初診時のMRIで診断できなかった大腿骨内顆骨壊死の2例

Initially MRI-negative Osteonecrosis of the Knee. Report of 2 Cases

　近年，半月板切除などの医療行為に続発する膝骨壊死の報告が散見される．しかしながら，疼痛発生時のMRIで骨壊死の評価が困難であるとすれば，必ずしも医療行為が原因ではない症例もこの中に含まれている可能性がある．筆者らは，遡及的には膝骨壊死発症の極早期に撮影したと考えられるMRIにおいて，壊死の所見が陰性であった2例を経験した．半月板切除やステロイドの関節内注射などは施行しなかったが，経過中に再検したMRIによって大腿骨内顆の骨壊死が明らかとなった．このようなMRIの経時的変化は，膝骨壊死の病態を脆弱性骨折とする学説を支持するものであるとも考えられた．

　Iatrogenic osteonecrosis of the knee following arthroscopic meniscal resection, etc., has become quite common in recent years.　However, we suspect that some cases of spontaneous osteonecrosis of the knee may have been misdiagnosed as iatrogenic. We report 2 cases of osteonecrosis of the knee in which MRI that was performed in what may have been the superacute stage showed no evidence of osteonecrosis.　Subsequent MRI examinations demonstrated the typical signs of osteonecrosis of the medial femoral condyle.　Thus, osteonecrosis of the knee may be MRI-negative in its superacute stage.　Our findings in these 2 cases also support the theory that so-called spontaneous osteonecrosis of the knee is the result of a subchondral insufficiency fracture, because such fractures are often MRI-negative in their superacute stage.