Extraskeletal para-articular osteochondroma adjacent to the cervical spine: illustrative case

Abstract

BACKGROUND

Extraskeletal osteochondromas are well-circumscribed osteocartilaginous lesions arising from soft tissues without bone continuity. Extraskeletal osteochondromas may present in a para-articular location, although few reported cases have occurred near the spine. Clinical diagnosis remains challenging as these tumors can be difficult to distinguish from other ossified soft tissue lesions. Treatment includes management by observation or resection. Here the authors present a rare case of an extraskeletal osteochondroma near the cervical spine.

OBSERVATIONS

A 56-year-old male presented with a palpable left paraspinal suboccipital mass that had slowly progressed over 5 years. The patient was asymptomatic and the neurological examination was nonfocal. MRI revealed an approximately 6-cm well-circumscribed heterogeneous mass in the left suboccipital area that lacked direct contact with the spine or calvarium. A hypointense capsule on T2-weighted MRI and multiple hypointense septations on T1- and T2-weighted MRI were identified. The slow rate of tumor growth suggested a benign tumor. CT imaging of the chest, abdomen, and pelvis was obtained to rule out malignancy. Surgical intervention was offered and accepted by the patient. At 2 months postoperatively, imaging demonstrated no recurrence, and the patient was asymptomatic and had improved range of motion.

LESSONS

Extraskeletal osteochondroma should be considered when diagnosing osteocartilaginous paraspinal masses.

https://thejns.org/doi/10.3171/CASE25468

Osteochondromas are a benign lesion that typically present with a bony external protrusion surrounded by a cartilage cap. They account for 20%–50% of benign bone tumors and 9% of all bone tumors.¹ The most common site of occurrence for an osteochondroma is the long bones, although 1%–9% of reported cases are found on the axial skeleton.^2,3 A rarer subset of axial osteochondromas presents as extraskeletal lesions, which are well circumscribed and not in direct contact with bone.^4,5 Diagnosis of osteochondromas at an atypical site through imaging is a challenge as typical imaging findings of osteochondromas, namely a bony outgrowth with a cartilaginous cap, are absent in this atypical presentation, making them difficult to distinguish from other pathologies occurring in the area.² Spinal osteochondromas, especially those located in the cervical or thoracic regions, can be easily misinterpreted on imaging due to their proximity to other structures such as nerve roots, muscles, and blood vessels. These lesions may also appear as soft tissue masses on MRI by displaying features like peripheral enhancement that can mimic other conditions such as soft tissue tumors, hematomas, or abscesses and complicate diagnosis by imaging.¹ Treatment of osteochondromas typically involves either observation or surgery, with pain, cosmesis, high risk for malignant transformation, and diagnostic uncertainty all being indications for resection.¹

The pathogenesis of extraskeletal osteochondromas is not fully understood.⁶ Proposed mechanisms include metaplasia of connective tissue or mesenchymal cells, which may lead to the development of these extraskeletal osteochondromas.^5,6 Additionally, repeat trauma has also been suggested as a contributing factor, due to its ability to stimulate metaplastic changes in connective tissue or mesenchymal cells, increasing the likelihood of extraskeletal osteochondroma formation.^5,6 Although primarily benign, the location of these lesions near critical anatomical structures, such as the cervical spine, can complicate diagnosis and management. Extraskeletal osteochondromas near the spine are sparsely reported, with only 4 cases described in the literature. In this report, we present a rare case of extraskeletal osteochondroma occurring adjacent to the cervical spine and treated with resection.

Illustrative Case

A 56-year-old male with no pertinent past medical history presented to our institution with a left paraspinal suboccipital mass that slowly enlarged over the prior 5 years. The patient reported that the mass had become more palpable, less fluctuant, and was causing greater discomfort while sleeping and lying down. The patient denied pain, tenderness, paresthesia, headaches, difficulty ambulating, neurological deficits, or trauma to the area. His neurological examination was normal and showed he was alert and oriented, had intact extra-ocular movements and facial symmetry, and had full motor strength both distally and proximally.

MRI of the cervical spine demonstrated an approximately 6-cm well-circumscribed heterogeneous peripherally enhancing mass in the left suboccipital area displacing the paraspinal muscles without invasion into the lamina of the bone or the suboccipital area of the spine or skull base. The lesion was well circumscribed and had a T2 hypointense capsule and multiple T1 postcontrast and T2 hypointense septations (continuous with the midportion of the left rectus capitis posterior muscle), as seen in Fig. 1. There was extensive susceptibility signal throughout the mass and in particular peripheral enhancing components, thus supporting its high vascularity. No CT or radiographic imaging was performed for this patient.

FIG. 1.

A: T2-weighted sagittal MR sequence showing T2 hyperintense parasagittal mass. B: T2-weighted coronal MR sequence showing T2 hypointense capsule and multiple T1/T2 hypointense septations continuous with the midportion of the left rectus capitis posterior muscle. C: T1-weighted contrast sagittal MR sequence showing heterogeneously enhancing mass with peripherally enhancing components. D: T1-weighted contrast coronal MR sequence redemonstrating heterogeneously enhancing mass with enhancement of the C3 spinous process.

The slow mass growth over the preceding 5 years suggested a benign tumor. To rule out other sources of malignancy, CT imaging of the chest, abdomen, and pelvis was performed. Resection was offered due to the long-standing nature of the mass, the patient’s discomfort when lying down, and the diagnostic utility of tissue collection. The risks and benefits were discussed with the patient, who elected to proceed with resection.

During the procedure, a Mayfield cranial clamp was used to immobilize the cervical spine. The patient was placed prone and the neck flexed. Following incision, electrocautery was used to expose the C5 spinous process, which was below the inferior-most aspect of the mass. Anatomical localization was conducted with fluoroscopy (Fig. 2). The fascia and muscle were separated from the inion to C6, revealing a calcified and immobile mass localized lateral to the C4 spinous process. The mass was circumferentially dissected from underlying muscle tissue and removed en bloc via gross-total resection. A portion of the mass was sent as a frozen specimen for pathological analysis. The incision was closed using a muscle advancement flap closure. The patient tolerated the procedure well with no complications occurring. The patient was extubated in the operating room, taken to the postanesthesia care unit for further monitoring, and eventually transferred to the floor. He was provided a figure-of-eight brace and discharged on postoperative day 1.

FIG. 2.

Intraoperative sagittal fluoroscopic image demonstrating the mass in close proximity to the cervical spine.

Grossly, a well-defined 7.3 × 5.3 × 4.8–cm bony mass was seen with a thin rim of surrounding soft tissue. Histopathological examination of the tumor showed a well-circumscribed proliferation of trabecular bone with a peripheral cartilaginous cap, as seen in Fig. 3. Enchondral ossification was present at the interface between the cartilaginous and osseous components. Neither component demonstrated any cytological atypia or mitotic activity. The tumor was surrounded by soft tissue with no identifiable communication with the underlying vertebra. These findings on pathology were characteristic of an osteochondroma. Given imaging showing a joint-sparing mass and the aforementioned histological findings, a diagnosis of extraskeletal osteochondroma was made.

FIG. 3.

A: The tumor (right) exhibits a well-demarcated border (arrows) from the adjacent soft tissues (left) (H&E, total magnification ×40). B: The tumor consists of a peripheral cartilaginous cap (black arrow), a central region of mature trabecular bone (blue arrow), and enchondral ossification at the interface between the two components (red arrow) (H&E, total magnification ×100). C: The chondrocytes within the cartilage cap are cytologically bland without atypia or mitotic activity (H&E, total magnification ×200). D: The trabecular bone similarly lacks cytological atypia or mitotic activity. Several more peripheral foci still show evidence of enchondral ossification (arrow) (H&E, total magnification ×200).

The patient was seen for follow-up 2 weeks after surgery and reported an increased range of motion in his neck.

Informed Consent

The necessary informed consent was obtained in this study.

Discussion

Observations

Etiology and Demographics

Osteochondromas are among the most common benign bone tumors and account for approximately 9% of all bone tumors.¹ Eighty-five percent of osteochondromas are solitary lesions, and 15% occur as multiple lesions resulting from hereditary multiple osteochondromas (HMOs).^1,7 HMOs result from mutations in the EXT1 (exostosis-1) or EXT2 (exostosis-2) gene and are an autosomal dominant genetic condition characterized by the development of multiple osteochondromas.⁷ Additionally, EXT1 mutations have also been identified in nonhereditary solitary lesions.⁸ Rarely, osteochondromas undergo malignant transformation into secondary chondrosarcoma.^9,10

Many osteochondromas are asymptomatic in their presentation, although compression of neural elements, fractures, bursa formation, or malignant transformation can result in symptoms such as pain, paresthesia, or bursitis.^1,11 Observation and surgery are the primary treatment options for osteochondroma.^1,3 Indications for resection include pain, cosmesis, osteochondromas at risk for malignant transformation, or an uncertain diagnosis.¹ The recurrence rate is estimated to be less than 2% following complete resection of the lesion.¹

Extraskeletal osteochondromas have a unique presentation relative to conventional osteochondromas and often occur in a para-articular location, as exemplified by the case described in the present report. Most reported extraskeletal osteochondromas are smaller than 2 cm.⁶ Typically occurring in adults without a history of trauma to the effected region, extraskeletal osteochondromas more frequently present in males and have a higher reported incidence in para-articular locations, including the hands, ankles, feet, and knees.^5,6 As a subset of extraskeletal osteochondromas, para-articular osteochondromas lack direct bony contact and are characterized by their close proximity to a joint. The concept of a para-articular osteochondroma was first introduced by Jaffe in 1958 in describing osteochondral metaplasia in a fibrous joint capsule or adjacent tissue.¹² Milgram and Dunn were the first to use the term “para-articular osteochondroma” and distinguish this diagnosis from intrasynovial osteochondromatosis, which is a neoplastic process resulting in many round cartilaginous loose bodies.^13,14 Reith et al. established the following criteria of a para-articular osteochondroma: 1) the lesion is radiographically and grossly a single mass; 2) the mass is composed of bone and cartilage, similar to the organization of osteochondroma on histopathology; and 3) the lesion occurs outside the synovium.¹⁵

On imaging, para-articular osteochondromas present as a well-circumscribed, mineralized oval or round mass lacking direct contact with bone.^4,16 On pathology, multiple osteochondral nodules are present, and each has a similar histological appearance to conventional osteochondromas.¹⁶ The nodules consist of lamellar, trabecular bone and are surrounded by a cartilaginous peripheral layer.¹⁶ Para-articular osteochondromas also have evidence of endochondral ossification at the intersection of the peripheral cartilage and the inner ossified areas.¹⁶ There may be focal chondrocyte atypia and hypercellularity in the cartilage.⁵ However, mitosis, infiltrative edges, isolated cartilage nodules in surrounding tissues, and other characteristic features of malignancy are absent.⁵

Recurrence and malignant transformation are rare outcomes following resection of an osteochondroma. These outcomes have not been identified in extraskeletal osteochondromas occurring near the spine, although rare instances of such complications in traditional osteochondromas do exist in the literature. Following complete resection of an osteochondroma, the local recurrence rate is reported to be less than 2%.¹ A cartilage cap thickness greater than 2 cm in adults or 3 cm in children, rapid mass growth, pain, elevated temperature, and local erythema have each been identified as potential indications of sarcomatous transformation of a previously asymptomatic osteochondroma.^1,2 Sarcomatous transformation occurs in around 10% of patients with HMO lesions compared to 1% of those with solitary lesions.¹ Should malignant transformation be identified, treatment is typically wide resection.²

Literature Review

Extraskeletal osteochondromas presenting near the spine are a rare diagnosis, as demonstrated through a review of the literature. A PubMed search using the term “extraskeletal osteochondroma” yielded 2 reported cases of extraskeletal osteochondroma in the posterior neck.^4,6 Both cases occurred in the cervical region of the neck and were surgically resected, with one case being a 66-year-old female and the other being a 42-year-old male.^4,6 A second PubMed search using the term “paraarticular osteochondroma” yielded 2 additional reported cases in the paraspinal area. In these 2 cases, a para-articular osteochondroma adjacent to a lumbar facet joint in a 55-year-old male and a para-articular osteochondroma adjacent to a cervicothoracic facet joint in a 69-year-old male were surgically resected.^16,17 On histological examination, an interior comprised of trabecular bone and surrounded by a hyaline cartilage cap was present in all 4 cases.^4,6,16,17 Outcomes across these reported cases have been excellent, as no recurrences or malignant transformations of extraskeletal osteochondromas near the spine have been reported in the literature. A summary of cases including the one described in this report can be found in Table 1.

TABLE 1.

Literature review and epidemiological data of reported extraskeletal osteochondromas occurring near the spine

Authors & Year	Sample Size	Sex	Age (yrs)	Presenting Sx	Tumor Location	Tumor Size (cm)	Treatments	Complications	Recurrence	FU Time (mos)
Lea et al., 20224	1	F	66	Round, movable, nontender, protruding mass	Posterior neck, from suboccipital region to C5	7.3 × 4.7 × 4.0	Direct excision	None	NR	NR
Singh et al., 20066	1	M	42	Localized pain, worsened by neck movement	Posterior to C2–5 spinous processes	3.5 × 3.0	Direct excision	None	No	36
Okamoto et al., 201116	1	M	69	Bilat lower extremity paresthesia & muscle weakness, gait disturbance, positive Babinski sign bilaterally	Medial to rt C7–T1 facet joint, dorsolat to spinal cord	1.5 × 0.9 × 0.6	Laminectomy of C7–1, laminoplasty of C3–6	None	No	12
Nakaya et al., 201817	1	M	55	Severe lt sciatica	Anteromedial to lt L3–4 facet joint	1.4 × 0.8 × 0.8	Microendoscopic hemilaminectomy, medial facetectomy	None	No	12
Present study	1	M	56	Upper neck discomfort, palpable nontender mass	Paraspinal, lt suboccipital region	7.3 × 5.3 × 4.8	Direct excision	None	No	2

FU = follow-up; NR = not reported; Sx = symptoms.

Differential Diagnosis

The differential diagnosis for an extraskeletal paraspinal mass like the one reported here is broad, including lipomatous lesion, myositis ossificans, synovial osteochondromatosis, metastasis, and primary sarcoma, among others.

Lipomatous lesions are the most common soft tissue neoplasm and can frequently impact the upper back.¹⁰ Around 13% of lipomas occur in the head and neck area.¹⁸ Most cases are asymptomatic and slow-growing.¹⁸ Lipomas appear as hyperechoic lesions on ultrasound and well-defined, encapsulated lesions that are isointense to surrounding adipose tissue on MRI.¹⁰ A lipomatous lesion was initially high on the differential diagnosis for the tumor described in this case report due to the hyperintense appearance on T2-weighted MRI, the suppressed appearance on fat-suppressed T2-weighted MRI, the slow growth of the described tumor, and the high prevalence of lipomatous lesions. However, the described tumor did not occur in an area of high adipose tissue presence and had a bony interior, indicating that a lipomatous lesion was not the cause of the reported paraspinal mass.

Myositis ossificans is a nonneoplastic extraskeletal bone formation most frequently identified in young adults.¹⁰ Trauma often contributes to the development of myositis ossificans.¹⁰ The paraspinal area is a rare location for myositis ossificans.¹⁹ Imaging of myositis ossificans evolves over time, with MRI of myositis ossificans initially demonstrating a poorly defined mass with high central signal intensity.¹⁰ After a few weeks, a peripheral mineralized ring often appears on MRI.¹⁰ The patient did not recall trauma occurring to the affected area, which lowered myositis ossificans on the list of potential diagnoses.

Synovial osteochondromatosis is a benign condition characterized by multiple cartilaginous and osteocartilaginous lesions within the synovial compartment of a joint. Pathogenesis involves metaplasia of the synovium and the production of cartilaginous nodules that can detach from their origin, mineralize, and ossify.²⁰ Patients can present asymptomatically or with symptoms including pain, swelling, and joint mobility limitations.²⁰ Intra-articular calcifications and bony erosions may be seen on CT imaging.²¹ On MRI, around three-quarters of cases display low to intermediate signal intensity on T1-weighted MRI and high signal intensity on T2-weighted MRI with hypointense calcifications.²¹ Resection is the definitive treatment and also the gold-standard diagnostic approach for synovial osteochondromatosis.²² Because the patient reported here did not have synovium involvement and the tumor occurred outside the joint space, synovial osteochondromatosis was a less likely cause of this paraspinal mass.

Metastasis to the paraspinal muscles is another condition that may result in an extraskeletal paraspinal mass. Trunk muscle metastases account for 27.8%–49% of all muscle metastases.¹⁰ Common primary malignancy sites resulting in muscle metastasis include the lungs, genitourinary system, and gastrointestinal system.¹⁰ Muscle metastasis presents with a ring enhancement pattern along with low to intermediate signal intensity on T1-weighted MRI and high signal intensity on T2-weighted MRI.¹⁰ Pain is the most frequent symptom at presentation of skeletal muscle metastases and can be managed through palliative excision and/or radiation therapy.²³ The patient presented in this case report lacked these symptoms, was not diagnosed with a primary malignancy, had imaging findings inconsistent with a metastatic lesion, and reported slow growth of the tumor over the prior 5 years, all of which decreased the likelihood of a paraspinal muscle metastasis.

Primary sarcomas can occur in the paraspinal region, although malignant neoplasms occur less frequently in this area than benign neoplasms.¹⁰ A single-institution retrospective review of paraspinal soft tissue sarcomas demonstrated that the most common diagnoses of a 25-patient cohort were synovial sarcoma (n = 6), unspecified high-grade sarcoma (n = 4), and malignant solitary fibrous tumor (n = 3).²⁴ Accurate pathological assessment is critical for diagnosing paraspinal soft tissue sarcomas as these tumors often share imaging features with benign lesions and other malignancies, making histological evaluation crucial for distinguishing between neoplastic and nonneoplastic conditions, guiding appropriate treatment strategies, and providing prognostic information. Treatment is multidisciplinary and centers around resection with negative margins.²⁴ For our case, a primary sarcoma remained low on the differential diagnosis due to the slow growth of the mass over a 5-year period coupled with its lack of irregular borders or bony destruction.

In our case, the slow progression of the lesion over a 5-year period suggested a benign process. On MRI, the round mineralized appearance of the mass and the lack of direct contact with the cervical spine were characteristic of an extraskeletal osteochondroma. Microscopic examination of the resected specimen demonstrated a well-circumscribed mass containing a cartilaginous periphery, an interior of trabecular bone, and enchondral ossification at their intersection. These pathological findings were characteristic of traditional osteochondromas and, coupled with imaging demonstrating a lack of direct contact with the nearby cervical spine and joints, supported the diagnosis of an extraskeletal osteochondroma.

Lessons

Treatment and Prognosis

Paraspinal masses near the cervical spine have a broad list of potential diagnoses. Extraskeletal osteochondroma is a rare but important diagnosis to be considered when evaluating paraspinal masses. Accurate identification of these lesions helps to ensure appropriate care, including surgery or observation. Additionally, while rates of malignant transformation are low, monitoring to identify occurrence of this complication should be performed following diagnosis of an osteochondroma. For patients receiving surgery, prognosis is good, with the rate of recurrence estimated to be less than 2% following complete resection.¹ This case highlights the importance of thorough evaluation and the potential for excellent patient outcomes when rare conditions like extraskeletal osteochondromas are correctly identified and managed.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Benjamin, Franklin, Attiah. Acquisition of data: Benjamin, Chamberlin, Attiah. Analysis and interpretation of data: Benjamin, Corliss, Chamberlin, Attiah. Drafting the article: Benjamin, Corliss, Murray, Franklin, Attiah. Critically revising the article: Benjamin, Corliss, Chamberlin, Attiah. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Benjamin. Administrative/technical/material support: Chamberlin, Franklin. Study supervision: Attiah.

Correspondence

Hughes W. Benjamin: School of Medicine, University of North Carolina at Chapel Hill, NC. hughes_benjamin@med.unc.edu.