Clinical characteristics.

Hereditary multiple osteochondromas (HMO) (also known as multiple hereditary exostoses [MHE]) is characterized by growths of multiple osteochondromas, benign cartilage-capped bone tumors that grow outward from the metaphyses of long bones. Osteochondromas can be associated with shortened stature, bony deformity, restricted joint motion, premature osteoarthrosis, and compression of peripheral nerves. The median age of diagnosis is three years; nearly all affected individuals are diagnosed by age 12 years. The risk for malignant transformation to osteochondrosarcoma increases with age, although the lifetime risk for malignant transformation is low (~2%-10%).

Diagnosis/testing.

The diagnosis of HMO is established in a proband with characteristic radiographic findings of multiple osteochondromas and/or a heterozygous pathogenic variant in EXT1 or EXT2 identified on molecular genetic testing.

Management.

Treatment of manifestations: Painful lesions in the absence of bone deformity may be treated with surgical excision that includes the cartilage cap and overlying perichondrium to prevent recurrence; forearm deformity may be treated with excision of the osteochondromas, corrective osteotomies, and/or ulnar-lengthening procedures; angular misalignment of the lower limbs may be treated with hemiepiphysiodeses (or osteotomies) at the distal femur, proximal tibia, or distal tibia; leg length inequalities may be treated with epiphysiodesis (growth plate arrest) of the longer leg; early treatment of ankle deformity may prevent or decrease later deterioration of function; sarcomatous degeneration is treated by surgical resection.

Surveillance: Clinical assessment for deformity, motor impairment, pain, neurologic manifestations, and/or other clinical manifestations can be considered; there is currently no accepted timeline for surveillance. Monitoring of the size of osteochondromas in adults may aid in early identification of malignant transformation, but no cost-benefit analyses are available to support routine surveillance. Some recommend a screening spine MRI in childhood to identify spinal lesions that may cause pressure on the spinal cord and would warrant close clinical follow up with excision of lesions that cause spinal cord impingement and/or symptoms; others recommend against screening spine MRI except in those with neurologic compromise or osteochondroma(s) of the ribs or pelvis. To date, there are no prospective studies to show benefit of systematic screening MRI in asymptomatic individuals.

Genetic counseling.

HMO is inherited in an autosomal dominant manner. Approximately 90% of individuals diagnosed with HMO have an affected parent; approximately 10% of individuals have the disorder as the result of a de novo pathogenic variant. Each child of an individual with HMO has a 50% chance of inheriting an HMO-causing pathogenic variant. If the HMO-causing pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Suggestive Findings

HMO should be suspected in individuals with the following radiographic features and/or family history.

• Multiple osteochondromas (cartilage-capped bony growths) arising from the area of the growth plate in the juxtaphyseal region of long bones or from the surface of flat bones (e.g., the scapula)
  • The key radiographic and anatomic feature of an osteochondroma is the uninterrupted flow of cortex and medullary bone (medullary continuity) from the host bone into the osteochondroma.
  • Osteochondromas possess the equivalent of a growth plate that ossifies and closes with the onset of skeletal maturity.
  • Approximately 70% of affected individuals have a clinically apparent osteochondroma about the knee, suggesting that radiographs of the knees to detect non-palpable osteochondromas may be a sensitive way to detect mildly affected individuals.
• Family history consistent with autosomal dominant inheritance (e.g., affected males and females in multiple generations). Absence of a known family history does not preclude the diagnosis; approximately 10% of affected individuals have no family history of multiple osteochondromas.

Establishing the Diagnosis

The diagnosis of HMO is established in a proband with characteristic radiographic features (see Suggestive Findings) and/or a heterozygous pathogenic (or likely pathogenic) variant in EXT1 or EXT2 identified by molecular genetic testing (see Table 1).

Note: (1) Per American College of Medical Genetics and Genomics / Association for Molecular Pathology variant interpretation guidelines, the terms "pathogenic variant" and "likely pathogenic variant" are synonymous in a clinical setting, meaning that both are considered diagnostic and can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this GeneReview is understood to include likely pathogenic variants. (2) Identification of a heterozygous EXT1 or EXT2 variant of uncertain significance does not establish or rule out the diagnosis.

Molecular genetic testing approaches can include a combination of gene-targeted testing (concurrent single-gene testing, multigene panel) and comprehensive genomic testing (exome sequencing, genome sequencing). Gene-targeted testing requires that the clinician determine which gene(s) are likely involved (see Option 1), whereas comprehensive genomic testing does not (see Option 2).

Clinical Description

Hereditary multiple osteochondromas (HMO) (also known as multiple hereditary exostoses [MHE]) is characterized by growths of multiple osteochondromas. Shortened stature compared to unaffected family members and angular deformities of the forearms and legs are common. The risk for malignant degeneration to osteochondrosarcoma increases with age, although the lifetime risk for malignant degeneration is low (~2%-10%). To date, more than 1,000 individuals with a pathogenic variant in EXT1 or EXT2 have been identified. The following description of the phenotypic features associated with this condition is based on these reports.

Onset. The proportion of individuals with HMO who have clinical findings increases from approximately 5% at birth to 96% at age 12 years [Legeai-Mallet et al 1997]. The median age at diagnosis is three years.

Osteochondromas. The number of osteochondromas varies widely even within families. Involvement is usually symmetric. The most commonly involved bones are the femur (30%), radius and ulna (13%), tibia (20%), and fibula (13%). Anatomic distribution and number of osteochondromas depends on genotype and sex of the affected person [Clement & Porter 2014b]. Osteochondromas grow in size and gradually ossify during skeletal development and stop growing with skeletal maturity, after which no new osteochondromas develop. One case report also suggests the possibility of regression of these lesions and symptomatic to asymptomatic transition with maturity. This supports the utilization of conservative management prior to skeletal maturity even for transiently symptomatic lesions [Ikeda et al 2025]. However, there is an overall lack of evidence that regression should be the expected outcome and families should be counseled appropriately.

Osteochondromas typically arise in the juxtaphyseal region of long bones and from the surface of flat bones (pelvis, scapula). An osteochondroma may be sessile or pedunculated. Sessile osteochondromas have a broad-based attachment to the cortex. The pedunculated variants have a pedicle arising from the cortex that is usually directed away from the adjacent growth plate. The pedunculated form is more likely to irritate overlying soft tissue, such as tendons, and compress peripheral nerves or blood vessels. The marrow and cancellous bone of the host bone are continuous with the osteochondroma.

Shortened stature. It has been stated that at least 40% of individuals with HMO have shortened stature (stature shorter than predicted based on the heights of unaffected parents and sibs). Although interference with the linear growth of the long bones of the leg often results in reduction of predicted adult height, the height of most adults with EXT2-related HMO and many with EXT1-related HMO falls within the normal range [Porter et al 2004]. Shortened stature is more pronounced in persons with EXT1-related HMO [Pedrini et al 2011, Clement et al 2012, Li et al 2017]. Height has been found to be directly proportional to leg length, and in many individuals with EXT1- and EXT2-related HMO, height is below the 10th centile [Li et al 2017]. Multivariate analysis determined that the presence of a distal femoral osteochondroma was an independent predictor of knee deformity, diminished knee joint range of motion, and short stature [Clement & Porter 2014a].

Bone deformity. Abnormal bone remodeling may result in shortening and bowing with widened metaphyses [Porter et al 2004]. Hand deformity resulting from shortened metacarpals is common. Abnormal growth and development of the forearm and leg in untreated individuals with HMO is common, including both proportionate and disproportionate shortening of the two bones of the forearm or leg, producing shortened and angulated limbs, respectively. In a study of 46 kindreds in Washington State, United States, 39% of individuals had a deformity of the forearm, 10% had an inequality in limb length, 8% had an angular deformity of the knee, and 2% had a deformity of the ankle [Schmale et al 1994]. Angular deformities (bowing) of the forearm and/or ankle are the most clinically significant orthopedic issues [Shin et al 2006]. Forearm deformities [Masada et al 1989, Jo et al 2017] and ankle deformities [Ahn et al 2019] have previously been classified. The classification reported by Ahn et al [2019] is the most widespread and accepted for HMO lower leg deformities. A combination of the Masada et al [1989] and Jo et al [2017] classifications are typically used for forearms with a preference for the Jo et al [2017] classification due to better reliability [Farr et al 2021]. Additionally, individuals with HMO and associated ankle deformities were reported to have up to 19% risk of early secondary arthritis by age 42 years [Noonan et al 2002]. A novel classification system has since been developed for forearm abnormalities caused by HMO that stratifies these deformities into three groups: high, moderate, and low risk of radial head dislocation. Individuals with distal ulnar lesions were classified as high or moderate risk based on radiographic parameter of proportional length (ulnar-to-radial length ratio), and those without distal ulnar lesions were considered low risk [Chan et al 2025].

Hip dysplasia frequently results from osteochondromas of the proximal femur and coxa valga. Decreased center-edge angles and increased uncovering of the femoral heads may lead to early thigh pain and abductor weakness and subsequent early arthritis [Makhdom et al 2014, Wang et al 2015]. Femoral-acetabular impingement may also arise from proximal femoral osteochondromas, limiting hip motion [Viala et al 2012, Higuchi et al 2016, Duque Orozco et al 2018].

Symptoms secondary to mass effect. Compression or stretching of peripheral nerves usually causes pain but may also cause sensory or motor deficits [Göçmen et al 2014, Onan et al 2014, Payne et al 2016]. Spinal cord compression and myelopathy from cervical osteochondromas have been reported [Aldea et al 2006, Giudicissi-Filho et al 2006, Pandya et al 2006, Ashraf et al 2013, Veeravagu et al 2017, Akhaddar et al 2018, Gigi et al 2019, Montgomery et al 2019], as has dysphagia from a cervical osteochondroma [Gulati et al 2013]. Bilateral inferior cervical osteochondromas have been found to produce neurogenic and vascular thoracic outlet syndrome [Abdolrazaghi et al 2018]. Syringomyelia and tethered cord / fibrolipoma in individuals undergoing screening imaging without evidence of spinal osteochondromas have also been described [Legare et al 2016]. Mechanical obstruction of joint motion may result from large osteochondromas impinging on the adjacent bone of a joint. Overlying muscles and tendons may be irritated or entrapped, resulting in pain and loss of motion [Andrews et al 2019]. Nerves and vessels may be displaced from their normal anatomic course, complicating attempts at surgical removal of osteochondromas. Rarely, urinary or intestinal obstruction results from large pelvic osteochondromas. Thoracic osteochondromas have been reported to lead to diaphragmatic rupture [Abdullah et al 2006], pneumothorax [Chawla et al 2013, Imai et al 2014, Dumazet et al 2018], hemothorax [Yoon et al 2015, Lin et al 2017], coronary artery compression [Rodrigues et al 2015], and severe chest pain [Kanthasamy et al 2020]. Osteochondromas have led to pseudoaneurysms [Oljaca et al 2019, Iqbal et al 2020] that can mimic sarcoma. Biopsy of a misdiagnosed pseudoaneurysm can have life-threatening consequences [Iqbal et al 2020].

Scoliosis. Some authors have documented scoliosis caused by or in the setting of osteochondromas with a prevalence of 72% [Matsumoto et al 2015, Veeravagu et al 2017, Gigi et al 2019]. The mean main curve was 15.3 degrees and minor curve 10.6 degrees. Despite the documented prevalence, a curve necessitating surgical intervention is uncommon.

Chondrosarcoma. The most serious sequela of HMO is malignant transformation of an osteochondroma. Axial sites such as the pelvis, scapula, ribs, and spine are more commonly the location of transformation of osteochondromas to chondrosarcoma [Porter et al 2004]. Rapid growth and increasing pain, especially in a physically mature person, are signs of sarcomatous transformation, a potentially life-threatening condition.

A bulky cartilage cap (best visualized with MRI or CT) thicker than 2-3 cm is highly suggestive of chondrosarcoma [Shah et al 2007, Bernard et al 2010]. After skeletal maturity, increased radionucleotide uptake on serial technetium bone scans may also be evidence of malignancy. High metabolic activity in the cartilage as evidenced by uptake of gadolinium on T₂-weighted MRI may also be indicative of malignancy [De Beuckeleer et al 1996]. FDG-PET imaging may be useful in the workup for malignant transformation in HMO. A standardized uptake value maximum (SUV_max) of 2 has been reported as the cutoff above which chondrosarcomatous transformation of an osteochondroma has likely occurred, although lesions with an SUV_max as low as 1.3 have been found in grade I chondrosarcoma [Aoki et al 1999, Feldman et al 2005, Purandare et al 2019].

The incidence of malignant transformation to chondrosarcoma, or less commonly to other sarcomas, is estimated at 2%-10%. In a large cohort of 529 affected individuals, the rate of malignant transformation was calculated to be 5% [Pedrini et al 2011]. A survey of an international heterogeneous cohort of 757 individuals with HMO revealed 21 (2.7%) with malignant transformation, with pelvis and scapula the most common sites of malignant change from benign osteochondromas [Czajka & DiCaprio 2015]. However, rates as high as 10% have been reported in individuals with HMO [Tepelenis et al 2021].

Malignant transformation can occur during childhood or adolescence, but the risk increases with age [Schmale et al 2010]. Based on a study of HMO in Washington State, United States, it was estimated that HMO may increase the risk of developing a chondrosarcoma by a factor of 1,000-2,500 over the risk for individuals without HMO.

Other

• Vascular abnormalities. In addition to vascular abnormalities from osteochondromas close to vascular structures, an association between osteochondromas and venous malformations has been reported [Albokhari et al 2023].
• Leukemia. There have been reports of a possible role of EXT1 and EXT2 in leukemogenesis through several mechanisms. To date, three individuals have been reported with HMO and leukemia [Comisi et al 2025].
• Bursal formation between osteochondromas and surrounding tissues occurs in at least 1.5% of individuals with HMO and can result in painful mass formation that tends to increase with size. This is most frequent in lesions of the scapula and shoulder joint [Murphey et al 2000]. It is important to differentiate symptomatic bursal formation from malignant transformation when an individual presents with new worsening pain.

Prognosis. Males tend to be more severely affected than females [Pedrini et al 2011]. Although pain is commonly reported, most individuals with HMO lead active, healthy lives.

Phenotype Correlations by Gene

Most studies have identified a higher burden of disease in persons with EXT1 pathogenic variants than in those with EXT2 pathogenic variants, including greater numbers of osteochondromas, more severe skeletal deformity, and shorter stature [Porter et al 2004, Pedrini et al 2011, Li et al 2017]. Pedrini et al [2011] suggested a clinical classification system based on the presence or absence of deformities and functional limitations (adapted in Mordenti et al [2013]). In 529 individuals with HMO a more severe phenotype was associated with pathogenic variants in EXT1 and male sex. The risk for chondrosarcoma may also be higher in individuals with an EXT1 pathogenic variant [Porter et al 2004], although this was not found in all studies [Pedrini et al 2011].

Genotype-Phenotype Correlations

No clinically relevant genotype-phenotype correlations for EXT1 or EXT2 have been identified.

Penetrance

The penetrance is estimated to be 96% in females and 100% in males [Schmale et al 1994]. Most published instances of reduced penetrance have occurred in females. However, comprehensive skeletal radiographs have not been performed in most of these instances.

Nomenclature

"Multiple osteocartilaginous exostoses" was used to convey the observation that the growths are composed primarily of cartilage in the child and ossify as skeletal maturity is reached.

In the United States, the terms "exostosis" and "hereditary multiple exostoses" have been used to denote the growths and the disorder, but the World Health Organization (WHO) has selected the nomenclature "osteochondromas" for exostoses and "multiple osteochondromas" for the disorder [World Health Organization 2020, Choi & Ro 2021]. These latter terms are preferable as they more precisely describe the lesions as cartilaginous in origin. However, "hereditary multiple exostoses" (HME) and "multiple hereditary exostoses" (MHE) are still frequently used to refer to this disorder.

EXT1 and EXT2 may be referred to as exostosin-1 and exostosin-2, respectively.

Prevalence

The reported prevalence of HMO ranges from as high as one in 100 in a small population in Guam to approximately one in 100,000 in European populations [Krooth et al 1961, Hennekam 1991]. The prevalence has been estimated to be at least one in 50,000 in Washington State [Schmale et al 1994]. These prevalence reports are felt to be an underestimation since many individuals may be asymptomatic.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with HMO, the evaluations summarized in Table 4 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Treatment of Manifestations

Supportive care to improve quality of life, maximize function, and reduce complications is recommended. This ideally involves multidisciplinary care by specialists in relevant fields (see Table 5).

Surveillance

To monitor existing manifestations, the individual's response to supportive care, and the emergence of new manifestations, the evaluations summarized in Table 6 are recommended.

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

A clinical trial of the retinoic acid receptor gamma agonist palovarotene for individuals younger than age 14 years with HMO was terminated secondary to concerns for early growth plate closure in a study utilizing the same drug for fibrodysplasia dissecans progressiva.

Roneparstat, a potent low-molecular-weight heparin derivative and inhibitor of enzyme heparinase, has also been investigated, as preclinical studies have shown that the drug can inhibit chondrogenesis, which is the initial step of osteochondroma development. However, no clinical studies have occurred.

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Mode of Inheritance

Hereditary multiple osteochondromas (HMO) (also known as multiple hereditary exostoses [MHE]) is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• Approximately 90% of individuals diagnosed with HMO have an affected parent.
• Approximately 10% of individuals diagnosed with HMO have the disorder as the result of a de novo pathogenic variant.
• If the proband appears to be the only affected family member (i.e., a simplex case), recommendations for the evaluation of the parents of the proband include physical examination, radiographs, and/or molecular genetic testing (if the causative pathogenic variant has been identified in the proband) to evaluate their clinical/genetic status and inform recurrence risk assessment. Note: A proband may appear to be the only affected family member because of failure to recognize the disorder in family members or reduced penetrance. Therefore, de novo occurrence of an EXT1 or EXT2 pathogenic variant cannot be confirmed unless molecular genetic testing has demonstrated that neither parent is heterozygous for the pathogenic variant identified in the proband.
• If the proband has a known EXT1 or EXT2 pathogenic variant that is not identified in either parent and parental identity testing has confirmed biological maternity and paternity, the following possibilities should be considered:
  • The proband has a de novo pathogenic variant.
  • The proband inherited a pathogenic variant from a parent with gonadal (or somatic and gonadal) mosaicism [Szuhai et al 2011, Sarrión et al 2013].* Note: Testing of parental leukocyte DNA may not detect all instances of somatic mosaicism and will not detect a pathogenic variant that is present in the germ (gonadal) cells only.
    * A parent with somatic and gonadal mosaicism for an EXT1 or EXT2 pathogenic variant may be mildly/minimally affected.

Sibs of a proband. The risk to sibs depends on the clinical/genetic status of the parents:

• If one parent of the proband is affected and/or is known to have the pathogenic variant identified in the proband, the risk to the sibs is 50%.
• If the proband has a known EXT1 or EXT2 pathogenic variant that cannot be detected in the leukocyte DNA of either parent, the recurrence risk to sibs is slightly greater than that of the general population because of the possibility of parental gonadal mosaicism [Szuhai et al 2011, Sarrión et al 2013].
• If the parents have not undergone molecular genetic testing but are clinically unaffected, sibs are still presumed to be at increased risk for HMO because of the possibility of reduced penetrance in a heterozygous parent and the possibility of parental gonadal mosaicism.

Offspring of a proband

• Each child of an individual with HMO has a 50% chance of inheriting an HMO-causing pathogenic variant.
• If the reproductive partner of an individual with HMO also has HMO (i.e., both parents are affected), each child has a 50% chance of inheriting an HMO-causing pathogenic variant from their mother and a 50% chance of inheriting an HMO-causing pathogenic variant from their father; the chance that offspring will inherit at least one pathogenic variant is 75%. Note: Offspring who inherit biallelic pathogenic variants in EXT2 are at risk for AREXT2 syndrome (see Genetically Related Disorders).

Other family members. The risk to other family members depends on the status of the proband's parents: if a parent has the pathogenic variant, the parent's family members may be at risk.

Related Genetic Counseling Issues

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown). For more information, see Huang et al [2022].

Prenatal Testing and Preimplantation Genetic Testing

If the HMO-causing pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Differences in perspective may exist among medical professionals and families regarding the use of prenatal and preimplantation genetic testing. While most health care professionals would consider use of prenatal and preimplantation genetic testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

EXT1 and EXT2 encode for transmembrane glycoproteins that together form a heterooligomeric heparan sulfate polymerase; the protein product also participates in cell signaling and chondrocyte proliferation and differentiation [Pacifici 2018]. Pathogenic variants in EXT1 or EXT2 cause cytoskeletal abnormalities that include actin accumulation, excessive bundling by alpha-actinin, and abnormal presence of muscle-specific alpha-actin [Bernard et al 2000].

Osteochondroma mouse models have shown that complete biallelic inactivation of Ext1 in a small fraction of chondrocytes is sufficient for the development of osteochondromas and other skeletal defects associated with hereditary multiple osteochondromas (HMO), and osteochondromas are composed of a mixture of EXT^+/- and EXT^-/- cells [Jones et al 2010, Matsumoto et al 2010, Pacifici 2018].

Mechanism of disease causation. Loss of function

Acknowledgments

The authors have not received financial support for the research, authorship, and/or publication of this chapter. We thank GeneReviews for allowing us to contribute in the form of this chapter.

Author History

Howard A Chansky, MD; University of Washington (2000-2026)
Nathan Donaldson, MD (2026-present)
Dawn Earl, ARNP (2026-present)
Wendy H Raskind, MD, PhD; University of Washington (2000-2026)
Gregory A Schmale, MD; University of Washington (2000-2026)
Ryan Sefcik, MD (2026-present)
Steven Thorpe, MD (2026-present)
Wim Wuyts, PhD; University Hospital of Antwerp (2000-2026)

Revision History

• 29 January 2026 (sw) Comprehensive updated posted live
• 6 August 2020 (sw) Comprehensive updated posted live
• 21 November 2013 (me) Comprehensive update posted live
• 5 September 2008 (me) Comprehensive update posted live
• 20 September 2005 (me) Comprehensive update posted live
• 2 July 2003 (me) Comprehensive update posted live
• 3 August 2000 (me) Review posted live
• 22 March 2000 (hc) Original submission

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