Here is the design of a clinical trial:

Official Title: Pre-emptive Scalp Infiltration With Ropivacaine Plus Methylprednisolone vs Ropivacaine Alone for Relief of Postoperative Pain After Craniotomy in Children (RP/MP vs RP)

Included conditions:
- Pain, Postoperative
- Post-Craniotomy Headache

Study Armgroups:
- {'label': 'MP plus RP group', 'type': 'EXPERIMENTAL', 'description': 'Patients in RP/MP group will receive a peri-incisional scalp infiltration with 0.125% methylprednisolone and 0.2% ropivacaine and normal saline miscible liquids. The assigned solution will be injected subcutaneously by surgeons along the incision and throughout the entire thickness of the scalp before skin incision. The volume of local infiltration solution will be decided by surgeons according to the cut length, and the capacity of the solution will be recorded by investigators.', 'interventionNames': ['Drug: Methylprednisolone', 'Drug: Ropivacaine']}
- {'label': 'RP group', 'type': 'ACTIVE_COMPARATOR', 'description': 'Patients in RP group will receive peri-incisional scalp infiltration with 0.2% ropivacaine alone. The assigned solution will be injected subcutaneously by surgeons along the incision and throughout the entire thickness of the scalp before skin incision. The volume of local infiltration solution will be decided by surgeons according to the cut length, and the capacity of the solution will be recorded by investigators.', 'interventionNames': ['Drug: Ropivacaine']}

Interventions:
- {'type': 'DRUG', 'name': 'Methylprednisolone', 'description': 'The local infiltration solution containing 1.25mg Methylprednisolone per milliliter.', 'armGroupLabels': ['MP plus RP group']}
- {'type': 'DRUG', 'name': 'Ropivacaine', 'description': 'The local infiltration solution containing 2mg Ropivacaine per milliliter.', 'armGroupLabels': ['MP plus RP group', 'RP group']}

Primary Outcomes:
- {'measure': 'Cumulative fentanyl consumption within 24 hours postoperatively', 'description': 'The PCIA device provides bolus (0.5µg/kg, 10min lock-out time) and the maximum dose will be limited as 2µg/kg per hour. If the patients feel inadequate analgesia after 5 times of fentanyl bolus, the bolus dose will be increased to 1 µg/kg and the maximum dose will be increased to 4 µg/kg per hour', 'timeFrame': 'Within 24 hours after the operation'}

Please estimate the sample size based on the assumption: 
The study is designed with a 90% power and a two-sided α level of 0.05. A dropout rate of 20% is also considered.
Sample size calculation
        According to Juâs article, the total fentanyl consumption during the first 24âhours after surgery was 103.9 (11.5) Âµg for children who received local infiltration with RP for tonsillectomy and adenoidectomy.37 Maxwell et al reported that paediatric craniotomy patients receive approximately 300âÂµg/kg of morphine on the first postoperative day.38 This dose of morphine is converted to an equianalgesic dose of fentanyl (the conversion factor is 1âmg morphine=10âÂµg fentanyl), thatÂ is, paediatric craniotomy patients receive approximately 3âÂµg/kg of fentanyl on the first postoperative day. Combined with the previous experience of our centre, we hypothesise that the total fentanyl consumption during the first 24âhours after craniotomy will be 110Â±30âÂµg for children who received local infiltration with RP.
        Although Ersayliâs study did not identify any extra effect of MP in patients undergoing lumbar discectomy,27 this study suggested a trend that local infiltration by MP could reduce the amount of postoperative analgesics in the 24âhours after lumbar discectomy by 20%. We hypothesise that the local application of MP will reduce the dosage of fentanyl by 20%, that is, 88Â±30âÂµg, within 24âhours after paediatric craniotomy.
        Therefore, we estimate that the accumulated fentanyl dose after 24âhours will be approximately 110Â±30âÂµg in the RP group and 88Â±30âÂµg in the RP/MP group. Forty patients per group will be needed based on a 90% power with a two-sided Î± level of 0.05 to show a relative between-group difference of 20% in the composite primary outcome measure. Considering a dropout rate of 20%, a total of 50 patients per arm will be required; thus, the total sample size will be 100 patients in this RP/MP versus RP trial.