First Reported Case of Nodular Fasciitis with Intraosseous Patellar Involvement: A Case Report

Introduction

Nodular fasciitis is a benign, self-limited myofibroblastic proliferation initially described by Konwaler et al. in 1955 as subcutaneous pseudosarcomatous fibromatosis ^[1]. This lesion typically presents as a rapidly growing, solitary soft-tissue mass arising in the subcutaneous tissues or fascia of the upper extremities, trunk, or head and neck region of young adults ^[2,3]. Despite its benign nature, its clinical presentation and histologic features may be alarming, often mimicking sarcoma with rapid growth, high cellularity, and prominent mitotic activity ^[4]. Although nodular fasciitis most commonly arises in superficial soft tissues, intra-articular occurrence is exceedingly rare and is rarely reported in the literature. Among documented cases, the knee is the most frequently affected joint, representing approximately 27% of intra-articular nodular fasciitis cases ^[5,6,7]. The rarity of this presentation, combined with non-specific and occasionally aggressive imaging findings, creates significant diagnostic challenges and often raises concern for malignancy.

The understanding of nodular fasciitis has evolved from being considered a reactive process to recognition as a self-limited neoplasm. In 2011, Erickson-Johnson et al. identified the recurrent ubiquitin-specific protease 6 (USP6) gene rearrangements in nodular fasciitis, with MYH9-USP6 being the most common fusion product ^[8]. USP6 gene rearrangements are now detected in up to 92% of cases and serve as a valuable diagnostic tool, particularly in morphologically or clinically atypical cases ^[9,10].

We report a unique case of nodular fasciitis arising in the subpatellar region with erosion into the undersurface of the patella in a 67-year-old man. To our knowledge, this represents the first reported case of nodular fasciitis with intraosseous involvement of the patella. This case highlights the diagnostic complexity of intra-articular nodular fasciitis and the critical role of molecular confirmation.

Case Report

Discussion

Nodular fasciitis is a benign, self-limited myofibroblastic proliferation that classically affects young adults between the second and fourth decades of life ^[2]. Presentations in patients over 60 years of age, as in our patient, are exceedingly uncommon. In the extremities, nodular fasciitis is typically found in the forearm (27%), thigh (17%), and upper arm (12%) ^[3]. Intra-articular occurrence is exceptionally rare, representing a diagnostic challenge for clinicians and pathologists.

A comprehensive review by Wang et al. identified only 21 reported cases of intra-articular nodular fasciitis in the literature ^[5]. Of these cases, the knee was the most commonly affected joint (67%), followed by the shoulder (14%), hand (10%), hip (5%), and ankle (5%). Of the lesions in the knee, the subpatellar fat pad appears to be a common site for nodular fasciitis to arise. The median reported age of 26 years (range 4–54 years) is substantially younger than in our case, underscoring the unusual demographic and clinical features of our patient ^[5,6,7,11].

The most striking and novel feature of our case is the presence of intraosseous erosion of the patella. While osseous involvement in nodular fasciitis has been rarely documented, it is typically seen in the cranial fasciitis variant, which affects the calvarium in young children ^[12]. A recent study by Zhang et al. reported rare cases of nodular fasciitis with bone involvement in locations other than the skull, including one case affecting the hallux ^[13,14]. However, to our knowledge, our patient represents the first reported case of nodular fasciitis with intraosseous involvement of the patella.

Radiographically, nodular fasciitis often demonstrates non-specific and at times aggressive features resembling those of malignancy. On MRI, lesions typically appear iso- to hypointense on T1-weighted images and hyperintense on T2-weighted images ^[15,16]. Heterogeneous enhancement is common, and some cases demonstrate aggressive features, including transcompartmental spread and osseous involvement ^[17]. In our patient, the presence of cortical erosion combined with his history of follicular lymphoma raised concern for malignancy, highlighting the importance of tissue diagnosis.

The histopathologic diagnosis of nodular fasciitis is characterized by a proliferation of plump, immature-appearing myofibroblasts arranged in short, irregular fascicles within a variably myxoid to collagenous stroma ^[18,19]. Scattered inflammatory cells, extravasated erythrocytes, and occasional multinucleated giant cells are common findings. Immunohistochemically, the cells typically stain positive for smooth muscle actin and vimentin but negative for desmin, distinguishing them from smooth muscle tumors ^[19].

The discovery of recurrent USP6 gene rearrangements has revolutionized our understanding of nodular fasciitis. In 2011, Erickson-Johnson et al. identified the MYH9-USP6 fusion gene as the primary genetic alteration in nodular fasciitis ^[8]. USP6 rearrangements are now detected in 86-92% of cases by FISH analysis, with a specificity approaching 100% ^[9,10]. The gene fusion results in overexpression of USP6, a deubiquitinase that promotes cell proliferation through activation of JAK-STAT and NF-kB signaling pathways ^[20]. In our patient, FISH confirmed USP6 (17p13) rearrangement, supporting the diagnosis of nodular fasciitis despite the unusual presentation with bone involvement.

Management of nodular fasciitis varies depending on the severity of symptoms and anatomic location. Although spontaneous regression has been reported, symptomatic lesions or those in critical locations often warrant surgical excision. Marginal resection is usually curative, and recurrence is exceedingly rare even with incomplete resection ^[2]. In our patient, the decision to proceed with surgical resection was based on the severity of symptoms refractory to conservative management and the presence of bony erosion into the patella.

This case highlights several important clinical principles. First, nodular fasciitis should be considered in the differential diagnosis of intra-articular knee masses, even in older patients and when aggressive features, such as bone erosion are present. Second, USP6 FISH analysis is an invaluable diagnostic tool when the morphologic diagnosis is uncertain or when the clinical presentation is atypical. Third, surgical treatment remains an appropriate option for symptomatic lesions despite the self-limited nature of nodular fasciitis, particularly when conservative management has failed. Ultimately, a close clinical correlation between imaging, histopathology, and molecular findings is crucial for accurate diagnosis and effective management.

Conclusion

We report the first documented case of nodular fasciitis with intraosseous involvement of the patella in a 67-year-old man. This unusual presentation highlights the importance of maintaining a broad differential diagnosis for intra-articular knee masses with bony erosion. Molecular confirmation through USP6 FISH analysis proved essential in establishing the correct diagnosis in this clinically challenging case. Surgical resection provided both diagnostic certainty and symptom relief for this patient with symptoms refractory to conservative management. This case expands our understanding of the anatomic diversity of nodular fasciitis and emphasizes the value of molecular diagnostics.

Clinical Message

Nodular fasciitis is a benign myofibroblastic proliferation that can closely mimic malignancy due to its rapid growth, high cellularity, and occasionally aggressive radiographic appearance. Although typically confined to superficial soft tissues, it can rarely present as an intra-articular knee mass with associated osseous erosion, creating significant diagnostic uncertainty, particularly in patients with a history of malignancy. This case emphasizes the importance of maintaining a broad differential diagnosis for intra-articular lesions with bone involvement and highlights the critical role of molecular confirmation using USP6 gene rearrangement analysis to establish an accurate diagnosis and avoid overtreatment. Recognition of this rare presentation allows for appropriate surgical management and symptom resolution while preventing misclassification as a malignant process.

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About the Authors

• 

  Dylan J Riley

• 

  Fernando O. Gonzalez

• 

  Alan T Blank