Intraoperative fracture in total knee arthroplasty (TKA) is a rare complication. However, when it happens, additional surgery to fix the fracture site is needed. Therefore, it is important to diagnose intraoperative fractures in TKA exactly. The authors experienced two cases of cortical perforation of medial femoral condyle misidentified as the fracture in TKA. Cortical perforation could be misdiagnosed as the fracture, which could lead to unnecessary surgery. This is the first report about cortical perforation in TKA. We report two cases of intraoperative cortical perforations and describe the radiological characteristics.