1. Background and Importance 
Cranioplasty is an essential procedure in neurosurgery, typically performed after decompressive craniectomy for severe traumatic brain injury, cerebrovascular accidents, or other causes of increased intracranial pressure. Beyond its protective and cosmetic roles, cranioplasty has been shown to improve cerebral hemodynamics and neurological function [1, 2].
Various materials have been employed for cranial reconstruction, including autologous bone, polymethylmethacrylate (PMMA), titanium mesh, and patient-specific three-dimensional (3D) implants [3]. Among these, polyetheretherketone (PEEK) has gained increasing popularity due to its biocompatibility, strength, radiolucency, and ability to be fabricated into custom implants with excellent cosmetic results [4, 5].
Despite these advantages, postoperative complications such as infection, bone resorption, hematoma, and fluid collection (epidural or subgaleal effusion) remain challenges for both clinicians and patients [6, 7]. While some effusions resolve spontaneously or with conservative treatment, others may persist and necessitate surgical intervention, including implant removal.
In this report, we describe two patients with traumatic brain injury who underwent delayed cranioplasty with patient-specific 3D PEEK implants and subsequently developed postoperative epidural and subgaleal effusions with divergent clinical outcomes. In addition, we provide a review of the literature on the management of effusions associated with PEEK implants.
2. Case Presentation
Case 1
A 27-year-old male patient was admitted following a motor vehicle accident with severe traumatic brain injury, diffuse cerebral edema, and an initial glasgow coma scale (GCS) of 8 with bilaterally mid-dilated pupils. He underwent emergency bilateral decompressive craniectomy in April 2024. Over the following four months, neurological recovery was achieved, the patient regained full consciousness, and tracheostomy was removed. In August 2024, he underwent cranioplasty using a three-dimensional custom-made PEEK implant. The postoperative course was uneventful, and the patient was discharged in good condition.
At three and six months follow-up, the patient presented with recurrent temporal swelling on the left side. Imaging revealed epidural and subgaleal effusion. Both episodes were successfully managed conservatively with aspiration, short-term corticosteroid therapy, and compressive bandaging, leading to complete resolution within two weeks (Figure 1).



Case 2
A 30-year-old male patient sustained a severe traumatic brain injury in a motorcycle accident in October 2024. Initial GCS was 7. Imaging revealed a large right frontal intracerebral hematoma associated with a severe open compound depressed skull fracture. The patient underwent bilateral decompressive craniectomy, evacuation of the hematoma, and removal of the bone fragments. Two months later, in November 2024, he underwent cranioplasty with a custom-made PEEK implant. Postoperative recovery was uneventful, with satisfactory cosmetic results.
During follow-up visits at two and four months, the patient reported mild swelling that resolved spontaneously without intervention. However, nine months postoperatively, he presented with marked swelling. CT scan revealed extensive bilateral epidural effusion extending into the subgaleal space. Despite two weeks of conservative management with aspiration and corticosteroids, the effusion progressed. The PEEK implant was removed, and cranioplasty was performed using a titanium mesh. Postoperatively, the swelling resolved completely, and no recurrence was observed during follow-up (Figure 2).
 

3. Discussion
Cranioplasty using PEEK has gained wide popularity as one of the most frequently employed alloplastic materials, alongside titanium mesh, PMMA, and bone cement [4, 8-11]. Its favorable characteristics—including biocompatibility, radiolucency, resistance to radiation, lightweight structure, and the ability to fabricate three-dimensional, patient-specific implants—make it particularly attractive for cranial reconstruction. PEEK also possesses high tensile strength and energy-absorbing properties similar to native bone, conferring a low risk of traumatic fragmentation. Unlike autologous bone or PMMA, it does not undergo resorption, and infection rates appear comparable to those of native bone [11]. Nevertheless, despite these advantages, complications such as infection, hematoma, and fluid collections remain clinically significant. Among these, recurrent epidural or subgaleal effusion following PEEK implantation is rare but increasingly recognized.
In a large series of 1,688 patients receiving custom-made prostheses, Morselli et al. documented a 20.6% overall complication rate, including 49 cases involving PEEK. The most common adverse events were infections, epidural hematomas, hydrocephalus, fluid collections, and graft displacement, with no allergic reactions identified [12]. Similarly, Jonkergouw et al. reported a 28% complication rate in 40 PEEK cranioplasties, mainly infections and postoperative hematomas, while Punchak et al. described a 15.3% complication rate in 183 cases, with implant failure and infections being predominant [4]. 
Two published cases have specifically implicated allergic or hypersensitivity mechanisms in the pathogenesis of post-PEEK effusions. Qiu et al. (2019) reported an adult patient who developed bilateral epidural effusions seven days after PEEK cranioplasty. The effusion fluid was light yellow and sterile, with elevated IgG protein concentration but normal glucose and negative bacteriological findings. The effusion resolved after dexamethasone therapy and drainage, suggesting a delayed-type allergic reaction without the need for implant removal [13]. In contrast, Shields et al. (2022) described a pediatric patient who developed combined epidural and subgaleal effusions three weeks after PEEK cranioplasty. Laboratory investigations revealed elevated ESR, CRP, and high IgG levels in the effusion [11]. Although corticosteroid therapy initially reduced the effusion, recurrence occurred after tapering, and definitive resolution was only achieved following removal of the PEEK implant and replacement with autologous bone.
Nevertheless, rare reports of hypersensitivity to PEEK have emerged. To date, only three cases of PEEK-related allergic reactions have been described in the literature: An intervertebral cage used in anterior cervical discectomy and fusion (ACDF) [14], a PEEK-containing device for rotator cuff repair [15], and a cranial implant following bilateral cranioplasty [3]. Symptoms appeared within hours to four weeks after implantation. Diagnostic findings often included sterile fluid collections with elevated immunoglobulin G (IgG) levels and negative bacteriological cultures. In most cases, removal of the PEEK implant resulted in symptom resolution, although Qiu et al. reported one patient in whom epidural effusion resolved with corticosteroid therapy and drainage without explantation [13].
Our two adult cases add further perspective to this spectrum of clinical outcomes. The first patient experienced recurrent effusions that responded well to conservative management including aspiration, short-term corticosteroids, and compressive dressing. The second patient, however, developed progressive effusions refractory to conservative therapy, ultimately requiring removal of the PEEK implant and reconstruction with titanium mesh. These divergent outcomes underscore the heterogeneity in the pathophysiology and clinical course of post-PEEK effusions.
Taken together, the literature and our cases suggest that post-PEEK effusions may have multifactorial etiologies, including hypersensitivity reactions, sterile inflammatory responses, or impaired fluid resorption. Management should therefore be individualized. Conservative treatment may suffice in transient or mild cases, whereas persistent or progressive effusions may necessitate implant removal. Importantly, allergic etiologies must be distinguished from infection or cerebrospinal fluid leak through appropriate imaging, sterile fluid analysis, and laboratory investigations.
Ultimately, careful follow-up and a high index of suspicion are essential for early recognition and appropriate management of effusions after PEEK cranioplasty. Future studies are needed to clarify the underlying immunological mechanisms and to identify patients at higher risk for this rare but significant complication.
4. Conclusion
Patient-specific PEEK implants provide excellent cosmetic and functional outcomes in cranioplasty. However, recurrent epidural or subgaleal effusions may complicate postoperative recovery. While some cases respond well to conservative management, others may require implant removal and reconstruction with alternative materials such as titanium mesh. Careful patient monitoring and individualized management strategies are crucial to achieving optimal outcomes.
Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.
Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 
Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.
Conflict of interest
The authors declared no conflict of interest.

References