Hereditary Multiple Exostoses: Current Insights

Introduction

Hereditary multiple exostoses (HME) is a rare genetic disorder where several benign cartilaginous tumors arise from the perichondrium and flank the cartilage growth. It has a prevalence of 1:50,000 in Western countries.^1–3 This autosomal dominant disease has also been called hereditary multiple osteochondromas, hereditary deforming dyschondroplasia, diaphyseal aclasis and multiple cartilaginous exostoses. It exhibits almost complete penetrance particularly in males.^1,2,4–8 In more than 50% of patients, the disease can be passed down directly from an affected parent, usually the father. Normally, an unaffected male does not transmit the disease, however, an unaffected female may have a latent or suppressed form of the disease and may transmit it.⁹ The disease occurs in only about 5% to 10% of the cases when compared to solitary exostosis, is more frequently seen in males (1.5:1) and is usually diagnosed before the patients reach the age of 10.^2,4 Osteochondromas may arise from every segment excluding facial bones. The disease may involve any bone which grows from endochondral ossification. Although exostosis origins more frequently in the scapulae, ribs, pelvis, and vertebrae, the most affected regions are the metadiaphyseal segments of long bones. No involvement of the skull, carpal and tarsal bones have been reported.⁴ Given the high variety of location, size and number, exostoses can cause several clinical problems.^1,10 HMEs usually impact the quality of life and physical activity levels since childhood, in addition to mental health, especially in female patients.^11–13 Resection, limb shortening and surgical deformity correction are the only available treatments and are advised when the cases are symptomatic or a malignant transformation is suspected.¹⁴

Epidemiology

The prevalence of HME in Western population has been assessed between 0.4 to 1 out of 50,000.^2,15 However, in the Chamorro people, a small population in Guam, the prevalence of HME reaches as high as 50 out of 50,000. The incidence of this pathology in the Western countries was reported as 1.5% per year.³ However, this information could be erroneous since these data were taken from hospital reports and health databases of the affected patients and relatives’ families, with a focus on people who reported the symptoms and were classified as affected. Since genetic screenings have been employed to our day, there may be several asymptomatic or paucisymptomatic patients who could be affected by HME, but not detected by hospitals or health databases. Moreover, 10% of the HME patients have not reported any family history of the pathology.¹ Schmale² found a penetrance of 96%, but a more accurate report by Wicklund¹ reported the HME penetrance as 100%. Although some reports¹⁶ have detected a male predominance in HME, these findings were proven wrong by other authors.^1,15

Pathogenesis

The main symptom of HME is the formation of exostosis. Progenitor cells of the chondrocytes in the growth plate that cause osteochondromas have recently been detected in the inner layer of the perichondrium.¹⁷ Genetic, molecular biology and clinical investigations are fundamental to understand how osteochondromas arise in patients with HME. Gene sequencing in patients affected by HME led to the discovery of the EXT1 gene, located on Chromosome 8 and at a later stage to the discovery of the EXT2 gene, located on Chromosome 11.¹⁸ EXT1 and EXT2 mutations have been detected respectively in 28 to 65% and in 21 to 61% of the affected patients.^19–21 EXT1 genes appear to be more susceptible to mutations, while EXT2 can support a higher mutation burden before leading to the development of the disease.²² Patients who have mutations in the EXT1 gene are also more prone to having higher number of exostoses, more cases of limb malalignment, a shorter stature and more pelvic involvement.²⁰ Surprisingly, in 5 to 34% of the HME patients, mutations in the EXT1 or EXT2 genes were not detected.^19–21,23 An explanation might be found in the mosaicism of the EXT genes. As a matter of fact, the mutation can lead to insufficient activity of the EXT1 and EXT2 genes in some patients.²⁴ Genetic transmission of HME follows a dominant autosomal pattern, loss of heterozygosis (LOH), haploinsufficiency and other models including mutations in the EXT regulatory genes, and thus post-transcriptional regulation pathways²⁵ have been proposed. In a mouse model, Jones²⁶ demonstrated a LOH in vivo with exostosis development, with the findings of several cells with homozygosis in EXT-allele in the exostosis tissue.²⁷ Reijinders²⁸ proved the hypothesis of LOH development in human osteochondromas, rejecting the haploinsufficiency model. In their study, a second hit mutation was found in 63% of the analyzed osteochondromas. Other studies that analyzed exostosis specimens did not find a second hit mutation in the studied cells. Furthermore, some population studies demonstrated that EXT1 added to EXT2 heterozygous mutations could result in exostosis formation in the affected patients.^25,29 In an effort to explain these findings, some authors hypothesized that a possible presence of not yet discovered epigenetic factors like post-translational EXT-products/inhibitors, heparan sulfate (HS) polymerase inactivation agents or miRNAs that could inhibit EXT-translation.³⁰ However, as Matsumuto suggested, the most likely hypothesis is the heterogeneity in the osteochondroma cells, which are usually a mixture of mutated and wild type cells that remain single. It would be reasonable to think that osteochondromas develop by the mutation of some cells only.^28,31 The molecular pathway that leads to HME and exostosis formation is strictly linked to the HS molecule. Several studies have asserted over the years that EXT-mutations lead to an insufficient elongation of the HS chains.^23,30,32,33 Products of the EXT1 and EXT2 genes form a Golgi reticulum-based complex that is essential for the production and activity of glycosyltransferase enzyme which is finally responsible for chain polymerization of HS. This explains why isolated mutations of EXT1 or EXT2 are sufficient to cause the pathology; in fact, the Golgi complex does not work appropriately without its components from EXT1 or EXT2.^32–34 The HS chains have several functions in the extracellular matrix. It is believed that the absence of HS chains or the presence of shorter ones could produce serious variation in chondrocyte differentiation or proliferation pathways. Extracellular HS chains could be involved in the Indian hedgehog (IHH) diffusion in the growth plate.²³ IHH is an important factor which regulates the growth and differentiation of chondrocytes.³⁵ HS chains also influence the distribution, range of action, stability and action on target of several other factors such as fibroblastic growth factors (FGFs), bone morphogenetic proteins (BMPs), Wnt signaling pathway (Wnts) and parathyroid hormone-related proteins (PTHrPs).^33,36–38 A severe interference to these signaling pathways leads to the development of osteochondromas and also to other manifestations of HME like a shorter stature.³⁹ Osteochondromas derive from an imbalance in the extracellular matrix of factors and signals that enhances chondrocyte proliferation or differentiation. Impairment of the HS chain has also been observed in autistic mice with a normal brain anatomy, and a function in glutamatergic synapses stability has been suggested for HS chains (Figure 1).⁴⁰

Pathology

Macroscopically, exostosis is covered by a cartilage cap that, until the end of the growth process, entirely covers the lesion. In the first years of life, instead, almost all of the lesions are constituted of a cartilage fused at the base with initial ossification tissue in continuity with the normal bone of the metaphysis. In a child, the cartilage cap is covered with a hyaline cartilage with blue light reflexes, similar to the normal child cartilage, with a variable thickness up to 2–3 cm. Along with the ageing of patients, the thickness of the cartilage cap tends to decrease and disappear in many sites, and the exostosis cap usually appears to be ossified except for some lenticular cartilage residuals with a thickness of few millimeters. Irregular networks of cancellous bone and yellow and red marrows are present under the cap. Insulae of cartilage tissue that ossify with age may be seen in the exostosis base. All osteochondromas are covered by the periosteum. Microscopically, during the growth phase of the exostosis, the cartilage cap has the same structure as the growth plate. Particularly, we can discern the proliferative, columned, hypertrophic and calcified cartilage layers from the cartilage cap surface followed by bone trabeculae generated by the endochondral ossification process. The difference with the normal growth plate is that the trabeculae in that case are irregular in shape and directions and could contain insulae of cartilage tissue. In large sized exostosis, we can also observe islands of necrotic bone trabeculae with precipitations of calcite that derive from local defects in the blood supply.^3,5,41–43

Clinical Presentation

Hereditary multiple osteochondromas patients are generally diagnosed before the age of 12. Usually, these patients come to the physician’s attention following the appearance of a palpable mass near the joints. Knees, shoulders, ankles and wrists are the most involved joints during diagnosis. An HME patient is generally burdened with an average of six exostoses.⁴¹ In recent years, a clinical classification system for HME has been designed and published to assess the burden and life limitations that follow the disease.⁴⁴ This system focuses on the deformities and functional limitations associated with HME and classifies the patients under three groups according to the number of involved segments. The effects of HME depend largely on the number and shape of the exostoses, which demonstrate a great variability. Usually, when an osteochondroma has an early onset, the lesion tends to be bigger and causes more problems. This happens because the lesion tends to grow in line with the growth plate and adjusts itself to the evolution of the diaphysis. Sometimes, exostoses could produce disturbances in the growing diaphysis giving rise to limb length discrepancies and characteristic deformations that usually affect the distal femur, radius, ankle and ulna. The ribs and the proximal tibia where exostoses are easily visible and palpable, are the segments which are more prone to esthetic issues.⁴⁵ The hip joint of the HME patients could be a “coxa valga” due to osteochondromas of the minor trochanter that alter the proximal femur shape by determining a verticalization of the neck-shaft angle, and more rarely the acetabulum exostoses could lead to acetabular dysplasia, femoroacetabular impingement and finally juvenile arthritis that may require arthroplasty.^46,47 The knee joint is generally involved, with genu valgum as the most frequently seen deformity that affects approximately one third of HME patients. Genu valgum usually derives from the angulation of the proximal tibia. In some cases, distal femur could also be angulated to form an oblique joint line.⁴⁸ These findings are associated with early knee arthritis and patellar dislocations or subluxations. The ankle joint is also burdened with valgus deformity. Generally, a relative shortening of the distal fibula compared to the distal tibia leads to an oblique growth plate and joint line with medial dislocation of the talus.⁴⁹ In the past, spinal exostoses were overlooked, however, it has been assessed that up to 68% of the patients could have spinal osteochondromas. However, data that associate scoliosis to HME are scarce.⁵⁰ Spinal exostoses could lead to severe and acute neurological syndromes as we have reported in the Prognosis and Diagnosis sections. HME patients, in particular those who have been diagnosed with EXT1 mutations, have a shorter stature compared to the general population. This finding does not seem to be related to limb length discrepancies and bone deformity, but might be a general characteristic of HME. It is well known that HS deficiency could interfere with physiological growth in several ways.⁵¹ In case of growth of osteochondromas in adult patients, malignant transformation should be suspected.^25,52 Many patients are also diagnosed with chronic inflammation syndromes, limited range of motion or paresthesia and nerve entrapment syndromes. That comes from the final shape of exostoses since it can interfere with nearby structures as nerves, vessels and tendons. Many reports showed that HME patients suffer from poor quality of life. Both children and adult patients reported difficulties with socializing at work and in school. Chronic pain syndromes are common and most of the patients have to abandon sports and leisure activities.^13,53,54

Diagnosis

Most of the patients are asymptomatic at birth, thus the diagnosis could be pursued only with genetic screenings of the newborns of affected families. The median age of diagnosis is 3 years.² Fifty percent of the patients have a visible tumor and get diagnosed by the age of 5 and 80% by the age of 10.³ In general, all patients are diagnosed by the age of 10–12. Conventional roentgenograms have a good accuracy in detecting the shape and presence of osteochondromas in most of the regions. X-rays are also useful to detect and study the HME-associated deformities. Kok⁵⁵ reported that in radiological studies, 33% of the patients with HME were also diagnosed with ulnar shortening and deformity focusing on bayonet hand or pseudo-Madelung deformities; and in the lower limbs it is generally easier to diagnose a typical distal femur deformity as the “Erlenmeyer flask” deformity. Axial deviations, shortenings and limb length discrepancies are also assessed by X-rays. Axial imaging with computed tomography (CT) scan or magnetic resonance imaging (MRI) is generally mandatory to study anatomical challenging regions such as the pelvis and the thorax. The spine should also be examined in HME patients (see the Prognosis section) with an MRI of the entire spine and the screening should be performed every two years during the follow-up period. An MRI can detect bursitis derived from the mechanical stress around the lesions. Entrapment of the nerves and vessels, their positions, and their anatomical relationships with exostoses are easily shown on the MRI. The nerves that suffer most from entrapment syndromes are the radial and peroneal nerves.⁵⁵ Performing an MRI is mandatory for planning the surgical removal of exostosis. Magnetic resonance imaging can highlight the shape of the exostosis and its continuity from the cortical bone; but before all, it allows a direct and precise study of the cartilage cap and the soft tissues surrounding the lesions. Cartilage cap can also be analyzed using CT images but an MRI offers a better resolution. Measurement of the cartilage cup is a reliable method to predict malignant transformation of osteochondromas.⁵⁶ The measurement should be performed from the osseous interface of the exostosis stalk to the edge of the cartilage cap at its thickest portion. During childhood and adolescence, the cap thickness can be more than 3 cm. However, in adults, HME should be suspected when the thickness is more than 1–2 cm. Various authors have set the limit at 1 or 2 cm. Finally, Kok⁵⁵ stated that a threshold value of 2 cm is enough to diagnose malignancy with 95% specificity and sensitivity. Other radiological signs of malignant transformation are an irregular surface of the cap, a lytic foci inside the exostosis or near the bone, and the presence of soft tissue mass surrounding the exostosis, especially if they contain scattered or irregular calcifications.⁵⁶ F-18 fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) may have a role in detecting malignancies, especially to evaluate the standardized uptake value (SUV) of glucose in a semi-quantitative manner. However, during childhood and adolescence, an abnormal uptake of 18F-FDG can be detected in absence of malignant lesions as the exostosis is growing. A bone scan is generally considered of no value in the study of HME.^55,56 Recently, Sonne-Holm⁵⁷ has suggested a full body MRI every two years throughout the follow-up period. Heparan sulfate and chondroitin sulfate (CS) could be the molecular target of the diagnostic tests in the future. Faruqi⁵⁸ used the HS/CS ratio; this could be used as a molecular predictor of HME in children since these patients have a half HS/CS ratio compared to the general population. Differential diagnosis is generally easy. Enchondromas have very different characteristics when compared with exostoses. In very rare syndromes, multiple exostoses could represent an epiphenomenon. To the best of our knowledge, we can cite Langer-Giedion syndrome⁵⁹ and metachondromatosis. In metachondromatosis, the patients are usually affected by enchondromas and exostoses that regress spontaneously with age. The exostosis histology in metachondromatosis syndrome could be similar to the same lesions in HME, but in certain cases the cartilage cap and in general the lesion differ from the typical osteochondromas in HME patients.⁶⁰

Prognosis

The burden of HME-related lesions varies according to the patients’ age, gender and specificity. A mean of six exostoses per patient has been reported in previous studies,^3,41 with an average of 2–3.5 exostoses removed per patient.^1,2 The shape, dimensions and the position of these lesions affect the prognosis in HME patients; limb deformities, involvement of the vessels and nerves, and surgery-related complications are the main factors which determine prognosis and quality of life. Although it is relatively rare, spinal involvement has to be screened since it determines serious damages also in acute settings.⁶¹ Malignant transformations and associations to other tumors are relatively rare.^52,62 Wiklund¹ showed that HME patients have a shorter stature compared to the general population. Thirty-seven percent of males and 44% of females fall under the fifth percentile. These characteristics seem to be intrinsic to the pathology and are not determined by limb length discrepancies and deformities shown by statistical tests. EXT1 and EXT2 mutations also affect patients’ growth. Patients with EXT1 mutations have been more frequently associated with exostoses, exostosis-related complications and a shorter stature.⁶³ Ten percent of the patients reported dimensional changes in exostosis during pregnancy and 63% of the pregnant women who had exostosis-related complications during delivery required a cesarean section.¹

Deformity

In the upper limbs, the distal ulna is usually involved with ulnar shortening that can lead to proximal dislocation of the radial head, as reported in 22 to 25% of the patients.^3,16 Radial or ulnar deviations were considered as “moderate” or “severe” in 60% of the cases, and as “normal” in the remaining 40%. As for the lower limbs, involvement of the proximal femur led to coxa valga deformity in 25% of the cases.^3,16 Cumulative deformities of the lower limbs caused limb length discrepancies in 50% of the patients, 23% of which required correction surgeries. Valgus knee was reported in 33% of the cases.³ Tibia “valga” with obliquity of the distal tibial epiphysis was described in 50 to 54%^3,16 of the cases, with 56% of fibular shortening linked to growth defects of the distal fibula.¹⁶ Schmale² reported that 9% of the patients could have severe deformities that impaired their walking ability. Schmale² also reported lower rates of deformity with 39% of forearm deformities, 8% of varus or valgus knee and 2% of ankle deformity. The hand is rarely involved, but in these cases, the ulnar side is more often affected and particularly the metacarpophalangeal joints. A mean of 12 tumors per hand was reported by Woodside⁶⁴ with an average of two ray shortenings and five ray angulations.

Neurovascular Complications

Involvement of the peripheral nerves and vessels are major indications for surgery when there is not any malignant transformation. Peripheral nerve compressions and neuropathy are common and observed in 22 to 23% of the cases.^1,3 Wiklund¹ also reported 11.5% of vessel-related complications. Nerves compression has a subtle clinical presentation with impairing symptoms in the long term. Involvement of the vessels can be more acute with micro-traumatisms that can lead to acute vessels damages.

Pain-Related Complications

Goud⁵³ reported chronic pain in 60% of the children and 80% of the adults (283 patients). Thirty percent of the children and 50% of the adults refrained from playing their preferred sport. Approximately 22% of the adults had to change workplace or get a tailored workplace due to pathology-related complications. Fifty percent of the children had problems with socializing. Darilek⁶⁵ reported about chronic pain that disturbed 80% of the patients. Thus, the social burden of HME is very high and the quality of life could be strongly impaired by the disease.

Spinal Involvement

Over the past years, several papers highlighted a very rare involvement of the spine in HME patients. In 1995, Wiklund¹ showed a very low rate of patients who had spinal cord compressions (0.6%). More recently, however, Roach⁶⁶ reported that of the patients who were performed axial screening of the spine, 68% had a spinal exostosis involvement. Twenty-seven percent of those patients had lesions that impinged their spinal cord; a much higher percentage than expected. Previous analyses were based on X-ray screening and since this screening method hardly detects spinal exostoses, data about the involvement of the spine were missing. Other authors reported myelopathy in 15% of the patients who had spinal exostosis.⁶¹ Ashraf⁶⁷ showed similar results to those of Roach and added that 5% of the HME patients in his study had compressive intracanal osteochondromas. Since, several patients with minor trauma showed acute neurological syndromes linked to exostosis, the authors proposed a systematic spinal canal screening using axial images (MRI or CT). The authors also reported extracanal lesions that determine pain, dysphagia, sleep-apnea and intracanal lesions that determine paresthesia, myelopathy, general weakness and gait disturbances.

Malignant Transformation

Malignant degeneration has been reported as a rare entity.^25,52 Usually a differentiated chondrosarcoma onset on the cartilage cap is diagnosed; however, very rarely, dedifferentiated chondrosarcomas or osteosarcomas from the bone base could arise in HME patients. Clinical suspicion of malignant degeneration comes out when the cartilage cap is more than 1.5–2 cm or when a dimensional growth of the exostosis is reported in adults.^25,52 Malignant degeneration is reported to involve 2 to 4% of the patients affected by HME, with a risk of 0.1% per year in the 30–50 year-old population.¹⁴ Other reports showed that 0.9% of the patients who were diagnosed with chondrosarcoma and with a lifetime risk of 1–6% or 1–5% develop a malignant degeneration.^2,3 Sciubba⁶¹ reported a much higher risk of 12% in a patient followed for spinal exostosis and Kivioja found a chondrosarcoma degeneration risk of 9% in a family affected by HME.⁶⁸ Other authors in the past reported a 25% risk of malignant degeneration, but these data are considered unreliable.

Treatment

To the best of our knowledge, nowadays a medical treatment for HME does not exist. However, there are several papers suggesting that some particular molecules involved in various signaling pathways could represent viable therapeutic options for HME patients in the future. In particular, Huegel⁶⁹ showed in a mice model that a higher level of heparanase inhibitor (SST0001) in chondrocytes could lead to an inhibition of unregulated chondrogenesis. The presence of heparanase inhibitors could interfere with HS, BMPs and chondrogenesis pathways. This finding suggests that there are more target areas for therapeutic agents other than EXT1, EXT2 and EXT-like genes and products.^70,71 Inubushi⁷² showed that palovarotene (PVO), a selective retinoic acid receptor gamma agonist, investigated as a potential target treatment in fibrodysplasia ossificans progressiva (FOP),⁷³ could be useful in HME. This drug reduced the exostosis formation in mice with EXT1-EXT2 deletion, concluding that palovarotene could be a potential therapeutic agent for HME. This result also showed and remarked the overlaps in pathogenesis between HME and FOP. At the moment, in this scenario it is clear that treatment in HME patients is symptomatic and surgical. Exostosis removal is advised in two cases; when there is a strong suspicion of malignant transformation or when there are symptomatic exostoses. A third case is represented by the intracanal exostoses of the spine which have to be removed due to possible acute neurological syndromes that can derive from microtraumas and represent a severe danger for HME patients. Excision of an osteochondroma is usually an easy process, but in some particular locations (like the proximal femur), it could represent a very challenging operation, especially when there is a complex geometry of the lesion that encompasses nerves or vessel (Figures 2–4, Case #1). Surgical excision was carried out by removing the exostosis at the bone base, with consequential removal of the cartilage cap. Usually, when the cartilage cap is completely removed, there are no recurrences. However, complete removal in some regions could lead to the necessity of reconstruction techniques like bone grafting and internal fixations. Recurrences may be seen when wide excision is not possible. Surgery is also needed in cases with moderate to severe limb length discrepancies, severe limb malalignment or segment shortenings.⁷⁴ In 2007, Jager⁷⁵ reported no significant problems after exostosis removal in a cohort of 52 HME patients. However, Wirganowicz⁷⁶ reported a complication rate of 12.5% after exostosis removal in a cohort of 285 HME patients, with neurapraxia as the most common complication. Given the fact that sporadic and HME-related osteochondromas could spontaneously regress,^5,77,78 an aggressive surgical approach is not recommended. A spontaneous regression of the exostosis has been reported in one third of the HME patients. The best management in HME patients should be an accurate follow-up and a wait-and-see approach. Without considering spinal osteochondromas, osteochondromas from other anatomical regions should be treated only in symptomatic patients (Figures 5–10, Case #2). In case of a malignant transformation, an immediate excision is advised. In those cases, usually a wide excision is curative given the low metastatic potential of differentiated chondrosarcoma onset on osteochondroma (more common tumor arising from exostosis). Adjuvant and neoadjuvant therapies are not commonly used in these cases. In case of a dedifferentiated chondrosarcoma, excision is recommended, but radiotherapy and chemotherapy have a poor effect on this kind of malignancy.

Current Trends

In recent years, several techniques in the field of Pediatric Orthopedics have been applied to children affected by HME in order to improve their functional activity levels during adulthood. In particular, we can report knee and radius stapling correction techniques that have been proposed to correct the genu valgus and the wrist deformities associated to HME during the growth process. The first reports have promising results.^79,80 Fibular lengthening has also been evaluated in the prevention and initial correction of ankle valgus deformities and talus instability with optimal results.⁸¹ Valgus deformities of the proximal femur could be corrected by several approaches with variable results.⁸² Apart from the removal of symptomatic osteochondromas and correction of established deformities, preventive and less invasive surgeries for the growing children could be proposed in the future more often, given the good results of the first reports. In brief, we prefer not to proceed with exostosis removal, except in suspected cases of malignant transformation. When a “symptomatic exostosis” is diagnosed in cases with neurovascular structure entrapments or damages, we recommend a surgical removal of the lesion. When functional or esthetical problems are raised, we debate with patients and parents to choose the best option. When the prevention or correction of the HME-related deformities are to be addressed in the growing child, we prefer to refer the patient to an orthopedic pediatric center.

Conclusions

Hereditary multiple osteochondromas is a genetic disorder where the orthopedist, the geneticist and the pediatrician should all study and treat the disease. The complexity and unresolved issues related to HME are a challenge for researchers and clinicians even though a significant progress has been made over the last years.^83,84 The knowledge about mutational spectrum affecting the EXT genes, especially EXT1, is progressing, showing that these genes are vulnerable and each variant has to be evaluated to arrive at a real evidence of pathogenic relevance.²² The complex genetic mechanism of the interactions between cells and molecules in the growth plate is still being investigated. The role of HS deficiency is well recognized in HME genesis, but we believe that, in the next few years, new discoveries on HS pathways may lead to a medical treatment of this pathology. Furthermore, results from recent mice studies are promising.^35,85 An accurate follow-up by experts in sarcoma centers and appropriate surgical indications are nowadays mandatory to guarantee the best possible prognosis for HME patients who have generally a low quality of life and several disabilities due to their pathology.

Figure 1 Brief description of the HME pathogenesis.

Figure 2 T1-weighted axial MRI showing the osteochondroma at the proximal femur with sciatic nerve entrapment (Case #1).

Figure 3 Intraoperative image of the osteochondroma (Case #1).

Figure 4 Resected specimen (Case #1).

Figure 5 T1-weighted axial MRI showing the exostosis in the proximal humerus, complicated with a massive brachial artery pseudoaneurysm following microtrauma (Case #2).

Figure 6 MRI with a STIR sequence showing the pseudoaneurysm after drainage of the hematoma (Case #2).

Figure 7 A TOF MRI sequence showing the brachial artery pseudoaneurysm (Case #2).

Figure 8 Intraoperative photograph showing the brachial artery pseudoaneurysm (Case #2).

Figure 9 Clinical case 2. Pre-operative radiograph. In the yellow circle, the exostosis that had to be removed.

Figure 10 Clinical case 2. Post-operative radiograph.