1. Imaging Findings

Based on the provided color Doppler ultrasound and lower limb CT angiography, the following observations can be noted:

• A “cystic” structure is visible in the right popliteal area, appearing round or oval in shape. There is a distinct blood flow signal inside, and it has a neck connecting to the adjacent artery.
• Color Doppler shows a “to-and-fro” flow pattern (the “yin-yang sign”), suggesting the presence of a pseudoaneurysm in the local artery.
• CT angiography reveals that this cystic lesion communicates with the popliteal artery. Part of the lesion contains thrombus, and there is surrounding soft tissue swelling.
• The integrity of adjacent muscles, tendons, and other soft tissue structures appears relatively preserved. No obvious bone destruction or joint structural abnormalities are observed.

2. Potential Diagnoses

Based on the above imaging findings and the patient’s history of right knee arthroscopy, possible diagnoses include:

1. Pseudoaneurysm
   • Vascular injury may result from surgical manipulation or excessive traction around the knee joint. Blood leaks out and is contained by surrounding tissue, forming a fibrous sac.
   • Typical “to-and-fro” signal on Doppler ultrasound; CT angiography shows a “saccular” structure communicating with the artery.
2. Thrombotic Arterial Dilatation or Arterial Aneurysm
   • If there is a pre-existing weakening of the arterial wall or conditions such as atherosclerosis or inflammation, intraoperative maneuvers could further damage the artery.
   • However, the history and imaging findings usually differ from a “true aneurysm,” and a pseudoaneurysm is more consistent with this clinical course and timeframe post-surgery.
3. Deep Vein Thrombosis (DVT) Accompanied by Surrounding Vascular Abnormalities
   • DVT is a standard differential diagnosis when considering lower limb swelling and pain. However, in this case, the prominent imaging feature is an aneurysm-like lesion in an artery.
   • Initial ultrasound indicated an arterial lesion rather than a simple venous thrombosis.

3. Final Diagnosis

Combining the onset of right popliteal pain and swelling on the second day post-surgery with ultrasound and CT angiography demonstrating a pseudoaneurysm communicating with the popliteal artery, the most likely diagnosis is:

Right Popliteal Artery Pseudoaneurysm (Postoperative Iatrogenic Pseudoaneurysm)

For further clarification of lesion details or preoperative evaluation, digital subtraction angiography (DSA) or repeat CT angiography can be considered under vascular surgery guidance to assist in treatment planning.

4. Treatment Plan and Rehabilitation

1. Treatment Strategy
   • Surgical or Interventional Management: Popliteal artery pseudoaneurysms often require urgent management to prevent bleeding or space-occupying complications. Common approaches include:
     • Surgical repair: Open surgical repair, patch angioplasty, or bypass grafting under direct visualization.
     • Endovascular embolization: Under angiographic guidance, endovascular embolization or covered stent placement can be performed.
   • Anticoagulant or Antiplatelet Therapy: If there is a risk of thrombus formation, anticoagulant or antiplatelet medication may be used under vascular surgical guidance, but the bleeding risk must be carefully weighed.
2. Postoperative Rehabilitation and Exercise Prescription
   • Early Protection Phase (Post-op 1-2 weeks):
     • The focus is on reducing tension in the popliteal fossa and avoiding further bleeding. Intermittent activity can be aided by braces or elastic bandages, avoiding prolonged weight-bearing.
     • Light ankle pump exercises and isometric contractions of the quadriceps can help promote lower limb circulation.
   • Intermediate Transition Phase (Post-op 2-6 weeks):
     • Once arterial repair or endovascular intervention is deemed stable, gradually increase range of motion exercises, such as seated knee extensions and straight leg raises.
     • As pain and swelling improve, low-intensity stationary cycling can be introduced to maintain joint flexibility and muscle strength.
   • Late Strengthening Phase (Post-op 6 weeks and beyond):
     • Avoid high-load or high-impact activities until the vascular condition is fully stable. Progress to low-intensity aerobic exercise, such as swimming or brisk walking.
     • In parallel, follow vascular surgery advice regarding lower limb strengthening and proprioception training, preventing knee instability.
   • FITT-VP Principle Reference:
     • Frequency (F): 3-5 sessions per week initially, gradually increasing to 4-6 times per week.
     • Intensity (I): Start at low intensity (40-50% of max heart rate), gradually increasing to moderate (50-70%).
     • Time (T): Begin with 10-20 minutes per session, gradually extending to 30 minutes or more.
     • Type (T): Low-impact activities that place minimal stress on ligaments/joints (e.g., cycling, swimming, elliptical), and later moderate-intensity walking.
     • Progression (P): Adjust exercise duration and intensity dynamically, such as increasing total time by 5-10 minutes every 1-2 weeks or slightly adding resistance.
     • Volume & Pattern (VP): Ensure a reasonable total volume of exercise. Sessions can be performed continuously or in segments, progressing as tolerated.
   • Individualized Safety Precautions:
     • If local swelling, increased skin temperature, worsening pain, or suspected bleeding occurs, stop exercising immediately and seek medical attention.
     • For patients at higher risk of bleeding, strictly follow vascular surgical advice, use elastic bandages appropriately, and maintain proper wound hygiene.

Disclaimer: This report is a reference analysis based on currently provided information and does not replace an in-person hospital consultation or the individualized opinion of a medical professional. If you have any doubts about diagnosis and treatment, please promptly consult a specialist or visit a hospital for further evaluation.