1. Imaging Findings

Based on the provided ultrasound, X-ray, and MRI images, a clear cystic lesion with fluid-like density or signal is observed in the posterior aspect of the knee (popliteal fossa). The specific characteristics are as follows:

• Ultrasound: A hypoechoic (or anechoic) area in the popliteal fossa is visible, sometimes showing septations or lobulated appearances. There may be small internal echoes that could indicate mild exudate or protein content.
• X-ray: Widened soft tissue shadow in the posterior joint region; in some cases, a slightly convex soft tissue outline can be seen. No obvious bone destruction or calcification is noted.
• MRI:
  • T1-weighted images: The cystic lesion appears as a relatively low signal intensity with a well-defined margin.
  • T2-weighted/STIR: The cyst contents display high signal intensity, consistent with a fluid-like (water) signal. A cystic pouch extending from the joint capsule or between the semimembranosus and gastrocnemius tendons is visible. In some cases, accompanying changes in the medial meniscus or cartilage can be noted.

This cystic lesion communicates with or is adjacent to the knee joint cavity, especially in the space between the semimembranosus and gastrocnemius tendons, consistent with the typical characteristics of a Baker’s cyst (popliteal cyst).

2. Potential Diagnoses

1. Baker’s Cyst (Popliteal Cyst): Patients with a history of meniscal issues often experience joint effusion and increased intra-articular pressure, leading to the formation of a synovial cyst behind the knee.
2. Meniscal Cyst: Generally related to meniscal degeneration or tears, presenting with cystic changes near the meniscus itself and often located close to the joint line.
3. Popliteal Artery Aneurysm: Appears as a popliteal fossa mass with palpable pulsation. Ultrasound typically shows abundant blood flow signals. (In this case, ultrasound suggests a classic fluid-filled cyst with no significant arterial flow signals, making this diagnosis less likely.)
4. Hematoma or Infectious Abscess: May form after acute trauma or infection, usually accompanied by significant clinical symptoms such as redness, local heat, pain, or a history of trauma. Less typical in this case.

3. Final Diagnosis

Considering the patient’s age (35 years), history of meniscal tear, and the imaging findings, the most likely diagnosis is Baker’s Cyst (Popliteal Cyst).

If further confirmation is needed, ultrasound-guided aspiration and pathological examination can be performed (to exclude other rare cystic lesions). However, clinical and imaging diagnoses are usually sufficiently clear.

4. Treatment Plan and Rehabilitation

1. Treatment Strategies

• Conservative Management: Includes rest, limb elevation, ice therapy, or non-steroidal anti-inflammatory drugs (NSAIDs) to control pain and inflammation.
• Joint and Cyst Aspiration: Under ultrasound or other imaging guidance, cyst fluid can be aspirated, and corticosteroids may be injected to reduce inflammation and fluid build-up. However, caution should be taken regarding potential infection risks.
• Surgical Treatment: If the cyst is large, causes significant symptoms, or if severe underlying pathology (e.g., a major meniscal tear) is present, arthroscopic repair of the meniscus or debridement may be considered, along with addressing the source of the cyst. This is typically considered if conservative methods fail or there are recurrent episodes.

2. Rehabilitation / Exercise Prescription Advice

Rehabilitation principles: gradual progression, individualized approach, and avoidance of excessive load on the knee joint.

• Acute Phase (when symptoms are pronounced):
  • Frequency: Several short sessions of joint movement daily, limiting activity to avoid exacerbating pain.
  • Intensity: Low-intensity, low-load exercises such as ankle pumps and straight leg raises.
  • Duration: 5–10 minutes per session, potentially multiple times per day.
  • Style: Avoid deep squats, high-impact running, jumping, or similar exercises.
• Subacute Phase (when the cyst and knee pain have subsided to some extent):
  • Frequency: 3–4 times per week.
  • Intensity: Low-to-moderate intensity exercises aimed at knee function, such as seated knee extensions or light resistance training, gradually increasing the load.
  • Duration: 15–20 minutes per session, extend gradually according to tolerance.
  • Style: Under professional guidance, introduce balance and muscle strengthening exercises, possibly with resistance bands or light weights.
• Recovery Phase (when functional activities are largely restored):
  • Frequency: 3–5 times per week.
  • Intensity: Progressively move to moderate-intensity training, focusing on muscular endurance and stability around the knee.
  • Duration: 20–30 minutes each session, adjustable based on individual response.
  • Style: Light running, cycling, swimming (all relatively knee-friendly), combined with stretching exercises to maintain flexibility.

Throughout the rehabilitation process, pay close attention to any knee pain or swelling. If significant discomfort or worsening occurs, seek medical evaluation promptly.

3. Special Considerations

• If the patient has other knee joint pathologies (e.g., severe cartilage damage, ligament instability), targeted treatment under professional guidance is necessary.
• In cases of osteoporosis or poor cardiopulmonary function, exercise intensity should be tailored to the individual for safety.

Disclaimer: This report is for reference purposes only and should not replace an in-person consultation or a professional medical opinion. If you have any concerns or if symptoms worsen, please seek further evaluation at a hospital.