Heterotopic bone formation within a muscle is designated as ‘myositis ossificans’ (MO) [1], and it is associated with multiple aetiologies, such as trauma, genetic predisposition [1], post-infection [2], or even undetermined causes [3]. When the disease develops as a result of a trauma, the MO is classified as ‘myositis ossificans traumatica’ (MOT), which is also known as focal or proliferative myositis. MOT is frequently reported as an orthopaedic outcome of chronic trauma to muscles, and it is rarely found in craniofacial sites, such as the temporal bone [1]. Focal MO in the head and neck often occurs in a defined muscle group [4]; when the disorder predominantly affects the temporal muscle, it is known as ‘MOT of temporalis’.

MOT affecting the temporal muscle is infrequent [3,5-14], and it is unusual to have MOT affecting the temporal muscle exclusively [15-23]. Trismus is one of the chief complaints reported in MO and MOT of temporalis [4,15-18,20,21], although swelling with or without pain can be present [19,23].

The differential diagnosis of MO, due to its radiographic features of radiopaque areas with ill-defined or infiltrative borders, primarily includes malignancies, such as sarcomas or chondrosarcomas, although other conditions may be considered, such as osteomas, osteochondromas, haemangiomas, or nodular fasciitis [24]. Hence, dentists should be aware of this unusual condition in order to determine a definitive diagnosis.

Thus, the objective of this report is to describe imaging features of a MOT of temporalis case, considering its characteristics in multislice computed tomography (MCT) and magnetic resonance imaging (MRI). Additionally, English language case reports of MO in different databases were reviewed, summarised and qualitatively assessed in order to allow for an overview of the main imaging features of MO in the literature.

An African descendant, 60-year old male patient was referred to the Maxillofacial Surgery service (Campo Limpo Hospital, São Paulo, Brazil) due to restricted mouth opening and pain during mastication. The patient noticed the symptoms ten years before the consultation. The extraoral examination showed pain with palpation of the bilateral masticatory muscles, mainly in the temporal region, and limited mouth opening. Intraoral examination did not reveal any associated abnormalities. The patient mentioned a history of a previous cranio-facial trauma with a fracture in the frontal bone followed by trismus, but no other concomitant systemic disease.

The patient initially underwent a MCT and MRI to identify possible causes of his complaints. In the MCT examination, an ectopic hyperdense area was observed with density similar to bone tissue, as demonstrated in Figure 1. In the coronal slice (Figure 1A), a bone protuberance arising from the frontal bone was noted; the sagittal slice (Figure 1B) demonstrated the same bone protuberance arising from the frontal bone and mandible coronoid process, which was the likely aetiology of the restricted mouth opening. Axial slices (Figure 1C) showed the radiopaque mass involved in the temporal muscle area. Figure 2 demonstrates a three-dimensional view of the ectopic bone formation.

MRI showed a hypointense area arising from the coronoid bone, suggesting the presence of the calcification observed in the MCT. The MRI is demonstrated in Figure 3.

Initially, these imaging findings suggested that possible diagnoses included malignant or benign neoplasms. The patient was referred to surgical treatment with full removal of the calcified areas and coronoidectomy. Histopathologic examination of the tissue removed confirmed the diagnosis of myositis ossificans traumatica in the temporal muscle.

Literature review search

The selection of MO case reports was performed using PubMed, Embase (Excerpta Medical Database), Cochrane Central Register of Controlled Trials, Scopus, Web of Science, and Google Scholar databases. These databases were searched for English language publications from August 1984 until April 2019. Original articles and literature reviews were excluded; only human cases affecting masticatory muscles were included in the review.

The keywords (considering Medical Subject Heading terms) applied were: “myositis“ AND “masticatory muscles“; “inflammatory myopathy“ AND “masticatory muscles“; “inflammatory myositis“ AND “masticatory muscles“; “infectious myositis“ AND “masticatory muscles“; “inflammatory muscle diseases“ AND “masticatory muscles“; “focal myositis“ AND “masticatory muscles“; “proliferative myositis“ AND “masticatory muscles“, exhibited in the flow chart in Figure 4.

The reports that were not available on the selected databases, as well as the library of São Paulo University, and the São Paulo University colaborative library service, were requested from the authors. In the cases with a lack of response by the authors, the case reports were excluded from the review. The search results and summarised data, mainly regarding MO imaging features, are available in Table 1.

The literature search found 53 English language articles reporting cases of MO [1-12,15,16,18-57], in which ten cases were bilateral [3,9,12,33,34,47,50]. MO Temporalis was reported in 12 cases [12,15,16,18-21,23,28-30,54], in which one case was bilateral [12]. The medial pterygoid muscles were the most affected muscle [1,3,5,8,26,27,32,35,38,39,42-46,48,51,55]. The summarised data regarding the muscles involved are available in Figure 5.

CT examinations were performed in all cases except three [11,29,57]. Ultrasonography (USG) was performed in two cases [20,27,29] and positron emission tomographic scans (PET) were used in two cases [4,49].

The qualitative assessment of the case reports included in the literature review is exhibited in Table 2. The case reports were evaluated according to “The Joanna Briggs Institute (2017)” critical appraisal checklist for case reports [58]. The checklist consists of 8 questions. Checklist items were marked as “yes, no, unclear or not applicable”. The question number 7 of the checklist was marked as “not applicable” for all the articles cited in this review as this question regards to a new condition or drug treatment [58]. Considering diagnostic imaging features qualitative evaluation (question 4: diagnostic methods), only one article was classified as “no” due to the fact that, despite of mentioning the imaging examinations applied, no description of the examinations was available, only USG [29].

Histopathology features of MOT were predominantly described as a novel formation of bone and osteoid tissue within the muscle fibers, [2] with the presence of inflammatory infiltrate, degenerative tissue and necrotic muscle fibres [4]. Only in 12 publications the histopathology examination were not proper described [15,16,21,26,29,33,36,40,45,47,50,53].

Cranial MO imaging may resemble malignant neoplasms due to imaging findings, which include the appearance of an ill-defined radiopaque mass, often related to reported symptomatology, such as trismus, pain, and oedema, especially when the pathological process is associated with the masticatory muscles. Furthermore, its rapid growth, followed by significant pain and joint mobility restriction [15] leads to concern among patients and professionals. The heterotopic bone formation inherent to MO is also present in malignant neoplasms, such as osteosarcomas [23], and MO is defined as the formation of a non-neoplastic, mature, lamellar bone in the extraskeletal soft tissue.

MO is usually observed in larger muscles and rarely in masticatory muscles [15]. According to the literature, 25% of cases of MO have an unknown aetiology [8] although MO can be a result of persistent inflammation or trauma [15]. In a few cases, the cause of MO was determined to be from dental origin, such as trauma during dental extraction surgery [8,26,35,43,46], repetitive infection in the third molar [24], dentoalveolar abscess [32], after a mandibular nerve block [10,45,50], intubation [47], or injection of absolute alcohol for trigeminal neuralgia [48]. In the case presented in this report, the cause of MO was traumatic.

It is rare that MO develops on both sides of the face [3,9,12,33,34,47,50], which requires multiple interventions.

Conventional radiographs are often requested as the first imaging examination to investigate patient symptomatology. For craniofacial MO, the conventional examination requested is a panoramic radiograph [15,18,20,23,24], which provides a dimensional observation of the radiopaque mass near the region affected [15,38,41]. Further imaging examinations, including MCT and MRI are needed to determine the extension of the radiopaque mass and the muscles involved in the heterotopic bone formation. However, in early development of MO, no pathological alterations can be observed by plain radiographs [10,56].

In MCT, the extension and the limits of the radiopaque mass observed in conventional radiographs can be fully determined, although the adjacent soft tissue compromised cannot be entirely evaluated. In the literature, imaging findings are often described as hyperdense areas related to the muscles involved, designated as calcifications [1,5-10,12,15,18,19,21,23,24,37] or heterotopic bone neoformation [3,16,26,32,38,41]. Other imaging findings were also reported, such as a hypodense central area within the hyperdense area [22], coronoid process hypertrophy [11] or coronoid process fusion with the temporal bone [18], diffuse [47] or irregular calcification [48] within the involved muscle, as well as coronoid process cortical thickening and periosteal reaction [28], and complete calcification of the muscle [45]. The absence of any calcification on CT was reported in two cases, which showed oedema in a diffuse area of the masticatory muscles [27] or no alterations [33]. Hounsfield values of the hyperdense area related to MO ranged from 1200 to 1400. In the case presented in this report, the hyperdense area noticed on CT also showed heterotopic bone formation, with a bone protuberance arising from the temporal bone within the temporal muscle.

On MRI, which is the imaging examination that provides the most accurate soft tissue evaluation, the imaging findings described are: partial bone ankylosis [11] an isointense area in the muscle involved on T1-weighted images [19], hyperintense areas on T2-weighted images [33], and haematoma-like findings within the muscle [5]. In the present report, a hypointense area was noted arising from coronoid bone, within the temporal bone.

Ultrasound examination was not usually requested; however, an USG can show the muscle enlargement [29] and muscle alterations as a heterogeneous echogenic area [20]. PET examinations, used as a complementary tool to CT and MRI, can show low-grade uptake [4] and spots in the muscle affected by MO [49].

Besides malignant neoplasms, the differential diagnosis of MO may include pathological processes such as nodular fasciitis, haemangiomas with multiple phleboliths [31] and benign neoplasms, such as osteomas [24]. While considering these hypotheses, some imaging features should be noted. For instance, osteomas are well-defined radiopaque lesions, in contrast to MO, which often exhibits ill-defined or diffuse borders [11,26,36,38,43,47,48,57], although some reports of MO did show round and well defined areas [24,31,41,51]. Nodular fasciitis, although of a similar aetiology to MO, does not appear as radiopaque masses [15]. Haemangiomas with multiple phleboliths can be excluded due to the main imaging feature of phleboliths, which usually include a radiopaque core with the appearance of concentric rings, also called ‘laminations’ [59,60]. However there is a case of MO in the masseter muscle which had imaging features similar to phleboliths [31]. When considering malignant neoplasms, despite the fact that calcifications can be noted, the destruction or invasion of adjacent structures or tissues is often observed, which is not seen in MO cases.

Other differential diagnoses should be considered when a case of MOT is under investigation, such as fibrous ankylosis in the temporal joint [11], and fibrodysplasia ossificans progressive [61], or progressive myositis ossificans, due to limited mouth opening and pain with mastication. However, these two pathologies have differences when compared to MO or even to MOT. Fibrous ankylosis in the temporal joint has its own imaging features, which include the anatomical alteration of the temporal joint and mandible head [61]. Fibrodysplasia ossificans progressive is a genetic disorder that may affect the patient in the childhood or in adult life and is recognised by two clinical features: progressive formation of extraskeletal bone and malformation of the great toes [61]. Progressive myositis ossificans is also a genetic disorder associated with several skeletal malformations, sexual disorders and deafness [21].

Finally, MO treatment usually includes surgical intervention, with the resection of the ossified mass [3,7,10,18] and eventually coronoidectomy [5,6,8,9,15,16,28], mainly if it affects temporal muscles, as in MOT of temporalis.

The limitation of the present case report and literature review was the impossibility to evaluate all the case reports available in the literature during the period of the time selected in the methodology due to the lack of response by some authors.

In conclusion, the main imaging feature of myositis ossificans is the presence of a radiopaque (on plain radiographs), hyperdense (on computed tomographic examination) or hypointense (on magnetic resonance imaging) mass in the affected muscle, which is demonstrated particularly well by computed tomography and magnetic resonance imaging. The final diagnosis is through histopathological examination, although imaging examinations can direct the most likely diagnosis.