A 7-year-old boy with a history of asthma presents for an emergency appointment after a fall in which he struck his anterior maxilla and lacerated his lips. The maxillary central incisors are mobile with bleeding from the gingival sulcus, and the maxillary lateral incisors are mobile without bleeding. During the initial exam he develops acute dyspnea. A radiograph of the maxillary central incisors reveals a small, calcified radiopaque mass between the roots of teeth #8 and #9. Which of the following is the MOST likely reason for this mass?