Heterotopic Ossification - An Abnormal Bone Pathology

What Is Heterotopic Ossification (HO)?

Heterotopic or ectopic ossification is defined as osteogenesis (bone formation) in soft tissues, especially near joints. It mostly occurs after an injury or repair, and there is a malformation in the repair process. Heterotopic ossification can lead to pain, contractures, impaired mobility, pressure injuries, and difficulty performing ADLs (activities of daily living). It most commonly occurs in the elbow, hip, and knee joints. Based on the severity, heterotopic ossification of the hip is classified into the following categories:

1. Grade I: In this stage, islands of bone are present within the soft tissues.

2. Grade II: In this stage, bone spurs arise from the proximal or pelvis, and a bone is present between the opposing bony surfaces.

3. Grade III: In this stage, bone spurs arise from the proximal or pelvis, and a bone less than one centimeter is present between the opposing bony surfaces.

4. Grade IV: In this stage, ankylosis (fusion) of the hip joint occurs.

What Are the Causes?

The cause of abnormal bone formation is unknown. However, some factors are associated with heterotopic ossification. They are:

1. Injury: Spinal cord injury and traumatic brain injury can cause heterotopic ossification. The frequency of heterotopic ossification depends on the magnitude of the injury.

2. Trauma: Direct trauma in the form of intramuscular bleeding and displaced fracture segments is the most common cause of heterotopic ossification.

3. Severe Spasticity: Spasticity is also known as the stiffness of the joints, and it occurs resulting from musculoskeletal trauma.

4. Genetic Cause: Heterotopic ossification occurs due to malformations in the spine, hands, and feet.

5. Autoimmune Disorders: In autoimmune disorders such as dermatomyositis and systemic sclerosis, heterotopic ossification can appear in the skin. Dermatomyositis is a condition in which there is muscle weakness. Systemic sclerosis can cause vascular abnormalities in the skin, joints, and organs.

6. Pressure Injuries: Prolonged pressure on the hips can lead to pressure injuries.

7. Joint Replacement Surgery: In patients who undergo joint replacement such as arthroscopy, the risk of heterotopic ossification is higher.

8. Burns: Thermal injury is usually related to the degree of burn and not necessarily the site of the burn.

9. Immobilization: Immobilization after an amputation following a traumatic injury. Paralysis patients also have this problem due to prolonged immobility.

10. Microtrauma: Microtrauma causes soft tissue and bone injuries. During the repair process, an ectopic (abnormal position) bone formation can occur in the joints.

What Are the Symptoms?

• Swelling at the affected site becomes firm and restricts motion.

• A spongy feeling of the joints during movements.

• Joint and muscle pain.

• Decreased range of motion.

• Erythema or redness.

• Local warmth near the joint.

• Compromised ability to perform various activities.

• Pressure injuries.

• Muscle contractures.

How Is It Diagnosed?

Following an injury, heterotopic ossification can be detected radiographically in the first four to six weeks. A technetium bone scan is also done along with a triple-phase bone scan to improve sensitivity. The maturity of the heterotopic ossification can be detected through a CT scan (computed tomography). Lab studies are done to see certain factors. In a technetium bone scan, a tracer is injected intravenously to detect for localizing the bone mass through gamma rays, which are helpful in this process.

How Is It Managed?

• Pharmacological Management: NSAIDs (non-steroidal anti-inflammatory drugs) like Indomethacin is effective in preventing the formation of heterotopic ossification by interrupting prostaglandin E2 synthesis (an enzyme) and also by inhibiting the differentiation of precursor cells into active osteoblasts (bone-forming cells). Bisphosphonates are used to intervene in this condition in the early stages when the radiographs are normal. Bisphosphonates inhibit the crystallization of hydroxyapatite (a mineral that is the base of bone), thus diminishing the chances of mineralization of the osteoid.

• Rest: Rest and activity modifications are advised to the patient for faster healing and reducing the symptoms. Avoiding heat and massage to the affected site is necessary because it may cause adverse reactions.

• Pulsed Low-Intensity Electromagnetic Field: External beam radiation is used to prevent heterotopic ossification and after excision of heterotopic ossification within 48 to 72 hours after resection to prevent a recurrence.

• Radiation Therapy: This has also been proven effective in treating heterotopic ossification.

• Surgical Excision: In case of severe limitations.

• Physical Therapy: Ultrasound, iontophoresis, and cryotherapy have been used to control the symptoms of heterotopic ossification. Splinting of the affected joint to restrict motion can help improve the range of motion. A compression bandage can also be applied to the affected joint.

• Iontophoresis: It is a process in which electricity is applied along with the medication delivering it to the affected site in a transdermal route.

• Cryotherapy: It is a procedure in which cold application to the affected site is made to remove abnormal tissue. It can be used effectively to treat heterotopic ossification that occurs due to underlying lesions.

• Ultrasound: Ultrasound involves the application of sound waves to the affected site at low intensity and regular intervals. This, in turn, produces heat that, in turn will promote healing.

Postoperative rehabilitation after surgical excision is done in three phases.

1. Phase I (Week One): This phase aims to prevent infection. The surgical site is protected to decrease stress. Pain and edema are controlled through pharmacological management. The range of motion is restored to 80 percent of the original function. Maintaining the range of motion to proximal and distal joints of the surgical site.

2. Phase II (Week Two to Eight): This phase aims to reduce pain and manage edema. Limited daily activities of living are encouraged. Proper remodeling and promotion of scar mobility are also induced in this phase. Muscle contractions that improve the quality are advised.

3. Phase III (Week Nine to Twenty Four): This phase aims for the self-management of pain by the individual. Patients are advised to withdraw activities that will aggravate the pain. Strength and range of motion are improved through some exercises.

How to Prevent Heterotopic Ossification?

It is better to prescribe treatments to reduce the incidence of heterotopic ossification in patients who are undergoing joint replacement surgery. Using NSAIDs (Non-steroidal anti-inflammatory drugs) before and after surgery will help prevent heterotopic ossification. Preventing injuries with strength training, stretching, and strengthening exercises. Once heterotopic ossification forms, surgery is the only option that can be long-lasting and effective in restoring function.

Conclusion:

Heterotopic ossification that occurs without an underlying genetic cause can be fully cured. Heterotopic ossification occurs after an injury goes away with ice, rest, and stretching exercises. With regular stretching exercises, the range of motion of the affected joints can be improved. Early mobilization after surgery is an important process to prevent the formation of heterotopic ossification. Education on mobilization after surgery to patients is mandatory to prevent this condition.