Results
A total of 24 volunteers were recruited, and one (male, age 30) was excluded due to unqualified data. Specifically, there was obvious deformation in the post-extraction facial scan, which may be due to the mandible position deviation or the defect of structural light scanning. A total of 23 volunteers were included, including 5 males (age 26.6 ± 2.5 years) and 18 females (age 27.3 ± 2.5 years). The T1 follow-up time was 3.86 ± 0.89 months (17 patients completed); the T2 follow-up time was 8.08 ± 1.71 months (21 patients completed). The characteristics of the third molars included are listed in Table 
The characteristics of the third molars*The number is counted by corresponding tooth surface (buccal, lingual/palatal, distal, mesial) which was involved in the cortical bone removal during extractionAs for the reliability of the projection method, the inter-observer ICC is 0.959, 95%CI 0.925–0.978, P < 0.01;and intra-observer ICC is 0.974, 95%CI 0.935–0.988, P < 0.01.Descriptive statistics of RMS on the extraction side and non-extraction side are displayed in Table 
Descriptive statistics of RMS on extraction side and non-extraction side (/mm)Two-way repeated measure ANOVA was used to assess the influence of extraction over time on STV. The interaction of treatment * time had no significant effect on STV, f (2,30) = 3.300, P = 0.051.One-way repeated measure ANOVA was used to analyze the STV over time. For the extraction side (n = 16): there was significant difference in STV between T0, T1 and T2, f (2,30) = 4.906, P < 0.05. Pairwise comparison indicated that, from T0 to T1, the STV decreased by 1.396 (95% CI: 0.323–2.470) ml (P < 0.05), and from T1 to T2 the STV increased by 1.753 (95% CI: − 0.01–3.507) ml (P = 0.05). There was no significant difference of STV between T2 and T0 (P > 0.05). For the non-extraction side (n = 16), there was no significant difference in STV between T0, T1 and T2, f (2,30) = 1.555, P > 0.05.Paired t-test was adopted to compare extraction and non-extraction side for mean distance (µ), standard deviation (SD), root mean square error (RMS), and STVC of the buccal region in each period (T1-T0, T2-T0, T2-T1). According to the Shapiro-Wilk test, besides the RMS of T2-T0 (P = 0.007) and T2-T1 (P = 0.006) did not follow the normal distribution, the other groups conformed to the normal distribution (P > 0.05). Wilcoxon signed-rank test was adopted for data of abnormal distribution. The results are listed in Table The HTVC on the extraction side was − 2.33 ± 0.46 ml of T2-T0. The Pearson correlation coefficient of the HTVC and STVC was − 0.397 (P > 0.05), proving no apparent correlation between the HTVC and the STVC.
Paired-t test of µ, SD, RMS, STVC of the extraction side and non-extraction side in three time period*The mean and standard deviation of the difference of the extracted side and the non-extracted side**P < 0.05, so the data did not follow the normal distribution***The data were analyzed by Wilcoxon signed rank test. The results showed that there was no significant difference in RMS of two sides (P > 0.05)