Strengths and Limitations
One of the key strengths of this meta-analysis is the uniformity of the results supporting the Shirodkar technique.
A significant limitation of this meta-analysis is the high risk of bias of many of the included studies. The overall quality of the evidence for the primary outcome was low; however, sensitivity analysis removing the impact of papers with high risk of bias improved the quality to moderate without altering the results for the primary outcome. There were two included papers judged to be of low risk of bias and both reported lower rates of PTB in the Shirodkar groups(40, 43). More than half the included studies were published prior to 2000, with 13 studies classified as having a high risk of bias; however, sensitivity analyses did not alter our primary results. All papers included were cohort studies except for one which used data from four randomised control trials(44), which reported lower rates of PTB in the Shirodkar arm when compared to both McDonald cerclage and bed rest.
As a result of the sparse number of events and unreported outcomes in the studies examined in this review, we were not able to report on some outcomes mentioned in our protocol paper(9). These included Apgar score, intra-operative membrane rupture, cervical stenosis, repeat cerclage rate, RDS rate and a sub-analysis of preterm birth at <36 weeks. It should also be noted that while cervical length and neonatal respiratory distress syndrome were included, the sample size in these analyses was small (Table 3) and these results should be interpreted with caution.
Another limitation was that all but one included paper did not control for the effect of surgical experience of different operators. The only study with a single surgeon for all procedures, who was equally experienced in both techniques, reported statistically significant better outcomes for PTB with the Shirodkar approach(43).
This review reported results, favouring the Shirodkar approach over the McDonald approach, which were consistent in effect sizes and direction of effects. The overall quality of the studies in the review, however, was low with the risk of bias rated from between moderate to high. Thus, there is a need for further well designed, randomised control studies to compare the outcomes between different cervical cerclage techniques.