Levator ani muscle (LAM) avulsion is defined as the disconnection of levator muscle fibers from their insertion on the inferior pubic ramus [1]. It is present in 10–35% of women after a vaginal delivery [2]. The most critical moment for the appearance of LAM avulsion is when the vertex is at station + 3 or + 4 [3], where the area of the levator hiatus acquires its greatest size [4]. LAM avulsion is a diagnostic challenge: although it is sometimes exposed by a vaginal tear [5], most often is that it goes unnoticed during childbirth. Therefore, diagnostic imaging tests have been gaining relevance for the detection of LAM avulsion, such as magnetic resonance imaging (MRI) [6, 7] and three-dimensional (3D) ultrasound [8]. Of the different modes of 3D ultrasound (introital and transperineal), transperineal or translabial ultrasound has shown a correlation with the diagnosis of LAM avulsion both clinically and by MRI [5]. The abdominal probes used in transperitoneal ultrasound also have clear advantages over vaginal transducers due to their larger size and better ergonomic properties of the perineal structures [9]. MRI is a less accessible test, is more expensive, and must be done in less anatomical postures, which leads us to think that transperineal ultrasound is the most desirable imaging test for the study of LAM avulsion.

As for the risk factors for LAM avulsion, instrumented delivery promotes it more than normal vaginal delivery (NVD). However, there are very different rates of LAM avulsion under forceps delivery (FD) vs. vacuum delivery (VD), possibly due to the lack of uniformity of the different study designs. Therefore, the objective of this meta-analysis was to establish the rates of LAM avulsion in patients after instrumented delivery (FD and VD), as assessed by 3D/4D transperineal ultrasound.

The search included 1225 studies that met the MeSH criteria, of which 26 were included in the review because they met the inclusion criteria (Fig. 1). Of all the excluded studies, we highlight the study by Greenbaum et al. [10] which, although it met the inclusion criteria, was excluded because it did not mention the type of instrumentation performed during vaginal delivery.

Fig. 1.

PRISMA flow diagram.

The results extracted from the different studies are shown in Table 1 (Ref. [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]). Five studies were retrospective and the rest prospective. The ultrasound examination was performed in most studies within 6 months after delivery [11, 12, 13, 14, 15, 16, 17, 19, 20, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35], though some performed an ultrasound examination only or also after this period [17, 18, 21, 24, 35, 36]. Some papers included a small number of VDs [12, 20, 27] and FDs [12, 13, 14, 27], a possible source of bias.

Twenty-three studies determined the LAM avulsion rate after VD and 19 after FD. The rates of LAM avulsion in the VD group ranged between 9% and 50% and in the FD group between 22% and 71%. Albrich et al. [12] was excluded because presented a single case of LAM avulsion in one patient with FD. The estimated joint OR for VD vs. NVD was 2.02 (95% CI: 1.34–3.03), including the exploration of Halle et al. [35] at 6 weeks postpartum (Fig. 2, Ref. [35]). If we include the exploration performed by Halle et al. [35] at one year, the joint OR was 1.93 (95% CI: 1.31–2.86) (Fig. 3, Ref. [35]). The estimated joint OR for FD vs. NVD was 5.33 (95% CI: 3.78–8.11) (Fig. 4, Ref. [23]). Finally, the joint OR for FD vs. VD was 2.36 (95% CI: 1.46–3.84) (Fig. 5).

Fig. 2.

Analysis of the general levator ani muscle (LAM) avulsion rates between vacuum delivery (VD) (including the exploration of Halle et al. [35] at 6 weeks postpartum) and normal vaginal delivery (NVD). Of the 13 studies included, log odds ratios ranged between –0.41 and 3.14, the majority being positive (85%). The estimated average log OR was 0.70 (95% CI: 0.29–1.11), the mean being different from 0 (z = 3.37, p = 0.0008). The Q test showed heterogeneous results (Q = 23.87, p = 0.02, $\tau$${}^2$ = 0.25, I${}^2$ = 48.53%), with a prediction interval of 95% from –0.36 to 1.76. Therefore, although the average result is estimated to be positive, in some studies the true result may be negative. The studentized residuals were all within $\pm$ 2.89 (without indications of outliers). Evaluating Cook’s distances, none of the studies could be considered too influential. There was no asymmetry in the funnel plot in the rank correlation (p = 0.95) or in the regression test (p = 0.54).

Fig. 3.

Analysis of the general levator ani muscle (LAM) avulsion rates between vacuum delivery (VD) (including the exploration of Halle et al. [35] 1 year after delivery) and normal vaginal delivery (NVD). Of the 13 studies included, log odds ratios ranged between –0.41 and 3.14, with the majority being positive (85%). The estimated average log OR was 0.66 (95% CI: 0.27 to 1.05), the mean being different from zero (z = 3.31, p = 0.0009). The Q test showed nonsignificant heterogeneous results (Q = 20.91, p = 0.05, $\tau$${}^2$ = 0.19, I${}^2$ = 41.36%), with a prediction interval of 95% from –0.29 to 1.61. Therefore, although the average result is estimated to be positive, in some studies the true result may be negative. The studentized residuals were all within $\pm$ 2.89 (without indications of outliers). Evaluating Cook’s distances, none of the studies could be considered too influential. There was no asymmetry in the funnel plot in the rank correlation (p = 0.59) or in the regression test (p = 0.39).

Fig. 4.

Analysis of the general levator ani muscle (LAM) avulsion rates between FD (forceps delivery) and normal vaginal delivery (NVD). Of the 8 studies included, log odds ratios ranged between 1.28 and 2.62, the majority being positive (100%). The estimated average log OR was 1.71 (95% CI: 1.33 to 2.09), the mean being different from zero (z = 8.78, p 0.0001). The Q test showed that there was no significant amount of heterogeneity (Q = 3.86, p = 0.796, $\tau$${}^2$ = 0.00, I${}^2$ = 0.00%). The study by Caudwell-Hall et al. [23] had a relatively large weight compared to the rest. The studentized residuals were all within $\pm$ 2.73 (without indications of outliers). Evaluating Cook’s distances, none of the studies could be considered too influential. There was no asymmetry in the funnel plot in the rank correlation (p = 0.39) or in the regression test (p = 0.86).

Fig. 5.

Analysis of the general levator ani muscle (LAM) avulsion rates between forceps delivery (FD) and vacuum delivery (VD). Of the 13 studies included, log odds ratios ranged from –0.61 to 1.82, the majority being positive (77%). The estimated average log OR was 0.86 (95% CI: 0.38 to 1.34), where the average was different from 0 (z = 3.49, p = 0.0005). The Q test showed heterogeneous results (Q = 36.44, p = 0.0003, $\tau$${}^2$ = 0.46, I${}^2$ = 70.52%), with a prediction interval of 95% of –0.55 to 2.28. Therefore, although the average result is estimated to be positive, in some studies the true result may be negative. The studentized residuals were all within $\pm$ 2.89 (without indications of outliers). Evaluating Cook’s distances, none of the studies could be considered too influential. There was no asymmetry in the funnel plot in the rank correlation (p = 0.68) or in the regression test (p = 0.28).

The type of instrument used during delivery was not reported in 13 studies with VD [11, 12, 13, 15, 16, 17, 18, 20, 21, 22, 23, 27, 35] and in 10 with FD [11, 12, 13, 14, 15, 16, 18, 21, 23, 27]. The majority of articles that specified the type of vacuum used in the RV reported that a metal vacuum bird’s cup was 50 mm [19, 24, 25, 26, 29, 30, 32, 33, 34], although one of them also assessed LAM avulsion after a Kiwi vacuum was used [32]. The majority of studies on FD described the rates of LAM avulsion with Kielland’s forceps [28, 29, 30, 31, 33, 34, 36], though one of them studied Anderson forceps and Wrigley forceps [19].

Disparate rates of LAM avulsion are described in VD (between 9% and 50%) and FD (between 22% and 71%), with a joint OR for FD vs. VD of 2.36 (95% CI: 1.46–3.84). Based on our findings, we agree with the results of a previous systematic review [37], where it was specified that FD was associated with a higher incidence rate and severity of LAM avulsion compared to NVD [37]. However, there is controversy about whether FD presents higher rates of LAM avulsion than VD. In fact, a recent meta-analysis found Kielland’s forceps to be safe and favored their use over vacuum-assisted delivery [38]. However, that meta-analysis did not study LAM. One of the main problems that we found in the previous reviews is that they included and compared different imaging techniques for the study of LAM avulsion [39, 40], such as MRI and 3D introital ultrasound. In addition, the vast majority of studies included in those reviews did not specify the type of instrument used (the type of forceps or vacuum) or how it was applied during delivery.

In this meta-analysis, we analyzed the type of instrument specified in the study. The most commonly used vacuum was the 50 mm bird’s cup [19, 24, 25, 26, 29, 30, 32, 33, 34]. Only one study made reference to the rate of LAM avulsion in deliveries with another type of vacuum (Kiwi vacuum) [32], so there were no differences in the LAM avulsion rates compared to those produced by Malmstrom’s vacuum, with an OR of 0.977 (0.426; 2.241; p = 0.957) and an adjusted OR of 2.90 (0.691; 12.20; p = 0.146). Something similar was true of the studies that specified the type of forceps used during the FD, as most describe the rate of LAM avulsion caused by only one type of forceps (Kielland’s forceps) [28, 29, 30, 31, 33, 34, 36]. Only one study included different forceps [19], with a small number of participants (20 Anderson forceps and 22 Wrigley forceps), and did not compare the rate of LAM avulsion between the two types.

Only five studies studied the rate of LAM avulsion according to the way in which the instrument was applied during instrumented delivery [26, 28, 29, 33, 36]. It was determined that the number of vacuum tractions necessary to complete the fetal extraction was not associated with a higher rate of LAM avulsion [26], nor was the position of the fetal head (anterior, posterior, or transverse) at the time of vacuum application [26]. In the case of FD, there are controversies. It has been described that the rate of LAM avulsion is not increased when rotational forceps are used (OR: 1.5 [0.6–3.6]; p: not significant), in the correction of asynclitism (OR: 0.8 [0.3–1.9]; p: not significant), or according to the station of the fetal head at the time of placement of the forceps (OR: 2.0 [0.8–5.1]; p: not significant) [28]. However, another study established that rotational forceps were associated with avulsion, with an OR of 2.45 (95% CI 1.22–4.93) [36]. One study analyzed whether the disarticulation of the forceps before the delivery of the fetal head could benefit the rate of LAM, and it found a crude OR (without disengagement vs. disengagement) for avulsion of 0.90 (95% CI 0.49–1.67; p = 0.757) and an adjusted OR (adjusted for maternal age, induced labor, epidural period, second stage of labor, perineal tear, and fetal head circumference) of 0.82 (95% CI 0.40–1.69; p = 0.603) [31]. However, when comparing the rates of LAM avulsion between VD and FD considering the position (anterior or other) and the station (low or medium instrumentation) of the fetal head at the time of instrument placement, no differences were found [29, 33].

Another noteworthy aspect of the current literature is the time when transperineal ultrasound is performed to diagnose LAM avulsion. It is advisable not to make a diagnosis of LAM avulsion until 3 months after delivery to avoid diagnostic errors [41], and this recommendation was qualified by a recent meta-analysis: Rusavy et al. [37] advised that the diagnosis of LAM avulsion be made 6 months after delivery, or 12 months after FD. On the other hand, different authors have mentioned that LAM avulsion can disappear over time [42, 43, 44, 45]. In fact, we recently reported that partial avulsions can present an improvement toward an intact LAM within 9 months postpartum [46]. There are two types of avulsions: Type I LAM avulsion is a lesion that can recover over time, and type II LAM avulsion is not recoverable [47]. This aspect has not only been described by transperineal ultrasound [47] but has also been established in anatomical studies and by MRI [48]. In this meta-analysis, the vast majority of studies performed ultrasound examinations up to 6 months after delivery [11, 12, 13, 14, 15, 16, 17, 19, 20, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35], but they did not assess its evolution over time. LAM avulsion produces an alteration in the support of the pelvic floor structures, however this fact should not influence the decision to implement a delivery. In fact, a recent Cochrane review establishes that there is no instrumentation that guarantees maternal and fetal safety [49]. However, instrumentation at delivery will depend on the skill of the operator, the selection of available instruments, and the clinical setting [49].

The main strength of this meta-analysis lies in its inclusion criteria: Only those studies with primiparous women after instrumented deliveries were included, and LAM avulsion was diagnosed only by 3D/4D transperineal ultrasound. One shortcoming is that the time of the ultrasound exam was not the same in all the articles, and most studies do not specify the type of instrument and the techniques used. In addition, there are no clinical trials that could provide solid evidence for the question that motivated this meta-analysis.

In conclusion, delivery instrumented with forceps or vacuum favors LAM avulsion. Current data cannot establish whether this injury is attributable to the specific instrument or to the type of delivery involved in the instrumentation itself.

LAM, Levator ani muscle; 3D, Three-dimensional; MRI, Magnetic resonance imaging; NVD, Normal vaginal delivery; FD, Forceps delivery; VD, Vacuum delivery.

JAGM and JASB designed the research study. JAGM performed the research. AFP analyzed the data. CLP, AFP and JAGM wrote the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

Not applicable.

Thanks to all the peer reviewers for their opinions and suggestions.

This research received no external funding.

The authors declare no conflict of interest. JAGM and JASB are serving as the Guest editors of this journal. We declare that JAGM and JASB had no involvement in the peer review of this article and have no access to information regarding its peer review. Full responsibility for the editorial process for this article was delegated to SM.