Myositis ossificans traumatica (MOT) is a non-neoplastic, heterotopic bone formation within muscle or fascia, presumably due to acute trauma, a burn, surgical manipulation, or repeated injury. A 36-year-old woman presented with progressive limitation of mouth opening over the past 5 months. Computed tomography revealed an irregular ossified mass attached to the medial aspect of the left ramus, extending to the medial and lateral pterygoid plates. MOT of the medial pterygoid is rare, with only four previously reported cases in the English language literature. Surgical excision of the ossification is the main treatment modality. The authors were able to establish a cleavage plane of 3 cm between the bony mass and the medial surface of the ramus. Abdominal fat was placed as an interpositional material to prevent fibrosis and heterotopic bone formation.
Bone has a unique capacity for growth, remodelling, and regeneration. When bone is formed within or in continuity with the skeleton, it is known as orthotropic bone, and when formed in tissues that do not normally ossify, it is termed heterotopic bone. Myositis ossificans traumatica (MOT) is often reported in the orthopaedic literature in young athletes. Its aetiology is linked to blunt injury to large skeletal muscles, chiefly the quadriceps femoris and the brachialis anticus . It presents as a palpable tumour-like calcified mass within the injured muscle. Common examples are horse-riders’ bone, cavalrymen’s osseous plate on the outer thigh and infantrymen’s drill bone on the deltoid. The lesion does not attach to the underlying bone, but it may become attached to the periosteal surface. Although the aetiopathogenesis of MOT is paradoxical, most authors consider it an aberrant form of physiological healing.
C arey proposed four main theories for its development: displacement of bony fragments into the soft tissue with subsequent proliferation; detachment of periosteal fragments into the surrounding tissue with proliferation of osteoprogenitor cells; migration of subperiosteal osteoprogenitor cells into surrounding soft tissue, through periosteal perforation induced by trauma; and metaplasia of extraosseous cells exposed to bone morphogenic proteins derived from the lysis of bone fragments displaced within the soft tissue during traumatic injury.
A rima et al. proposed that autolysis of scattered bone fragments releases bone morphogenic protein (BMP), inducing the differentiation of perivascular mesenchymal cells into muscular tissue, resulting in a relatively homogenous bony mass. They also found that the interval between trauma and first detection of a calcified mass ranged from 3 weeks to more than 20 years.
In the case presented, trauma caused by extraction of a lower molar and subsequent manipulation has produced the myorrhexia that stimulated the sequence of events.
A 36-year-old woman presented in March 2009 with progressive, painful limitation of jaw opening over the past 3 months. She traced her problem to a traumatic extraction of a left lower molar approximately 5 months previously. In November 2008, the patient underwent extraction of the lower third molar at a dental office. She reported that three injections were necessary to achieve anaesthesia and her mouth had been widely held open for an extended period of time. The patient denied a history of joint sounds, pain or any previous limitations in mandibular motion. She returned 1 week postoperatively with complaints of pain and trismus. She was advised to undergo physiotherapy and take non-steroidal anti-inflammatory drugs (NSAIDs). She returned 6 weeks later, with a maximal incisal opening (MIO) of 3 mm. An intra-articular injection of local anaesthetic was given, and the lower jaw forcefully manipulated. She showed temporary improvement in MIO post manipulation, but within 2 or 3 weeks her MIO was 3–5 mm.
Physical examination revealed a well-nourished woman without obvious developmental abnormalities. MIO at presentation was 3 mm ( Fig. 1 ). She was unable to protrude or produce excursive movements. Palpation of the left coronoid process elicited mild pain. Palpation of the masseter and temporalis muscles was normal. The occlusion was stable.
A panorex revealed an elongated left coronoid process and diffuse radio-opaque structures at the pterygoid plates and the sigmoid notch. Computerized tomography (CT) scans depicted an irregular ossified mass attached to the medial aspect of left ramus of the mandible extending medially to the medial and lateral pterygoid plates ( Fig. 2 ). The bony mass showed a zonal pattern characterized by multiple foci of central noncalcified regions of low attenuation, surrounded by a peripheral ring of high density, consistent with mature bone.
After finding extensive calcifications within the masticatory musculature, the laboratory tests included calcium, alkaline phosphatase, phosphorus, and vitamin D. All reports were within normal limits. The working diagnosis was traumatic myositis ossificans. Surgical excision of the ossification was planned under general anaesthesia. After fibre optic assisted intubation, the mandibular ramus was approached via a submandibular incision. The masseter was noted to be tightly bound to the lateral aspect of the ramus. Reflection of this muscle did not show any calcifications within the muscle. An attempt was made to reflect and access the calcified medial pterygoid muscle. This could not be accomplished completely because of extensive calcification of the muscle. An additional intra oral paramedian osteotomy was performed and the mandible swung aside to provide access to the medial aspect of the ramus.
A calcified mass within the medial pterygoid muscle was identified. Biopsy specimens were obtained, and a cleavage plane of approximately 3 cm was achieved between the bony mass and the medial surface of the ramus. An osteotomy was performed on the medial aspect of the ramus at the area of insertion of the medial pterygoid. An interpositional abdominal fat graft (full thickness skin and subcutaneous fat) was placed on the medial aspect of the ramus ( Fig. 3 ). The paramedian osteotomy, reduced into anatomical position with miniplates. A mouth prop was inserted between the teeth, and the mouth was opened to 32 mm.
Specimens were submitted for histopathological analysis. Microscopically, section A shows bone tissue with numerous lacunae filled with osteocytes ( Fig. 4 ). The periphery shows fibrous connective tissue with numerous spindle fibrocytes and mature collagen fibres arranged in moderately thick bundles. Few blood vessels are also seen. Section B shows trabeculae of bone tissue with osteocytes and numerous reversal lines ( Fig. 5 ).