Mobile Subcutaneous Hip Lump That "Popped" and Decreased in Size
This is most likely a post-traumatic hematoma or seroma that partially decompressed when you felt the "pop," and you should start with plain radiographs followed by ultrasound to confirm the diagnosis and rule out other pathology.
Most Likely Diagnosis
The clinical scenario you describe—a mobile subcutaneous lump appearing after trauma, experiencing a "pop" sensation, then becoming smaller—is highly characteristic of a post-traumatic fluid collection (hematoma or seroma) that has partially ruptured or decompressed 1, 2. The hip/buttock region is particularly prone to this phenomenon, especially following blunt trauma 2.
Key Diagnostic Considerations:
- Post-traumatic cyst/seroma: These develop when subcutaneous hematomas fail to resolve, creating fluid-filled cysts lined with fibrous tissue at the junction between subcutaneous fat and underlying fascia 1
- Traumatic lipoma/pseudolipoma: Can develop approximately 6 months after significant trauma to the hip/buttock area, though your acute presentation with size change makes this less likely 2
- The "pop" sensation followed by size reduction strongly suggests fluid decompression rather than a solid mass
Recommended Diagnostic Algorithm
Step 1: Plain Radiographs (Initial Study)
Start with plain X-rays of the hip, as this is the fundamental first step emphasized by ACR guidelines 3. While often considered unrewarding, radiographs can:
- Identify unsuspected skeletal abnormalities (22% show bone involvement) 3
- Detect calcifications (present in 27% of soft-tissue masses) 3
- Rule out foreign bodies or other mineralized lesions
Step 2: Ultrasound (Primary Diagnostic Tool)
Ultrasound is the most appropriate next imaging study for superficial/subcutaneous masses like yours 3. Recent evidence shows:
- 95.3% sensitivity and 100% specificity for characterizing lipomas 4
- 94.1% overall sensitivity for superficial soft-tissue masses 3
- Excellent for differentiating solid from cystic lesions 3
- Can confirm fluid content and assess vascularity
Ultrasound will definitively show whether this is a fluid collection (hematoma/seroma) versus a solid mass.
Step 3: MRI (Only If Needed)
MRI is not indicated as initial imaging 3 but would be appropriate if:
- Ultrasound shows atypical features
- The mass is deep to fascia
- Clinical concern for malignancy arises
Management Based on Diagnosis
If Confirmed Post-Traumatic Fluid Collection:
Conservative management first:
Intervention if needed:
- Needle aspiration for symptomatic collections 5
- Surgical excision only if recurrent or causing symptoms 1, 5
Critical Caveat:
Post-traumatic cysts have a high recurrence rate after surgical excision 1. If aspiration or surgery is performed, careful monitoring for seroma formation and early treatment of persistent postoperative fluid collections is essential 1.
Red Flags Requiring Urgent Evaluation
While your presentation sounds benign, seek immediate evaluation if you develop:
- Rapid enlargement despite the initial size decrease
- Firmness harder than surrounding muscle 6
- Pain or neurological symptoms (suggesting nerve compression) 5
- Signs of infection (warmth, erythema, fever)
Why Size Alone Doesn't Drive Management Here
Recent evidence challenges the traditional "≥5 cm = high-risk" guideline for superficial masses. A 2025 study found that 99.8% of low-risk subcutaneous lesions <5 cm were benign, and size alone should not be the sole indicator for escalating to MRI when clinical assessment and ultrasound are reassuring 4. Your mobile, post-traumatic presentation with dynamic size change places this in the low-risk category.
The combination of trauma history, mobility, superficial location, and the "pop" with size reduction makes this overwhelmingly likely to be a benign post-traumatic fluid collection rather than a neoplastic process 1, 2.