Case report: a case of epidermoid cyst at the incision site following total knee arthroplasty

Epidermoid cysts, also termed epidermal inclusion cysts, rank among the most prevalent benign cutaneous cystic lesions. They arise from aberrant epidermal cell proliferation and keratin buildup within the dermis, clinically manifesting as mobile subcutaneous nodules usually with a central punctate depression. While these cysts may arise anywhere on the body, they predominantly affect the face, neck, upper back and scalp. Notably, epidermoid cysts emerging at atypical sites like knee surgical incisions possess distinct pathogenetic mechanisms from standard spontaneous lesions. Total knee arthroplasty incision sites represent a rare atypical location for secondary epidermoid cysts, whose clinical features, pathogenesis and preventive approaches remain insufficiently elaborated. We report a rare case of an epidermoid cyst arising at the surgical incision site after total knee arthroplasty. We systematically describe the clinical and histopathological features of this uncommon postoperative complication and highlight the role of preoperative musculoskeletal ultrasonography in lesion characterization and in assessing communication with the joint cavity, thereby aiding the exclusion of sinus tract formation or periprosthetic joint infection. Postoperative incision-site epidermoid cysts following total knee arthroplasty differ fundamentally from spontaneous cutaneous epidermoid cysts in their etiology. This report expands the clinical understanding of this rare arthroplasty-related complication and highlights the value of musculoskeletal ultrasonography in preoperative lesion characterization and assessment of potential communication with the joint cavity. The preventive strategies discussed may help orthopedic surgeons reduce the occurrence of such postoperative cystic lesions.