1. Imaging Findings

Based on the provided lumbar spine MRI (sagittal T1 and T2-weighted sequences), the following observations are noted:

• In the subcutaneous tissue and fascial compartment of the lumbar region (approximately the mid to upper lumbar area), a relatively well-defined cystic fluid signal is visible, occupying the potential space between the subcutaneous tissue and the deep fascia.
• On T1-weighted images, the lesion demonstrates relatively low or iso-signal intensity, while on T2-weighted images it appears hyperintense. Some fibrous band-like low signals or heterogeneous areas are visible within.
• A thin low-signal rim is observed around the lesion, suggesting the possible formation of a fibrous capsule.
• No apparent bony destruction or vertebral structural abnormality is noted.

Overall, the imaging findings correlate with the clinical presentation of back pain, local soft tissue swelling, and tenderness.

2. Potential Diagnoses

Considering the patient’s age of 16, her high-intensity ballet training, clinical presentation, and imaging features, the following differential diagnoses are proposed:

1. Morel-Lavallée Lesion (Closed Subcutaneous Soft Tissue Degloving Injury):
   Caused by repeated impact or shearing forces resulting in a degloving of the subcutaneous tissue from the deep fascia, leading to hematoma and lymphatic fluid accumulation. Commonly seen in lateral hip or lumbar regions. MRI typically shows fluid collection with a fibrous capsule, which aligns well with this case’s imaging and history.
2. Chronic Hematoma or Vascular Lesion:
   Repeated minor trauma can result in a chronic hematoma forming a similar fluid collection. However, such lesions usually lack the typical subcutaneous-fascial degloving appearance, and the capsule formation is less pronounced.
3. Soft Tissue Cyst or Infection (Abscess):
   If the patient had significant signs of redness, heat, pain, or elevated laboratory markers (e.g., WBC count, CRP), an infectious lesion would need to be ruled out. However, the current history does not support a pronounced infectious process.

3. Final Diagnosis

Taking into account the patient’s young age, high-intensity dance training, 6-week duration of symptoms, and imaging findings indicating a fluid collection with a fibrous capsule between the subcutaneous tissue and deep fascia, the most likely diagnosis is:

Morel-Lavallée Lesion (Closed Degloving Injury).

If there is still uncertainty, further assessment can be done with ultrasound or, if needed, aspiration and analysis of the lesion’s contents (blood, lymphatic fluid, or inflammatory exudate) to confirm the diagnosis.

4. Treatment Plan and Rehabilitation

Depending on the phase and severity, the recommended management strategies for a Morel-Lavallée lesion include:

1. Conservative Treatment (for relatively short clinical course, mild symptoms):
   Use compression bandages, elastic wraps, or braces for support, and apply ice to reduce pain and swelling. High-intensity dance or weight-bearing activities should be limited to allow time for soft tissue recovery.
2. Interventional Therapy (for prolonged cases, recurrent fluid accumulation, or larger cystic collections):
   Under ultrasound or CT guidance, perform aspiration of the fluid collection, followed by possible injection of a sclerosing agent (e.g., sclerosant or fibrin glue) to promote adherence of the cyst walls.
3. Surgical Treatment (in severe cases, or where infection or persistent fluid accumulation is involved):
   Surgical intervention involves open drainage, thorough debridement of the lesion, and possibly repair or secondary suturing of the damaged tissues.

During rehabilitation, a gradual progression (FITT-VP principle) is recommended, as illustrated below:

• Frequency: Begin with 2–3 sessions per week of low-intensity core stability exercises. Increase to 3–5 sessions per week as pain subsides and healing progresses.
• Intensity: Start with low-resistance/low-impact exercises (e.g., bridges, four-point kneeling). Gradually progress to moderate-resistance drills or basic dance movements, avoiding excessive stretching or high-impact actions.
• Time: Allocate about 20–30 minutes for each core and lower-limb workout, focusing on improving lumbar spine stability and flexibility. Extend the duration as local recovery permits.
• Type: Initially focus on eccentric muscle control and core activation exercises (e.g., planks, Bird-Dog). Gradually introduce dynamic balance drills and lower-limb flexibility/strength training to meet the demands of dance activities.
• Volume: Increase the total training load as rehabilitation advances, ensuring it remains within the patient’s tolerance and does not trigger significant pain or swelling.
• Progression: Once pain is significantly reduced, with no new fluid accumulation or swelling and improved strength and stability, increase the number of sets, escalate exercise difficulty, and gradually resume basic ballet moves. High-impact or rotational jump movements should be avoided until full recovery is achieved.

Disclaimer

This report is based on the current medical history and imaging data provided and aims to offer preliminary references for healthcare professionals and patients. It is not a substitute for in-person diagnosis and treatment by a clinical physician. Please follow the advice of your healthcare provider for specific treatment decisions. If you have any concerns or changes in your condition, you should seek medical attention promptly.