Eosinophil-rich sterile fluid collection after calcium phosphate cement use in cranial reconstruction: A case suggesting implant-associated inflammatory reaction

Background: Calcium phosphate cement (CPC) is widely used in cranial reconstruction; however, postoperative fluid collection occasionally occurs, and its biological characteristics remain poorly understood. Case Description: A 70-year-old woman underwent microsurgical clipping of an unruptured middle cerebral artery aneurysm. CPC was applied during cranial reconstruction. Early postoperative imaging showed subcutaneous fluid accumulation, which became clinically evident by postoperative day 4 and increased until day 17. Aspiration yielded clear, culture-negative fluid. Cytological analysis revealed lymphocyte-predominant inflammation with a notable eosinophilic component and granuloma-like CD68-positive histiocytic aggregates surrounding amorphous material. No clinical or radiological findings suggested infection or cerebrospinal fluid (CSF) leakage. The collection resolved after a single aspiration without recurrence. Conclusion: Eosinophil-rich, culture-negative fluid collection after CPC use may represent a self-limited implant-associated inflammatory reaction. Recognition of this pattern may support conservative management when infection and CSF leakage are unlikely.