What is Heterotopic Ossification?
Heterotopic ossification (HO) is a common issue that occurs during physical therapy. In simplest terms, it’s the growth of mature bone in places outside the skeleton, where bone shouldn’t normally be. This problem often affects patients who have suffered from burns, strokes, spinal cord injuries, traumatic amputations, joint replacements, and traumatic brain injuries. However, there are also certain genetic conditions that can cause HO, like fibrodysplasia ossificans progressiva, Albright’s hereditary osteodystrophy and progressive osseous heteroplasia. But these genetic conditions aren’t related to complications from physical therapy and hence, won’t be mentioned in this context.
What Causes Heterotopic Ossification?
Heterotopic ossification (HO), or the formation of bone in soft tissue where it’s not normally found, can be divided into two main categories: traumatic, which is usually a result of injury or surgery, or neurogenic, which is related to nerve diseases or neurological conditions. The types of trauma that can lead to HO include fractures, joint replacements, muscle injuries, joint dislocation or burns. On the other hand, neurogenic HO might occur with conditions like stroke, spinal cord injury (SCI), traumatic brain injury (TBI), or brain tumors.
Among joint replacement surgeries, the most common sites of HO are in the hip, knee, elbow, and shoulder. Chronic muscle injury can lead to a specific type of HO known as traumatic myositis ossificans – this mostly affects the large muscle at the front of the thigh (the quadriceps) and the muscle on the front of the arm (the brachialis muscle).
In those with neurogenic conditions, HO often occurs in the hips, elbows, shoulders, and knees. Uncommonly, HO could potentially happen in the kidneys, uterus, certain parts of the male genitalia, the gastrointestinal (GI) tract, or even the site of a surgical incision. The exact cause and way neurogenic HO happens is not completely understood.
Certain factors increase the risk of developing HO. These include muscle spasms or stiffness (spasticity), being older in age, having a pressure sore or deep vein thrombosis (DVT – a blood clot in a deep vein), having a tube inserted into the windpipe to assist with breathing (tracheostomy), injuries to long bones, a previous injury to the same area, swelling (edema), being immobilized, long-term unconsciousness (coma), the severity of the injury (e.g. trauma, TBI, SCI, stroke).
In patients who have had total hip arthroplasty (THA – a hip replacement operation), some factors are associated with a higher or moderate risk of developing HO. These include being a man having both hips replaced, previous history of HO, conditions like ankylosing spondylitis, diffuse idiopathic hyperostosis, or Paget’s disease.
Risk Factors and Frequency for Heterotopic Ossification
In people who have had a total hip replacement, the rates of a condition known as heterotopic ossification (HO) can vary. For those who have high-risk factors, the rates can be as high as 90%. On average, around 53% of people having total hip arthroplasty develop HO. It’s generally twice as common in males as compared to females. However, women older than 65 have a higher risk of developing HO. There is a 10% to 20% chance of HO occurring in individuals with nerve-related conditions.
- Heterotopic ossification rates differ in the total hip replacement population.
- In people with high-risk factors, this rate can spike up to 90%.
- The average incidence of HO in this group is about 53%.
- HO is twice as common in males as compared to females.
- Females older than 65 have an increased risk of developing HO.
- The chance of people with nerve-related conditions developing HO is between 10% and 20%.
There are variations in HO rates when considering different patient groups. The rates for adults with spinal cord injuries (SCI) are around 20% to 30%. For those with traumatic brain injuries (TBI), the rates are about 10% to 20%. The rates of HO in children with TBI range between 3% and 20%.
Signs and Symptoms of Heterotopic Ossification
Heterotopic ossification (HO), or abnormal bone growth in the body, usually happens 3 to 12 weeks after an injury, but can sometimes take up to 6 months to notice. It often occurs after surgeries like hip or knee replacement, or following a stroke, spinal cord injury, traumatic brain injury, or severe burn. The most common signs are pain and a decrease in the ability to move the affected joint. Patients often feel stiffness. Other symptoms to watch out for include swelling, fluid buildup, redness, warm skin, tenderness in the affected area, and low-grade fever.
- Pain in the affected area
- Decreased range of motion (ability to move a joint)
- Joint stiffness
- Swelling
- Fluid build-up
- Redness
- Warm skin
- Tenderness
- Low-grade fever
The swelling in the affected area can look like a deep vein thrombosis (DVT), a condition where a blood clot forms in a deep vein. If muscle stiffness or spasms are present near the affected joint, they are also risk factors for HO. Other things that might raise your risks are being in a coma for a long time, having a breathing or feeding tube, lack of mobility, pressure ulcers, and fractures in the long bones of the body. The greatest risk for developing HO is between the 3 to 4 month period after the injury.
Testing for Heterotopic Ossification
When dealing with the possibility of heterotopic ossification (HO), a condition where bone grows in the wrong place, doctors often order lab tests to check the levels of specific substances in your blood. One of these is alkaline phosphatase, a substance that can increase in cases of HO. However, this increase might not happen immediately and can take up to two weeks. Even then, the level of this substance doesn’t tell us how severe the injury is. Moreover, it can also give false results if there are associated injuries to the long bones.
Other substances that doctors commonly check are erythrocyte sedimentation rate (ESR) and C-reactive protein, both markers of inflammation in the body. If these are higher than normal, it can suggest the development of HO. However, these tests are not specific and can be high in many different conditions. Similarly, another substance, creatine kinase (CK), can indicate the severity of HO but is also not very specific. There is some evidence to suggest that a high CK level may indicate a more aggressive course of HO that might not respond well to a particular therapy called etidronate.
In addition to lab tests, imaging tests are also commonly used. X-ray is often used first because it can show bone growing in the wrong place, a distinctive sign of HO. However, X-rays may not show early stages of HO and usually can’t detect the condition until 3 to 4 weeks after it appears on a different imaging test called a bone scan. Early HO can be detected as soon as 2.5 weeks after injury with a bone scan, making it the most sensitive test for this condition.
Other imaging techniques, like computed tomography (CT) scans, may be used to outline the area where the new bone is growing, particularly if surgery is planned. However, their role in diagnosing HO is not well established. Magnetic resonance imaging (MRI) may also be used in some situations, but it is not cost effective unless the new bone is growing around nerve structures. Ultrasound and 3-dimensional stereolithography, two other types of imaging, are not used very often.
Treatment Options for Heterotopic Ossification
The goal of prophylaxis, or preventive care, is to identify patients who are at a high risk of developing a condition called heterotopic ossification (HO). Heterotopic ossification is an abnormal growth of bone in the soft tissues of the body. However, routine preventive care is not recommended for everyone.
Currently, ways to prevent HO include gentle range of movement exercises, medications such as indomethacin and etidronate, and a type of radiation therapy called external beam radiation. Indomethacin is a type of medicine known as a non-steroidal anti-inflammatory drug (NSAID). It’s most commonly used to prevent HO. Other effective NSAIDs include meloxicam, celecoxib, rofecoxib, and ibuprofen. However, these medications should be used with caution as they can increase the risk of bleeding.
Etidronate is a type of medicine known as a bisphosphonate. It’s approved for preventing HO in patients with spinal cord injuries and those who have had a total hip replacement. External beam radiation is a treatment that uses high-energy rays to destroy abnormal cells. It’s mainly used after joint replacement surgery.
Managing risk factors like muscle stiffness, also known as spasticity, is also crucial in the prevention of HO.
If HO does form, it’s usually treated with range of movement exercises, medications like indomethacin and etidronate, and surgery to remove the extra bone. This surgery is only recommended if it will improve the patient’s quality of life, like their ability to move, transfer from a bed to a chair, maintain personal hygiene, and perform activities of daily living.
Like in prevention, indomethacin and etidronate are also used in the treatment of HO to stop further bone growth. However, it’s not clear how effective these drugs are in patients who have suffered a traumatic brain injury. In patients with spinal cord injury, surgery to remove the extra bone is the most effective treatment option.
What else can Heterotopic Ossification be?
When dealing with individuals who have suffered a spinal cord injury, certain conditions are commonly considered before reaching a final diagnosis. These conditions are typically evaluated, especially if initial x-rays don’t show anything unusual:
- Deep vein thrombosis (DVT)
- Skin infection (Cellulitis)
- Collection of blood outside of blood vessels (Hematoma)
- Types of bone tumors such as osteosarcoma and osteochondroma
- Prosthetic device-related infection (Hardware infection)
- Vein inflammation due to a blood clot (Thrombophlebitis)
- Bone infection (Osteomyelitis)
A Doppler ultrasound test is often done to swiftly rule out deep vein thrombosis. Identifying skin infections and abscesses can be quite hard as they can cause similar inflammation markers like Hetrotropic Ossification (HO). But, an elevated level of a specific enzyme known as alkaline phosphatase can help distinguish HO from other infection-related conditions.
An imaging technique like CT or MRI scan with contrast can help differentiate HO from conditions such as deep thrombophlebitis, bone infection, abdominal hematoma, and tumors. However, these methods might not always be the best option due to their high cost. A triple-phase bone scan is usually more sensitive in detecting HO, especially in its early stages, because it can take up to 6 weeks from the first sign of symptoms for HO to show up on an x-ray.
What to expect with Heterotopic Ossification
Complications from HO (Heterotopic Ossification, a condition where bone grows abnormally inside muscle and soft tissues) can lead to reduced movement, nerve damage and pressure sores. Up to 70% of HA (Hip Arthroscopy, a surgical procedure used to diagnose and treat problems in the hip joint) cases are symptom-free.
HO can further lead to additional issues such as ankylosis (stiffening and immobility of a joint due to bone fusion), vascular compression (pressure on blood vessels), and lymphedema (swelling due to a blockage in the lymphatic system).
However, the outlook after surgery for HO is often positive. On average, the time from injury to surgery is 3.6 years. Surgeries have proven to improve hip movement from 24.3 degrees to greater flexibility. This improvement persists even six months after surgery.
However, like any surgery, removing HO may result in complications such as infection, severe bleeding, and DVT (Deep Vein Thrombosis, a blood clot in a deep vein, often in the legs).
