What is Radiation Therapy for Heterotopic Ossification Prophylaxis?
Heterotopic ossification (HO) is a condition where mature bone forms in soft tissues where it’s not normally supposed to be. This often happens after an injury or surgery near the joints, and it’s particularly linked with hip injuries. Three main causes can lead to HO; these are physical injuries, damage to the nervous system, and certain genetic factors.
HO can be caused by different types of physical injuries like bone fractures, dislocations, and surgeries like open reduction-internal fixation (a procedure to set a broken bone) and total hip replacement. In terms of location, it most frequently affects the muscle that helps move the hip away from the body (the abductor compartment). Following injuries or surgery to the elbow, it’s the second most common place for HO to occur.
Damage to the nervous system, like spinal trauma and head injuries, can also lead to HO, particularly around major joints. The connection between the elements of the nervous system and the development of HO isn’t entirely clear. Still, it seems to involve an overactive response from cells called osteoblasts that produce new bone tissue where it shouldn’t be.
Lastly, genetic disorders like fibrodysplasia ossificans progressiva, progressive osseous heteroplasia, and Albright hereditary osteodystrophy can also cause HO. These rare genetic conditions cause HO to develop early in life and can lead to major health issues.
What Causes Radiation Therapy for Heterotopic Ossification Prophylaxis?
Back in the 1950s, scientists discovered that certain cells involved in bone repair, referred to as ‘osteogenic progenitors’, are sensitive to radiation therapy. These early studies explored the process of a phenomenon called heterotopic ossification (HO), where certain cells in the surrounding soft tissues turn into a type of bone-forming cell called osteoblasts.
In this process, a tissue that’s typically involved in creating bone starts to form a different type of bone due to an abnormal cycle of bone formation and remodeling. Researchers think that these cells are sensitive to radiation because they divide and mature into bone and cartilage cells at a high rate.
Because of this, low-dose radiation is used as a preventive treatment to decrease the chance of heterotopic ossification, where bone grows in the wrong place, a detail we’ll talk more about below.
Risk Factors and Frequency for Radiation Therapy for Heterotopic Ossification Prophylaxis
Heterotrophic ossification, or the abnormal growth of bone in the soft tissues, is highly common in patients undergoing hip replacement, especially those with moderate to severe bone spurs in the hip joint. This condition occurs most frequently in the hip, and sometimes affects the elbow and other major joints. It’s rare in soft tissue areas not connected to joints.
There are several risk factors for this condition. We can categorize risk levels:
- High-risk patients:
- Those with bone spurs in both hip joints
- Those with a prior history of heterotrophic ossification
- Those with arthritis caused by trauma leading to the development of bone spurs
- Moderate-risk patients:
- Those with bone spurs in one hip joint
- Those with stiffening of the spine called ankylosing spondylitis
- Patients with Paget’s disease, which disrupts the normal cycle of bone growth
- Those with Diffuse Idiopathic Skeletal Hyperostosis (DISH), which is characterized by bony growths on ligaments in the body
Interestingly, heterotrophic ossification is twice as likely to occur in males over the age of 65 who undergo hip surgery, particularly if the surgery is performed from the side of the hip.
Signs and Symptoms of Radiation Therapy for Heterotopic Ossification Prophylaxis
Hip stiffness after surgery or an injury is one of the major symptoms that patients present with. This stiffness can cause a decrease in the range of motion, making it hard to move freely and carry out daily activities. In severe cases, patients with HO, also known as hip ossification, might experience other symptoms such as high temperature, skin redness and warmth around the hip, sensitivity, and swelling. A less common symptom is pain that occurs just a few days after the surgery.
Sometimes patients may not show any physical symptoms of HO but may be diagnosed based on a routine X-ray. These X-rays can show structures around the hip becoming hardened and having fuzzy edges as early as 2 to 6 weeks after the surgery. Bone scans may also exhibit an increase in activity in the soft tissues around the hip. Bone scans can also catch HO a few days before it becomes visible on a normal X-ray; however, these findings are not exclusive to HO. The maturation of the HO may take up to 1 to 2 years.
Furthermore, there have been rare cases where there have been links with severe HO and a sedimentation rate higher than 35 mm/hr and a serum alkaline phosphatase level above 250 international units/L.
Testing for Radiation Therapy for Heterotopic Ossification Prophylaxis
In cases of HO (a condition where bone grows in places where it shouldn’t), doctors often use x-rays for evaluation. A popular method to classify the severity of this condition is the Brooker Staging system. This system categorizes the condition into four grades based on the x-ray images of the pelvis and hip. The impact of grades 3 and 4 is significant because they affect movement and cause pain.
Here’s what each grade in the Brooker Staging system means:
- Grade 1: Small pieces of bone are found in the soft tissues around the hip.
- Grade 2: Bony outgrowths (exophytes) are found in the pelvic region or upper part of the thigh bone (femur), with at least 1 cm of space between opposing bone surfaces.
- Grade 3: Bony outgrowths are found in the pelvic region or upper part of the thigh bone, but the space between opposing bone surfaces is less than 1 cm.
- Grade 4: The upper thigh bone and pelvis have joined together abnormally due to bone growth.
Understanding these stages can help doctors plan the most appropriate treatment for patients with HO.
Treatment Options for Radiation Therapy for Heterotopic Ossification Prophylaxis
Treatment for heterotopic ossification (HO), or the abnormal growth of bone in the soft tissues of the body, varies based on how severe the condition is, what symptoms the patient is experiencing, or the patient’s risk of developing HO. The orthopedic surgeon, a doctor who specializes in the skeletal system, often decides the best preventive measures for the patient. Early stage HO that isn’t causing any symptoms might just be monitored. However, if HO is advanced, the patient may need to have the extra bone surgically removed. After the operation, the patient might receive other treatments like medications or radiation therapy to reduce the chance of the condition returning.
The main medicines used to prevent HO are nonsteroidal anti-inflammatory drugs (NSAIDs), a type of medication that can relieve pain, decrease inflammation, and lower fever. While indomethacin is the most commonly used NSAID, other NSAIDs like ibuprofen can also be effective for preventing HO. These alternatives also are easy to administer and don’t cost much. The patient should take these medicines after their operation, but be aware of potential side effects such as gastritis and ulcer formation, which involve inflammation and sores in the stomach.
What else can Radiation Therapy for Heterotopic Ossification Prophylaxis be?
When going through radiation therapy to prevent abnormal bone growth (heterotopic ossification), some conditions may present similar symptoms and should be considered. These conditions include:
- Cellulitis
- Fracture
- Hematoma
- Local trauma
- Septic arthritis
- Thrombophlebitis
