Heterotopic ossification: a review

Myositis ossificans revisited - The largest reported case series

INTRODUCTION

Myositis ossificans (MO) is a condition characterised by the formation of non-neoplastic heterotropic ossification in extraskeletal soft tissues.

MATERIAL AND METHODS

We performed a retrospective study of our radiology databases within our tertiary orthopaedic centre to identify all cases of MO, reported on X ray, Magnetic resonance imaging (MRI) and Computed tomography (CT) over the past 13 years (2007-2020).

RESULTS

We identified 68 cases of MO, which were included into our cohort. The average age of our patients was 36 years (range 4-84 years). 73% of cases (n = 50) were found to affect the lower limb muscles with the majority in the quadriceps.

CONCLUSION

We report the largest case series of MO and discuss the demographics, diagnoses and management.

A nontraumatic myositis ossificans case of the forearm: Case report and literature review

Myositis ossificans (MO) is a rare, benign ossifying lesion characterized by focal formation of heterotopic bone and cartilage in extraskeletal soft-tissue that most commonly occurs in young adults. In most cases, no causative factor can be identified. The diagnosis of MO is usually based on the patient's history of trauma, clinical signs, on imaging appearance and histological examination. We present a non-traumatic MO case of the forearm in a 40-year-old man with weakness in left finger motion, a decrease in prehension for more than three weeks, without any weight loss, malaise, anorexia or fever. The clinical symptoms and radiological findings can be easily confused with malignant lesions. Treatment is usually conservative but, due to the limited strength and range of motion of the left hand, the tumor was extirpated and the diagnosis of MO was made by biopsy. The patient had no neurological deficits after surgical treatment and was discharged on the fifth day after the surgery in good condition with the recommendation to begin a rehabilitation program.

A case report and literature review of heterotopic mesenteric ossification

Introduction and importance

Heterotopic mesenteric ossification is a benign bony tissue growth in the mesentery that mostly follows repetitive or severe abdominal injuries leading to reactive bone formation in the mesentery. There are only 73 cases (51 publications) identified in the literature up to the beginning of 2020.

Case presentation

45-year-old Saudi male underwent multiple laparotomies to manage complicated appendicitis which ended with a diverting ileostomy and a colostomy as a mucus fistula. After 9 months, the patient was admitted to the General Surgery department in Al-Hada Armed Forces Hospital for an open ileostomy and colostomy reversal surgery where several irregular bone-like tissues of hard consistency and sharp edges with some spindle-shaped structures resembling needles were found in the mesentery of the small intestine and histopathology revealed of trabecular bone fragments confirming the diagnosis.

Clinical discussion

The majority of cases occur mid to late adulthood with a predilection in the male gender, and usually present with bowel obstruction or an enterocutaneous fistula. Although it has no malignant potential, it may cause severe bowel obstruction that can lead to mortality, it's a rare occurrence and, therefore, is difficult to diagnose among many common abdominal disturbances.

Conclusion

Here we report a rare case of heterotopic mesenteric ossification, which should be considered as one of the delayed complications of abdominal surgery or trauma. The time range of expecting the presentation of heterotopic mesenteric ossification following major abdominal trauma or surgery should be extended and continuously considered during differential diagnosis.

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One of the delayed complications of abdominal surgery or trauma is Heterotopic mesenteric ossification.

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Since 1983 there are only 73 cases of Heterotopic mesenteric ossification published to date.

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Preoperative diagnosis of Heterotopic mesenteric ossification should be based mainly on the characteristic radiographic findings without relying on the levels of calcium or alkaline phosphatase.

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The only way to reach the definitive diagnosis is through excision and histopathological analysis.

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Heterotopic mesenteric ossification has no malignant potential, but it can cause severe bowel obstruction that can lead to mortality in already sick patients.

Strategies to reduce re-ankylosis in temporomandibular joint ankylosis patients

The purpose of the study was to define a protocol for the prevention of re-ankylosis after surgical management of temporomandibular joint ankylosis (TMJA). The investigators designed a retrospective study on all TMJA patients treated with different treatment modalities from 2013 to 2019. The investigators observed that complete removal of the ankylotic mass particularly on the medial side; use of a piezoelectric scalpel for a clean and smooth osteotomy with copious irrigation to remove bone chips and slurry; less trauma to the local tissue; osteotomy design parallel and inferior osteotomy at the narrowest part, which mostly corresponds to the condylar neck; performance of a coronoidectomy (if mouth opening is <30mm), fat interposition; no intraoperative correction of any pre-existing chin deviation when treated with costochondral graft; patient motivation; and aggressive physiotherapy, and use of a vacuum drain are all important to prevent re-ankylosis, irrespective of the treatment modality. A total of 114 patients (n=152 joints), [bilateral (n=38), unilateral (n=76)] were evaluated retrospectively. Interpositional arthroplasty with fat was performed in n=43, CCG was used for reconstruction in n=30 and total joint replacement (TJR) was done in n=41 patients. Re-ankylosis was seen in n=3 (2.6%) patients (2 in CCG and 1 patient in interpositional arthroplasty). The follow-up ranged from 12-80 months. The results conclude that following the suggested best practice protocol is effective in reducing re-ankylosis.

Activin A promotes the development of acquired heterotopic ossification and is an effective target for disease attenuation in mice

Heterotopic ossification (HO) is a common, potentially debilitating pathology that is instigated by inflammation caused by tissue damage or other insults, which is followed by chondrogenesis, osteogenesis, and extraskeletal bone accumulation. Current remedies are not very effective and have side effects, including the risk of triggering additional HO. The TGF-β family member activin A is produced by activated macrophages and other inflammatory cells and stimulates the intracellular effectors SMAD2 and SMAD3 (SMAD2/3). Because HO starts with inflammation and because SMAD2/3 activation is chondrogenic, we tested whether activin A stimulated HO development. Using mouse models of acquired intramuscular and subdermal HO, we found that blockage of endogenous activin A by a systemically administered neutralizing antibody reduced HO development and bone accumulation. Single-cell RNA-seq analysis and developmental trajectories showed that the antibody treatment reduced the recruitment of Sox9+ skeletal progenitors, many of which also expressed the gene encoding activin A (Inhba), to HO sites. Gain-of-function assays showed that activin A enhanced the chondrogenic differentiation of progenitor cells through SMAD2/3 signaling, and inclusion of activin A in HO-inducing implants enhanced HO development in vivo. Together, our data reveal that activin A is a critical upstream signaling stimulator of acquired HO in mice and could represent an effective therapeutic target against forms of this pathology in patients.

Unintended fusion in cervical artificial disk replacement: a prospective study on heterotopic ossification, progression, and clinical outcome, with 5-year follow-up

PURPOSE

Heterotopic ossification (HO) may cause unintended fusion in artificial disk replacement (ADR), failing to preserve motion. The reported incidence of HO varies hugely. This study aimed to determine prevalence of HO, progression, predisposing factors to occurrence and to progression, and potential effect on clinical outcomes.

METHODS

Eighty-three patients treated with ADRs for cervical radiculopathy at one or two segments were extracted from a previously published RCT. Of the 83 patients, 59 had remaining ADR (79 implants) and sufficiently high-quality X-rays at 5 years of follow-up to allow analysis. HO was graded on plain films according to Mehren/Suchomel. Prevalence, progression, predisposing factors and effect on clinical outcome (Neck Disability Index-NDI) were analyzed.

RESULTS

At 2 years, HO was seen in 46/55 ADR implants (84%), severe enough to affect mobility in 27/55 (49%). At 5 years, HO was seen in 92% of 79 implants, severe in 71%. Male sex was a predisposing factor to HO. No predisposing factors to progression were identified. NDI was not affected by the severity of HO.

CONCLUSION

Almost all ADR implants in our study have HO at 5 years of follow-up. Male sex is a risk factor. Severe HO did not affect clinical outcome.

TRIAL REGISTRATION

Study registered at ISRCTN (registration number: 44347115).

Bridging Myositis Ossificans After Supracondylar Humeral Fracture in a Child: A Case Report

Background: Myositis ossificans is an uncommon complication of trauma and surgery, defined as ossifying changes in a non-osseous tissue such as muscles. It happens after tissue injury, with or without fractures. When myositis ossificans occurs around a joint, it can cause ankylosis, leading to complete dysfunction of the joint. Though it has been described in most parts of the body, bridging myositis ossificans involving the elbow joint were scarcely reported. Case Presentation: We report a severe case of myositis ossificans after a supracondylar humerus fracture in a 9-year-old child. In this case a palpable painless mass appeared following the fracture and surgical trauma. Ultrasound or X-ray is of significant diagnostic value. The brachialis was completely ossified and formed a bony bridge around the elbow, causing complete ankylosis. The bone mass was surgically removed through a bilateral less-invasive approach with less surgical trauma 9 months after initial presentation. we applied bone wax to the fresh bone wounds to prevent the formation of hematocele. Indomethacin, a non-steroidal anti-inflammatory drug, was administered after the operation to suppress bone proliferation in our case. Our patient had the best possible functional status and no recurrence at 2 years' follow-up. Conclusion: Elbow myositis ossificans in children may mainly affects the brachialis. A bilateral less-invasive approach is sufficient to remove the bone mass with less surgical trauma. This case also provides a new reference for the treatment of myositis ossificans after the elbow injuries.

Muscle Imaging

Evaluation and characterization of skeletal muscle pathology is a frequently encountered indication for musculoskeletal imaging. Causes of muscle pathology are diverse and include traumatic, autoimmune, infectious, inflammatory, neurologic, and neoplastic. Each etiology while dramatically different in the pathophysiology may present with similar imaging features. An understanding of the subtle differences in imaging features between the pathologic conditions may serve to guide diagnosis and treatment in these often complex cases. In this section, we will discuss the various skeletal muscle pathologies and the imaging features associated with each.

Genetic and Acquired Heterotopic Ossification: A Translational Tale of Mice and Men

Heterotopic ossification is defined as an aberrant formation of bone in extraskeletal soft tissue, for which both genetic and acquired conditions are known. This pathologic process may occur in many different sites such as the skin, subcutaneous tissue, skeletal muscle and fibrous tissue adjacent to joints, ligaments, walls of blood vessels, mesentery and other. The clinical spectrum of this disorder is wide: lesions may range from small foci of ossification to massive deposits of bone throughout the body, typical of the progressive genetically determined conditions such as fibrodysplasia ossificans progressiva, to mention one of the most severe and disabling forms. The ectopic bone formation may be regarded as a failed tissue repair process in response to a variety of triggers and evolving towards bone formation through a multistage differentiation program, with several steps common to different clinical presentations and distinctive features. In this review, we aim at providing a comprehensive view of the genetic and acquired heterotopic ossification disorders by detailing the clinical and molecular features underlying the different human conditions in comparison with the corresponding, currently available mouse models.

Tendon-derived cathepsin K-expressing progenitor cells activate Hedgehog signaling to drive heterotopic ossification

Heterotopic ossification (HO) is pathological bone formation characterized by ossification within muscle, tendons, or other soft tissues. However, the cells of origin and mechanisms involved in the pathogenesis of HO remain elusive. Here we show that deletion of suppressor of fused (Sufu) in cathepsin K-Cre-expressing (Ctsk-Cre-expressing) cells resulted in spontaneous and progressive ligament, tendon, and periarticular ossification. Lineage tracing studies and cell functional analysis demonstrated that Ctsk-Cre could label a subpopulation of tendon-derived progenitor cells (TDPCs) marked by the tendon marker Scleraxis (Scx). Ctsk+Scx+ TDPCs are enriched for tendon stem cell markers and show the highest self-renewal capacity and differentiation potential. Sufu deficiency caused enhanced chondrogenic and osteogenic differentiation of Ctsk-Cre-expressing tendon-derived cells via upregulation of Hedgehog (Hh) signaling. Furthermore, pharmacological intervention in Hh signaling using JQ1 suppressed the development of HO. Thus, our results show that Ctsk-Cre labels a subpopulation of TDPCs contributing to HO and that their cell-fate changes are driven by activation of Hh signaling.

The Impact of Heterotopic Ossification on Self-Reported Outcomes After Total Hip Arthroplasty Using the Direct Anterior Approach

BACKGROUND

Heterotopic ossification (HO) is a known finding after total hip arthroplasty (THA) that potentially affects clinical outcomes. The incidence of HO is related to various factors, including soft-tissue trauma, postoperative medication, surgical approach, and the surgeon's experience. Because use of the direct anterior approach (DAA) is gaining popularity, we aimed (1) to determine the incidence of HO after DAA-THA and (2) to evaluate the impact of HO on patient-reported outcomes (PROs).

METHODS

We retrospectively reviewed 401 THAs (67 ± 10 years old, 210 men). The incidence and grade of HO were evaluated using the Brooker classification with anteroposterior and lateral radiographs. PROs were collected with use of the Oxford Hip Score (OHS) and the Core Outcome Measures Index (COMI)-Hip and were correlated with HO grades using generalized multiple regression models.

RESULTS

The incidence of HO was 29.9% on the anteroposterior radiographic views (Brooker grade 1, 14.5%; grade 2, 11.1%; grade 3, 2.7%; and grade 4, 0.57%); the lateral radiographic views detected 9% additional HO when compared with the anteroposterior radiographs alone (Brooker grade 1, 8%, and grade 2, 1%). HO was more frequent in men. The American Society of Anesthesiologists (ASA) grade, age in women, and higher body mass index (BMI) in men were associated with higher HO grades. A transverse "bikini" incision was associated with a lower rate (4%) of Brooker grades 3 and 4 HO when compared with a longitudinal incision (12%). A Brooker grade of 0 to 3 HO did not impact outcomes. Only Brooker grade-4 HO was associated with significantly worse PROs; however, pain was not affected.

CONCLUSIONS

Low-grade HO after DAA-THA is not uncommon, and its rate is underestimated when anteroposterior radiographic views are evaluated without lateral views. The risk factors for developing HO in our cohort were an ASA grade of >3, male sex (especially with a high BMI), older age in women, and use of longitudinal incisions. With use of the bikini incision, patients had lower rates of severe HO, but mostly senior surgeons with more experience performed their surgery. Age, ASA grade, experience level of the surgeon, and Brooker grade-4 HO all had a significant influence on PROs. Clinically important HO (Brooker grade 4) remains rare after DAA-THR.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Is dual mobility cup total hip replacement associated with increased incidence of heterotopic ossification compared to conventional total hip replacements in fracture neck of femur patients?

INTRODUCTION

The incidence of heterotopic ossification after total hip replacement is variable in the literature. If symptomatic, it may cause pain and reduced range of motion. Dual mobility total hip replacements have been considered a valuable option for the treatment of femoral neck fractures in the active patients, achieving good range of motion with reduced risk of dislocation. The occurrence of HO may have detrimental effect on this type of articulation and may accelerate polyethylene wear and predispose to intra-prosthetic dislocation. We compared the incidence of HO in DMC versus conventional THR in femoral neck fracture patients across 3 large trauma institutes PATIENTS AND METHODS: This is a retrospective cohort study which included 334 patients, 223 received DMC replacements, 111 received conventional total hip replacement (THR). Mean age for the DMC group was 72.3 ± 9.9 years and was 72.1 ± 6.8 years for the THR group. Patients were stratified according to specific inclusion and exclusion criteria, and the 1-year follow up X rays were evaluated for the presence of heterotopic ossification by 3 assessors.

RESULTS

The incidence of HO in the DMC group was 27.1% compared to 12.2% in the THR group (χ² p value = 0.0001, Relative Risk = 2.22, 95% CI 1.22-4.03, p value = 0.0091). There was no significant effect of gender or age on the incidence of HO in both groups CONCLUSIONS: Patients receiving DMC hip replacements are more likely to develop HO as compared to patients receiving conventional THR for femoral neck fractures. Further prospective studies are required to confirm our findings and to assess the indication of HO prophylaxis in patients receiving DMC hip replacements, and the long-term outcomes in these patients.

Extracorporeal Shockwave Therapy for Painful Chronic Traumatic Heterotopic Ossification after Right Acetabulum Fracture Fixation: A Case Report and Literature Review

BACKGROUND: Heterotopic ossification (HO) usually occurs in trauma, such as fractures. To the best of our knowledge, there is a lack of studies about the effects of shockwave on the size of HO in the literature. AIM: The aim of this study was to describe the effects of extracorporeal shockwave therapy (ESWT) on the size of HO as well as the lower extremity functions in patients with fracture right acetabulum. CASE REPORT: A 36-year-old gentleman had a traffic accident on August 30, 2015, resulted in a fracture of the right acetabulum treated by open reduction and internal fixation on September 2, 2015. The patient has complained of severe right hip pain with limitation of daily activities, especially walking, standing, and sitting, visual analog scale about 7-8/10 with painful and restricted ROM of the right hip. Plain X-ray of the right hip revealed HO at greater trochanter with a long axis length 37.3 mm. ESWT was applied for HO. ESWT was administered 6 times each weekly for 6 weeks. At 6-month follow-up, the size of HO had become slightly smaller with the improvement of pain, and lower extremities functions. DISCUSSION: Soft tissues around HO have been regenerated from ischemia for several reasons such as ESWT stimulating angiogenesis and neurogenesis and the size of HO had become slightly smaller by radial shockwave therapy. CONCLUSION: ESWT is a novel non-invasive and safe treatment for HO. The effects of ESWT on the size of HO had become slightly smaller with the improvement of lower extremities functions.

Mechanism of traumatic heterotopic ossification: In search of injury-induced osteogenic factors

Heterotopic ossification (HO) is a pathological condition of abnormal bone formation in soft tissue. Three factors have been proposed as required to induce HO: (a) osteogenic precursor cells, (b) osteoinductive agents and (c) an osteoconductive environment. Since Urist's landmark discovery of bone induction in skeletal muscle tissue by demineralized bone matrix, it is generally believed that skeletal muscle itself is a conductive environment for osteogenesis and that resident progenitor cells in skeletal muscle are capable of differentiating into osteoblast to form bone. However, little is known about the naturally occurring osteoinductive agents that triggered this osteogenic response in the first place. This article provides a review of the emerging findings regarding distinct types of HO to summarize the current understanding of HO mechanisms, with special attention to the osteogenic factors that are induced following injury. Specifically, we hypothesize that muscle injury‐induced up‐regulation of local bone morphogenetic protein‐7 (BMP‐7) level, combined with glucocorticoid excess‐induced down‐regulation of circulating transforming growth factor‐β1 (TGF‐β1) level, could be an important causative mechanism of traumatic HO formation.

BMP signalling in a mechanical context - Implications for bone biology

Bone Morphogenetic Proteins (BMPs) are extracellular multifunctional signalling cytokines and members of the TGFβ super family. These pleiotropic growth factors crucially promote bone formation, remodeling and healing after injury. Additionally, bone homeostasis is systematically regulated by mechanical inputs from the environment, which are incorporated into the bone cells' biochemical response. These inputs range from compression and tension induced by the movement of neighboring muscle, to fluid shear stress induced by interstitial fluid flow in the canaliculi and in the vascular system. Although BMPs are widely applied in a clinic context to promote fracture healing, it is still elusive how mechanical inputs modulate this signalling pathway, hindering an efficient and side-effect free application of these ligands in bone healing. This review aims to summarize the current understanding in how mechanical cues (tension, compression, shear force and hydrostatic pressure) and substrate stiffness modulate BMP signalling. We highlight the time-dependent effects in modulating immediate early up to long-term effects of mechano-BMP crosstalk during bone formation and remodeling, considering the interplay with other already established mechanosensitive pathways, such as MRTF/SRF and Hippo signalling.

TGF-β1 plays a protective role in glucocorticoid-induced dystrophic calcification

Dystrophic calcification (DC) is the deposition of calcium in degenerated tissue which occurs as a reaction to tissue damage. Sometimes if tissue repair fails, it can progress into heterotopic ossification (HO), a pathological condition of abnormal bone formation. HO happens frequently in severe trauma patients such as in blast injury, central nervous system injury and burn injury, in which excessive endogenous glucocorticoid production has always been found. Glucocorticoids have a big impact on bone and muscle. However, few studies have investigated the impact of glucocorticoids on DC/HO formation in muscle. This study aimed to determine the role of glucocorticoids in DC/HO pathogenesis following muscular injury and the possible underlying mechanism. In this study, we administered a high dose of a synthetic glucocorticoid, dexamethasone (DEX), to animals with muscle injury induced by cardiotoxin (CTX) injection to mimic a glucocorticoid excess state following severe muscle trauma. The findings reported here showed that DEX treatment together with CTX-induced muscle injury led to a significant amount of DC in muscle. This effect was likely related to protein level alterations in the fibrinolytic system and resultant decreased circulating transforming growth factor-beta 1 (TGF-β1), given that supplementation of recombinant TGF-β1 markedly rescued this phenomenon. In summary, our results suggest that glucocorticoid excess impairs muscle regeneration and promotes DC/HO, and that TGF-β1 could be a key factor in modulating this process.

CT imaging features of skeletal muscle metastasis: A rare tumour group with different patterns

INTRODUCTION

Skeletal muscle metastasis (SMM) are rare and can have different patterns on computed tomography (CT). Our aim was to estimate the frequency of the different patterns, primary tumours and localizations of SMM in a large single-centre sample.

METHODS

A retrospective search in the database of our radiological institution was performed for patients with SMM. The primary tumour, affected muscles and CT patterns were estimated.

RESULTS

Overall, 104 SMM were diagnosed in 82 patients. In 11 patients (13.4%), SMM were the isolated manifestation of metastatic disease in clinical work up. Regarding primary tumours, gastrointestinal tumours (24.4%), lung cancer (20.4%) and malignant melanoma (13.4%) were identified most frequently. Other malignancies were rare. Most commonly, the trunk musculature was affected (41.3% of the cases). SMM type 1 were identified in 37 cases (35.6%), type 2 in 64 cases (61.5%), type 3 in 2 cases (1.9%) and type 4 in one case (1%).

CONCLUSIONS

The present study shows typical imaging appearances of SMM based on a large patient sample in a single centre. SMM has a rare occurrence with several different patterns, which can impose diagnostic difficulties and varies possible differential diagnoses.

Local injection of a hexametaphosphate formulation reduces heterotopic ossification in vivo

Heterotopic ossification (HO), the pathological formation of ectopic bone, is a debilitating condition which can cause chronic pain, limit joint movement, and prevent prosthetic limb fitting. The prevalence of this condition has risen in the military population, due to increased survivorship following blast injuries. Current prophylaxes, which aim to target the complex upstream biological pathways, are inconsistently effective ​and have a range of side-effects that make them unsuitable for combat-injured personnel. As such, many patients must undergo further surgery to remove the formed ectopic bone. In this study, a non-toxic, U.S. Food and Drug Administration (FDA) -approved calcium chelator, hexametaphosphate (HMP), is explored as a novel treatment paradigm for this condition, which targets the chemical, rather that biological, ​bone formation pathways. This approach allows not only prevention of pathological bone formation ​but also uniquely facilitates reversal, which current drugs cannot achieve. Targeted, minimally invasive delivery is achieved by loading HMP into an injectable colloidal alginate. These formulations significantly reduce ​the length of the ectopic bone formed in a rodent model of HO, with no effect on the adjacent skeletal bone. This study demonstrates the potential of localized dissolution as a new treatment ​and an alternative to surgery ​for pathological ossification and calcification conditions.

The Survey of Cells Responsible for Heterotopic Ossification Development in Skeletal Muscles-Human and Mouse Models

Heterotopic ossification (HO) manifests as bone development in the skeletal muscles and surrounding soft tissues. It can be caused by injury, surgery, or may have a genetic background. In each case, its development might differ, and depending on the age, sex, and patient's conditions, it could lead to a more or a less severe outcome. In the case of the injury or surgery provoked ossification development, it could be, to some extent, prevented by treatments. As far as genetic disorders are concerned, such prevention approaches are highly limited. Many lines of evidence point to the inflammatory process and abnormalities in the bone morphogenetic factor signaling pathway as the molecular and cellular backgrounds for HO development. However, the clear targets allowing the design of treatments preventing or lowering HO have not been identified yet. In this review, we summarize current knowledge on HO types, its symptoms, and possible ways of prevention and treatment. We also describe the molecules and cells in which abnormal function could lead to HO development. We emphasize the studies involving animal models of HO as being of great importance for understanding and future designing of the tools to counteract this pathology.

Heterotopic ossification: a preventable case of gossypiboma in spinal cord injury

OBJECTIVE

Heterotopic ossification is the formation of ectopic bone in soft tissues. It has three established aetiologies: genetic, traumatic and neurogenic. A gossypiboma is defined as a retained foreign body, such as a mass or sponge, usually after a surgical procedure. In this article, we present a unique, preventable case of a patient admitted for newly developed heterotopic ossification in the gluteus maximus muscle caused by a retained piece of foam from negative pressure wound therapy (NPWT). The heterotopic ossification lesion, together with the retained foreign body, was completely excised and reconstructed using a posterior thigh fasciocutaneous advancement flap. This is the first reported case of heterotopic ossification caused by a retained foreign body and may be helpful to better understanding of the aetiology of heterotopic ossification.

Glossary of terms for musculoskeletal radiology

Members of the International Skeletal Society compiled a glossary of terms for musculoskeletal radiology. The authors also represent national radiology or pathology societies in Asia, Australia, Europe, and the USA. We provide brief descriptions of musculoskeletal structures, disease processes, and syndromes and address their imaging features. Given the abundance of musculoskeletal disorders and derangements, we chose to omit most terms relating to neoplasm, spine, intervention, and pediatrics. Consensus agreement was obtained from 19 musculoskeletal radiology societies worldwide.

Extraskeletal osteosarcoma misdiagnosed as heterotopic ossification after periprosthetic femoral fracture: A case report

Extraskeletal osteosarcoma is a malignant tumor of soft tissue characterized by osteoid production and has a very low prevalence, comprising approximately 4% of all osteosarcomas and about 1% of all soft tissue sarcomas, and a total of about 350 cases have been reported until now. Heterotopic ossification is a pathological finding of bony tissue in soft tissue regions such as muscle, skin and subcutaneous tissue. We report a case of an 86-year-old woman with a history of total hip arthroplasty (THA), in which open reduction and internal fixation were done for periprosthetic femoral Fracture. The ossified lesion misdiagnosed as heterotopic ossification initially was diagnosed as extraskeletal osteosarcoma at 6 months after the surgery. Both extraskeletal osteosarcoma and heterotopic ossification have no definite symptoms, but show radiopaque shadows on simple radiograph. Therefore, careful attention and thorough evaluation with multiple imaging tests may be necessary for the differential diagnosis of these entities.

Inflammation in Fibrodysplasia Ossificans Progressiva and Other Forms of Heterotopic Ossification

PURPOSE OF REVIEW

Heterotopic ossification (HO) is associated with inflammation. The goal of this review is to examine recent findings on the roles of inflammation and the immune system in HO. We examine how inflammation changes in fibrodysplasia ossificans progressiva, in traumatic HO, and in other clinical conditions of HO. We also discuss how inflammation may be a target for treating HO.

RECENT FINDINGS

Both genetic and acquired forms of HO show similarities in their inflammatory cell types and signaling pathways. These include macrophages, mast cells, and adaptive immune cells, along with hypoxia signaling pathways, mesenchymal stem cell differentiation signaling pathways, vascular signaling pathways, and inflammatory cytokines. Because there are common inflammatory mediators across various types of HO, these mediators may serve as common targets for blocking HO. Future research may focus on identifying new inflammatory targets and testing combinatorial therapies based on these results.

Benign peripheral nerve tumors

Benign peripheral nerve tumors encompass a wide range of neoplasms and non-neoplastic tumor like lesions. Some of these lesions if not encountered in the setting of genetic syndromes, are occurring sporadically. The principles of oncology should be respectfully followed in every step of diagnostic approach and surgical management. Albeit, classified as benign, some of them do have different level of malignant potential, thus the treating physicians should be aware of that to avoid possible pitfalls with devastating outcomes. This article reviews the most common benign peripheral nerve tumors discussing the clinicopathological findings, imaging appearance and the current trend in their approach.

Unusual osseous metaplasia following hernia repair with Prolene mesh: a case report

The use of Prolene mesh for repair of various hernias is very common. As with any surgical procedure, it can be associated with complications such as seroma, hematoma and wound infection. However, it is uncommon to develop heterotopic ossification following incisional hernia repair with Prolene mesh. Herein, we report a case with chronic abdominal pain secondary to heterotopic ossification occurring after incisional hernia repair with Prolene mesh. It is crucial to report uncommon reactions to the mesh in order to aid in diagnosis of unexplained abdominal pain post hernia repair, after common etiologies are ruled out.

Myositis ossificans traumatica of the piriformis muscle: a rare mature case in an adult African male

Myositis ossificans traumatica (MOT) is a common form of heterotopic ossification associated to trauma. Rare mature manifestations and topographically atypical presentations of MOT are often misdiagnosed as osteosarcoma. This case study discusses a rare, mature case of MOT of the piriformis muscle, potentially clinically associated with piriformis syndrome. The ossification was observed on a dry sacral bone of an adult skeleton belonging to a South African male during routine inventory of the Raymond A. Dart Collection of Human Skeletons, the University of the Witwatersrand, Johannesburg. The MOT was located on the anterior aspect of the sacrum at a site corresponding to the upper portion of the origin of the muscle and extended laterally towards the greater trochanter, beyond the greater sciatic notch. It was cylindrical in shape and measured approximately 52.70 mm in length and 12.10 mm in diameter. Micro-focus CT revealed an extensive and mature bony development of the piriformis muscle with distinct outer cortical and inner trabecular bone. In addition, the skeleton showed widespread healed skeletal trauma, suggesting a history of trauma. The MOT was completely fused to the sacral bone excluding the possibility of congenital anomalies. Information on the MOT of the piriformis muscle is vital to clinicians and radiographers to aid in successful diagnosis and management of the piriformis syndrome and sciatica in the gluteal region. This case also provides a rare example to biological anthropologists, paleoanthropologists and bioarchaeologists of the representation of pathologies like these on a dry bone sample.

Muscle injury promotes heterotopic ossification by stimulating local bone morphogenetic protein-7 production

Background

Heterotopic ossification (HO) is a pathological condition of abnormal bone formation in soft tissue, which causes pain and restricted range of motion in patients. There are two broad categories of HO, hereditary and acquired. Although different types of HO do not use identical mechanistic pathways of pathogenesis, muscle injury appears to be a unifying feature for all types of HO. However, little is known about the mechanisms by which muscle injury facilitates HO formation.

Objective and method

This study aimed to explore the cellular and molecular mechanisms linking muscle injury to HO by using cardiotoxin to induce muscle injury in a bone morphogenetic protein-2 (BMP-2)-induced HO mouse model.

Results

We found that muscle injury augmented HO formation and that this effect was correlated with BMP signalling activation and upregulation of BMP-7 expression at the early phase of HO progression. We further demonstrated that inhibition of BMP-7 activity in vitro suppressed the osteogenesis-promoting effect of conditioned medium derived from injured muscle tissue and in vivo reduced the volume of HO formation. We also showed that antiinflammatory drug treatment reduced the volume of HO with concomitant reduction in BMP-7 production.

Conclusion

In summary, our study has identified BMP-7 as a key osteoinductive factor in injured muscle that facilitates HO formation.

The translational potential of this article

Our results provide a candidate mechanistic rationale for the use of antiinflammatory drugs in the prevention of HO.

[Muscle injuries in athletes : The value of magnetic resonance imaging]

Competitive sports yield high demands on the musculoskeletal system, accordingly muscle injuries are a common complication. Early imaging clarification of the muscles in cases of a trauma is essential in order to define the exact location of the lesion, the affected muscles, the extent and the degree of the injury as well as to define possible concomitant complications. In the case of a professional athlete, the assessment made by MRI is important for defining the individually required resting period for a riskless resumption of the sporting activities.

Bilateral fibrodysplasia ossificans affecting the masticatory muscles and causing irreversible trismus in a domestic shorthair cat

Case summary

An 8-year-old spayed female domestic shorthair cat was referred for trismus of progressive onset, which had started at least 1 month previously. The patient presented with weakness, anorexia, chronic bilateral purulent nasal discharge and concurrent reduced nasal airflow. Upon physical examination, painful mouth opening, bilateral swelling of the temporal areas, with an inability to open and close the mouth completely, were apparent. A vertical mandibular range of motion (vmROM) of 22 mm was noted. Complete blood count, biochemistry, electrolytes and various serology tests were unremarkable. CT revealed multiple mineralised lesions within the masticatory muscles. Histopathological features were consistent with those seen in the human disease fibrodysplasia ossificans progressiva (FOP). Supportive treatment did not improve the vmROM, eventually resulting in a 13 mm open bite, and total inability to close and open the mouth. Cardiac arrest occurred at the induction of an anaesthesia procedure aiming to perform tracheostomy and nasal lavage. Despite emergency tracheotomy and cardiac resuscitation, humane euthanasia was elected by the owners. Post-mortem molecular investigations highlighted a heterozygous deletion, compatible with a splicing site mutation in ACVR1, which is also associated with FOP in humans.

Relevance and novel information

This is the first report in the veterinary literature of FOP-like disease selectively affecting the masticatory muscles. This condition is associated with a poor prognosis, as no medical or surgical treatment has currently proven to be of any prophylactic or curative benefit. Although rare, FOP-like disease should be included in the differential diagnosis of trismus in the cat. Any further muscle injury should be avoided.

A Case of Soft Tissue Ossifications: A Case Report

CASE

A patient who had previously been diagnosed with fibrodysplasia ossificans progressiva was seen for hip pain and progressive soft tissue ossifications. Through a careful clinical examination, by which a subtype of brachydactyly was noted, the Albright hereditary osteodystrophy phenotype was recognized, and a new diagnosis of pseudopseudohypoparathyroidism was established. This paucisymptomatic condition often remains unidentified; however, its transmission can lead to more potentially serious diseases.

CONCLUSIONS

A careful diagnostic process, including physical examination, is essential. Even if advanced tests exist, small clinical findings can lead to the proper conclusion. In our case, a finger pointed us in the right direction.

Heterotopic Ossification: A Comprehensive Review

Heterotopic ossification (HO) is a diverse pathologic process, defined as the formation of extraskeletal bone in muscle and soft tissues. HO can be conceptualized as a tissue repair process gone awry and is a common complication of trauma and surgery. This comprehensive review seeks to synthesize the clinical, pathoetiologic, and basic biologic features of HO, including nongenetic and genetic forms. First, the clinical features, radiographic appearance, histopathologic diagnosis, and current methods of treatment are discussed. Next, current concepts regarding the mechanistic bases for HO are discussed, including the putative cell types responsible for HO formation, the inflammatory milieu and other prerequisite “niche” factors for HO initiation and propagation, and currently available animal models for the study of HO of this common and potentially devastating condition. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Postoperative Heterotopic Ossification After Cervical Disc Replacement Is Likely a Reflection of the Degeneration Process

BACKGROUND

The cause of postoperative heterotopic ossification (HO) after cervical disc replacement (CDR) is unclear. The aim of the present study was to investigate the incidence of HO, its clinical significance, and whether the degree of preoperative degeneration was associated with its occurrence.

METHODS

Patients who had undergone CDR using the Prestige-LP disc with a minimum of 48 months of follow-up were included. The patients were divided into 2 groups stratified by the presence of HO. The clinical outcomes (visual analog scale for neck and arm pain, neck disability index, and Japanese Orthopaedic Association scores) and radiographic parameters were collected and analyzed pre- and postoperatively and compared between the 2 groups. The preoperative degeneration at the operated levels was evaluated using the Kellgren-Lawrence scale and Miyazaki magnetic resonance imaging grading system.

RESULTS

A total of 69 patients (89 levels) were identified, and the average age at surgery was 45.8 years. The overall incidence of HO at the final follow-up visit was 37.7% (26 of 69 of patients), and an increasing incidence and severity of HO was observed during the follow-up period. Significant and maintained improvements for all clinical outcome measures were observed, and the presence of HO did not influence these improvements. Segments with greater severe preoperative degeneration had a greater incidence of HO (P < 0.01). However, no statistical significant correlation between the degree of preoperative degeneration and the grade of HO was observed.

CONCLUSION

The presence of HO after CDR was a relatively frequent finding at the final follow-up visit but did not influence patients' improvements. Segments with higher grades of preoperative degeneration had a greater incidence of HO; thus, HO is likely a reflection of the degeneration process.

Musculoskeletal "don't touch" lesions: pictorial essay

Focal bone lesions are not uncommon findings in the daily practice of radiology. Therefore, it is essential to differentiate between lesions with aggressive, malignant potential that require action and those that have no clinical significance, many of which are variants or benign lesions, sometimes self-limited and related to reactive processes. In some cases, a diagnostic error can have catastrophic results. For example, a biopsy performed in a patient with myositis ossificans can lead to an incorrect diagnosis of sarcomatous lesions and consequently to mutilating surgical procedures. The present study reviews the main radiological aspects of the lesions that are most commonly seen in daily practice and have the potential to be confused with aggressive, malignant bone processes. We also illustrate these entities by presenting cases seen at our institution.

Uncommon Complications after Anterior Cruciate Ligament Reconstruction

Anterior cruciate ligament reconstruction (ACL-R) is a common surgical procedure, with good outcome in 75 to 97% of the cases. However, different complications have been described including infection, hemarthrosis, deep vein thrombosis (DVT), and pulmonary embolism (PE) with a rate ranging from 1 to 15%. There are few case reports in the literature describing rare complications after ACL-R and they can be divided into: (1) complications related to the fixation device (rupture, migration); (2) fractures (tibial or femoral side); (3) infections due to uncommon bacteria, mycobacterium, and mycosis; (4) rare vascular injuries; (5) nerve injuries; and (6) other rare complications. In case of fixation device rupture or migration, device removal can be easy but the diagnosis may be challenging. Patellar fracture after ACL-R may be related to harvesting and it is not uncommon. Conversely, femoral or tibial fractures are most frequently due to bone weakness related to bone tunnels. Some rare infections related to uncommon bacteria or mycosis are also described with potentially devastating joint damage. Popliteal artery injuries are uncommon in ACL-R but minor vessels damages are described with possible severe consequences for patients. Injuries to the infrapatellar branch of the saphenous nerve are not uncommon in ACL-R. However, there are few case reports also describing injuries to the saphenous nerve, the common peroneal nerve and the sciatic nerve. The aim of this paper is to review the literature describing uncommon complications after ACL-R, giving some more information about diagnosis and treatment.

NF-κB/MAPK activation underlies ACVR1-mediated inflammation in human heterotopic ossification

BACKGROUND

Inflammation helps regulate normal growth and tissue repair. Although bone morphogenetic proteins (BMPs) and inflammation are known contributors to abnormal bone formation, how these pathways interact in ossification remains unclear.

METHODS

We examined this potential link in patients with fibrodysplasia ossificans progressiva (FOP), a genetic condition of progressive heterotopic ossification caused by activating mutations in the Activin A type I receptor (ACVR1/ALK2). FOP patients show exquisite sensitivity to trauma, suggesting that BMP pathway activation may alter immune responses. We studied primary blood, monocyte, and macrophage samples from control and FOP subjects using multiplex cytokine, gene expression, and protein analyses; examined CD14+ primary monocyte and macrophage responses to TLR ligands; and assayed BMP, TGF-β activated kinase 1 (TAK1), and NF-κB pathways.

RESULTS

FOP subjects at baseline without clinically evident heterotopic ossification showed increased serum IL-3, IL-7, IL-8, and IL-10. CD14+ primary monocytes treated with the TLR4 activator LPS showed increased CCL5, CCR7, and CXCL10; abnormal cytokine/chemokine secretion; and prolonged activation of the NF-κB pathway. FOP macrophages derived from primary monocytes also showed abnormal cytokine/chemokine secretion, increased TGF-β production, and p38MAPK activation. Surprisingly, SMAD phosphorylation was not significantly changed in the FOP monocytes/macrophages.

CONCLUSIONS

Abnormal ACVR1 activity causes a proinflammatory state via increased NF-κB and p38MAPK activity. Similar changes may contribute to other types of heterotopic ossification, such as in scleroderma and dermatomyositis; after trauma; or with recombinant BMP-induced bone fusion. Our findings suggest that chronic antiinflammatory treatment may be useful for heterotopic ossification.

Prophylactic Celecoxib Administration Is Associated With Decreased Incidence and Severity of Heterotopic Ossification After Hip Resurfacing by Direct Lateral Approach in Male Patients

Heterotopic ossification is a potential complication that may have a particularly higher association with hip resurfacing. The 2 current mainstays for heterotopic ossification treatment and prophylaxis are administration of nonsteroidal anti-inflammatory drugs and radiotherapy. Recent studies have determined that celecoxib is effective in heterotopic ossification prophylaxis after total hip arthroplasty. However, considering the reportedly higher incidence and severity of heterotopic ossification in these patients, relatively few studies have evaluated its role in hip resurfacing. Therefore, the authors assessed the incidence, severity, and risk factors of heterotopic ossification in patients who had hip resurfacing and did or did not receive celecoxib. Of the 198 patients, 83 received celecoxib and 115 did not. Radiographs were examined to grade heterotopic ossification using the Brooker classification system. The rate of heterotopic ossification differed between patients who did and patients who did not receive celecoxib prophylaxis (25% vs 65%, P<.001). Celecoxib was an independent predictor of decreased heterotopic ossification (odds ratio, 0.16; 95% confidence interval, 0.08-0.35). Celecoxib administration is associated with decreased incidence and severity of heterotopic ossification after hip resurfacing performed using the direct lateral approach in male patients. [Orthopedics. 2018; 41(6):e807-e812.].

ACVR1R206H FOP mutation alters mechanosensing and tissue stiffness during heterotopic ossification

An activating bone morphogenetic proteins (BMP) type I receptor ACVR1 (ACVR1^R206H) mutation enhances BMP pathway signaling and causes the rare genetic disorder of heterotopic (extraskeletal) bone formation fibrodysplasia ossificans progressiva. Heterotopic ossification frequently occurs following injury as cells aberrantly differentiate during tissue repair. Biomechanical signals from the tissue microenvironment and cellular responses to these physical cues, such as stiffness and rigidity, are important determinants of cell differentiation and are modulated by BMP signaling. We used an Acvr1^R206H/+ mouse model of injury-induced heterotopic ossification to examine the fibroproliferative tissue preceding heterotopic bone and identified pathologic stiffening at this stage of repair. In response to microenvironment stiffness, in vitro assays showed that Acvr1^R206H/+ cells inappropriately sense their environment, responding to soft substrates with a spread morphology similar to wild-type cells on stiff substrates and to cells undergoing osteoblastogenesis. Increased activation of RhoA and its downstream effectors demonstrated increased mechanosignaling. Nuclear localization of the pro-osteoblastic factor RUNX2 on soft and stiff substrates suggests a predisposition to this cell fate. Our data support that increased BMP signaling in Acvr1^R206H/+ cells alters the tissue microenvironment and results in misinterpretation of the tissue microenvironment through altered sensitivity to mechanical stimuli that lowers the threshold for commitment to chondro/osteogenic lineages.

Musculoskeletal Pitfalls on Fluorodeoxyglucose F 18 PET-Computed Tomography: Pictorial Review

A wide range of musculoskeletal processes can demonstrate increased uptake on PET-computed tomography (CT) with fluorodeoxyglucose (FDG) F 18, including reactive, benign neoplastic, inflammatory, traumatic, posttreatment, and arthritic conditions that may mimic malignancy. In addition, physiologic causes of increased FDG uptake such as asymmetric muscle use and presence of brown fat can lead to increased FDG uptake and potential false-positive results. This article presents various case examples of non-malignant musculoskeletal hypermetabolism on 18F-FDG PET-CT and describes useful tools to avoid the potential pitfall of misinterpreting these as malignancy.

Elongated odontoid process in late Holocene skeletal remains from B6 archaeological site, Mendoza, Argentina

The odontoid process of the axis can be affected by congenital or acquired pathologies. While abnormalities such as os odontoideum, agenesis, and fractures are reported in archaeological remains, the abnormality of an elongated length of the odontoid process has not been described in the paleopathological literature. The aim of this paper is to evaluate two individuals with elongated odontoid processes from a skeletal assemblage from the B6 archaeological site (Mendoza, Argentina), and to discuss the possible etiologies of the condition, with particular attention given to the relation to trauma and Crowned Dens Syndrome (CDS), a condition characterized by the ossification of ligaments of the odontoid process of the second cervical vertebra.

Digit-Like Bony Anomaly of the Hindfoot: A Case Report

We report the case of an asymptomatic digit-like bony anomaly located on the plantar aspect of the calcaneus, which was incidentally found on radiographs of a 50-year-old male with a tibial shaft fracture. To the best of our knowledge, this is the first description of such an anomaly in the foot. The differential diagnosis includes accessory ossicles, polydactyly, heel spur, heterotopic ossification, osteochondroma, and pelvic digits.

Trauma induced heterotopic ossification patient serum alters mitogen activated protein kinase signaling in adipose stem cells

Post-traumatic heterotopic ossification (HO) is the formation of ectopic bone in non-osseous structures following injury. The precise mechanism for bone development following trauma is unknown; however, early onset of HO may involve the production of pro-osteogenic serum factors. Here we evaluated serum from a cohort of civilian and military patients post trauma to determine early induction gene signatures in orthopaedic trauma induced HO. To test this, human adipose derived stromal/stem cells (hASCs) were stimulated with human serum from patients who developed HO following trauma and evaluated for a gene panel with qPCR. Pathway gene analysis ontology revealed that hASCs stimulated with serum from patients who developed HO had altered gene expression in the activator protein 1 (AP1) and AP1 transcriptional targets pathways. Notably, there was a significant repression in FOS gene expression in hASCs treated with serum from individuals with HO. Furthermore, the mitogen-activated protein kinase (MAPK) signaling pathway was activated in hASCs following serum exposure from individuals with HO. Serum from both military and civilian patients with trauma induced HO had elevated downstream genes associated with the MAPK pathways. Stimulation of hASCs with known regulators of osteogenesis (BMP2, IL6, Forskolin, and WNT3A) failed to recapitulate the gene signature observed in hASCs following serum stimulation, suggesting non-canonical mechanisms for gene regulation in trauma induced HO. These findings provide new insight for the development of HO and support ongoing work linking the systemic response to injury with wound specific outcomes.

Palovarotene inhibits connective tissue progenitor cell proliferation in a rat model of combat-related heterotopic ossification

Heterotopic ossification (HO) develops in the extremities of wounded service members and is common in the setting of high-energy penetrating injuries and blast-related amputations. No safe and effective prophylaxis modality has been identified for this patient population. Palovarotene has been shown to reduce bone formation in traumatic and genetic models of HO. The purpose of this study was to determine the effects of Palovarotene on inflammation, progenitor cell proliferation, and gene expression following a blast-related amputation in a rodent model (n = 72 animals), as well as the ability of Raman spectroscopy to detect early HO before radiographic changes are present. Treatment with Palovarotene was found to dampen the systemic inflammatory response including the cytokines IL-6 (p = 0.01), TNF-α (p = 0.001), and IFN-γ (p = 0.03) as well as the local inflammatory response via a 76% reduction in the cellular infiltration at post-operative day (POD)-7 (p = 0.03). Palovarotene decreased osteogenic connective tissue progenitor (CTP-O) colonies by as much as 98% both in vitro (p = 0.04) and in vivo (p = 0.01). Palovarotene treated animals exhibited significantly decreased expression of osteo- and chondrogenic genes by POD-7, including BMP4 (p = 0.02). Finally, Raman spectroscopy was able to detect differences between the two groups by POD-1 (p < 0.001). These results indicate that Palovarotene inhibits traumatic HO formation through multiple inter-related mechanisms including anti-inflammatory, anti-proliferative, and gene expression modulation. Further, that Raman spectroscopy is able to detect markers of early HO formation before it becomes radiographically evident, which could facilitate earlier diagnosis and treatment. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1135-1144, 2018.

Heterotopic ossification and lessons learned from fifteen years at war: A review of therapy, novel research, and future directions for military and civilian orthopaedic trauma

Heterotopic ossification, the formation of bone in soft tissues, is a common complication of the high-energy extremity trauma sustained in modern armed conflict. In the past 15years, military treatment facilities and aligned laboratories have been in a unique position to study and treat this process due to the high volume of patients with these injuries secondary to blast trauma. The devastating nature of these wounds has limited traditional therapeutic options, necessitating alternative solutions to prophylaxis and initial treatment producing substantial advances in modeling, prophylaxis, detection, and therapy. Specific developments include establishment of an animal model that reproduces the systemic and local tissue injury of blast injuries, the use of molecular assays and predictive modeling in clinical decision making, advances in early detection including Raman spectroscopy, and investigation of prophylactic and therapeutic pharmacotherapy targeting the molecular pathways of aberrant bone formation. In this review article, we will present the literature to date, ongoing studies, and future directions for investigation of heterotopic ossification, with a focus on military-specific research.

Neurological heterotopic ossification: Current understanding and future directions

Neurological heterotopic ossification (NHO) involves the formation of bone in soft tissue following a neurological condition, of which the most common are brain and spinal cord injuries. NHO often forms around the hip, knee and shoulder joints, causing severe pain and joint deformation which is associated with significant morbidity and reduced quality of life. The cellular and molecular events that initiate NHO have been the focus of an increasing number of human and animal studies over the past decade, with this work largely driven by the need to unearth potential therapeutic interventions to prevent the formation of NHO. This review provides an overview of the present understanding of NHO pathogenesis and pathobiology, current treatments, novel therapeutic targets, potential biomarkers and future directions.

Retinoid roles and action in skeletal development and growth provide the rationale for an ongoing heterotopic ossification prevention trial

The majority of skeletal elements develop via endochondral ossification. This process starts with formation of mesenchymal cell condensations at prescribed sites and times in the early embryo and is followed by chondrogenesis, growth plate cartilage maturation and hypertrophy, and replacement of cartilage with bone and marrow. This complex stepwise process is reactivated and recapitulated in physiologic conditions such as fracture repair, but can occur extraskeletally in pathologies including heterotopic ossification (HO), Ossification of the Posterior Longitudinal Ligament (OPLL) and Hereditary Multiple Exostoses (HME). One form of HO is common and is triggered by trauma, invasive surgeries or burns and is thus particularly common amongst severely wounded soldiers. There is also a congenital and very severe form of HO that occurs in children with Fibrodysplasia Ossificans Progressiva (FOP) and is driven by activating mutations in ACVR1 encoding the type I bone morphogenetic protein (BMP) receptor ALK2. Current treatments for acquired HO, including NSAIDs and local irradiation, are not always effective and can have side effects, and there is no effective treatment for HO in FOP. This review article describes the research path we took several years ago to develop a new and effective treatment for both congenital and acquired forms of HO and specifically, the testing of synthetic retinoid agonists to block the initial and critical chondrogenic step leading to HO onset and progression. We summarize studies with mouse models of injury-induced and congenital HO demonstrating the effectiveness and mode of action of the retinoid agonists, including Palovarotene. Our studies have provided the rationale for, directly led to, an ongoing phase 2 FDA clinical trial to test efficacy and safety of Palovarotene in FOP. Top-line results released a few months ago by the pharmaceutical sponsor Clementia are very encouraging. Given shared developmental pathways amongst pathologies of extraskeletal tissue formation, Palovarotene may also be effective in HME as preliminary in vitro data suggest.