Background and objective : The coronoid process hyperplasia is an uncommon finding in the literature. It is characterized by an enlargement of the coronoid process, causing a mechanical obstacle by interposition of this structure in the posterior portion of the maxilla or zygomatic arch, resulting in limitation of mandibular opening, pain and trismus.

Case : A 21-year old man presented to The Department of Oral and Maxillofacial Surgery, complaining of limited mouth opening. During patient assessment, the individual did not report any alterations in his medical history, including systemic or endocrine disorders and haematologic alterations or allergies, and no signs of coronoid hyperplasia were present in his family history. Gene examination for differential diagnosis from Jacob syndrome showed that the patient have normal karyotype. Mouth opening was equivalent to 19 mm. Computed tomography with three-dimensional reconstruction was required, and an enlargement of the coronoid process was observed bilaterally. Under general anesthesia with nasotracheal intubation, the coronoid processes on both sides were removed by intraoral approach.

Result : In the immediate postoperative period, an evident improvement in the buccal opening was observed with 33 mm. After one month a mucofibrotic band from mandibular posteior buccal mocosa to maxillar side was detected and cut. The last mouth opening was equivalent to 35.5 mm.

Discussion and conclusion : To clarify the diagnosis, conventional imaging examinations can be used; however, because of the superimposition of the anatomic structures caused by its bidimensional image. Many theories have been described aiming at clarifying the origin of the coronoid process hyperplasia, such as temporal muscle hyperactivity, previous facial trauma, presence of hormonal alterations in puberty, family history. This also can be related to Jacob’s disease, which is characterized by elongation of the coronoid process and temporomandibular joint dysfunction. Treatment is exclusively surgical.