1. Radiological Findings

• Based on MRI cross-sectional, coronal, and sagittal images, a well-defined cystic lesion is observed in the medial intercondylar notch area of the right knee.
• The internal signal of the lesion appears uniformly hyperintense (high signal on T2-weighted and STIR sequences, low or isointense on T1 and PD sequences), consistent with a cystic or fluid-containing lesion.
• The cyst wall is relatively thick and closely associated with the fibers of the anterior cruciate ligament. There is slight bulging in this area, which may cause deep knee joint pain.
• No significant swelling of the surrounding soft tissues or substantial bone destruction is seen, and there is no obvious bone marrow edema signal.

2. Potential Diagnoses

1. Cystic lesion originating from the cruciate ligament at the medial intercondylar notch of the knee (i.e., intra-articular ganglion cyst)
   Cause: Due to mechanical friction and degenerative changes, the tissue within or near the ligament fibers may form a cystic lesion rich in mucin.
   Imaging features: High signal on T2/STIR sequences and low/isointense on T1. It is adjacent to the anterior/posterior cruciate ligament and often presents as an oval or multiloculated cyst.
2. Perimeniscal cyst (parameniscal cyst)
   Cause: When there is meniscal degeneration or tear, joint fluid may pass through the tear to form a cyst; commonly found near the meniscus of the knee.
   Imaging features: Often located at the periphery of the meniscus (lateral or medial side), differing in location from cruciate ligament-related cysts.
3. Synovial cyst or synovial tumor
   Cause: Synovial hyperplasia or other synovial lesions can exhibit cystic changes; these include synovial sarcoma and benign lesions such as synovial hemangioma.
   Imaging features: The signal intensity may be heterogeneous or show enhancement, differing from the purely high T2 signal typical of simple cysts.
4. Other intra-articular myxoid lesions (e.g., loose bodies, chondroma-like lesions)
   Imaging features: These may be associated with calcification or cartilage-like signals, or appear as multiple lesions. Their relationship to the ligament is usually less pronounced compared to a ganglion cyst.

3. Final Diagnosis

Combining the patient’s age (50 years), primary complaint of deep knee pain exacerbated by full flexion, and the MRI findings (a uniform high-signal cystic lesion within or adjacent to the fibers of the anterior cruciate ligament), the most likely diagnosis is an intra-articular ganglion cyst originating from the anterior cruciate ligament (knee ganglion cyst).

For further confirmation, arthroscopic evaluation and pathological biopsy can be performed if necessary. However, given the clinical history and characteristic imaging features, the diagnosis here is quite clear.

4. Treatment Plan and Rehabilitation

4.1 Treatment Strategies

• Conservative management: For patients with smaller cysts and less severe symptoms, oral anti-inflammatory and analgesic medications, local corticosteroid injections, and physical therapy (e.g., ultrasound therapy) can be considered to alleviate discomfort.
• Arthroscopic evaluation and surgical excision: If the cyst is large or if there is significant mechanical locking, restricted range of motion, or persistent pain, arthroscopic exploration and cyst excision, along with removal of the mucinous contents, may be considered to reduce mechanical irritation.

4.2 Rehabilitation/Exercise Prescription

During the gradual recovery of joint function, a progressive exercise regimen (FITT-VP) is recommended as follows:

1. Type of exercise (Type):
   Focus on improving joint flexibility and muscle strength. This includes non-weight-bearing or low-weight-bearing knee flexion and extension exercises, wall-assisted static squats, and lower-limb muscle training (e.g., quadriceps).
2. Exercise frequency (Frequency):
   Begin with 3-4 sessions per week and gradually increase to 5 sessions per week as tolerated. Reduce frequency during the early postoperative phase or when pain is severe, avoiding overly intense activities.
3. Exercise intensity (Intensity):
   Start with low to moderate intensity, ensuring pain is not provoked or is only minimal. Monitor intensity using heart rate or perceived exertion levels (RPE 11-13).
4. Exercise duration (Time):
   Each session should last 10-20 minutes initially, extending as tolerated, avoiding prolonged weight-bearing exercises.
5. Mode of exercise (Mode):
   Consider seated knee extension exercises, straight-leg raises, and resistance band training. Transition to low-impact activities like cycling or swimming as symptoms improve.
6. Progression (Progression):
   Once pain subsides and range of motion improves, gradually introduce weight-bearing exercises (e.g., partial or half squats), followed by higher-level strength and proprioceptive training, such as balance board exercises or single-leg stance stability drills.
7. Precautions:
   For patients with lower bone density, take measures to prevent falls. For those with concerns about cardiovascular fitness, exercise intensity should be adjusted under professional guidance.

Disclaimer

This report provides a reference analysis based on imaging and clinical information. It does not replace an in-person consultation or the advice of a professional physician. If further treatment is needed or any discomfort occurs, please seek medical attention promptly and proceed with diagnosis and rehabilitation under professional guidance.