A 30-year-old runner is referred for a magnetic resonance imaging (MRI) of his right forefoot by an orthopaedic surgeon. The patient complained about having a soft lump, painful with palpation, on the plantar aspect of the first metatarsal head.
MRI of the forefoot showed a well-defined fluid collection located within the subcutaneous plantar fat pad of the first metatarsal head. The lesion was 2 cm wide and had homogeneous low-signal intensity on T1-weighted images (WI) and high-signal intensity on T2WI sequences, and sharp margins (Fig. 1).
Adventitious bursitis is an acquired soft-tissue lesion caused by friction or excessive pressure and microtrauma between bone surface and overlying soft tissues. It is a fluid collection formed within the subcutaneous fat, more frequently on the forefoot, at the lateral or plantar aspect of the first metatarsal head, and sometimes of the fifth, even in asymptomatic individuals [1]. Because of this, several sport activities are often related to it, such as running or playing tennis.
The most common symptoms related to this lesion are pain when the foot is planted, similar to metatarsalgia, and palpable lump. The acute onset of the symptoms and clinical background make clinical suspicion normally easy. However, its association with other concomitant lesions, such as Morton´s neuroma or stress fracture [2], make radiological examination necessary. Moreover, some less likely diagnosis, like epidermal cyst of the sole, haematoma or soft-tissue tumours should be ruled out.
Although a soft-tissue ultrasound can easily get to the right diagnosis, MRI is recommended as the diagnostic test of choice when adventitious bursitis is suspected, mainly because of the best advice for concomitant lesions [3]. Intravenous contrast agent administration is not usually necessary, though it may be useful when the lesion has complex characteristics, to make differential diagnosis with soft-tissue tumours.
Imaging findings are typical, and generally consist of a simple fluid collection within the plantar fat pad of the metatarsals, well-defined, hypointense on T1WI sequences and hyperintense on T2WI.
The treatment of choice for this disease is conservative, and should include a combination of analgesics/anti-inflammatory drugs (NSAIDs or COX-2) and physical therapy. It is recommended as well to avoid painful shoes and apply heat and cold. Surgical excision of the bursa may be an option when conservative treatment is not enough.
In conclusion, adventitious bursitis should be the main diagnosis to consider when a palpable painful lump on the plantar fat of the first metatarsal is noticed, especially in sportsmen or patients undergoing repeated trauma over that area. MRI is the technique of choice in these cases, mainly to assess accompanying injuries.
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Adventitious bursitis
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Based on the provided MRI images of the right forefoot, a clearly demarcated fluid-signal lesion can be observed in the subcutaneous fatty layer on the plantar side of the first metatarsal head:
These imaging characteristics suggest a fluid-containing cystic lesion in the subcutaneous tissue, consistent with bursitis or a bursal-like lesion.
This condition arises due to repeated friction, pressure, or minor trauma between a bony prominence and the skin or soft tissue. It is relatively common among runners and other physically active individuals, often presenting with plantar-side pain and tenderness. MRI findings typically reveal a simple fluid-signal lesion.
Usually occurs in the interdigital nerve sheath, with clinical symptoms including forefoot pain and numbness. Although the patient in this case complains of forefoot pain, Morton’s neuroma typically appears on MRI as thickening of the tissue around the nerve, which differs in shape from this case.
Often associated with bone marrow edema signals or cortical interruption. As no notable bone signal abnormality is found in this case, the likelihood of a stress fracture is relatively low.
Examples include epidermoid cysts or benign soft tissue tumors, which often have characteristic signal or enhancement features. Further differentiation would require correlation with clinical symptoms and imaging characteristics. However, this case more closely aligns with a simple fluid presentation indicative of bursitis.
Considering the patient’s age (30 years old), history of running, clinical presentation (pain on the plantar side of the first metatarsal head with a palpable soft mass), and the MRI demonstration of a simple fluid signal, the most likely diagnosis is: Acquired Bursitis (Adventitious Bursitis).
No additional contrast-enhanced imaging or invasive examinations are currently necessary. If pain persists despite clinical treatment or if the lesion exhibits more complex signal changes, further evaluation or a biopsy may be considered to rule out other rare soft tissue tumors.
The primary approach is conservative treatment, complemented by a stepwise exercise program following an appropriate exercise prescription.
If long-term conservative treatment fails and symptoms persist, surgical removal of the bursa may be considered. Proper wound healing and a structured foot-loading rehabilitation program are essential post-surgery.
Disclaimer: This report provides a reference for medical analysis and is not a substitute for an in-person consultation or professional medical advice. Any specific treatment plan should be determined by a qualified physician or legitimate hospital based on the patient’s actual condition.
Adventitious bursitis
