Myxochondroid metaplasia of the plantar foot: a distinctive pseudoneoplastic lesion resembling nuchal fibrocartilaginous pseudotumor and the equine digital cushion.

This study investigates an unusual benign chondroid lesion found in the plantar foot that occurs due to chronic mechanical stress, termed ‘myxochondroid metaplasia.’ Unlike a typical soft-tissue chondroma, these lesions resemble nuchal fibrocartilaginous pseudotumor and the equine digital cushion, suggesting a reparative process. Recognizing the distinct features of this lesion may aid in distinguishing it from other pseudoneoplastic and neoplastic bone-like growths in the feet.

Background and Methodology

• The researchers worked on the understanding that cartilaginous tumors in soft tissue are not common, with benign chondromas or benign cartilaginous growth outnumbering the rare soft-tissue chondrosarcomas, cancerous cartilage tumors.
• In recent years, researchers have noted a unique form of chondroid lesion of the plantar foot (the bottom of the foot), which appeared benign but differed from the chondroma of soft parts.
• The research team conducted a retrospective review of cases from their institutions’ soft tissue consultation and surgical pathology archives, looking for similar cases.
• They were able to identify nine cases, most of which had been previously classified as ‘fibroconnective tissue with chondroid metaplasia’, indicating a fibrous connective tissue undergoing change to resemble cartilaginous tissue.

Results and Findings

• Patients comprised of 4 young males and 5 older patients who presented with non-specific and variably painful masses on their plantar feet subcutaneous tissues, with no evidence of bone involvement as per the radiographic studies.
• On examination, these lesions had unique features contrary to chondroma of soft parts or fibro-osseous pseudotumor, including small foci of true cartilaginous metaplasia with lacuna formation and confirmed cartilaginous differentiation in three cases with immunohistochemistry for S100 and ERG proteins.
• The lesions also showed intralesional cystic changes and various other reactive-appearing changes in the surrounding connective tissue.

Conclusion

• The researchers concluded that they had identified a unique type of lesion on the foot that resembled a reactive, metaplastic process likely due to chronic mechanical stress.
• The unique morphological features of this lesion, myxochondroid metaplasia of the plantar foot, are reminiscent of nuchal fibrocartilaginous pseudotumor and the equine digital cushion.
• This similarity suggests a reactive or reparative etiology, which may aid in distinguishing such lesions from other neoplastic or pseudoneoplastic cartilaginous lesions of the feet.
• Further investigation and awareness of these distinctive lesions may enable more accurate diagnosis and perhaps treatment avenues in the future.