膝蓋下脂肪体骨化症の1例

Ossification of Infrapatellar Fat Pad. Case Report

　膝蓋下脂肪体骨化症は比較的稀な疾患であり，成因もいまだに明らかではない．症例は47歳の女性で，1カ月前からの右膝痛と違和感を主訴に来院した．X線像にて右膝蓋骨下に骨化像を認め，MRIにて内部に石灰化を推察させる低信号領域を示す腫瘤を認めた．鏡視にて他に関節内の異常がないことを確認し，観血的に摘出した．易剝離性で膝蓋骨，膝蓋腱との連続性は認めなかった．病理組織では層板状構造をもつ骨梁が認められた．本症の成因にはいくつかの仮説があるが，今回の症例では脂肪体のmetaplasiaにより骨化したものと推察された．

　Ossification of the infrapatellar fat pad is a rare disease, and its origin has not yet been clarified. A 47-year-old woman had felt pain and incongruity in her right knee for one month before consulting us. An X-ray showed ossification just below the right patella, and a very low-intensity area on a magnetic resonance image suggested a calcified tumor. Arthroscopic observation showed no morphological abnormalities in the joint except for the existence of the tumor. The tumor was not connected with the patella or the patellar tendon, so the tumor was easily resected from the peripheral tissues during a surgical procedure. Pathologic examination of the resected tumor showed cancellous bone with a lamellar structure. Among the several mechanisms of ossification that have been proposed so far, this case supports the development of ossification through metaplasia of the fat pad.