Quality assessment
According to the NOS scale (in Table 1), six studies with scores >=7 were considered to be of high quality with low risk of bias, while the remaining three studies were of moderate quality with a score of 6, with some risk of bias. This was mainly due to the fact that four of the nine included studies were single-centered and not representative in terms of patient selection. Two studies did not conduct heterogeneity analysis on other characteristics other than age. Six studies did not mention the postoperative follow-up rate or did not have a long enough follow-up time, which may be the source of information bias.