1. Imaging Findings

Based on the provided knee photographs and ultrasound images, a localized protrusion can be observed in the left knee when fully flexed. The ultrasound findings show:

• In the area below the left patella and around the joint, echoes from adipose tissue are visible, presenting a continuous form, possibly connected to the anterior portion of the knee (the patellar ligament or the fat pad region).
• The fat echo locally protrudes into the subcutaneous tissue, with mild mobility or dynamic features.
• The overall shape has clear margins, and no significant abnormal blood flow signals are noted.

These findings suggest a defect or weakness in the patellar lateral supporting band (lateral patellar ligament) of Hoffa’s fat pad, causing a herniation-like protrusion into the subcutaneous tissue. No obvious abnormalities were found in the surrounding bone or soft tissue on ultrasound, and no evident fractures, soft tissue tears, or significant inflammatory changes were observed.

2. Potential Diagnoses

Taking into account the patient’s age, clinical history (no notable trauma history, no pain), and imaging findings, the following potential diagnoses can be considered:

1. Hoffa’s Fat Pad Herniation
   This matches the clinical and ultrasound findings. It commonly occurs when there is a defect or weakness in the patellar ligament or lateral patellar supporting structures, allowing the fat to protrude into the subcutaneous tissue.
2. Lipoma or other benign soft tissue masses
   These typically arise within subcutaneous fat tissue and may present as a localized protrusion, but they usually do not have a significant relationship with joint movement dynamics.
3. Synovial cyst or synovial protrusion (e.g., meniscal cyst or other rare synovial cysts)
   Certain internal structures of the knee can develop cystic changes, forming a mass, often located inside or around specific areas of the synovial membrane. Mild discomfort or tenderness may accompany these.

3. Final Diagnosis

Taking into consideration the patient’s age, the painless nature of the protruding mass that appears with knee flexion, and the ultrasound demonstration of fat pad continuity protruding, the most likely diagnosis is:

Hoffa’s Fat Pad Herniation.

This type of finding is generally benign and often related to a local weakness in the structural tissues. If the patient does not exhibit additional symptoms, observation can be considered initially.

4. Treatment Plan and Rehabilitation

Currently, the patient experiences no pain or functional impairment. The overall treatment approach can be delineated as follows:

1. Conservative Treatment
   • If the mass remains stable, without marked pain or activity limitations, periodic follow-up observation is advised.
   • Parents should monitor any changes in the size or shape of the mass. If pain or restricted movement appears, a prompt medical consultation is recommended.
2. Indications for Surgery
   • If the mass continues to enlarge, causing persistent pain or functional limitations, surgical repair of the weakened area may be considered to prevent further herniation of the fat pad.
3. Rehabilitation and Exercise Prescription
   • In the absence of significant symptoms, moderate low-impact exercise (e.g., walking, gentle play activities) can help maintain normal lower limb development.
   • Avoid prolonged and repeated deep knee flexion (like squatting or kneeling), especially when the mass appears repeatedly or increases in size.
   • If discomfort occurs at a later stage, gradual introduction of knee extension exercises and quadriceps strengthening (for example, seated knee extensions within tolerance) can be considered. Start at 2–3 times per week, each session 10–15 minutes, and increase as tolerated.
   • If further intervention is necessary, post-surgery or after treatment, undertake joint range of motion and muscle strength training under professional rehabilitation guidance. Utilize the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, and Progression) to customize the regimen, ensuring safety and effectiveness.

5. Disclaimer

This report, based on the provided clinical history and imaging data, is for reference only and does not replace an in-person evaluation or professional medical advice. If any symptom changes occur or if there are any questions, it is recommended to consult an orthopedic or pediatric surgery specialist.