels is similar of a "cyclops." (1)
In patients following ACL repair, Cyclops lesions are rare with an estimated frequency of ~5% They have also been described in patients without ACL reconstruction. (2)
After surgery, a fibroproliferative process has been linked to the formation of cyclops lesions. Fibrous tissue, fibrocartilage, bone, synovium, and fat from the infrapatellar fat pad all contribute to the formation of a cyclops lesion. A smaller intercondylar notch may also contribute to cyclops formation.
Even when the ACL is clinically or radiologically intact, this lesion is thought to originate from the microtrauma of sub-clinically damaged ACL fibres.(3)
Generally, the majority of the patients present with pain, progressive loss of extension of the knee joint and audible clunk at terminal extension. (4)
On MRI, a soft tissue mass can be detected anteriorly within the intercondylar notch in proximity to the reconstructed ACL at the tibial insertion.
These lesions exhibits intermediate to low signal intensity with all sequences and there is variable contrast enhancement. (5,6) Due to local irritation of the lesion because it is pinched between the two bones, cyclops lesions have higher signal intensity than is typically seen in fibrous lesions.
Arthroscopic excision is the treatment of choice with an excellent prognosis.
Cyclops lesion
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1. On the right knee MRI sequences (sagittal, coronal, and axial views), a soft tissue nodule is observed near the tibial attachment of the anterior cruciate ligament (ACL), located in the anterior portion of the intercondylar notch of the femur.
2. The nodule shows relatively low-to-intermediate signal intensity on both T1-weighted and T2-weighted images, with fairly clear margins and mild enhancement in some areas.
3. The lesion is situated close to the reconstructed ACL graft, appearing to potentially cause some mechanical interference with joint flexion-extension movements.
4. Other joint structures, including the femoral condyle, tibial plateau, and menisci, show no obvious new fractures, significant meniscal tears, or other notable abnormalities.
Based on the patient’s history of ACL reconstruction and the MRI findings indicating a nodular lesion, the following diagnoses are considered:
1. “Cyclops” lesion (anterior arthrofibrosis or nodular fibrosis of the ACL): This lesion commonly occurs after ACL reconstruction, presenting as a fibrous or fibrocartilaginous nodule attached to the tibial insertion of the graft, potentially causing limited knee extension and pain.
2. Local scar proliferation or small nodular neoplasm: Focal scar or fibrous tissue proliferation is common after joint surgery, but if signal irregularity is more pronounced or abnormally located, other nodular lesions should be considered.
3. Other intra-articular soft tissue masses: Such as intra-articular loose bodies, synovial cysts, or inflammatory changes in the fat pad. However, these conditions usually have distinct morphological features on imaging, which do not fully match the appearance in this case.
Considering the patient is a 39-year-old female who underwent ACL reconstruction 10 years ago, now presenting with limited knee extension and some degree of pain, along with MRI findings of a low-to-intermediate signal soft tissue lesion near the ACL tibial attachment in the femoral intercondylar notch, the most likely diagnosis is:
“Cyclops” lesion (also known as anterior arthrofibrosis or Cyclops lesion).
This lesion is often associated with fibrous or cartilaginous tissue proliferation post-ACL reconstruction and can lead to reduced joint mobility and pain. For further confirmation, arthroscopic evaluation and histopathological examination may be considered.
1. Treatment Strategy:
- Conservative Management: For patients with mild symptoms and minimal functional limitations, conservative treatments such as joint mobilization, physical therapy, and anti-inflammatory/analgesic medications can be attempted first.
- Arthroscopic Surgical Resection: If joint mobility is significantly restricted, the lesion is large, or symptoms notably affect daily activities, arthroscopic resection of the fibrous nodule is often effective and can improve joint function.
2. Rehabilitation/Exercise Prescription Suggestions (FITT-VP principle):
- Phase 1 (Early Recovery Phase):
• Frequency: 1–2 times per day of gentle joint mobility exercises.
• Intensity: Perform passive or active range of motion exercises within a pain-free or slightly uncomfortable limit. Avoid forceful knee extension or excessive weight bearing.
• Time: About 5–10 minutes per session, gradually increasing.
• Type: Joint mobilization exercises, gentle isometric quadriceps contractions (e.g., seated tightening and relaxing of the quadriceps), with local cold/heat therapy as appropriate.
• Progression: Gradually increase motion range and repetitions based on pain and swelling response.
- Phase 2 (Functional Strengthening Phase):
• Frequency: 3–4 times per week of rehabilitation exercises, with additional lower limb strength training sessions as needed.
• Intensity: Increase resistance (e.g., resistance bands, light weights) within a pain-free range to strengthen the muscles around the knee, especially the quadriceps and hamstring groups.
• Time: 15–20 minutes per session, increasing training volume as tolerated.
• Type: Seated or supine straight leg raises, half-squat exercises, slow cycling, or other low-impact aerobic activities.
• Progression: Gradually increase flexion-extension range and loading as symptoms improve.
- Phase 3 (Return-to-Activity Phase):
• Frequency: 2–3 times per week of sport-specific or skill-based exercises.
• Intensity: Gradually approach the intensity required for daily or athletic activities without causing notable pain or swelling.
• Time: 20–30 minutes per session or according to the needs of the specific activity.
• Type: Proprioception training, core stability exercises, and a gradual return to running, jumping, or other functional movements. If the patient has specific athletic demands, proceed under the supervision of a specialist.
• Progression: Emphasize accuracy and stability of movements, steadily returning to normal training loads.
- Precautions:
• Carefully monitor joint swelling, pain, and range of motion. In case of significant discomfort or joint effusion, consult with the physician or rehabilitation therapist promptly and adjust the exercise plan accordingly.
• For patients with osteoporosis or other underlying conditions (e.g., cardiovascular or pulmonary issues), reduce exercise intensity and incorporate additional rest periods as needed to ensure safety.
This report is based on the clinical information and imaging data provided and serves as a reference. It does not replace an in-person consultation or professional medical advice. If you have any further concerns or if your condition changes, it is recommended to seek medical attention and undergo appropriate examinations and treatments promptly.
Cyclops lesion
