A rare case of inverted follicular keratosis on the knee

INTRODUCTION

Inverted follicular keratosis (IFK) is a rare benign epithelial tumor arising from the hair follicle infundibulum.^[1] First described by Helwig in 1954, it classically presents as a solitary papule on the face of elderly males.^[2] It most commonly occurs on the face and may clinically mimic squamous cell carcinoma (SCC) or keratoacanthoma.^[3,4] Histologically, IFK shows endophytic growth with squamous eddies and no cellular atypia.^[5]

We report a rare case of IFK on the knee of an 18-year-old female. The purpose of reporting this case is to highlight this unusual anatomical presentation, which poses a diagnostic challenge and carries a high risk of misdiagnosis, potentially leading to inappropriate treatment. Emphasizing such atypical presentations aims to enhance clinician awareness and ensure accurate diagnosis through histopathological evaluation.

CASE REPORT

An 18-year-old female presented with a 3-year history of asymptomatic lesion on the left knee. It began as a small papule and progressed to a 1 × 2 cm well-demarcated erythematous plaque with central hyperkeratosis and superficial erosions [Figure 1]. Recurrent trauma caused secondary infections. Multiple courses of oral/topical antibiotics and corticosteroids were ineffective. No pain, pruritus, or systemic symptoms were reported. Medical and family history were unremarkable.

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Figure 1: Clinical image: Well-demarcated erythematous plaque on left knee with hyperkeratosis and erosions.

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Differential diagnoses included irritated seborrheic keratosis, verruca vulgaris, keratoacanthoma, and SCC. Excisional biopsy under local anesthesia was performed.

Histopathology showed hyperplastic squamous epithelium forming endophytic lobules of polygonal cells with numerous squamous eddies and mild inflammation [Figures 2 and 3]. No atypia, mitoses, or invasion was seen. Margins were clear.

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Figure 2: Histopathology (Hematoxylin and Eosin, ×10): Endophytic squamous lobules with squamous eddies. No atypia.

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Figure 3: Histopathology (Hematoxylin and eosin, ×40): Endophytic squamous lobules with squamous eddies, no atypia (arrow).

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The wound healed completely. At the 6-month follow-up, no recurrence was noted, and the cosmetic outcome was satisfactory.

DISCUSSION

IFK is a rare entity, with fewer than 500 cases documented.^[1]It predominantly affects elderly males and typically involves the head-and-neck region, particularly the face. This case is unusual because it occurred in a young (18-year-old) female and at an extrafacial site (knee), both of which are distinctly uncommon for IFK.^[3,4] Such deviation from the classic demographic and anatomical distribution makes this presentation exceptionally rare and increases the likelihood of clinical misdiagnosis. The lesion’s chronicity, recurrent trauma, and secondary infections likely exacerbated its eroded appearance, further complicating the clinical diagnosis. In addition, its location over the knee, a site prone to friction and trauma, may have altered its morphology, making it clinically resemble more aggressive entities. The lack of response to antibiotics and corticosteroids further necessitated a biopsy, which is critical for differentiating IFK from malignant mimics.

Histologically, the presence of squamous eddies and absence of atypia distinguish IFK from squamous cell carcinoma, which shows invasion, or keratoacanthoma, which typically has a crateriform architecture.^[5] The clear margins and complete healing post-excision align with IFK’s benign prognosis, with recurrence being rare following adequate removal.^[4] The absence of syndromic associations (e.g., Cowden syndrome) is consistent with a solitary lesion.^[6] This case underscores the importance of considering IFK in the differential diagnosis of solitary keratotic or eroded lesions, even at atypical sites and in younger patients. Early histopathological evaluation is essential in such unusual presentations to avoid misdiagnosis and unnecessary aggressive treatment.

CONCLUSION

IFK on the knee in a young female is exceptional. Histopathology is crucial to confirm the diagnosis and exclude malignancy. Excision offers a definitive cure.