Myositis ossificans revisited – The largest reported case series

Abstract

Introduction

Myositis ossificans (MO) is a condition characterised by the formation of non-neoplastic heterotropic ossification in extraskeletal soft tissues.

Material and methods

We performed a retrospective study of our radiology databases within our tertiary orthopaedic centre to identify all cases of MO, reported on X ray, Magnetic resonance imaging (MRI) and Computed tomography (CT) over the past 13 years (2007–2020).

Results

We identified 68 cases of MO, which were included into our cohort. The average age of our patients was 36 years (range 4–84 years). 73% of cases (n = 50) were found to affect the lower limb muscles with the majority in the quadriceps.

Conclusion

We report the largest case series of MO and discuss the demographics, diagnoses and management.

1. Introduction

Myositis ossificans (MO) is a rare condition characterised by the formation of non-neoplastic heterotropic ossification in extraskeletal soft tissues.¹ It can be idiopathic, or present 4–12 weeks following trauma. MO is most commonly found in young athletic males, between the second and third decades of life.²

The clinical presentation of MO is often non-specific, and can present as a rapidly enlarging tender mass, closely mimicking more sinister pathological lesions. Therefore, a high index of suspicion, awareness of disease progression and knowledge of radiological features is essential to ensure appropriate early diagnoses and management is initiated. The incidence of MO following trauma is reported to be between 9 and 17%. MO can affect any part of the body. This study highlights the different regions of where MO can arise, the best methods of diagnosis based on the literature and management.

2. Material and methods

We performed a retrospective study of our radiology databases within our tertiary orthopaedic centre to identify all cases of MO, reported on X ray, Magnetic resonance imaging (MRI) and Computed tomography (CT) over the past 13 years (2007–2020). The data was categorised according to patient demographics and the site of the lesions. Furthermore, all images were reviewed by a senior radiology consultant with over 8 years’ experience.

3. Results

We identified 68 cases of MO, which were included into our cohort. The average age of our patients was 36 years (range 4–84 years). There was a male predominance with a ratio of 1.4 males to 1 female (42 male, 29 female).

MO was found in multiple different sites within the body. 73% of cases (n = 50) were found to affect the lower limb muscles, whereas 26% (n = 17) were identified in the upper limb muscles (Fig. 1). There was 1 case reported within the paraspinal muscles (Fig. 2).

Fig. 1.

Axial T1(a), STIR(b) and CT(c) showing myositis ossificans (arrow) in the vastus medialis.

Fig. 2.

Longitudinal ultrasound (a) and axial CT(b) showing myositis ossificans (arrow) in left paravertebral muscles (erector spinae).

The majority of cases in the lower limb muscles were located within the Quadriceps group of muscles (n = 27), of which 12 lesions were found to affect the Vastus intermedius, 7 in the vastus lateralis, 6 cases in the vastus medialis and 2 cases affecting the rectus femoris muscle. The second most common location was the superficial posterior compartment of the leg, where 5 lesions were identified in the soleus muscle and 2 lesions in the gastrocnemius. There were 4 lesions found to affect the hamstring group of muscles. All other lower limb lesions are described in Table 1 (Fig. 3, Fig. 4, Fig. 5).

Table 1.

Site of Myositis ossificans within our cohort.

Myositis Ossificans location	Total
Upper Limb
Biceps	1
Brachialis	4
Brachoradialis	2
Deltoid	1
Flexor DP	3
Infraspinatus	1
Lattismus dorsi	1
Triceps	4
Trunk
Paraspinal Muscles	1
Lower Limb
Adductor brevis	1
Adductor Magnus	1
Gastrocnemius	2
Gluteus medius	3
Gluteus Minimus	1
Hamstrings	4
Iliopsoas	3
Obturator Externus	1
Plantar fascia	1
Sartorius	1
Soleus	5
Rectus femoris	2
Vastus intermedius	12
Vastus lateralis	7
Vastus Medialis	6
Total	68

Fig. 3.

Axial T1(a), STIR(b) and coronal STIR(c) showing myositis ossificans (arrow) in the right iliacus.

Fig. 4.

Axial T1(a), STIR(b) and CT(c) showing myositis ossificans (arrow) in the infraspinatus.

Fig. 5.

Axial T1(a), STIR(b) and CT(c) showing myositis ossificans (arrow) in the soleus.

Of the 18 cases affecting the upper limb muscles, MO was found most commonly within the triceps and Brachialis (n = 4). This was followed by 3 lesions in the flexor digitorum profundus muscle (FDP). All other lesions can be found in Table 1.

4. Discussion

MO is a rare, self-limiting, pseudo-inflammatory ossifying lesion, characterised by anomalous benign heterotopic bone formation within soft tissue, and despite their name, are void of any primary muscle inflammation. MO most commonly originates within skeletal muscle,^3,4 however, can also be derived from tendons, nerves and subcutaneous fat. The exact causes of MO are not well understood,⁵ and in most cases, no contributory factors are found.⁶ Nevertheless, several factors have been documented in the literature to play a major role in its development. Triggering factors may be initiated following soft tissue trauma (usually 4–12 weeks following injury), burns, bony fractures, dislocations, or repetitive trauma, the latter documented in up to 65% of cases. Other non-traumatic causes ensue from neurogenic conditions such as spinal cord injuries, strokes, brain tumours and other neurological etiologies.^7,8

5. Epidemiology

MO is the most common form of heterotopic ossification. It has a prevalence of less than one in one million⁹ Males are more commonly affected, and although they can arise at any age, over 50% of patients present within the second and third decades of life.¹⁰ The site of origin of MO is variable, however, approximately 80% are found within large skeletal muscles of the extremities. The two most commonly affected regions are the quadriceps (incidence of 52–56%), and brachialis muscles (incidence of 12–23%).^11,12 Other common areas susceptible to trauma include the pelvis, elbow and shoulder,^{13, 14, 15} however, the exact incidence in the latter regions are unknown, and to the best of the authors knowledge, have only been reported in the literature on multiple case reports.^{16, 17, 18, 19, 20}

6. Pathology

MO develops through three distinct phases that lead to a characteristic ‘zone phenomenon’ observed on histological tissue samples.²¹ The zones are divided into a well organised mature lamellar bone surrounding the periphery, an intermediate osteoid region and a central immature non-ossified fibroblastic focus. The phases of transition of MO usually last over an average of three months.

In the initial acute phase, tissue injury causes organisation and proliferation of granulation tissue, with differentiation of both osteoblastic and fibroblastic cells and osteoid bone formation. On histological diagnoses, MO appears as a pseudo-fibrosarcomatous lesion during this stage,²² leading to diagnostic confusion, as it is often very difficult to differentiate from other sarcomatous lesions.

Following this phase, the osteoid matrix continues to develop with immature lamellar bone to give a pseudo-osteosarcomatous appearance.²² Between 5 and 24 weeks, the immature bone growth progresses to form mature cortical and trabecular bone, which on histological diagnoses distinctively shows the three characteristic zones of MO.

7. Clinical presentation

The clinical presentation of MO is variable. It can often be an incidental finding on radiological imaging. In other patients, presentation can be more dramatic, with a rapidly enlarging painful mass at the region of the sustained trauma, with or without limitations of movement. Overall, clinical symptoms of MO can highly mimic that of aggressive pathological tumours, such as sarcomas and requires a high index of suspicion (9,10). MO is generally self-limiting, and often the lesion decreases in volume within 1–2 years²³

8. Diagnoses

Diagnoses of MO are highly dependent on appropriate history taking (preceding a traumatic cause) clinical symptoms, aided by radiological and histological diagnoses, (when the zone phenomenon can be illustrated).²⁴

The radiological features of MO must be interpreted with caution, especially during the early phases of the disease. On Computed tomography (CT) and radiographs, MO depict as round calcified lesions with a radiolucent centre, and the classical ‘string sign’, typified by a radiolucent cleft separating lesion from adjacent bone. It can take up to 2 weeks for the abnormality to appear on radiographs, as initially calcification is absent.²⁵ Appearances on Magnetic resonance imaging (MRI) show a temporal change with time and demonstrate heterogeneity.²⁶ More importantly, radiologists need to be aware that early changes can be misleading, since the peripheral calcification may not be well visualised, and soft tissue oedema often extends well beyond the calcific rim for up to 8 weeks. Late features mimic bone, where on the peripheral aspect, the lesions may produce low signal intensities, signifying mature lamellar bone, with intermediate to high signal intensities more centrally on T1WI.²⁷

9. Differential diagnoses

As mentioned above, MO can have a dramatic clinical presentation, closely mimicking that of more sinister pathological entities. In general, appropriate diagnoses of MO must be devoid of multiple differential diagnoses to ensure unnecessary investigations are avoided and appropriate management is undertaken. The main differential diagnoses of MO are extraskeletal osteosarcomas, which mirror both imaging and histological presentation.

Parosteal osteosarcomas (PO) are classically described as ‘cauliflower-like’ osseous lesions. They have a female predominance, and are found most commonly in young adults. Typically, they have a tendency to grow in long bones, with 90% of lesions identified in the metaphyseal region. On radiological imaging, PO may also display the classical ‘string sign’ described above, but in contrast to MO, calcifications are more centrally located.²⁷

Synovial sarcomas are another rare group of highly aggressive soft tissue lesions that mimic MO. They can occur in almost any part of the body. Nearly all SS have a genetic predisposition that are related to the disease morphology and outcome. Diagnoses of these lesions are dependent on a combination of immunohistochemical, genetic and radiological imaging. Similar to MO, calcifications of those lesions are often more concentrated at the periphery. However, SS tend to cause bony erosion and destruction, a feature that is never observed in MO.²⁷

10. Management

The management of MO remains controversial and depends on the severity of patient symptoms. In most cases, MO requiring no intervention.²⁶

Conservative management can be offered to symptomatic patients with minimal pain, or those deemed unfit for surgical intervention. This would normally include a combination of analgesia, in the form of nonsteroidal anti-inflammatory drugs (NSAID) and physiotherapy.²⁸ Other non convential treatment therapies which have been reported to be suggestive include acetic acid therapy, iontophoresis treatment, magnesium therapy, and etidronate disodium.^29,30

Patients with severe persistent symptoms may benefit from surgical intervention.³¹ The definitive treatment of symptomatic MO is surgical excision. Orthopaedic surgeons need to be aware that it is critical this is carried out once the ossification has fully developed, as early excision often leads to local recurrence.^32,33 Surgical excision can often be followed by short doses of radiotherapy, with some studies proving this to be beneficial in limiting pain and restoring mobility.^34,35

11. Conclusion

MO are non-neoplastic lesions that can affect any extraskeletal compartment. Our study showed that they are most commonly found with the anterior thigh region surrounding the quadriceps group of muscles. It Is essential for clinicians to have a high index of suspicion to be able to correctly diagnose MO and initiate appropriate management.

Declaration of competing interest

No conflicts of interest.

No financial disclosures.