The term heterotopic ossification refers to the aberrant formation of mature, lamellar bone in nonosseous tissue. Translated from its Greek (heteros and topos) and Latin (ossificatio) etymologic origins, heterotopic ossification can be literally defined as "bone formation in other location." The first written account of heterotopic ossification describes the treatment of symptomatic lesions. Al-Zahrawi (more commonly known in Western cultures as Albucasis), widely considered the father of surgery, wrote in the year 1000 C.E., "This callus often occurs after the healing of a fracture…and sometimes there is limitation of the natural function of the limb…if the callus is stony hard and its removal is urgent, incise the place and cut away the superfluous prominence, or pare it away with a scraper until it is gone; and dress the wound until it heals."1 Currently, orthopaedic surgeons faced with treating mature, refractory, symptomatic heterotopic ossification are left with few options other than operative excision. Although it is remarkable that the treatment of heterotopic ossification has scarcely changed in the last millennium, it is generally accepted that prophylaxis against heterotopic ossification is far preferable than the later treatment of symptomatic lesions. As such, the focus of scientific effort in recent years has been directed toward prophylaxis, not treatment.
The formation of heterotopic ossification has been observed following total hip arthroplasty, acetabular and elbow fracture surgery, electrocution and burn injuries, and traumatic brain injury or spinal cord injury2. Following most traumatic injuries in the civilian population, the formation of heterotopic ossification is relatively rare in the absence of head injury. Even following traumatic brain injury or spinal cord injury, heterotopic ossification develops in only 20% and 11% of patients, respectively3. Rates of heterotopic ossification formation exceed 50% only in the setting of femoral shaft fractures with concomitant head injury4, although reported rates following acetabular and elbow fractures vary substantially. Numerous combat-related injury and amputation studies from the latter half of the twentieth century make no specific mention of heterotopic ossification, suggesting that it was not a common occurrence in prior conflicts5-8. However, military medical texts from the U.S. Civil War and World War I9,10 make specific mention of heterotopic ossification as a common problem following amputation, as described by Huntington: "…the stumps became conical sooner or later; short stumps sometimes remained well-rounded, long stumps rarely; but when they remained full it was often due to osteophytes, which in time became troublesome."10
Thus, since the birth of both surgery and modern warfare, heterotopic ossification has been recognized as a nameless condition that occurs following trauma. Indeed, in the current conflicts in Iraq and Afghanistan, heterotopic ossification has proven to be a frequent occurrence and a common clinical problem. The goals of the present manuscript are to summarize recent findings and the current state of science with regard to combat-related heterotopic ossification as well as to present the preliminary findings of ongoing studies and future directions.
Source of Funding
The aforementioned studies were supported, in part, by research grants from USAMRAA OTRP W81XWH-07-1-0222, the Office of Naval Research and U.S. Navy BUMED Advanced Medical Development 6.4/5 Program 604771N.0933.001.A0604.
The trend in modern warfare has shifted toward a higher percentage of extremity injuries11-18. This, coupled with the introduction of improved body armor, the judicious use of tourniquets, and the availability of forward-deployed surgical units, has given rise to highly survivable yet paradoxically devastating extremity injury patterns19,20. As a result, the current prevalence of heterotopic ossification has been found to be higher than expected in the combat-wounded patient population21-23.
We were the first to report this increased prevalence of heterotopic ossification in a cohort of 330 patients with 373 combat-related amputations, which are an important subset of combat casualties23. The study evaluated whether the mechanism of injury (blast or nonblast) correlated with either the presence or severity of heterotopic ossification. Surgery-related variables, such as the level of amputation (either within or proximal to the zone of injury), the number of irrigation and debridement procedures, the time from injury to definitive wound closure, and the prevalence of early wound complications following definitive closure, were also evaluated. The study found clinically detectable heterotopic ossification in 63% of residual limbs and, with the use of univariate analysis, identified the following important risk factors for the eventual development of heterotopic ossification: blast mechanism of injury (p < 0.05), and amputations performed in the zone of injury (p < 0.05). The latter risk factor also correlates with the severity of lesions, with use of the newly described Walter Reed Classification of heterotopic ossification23 (Figs. 1-A, 1-B, and 1-C). Interestingly, only twenty-five limbs (6.7%) required surgical excision of symptomatic lesions and a variety of prophylactic measures against secondary recurrence were used, including a single fraction of radiation therapy (700 cGy), indomethacin, cyclooxygenase (COX)-2 inhibitors, and other nonsteroidal anti-inflammatory drugs. Importantly, despite the various means of secondary prophylaxis used, the rate of symptomatic recurrence in this subgroup was low at 0%.
We also defined the prevalence of heterotopic ossification in a separate cohort of patients with combat-related extremity trauma requiring orthopaedic intervention21. Two hundred and forty-three patients requiring amputation, external fixation, or internal fixation were evaluated. This cohort study compared 157 patients who developed heterotopic ossification (the study group) to eighty-six patients who did not (the control group). This design enabled us to estimate the prevalence of heterotopic ossification not only in amputees (66% of lower-extremity amputees and 30% of upper-extremity amputees), but also in patients undergoing limb salvage (60.1%). The findings from the former study21 corroborated our earlier findings23. The data from the later study21 suggested that combat-related injuries, in general, are associated with a higher-than-expected prevalence of heterotopic ossification, when compared with civilian data3,4,24-29.
This study21 also identified several important risk factors for the development of heterotopic ossification in this patient population. Traumatic brain injury was associated with both the presence (p = 0.006) and the severity (p = 0.003) of heterotopic ossification on univariate but not multivariate analysis. Regression analysis revealed that the Injury Severity Score (as a continuous variable) and an Injury Severity Score of ≥16 (p = 0.02, odds ratio = 2.2) were significant predictors, as was multiple limb trauma requiring orthopaedic intervention (p = 0.002, odds ratio = 3.9). The most striking observation, however, was that the presence of an amputation was, itself, independently associated with the development of heterotopic ossification (p = 0.048, odds ratio = 2.9).
The association between heterotopic bone growth and the number and method of surgical debridement procedures, including the use of negative-pressure wound therapy, is controversial. We have observed, on univariate analysis, that patients who subsequently developed heterotopic ossification have undergone more debridement procedures (p < 0.001) and thus have been exposed to a longer duration of negative-pressure wound therapy (p < 0.001)21. Nevertheless, these results should be interpreted with caution because the increases in both the number of debridement procedures and the duration of negative-pressure wound therapy are likely more indicative of the severity of injury than they are causal. This is supported by the nonsignificant relationship between the formation of ectopic bone and these wound-care modalities as revealed by multivariate analysis21. Although local factors may play a supporting role, current data21,23 suggest that the development of heterotopic ossification in this patient population is largely due to systemic factors.
Given the high prevalence of combat-related heterotopic ossification in our war-wounded patients, a primary prophylaxis regimen including radiation therapy, conventional nonsteroidal anti-inflammatory drugs, and/or etidronate (an older, nonselective bisphosphonate and the only medication currently approved by the U.S. Food and Drug Administration for the treatment or prevention of heterotopic ossification) would be extremely appealing. Local radiation therapy, generally administered within twenty-four hours preoperatively to forty-eight hours postoperatively in patients at risk for heterotopic ossification, and nonsteroidal anti-inflammatory drugs have well-documented efficacy in the prevention of primary heterotopic ossification21,23,30-40. Recent meta-analyses and reviews have suggested that radiation therapy may be slightly more effective than nonsteroidal anti-inflammatory drugs for this purpose, although most of this difference is thought to be related to patient noncompliance with medication35,41. Unfortunately, the vast majority of severely injured combatants have multiple medical contraindications to prophylaxis against heterotopic ossification that make the interventions listed above difficult to accomplish. Commonly encountered relative and absolute contraindications to prophylaxis against heterotopic ossification include severe systemic polytrauma, open and contaminated wounds, concomitant traumatic brain injury and/or long bone fractures or spinal column injuries requiring operative stabilization and fusion, and the need for serial surgical procedures. Additionally, substantial potential for impaired renal function, bleeding, and stress gastritis have precluded widespread use of nonsteroidal anti-inflammatory drugs to date. Logistical limitations at remote, far-forward medical facilities in the theater of war are additional hurdles, particularly for the timely administration of radiation therapy. Etidronate can be administered later in the period after injury, but there can be problems with late mineralization or formation of heterotopic ossification after the cessation of medication. Thus, the efficacy of etidronate has recently been called into question by, among others, a recent Cochrane Database review42. Etidronate is also a relatively nonselective osteoclast inhibitor, potentially owing its limited efficacy to osteoblast inhibition, and may inhibit fracture-healing and spinal fusion. Corticosteroids, colchicine, retinoid agonists, coumarin derivatives, and calcitonin have all been studied for their value with regard to prophylaxis against heterotopic ossification; however, the data on human usage is either limited or absent and these drugs share many of the same medical concerns and contraindications as nonsteroidal anti-inflammatory drugs23,43-48. Thus, no practicable primary prophylactic regimen for combat-related heterotopic ossification has been identified, although a trial of the COX-2 inhibitor celecoxib is in development, as discussed below.
Fortunately, some patients with combat-related heterotopic ossification remain entirely asymptomatic and no specific treatment is indicated. In many others, lesions are transiently painful or bothersome and symptoms subside as adjacent joint motion improves, residual limb "toughening" occurs, and the combat-related heterotopic ossification transitions from its more inflammatory formative phase into a more quiescent maturation phase. Many patients, however, develop symptoms directly attributable to their combat-related heterotopic ossification that persist indefinitely. These symptoms may be localized pain, including, but not limited to, residual limb pain and prosthetic fitting difficulties; ulceration, particularly when the combat-related heterotopic ossification develops beneath an overlying skin graft (Figs. 2-A and 2-B); overt joint ankylosis; secondary arthrofibrosis due to osseous impingement (Figs. 3-A and 3-B); muscle entrapment (Fig. 4); or neurovascular entrapment (Figs. 5-A and 5-B). The treatment of symptomatic combat-related heterotopic ossification is individualized to the patient and the symptoms associated with their combat-related heterotopic ossification.
In the absence of overt ulceration causing concomitant deep infection or overt joint ankylosis, the initial management of symptomatic combat-related heterotopic ossification is nonoperative. This generally includes a period of rest, physical therapy, and gentle stretching and splinting to treat secondary contractures. Taking pressure off of symptomatic areas by positioning, pads or prosthetic socket adjustments, and optimizing pain control and medication regimens are also important. Other potentially contributing causes of pain should be investigated and treated appropriately including infection, fracture nonunion, internal derangement of adjacent joints, symptomatic neuromata, phantom pain, and complex regional pain syndrome. When nonoperative measures fail, concurrent procedures in addition to "simple" heterotopic ossification excision are often indicated and appropriate including amputation revision, neuroma excision, quadricepsplasty, contracture release, and/or skin graft excision. The most common indications for excision of combat-related heterotopic ossification in our military patients are pain that is caused by wearing a prosthesis and that has proven to be refractory to multiple socket adjustments, and arthrofibrosis in patients for whom limb salvage will be attempted.
Abundant recent evidence suggests that a prolonged waiting period of twelve to twenty-four months for heterotopic ossification "maturation" prior to excision is not necessary in the post-extremity trauma setting, particularly in the absence of traumatic injury to the brain or spinal cord23,49-58. Instead, a waiting period of six months after injury appears to be adequate to permit sufficient bone maturation to ensure the presence of a gross cortical rind to facilitate marginal excision and to permit a diligent trial of nonoperative treatments. Patients with recalcitrant ulcerations or severe secondary joint contractures may become operative candidates as early as three to four months after injury.
The operative approach to combat-related heterotopic ossification must account for the local anatomy, the location and severity of the heterotopic ossification, and the patient's prior wounds and incisions. We advocate a direct approach utilizing, when feasible, existing incisions, followed by marginal excision of the symptomatic lesion(s). The entirety of the heterotopic ossification need not be removed when the patient's symptoms are focal, particularly when access to the entire lesion would be difficult and the combat-related heterotopic ossification has a mature cortical shell. Wide excision to prevent recurrence would not be feasible in most patients because of the magnitude of tissue sacrifice that would be required. We have noted infrequent recurrences following excision about the elbow, but no symptomatic recurrences in residual limbs or about the thigh in more than 100 patients (unpublished data). This surgical approach has been coupled with the judicious use of secondary prophylaxis with nonsteroidal anti-inflammatory drugs. The decision was made to incorporate the use of secondary prophylaxis into our treatment protocol because of the high rate of wound complications in our initial series of amputees who were treated with radiation, which is used along with nonsteroidal anti-inflammatory drugs in patients with the most severe conditions23,38. However, the prognosis for concomitant excision and quadricepsplasty of the thigh for treatment of combat-related heterotopic ossification is guarded as a result of anecdotally high rates of wound complications, recurrent arthrofibrosis, extensor mechanism compromise, or preexisting arthrosis due to prior fractures, prolonged immobilization, or ankylosis.
Computed tomography-based three-dimensional modeling is very useful in the evaluation and treatment of symptomatic combat-related heterotopic ossification (Figs. 6-A and 6-B). Clinically, it is useful for allowing patients, therapists, and prosthetists to anatomically localize symptomatic areas and take efforts to avoid irritation through the use of activity modification, prosthetic socket relief, or model-assisted stereolithographic socket design. For the patient with symptomatic combat-related heterotopic ossification, resin models are useful for both preoperative planning and intraoperative referencing. The formation of combat-related heterotopic ossification distorts normal anatomy, placing critical neurovascular structures and muscle groups at risk for inadvertent injury. In some instances, major nerves and vessels may pass directly through and be incarcerated in the combat-related heterotopic ossification. The physical resin models serve as intraoperative anatomical guides that are based on heterotopic ossification topography. This is particularly helpful for patients in whom the condition is severe or for patients in whom excision of the focal, symptomatic combat-related heterotopic ossification is attempted in an effort to avoid complete takedown of the myodesis and revision of the amputation stump.
It is generally accepted that systemic inflammation is associated with the development of heterotopic ossification. Evans et al.59 examined the systemic and local wound inflammatory response in twenty-four patients with high-energy, penetrating extremity wounds. In preparation for the development of a prognostic clinical decision model, the goal of this pilot study was to identify whether a particular cytokine and chemokine profile could be identified in those at risk of developing heterotopic ossification. Serum and wound effluent samples were collected prior to each of these procedures in a manner previously described60,61. Twenty-two cytokines and chemokines (including interleukin [IL]-1 through 8, 10, 12, 13, and 15; interferon [IFN]-γ; eotaxin; tumor necrosis factor [TNF]-α; monocyte chemotactic protein [MCP]-1; granulocyte colony stimulating factor [GCSF]; macrophage inflammatory protein [MIP]-1α; the protein regulated on activation, normal T expressed and secreted [RANTES]; and IFN-γ inducible protein-10) were quantified. After a minimum follow-up of two months, subjects were then stratified according to a two-author (K.N.E. and J.A.F.) blinded review of radiographs into a study group and a control group, on the basis of the presence or absence of heterotopic ossification within the wounded extremity.
Serum analysis demonstrated a profound systemic inflammatory response in the study group. Of the twenty-two serum cytokines and chemokines analyzed, only three (IL-6, IL-10, and human MCP-1 [also known as chemokine (C-C motif) ligand 2, or CCL-2]) differed significantly between the two groups (unpublished data). Specifically, IL-6 remained elevated at all time points, as did MCP-1. Both are inflammatory agents and recruit monocytes and macrophages to the site(s) of injury, indicating sustained inflammation throughout the debridement process. MCP-1, however, is also involved in bone remodeling and may be an early indicator of this process. Interestingly, concentrations of IL-10, an anti-inflammatory cytokine that is important in inhibiting the production of pro-inflammatory cytokines, became significantly more concentrated in the study group approximately two weeks after injury, as compared that in the control group. The late upregulation of this anti-inflammatory mediator also signifies the presence of persistent systemic inflammation and supports our previous observation62 that the systemic inflammatory response in this patient population has aberrant regulation.
An analysis of the local wound effluent during the serial debridement process produced similar findings. Wounds that developed heterotopic ossification expressed significantly higher concentrations of MIP-1α and lower concentrations of IFN-γ-inducible protein-10 in the study group as compared with controls. Although both are considered pro-inflammatory, their prolonged, discordant expression, without a demonstrable compensatory anti-inflammatory component, also appears dysregulated. The change in the regulation of the inflammatory response that was observed systemically also seems to persist within the local wound environment. As such, the association between these biomarkers and heterotopic ossification may not simply be related to the difference in concentration between the two groups, but rather, the relationship between pro-inflammatory and anti-inflammatory mediators as well as their relative time-dependent concentrations as measured throughout the debridement process.
The cellular and biochemical etiology and pathophysiology of heterotopic ossification remain unclear. A postulate theory is that heterotopic ossification results from the presence of osteoprogenitors that pathologically are induced by an imbalance of local and/or systemic factors in soft tissue following traumatic injury. It is thought that a systemic increase in the osteogenic potential of endogenous muscle progenitor cells occurs in these severely injured patients in the immediate period after trauma.
Mesenchymal stem cells are multipotent, adult progenitor cells of great interest because of their unique immunologic properties and regenerative potential63. Mesenchymal stem cells reside within most adult connective tissues and organs64. Muscle-derived mesenchymal progenitor cells have been shown to be inherently plastic, enabling them to differentiate along multiple lineages; they promote wound-healing and regeneration of surrounding tissues by migrating to the site of injury, promoting repair and regeneration of damaged tissue, modulating immune and inflammatory responses, stimulating the proliferation and differentiation of resident progenitor cells, and secreting other trophic factors that are important in wound-healing and tissue remodeling63,65-68. Several recent reports describe the isolation and characterization of extensively passaged mesenchymal cell-like progenitor cells (MPCs) isolated from tissue collected following surgical debridement of traumatic orthopaedic extremity wounds67,69-71. Yet, the effects of acute and often prolonged aberrant inflammation62 on muscle-derived mesenchymal cells are unclear.
We speculate that the initiation of heterotopic ossification involves a complex interplay of signaling molecules secreted from the injured tissue. Proliferation and recruitment of local and/or circulating progenitor cells and the aberrant commitment, growth, and differentiation of these cells into bone occur early in the process of wound-healing and repair. In a series of preliminary studies, we have found that wound effluent collected strictly from patients with heterotopic ossification at times of early wound debridement is highly osteogenic, which accelerates the directed in vitro osteogenic differentiation of multipotent bone-derived mesenchymal stem cells in culture (Fig. 7). In contrast, mesenchymal stem-cell cultures treated with wound effluent alone, without exogenous exposure to standard induction media, had no measurable effect on the induction of bone formation. Therefore, there is some strong preliminary evidence that severe trauma leads to the release and delivery of osteogenic factors. The identity of these factors and how they interact with progenitor-cell signaling remain unknown.
Currently, little is known about the precursor cell to heterotopic ossification or the environment that permits formation of heterotopic ossification. Understanding the signaling pathways and the involvement of MPC differentiation is essential for the development of early diagnostic and prognostic tests and the development of novel prophylactic therapies. We have developed a unique cell-isolation process and in vitro culture system to easily quantify functionally assayable multipotent muscle-derived progenitor cells at the clonal level. The multipotent differentiation capacity of individual clonal cell-derived colonies can be easily assessed by their ability to undergo osteogenic, chondrogenic, and adipogenic differentiation when incubated with specific differentiation induction media. Using this system, we tested the hypothesis that endogenous muscle-derived progenitor cells following severe blunt trauma are greater in number and have a stronger osteogenic potential in patients in whom wound-healing is associated with the formation of combat-related heterotopic ossification than in patients whose wounds heal uneventfully. We prospectively collected wound muscle biopsies during debridements of ten active-duty service members who sustained high-energy penetrating injuries of an extremity during combat operations. Ectopic bone formation was determined by follow-up radiographic assessment at various intervals during the recovery period and was compared with culture results. We also collected hamstring muscle as control tissue from five healthy patients undergoing elective anterior cruciate ligament reconstruction with hamstring autograft. We found that the number of adherent colony-forming progenitor cells that could be isolated per gram of tissue from wartime wounds was profoundly increased (range, thirty-twofold to fiftyfold) compared with the number in the uninjured muscle tissue of the control group (Fig. 8). Quantification of progenitor cells with osteogenic potential showed that the measured 2.3-fold increase in osteogenic progenitors in tissue from patients with combat-related heterotopic ossification compared with that in tissue from patients with noncombat-related heterotopic ossification was significant (p < 0.007)72. Therefore, these findings suggest that wounds that present with a higher prevalence of resident assayable osteoprogenitors in the tissue, presumably supported through local and/or systemic reactions, correlate with the eventual formation of ectopic bone in traumatized tissue.
Raman spectroscopy is a scattering technique that can be used to gain information about the structure and composition of molecules from their vibrational transitions. A Raman spectrum can be thought of as a chemical "fingerprint" and is thought of as a complementary technique to the more widely known infrared spectroscopic techniques. The vibrational (and rotational) bands in a Raman spectrum are specific to the chemical bonds and particular structure of the molecule(s) being investigated. In addition, the band area of a Raman vibrational band is proportional to the amount of analyte present. Thus, Raman spectroscopy can identify the components that are present and quantify the amount of each component.
Over the past fifteen years, Raman spectroscopy has become an attractive technology for probing biomedical samples for several reasons. First, Raman spectroscopy can be used to study both organic and inorganic components (i.e., protein and mineral). Second, Raman spectroscopy can be applied noninvasively. Recent in vivo Raman spectroscopic studies include incorporation of a Raman probe into an endoscope for examination of the esophagus and stomach73,74, detection of cervical dysplasia75, diagnosis of nonmelanoma skin cancer76, characterization of psoriatic skin77, observation of human-swine coronary xenografts after transplantation78, measurement of macular carotenoids in the eye79-82, and transcutaneous monitoring of bone83. Third, most biological samples contain water, and unlike infrared spectroscopy, the Raman spectra of biological samples do not suffer from spectral interference of water vibrational bands. Fourth, Raman spectroscopy is a scattering technique, requiring very little, if any, sample preparation. Finally, the technological advances during the past fifteen years, such as holographic notch filters, small-form diode lasers, and thermoelectrically cooled charge-coupled device detectors, have enabled the production of less expensive, compact, and portable Raman spectroscopic systems.
While Raman spectroscopy has been used extensively to study the process of biomineralization84-98, it has not been previously used to provide insight into the pathologic process of heterotopic ossification. We have collected Raman spectra of uninjured muscle, injured muscle, and combat-injured tissue with pre-heterotopic ossification (defined as palpably firm or "woody" tissue without roentgenographic evidence of heterotopic ossification) found within high-energy penetrating wounds (Fig. 9). When comparing uninjured to injured muscle, there is an apparent decrease in the 1340 and 1320 cm-1 vibrational bands in the injured muscle as well as an increase in the 1266 cm-1 vibrational band. This suggests collagen-specific alterations within the tissue, as a result of traumatic injury. In one case, a patient exhibited combat-injured muscle with pre-heterotopic ossification during a debridement procedure. On Raman spectroscopic examination, it was clear that the tissue was indeed mineralized, even in "soft" tissue areas. Mineral vibrational bands at 1070, 960, and 591 cm-1, typical of a carbonated apatite, were prominent in the spectrum. These vibrational bands are attributed to the phosphate and carbonate stretching modes of bone99. Thus, Raman spectroscopy can potentially be utilized to identify areas of tissue affected by early combat-related heterotopic ossification as well as areas of tissue that may be predisposed to the formation of combat-related heterotopic ossification.
A critical hurdle in our investigation of the etiology, treatment, and prevention of combat-related heterotopic ossification is the absence of a reliable and reproducible small animal model in which to further characterize the formation of combat-related heterotopic ossification, potentially identifying new therapeutic targets, and to test new therapeutic interventions. Currently, several small animal models exist. These models include the forcible passive manipulation of the hindlimbs of paralyzed rabbits100,101; implantation of Matrigel (basement membrane/collagen-IV matrix; BD Biosciences, Bedford, Massachusetts) impregnated with recombinant human bone morphogenetic protein (rhBMP)-248 or BMP-4102 in genetically predisposed mice; implantation of genetically engineered, BMP-2-producing human or murine fibroblasts into immunocompromised mice103; and crush injury of the quadriceps augmented with syngeneic bone-marrow stem cells in inbred rats104. Our current understanding of heterotopic ossification suggests that these models have important limitations that may make them unsuitable proxies for combat-related heterotopic ossification. Specifically, these models all lack the systemic injury (e.g., blast injury and/or traumatic brain injury) components commonly seen in injured military personnel with combat-related heterotopic ossification. Additionally, no induced wound with associated bacterial contamination and resulting bioburden, as is the rule rather than the exception for combat injuries, is associated with these models. Moreover, there is a lack of general agreement within the current literature as to whether all inciting events lead to heterotopic ossification via the same cellular mechanisms and even whether all heterotopic ossification occurs via enchondral ossification2,22,105. Finally, these models are non-physiologic in that they artificially induce bone growth in the soft tissues of small animals through augmentation or manipulation of cellular signals, genetic predispositions, and/or cell presence. Previously described small-animal models therefore produce ectopic bone growth that may have little, if any, relationship to the clinical heterotopic ossification that is seen in human patients in general or in combat-injured patients in particular.
We have conceived of a physiologic rodent model that we hope will fill this research gap. Our model incorporates blast exposure, soft-tissue crush injury, and bacterial contamination, augmented with one of several additional systemic insults to invoke additional systemic inflammation. The fracture component will be omitted to limit rodent mortality as well as avoid confounding results due to exuberant fracture callus, if it were to occur. It is our hope that this model will reliably produce heterotopic ossification through mechanisms similar to that seen in our combat-wounded patients and that it will allow further characterization of this pathologic process. Once validated, this model may permit future identification of novel therapeutic targets as well as testing of various described and original therapeutic modalities.
As noted, due to medical contraindications and logistical constraints, no practicable primary prophylaxis regimen has yet been developed, tested, or widely utilized in wounded servicemen despite the exceedingly high rate of heterotopic ossification in this population. Recently, COX-2 inhibitors such as celecoxib have been shown to be safe and efficacious for the prevention of heterotopic ossification following hip and acetabular surgery106-108 and may be useful in our patient population. COX-2 is required for endochondral bone formation, a mechanism implicated in the development of heterotopic ossification103. Although not prescribed as prophylaxis against heterotopic ossification, COX-2 inhibitors are currently used in this institution as part of a comprehensive pain-management regimen and are thought to decrease the patient's opioid requirement. Concerns about COX-2 inhibitors in an orthopaedic population stem from the blunting of "helpful" inflammation necessary for endochondral ossification in early fracture-healing109-113. Nevertheless, several studies evaluating COX-2 inhibitors found little or no deleterious effect on fracture-healing or osseous healing99,114-117.
In order to address the safety and efficacy of COX-2 inhibition in combat-related heterotopic ossification, a prospective randomized trial of celecoxib, started within five days after injury for a two-week treatment period (200 mg by mouth twice a day), will begin enrolling patients this year. The study will enroll as many as 100 patients (fifty in the treatment arm and fifty in the control arm), and is powered to detect a 30% relative decrease in the rate of formation of combat-related heterotopic ossification (e.g., a decrease from 60% to 40%). Primary study end points are the overall prevalence and severity of combat-related heterotopic ossification in study patients. This decrease in combat-related prevalence of heterotopic ossification is well below that anticipated on the basis of a putative 56% to 67% reduction in heterotopic ossification rates with use of nonsteroidal anti-inflammatory drugs in other populations31. Testing for this lower rate of reduction appears reasonable because the actual effect of nonsteroidal anti-inflammatory drugs may differ in patients with combat-related heterotopic ossification, because treatment cannot practicably be initiated at the point and time of injury, and because testing would serve to ensure adequate power of the trial. Secondary end points include fracture nonunion, time to fracture union, rate of impaired wound-healing, medical and drug-related complications (e.g., gastrointestinal problems and renal dysfunction), and patient pain ratings and opioid requirements.
The concern that celecoxib may delay fracture-healing is an important one. Nevertheless, its effect may be tempered by the timing of fracture fixation in a high-energy penetrating injury of an extremity. In most cases, fracture fixation is not performed until ten to fourteen days after injury, which is the time it takes to debride and prepare the wound for closure or flap coverage. Prophylaxis against heterotopic ossification is started as soon after injury as possible and continued for fourteen days. Ideally, celecoxib dosing for the purpose of prophylaxis against heterotopic ossification will be complete prior to definitive fracture fixation. Goodman and colleagues demonstrated that COX-2 inhibitors, if given within the first fourteen days following fracture, did not result in appreciable impairment of fracture-healing118. There is also no evidence to suggest that celecoxib significantly affects fracture-healing in our combat-wounded patient population. Nevertheless, "nonunion" and "time to union" will be reported as secondary outcomes in this study. Of note, short-term use of celecoxib in patients without fractures, including amputees, has not been associated with any negative effects.
Combat-related heterotopic ossification is exceptionally common and is often a harbinger of a complex and difficult clinical course. Operative excision of symptomatic combat-related heterotopic ossification lesions is generally successful and associated with low rates of recurrence but can be technically demanding and fraught with complications, particularly about the knee when concurrent quadricepsplasty is required. We have identified numerous clinical factors and biological markers that are predictive of eventual formation of combat-related heterotopic ossification. Through ongoing and future research efforts by our consortium, we hope to further elucidate the biochemical and cellular basis for the formation of combat-related heterotopic ossification, further define the relative roles of local and systemic inflammation, continue to develop new means of early diagnosis and prognostication, and test and validate both conventional and novel practicable primary prophylactic treatment modalities. It is our hope that, through a combination of new diagnostic and therapeutic interventions, we can affect both the incidence and clinical management of combat-related heterotopic ossification.
Note: The authors acknowledge the research and/or clinical support of the following individuals: Fred Gage; Wesley Stepp, MS; Steve Ahlers, PhD; COL James Ficke, MD; COL(ret) William Doukas, MD; LTC(P) Romney Andersen, MD; LTC(P) Dan White, MD; LTC Scott Shawen, MD; CDR John Keeling, MD; MAJ Wade Gordon, MD; Stephen Rouse, PhD; Peter Liacouras, PhD; LTC(P) Tad Gerlinger, MD; LTC(P) Donald Gajewski, MD; MAJ(P) Joseph Hsu; and H. Michael Frisch, MD.
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