Heterotopic ossification
Formation of bone tissue outside of the skeleton
From Wikipedia, the free encyclopedia
Heterotopic ossification (HO) is the abnormal process by which bone tissue forms outside of the skeleton in muscles and soft tissue. It forms following a traumatic injury, neurogenic injury, or genetic predisposition. The likelihood and severity of HO is linked to the severity of the injury. The most common sites of developing HO are the hip, knee, elbow and shoulder. While the exact mechanism of how HO develops is not known, it is believed to be a result of an abnormal muscle repair system, abnormal neurologic healing response below the level of injury, an abnormal inflammatory response, or a combination of them.
| Heterotopic ossification | |
|---|---|
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| Heterotopic ossification around the hip joint in a patient who has undergone hip arthroplasty |
Heterotopic ossification presents differently in different stages. In the initial inflammatory stage, there is pain, redness, swelling, warmth, generalized tenderness; this can mimic other more serious medical conditions and should require urgent evaluation. In the late stage, there is more localized pain, decreased range of motion, and underlying tissue damage and complications. There are various blood or urine tests that can be useful in helping predict HO, but there is not one single gold standard lab. Triple bone scan, X-rays and CT scans are the most useful and reliable imaging studies to rule out other similar medical conditions and confirm the diagnosis of HO.
Heterotopic ossification does not have a single treatment to prevent its development or cure its progression. The goal of management should be to control pain, prevent long term complications, and allow the patient to continue to perform their own daily activities of living. This can be achieved through an array of options, ranging from physical therapy to prophylactic radiation and medications to surgical interventions. There are many emerging management styles of HO, but coordinating care between the patient, their support system, physicians, therapists, and other care takes should always remain the central focus.
Epidemiology
Heterotopic ossification (HO) is a result of one of three initial insults: a traumatic injury, a neurogenic injury, or a genetic predisposition. It is more common in men (3:2 male to female ratio),[1] likely a result of increased muscle mass, increased physical activity, and hormonal signally pathways. It is also more commonly in those that suffer from prolonged intubation,[2] likely from an anoxic brain injury or prolonged immobilization. It is a hard diagnosis to make due to its overlap in symptoms with other common conditions.[3]
In patients that suffered a forearm fracture, 20% of patients will likely develop HO as a complication.[4] In patients that suffered a femoral shaft fracture, that percentage jumps to 52% of patients.[4] However, HO is even more common in military members that suffered combat related injuries. Military patients that suffered any combat related trauma were likely to have HO as a complication 68% of the time; that number increases to 86% if they had a concomitant traumatic brain injury with their orthopedic injury.[5]
Without a specific trauma or inciting event, patients will frequently develop arthritis in major joints (knees, hips and shoulders). This subjects them to eventual surgical treatment or joint replacement. In those patients, their ability to place weight on a joint or move their joint may be limited for a significant period of time. Therefore, this patient population is also subject to developing HO as a surgical complication. It was found that 44% of patients that received either a hip arthroscopy or total hip replacement developed HO.[6]
In patients that develop a lesion or injury to the nerves of their spinal cord, they can develop neurogenic heterotopic ossification. Neurologenic HO effects 20-29% of patients that developed a spinal cord injury (SCI). In an SCI, the spinal level of injury is important to know since complications typically arise distal or below the level of the injury. This is thought to be due to an impaired or abnormal healing response below the injury level. For this reason, the most common sites of HO in the SCI population are the hips and knees.[3]
In patients that develop an injury to their brain, they can also develop neurogenic heterotopic ossification. In traumatic brain injury (TBI) patients, neurogenic HO effects 5-20% of patients, specifically in those that suffered a severe TBI (initial GCS <15).[3] Patients that suffered a TBI due to a blast, commonly military members, have HO as a complication higher than 20% of the time.[3] Similar to SCI patients, TBI patients have HO commonly in sites below the level of neurologic injury. For this reason, the most common sites of HO in the TBI population are the hips, elbows, shoulders, and knees.
In patients that suffer a burn, there are various degrees to the severity of a burn; the depth to the burn and the surface area which the burn covers are two important factors. For HO complications, the surface area is the more influential variable. The likelihood of developing HO as a complication of a burn is between 1-4% if the burn covers at least 30% of the person's body.[6][7] This complication percentage increases as the burn percentage also increases. For burn patients, the most common site of HO is the elbow, likely due to increased motion at the joint to prevent flexure contractures. The elbow is a clinically significant site to monitor since this specific complication of HO can lead to cubital tunnel syndrome; other common, but less likely, sites are the shoulders, knees, forearms, and temporamandibular joint.[6][7]
Presentation of symptoms
While heterotopic ossification is not a direct result of a traumatic injury, it also does not have to present with symptoms. In the population that is symptomatic, it commonly follows a standard progression of symptoms.
Following the initial inciting traumatic or neurologic injury, there is a heightened inflammatory response. An abnormal or prolonged inflammatory response can lead to the early stages of HO.[8] Typically within the first 6 to 12 weeks, symptoms will start to arise; this is considered early HO. Here, immature bone forms within the affect joint or tissue, leading to generalized inflammatory symptoms. This manifests as symptoms of redness, warmth, and swelling (37% of patients), pain (35% of patients), or generalized tenderness and decreased range of motion (49% of patients) in the affected joint or tissue.[9][10] While these symptoms are more annoying than troubling in the context of HO, these symptoms can also be red flag signs for a more serious underlying condition and require medical evaluation.
As time continues, symptoms will typically progress from a more generalized inflammatory response to a localized pain center. This presents as localized tenderness, pain, and painful or decreased range of motion in the tissue or joint affected.[8] This is considered late HO. In all stages, if the abnormal bone formation grows to imping on surrounding structures, complications can arise depending on the structure involved. This can lead to nerve compression, vascular compression, pressure sores, lymphedema, ankylosis and present as pain, numbness, tingling, weakness, swelling, spasticity, decreased range of motion.[8] Presentation of physical exam symptoms will depend on the location and severity of the HO.[9]
Diagnosis
Since heterotopic ossification can present similar to other more serious pathologies, a physical exam alone may not confirm the diagnosis. Utilizing laboratory data and medical imaging together can provide a better picture to rule out more serious conditions and correctly identify the stage and severity of HO.[6]
Labs
Before treatment can begin for heterotopic ossification, more serious conditions must be ruled out. Lab work may begin with a routine complete blood count and comprehensive metabolic panel to monitor blood, immune system, and organ function. With serious conditions ruled out, more specific labs may be ordered to assist in diagnosing the severity and progression of HO. Since HO is an abnormal growth of bone outside the skeletal system, it is theorized that calcium, a major mineral of bones, may be abnormal during different stages of HO. However, tracking calcium is not a reliable method as calcium is strictly hormonally regulated in the body and will not be elevated in HO.[11] Alkaline phosphatase is an enzyme whose activity is linked to bone growth and has been found to be elevated in HO; however, it is not statistically significant in identifying HO alone.[12] However, it was found that in HO patients, alkaline phosphatase was elevated 2 weeks following their initial injury, peaked at 10 weeks post-injury, and returned to baseline by the 18 week post-injury mark.[13][11] Osteocalcin is a hormone produced during bone turnover that is neither sensitive nor specific for HO and cannot accurately predict its occurrence.[12] Understanding that HO is theorized to be a result of increased inflammation, blood inflammatory markers such as C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) can identify a body's inflammatory state; however, this is not a sensitive marker for HO.[14] While it is not specific to HO, it was found that 12 weeks after a total hip replacement, an ESR above 35mm/hr was a reliable predictor for developing severe HO in patients.[14] Similarly, elevation in creatine kinase (CK) was found to be associated with more aggressive forms of HO.[15] CK is an enzyme found in muscle tissue that, when found in the blood, can lead to believe there is active muscle damage.
In addition to blood work, a urinalysis can identify certain byproducts of HO that a clinician may use to rule in or rule out its diagnosis. A 24 hour urine hydroxyproline measurement can detect if one is building and breaking down bone at an increased rate compared to normal; however, this is not a reliable method to detect HO.[12] Contrasting, a 24 hour urine prostaglandin E2 lab can be elevated in the early stage of HO and will warrant a triple bone scan for further workup.[13][16] While prostaglandin E2 is not specific for HO and is more associated with inflammatory conditions, its elevation has been associated with early HO.
Imaging
While laboratory studies are not reliable for diagnosing heterotopic ossification, medical imaging studies are more sensitive and specific for accurately diagnosing HO. Although it may be the simplest, most inexpensive method, an x-ray will not be helpful during the early stage. The only definitive diagnostic test in the early stage is a bone scan, which will show heterotopic ossification as early as 2.5 weeks post-injury, which is over 2 weeks sooner than detected by x-ray.[17] While a bone scan is more specific less than one month post-injury, its use will be dependent on when symptoms begin. If symptoms arise after 4 weeks post-injury, and if the treatment team wants to rule out more serious complications, the team may begin with radiography or a CT scan. Their low cost and ability to detect immature bone formation while also checking for other diagnosis make them a useful tool to aid in the diagnosis.[18] A treatment team may also utilize other imaging techniques such as an ultrasound or MRI to aid in the diagnosis process, more commonly used to rule out more serious, similarly presenting underlying conditions.
When the initial presentation is swelling and increased temperature in a leg, thrombophlebitis or a deep vein clot cannot be ruled out with a clinical exam alone; therefore, ultrasound imaging may be necessary to differentiate.[18] Similarly, osteomyelitis and malignant soft tissue masses can present similarly to HO; in this case, MRI has been found helpful in correctly diagnosis HO.[18]
Classification types
Once heterotopic ossification has been diagnosed via imaging, the next step is assessing its severity and monitoring for progression of symptoms. There have been several classification systems identified, each dependent on the joint affected.[1]
The Brooker Classification System is utilized following total hip replacement to grade HO formation.
| Classification | Description |
|---|---|
| Class 1 | Island of bone within the soft tissue around the hip |
| Class 2 | Bone spurs on the hip or proximal femur, where the distance between the spurs is > 1cm |
| Class 3 | Bone spurs on the hip or proximal femur, where the distance between the spurs is < 1cm |
| Class 4 | Boney fusion of the hip and proximal femur |
The Hastings and Graham Classification System is utilized to grade HO formation at the elbow and forearm.
| Classification | Description |
|---|---|
| Category 1 | Asymptomatic (no functional limitations), HO present on radiographs |
| Category 2 | Symptomatic (functional limitations), HO present on radiography |
| Category 2A | limitations in elbow flexion/extension |
| Category 2B | limitations in forearm pronation/supination |
| Category 2B | limitations in elbow flexion/extension AND forearm pronation/supination |
| Category 3 | Symptomatic (functional limitations), complete boney fusion of the joint on radiograph |
Management
The only curative treatment is surgical resection when possible depending on location and size of the heterotopic ossification.[21] However recurrence may occur with rates between 2% and 31% depending on the report.[22] Additional treatment with bisphosphonate (etidronate) or indomethacin after resection surgery do not significantly reduce the rate of recurrence.[22]
There is no established preventive treatment mostly because the cellular and molecular mechanisms leading to heterotopic ossifications have remained poorly understood. Originally, bisphosphonates were expected to be of value after hip surgery but there has been no convincing evidence of benefit, despite having been used prophylactically.[23]
Radiation therapy
Prophylactic radiation therapy for the prevention of heterotopic ossification has been employed since the 1970s. A variety of doses and techniques have been used. Generally, radiation therapy should be delivered as close as practical to the time of surgery. A dose of 7-8 Gray in a single fraction within 24–48 hours of surgery has been used successfully. Treatment volumes include the peri-articular region, and can be used for hip, knee, elbow, shoulder, jaw or in patients after spinal cord trauma.
Single dose radiation therapy is well tolerated and is cost effective, without an increase in bleeding, infection or wound healing disturbances.[24]
Other possible treatments
Non-steroidal anti-inflammatory drugs, such as indomethacin and ibuprofen, have shown some effect in preventing recurrence of heterotopic ossification after total hip replacement[25] or spinal cord injury.[26]
Conservative treatments such as passive range of motion exercises or other mobilization techniques provided by physical therapists or occupational therapists may also assist in preventing HO. A review article looked at 114 adult patients retrospectively and suggested that the lower incidence of HO in patients with a very severe TBI may have been due to early intensive physical and occupational therapy in conjunction with pharmacological treatment.[27] Another review article also recommended physiotherapy as an adjunct to pharmacological and medical treatments because passive range of motion exercises may maintain range at the joint and prevent secondary soft tissue contractures, which are often associated with joint immobility.[28]
Patho-mechanism
The mechanisms by which neurogenic heterotopic ossification develop following a spinal cord injury have been studied in a mouse model. In this model, heterotopic ossifications develop after a dual injury of the central nervous system and muscle, with ossifications developing exclusively in injured muscles.[21] Mechanistically, heterotopic ossification is caused by a perversion of the muscle repair program. During normal muscle repair, muscle satellite cells, which are muscle stem cells, are induced to divide and then differentiate into myoblasts and myotubes to regenerate muscle fibres. This process is controlled by inflammatory macrophages and supportive myogenic growth and differentiation factors produced by muscle-specific mesenchymal progenitor cells called fibro-adipogenic progenitors or interstitial cells.[29] A key step during normal muscle repair is the programmed cell death (apoptosis) of these fibro-adipogenic progenitors triggered by tumor necrosis factor TNF secreted by inflammatory macrophages accumulating in the injured muscle, a process that prevents the development of muscle fibrosis.[30] However, following a spinal cord injury, fibro-adipogenic progenitors fail to undergo apoptosis and instead accumulate and differentiate into bone forming osteoblasts.[31] How a spinal cord injury perverts muscle repair to osteogenesis has recently been elucidated. The spinal cord injury stimulates the adrenal glands[32] to release the glucocorticoid corticosterone into the circulation. Excessive corticosterone causes an exacerbation of inflammation in the injured muscle with excessive release of oncostatin M and interleukin-1β.[33] Oncostatin M and interleukin-1 bind to their cognate receptors OSMR[34] and IL1R1[35] expressed by muscle fibro-adipogenic progenitors which in turn promote their proliferation and osteogenic differentiation. In support of this model, treatment with glucocorticoid receptor antagonists such as mifepristone or relacorilant or conditional deletion of the glucocorticoid receptor gene strongly inhibit the development of neurogenic heterotopic ossification after spinal cord injury in this mouse model.[33] Treatment with ruxolitinib, an inhibitor of JAK1 and JAK2 tyrosine kinases, which are activated downstream of OSMR, also reduces neurogenic heterotopic ossification in this model.[36] This mechanism also explains why infections, particularly with gram-negative bacteria, are associated with higher prevalence of neurogenic heterotopic ossifications in victims of traumatic brain and spinal cord injuries.[37][38][39][40] Lipopolysaccharides from gram-negative bacteria worsen heterotopic ossification by binding to their receptor Toll-like receptor 4 expressed by macrophages and muscle fibro-adipogenic progenitors and further increase oncostatin M and interleukin-1β release by macrophages.[40]
There are also rare genetic disorders causing heterotopic ossification such as fibrodysplasia ossificans progressiva (FOP), a condition that causes injured bodily tissues to be replaced by heterotopic bone. Characteristically exhibiting in the big toe at birth, it causes the formation of heterotopic bone throughout the body over the course of the sufferer's life, causing chronic pain and eventually leading to the immobilisation and fusion of most of the skeleton by abnormal growths of bone.[41]
Another rare genetic disorder causing heterotopic ossification is progressive osseous heteroplasia, is a condition characterized by cutaneous or subcutaneous ossification.[42]