An 11-year-old child has a thickening in the frontal region of the lower jaw that appeared after the child fell down. The trauma occurred one month ago. Objectively, on the lower jaw, in the area of the mucogingival junction, there is a circumscribed dense spot with smooth surface that is slightly painful on palpation. The teeth in the pathologic focus are intact and vital. X-ray detects an osteoporotic focus with thickened periosteum in the frontal region of the lower jaw. What is the most likely diagnosis in this case?