Inclusion criteria: (1) 18–30 years old; (2) Immature necrotic permanent teeth [12]: tooth development is in stage 7, 8 or 9 of Nolla staging. The Nolla staging method is as follows: stage 0: no dental follicle appears; stage 1: imaging of the dental follicle appears; stage 2: beginning of calcification of the tooth tip; stage 3: crown formation of 1/3; stage 4: crown formation of 2/3; 5 Stage: the crown is almost formed; stage 6: the crown is formed and the root begins to develop; stage 7: the root is formed 1/3; stage 8: the root is formed 2/3; stage 9: the root is almost formed, the apical foramen is not closed; 10 Stage: Tooth root formation, apical foramen closed; (3) Adult permanent teeth with mature roots but with absorption damage to the apex, and the diameter of the apical hole is greater than 1 mm; (4) A restorable tooth; (5) There is no need to leave space for the final post/core restoration; (6) Anterior teeth or premolars with single canal; (7) A cooperative and compliant patient; (8) Patients are not allergic to the drugs and antibiotics which needed to complete treatment; (9) No periodontal disease or root cracking.

Exclusion criteria: (1) patients with other serious organ diseases, such as cardiopulmonary, kidney and other major diseases; (2) patients with apical cyst; (3) patients with poor cooperation and those who quit the study halfway.

20 patients with mature permanent teeth treated in our hospital (may 2019 ~ may 2021) were randomly divided into control group and experimental group, with 10 cases in each group. The general data are shown in Table 1 below. There is no significant difference between the two groups (p > 0.05). The study protocol was approved by the Ethics Committee of our institution (No. KS20220606001), and it meets the ethical requirement of the Helsinki Declaration.

The experimental group used PRF to assist mature permanent teeth revascularization treatment: (1) At the first visit (Root canal sealing): perfect X-ray, blood routine, and coagulation function examination before operation. Use articaine to local anesthetize the patient’s oral cavity, expose the decayed pulp and uncover the crown with a rubber dam, and construct a crown approach. Use 20 ml of 1.25% NaOCI to wash the root canal repeatedly for 5–10 min, then rinse the root canal with normal saline for 5 min, and then dry the root canal. Ciprofloxacin, metronidazole and minocycline powder were mixed at a ratio of 1:1:1, and saline was added to prepare a 0.1 g/L triple antibiotic paste. Seal the triple antibiotic paste into the root canal, use a conveyor to feed the catheter, cover the mouth of the root canal with a sterile cotton ball, and temporarily seal the cavity with a glass ionomer cement. The patient will follow up within 3–4 weeks after surgery. The paste was removed one week later. If the gums are swollen, painful percussion, etc., root canal disinfection and sealing medicine should be repeated until the patient’s teeth are healed. (2) At the second visit (PRF implement): Before drawing blood, confirm that the patient has no symptoms such as red and swollen gums, and the examination has no positive characteristics. Local anesthetize the patient’s oral cavity, remove the temporary sealing material, rinse the root canal with 20 ml of 17% ethylenediamine tetraacetic acid, and dry it with absorbent paper. Remove 5 ml of the patient’s median venous blood and centrifuge. After centrifugation (see Fig. 1a), the middle layer of PRF gel (see Fig. 1b) is taken out, take the supernatant (see Fig. 1c), taken out with sterile tweezers, and the gel is squeezed with sterile gauze to obtain a PRF film. Perform X-ray examination to detect the length of the patient's tooth root. Use a sterile 40# root canal file to puncture the root canal tissue beyond the root tip tissue 3-5 mm to allow blood to flow into the root canal. After that, the PRF membrane was cut into pieces and placed in the root canal (see Fig. 1d). iRoot BP Plus (Innovative Bioceramix Inc., Vancouver, Canada) was placed 4 mm below the enamel bone boundary and no more than 1-2 mm below the enamel cementum boundary (see Fig. 1e). A wet cotton ball was placed above the iRoot BP Plus, and the cavity was temporarily sealed with a glass ionomer cement (see Fig. 1f). X-rays were taken in parallel after operation. Close the crown and review the patient's constant pressure. One day after the operation, the glass ionomer cement was taken out, the hardness of the iRoot BP Plus was checked, and permanent filling was performed with light-cured resin. The patient will be reviewed 3–6 months after surgery.

PRF assisted mature permanent teeth revascularization operation diagram. a Centrifugal treatment; b Preparation of PRF: After collecting blood and centrifuging, let it stand for stratification, and take the middle Layer PRF gel; C Take the supernatant; d Put the PRF membrane into the root canal; e Tooth with PDF film implanted; f Closed crown

The control group was treated with classic revascularization: the root canal sealing was the same as the experimental group. The root apical hole was pierced, and the blood was drawn so that the blood reached 4 mm below the border of the enamel bone. No PRF was placed in the root canal. The other steps were the same as the experimental group.

At six months after the operation, the patient had no symptoms such as pain, red and swollen gums, tooth tapping pain and no pain, no sinus in the gums, loose teeth consistent with normal teeth. X-ray examination of the root apex periodontal disease disappeared, the apex was gathered, the root canal cavity was reduced, and the root was extended. It is judged to be remarkable effect.

The patient has no symptoms such as pain, red and swollen gums, tooth percussion and no pain, no sinus in the gums, loose teeth consistent with normal teeth. X-ray examination of the root apex periodontal disease disappeared, and the root can not be continued. It is judged to be effective.

The patient has gum swelling and pain, hot and cold tingling, etc., gums have sinus. X-ray film shows the existence of apical periodontal disease, and the root can not be extended. It is judged to be invalid [13].

(1) The root improvement (root canal wall thickness and crown root length) was recorded 6 months after operation. The root canal wall thickness and crown root length were within the normal range. The greater the root canal wall thickness, the shorter the crown root length, indicating the better effect of the treatment. (2) the improvement of tooth function and patient satisfaction score were recorded.

In the initial pretrial, three patients who needed mature permanent tooth treatment adopted autologous PRF technology, and 100% achieved effective results after three months. In the same period, 3 patients received treatment without autologous PRF technology, and only 1 patient achieved effective results after three months. The required sample size was calculated based on a two-tailed significance level of 0.05 and a statistical power of 0.8, resulting in a minimum sample size of 6 in each group. The present study protocol further referenced a recently published systematic review related to autologous platelet concentrates for regenerative endodontic treatment [14]. In the included studies, the sample size of each arm was approximately 5 to 15. Therefore, we selected 10 samples from each group in this study.

The data was analyzed by SPSS18.0 statistical software, the measurement data was described by (\(\overline \pm s\)), and the comparison was performed by t test; the count data was described by percentage (%), and the comparison was performed by χ2 test. P < 0.05 indicated that the difference was statistically significant.