case_images: PMC6849066_01
case_images: (A) Initial anteroposterior radiograph showed no significant findings. (B) T2-weighted magnetic resonance images showed a straight high intensity line indicating a horizontal tear of the medial meniscus (red arrow). (C) Intraoperative image of arthroscopic meniscectomy. Meniscus height is obviously uneven in between the middle portion of the meniscus and the resected posterior portion.
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case_images: Articular and osseous anatomy was unremarkable (Fig. 1A,B). MRI findings of a horizontal tear in the middle and posterior segments of the medial meniscus were substantiated, and basket forceps and a radiofrequency assisted shaver (DYONICS ELECTROBLADE, Smith & Nephew, London, United Kingdom) were used to perform a partial medial meniscectomy (Fig. 1C).
case_images: (A) AP radiograph 10 months after arthroscopic surgery showed focal radiolucency in the weight-bearing area of the medial femoral condyle. (B) Multiplanar reconstruction indicated a sclerotic band in the weight-bearing area of the medial femoral condyle. (C) The T1-weighted image showed diffuse low signal intensity in the medial femoral condyle, and an associated band of lower signal intensity was seen in the area of the lesion. (D) The T2-weighted image had an inhomogeneous area of high signal intensity in the corresponding region and showed a focal area of low signal intensity underlying the articular cartilage. (E) T2-weighted images generally showed that the posterior root of the medial meniscus had been preserved (arrowhead).
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case_images: Radiographs from 10 months after the index surgery showed a small radiolucent area in the medial femoral condyle of the right knee (medial side of weight-bearing area) (Fig. 2A). Computed tomography was used to create a multiplanar reconstruction image showing a sclerotic band at the same part of the radiograph (Fig. 2B). T1-weighted MRI, performed immediately after the radiographic check-up, showed a low-intensity rounded band within the weight-bearing area of the medial femoral condyle and surrounded by a diffuse low-intensity area (Fig. 2C). T2-weighted MRI showed the same band, surrounded by an edematous lesion in the bone marrow (Fig. 2D). We judged that the posterior root of the medial meniscus had been preserved as described in the classification by Robertson et al. (Fig. 2E), and we diagnosed stage 2 SONK based on to the classification by Koshino.
case_images: (A) Intraoperative findings of for the cartilage surface of the medial femoral condyle showed a mild cartilage wear but no detachment from the osteochondral lesion (arrow). (B) Although the posterior portion of the medial meniscus was thinned by arthroscopic meniscectomy, the posterior root was intact. The middle portion of the medial meniscus was of uneven height (black arrowheads) and the higher side (asterisk) was significant stiffer than the other portions. (C) The stiffened area of the medial meniscus corresponded to the subchondral fracture of the medial femoral condyle (dotted circle).
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case_images: During surgery, we noted slight irregularities in the cartilage surface of the medial femoral condyle but no detachment of the osteochondral lesion (Fig. 3A). Pinching showed the stiffened area in the middle part of the meniscus to be much harder than other parts of the resected meniscus (Fig. 3B). The subchondral fracture of the medial femoral condyle was located at just over the stiffened area of the medial meniscus (Fig. 3C).
case_images: (A) Photomicrograph of the resected medial condyle demonstrating a compression fracture of the medial femoral condyle with intact overlying articular cartilage. (B) Enlarged image of the left area (box) in Fig. 4A. Prominent callus formation (arrows) was seen, consisting of reactive woven bone and granulation tissue. We found no evidence of antecedent bone infarction such as creeping substitution or bone marrow necrosis. (Hematoxylin and eosin stain, A; x40. B; x100) (C) Photomicrograph of the resected meniscus near the area of uneven height (D) Enlarged image of the left area (box) in Fig. 4C; there was visible aggregation of fibroblasts and collagen fibers.
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case_images: Fracture-related bone debris was focally observed on the osteochondral side of the fracture (Fig. 4A,B). The resected meniscus showed a proliferation of fibroblasts and collagen fibers corresponding to the stiffened area (Fig. 4C,D).