Introduction

Enchondroma is a neoplasm of intramedullary cartilage most common in the hand or wrist of children and young adults and second most common benign chondral tumor comprising 12–24% of benign bone tumors [4, 22]. This relatively common lytic bone lesion is rarely found in the axial skeleton, with limited reports in the literature. Here, we present the first case in North America of enchondroma of the thoracic spine.

Case Presentation

A 31-year-old man with no medical history presented to our institution’s clinic with a slow growing lesion of his midline thoracic spine causing an obvious lump under the skin. The lesion was noted 2 years prior and was referred for continued growth. The lesion was not associated with pain or tenderness.

Physical examination revealed a firm, immobile, palpable lesion between spinous processes located on the high thoracic spine without erythema or fluctuance. Neurological exam was unremarkable.

Neuroimaging

CT (Fig. 1) demonstrated a well-defined 4.5 cm X 4.0 cm partially ossified exophytic dorsal paraspinal lesion arising from the outer cortex of the T3 spinous process. Peripherally dense calcification and/or ossification was present in a ring and arc configuration, typically described in enchondromas. There was no destruction of the adjacent T4 spinous process or periosteal reaction. MRI findings (Fig. 2) were consistent with peripheral calcification and/or ossification and central fatty bone marrow signal intensity. Mild surrounding soft tissue edema and inflammation were present, without evidence of associated soft tissue mass.

Fig. 1
figure 1

Sagittal non-contrast CT (A, B) and axial CT (C, D) demonstrate a heterogenous exophytic dorsal paraspinal mass originating from the outer cortex of the T3 spinous process. Peripherally dense calcification/ossification are present in a ring and arc configuration typically described in enchondromas (long arrows, A & B). Central relative low density reflects internal fatty marrow and cellular elements (short arrows, C & D). The margins of this mass are well defined and sharp and a very short zone of transition with the normal bone of the spinous process is noted

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Fig. 2
figure 2

Sagittal T1-weighted (A), sagittal T2 weighted STIR (B) and axial T2 weighted (C) non-contrast MRI sequences of the thoracic spine demonstrate a heterogenous exophytic dorsal paraspinal mass originating from the T3 spinous process. Peripherally T1 and T2 hypointensity reflects dense calcification/ossification (long arrows). Central T1 and T2 hyperintensity reflects fatty marrow and cellular elements (short arrows). There is mild surrounding soft tissue edema and inflammation present (arrowhead). No associated soft tissue mass is evident

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The overall findings were consistent with a low grade osteochondromatous process with the characteristic ring and arc calcification supportive of an enchondroma.

Surgical Resection

The patient underwent T3/4 laminectomy and resection of bony tumor from a posterior approach in the prone position. A subperiosteal dissection was performed to expose the posterior elements of T3 and T4. A large fungating calcified bony lesion was identified between the T3 and T4 spinous processes extending down to and partially involving the lamina (Fig. 3a). Using a high-powered drill and Leksell rongeurs, the lesion and lamina were resected piece-meal without obvious residual lesional tissue remaining. Care was taken to perform as minimal a laminectomy as possible to preserve facet joints and posterior structural elements while ensuring gross total resection.

Fig. 3
figure 3

(A) Fungating calcified lesion identified between the T3 and T4 spinous processes extending down to and involving the lamina. Specimen stained with hematoxylin and eosin: (B) Low power (4x) slides demonstrate predominantly cartilage with extensive calcification and ossification at the periphery of the mass. (C) High power (20x) slides demonstrate cartilaginous lacunae containing chondrocytes without significant cytologic atypia

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Pathology

On gross examination, the specimen was a 5.5 × 4.5 × 3.0 cm aggregate of tan-pink, irregular bony and soft tissue fragments without discrete masses or lesions. Histologically the biopsy sample revealed a well-differentiated lesion composed of cartilage with extensive internal calcification (Fig. 3b). In some fragments, the lesion demonstrated a nodular growth pattern arising between the periosteum and bone. In many fragments, endochondral ossification was identified with cartilaginous lacunae containing chondrocytes without significant cytologic atypia (Fig. 3c). The overall findings suggest a long-standing, low-grade cartilaginous lesion with endochondral ossification consistent with enchondroma.

Post-Operative Course

The postoperative course was unremarkable, and the patient was discharged home on postoperative day one without complications. Post-operative CT confirmed gross total excision with preservation of the normal thoracic kyphosis and no violation of the facet joints (Supplemental Fig. 1).

The patient was seen in clinic on postoperative day 14 with no further complaints or concerns with plans for surveillance imaging and follow-up in 1 year.

Discussion

Primary tumors of the bone offer a wide differential diagnosis. According to the 2013 WHO classification, the initial branch point separating large categories of primary bony lesions rests on the cell of origin or histogenesis. Generally, primary bone lesions arise from bone-forming tumors, cartilage-forming tumors, fibrous bone lesions, or bone marrow tumors (Table 1) [There are a variety of primary bone tumors or lesions not included here and grouped in additional smaller categories, including aneurysmal bone cysts, chordoma, giant cell tumors, etc.] [4, 17, 22].

Table 1 Differential diagnosis of primary bone tumors
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Cartilage forming tumors are comprised mostly of benign lesions, including chondroblastoma, chondromyxoid fibroma, enchondroma, juxtacortical, and chondroma, while the malignant cartilage forming tumor is chondrosarcoma. Benign chondrogenic tumors are the most common benign primary bone tumors. Within this category, enchondromas, second only to osteochondroma, are the most common osseous neoplasms [4, 20, 22]. Enchondromas have a peak incidence in ages 10–40 years, yet since many patients are asymptomatic the true prevalence is unknown [4, 12, 13]. Brien et al. reviewed 3067 primary bone tumors and lesions, and reported enchondroma in 19.7% of all cartilaginous tumors and in 7.7% of all cases [2]. Although enchondromas tend to occur as solitary lesions, there are two enchondromatosis syndromes, Ollier disease, and Maffucci syndrome, associated with multiple enchondromas found in unilateral long bones or associated with soft tissue hemangiomas, respectively. These enchondroma syndromes highlight the risk of sarcomatous degeneration with rates as high as 25% in Ollier and Maffucci, whereas isolated enchondroma malignant transformation to chondrosarcoma, usually low-grade, is rare [6, 14, 19].

Most often occurring in the appendicular skeleton, enchondromas are the most common benign bone tumor of the hand and wrist (40–65%), followed by the major long bones (25%)—femur, humerus, tibia, in descending order—and the small bones of the feet (7%) [4, 8]. Other body sites of enchondroma are uncommon, and they are only very rarely found in the spine [4, 8, 10, 14]. There are only a few documented cases of spinal enchondroma [7, 9,10,11, 23].

The diagnosis of benign cartilaginous bone tumors, including enchondroma, is multifactorial and when uncertain requires tissue diagnosis. Chondrosarcoma (8–17% of bone tumors) can be histologically and radiographically difficult to distinguish from enchondroma [5]. Histologically, enchondroma consists of neoplastic chondrocytes among a hyaline-myxoid background, with focal calcifications and rare cellular mitoses and minimal nuclear atypia, whereas chondrosarcoma has similar features and is locally invasive [1, 10, 11]. Radiographically, the only established differentiating characteristic is size—enchondroma < 5 cm and chondrosarcoma > 4 cm [3, 15, 20, 22]. Clinically, 95% of patients with chondrosarcoma have associated pain, while enchondroma patients are generally asymptomatic [16, 18, 21].

Conclusions

Given the aforementioned diagnostic dilemma and possibility of malignant transformation, surgical excision is generally recommended, especially in the presence of a symptomatic lesion greater than 4 cm. Overall, as this case demonstrates, the surgical goal is to determine a final diagnosis and achieve a gross total resection to eliminate the risk of sarcomatous degeneration.