Osteochondroma of the Scapula with Accessory Nerve (XI) Compression

Osteochondroma is the most common benign bone tumor and is characterized as a cartilage-capped bony stalk. This lesion usually develops from the growth plate of long bones. Most osteochondromas are asymptomatic. Neurovascular compressions or cosmetic issues can occur in specific locations. Malignant transformation is extremely rare, and MRI can help evaluate these lesions. Symptomatic mass and malignancy features are the main surgical indications. Uncommonly, an osteochondroma can develop from flat bones. We present the case of a 25-year-old patient with a right scapula osteochondroma causing an accessory nerve compression. The mass was surgically removed, and the diagnosis was confirmed. The patient fully recovered at the latest 3-year follow-up visit.


Introduction
Osteochondromas account for 35 to 46% of benign bone tumors [1]. is tumor is characterized as a cartilage-capped osseous stalk with a bone marrow cavity in continuity to the underlying bone.
is lesion usually develops from the growth plate of long bones in the rst two decades of life, when the cartilage solidi es into the bone. Solitary osteochondromas are by far the most common presentation, and numerous lesions in the same patient are observed in the multiple hereditary osteochondromatosis.
Osteochondroma is generally asymptomatic and discovered during diagnostic radiological exams ordered for other purposes (i.e., search for fractures after a trauma). Two di erent morphological types are described: sessile, with an increased risk for malignant transformation, and pedunculated. e risk for malignant transformation into chondrosarcoma is thought to be less than 1%. Osteosarcoma transformation is very uncommon.
Depending on its location, an osteochondroma can sometimes cause neurovascular compression, cosmetic issue, and/or pain. is often leads to surgical removal with histopathologic analysis to con rm the diagnosis. Symptom disappearance with fully recovered function is generally expected [1][2][3][4][5].
Uncommonly, an osteochondroma can develop from at bones. We present the case of a 25-year-old patient with a right scapula osteochondroma causing an accessory nerve (XI) compression. e mass was surgically removed through a posterior incision, and the diagnosis was conrmed. e patient fully recovered at the latest 3-year follow-up visit.

Case Report
A 25-year-old male presented at our clinics for a right shoulder pain related to a dorsal scapular mass rst observed 4 months earlier. e main symptom was a shoulder discomfort when lying on his back, sometimes compromising the sleep. e patient also complained of some weakness when using his right arm under the shoulder level. e medical history consisted in a unique kidney and past treatments for a nodular sclerosis-subtype Hodgkin lymphoma (chemotherapy and radiotherapy, last treatment twelve years ago and no recurrence). e physical examination revealed a nonmobile 3 cm T × 3 cm AP × 2 cm CC hard mass on the posterior superomedial angle of the right scapula, sensitive to palpation ( Figure 1). Muscle trophicity was symmetrical. e right shoulder presented a full range of motion in all directions without scapulothoracic dyskinesis. Weakness and pain were observed when raising the right shoulder or during active abduction (at all levels).
A right shoulder radiological series including AP, lateral, and Neer views was described normal and conducted additional investigations. A thoracic computed tomography was performed and described a 2.6 cm T × 2.6 cm AP × 2.2 cm CC solid bone mass at the posterior aspect of the superomedial angle of the scapula. e diagnosis of an osteochondroma without signs of malignancy was stated ( Figure  2). No adenopathy or recurrence of the past Hodgkin lymphoma was observed. Retrospectively, the mass was apparent on the shoulder Neer view. A magnetic resonance imaging (MRI) including the right shoulder and scapula con rmed the benign character of the osteochondroma. e accessory nerve (XI) was compressed between the osteochondroma and the deep layer of the trapezius; no muscle atrophy was described ( Figure 3). e osteochondroma was surgically removed under general anesthesia. e patient was prone, and a 10 cm incision was centered on the mass. e trapezius muscle was split at its superior part at the level of the scapula spine. e accessory nerve was visualized and protected in the deep layer of the trapezius muscle. e osteochondroma was exposed and resected from the supraspinatous fossa with an osteotome. e mass was perfectly smooth and measured 3 cm T × 3 cm AP × 2 cm CC ( Figure 4). e integrity of the accessory nerve was checked before closure. e histopathologic analysis con rmed the diagnosis of a benign osteochondroma ( Figure 5). e shoulder was immobilized in a sling 10 days for wound care. Free mobilization was then granted. No physiotherapy was needed. e patient was followed annually up to 3 years. He regained full shoulder range of motion and complete symmetric trapezius and rotator cu strength. e patient has no clinical and radiological recurrence of the osteochondroma.

Discussion
Osteochondroma was initially thought to represent a misplaced epiphyseal cartilage plate herniated in a bone with a periosteal defect. Recent basic research on gene analytics considered these tumors as true primary neoplasms. Most cases represent solitary lesions. Multiple lesions are observed in the multiple hereditary osteochondromatosis. A familial link with germline mutations of EXT1, EXT2, and EXT3 was described in 90% of patients. EXT1 mutation (8q24) was even found in some cases with a solitary lesion.
In this case, we questioned a possible genetic link between the past diagnosis (unique kidney and Hodgkin lymphoma) and the osteochondroma. However, no association was found on the Online Mendelian Inheritance in Man (OMIM) database.
Secondary osteochondromas have been reported with autologous hematopoietic stem cell transplant, trauma on the growth plate, and irradiation. Osteochondroma is the most common benign radio-induced tumor. Dosage from 1.25 grays (Gy) up to 64.25 Gy was described for radioinduced osteochondroma, with a mean latent time of 8 years. In this case, the patient received chemotherapy and mantle eld radiotherapy for his nodular sclerosis-subtype Hodgkin lymphoma. Even though mantle eld irradiation is relatively precise, no speci c shields are used to block the radiations outside the dedicated zone (scapula, lungs, and humeral head). e patient received 14 cycles of mantle eld irradiation for a total absorbed dose of 21 Gy. Considering that the last irradiation dated twelve years ago and no lesion was observed on the last chest tomodensitometry (seven years ago), a link between the irradiation and the osteochondroma is less likely.
Flat bone is an uncommon site for solitary osteochondroma [1][2][3][4][5][6][7][8][9][10][11][12]. Most reported cases on the scapula tend to develop on the ventral side, causing snapping and   Case Reports in Orthopedics pseudowinging. To our knowledge, this case represents the rst osteochondroma arising from the posterior superomedial angle of the scapula. is speci c location is propicious to a ect the surrounding neurologic structures, mainly the accessory and suprascapular nerves. e patient demonstrated trapezius weakness, and the imaging illustrated the compressed accessory nerve between the osteochondroma and the deep layer of the trapezius. We did not ask for an electromyogram to assess the level of impairment of the accessory nerve as it would not have changed the need for surgical removal. Only the surgical approach was adapted to protect the nerve during the deep dissection. e patient recovered full strength at the latest follow-up, con rming full decompression of the accessory nerve.
Various tumors can a ect the scapula. Osteochondroma is the most frequent benign tumor, and chondrosarcoma is the most frequent malignant one [13]. e location on the scapula can help for the di erential diagnosis.
e Musculoskeletal Tumor Society divided the scapula into two zones: the S1 zone includes the blade-spine portion of the scapula and the S2 zone the acromioglenoid complex. Although benign tumors are more likely to sit in S2 zone (including aggressive benign tumors like aneurysmal bone cyst and giant cell tumor), osteochondromas are often observed in S1 zone, a zone more prone to malignancies (chondrosarcoma, Ewing's sarcoma, multiple myeloma, and lymphoma). In addition to the location, the clinical presentation and imaging (mainly MRI) are very helpful to di erentiate malignancies from benign tumors.
A rare entity called Nora lesion can mimic an osteochondroma. ese lesions are outgrowth from the cortical surface characterized by the bone, cartilage, and brous tissue and a ect mostly patients in their 20s [14]. e most common a ected bones are phalanges and metacarpal and metatarsal bones [14]. Otherwise, osteochondromas are continuous to the cortical bone, and the histopathologic analysis is essential for the nal diagnosis.
Osteochondromas are best illustrated with MRI, mainly the extension of the native medullary cavity into the tumor's marrow cavity. e cartilaginous cap is richer in water than the bone part, and the cartilage thickness can be easily measured: a thickness higher than 1.5 to 2 cm in adults and 3 cm in children is suspect for a malignant transformation into a low-grade osteosarcoma [15]. Other malignant features well detected on MRI are prominent myxoid changes, wide brous bands, lobulations, and enhancement after gadolinium injection. Furthermore, MRI is useful to analyze the tumor's impact on the local contiguous structures: reactive bursitis, nerve compression, vascular compression, and muscle denervation [15]. In this case, MRI was very helpful to describe the benign character of the lesion and the accessory nerve compression. To our knowledge, this case is the rst case illustrating a posterior scapular osteochondroma with accessory nerve compression. is demonstrates the need for careful clinical and radiological evaluation of these tumors, mostly to identify the possible compression of the surrounding neurovascular structures.

Conflicts of Interest
e authors declare that they have no con icts of interest to disclose.