This will be a multicenter prospective randomized study of adult patients with an acquired skull defect as a result of craniectomy and considered suitable for cranioplasty, i.e. reconstruction of the skull defect, at all seven Hong Kong Hospital Authority neurosurgical units. Patients that underwent their primary craniectomy operation at any of the Hospital Authority neurosurgery centers from the 1st March 2019 and considered suitable for cranioplasty will be included in this study. Those who underwent their primary craniectomy before 1st March 2019 or at an institution other than the aforementioned neurosurgical units will be excluded. Data from clinical records, operation notes, medication-dispensing records, laboratory records and radiological reports will be collected.

30 adult patients with craniectomy will be recruited and randomized into two groups: "early" cranioplasty, i.e. performed within 3 months of craniectomy, and "late", i.e. cranioplasty performed more than 3 months after the operation. The aim of the study is to determine whether early cranioplasty can improve on patient's cognitive performance compared to those who undergo the procedure after 3 months.

Decompressive craniectomy, a neurosurgical procedure where a portion of the skull calvarium is removed, is a life-saving procedure. The complication rate of cranioplasty, a neurosurgical procedure where the acquired skull defect is reconstructed, ranges from 11% to 26% and includes postoperative hemorrhage and infection. (4) The syndrome of the trephined is a recognized long-term complication in which certain groups of patients, experience debilitating neurocognitive deficits in addition to chronic headache, dizziness, fatigability and clinical depression. (2) It is believed that the lack of an overlying bone may cause undue significant atmospheric pressure on the underlying cortex, thereby reducing cerebral perfusion and cerebrospinal fluid flow. There are reports that cognitive improvement can be observed in up to 30% of patients after cranioplasty yet the underlying mechanism for this observation is unclear. (1) Some studies have demonstrated enhanced cerebral perfusion by non-invasive investigations, but there is a lack of large scale systematically performed studies to verify such cerebral hemodynamic effects. (1-3) Clinical equipoise exists regarding the optimum timing of cranioplasty procedures after craniectomies. While the anecdotal practice of delaying cranioplasty for at least 3 months after a craniectomy is common, local and overseas observational studies suggest that performing early cranioplasties (i.e. within 3 months) is equally safe in terms of infection and other operative complications. (4-6)

Inclusion Criteria:

1. Age greater than or equal to 18 years-old,
2. Craniectomy was performed due to head injury, infarct or spontaneous intracerebral hemorrhage, and benign tumors.
3. Craniectomy skull defect size of >10cm at its longest diameter
4. Craniectomy performed at any of the Hospital Authority's Neurosurgical Centers after 1st March 2019

Exclusion Criteria:

1. Age older than 80 years-old,
2. Patients cannot communicate by obeying simple command,
3. Patients who are unfit for cranioplasty as decided by the treating neurosurgeon
4. Posterior fossa craniectomy
5. Craniectomy performed before 1st March 2019
6. Craniectomy performed at an institution outside the Hospital Authority
7. Any pre-existing illness that renders the patient moderately or severely disabled before the brain insult.
8. Patients that need an additional procedure e.g. cerebrospinal fluid shunting with cranioplasty in the same setting.
9. History of central nervous system infection
10. Craniectomy-related complications such as hemorrhage or surgical site infection requiring surgical intervention or deemed to affect patient's long-term cognitive outcome
11. Claustrophobia or any other medical condition that prohibits the patient from undergo MRI scanning
12. Patients who cannot understand spoken English or Chinese

1:1 block central randomisation with assignments written on a card sealed in an opaque envelop.