Right thumb injury six weeks ago now with pain overlying dorsum of the IP joint and possible foreign body seen on x-ray. The density seen on recent radiographs along the dorsal surface of the diaphysis of the proximal phalanx corresponds to plaque-like nonenhancing low signal intensity on this MRI study. This abnormal low signal intensity appears to contact to the cortex both proximally and distally, with subjacent relatively high signal intensity between it and the underlying cortical bone. While it is possible that this could represent a foreign body, its margins are relatively poorly defined and it does not have the typical geometric appearance of a foreign body. I favor focal posttraumatic periosteal reaction, perhaps with a small subperiosteal hematoma. Of note, the adjacent cortex itself appears intact, and the signal intensity of the medullary bone of the proximal phalanx is normal. Additional abnormal signal intensity extends through the soft tissues dorsal to the proximal phalanx to the skin surface that is isointense to skeletal muscle on T1-weighted images, heterogeneously hyperintense on T2-weighted images, and shows enhancement following gadolinium administration, indicating inflammation. I see no rim-enhancing fluid collection to suggest an abscess. The extensor tendon is bowed dorsally by the underlying cortically-based mass/periostitis, and its fibers dorsal to the mid diaphysis of the proximal phalanx are indistinct suggesting at least partial if not full-thickness tearing of the tendon. The flexor pollicis longus appears normal. The collateral ligaments of the interphalangeal joint appears intact. A small focus of high signal intensity separates the dorsal fibers of the ulnar collateral ligament of the first metacarpophalangeal joint from the base of the proximal phalanx, but this finding is of questionable significance, as the volar fibers of the ligament appear to attach in normal fashion to the bone.
1.Density along the dorsal margin of the proximal phalanx of the thumb seen on recent radiographs is favored to represent periosteal reaction/heterotopic ossification over foreign body.2.Soft tissue inflammation dorsal to the proximal phalanx with tear of the extensor tendon and other findings as above.