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).