Coronoid process hyperplasia (CPH) of the mandible is a rare condition in which abnormal elongation of the coronoid process leads to chronic mandibular hypomobility. CPH causes impingement or impedance of the coronoid process on the inner surface of the frontal part of the zygomatic bone during mouth opening [1]. Its etiology remains inconclusive despite several theories being suggested. The temporalis muscle inserts onto the coronoid process, and its hyperactivity may contribute to bony overgrowth. Pregarz et al. [2] provided evidence of temporalis hypertrophy using magnetic resonance imaging (MRI) scans. However, there are contradicting results from electromyography studies of patients with CPH and healthy controls [3,4]. Some studies based on reports of CPH in patients with ankylosis of the temporomandibular joints (TMJs) have highlighted the influence of mandibular hypomobility [5]. Trauma to the maxillofacial area can induce tensile forces of the temporalis, lengthening the coronoid process lengthens, as in cases of distraction osteogenesis [6]. Hormonal and genetic factors have also been suggested, but none of the above-mentioned theories solely explain its pathogenesis or establish a clear causal relationship [7].

According to a recent literature review, CPH is five times more prevalent in men than in women. The mean age of diagnosed patients is reportedly 22.6 years, mostly 11 to 20 years, followed by 21 to 30 years. Bilaterally affected cases are four times more than unilateral cases. Most cases are idiopathic [8]. CPH usually presents no symptoms apart from a progressive reduction of mouth opening over time. The lack of significant symptoms makes the diagnosis challenging. Patients may not notice the inhibition until their daily activities are severely disturbed. Thus, a considerable period exists between its onset and diagnosis.

In this report, three reviewed cases of CPH are presented. The study was approved by the Institutional Review Board of Seoul National University Dental Hospital (ERI23022). The IRB waived the need to obtain the informed consent again because the study was performed by clinical chart review.

1. Case 1

A 31-year-old man visited the clinic at the Department of Oral Medicine of Seoul National University Dental Hospital with limited mouth opening as the chief complaint. He reported that his symptoms worsened two years prior and that he had undergone physical therapy and medication at another clinic, but without improvement. He had no significant medical or trauma history.

His mouth opening was limited to 23 mm, and protrusive movement of the mandible was restricted. He experienced pain on palpation of the right masseter near its origin. There was no obvious facial asymmetry or deviation in mouth opening. Under the impression of temporomandibular disorder (TMD) of muscular origin, the attending doctor ordered physical therapy and plain radiographs. Radiographic imaging revealed bilateral elongation of the coronoid process, and computed tomography (CT) and MRI were performed for further evaluation. CT scans showed anterior flattening of the coronoid process, forming a joint-like structure with the inner surface of the zygomatic bone on both sides (Fig. 1). There were no signs of disc displacements in both joints or any pathological changes in the masticatory muscles, including inflammation or hypertrophy on MRI. The patient was diagnosed with bilateral CPH, resulting in hypomobility of jaw. He was referred to the Department of Oral Maxillofacial Surgery and underwent coronoidectomy on both sides. His mouth opening increased to 47 mm four months after the operation without any discomfort.

2. Case 2

A 35-year-old man presented to the clinic complaining of limited mouth opening. He recalled that his mouth opening had gradually decreased over the past ten years, but had no discomfort in his daily activities. He had no specific medical or trauma history.

His maximum mouth opening was 28 mm, and his mandible deviated to the left when he opened his mouth. Lateral movement to the right side was restricted. He experienced pain on palpation of the left condylar area. The elongation of the coronoid process on both sides was visible on the panoramic radiograph. CT imaging confirmed anterior flattening of the left coronoid process and joint formation with the opposing zygomatic bone, which was erosive and sclerotic, as well as a hyperplastic tendency of the right coronoid process (Fig. 2). The patient was given a detailed explanation of his condition and surgical treatment plan.

3. Case 3

A 6-year-old woman was referred from the Department of Pediatric Dentistry because of restricted mouth opening. Her parents reported that she had limited use of the perioral muscles since her neonatal period, making food swallowing and chewing difficult. It was unclear why or whether it was related to her history of epilepsy, and her pediatricians had not found any abnormal findings on her brain MRI.

Her maximum mouth opening was 25 mm, and her protrusive mandibular movement was restricted. She complained of pain upon mouth opening and palpation of her left condylar area, both the masseter and temporalis muscles. Initial diagnoses of arthralgia and myalgia were made. She had undergone physical therapy and performed mouth opening exercise, but without any improvement. The presence of overlapping anatomic structures on panoramic radiograph, and the patient’s painful symptoms made the diagnosis of CPH difficult. Her mouth opening gradually decreased over the next few visits until CT was performed, confirming the diagnosis. The coronoid processes were elongated over the zygomatic arches on both sides, which was considered to have caused the patient’s chronic mandibular hypomobility (Fig. 3). The patient was then referred to the Department of Oral Maxillofacial Surgery and underwent coronoidotomy on both sides. Her passive mouth opening was up to 39 mm during surgery after osteotomy.

Differential diagnosis is crucial for treating patients with chronic mandibular hypomobility. Disc displacement without reduction (also termed a closed lock) should be considered first. The displaced disc does not allow full translation of the condyle. A typical interincisal opening would be 25 to 30 mm, and the mandible would deflect to the affected side while opening [1]. A history of joint clicking usually precedes this condition, and patients are mostly aware of the onset of symptoms. It does not necessarily accompany pain, but can cause retrodiscal tissue inflammation [9].

Ankylosis may also prohibit mandibular movements. It is a fibrous or osseous adhesion in TMJ and is commonly trauma-related, which can cause tissue damage, inflammation, and hemarthrosis, leading to fibrotic changes in the joint [10]. Bony ankylosis can be visualized on a radiograph. Movement is restricted in all directions, and if unilaterally affected, there will be midline deflection to the affected side during mouth opening and limited movement to the contralateral side.

Muscle contracture refers to the painless shortening of the functional length of a muscle. Muscle contracture of the closing muscles can also cause chronic mandibular hypomobility. Myostatic contracture usually results from the long-term restriction in the range of motion due to pain or an intermaxillary fixation. Myofibrotic contracture is caused by tissue adhesions within a muscle, commonly following myositis or trauma [11]. Both myostatic and myofibrotic contractures are painless and do not affect lateral condylar movements.

The coronoid process passes anteroinferiorly between the zygomatic bone and the posterolateral surface of the maxilla; an abnormally long process causes chronic hypomobility of the mandible [1]. CPH can be misdiagnosed as other types of TMDs, including the previously mentioned conditions. Since it is difficult to differentiate them, visualization with advanced three-dimensional imaging, such as CT or MRI, is crucial for their diagnosis. In this report, CT scans revealed bilateral CPH in all cases and joint-like structures between the coronoid process and zygomatic bone in two of the three cases. This rare condition is known as Jacob’s disease, in which a new joint formation is established between an enlarged coronoid process and the zygomatic arch [12].

In cases in which CPH is the likely cause of mandibular hypomobility, surgery is preferable and a true definitive treatment [1,13]. However, it should only be considered when a patient’s jaw function is severely impaired because of its irreversible and invasive nature. For the surgical treatment of CPH, coronoidectomy or coronoidotomy is usually recommended [8,14]. The former involves stripping the temporalis muscle fibers from the coronoid process and removing the bone fragment after osteotomy at the base of the coronoid process, while the latter does not, leaving the osteotomized bone fragment inside. Coronoidectomy allows the complete removal of the mechanical cause obstructing the mouth opening pathway, along with the histological examination of the specimen. However, releasing the temporalis insertion can be a difficult and traumatic procedure. Coronoidotomy is less traumatic and time-consuming, while there is a risk of recurrence by reattachment of the process [8,14,15]. Yura et al. [15] reported a case where coronoidotomy was performed in a patient with CPH and the osteotomized fragment was united with the ramus with moderate dislocation and inclination posteriorly at 15 months after operation; the authors suggested that given adequate postoperative physiotherapy (mouth-opening exercises), unison may occur without causing further obstruction. Gap (modified) coronoidotomy has now been introduced, which is the removal of a bone segment using two separate osteotomies 6-7 mm apart; the undesirable outcome of an improper direction of bone cutting and less upward displacement from a conventional coronoidotomy is avoidable [16].

The operation site can be approached either extraorally or intraorally [8,14]. The extraoral approaches used in previous literature include submandibular, preauricular, and coronal approaches. They provide better exposure of the operative field during surgery and are thus preferred in severe cases of limited mouth opening. However, they leave visible scars and pose risks of facial nerve damage. The intraoral approach requires an incision along the external oblique ridge; however, there is a supposed risk of postoperative hematoma and fibrosis.

Postoperative physiotherapy is important in improving patients’ mandibular mobility, regardless of the type of surgical treatment. It can be performed using spatulas [13], wedges [15], or a specific device [17]. Currently, there are no specific guidelines, but many clinicians advocate that aggressive physiotherapy must be performed during the first year after surgery [14]. The pediatric patient presented in this report was uncooperative in mouth opening exercise, according to her parents. Her maximum mouth opening decreased from 39 mm during surgery to 28 mm at five days after the operation, then to almost 10 mm two months after the operation, from which point she was lost to follow-up.

Coronoid process hyperplasia is a rare condition that causes chronic mandibular hypomobility. Its diagnosis is often delayed, and patients may undergo unnecessary treatment procedures. Therefore, clinicians should be aware of this condition and exercise caution in the differential diagnosis. Also, surgical intervention should be performed when needed.

No potential conflict of interest relevant to this article was reported.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

None.

Conceptualization: JC, JWC. Data curation: JC. Methodology: JWC. Visualization: JC. Writing original draft: JC. Writing review & editing: JWC.