I. Radiological Findings

In this case, MRI images (sagittal, axial, and other sequences) show a relatively well-defined cystic lesion adjacent to the bone cortex in the posterior aspect of the right knee:

• The lesion appears multi-compartmental or lobulated. On T2-weighted images, it demonstrates notable hyperintensity, and on T1-weighted images, it shows medium to low signal intensity.
• The lesion is confined to the outer cortical surface of the bone and shows a cystic change under the local cortex, with no obvious involvement of the bone marrow cavity.
• The lesion extends slightly into the surrounding soft tissue, but the boundary remains relatively clear, with no marked soft tissue mass-like enhancement.
• No significant meniscal tears or other ligamentous abnormalities were detected.
• No obvious bone destruction or significant cartilage surface damage was observed.

II. Potential Diagnoses

Based on the patient’s age, symptoms (posterior knee pain), imaging presentation, and characteristics of the lesion, possible differential diagnoses include:

1. Subperiosteal/Periosteal Ganglion Cyst
   Rationale: On MRI, it typically appears as a cystic lesion with high signal intensity on T2, located along the outer cortical surface or subperiosteally, and often shows no obvious enhancement. It may be associated with neighboring joint degeneration or chronic irritation.
2. Baker’s Cyst (Popliteal Cyst)
   Rationale: This also presents a cystic change. However, it usually communicates with the meniscus or joint cavity and is most frequently found in the popliteal fossa. In this case, there is no definitive sign of communication with the joint.
3. Soft Tissue Tumors (e.g., Synovial Sarcoma)
   Rationale: Theoretically, soft tissue tumors can present as mass lesions, sometimes with cystic components. However, these lesions typically show heterogeneous signals and special enhancement patterns. In this case, the lesion characteristics are more consistent with a benign cystic lesion.
4. Meniscal Lesion with Associated Cystic Change
   Rationale: In certain circumstances, complex meniscal tears may produce cystic extensions. However, no clear meniscal tear or related changes are seen in this case, making this diagnosis less likely.

III. Final Diagnosis

Considering the patient’s age, clinical presentation (mild, non-traumatic, posterior knee pain), and the imaging findings (subperiosteal cystic lesion with T2 hyperintensity and no significant destructive changes), the most likely diagnosis is:

“Subperiosteal/Periosteal Ganglion Cyst”

If clinical symptoms are significant or further clarification is needed, image-guided aspiration or surgical removal with pathological examination may be considered to rule out other rare soft tissue tumors.

IV. Treatment Plan and Rehabilitation Program

1. Treatment Strategies

• Conservative Treatment: If symptoms are mild, close observation or local immobilization can be adopted first, complemented by non-steroidal anti-inflammatory drugs (NSAIDs) for pain relief and physical therapy.
• Image-Guided Intervention: Under ultrasound or CT guidance, cyst aspiration, drainage, and irrigation can be performed. If necessary, a small amount of corticosteroid can be injected to reduce local inflammatory response.
• Surgical Treatment: For severe cases, recurrent lesions, or suspected communication with the joint space, surgical exploration and complete cyst removal may be considered, with repair or disruption of any channel between the cyst and the joint to reduce the risk of recurrence.

2. Rehabilitation/Exercise Prescription Suggestions

Systematic and progressive rehabilitation during treatment and postoperatively can help restore joint function and reduce late complications:

• Early Stage (acute phase or first 2–4 weeks postoperatively):
  • Focus on swelling reduction, pain relief, and protection of the affected area.
  • Use braces or knee supports as appropriate to limit excessive flexion-extension.
  • Practice isometric quadriceps contractions and active ankle exercises to promote blood circulation in the lower limbs.
• Intermediate Stage (4–8 weeks postoperatively or when stable):
  • Gradually increase the range of motion exercises, such as seated or supine knee flexion-extension.
  • Introduce mild resistance training with a low-resistance band to strengthen the quadriceps, hamstrings, and calf muscles.
  • Ensure rehabilitation exercises three to four times per week, with intensity controlled to avoid provoking significant pain.
• Late Stage (8 weeks postoperatively or during symptom improvement):
  • Progressively resume weight-bearing activities; under the guidance of a physical therapist, try low-impact aerobic exercises such as cycling or swimming.
  • If recovery is favorable, gradually incorporate lateral stability exercises, balance training, and light jogging/jumping.
  • Follow the FITT-VP principle to arrange training Frequency, Intensity, Time, Type, and Progression:
    — Exercise 3–5 times a week, with low to moderate intensity for 20–30 minutes, then gradually increase.

Throughout the rehabilitation process, exercises should be tailored to individual circumstances (e.g., bone quality, cardiopulmonary function, current pain levels, and surgical scope). If persistent or worsening pain occurs, discontinue the specific exercise and seek medical review.

Disclaimer: This report is a reference-based analysis using limited information and cannot replace in-person consultation or a professional medical opinion. If you have further questions or if symptoms worsen, please consult a specialist promptly.