Asymptomatic Myositis Ossificans of the MedialPterygoid Muscles: A Case Report

Karen A, Tong, MDDepartment of RadiologyLoma Linda University Medical CenterPO Box 2000Loma Linda, California 92354

Edwin L Christiansen, DDS, PhDDepartment of Oral Diagnosis,

Radiology, Pathology

William Heisler, DDSAssociate Dean for Clinical

Administration

School of DentistryLoma Linda University

David B, Hinshaw. Jr. MDDepartment of RadiologyChiefSection of Magnetic Resonance

Imaging

Anton N, Hasso, MDDepartment of RadiologyChief

Section of N euro radio logy

Loma Linda University Medical Center

Correspondence to Dr Tong

Traumatic myositis ossificans, also known as myositis ossificanscircumscripta or fibrodysplasia ossificans circumscripta, is a formof dystrophic calcification leading to heterotopic ossification ofintramuscular connective tissue. This is usually due to a singlesevere injury or repeated minor injuries to muscle, although caseswithout a history of injury have been reported. Heterotopic ossifi-cation is rare in the orofaciai region, especially in the medial ptery-goid muscles, A case of medial pterygoid myositis ossificans withunique computed tomography findings is described.,1 OROFACIAL PAIN 1994;H;223-226,

A73-yeat-old healrhy wotnan presented for routine prostheticdental care. She reported occasional difficulry swallowingand minor "ear problems," On oral examination, an abnot-

mality was nored in the patient's left posterolateral oral cavity (Figla). Palparion revealed a firm,, painless mass without evidence ofmucosal ulcération or tissue discoloration. Jaw movement wasasymmetrical and restricted, with 10 mm Hghr lateral rranslarioo,15 mm left lateral translation, 10 mm protrusion, and 40 mm ven-tral opening. No head or neck lymphadenopatby was clinicallydetected. Prior medical history included complicated tonsillectomyas a young adult and a history of prior multiple, bilateral dentalanestheric block of the mandibular nerves.

Comptited tomography (CT) witb intravenous contrast was per-formed on a Somatom DRH scanner (Siemens, Erlangen,Germany), using sequential, 2,0-mm-rhick axial scans. The scansrevealed bilaterally symmetrical, diffusely increased density of tbemedial ptetygoid muscles, with Hounsfield units of apptoximately286, as is consistent with calcification or ossification. No othermasricarory muscles were involved (Figs lb and lc). This was feltto be a benign process and altbough the oprion of biopsy was givento the patient, she declined.

Discussion

Calcifications commonly occur in various pathologic conditionsand ate usually of tbree main types: (1) dystrophic calcificatioti; (2)metastatic calcification; and (3) generalized calcinosis. Dystrophiccalcification occurs in damaged tissues in the absence of a general-ized metabolic derangement, Heterotopic bone formation canoccur in the focus of calcification. Metastatic calcification occurs invital tissues and is telared to abnormal calcium or phosphorusmetabolism, resulting in hypercalcemia, wbich has numerouscauses,' Generalized calcinosis occurs in skin and subcutaneous tis-

Joumal of Orofaciai Pain 223

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Figs la to lc Myositis ossificans involving bilateralmedial pterygoid muscles; (a) intraoral photoeraphshows smooth, rounded, submucosal mass projectingfrom superior aspect of the left postero latera I oral cav-ity; (b and cj contrast-enhanced axial CT (soft tissueand bone windows) demonstrates amorphous increaseddensiti' of the medial prerygold muscles bilaterally, withHounsfield units of 286, consistetit vi\ú\ calcification orossification. There is mild asymmetry of the muscle cal-cifications with the left slightly greater than the right,correlattng with the palpable left mass.

Fig la

sue in the presence of normal calcium metabolismand includes connective tissue disorders, idiopathictumoral calcinosis, and idiopathic calcinosis uni-versahs.-

Myositis ossificans is an example of dystrophiccalcification that can lead to heterotopic ossifica-tion. This entity is not to he confused with myosi-tis ossificans progressiva, aiso known as fibrodys-plasia ossificans progressiva, which is anautosomal dominant disorder resulting in progres-sive ossification of the connective tissue of volun-tary muscles and ligaments in association withskeletal anomalies.^ Many types of injury in trau-matic myositis ossificans have been described,from hlunt trauma to lnfectton/inflammation. Themechanism of calciftcation is unclear, but hypothe-ses include: traumatization of periosteum with dis-placement of osteoblasts into muscle, activation ofperiosteal implants already within muscle, meta-plasia of pluripotential intramuscular connectivetissue into bone, and metaplasia of fibrocartiiageinto bone.' Within 7 to 10 days after injury, a softtissue mass develops that may be associated withperiosteal reaction of adjacent bone, hlocculentcalcifications atise in the mass from 11 days to 6weeks after trauma. The calcifications graduallyenlarge, and in 6 to 8 weeks a lacy pattern of het-erotopic new bone is formed around the peripheryof the lesion with a tnore lucent central zone.'

Heterotopic localized ossification is rare in theorofacial region. Reviews covering 1945 to 1989revealed only three cases of medial pterygoid calci-fication, two of which were traumatic,•*•' and thethird occurred in fibrodysplasia ossificans progres-siva.' Only two cases involved CT scans. The firstcase involved a patient presenting with chronicprogressive facial pain and restriction of mandibu-lar range of movement. Computed tomography

Figlb

Fig lc

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Tong

scans showed a small, round, dense calcificationwithin the right medial pterygoid muscle and anipsilateral enlargement of the pterygoid process.This was thought to be a case of traumatic myosi-tis ossificans resulting from a series of injections oflocal anesthesia and alcohol in the region of themandibular foramen,' The second case (attributedto fibrodysplasia ossificans progressiva) demon-strated bilateral, linear, streaklike calcificationswithin the medial pterygoid muscles."

Definitive etiology of the medial pterygoid calci-fication could not be pathologically proven in ourcase. A review of the patient's medical history sug-gested several possibly related events. She had ahistory of prior multiple, bilateral dental anestheticblocks of the mandibular nerves, and it is possiblethat the anesthetic blocks resulted in intramusculartrauma with hemorrhage and subsequent dys-trophic calcification, although the bilaterally sym-metrical diffuse involvement would make thistmlikely. She also had a history of a complicatedtonsillectomy as a young adult, which may haveresulted in intramuscular trauma. Other possibili-ties are that these findings are atypical manifesta-tions of degeneration related to aging or are simplyidiopathic.

Certain featutes make this case unique from pre-viously reported cases. The involvement of themedial pterygoid muscles is bilaterally symmetricaland diffuse. The pattern of calcification on CT wasunlike that described in the two previously men-tioned cases. A new CT appearance of this entity istherefore demonstrated. Although Reinig et al," in1986, reported the CT appearance of fibrodyspla-sia ossificans progressiva, there have been noknown studies documenting the CT appearance ofmyositis ossificans, especially involving the masti-catory muscles. Further investigation and clarifica-tion are warranted.

Although this process is extremely rate, oralclinicians should be aware of the possibility of

medial pterygoid or other masticator muscle calci-fication or ossification in the evaluation of disor-ders of mandibular range of movement. The clini-cal presentation and absence of other medicalpathology are helpful in narrowing the differentialdiagnosis of this radiographie entity. The mostimportant differential diagnosis is a malignantneoplasm, such as an osteogenic sarcoma.Malignant degeneration of myositis ossificans cir-cumscripta into osteogenic sarcoma, althoughunhkely, has been reported, Leilo and Makek' rec-ognized rhat these early cases may have beenincorrectly diagnosed as myositis ossificans, result-ing in the impression that degeneration hadoccurred later. Although wide excisional biopsyyields the definitive diagnosis and may also betherapeutic for severe symptoms, it should be rec-ognized that myositis ossificans has a tendency torecur.'

References

1, Rubbins SL, Cotran RS, Kumar V, Pathologic Basis cfDisease, ed 3- PKikdelphia; Saunders, 1984:35-36.

2, Kesnick D, Bone and Joint Imaging. Philadelphia; Saunders,1989:1231-1283,

3, LeIlo GE, Makek M, Traumanc myosilis ossificans in mas-ticatüry mu5cles, J Maxillofac Surg 19Sé;14:131-237,

4, Narang R, Dixon RA Jr. Myositis ossificans: Medial pcery-güid muscle—A case report, Br J Oral Sur^ 1974;12:229-234,

5, Nilner MN, Petersson A, Mandibuiar limitation due toenlarged pterygoid process and calcification of the medicalpcerygoid muscle, A ca^e report, J Craniomand Pract1989;7:230-234,

fi. Kabala JE, Watt I. Hollingworth P, Ross JW. Case report;Trismus and mulcifocal soft tissue ossification, A presenta-rion of fibrodysplasia ossificans ptogressiva? Clin Radiol1989;40:i23-527,

7, Reinig JW, Hill SC, Fang M, Marini J, Zasloff MA.Fibrodysplasia ossificans progressiva; CT appearance,Radiol 1986il59:l,îî-157.

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Resumen

Reporte de un caso de miositis osificante en los múscu.los pterigoideos medios

La miositis osificante traumática, también conocida comomiosilis osificante circunscrita o fibrodisplasia osificante circun.sonta, es un tipo de calcificación distrofica que conduce a la osi.ficación heterotópica del tejido conectivo intramuscular. Esto esdebido usjalmente a una sola lesión severa o a lesiones muscu.lares menores repelidas, aunque lambién se han reportadocasos sin historia de lesión La osificación helerolópica es raraen la región orofacial. especialmente en los músculospterigoideos medios Se descnbe un caso de miosilis osificantedel pterigoideo medio, con características únicas suministradaspor un examen de tomografia computerizada.

Zusammenfassung

Asymptomatische Myositis ossiticans des Musculusplerygoideus medialis: Eine Fallbeschreibung

Die traumatiscbe t\^yositis ossificans, auch bekannt unter denBezeichnungen Myositis ossifiosns circumscripta oderFibrodysplasia ossifiGans circumscripta, ist eine Form der dys.trophischen Kalzifizierung und führt zu heterotopischenVerknöcherungen des intramuskuläre Bindegewebes Schuldtragt meist eine einzelne schwerere Verletzung oder aher eineAnzabi wiederbolter kleinerer Verletzungen des betroffenenMuskels. Es sind auch Falle ohne vorangegangene Verletzungbeschrieben worden Im orofacialen Gebiet, insbesondere imMusculus pterygoideus medialis. treten heterotopischeOssifikationen selten auf. Im vorliegenden Artikel wird ein Fallvon Myositis ossificans des M. Pterygoideus medialisbeschrieben, der einzigartige CT.Befunde zeigte.

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