case_images: PMC6071151_01
case_images: Plain radiograph showing irregular exophytic ossification at the
  posteromedial side of the medial femoral condyle of the right knee.
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case_images: A plain radiograph revealed irregular exophytic ossification at the posteromedial side of the MFC (Figure 1).
case_images: Computed tomography (CT) images revealed lobulated irregular osseous
  hypertrophy of the medial side of the medial femoral epiphyses: coronal (a),
  sagittal (b), axial (c), and volume rendering imaging (d).
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case_images: Computed tomography (CT) also revealed lobulated irregular osseous hypertrophy of the medial side of the femoral epiphyses (Figure 2).
case_images: Magnetic resonance imaging showing asymmetrical osteocartilaginous lesions,
  continuous with the distal femoral epiphyses: coronal view of T2*-weighted
  image (WI) (a), sagittal view of fat suppression proton-density WI (b), and
  T2*-WI (c), and axial view of T2 turbo spin echo WI (d).
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case_images: Magnetic resonance imaging (MRI) showed asymmetrical osteocartilaginous lesion, continuous with the distal femoral epiphysis (Figure 3). T2*-WI (Figure 3(a) and (c)).
case_images: Arthroscopic findings: (a) protuberance of the cartilage surface was seen at
  the posterior medial femoral condyle (MFC) without cartilage injury. (b) The
  extensive chondral protuberance of the MFC was over-riding the posterior
  horn of the medial menisci during deep knee flexion. (c) The protruding
  region was impinging on the posterior medial tibia during internal rotation
  of the knee, and the locking phenomenon was elicited. (d) After arthroscopic
  removal and contouring of the posteromedial protuberance, impinging was
  eliminated during full range of motion.
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case_images: The cartilaginous surface of the MFC was smooth without chondral injury or instability, but at the posterior part of the MFC, protuberance of the cartilaginous surface was seen without obvious injury, including fissures and defects (Figure 4(a)). The anterior part of the extensive chondral protuberance of the MFC impinged on the posterior medial tibia and menisci at about 70  of knee flexion while externally rotating the tibia (Figure 4(b)). The lesion rode over the posterior horn of the medial menisci on bending the knee (Figure 4(c)). Arthroscopic removal and contouring of the protuberance were performed until the phenomena were completely eliminated during the full range of motion (Figure 4(d)).