Postoperative Imaging of Complications Following Cranial Implants

A wide range of neurosurgical implants, cranioplasty materials and catheters have been developed to treat a variety of intracranial disorders. Interpretation of postoperative imaging can be challenging and confounded by postoperative changes and implant-related complications. Review of recent literature suggested that there is a paucity of data on postoperative cranial implant-related complications. If not addressed appropriately in a timely manner, these complications may cause a delay in the patient’s treatment with subsequent prolongation of hospital stay. It is therefore paramount for clinicians and radiologists to be aware of the appearance of these implant-related complications on imaging during postoperative surveillance.

A burr hole is created in the skull for evacuation of subdural hemorrhage or stereotactic tumor biopsy. After surgery, the burr hole can be repaired by applying patient's autologous bone dust or titanium cover plates to prevent sinking of the overlying skin.

Cranioplasty
Craniotomy is an operation for temporary removal of a bone flap to expose the underlying brain. Titanium Crani-oFix (Aesculap) plates are widely used to fix the bone flap to the cranium after craniotomy. Rarely, late complication may occur due to skin erosion overlying these plates. Patients with poorly vascularized or fragile skin are par-ticularly vulnerable, risking infections such as meningitis and subdural empyema.
Craniectomy is a similar surgical approach as craniotomy except that the skull flap is not replaced after surgery (Figure 2A). It is commonly used to treat medically intrac- If not addressed appropriately in a timely manner, these complications may cause a delay in the patient's treatment with subsequent prolongation of hospital stay. It is therefore paramount for clinicians and radiologists to be aware of the appearance of these implant-related complications on imaging during postoperative surveillance.
Keywords: Artifacts; cranioplasty; endovascular embolization materials; cranial implants; titanium; ventriculoperitoneal shunts malignant infarction or intracranial hemorrhage. A late complication following craniectomy is the " sinking" of the skin flap over the surgical site, known as the "Sunken brain and Scalp Flap Syndrome"(SSFS) or "Motor Trephine Syndrome" (MTS) (Figure 2A). These patients present with sensorimotor deficit and neurological deterioration, which can be aggravated by CSF diversion procedures such as ventriculoperitoneal (VP) shunts (Figure 2A and 2B).
According to some authors [2], patients with MTS or SSFS may benefit from an early cranioplasty, which has been shown to improve the "sunken" brain with subsequent improvement of patients' neurological and cognitive functions ( Figure 2B). Early cranioplasty for MTS may therefore serve as a therapeutic procedure rather than being merely cosmetic [2]. The material used for cranioplasty are either patient's autologous bone flap, titanium mesh (Figures 2 and 4), customized acrylic ( Figure 3A and 3B) or poly-etherether-ketone (PEEK) ( Figure 3C).
The most commonly used acrylic resin is methylmethacrylate (MMA), which is prefabricated and created with computer-aided 3D CT data of the skull defect. Acrylic cranioplasty may appear radiolucent on CT and contain gas bubbles formed during exothermic polymerization hence, it should not be mistaken for infection ( Figure 3A and 3B). Titanium mesh is one of the most Figure 2: Axial brain CT of a 22-year-old man who underwent right hemicraniectomy. After resolution of the cerebral edema and mass effect, the pathological side of the brain appears "sunken" (A). This appearance is also known as "Motor Trephine Syndrome (MTS)" or "Sunken Brain and Scalp Flap Syndrome" (SSFS). This condition is further aggravated by CSF diversion procedures such as ventriculo-peritoneal shunt. Note of the ventricular catheter tip in the left frontal horn. He subsequently underwent titanium cranioplasty with pneumocephalus underneath the titanium implant (B). There is interval improvement following cranioplasty with improvement of the patients' neurological and cognitive functions. A few weeks later, he developed a late postoperative complication with hydrocephalus due to shunt dysfunction (C). widely used materials for calvarial reconstruction as it is considered safe and avoids cosmetic deformity. Immediate postoperative complications following cranioplasties are intracranial hemorrhages and infections ( Figure 3C). Staphylococcus and streptococcus are two most common pathogenic gram positive cocci responsible for infections following cranioplasty ( Figure 3C). Late complications of titanium hardware include palpability, exposure, pain and hardware malfunction, which may necessitate hardware removal. Figure 4 reports the anecdotical case of an elderly patient with a titanium cranioplasty mesh in whom tension pneumocephalus occurred five years after surgery, when a frontal scalp wound exposed the underlying titanium mesh.

Brain drains, shunts and probes
These are among the most common cranial implanted materials. Immediate postoperative complications are hemorrhages along the shunt tracks (Figures 5 and 6A) particularly in patients with history of bleeding diathesis.   ventriculitis and abscess formation (Figure 7). Staphylococcus aureus and streptococci are the usual pathogens, particularly vulnerable to the premature, infants and immunocompromised. Early complications are mechanical dysfunctions from shunt obstruction or disconnection leading to hydrocephalus. Dysfunctions of programmable shunt valves may necessitate adjustment of the valve threshold. Overshunting of the cerebrospinal fluid (CSF) may result in slitlike ventricles and intracranial hypotension (Figure 6). By adjusting or increasing the valve threshold, the patient's ventricular size may improve with subsequent improvement of symptoms. If shunt failure persists despite these measures, surgical shunt revision may be necessary.

Deep Brain Stimulator (DBS)
DBS is inserted to treat medically intractable Parkinson disease. The tips of DBS electrodes are ideally positioned in the subthalamic nuclei. The electrodes are subsequently tunneled under the skin to the chest to be connected to a battery powered internal pulse generator (IPG) in a subcutaneous pouch (Figure 8). Intermediate and late postoperative complications may occur from electrode disconnection. Occasionally, infections may occur along the DBS tracks; most worrisome of all is intracranial abscess formation, which may necessitate implant removal and administration of long-term appropriate antibiotics (Figure 8).

Endovascular treatment (EVT) materials and surgical clips
Endovascular detachable coils have been widely used for treatment of intracranial aneurysms while liquid agents (include N-butyl cyanoacrylate and Onyx) are  being used for treatment of arteriovenous malformations. Artifacts from these embolization materials and clips may pose some challenges during postoperative surveillance.

Conclusion
Postoperative imaging can be challenging and confounded by postoperative changes and implant-related complications. Awareness of these complications are paramount to clinicians and radiologists during postoperative surveillance.