case_images: PMC6626976_01
case_images: (A) Plain radiography of the left distal femur. The nidus was undetectable on the frontal view. (B) Mild radiolucent area with slight central calcification was detected on the lateral view. (C) Computed tomography coronal view (bone condition). (D) Coronal T1-weighted image. (E) Coronal T2-weighted image with fat suppression. (F) Coronal Gadolinium contrast image.
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case_images: A plain radiography revealed, though extremely difficult to discern, a small, relatively radiolucent area with a tiny central calcification in the distal lateral femur (Fig. 1A and B). This was clearly visible as an approximately 1 cm oval nidus, including a mineralized osteoid in the center, on CT scan (Fig. 1C). Magnetic resonance imaging (MRI) revealed a low-signal oval lesion that reflected the osteoid on T1-weighted imaging (Fig. 1D), and a surrounding high-signal area on T2-weighted imaging that reflected the adjacent bone marrow edema (Fig. 1E). Only the nidus was well-enhanced on contrast MRI, which might indicate the hypervascularity of the nidus (Fig. 1F).
case_images: (A) An instant marker was set on the lateral side of the knee. (B) Confirmation of tumor location with cone-beam computed tomography coronal images. (C) Axial image. (D) First attempt. The Kirschner wire was inserted proximal to the tumor. (E) Second attempt using wire sleeves for parallel insertion. The Kirschner wire was inserted to the center of the tumor. (F) Sagittal image.
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case_images: An incision line was marked on the skin above the tumor under CBCT imaging guidance by using an instant marker made by arranging Kirschner wires at 1 cm intervals (Fig. 2A, B, and C). An approximately 1 cm skin incision was made, and a Kirschner wire was inserted toward the tumor, with CBCT guidance at each step (Fig. 2D, E, and F).
case_images: (A) The soft-tissue was dilated with the dilator from the microendoscopic discectomy (MED) system. (B) Retraction with the MED retractor. (C) Drilling with the 120 mm trephine tip. (D) Confirmation of the tumor in the drilled route. (E) Tumor bone resection. (F) Insertion of a beta-tricalcium phosphate block into the bone defect. Coronal cone-beam computed tomography (CBCT) image. (G) Sagittal CBCT image.
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case_images: The soft-tissue was dilated bluntly in small parts by using the dilator originally used for MED surgery to minimize soft-tissue damage (Fig. 3A). After exposing the lateral side of the femur, 120 mm trephine tip for bone biopsy was drilled according to the MED retractor (Fig. 3B and C). After confirming on the CBCT image that the tumor was in the drilled route (Fig. 3D), the tumor-bearing bone was resected en-bloc with the trephine tip and additional use of a round chisel (Fig. 4A and B).
case_images: (A) Resected bone. (B) Vertical split of the tumor bone. (C) Additional curetted bone. No tumor tissue was detected. (D, E) Hematoxylin and eosin staining. Scale bars indicate 100 mum. The nidus included anastomosing trabeculae of the osteoid and woven bone.
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case_images: After the resection, additional curettage and cauterization with an electric scalpel were performed to ensure complete tumor removal (Fig. 4C), although no tumor remnants were detected on CBCT (Fig. 3E). A histological evaluation confirmed the diagnosis of osteoid osteoma with differentiating osteoblasts and immature osteoid trabeculae in the nidus surrounded by reactive bones (Fig. 4D and E). Absence of chondrocyte or cartilage formation allowed exclusion of other benign bone lesions such as enchondroma or Bizarre parosteal osteochondromatous proliferation.
case_images: (A) Fluoroscopic image of cone-beam computed tomography immediate after the surgery. (B, C) Radiographs of the final follow-up. The bone defect was completely filled by new bone.
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case_images: Plain radiography at the final follow-up revealed new bone formation completely filling the bone defect, without local recurrence (Fig. 5B and C).