What are the differential diagnoses for a soft, non‑tender, mobile mass anterior to the right patella in a patient after kneeling, with no erythema, warmth, or fever?

Differential Diagnoses for Soft, Non-Tender Mobile Mass Anterior to Right Patella

The most likely diagnosis is prepatellar bursitis (chronic or resolving), followed by lipoma, with less common possibilities including ganglion cyst, synovial plica/septum, or prepatellar neuroma.

Primary Differential Diagnoses

Most Likely: Prepatellar Bursitis

• Prepatellar bursitis classically presents as a soft, mobile, non-tender mass anterior to the patella following repetitive kneeling trauma 1
• The absence of erythema, warmth, and fever effectively rules out acute septic or inflammatory bursitis 1
• Chronic or resolving prepatellar bursitis remains soft and mobile but loses the acute inflammatory signs 1

Lipoma

• Lipomas are the most common soft-tissue tumor and present as soft, mobile, non-tender masses that can occur in the prepatellar region 1
• Physical examination alone correctly identifies only 85% of lipomas, necessitating imaging confirmation 1
• The superficial location and characteristic soft consistency make this a strong consideration 1

Ganglion Cyst

• Ganglion cysts can arise from the knee joint capsule or adjacent structures and present as soft, mobile masses 2
• These are typically non-tender unless compressed and can occur in the prepatellar region 2

Synovial Plica or Non-Perforated Septum Suprapatellaris

• Incomplete involution of embryologic synovial membranes can create a suprapatellar septum that presents as a soft-tissue swelling above the knee, sometimes mistaken for a tumor 3
• These are typically mobile and non-tender unless symptomatic 3

Prepatellar Neuroma

• Traumatic prepatellar neuroma from injury to the infrapatellar branch of the saphenous nerve presents as a palpable fusiform nodule with point tenderness and positive Tinel's sign 4
• The absence of tenderness in your case makes this less likely but not excluded 4

Less Common but Important Considerations

Glomus Tumor

• Glomus tumors anterior to the patellar tendon present as painful, soft, mobile masses that are typically exquisitely tender to light touch 5
• The absence of pain in your case makes this diagnosis unlikely 5

Soft-Tissue Sarcoma

• While rare (<1% of malignancies), soft-tissue sarcomas must be considered in any unexplained soft-tissue mass, particularly when size or symptoms seem disproportionate to injury 2
• The benign clinical features (soft, mobile, non-tender) make sarcoma less likely but do not exclude it 1

Recommended Diagnostic Approach

Step 1: Plain Radiographs (Initial Study)

• Plain radiographs must be obtained first for any suspected musculoskeletal soft-tissue mass, as they reveal calcifications in 27% of cases, bone involvement in 22%, and intrinsic fat in 11% 1, 6
• Radiographs can identify characteristic patterns such as phleboliths (hemangioma) or peripheral ossification (myositis ossificans) 1
• They exclude skeletal abnormalities that may masquerade as soft-tissue lesions 6

Step 2: Ultrasound for Superficial Masses

• Ultrasound is highly accurate for superficial prepatellar masses, with 94.1% sensitivity and 99.7% specificity for characterizing lipomas, vascular malformations, and cysts 1
• US can confirm prepatellar bursitis by demonstrating fluid-filled bursal distension 1
• Characteristic lipoma features on US include minimal acoustic shadowing, minimal vascularity, and simple curved echogenic lines within an encapsulated mass 1

Step 3: MRI if Diagnosis Remains Uncertain

• MRI is the definitive imaging modality when US or clinical features are atypical, providing tissue characterization and relationship to neurovascular structures 6
• MRI sequences (T1, T2, STIR) without contrast are typically sufficient for diagnosis 6

Critical Pitfalls to Avoid

• Do not rely solely on clinical examination; even experienced clinicians misidentify 15% of lipomas by palpation alone 1
• Do not dismiss the mass as benign without imaging; soft-tissue sarcomas can occasionally present with deceptively benign features 2
• If imaging raises any suspicion for sarcoma, refer to a musculoskeletal oncologist before performing biopsy, as poorly planned biopsies adversely affect prognosis and may necessitate amputation 2, 6
• Do not perform MRI as the initial study; plain radiographs must precede MRI to assess for mineralization and bone involvement 6
• Special vigilance is warranted when the mass is not in a typical position, when size seems disproportionate to injury, or when symptoms persist beyond expected duration 2