Main findings
Cervical cerclage is an effective method of PTB prevention(26, 27). Until now, however, there has not been sufficient evidence from individual small studies to reach a consensus opinion amongst experts that one cerclage technique is more effective than the other. This systematic review and meta-analysis of all available data shows that pregnancies requiring a cervical cerclage are significantly less likely to result in preterm birth when the Shirodkar technique of cerclage is utilised compared to the McDonald approach. The number needed to treat (NNT) was calculated using the risk difference. It was estimated that 38 (95% CI 23-171) additional Shirodkar cerclages would be required in order to prevent one additional preterm birth <37 weeks, while an additional 28 (95% CI 18-56) Shirodkar cerclages would need to be performed to prevent one birth <32 weeks(28, 29). This finding is reinforced by a statistically significant reduction in rates of preterm birth before 37, 35, 34 and 32 weeks, PPROM, difference in cervical length, cerclage to delivery interval and an increase in birthweight in the Shirodkar group. Secondary analyses showed the greatest statistical differences in PTB exist at <32 and <34 weeks with approximately three-fold reductions in PTB rates when a Shirodkar cerclage was used.
It is well established that the risk of spontaneous PTB is increased for women with a short cervix on transvaginal ultrasound(30-32). Previous studies have shown a correlation between increased cervical length post cerclage and later gestation at delivery(33, 34). A cerclage height of at least 18mm (measured from the cerclage to the external os in a mid sagittal plane on transvaginal ultrasound) has been shown to be associated with a reduction in PTB when compared to cerclages placed closer to the external os(35). In two separate publications by Sheib et al. and Miroshnichenko et al., the McDonald cerclage has been shown to fail to achieve this height in the majority of women(35, 36). The Shirodkar approach places the cerclage higher and closer to the internal os, hence it is more likely to result in a longer post-cerclage cervical length. This is supported by the two included studies comparing cervical length post cerclage(37, 38) in this review, which showed a significant increase in cervical length post cerclage for the Shirodkar technique when compared to the McDonald approach (mean difference 5.25mm, 95% CI: 4.68-5.83). Intuitively, the ability to achieve a longer cervix with cerclage placement closest to the internal os is more likely with the Shirodkar procedure as it allows clinical estimation of the internal os after dissection of the urinary bladder away from the cervix.
Respiratory distress syndrome (RDS) was not included in the final analysis as only one included paper reported on the outcome. However, the study by Wong et al., showed a statistically significant reduction in RDS rates when the Shirodkar technique was used. This is in keeping with literature which links increasing prematurity with RDS rates.
The interval between cerclage and birth, was significantly longer in the Shirodkar cerclage. This outcome was reported on by two studies; one of which favoured McDonald(39) and the other favouring Shirodkar(40). It should, however, be noted that in the study by Odibo et al. that favoured the McDonald cerclage, the McDonald group had a significantly longer cervical length at entry into the study and less advanced gestational age when compared to the Shirodkar group (20 versus 23 weeks)(39).
Even though the Shirodkar technique requires greater surgical expertise than the McDonald approach, the procedure complication rate is generally low(26). This review was not able to compare rates of intraoperative rupture of membranes or repeat cerclage due to small numbers. However, there was no difference in the rates of cervical laceration between the two techniques. Furthermore, PPROM, arguably an important delayed technique-related complication, occurs less frequently with the Shirodkar technique without a previously reported increase in the Caesarean section rate(37, 40-43).