Solitary Osteochondroma

Updated: Jun 13, 2017
  • Author: Ian D Dickey, MD, FRCSC; Chief Editor: Harris Gellman, MD  more...
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Overview

Background

Osteochondroma is a benign, cartilaginous neoplasm that is found in any bone that undergoes enchondral bone formation in its development. The World Health Organization (WHO) defines osteochondroma as a cartilage-capped bony projection on the external surface of a bone. [1] It is found most commonly around the knee and the proximal humerus; however, it can occur in any bone.

Osteochondroma is the most common benign bone tumor. Osteochondromas account for approximately 35% of benign bone tumors and 9% of all bone tumors. Most are asymptomatic, but they can cause mechanical symptoms depending on their location and size.

As benign lesions, osteochondromas have no propensity for metastasis. In fewer than 1% of solitary osteochondromas, malignant degeneration of the cartilage cap into secondary chondrosarcoma has been described and is usually heralded by new onset of growth of the lesion, new onset of pain, or rapid growth of the lesion. [2, 3, 4]

Historically and currently, most osteochondromas are incidental findings and are treated solely with observation. If they remain asymptomatic, they can be ignored. Lesions that create mechanical symptoms, become painful, begin to enlarge, or cause growth disturbance have historically been treated with surgical removal, and this remains the mainstay of treatment. Biologic therapies for osteochondromas may be possible in the future.

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Anatomy

Osteochondromas can occur in many different locations in the body. Thus, a complete understanding of local anatomy is paramount to ensure that local structures are not harmed during surgical resection. Because these lesions arise from the metaphysis, particular care must be taken to avoid damage to the growth plate in the skeletally immature patient.

The osteochondroma may have a stalk and be defined as pedunculated, or it may have a broad base of attachment and be considered sessile. Whether the neoplasm is sessile or pedunculated, the medullary canal of the stalk and the bone are in continuity by definition. [5]

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Pathophysiology

Osteochondroma is a hamartoma, and patients most commonly present in the second decade of life. Osteochondromas are located adjacent to growth plates and develop away from the growth plate with time because they are essentially isolated growth plates. They are affected by, and respond to, various growth factors and hormones in the same manner as epiphyseal growth plates.

Osteochondromas grow until skeletal maturity; growth generally stops once the growth plates fuse. [6]  Slow growth from the cap may continue over time, as described by Virchow, but this usually stops by age 30 years.

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Etiology

Although the exact etiology of these growths is not known, a peripheral portion of the physis is thought to herniate from the growth plate. [7] This herniation may be idiopathic or may be the result of trauma or a perichondrial ring deficiency. Whatever the cause, the result is an abnormal extension of metaplastic cartilage that responds to the factors that stimulate the growth plate and thus results in exostosis growth.

This island of cartilage organizes into a structure similar to the epiphysis (see Histologic Findings). As this metaplastic cartilage is stimulated, enchondral bone formation occurs, developing a bony stalk. The histology of the cartilage cap reflects the classic defined zones observed in the growth plate—namely, proliferation, columniation, hypertrophy, calcification, and ossification.

This theory is thought to explain the classic finding of the osteochondroma associated with a growth plate and growing away from the physis while maintaining its medullary continuity. The theory is also thought to explain the clinical behavior of the exostosis growing only until skeletal maturity.

Genetic karyotyping has suggested that reproducible genetic abnormalities are associated with these benign growths and that they may actually represent a true neoplastic process, not a reactive one. [8, 9] This research is in the early stages, and further investigation is necessary. [10, 11, 12, 13]

Heinritz et al reported on the clinical findings and results of molecular analyses of the EXT1 and EXT2 genes—mutations of which lead to multiple osteochondroma—in 23 patients. [14] In 17 of the 23 patients, novel pathogenic mutations were identified (11 in the EXT1 gene; six in the EXT2 gene). According to the authors, findings of this study extend the mutational spectrum and understanding of the pathogenic effects of EXT1 and EXT2 mutations.

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Epidemiology

Osteochondromas are the most common benign bone tumors, representing 35% of all benign tumors and 9% of all bone tumors. However, their actual frequency is not known, because many osteochondromas are not diagnosed. Osteochondromas can occur in any bone that undergoes enchondral bone formation, but they are most common around the knee. Most are diagnosed in patients younger than 20 years (see the image below). A marked predilection for males exists (male-to-female ratio, 3:1). [15]

Solitary osteochondroma. Anatomic and age distribu Solitary osteochondroma. Anatomic and age distribution of solitary osteochondromas.
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Prognosis

For solitary osteochondromas, the outcome and prognosis after surgery are excellent, with excellent local control and a local recurrence rate of less than 2%. [16, 17, 18, 19]  The process is a benign one; thus, the prognosis is usually one of complete recovery. Poorer outcomes usually are related to the morbidity associated with the exposure required to remove the lesion or associated with secondary bone deformity, but the latter is usually observed in the multiple hereditary form of the disease. [20, 21, 22, 23, 24, 25, 26]

Florez et al performed a retrospective study of 113 solitary osteochondromas (most frequently located in the distal femur) that were treated between 1970 and 2002. Six patients had a recurrence after treatment, and in two patients, the lesions became malignant and developed into chondrosarcoma. The authors noted that relapse of the exostosis is rare, occurring in approximately 2% of resections, and that growth of an osteochondroma or the presence of pain in older patients suggests a possible malignancy. [17]

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