What Is Myositis Ossificans?

To explain myositis ossificans (MO) as a condition, let’s take apart its complicated name and by doing so, reveal the condition itself.

When you break down the words myositis ossificans (pronounced MY-o-SI-tis o-SIF-I-cans), it  describes the condition quite nicely:

• Myo (muscle)
• Itis (inflammation)
• Ossificans (ossification being the process of bone formation - turning to bone)¹  

So, the name myositis ossificans explains the condition as being the result of inflammatory bone forming in muscle.²  Kransdorf et al. defined MO as: “a benign solitary, self-limiting, ossifying soft-tissue mass typically occurring within skeletal muscle.” ³

This definition means – a lump that isn’t cancerous, usually found in just one place, and made of bone cells; it can develop in different soft tissues (e.g., fatty tissue, tendon, ligament), but usually forms in muscle tissue.

The general term heterotopic ossification is also worth mentioning.⁴ This means bone formation (ossification) occurs abnormally in tissues where it typically wouldn't.

What causes myositis ossificans?

Myositis ossificans is classified into 3 categories based on the causative mechanism:

1. Traumatic
2. Non-traumatic
3. Progressive (also known as fibrodysplasia ossificans progressiva)

Traumatic myositis ossificans

This type of MO is by far the most common (although it is still an uncommon condition overall) and is caused by direct trauma or injury to a muscle.²  Most often, this type of MO occurs in younger athletic people where muscle trauma from a sports injury is a risk. An example of this would be a football player who received blunt-force trauma to the thigh. ²

Smaller muscle traumas over time can also trigger traumatic MO when certain muscles are repeatedly overused. An example of this would be MO in the adductor muscles (inner thigh muscles) of horse riders²

Non-traumatic myositis ossificans

This type of MO is rarer and is not linked to a known trauma in the muscle(s). Its cause is not fully understood, but it has been associated with different conditions, including:^{5, 6}  

• Burns
• Poliomyelitis
• Paraplegia
• Infections
• Neuromuscular disorders
• Haemophilia

Progressive myositis ossificans

This type of MO is also referred to as Fibrodysplasia Ossificans Progressiva (FOP) and is an extremely rare genetic disease, affecting approximately 1 in 2 Million people worldwide.⁷ It is a disabling and often fatal disease that presents itself in early childhood.^8,9 Whereas traumatic and non-traumatic MO occur in solitary muscles or a single muscle group, FOP progresses to many different muscles, as well as other soft tissues, including tendons, ligaments and fascia, and can be triggered by minor trauma (bruises and falls).^{8, 9}  

Symptoms of FOP

Children with FOP often have deformities in both big toes at birth,^8,9 which is the first indication of FOP. They then have periods of painful flare-ups of inflammatory symptoms throughout their lives, followed by bone-forming throughout their soft tissues.^8,9

Treatment of FOP

There is currently no cure and very little effective treatment for FOP; people live, on average, until the age of 40.⁸

This is an extremely rare type of MO that doesn’t have the same symptoms or treatment as traumatic and non-traumatic MO. Therefore, it will not be discussed in further detail in this article.

What happens in the body with myositis ossificans?

It is not fully understood how MO develops in muscle, but you can think of it as muscle tissue repair going awry. ⁴  

Here is a breakdown of the physiological process of MO development:

1. A muscle is injured,
2. The muscle injury causes an inflammatory response in the body,
3. The inflammatory response releases proteins known as cytokines into the muscle which control the inflammation. 
4. Specific types of these cytokines stimulate cells known as vascular endothelial cells that sit within the muscle.
5. The vascular endothelial cells differentiate (turn into) chondrocytes (cartilage cells) or osteoblasts (bone-forming cells). ^{2, 5}

The result of this process is skeletal bone cells forming inside muscle cells—hence the term heterotopic ossification. ⁴ 

What are the stages of myositis ossificans?

The process of bone cells forming in muscle is suggested to occur over 3 stages: early, intermediate, and late. Each stage corresponds to different symptoms and clinical presentations, as well as different levels of visibility on diagnostic images.

Early stage

The early stage of MO is thought to happen within the first 4 weeks ² of muscle injury when the body’s inflammatory response is underway. Patients may have swelling, redness and pain in the affected muscle. Calcification doesn’t generally show up in diagnostic imaging at this stage. ^{2, 5} 

Intermediate stage

Generally, within 4-8 weeks of muscle injury, a soft-tissue mass may be present with increased stiffness around the joint nearest the affected muscle, and bone formation in the muscle starts to become visible with diagnostic imaging. ² 

Mature/Late stage

After approximately 8 weeks ², the mass in the muscle may still be present, but joint stiffness, pain, and swelling of the muscle should have been reduced. A dense soft-tissue mass will be visible and identifiable with diagnostic imaging at this stage. ^{2, 5}

What are the symptoms of myositis ossificans?

Traumatic and non-traumatic myositis ossificans generally present with the following symptoms: ^{2, 4, 5,10}

• Pain and swelling in the affected muscle
• A rapidly swelling mass in the muscle
• Redness in the skin around the affected muscle
• Heat radiating from the affected muscle
• Joint stiffness and reduced range of movement in the nearest joint

Patients often report pain in the muscle that lasts longer than an ordinary muscle sprain or strain. ²  Traumatic MO is by far the most common of the 3 types, so patients with it will often remember the event that caused the MO. ²  However, patients in the late stage may not have many symptoms because the inflammatory process usually peaks in the early stage.

How is myositis ossificans diagnosed?

To successfully diagnose MO, doctors need to correlate the results of physical examination, patient history, biopsies of the muscle mass, and diagnostic imaging techniques. Also, because MO has distinct stages of development, different diagnostic methods may be more or less effective depending on the stage of the condition.

Physical examination

A doctor asks the patient about their symptoms as well as observing and feeling the specific muscle that is affected. A mass may be noticeable depending on the stage of MO, and the joint associated with the injured muscle may be stiff and have a reduced range of movement. ^{2, 5}

Imaging

Different imaging methods may be used including:

• Computerised Tomography (CT)
• Magnetic Resonance Imaging (MRI)
• Ultrasound  

Generally, MRI is the preferred method for looking at soft tissue, including muscle. However, because MO is the development of bone within muscle, a CT may be the best imaging method, as it can clearly define patterns of calcification. ²

However, imaging is deployed diagnostically most effectively in the late stage of the condition as the mass will have had time to develop in the muscle and will show clear calcification at this stage. ^{2, 4, 11}  At the early stage, the inflammation and swelling caused by MO, and the early stage of bone formation within the muscle mass, does not always appear on MRI or CT scans. ² 

Muscle biopsy

In some cases, a patient’s symptoms and imaging results may be clear enough to make a definitive diagnosis, but in other cases, MO presents itself differently at different stages, and patients will require a muscle biopsy to allow doctors to look at the specific types of cells within the mass under the microscope and confirm or deny the diagnosis. A muscle biopsy may be particularly necessary for patients who have non-traumatic MO and patients who are in the early stage, where the pattern of bone formation in the muscle isn’t clear on an MRI or CT. ²

Using several of these diagnostic tools in combination is important to diagnose MO, particularly if the patient does not report an event that caused the muscle injury. This is because MO can mimic other conditions and has been referred to as a “pseudotumour”¹¹ (false tumour). Even with MRI and CT scans, MO can mimic a type of cancerous tumour in the soft tissue known as a soft tissue sarcoma.²

How do you treat myositis ossificans?

Myositis ossificans has a good prognosis and is known as a self-limiting disease, meaning it tends to resolve by itself in time. ^{2, 5} In the first instance, non-surgical treatments focus on reducing the symptoms of inflammation and maintaining the function of the joint associated with the damaged muscle, ²       

They include:

Rest, Ice, Compression, Elevation (RICE)

In the early stage (week 1), the RICE protocol of rest, ice, compression, and elevation is recommended to reduce swelling and inflammation. ^{2, 5}

Gentle movement

Following this, gentle and assisted mobilisation of the muscle and its nearest joint is recommended to keep as much range of movement in the joint as possible. This can begin a few days after the muscle injury, once the initial inflammation and swelling have subsided. ^{2, 5}

Physiotherapy and rehabilitation

Rehabilitation with physiotherapy can be introduced gradually and in an exercise program with different exercises to improve a dynamic range of movement at the joint and strengthening exercises for the affected muscle. ^{2 5}

Do you need surgery for myositis ossificans?

Surgery may be considered for patients who continue to have pain, swelling, or reduced range of movement in the nearby joint even after conservative management. It may also be necessary for patients in whom the mass in the muscle is compressing nerves and blood vessels. ²

Case studies of patients with the rarer non-traumatic MO type had, surgical excision of the muscle mass, radiotherapy, and physiotherapy included in the patients’ treatment program.^{12, 13}

How can you prevent myositis ossificans?

Avoiding trauma to muscles is the best way to prevent MO from developing, and effective use of all available diagnostic tools is essential for getting the correct diagnosis as it can mimic other soft-tissue diseases including cancerous soft tissue sarcomas, particularly in the early stage.

Summary

Traumatic myositis ossificans is an uncommon condition that arises from trauma to a muscle. When the muscle is injured, the healing process takes a different turn and bone cells form within the injured muscle. Its development has different stages, making it tricky for doctors to diagnose early on, but it doesn’t spread to other places and generally heals on its own in time.

Even rarer forms of MO include non-traumatic MO and fibrodysplasia ossificans progressiva. The cause of non-traumatic MO is unknown, but it has the same physiological pathway, symptoms, diagnosis, and treatment as traumatic MO, but differs in that surgical excision and radiotherapy may be used in the first instance.

Fibrodysplasia Ossificans Progressiva is an extremely rare genetic form of MO, which becomes apparent in early childhood and is not limited to a single muscle. There is no cure for this disease.