Fibroma of the tendon sheath in the knee
Moderately tender mass below the knee cap, that had been present for about a year.
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Rounded mass in the lateral infrapatellar fat pad of the knee contacting the dorsal surface of the patellar tendon. T1 isointense and T2 heterogeneously hyperintense signal, with some oedema in the adjacent fat tissue. Note the absence of susceptibility artifacts on the T2* gradient echo sequence.
The lesion was removed and histology showed spindle-shaped myofibroblasts in a collagen matrix with some hyalinisation and slit-like vascular channels. No giant cells or other features of PVNS.
Fibroma of the tendon sheath is a rare differential of PVNS, that had been first described in the finger tendons, but is also known to occur in large joints 1.