Intraosseous Schwannoma of the Phalanx of the Left Third Toe: A Case Report

Introduction

Schwannomas are benign tumors originating from Schwann cells, the cells forming the neural sheath of the peripheral nervous system. They are among the most prevalent tumors of the nervous system, yet their intraosseous presentation is rare. It has been estimated that schwannomas account for only 0.2% of primary bone tumors. ³

After a literature review, we can state that this is the first described case of an intraosseous schwannoma involving the middle phalanx of a toe where amputation was the surgical solution selected.

The purpose of this report is to describe the case of a patient with an intraosseous schwannoma involving the phalanx of the left third toe and to review the diagnostic and treatment process followed for these tumors.

Case Presentation

The patient was a 33-year-old man without any relevant medical history or any history of congenital tumors or bone lesions such as multiple hereditary exostoses, enchondromatosis, neurofibromatosis, or Ollier disease. He presented to our hospital with a painful neoplasm in the left third toe of 8 years’ evolution.

Clinically, the patient experienced minimal pain and an alteration of the gait pattern with limitations in the performance of certain everyday life and sports activities.

Physical exploration (Figure 1) revealed a soft mass attached to deep tissue layers at the level of the second phalanx of the third toe of the foot, which covered the phalanx’s dorsal and plantar aspect, as well as dysesthesias and paresthesias along the plantar aspect of the third metatarsal.

Figure 1.

Swelling on the third toe of the left foot.

The radiographs showed a lesion with lytic characteristics, ill-defined margins and cortical involvement, resulting in almost complete destruction of the middle phalanx of the third toe of the left foot (Figure 2). The lesion did not seem to affect either the first or the third phalanx.

Figure 2.

Radiographs showing lytic lesion involving the second phalanx of the third toe.

Based on the Modified Lodwick-Madewell Grading System, ² the observed lesion would correspond to grade II, as it was a geographic lytic lesion with a partially ill-defined margins.

The contrast-enhanced magnetic resonance imaging (MRI) (Figure 3) revealed an expansive, uniform, bone neoplasm, 2.8 × 2.7 × 2.6 cm in size, which was destroying all of the middle phalanx. Slight involvement of the base of the third phalanx was also observed. The lesion appeared to be heterogeneous on T1-wiehgted images and hyperintense on T2-weighted images.

Figure 3.

(A) Coronal T1-weighted sequences showing hypointense heterogeneous lesion. (B) Coronal, (C) sagittal, and (D) axial T2-weighted images showing hyperintense lesion that involves the entire phalanx.

The characteristics of the lesion prompted consideration of both benign and malignant differential diagnoses. However, because of the extensive bone destruction and the challenges associated with potential reconstruction, amputation of the toe was selected. Thus, a single surgical procedure enabled both diagnosis and treatment of the lesion.

A pathologic examination showed a biphasic proliferation of spindle cells consisting of hypercellular areas with palisading nuclei as well as hypocellular areas (Antoni A and B patterns, respectively), corresponding to a schwannoma.

Currently, the patient is pain-free and has been able to return to his daily and sporting activities normally. After 3 years’ follow-up, no signs of recurrence have been detected.

Discussion

A schwannoma (or neurilemoma) is a benign tumor originating in Schwann cells. These cells are in charge of synthesizing the myelin sheath around peripheral nerve cells. They reside mainly in sensory nerves.^5,6 The schwannomas develop in the soft tissues of the head, neck, and appendicular axis, with only a few cases of intraosseous involvement having been reported. ⁶ It has been estimated that intraosseous schwannomas account for around 0.2% to 1% of all primary bone tumors.^5,6 According to Drumond et al, ³ intraosseous involvement of schwannomas may develop in one of 3 locations: in the bone’s nutrient foramen, directly on the bone, or in the extraosseous space but eventually affecting the bone.

Intraosseous schwannomas may affect virtually any bone. They have most frequently been described in the mandible, a befitting location given the proximity of the mandibular nerve, a predominantly sensory nerve; and in the sacrum, an area crossed by a large amount of sensory nerve roots. ⁵ Other locations include the humerus, the radius and the metacarpals in the upper limb, and the proximal femur, the fibula, the cuneiforms, the metatarsals and, above all, the calcaneus in the lower limb. They usually appear between the 20th and 60th years of life, with a slight predilection for the female sex.^3,7

The majority of intraosseous schwannomas present asymptomatically or as unspecific insidious pain with sensory-motor alterations. They may at times develop as pathologic fractures following trauma.

On radiographic images, intraosseous schwannomas appear as well-demarcated lytic lesions characterized by sclerotic edges that, on some occasions, present with cortical trabeculations and erosions.^3–5,8 They do not normally contain intralesional calcifications. ⁸ In contrast, the case presented here appeared as a lesion with partially defined margins, thus classified as a grade II as per the Modified Lodwick-Madewell Grading System. ² According to Caracciolo et al, ² grade II lesions have a 46% probability of being benign.

Given the lesion’s characteristics and its similarity to other bone lesions, Wang et al ⁸ suggested including schwannomas in the broad differential diagnosis of osseous conditions affecting the toe, together with other entities such as simple bone cysts, aneurysmatic bone cysts, giant cell tumors, and benign chondroblastomas, among others. Additionally, glomus tumor should be considered in the differential diagnosis of lytic lesions of the phalanges, as 10 cases have been described in the literature. ¹ However, pathologic anatomy ruled out this diagnosis.

Grade II lesions carry a moderate risk of malignancy. Chondrosarcoma, myeloma, osteosarcoma, and metastases need to be included in the differential diagnosis.

Lastly, differential diagnosis should also be made with osteomyelitis, given its wide range of clinical presentations. In this case, it would be advisable to perform an analysis with systematic markers of inflammation. ¹

For definitive diagnosis, other complementary tests such as MRI and biopsies have to be included.

According to Knight and Birch, ⁷ performance of an MRI is essential as radiographic images cannot effectively differentiate between benign and malignant lesions. ⁴ On MRI, schwannomas appear as heterogeneous masses typically hypointense on T1-weighted sequences (corresponding to hypercellularity areas) and hypointense on T2-weighted sequences (corresponding to myxoid areas).^4-6,8

We have observed that it is often more productive to discuss hand or foot bone lesions in radiology conference rather than in tumor committees, as imaging occurs prior to obtaining samples. This approach can facilitate obtaining diagnostic samples without compromising definitive treatment.

Pathologic analysis is indispensable for an accurate diagnosis of schwannoma. The pathognomonic histologic pattern of schwannomas has been described thoroughly in the literature as a double histologic pattern, with Antoni A and B areas.^3,5,6,8 Antoni A areas are made up of spindle cells whose nuclei may be arranged in palisade forming 2 parallel rows separated by Schwann cell prolongations. These are known as Verocay bodies. Hypercellular areas are characterized by low mitotic activity without cellular atypia. On the other hand, Antoni B areas are hypocellular, with cells set in a somewhat random myxoid arrangement. According to Flores et al and Haberal et al, schwannomas may also present with intense positivity for the S-100 protein, which serves as diagnostic confirmation and makes it possible to distinguish them from lesions such as fibrosarcoma, fibrous dysplasia, and nonossifying fibroma.^4,5

Different treatment options have been described in the literature, which vary depending on the bone involved. Treatment of schwannomas often consists in curetting the lesion and the filling of the resulting defect with either an autologous iliac crest graft or a bone graft from a tissue bank.^5-7

In cases where the defect resulting from the curettage procedure is too large, bone grafting may be compounded with osteosynthesis hardware to stabilize such a defect, as described by Drumond et al ³ and Wang et al. ⁸

In the case of our patient, a decision was made to amputate the toe given the body part’s acral location, the few clinical repercussions associated with the procedure, and the impossibility of achieving an optimal surgical reconstruction on account of the extensive lytic involvement of the bone and of the surrounding soft tissues.

The prognosis of the procedure is usually satisfactory, with no instances of malignant progression having been described. Recurrence is unusual, with a 16% rate of reported incomplete resections. ⁴

Conclusions

On radiographic images, intraosseous schwannomas typically appear as lytic lesions with well-defined margins. For that reason, despite constituting an infrequent finding, they should be included in the differential diagnosis of osseous toe conditions.

Their infrequency and nonspecificity make interosseous schwannomas difficult to diagnose based exclusively on the patient’s symptoms and imaging findings. An accurate diagnosis must also be informed by the results of pathologic examinations, which can only be performed after the lesion has undergone surgical treatment.

Supplemental Material

sj-pdf-1-fao-10.1177_24730114241255358 – Supplemental material for Intraosseous Schwannoma of the Phalanx of the Left Third Toe: A Case Report

Supplemental material, sj-pdf-1-fao-10.1177_24730114241255358 for Intraosseous Schwannoma of the Phalanx of the Left Third Toe: A Case Report by Andrea Iglesias Ruiz, Xavier Llorens Martinez, Raul Figa Barrios and Andrea Rodriguez Fernandez in Foot & Ankle Orthopaedics