Endovaginal Imaging: Slings

Fig. 10.1

180° scan of anterior compartment: midurethral retropubic sling seen as a hyperechogenic structure beneath midurethra. Bladder (B), urethra (U), pubic symphysis (P), urethrovesical junction (UVJ), midurethral retropubic sling (S)

Fig. 10.2

180° scan of anterior compartment: retropubic sling seen proximal to the urethrovesical junction. Bladder (B), urethra (U), pubic symphysis (P), urethrovesical junction (UVJ), retropubic sling (S)

360° Scan

Similarly, while scanning with the 2052 probe , it is important to ensure that the probe is inserted in the vagina in the neutral position and has been inserted cephalad enough to capture all important details. Though normally the cephalad extent of the 3D scan should begin just proximal to the urethrovesical junction, so that in the sagittal cut of the data volume a small portion of the bladder is seen narrowing into the urethrovesical junction, it must be kept in mind that in the case of slings that have not fixated suburethrally and hence are located proximally beneath the bladder, the 3D scan may need to begin even more cephalad. At the same time, it is necessary to ensure that the caudad extent of the 3D data volume is beyond the external urethral meatus so that the urethra is imaged along its entire length. Hence often it may be necessary to capture two 3D data volumes along the length of the urethra to ensure that all important structures are included. Increasing the depth may help to include the entire extent needed in a single data volume, but it must be remembered that increasing the depth reduces the resolution and decreases the image size.

Manipulation of the 3D Data Volume to Trace the Intrapelvic Course of Slings, Retropubic, and Transobturator

The 180° 3D data volume of the anterior pelvic compartment (probe 8848) can be manipulated in the sagittal , axial, or coronal planes. However, to track the intrapelvic course of a sling, it is preferable to begin with manipulation in the sagittal plane. It is important to first orient to the egocentric coordinates, i.e., the relative directions of the data volume, or more simply put, it is important to first understand which sagittal surface of the data volume denotes the left of the patient, and which surface the right. Depending on how the 3D external mover moves the probe, the 3D scan may begin on the left or right of the patient. If the 3D external mover is programmed to begin the scan on the left of the patient, then the data volume is constructed progressively in real time during scanning from the left to the right.

As one begins manipulating the 3D data volume in the sagittal plane, the arm of the sling on that side progressively comes into vision. The arm of the sling, in case of retropubic slings, can be seen, exhibiting a mesh-like weave, extending until the pubic symphysis. In the case of transobturator sling, the arm can be seen extending at a more obtuse angle beyond the pubic symphysis. The sling can be tracked behind the urethra in the midsagittal plane and then can be seen extending on the other side to the pubic symphysis in case of retropubic slings or beyond the pubic symphysis at a more obtuse angle in case of transobturator slings.

The location of the sling behind the urethra in the midsagittal plane will vary, depending on the type of sling. In case of TVT slings (see Fig. 10.1) and transobturator slings (Fig. 10.3), the sling will be seen as a hyperechogenic horizontal structure beneath the midurethra, and in the case of a bladder neck sling, it can be seen beneath the proximal urethra with its proximal end at the urethrovesical junction (Fig. 10.4).

Fig. 10.3

180° scan of anterior compartment: transobturator sling seen as a hyperechogenic structure beneath midurethra. Bladder (B), urethra (U), pubic symphysis (P), urethrovesical junction (UVJ), transobturator sling (S)

Fig. 10.4

180° scan of anterior compartment: pubovaginal sling seen at bladder neck. Bladder (B), urethra (U), pubic symphysis (P), urethrovesical junction (UVJ), pubovaginal sling (S)

The intrapelvic course of the slings can be tracked more easily when the rendered volume of the data volume is manipulated (Fig. 10.5). Often one may need to manipulate the data volume in oblique parasagittal planes to be able to track the sling course better.

Fig. 10.5

180° scan of anterior compartment: rendered volume, single incision sling arm seen on the right of the patient extending beyond the pubic symphysis until the obturator foramen. Pubic symphysis (P), obturator foramen (OF)

Manipulation of the 3D data volume obtained with the 2052 probe can also be done in the axial, sagittal, and coronal planes. However, transobturator and retropubic slings can be more easily differentiated in the axial plane. Manipulation in the axial plane adds to the information obtained from sagittal manipulation of the data volume obtained via the 8848 probe as we are able to look at the sling from a different angle. The retropubic sling can be seen in the axial plane in a u-shaped curve hugging the urethra (Fig. 10.6), while the transobturator sling can be seen extending hammock-like to the obturator foramen bilaterally (Fig. 10.7). The slings can be seen better when the rendered volume of the data volume is manipulated. We may also need to manipulate the data volume in oblique planes to follow the course of the sling until its insertion points bilaterally.

Fig. 10.6

360° scan. Retropubic sling seen hugging the urethra in a u-shape. Urethra (U), anal canal (A), levator ani muscles (LA), probe (T), midurethral retropubic sling (S)

Fig. 10.7

360° scan. Transobturator sling seen extending hammock-like to the obturator foramina bilaterally. Urethra (U), anal canal (A), levator ani muscles (LA), probe (T), transobturator sling (S)

2D Dynamic Functional Assessment of the Slings

The in vivo behavior of the sling during periods of stress, namely, cough, Valsalva, and squeeze maneuvers, can be assessed by recording 20s 2D cineloops in the midsagittal plane or the axial plane using the 8848 probe (endovaginal ultrasound of the anterior compartment) or an abdominal curvilinear probe (transperineal ultrasound). These cineloops can also be stored for offline analysis and are available as a permanent record of in vivo sling behavior.

2D Dynamic Functional Assessment of Slings and Its Correlation with Outcome

Dynamic interaction of the urethra with the midurethral sling is a crucial determinant of the outcome following sling surgery [8]. Midurethral positioning of the sling has been regarded as important in achieving urinary incontinence , as it allows the tape to act as a fulcrum to produce dynamic kinking of the urethra [9] or as a mechanical device to enhance the increase of intraurethral pressure [10, 11] with stress. Dynamic kinking of the urethra with straining has been seen in 87–92% of women in whom a midurethral sling has been implanted [12]. Therefore, theoretically, dynamic changes in the interaction of the urethra with the sling during periods of sudden and/or sustained stress appear to be a crucial factor in ensuring successful outcomes following midurethral sling surgery [8]. Thus dynamic functional assessment of in vivo sling behavior may prove crucial for improving our understanding of the mechanism of action of the midurethral sling and help delineate reasons for failure [8].

We conducted an unmatched case study of 100 patients returning for their 1–2 year follow-up visit following transobturator sling surgery (Monarc, American Medical Systems, Minetonka, Minnesota, USA) in which we compared deformability of the sling on Valsalva, concordance of urethral movement with the sling, and sling location on maximal Valsalva between two groups: group A (n = 50) patients had successful outcomes and group B (n = 50) patients had suboptimal outcomes 1 year following surgery [8]. 3D cubes were manually obtained using transperineal probe over 30 s at rest and over 6 s in maximal Valsalva in addition to 2D cineloops on Valsalva.

Deformability of the Sling

The dynamic change in shape of the tape in the 2D cineloop film was used to categorize three types of sling deformability :

1.

Parallel to the urethral lumen at rest (flat or slightly curved in shape along its width at rest) and deforms to a c-shape on maximal Valsalva
2.

Parallel to the urethral lumen, both at rest and during maximal Valsalva: The tape remains flat or slightly curved in shape along its width and does not deform to a c-shape on maximal Valsalva
3.

C-shaped at rest and during maximal Valsalva : The tape remains c-shaped along its width both at rest and during maximal Valsalva.

Location of the Sling on Maximal Valsalva

The 3D cube obtained over 6 s during maximal Valsalva across the mid sagittal plane was analyzed to determine location of the sling on maximal Valsalva.

Concordance of Urethral Movement with the Sling During Maximal Valsalva

The 3D cube obtained over 30 s at rest was analyzed to determine the location of the sling relative to the urethral length at rest. The sling location at rest was compared to that on maximal Valsalva . If the sling location on maximal Valsalva relative to the urethral length was identical to that at rest, the urethra was considered to move concordant with the sling [8]. If the sling location on maximal Valsalva relative to the urethral length differed from that at rest, the urethral movement in relation to the sling was considered discordant [8]. Concordance of urethral movement with sling was assessed on 2D dynamic assessment also.

When compared with group B, group A had a significantly greater number of patients in whom the sling deformed on Valsalva (flat at rest and curved into a c-shape on Valsalva), the urethral movement was concordant with the sling, and the sling was located beneath the midurethra (p < 0.0001). The urethrovesical junction moved distal to the sling in 8 (26.7%) patients in group B who had discordant movement of the urethra relative to the sling. Therefore the data suggest that on 2D and 3D transperineal ultrasound, the best outcomes following midurethral transobturator sling surgery are found to be associated with concordance of urethral movement with the sling and midurethral location at maximal Valsalva followed by deformability of the sling on dynamic assessment [8]. Thus dynamic assessment of sling function helps to understand the mechanism of sling action. Significantly, though dynamic kinking of the urethra is what has been considered the reason for continence achieved with the help of the sling previously, dynamic assessment shows that the effect is due more to dynamic compression than to actual kinking at the urethral knee.

Interestingly, a patient in whom the sling does not deform on Valsalva (i.e., does not curve into a c-shape from flat at rest along its width) may still have a successful outcome if the sling is located in the correct location (midurethral) at rest and the urethra moves in a concordant manner with the sling. Conversely, a patient, in whom the sling deforms on Valsalva may still have a poor outcome if the urethra moves in a discordant manner with the sling and/or the sling is not located beneath midurethra. We observed that this is because the three parameters often work together to compensate for the failure of an individual parameter to ensure successful outcome. Hence it is important to examine all three parameters of dynamic assessment while assessing a patient.

When the urethra and the sling do not move in a concordant fashion, i.e., the urethra moves independently of the sling, it may be that the sling has not fixated itself well to the suburethral connective tissue or that the sling has been inserted too loosely; therefore, even though the sling has scarred in following surgery, the urethra and surrounding tissue move independently of it [8]. Accordingly, even if the midurethral or bladder neck sling is confirmed on static 2D and 3D ultrasound to be placed in the correct location, dynamic assessment may show that the urethra moves independently of it on dynamic assessment. As seen in our study (unpublished data), in some patients with failed slings, the urethrovesical junction may even move distal to the sling on dynamic assessment. Therefore the sling does not have the desired functional effect and may fail.

Kociszewski et al. correlated the dynamic changes in TVT sling shape seen on transperineal ultrasound with outcomes following TVT sling surgery in 72 women [9]. They found that 98% of patients, in whom the tape was flat at rest along its width in the midsagittal plane and curved into a c-shape during straining, were continent after surgery. There was improvement in one case (2%), and none of these patients was classified as failure. However, in 39% of the patients, no change was visible in the sling shape along its width on straining in the midsagittal plane. In the 11% of patients in whom the tape position was flat along its width at rest and during straining (i.e., too far away from the urethra), the failure rate was highest at 25%. In the 28% of patients in whom the sling was c-shaped along its width at rest and on straining, the failure rate was 10%.

We followed up the previous study with a second study in which we correlated the dynamic assessment of sling function on transperineal ultrasound with outcomes 1 year following surgery in 94 patients who had undergone retropubic midurethral sling surgery (Gynecare TVT Retropubic System, Ethicon, Somerville, New Jersey, USA) [13]. Our hypothesis was that, due to its retropubic location, the TVT sling procedure may be associated with increased tape tension and urethral compression that may compensate for any inappropriate sling location while still maintaining continence [13]. We found that even in the case of retropubic midurethral slings, the best outcomes following surgery are found to be associated with concordance of urethral movement with the sling and midurethral location at maximal Valsalva followed by deformability of the sling on dynamic assessment.

2D Dynamic Assessment of Deformability of Different Sling Types

Since deformability of slings has been shown to have an impact on the outcomes following sling surgery [8], it may be beneficial to compare the deformability of different slings available on the market. This is especially important given the fact that surgeons differ in the sling types they use. At our center, we use an inelastic retropubic sling (I-STOP, CL Médical, Sainte Foy Les Lyon, France) placed at the bladder neck in patients with intrinsic sphincter deficiency. The I-STOP sling has lower elasticity and lower deformability as compared to other slings [14]. We find that an I-STOP sling lies flat at rest (see Fig. 10.4) against the urethra and, on dynamic assessment, it moves with the urethra and constricts the bladder neck without deforming or bending into a c-shape along its width. This is synonymous with its mechanism of action, which is increasing resistance at the bladder neck during periods of stress as opposed to that of elastic midurethral slings, which act by causing dynamic compression. Thus it is easy to distinguish the I-STOP sling from other slings that have higher elasticity and greater deformability: e.g., TVT, Monarc, and the SPARC Sling System (American Medical Systems, Minnetonka, Minnesota, USA) slings.

In an unmatched case-control study of 120 patients returning to our center for one-year follow-up following sling surgery , we compared the in vivo deformability of three different slings [15]. The study group A consisted of 40 patients who had undergone retropubic bladder neck sling surgery (I-STOP) and groups B and C consisted of 40 patients who had undergone retropubic midurethral TVT sling surgery and transobturator Monarc sling surgery, respectively. The change in the distance of the proximal, midpoint, and distal ends of the tape from the midpoint of the urethral lumen and the change in the TSd (distance between the midpoint of the tape and the inferior border of the symphysis pubis in the sagittal plane) and TSa (the angle between a line from the midpoint of the tape to the inferior border of the symphysis pubis and the midline of the symphysis pubis in the sagittal plane) did not vary between the three groups. The tape width at rest was significantly more in group A when compared with group B and C (p < 0.001). The number of patients in whom the tape lay flat against the urethra at rest was significantly more in group A than the other groups (p < 0.001). The change in tape angle was significantly less in group A when compared with groups B and C (p < 0.001).

We therefore concluded that different sling types vary in their deformability. The I-STOP sling is more likely to lie flat at rest and not to deform during dynamic stress events when compared with the TVT and Monarc sling tapes. However, we suggested that since the I-STOP sling is inserted under proximal urethra, the impact of the interaction between the location of the sling and deformability of the sling on outcomes needs to be assessed [15].

We also conducted a prospective study in which we compared an inelastic, i.e. non-deformable pubovaginal sling (I-STOP), with a deformable midurethral sling (TVT) with respect to dynamic assessment of the sling function (deformability, location of sling, and concordance of urethral movement with sling) on 2D and 3D transperineal ultrasound [16]. We found that on 2D and 3D transperineal ultrasound, concordance of urethral movement with the sling was correlated with successful outcome following sling surgery with both non-deformable and deformable slings. Also proximal placement of sling that does not deform on straining seems to enable application of steady compression effect at the proximal urethra in patients with intrinsic sphincter deficiency and achieve similar effects as that achieved with dynamic compression midurethrally with a deformable sling [16].

Diagnosis and Planning of Future Treatment in the Case of Failed Sling Surgery

In patients who have poor outcomes following sling surgery, multicompartment 3D imaging can often be invaluable in delineating the cause for failure and also in planning future treatment. In this section we will discuss various scenarios we encounter in such patients at our center.

Unknown Sling Type

Often the patient is unaware of the exact nature of the previous sling surgery. 2D imaging cannot delineate the type of sling on either transperineal ultrasound or endovaginal ultrasound. A midurethral sling, whether retropubic or transobturator, will appear as a hyperechogenic horizontal structure behind the midurethra (see Figs. 10.1 and 10.3). A bladder neck sling that is located correctly at the urethrovesical junction should ideally be easy to distinguish from a midurethral sling based on location. But in patients with poor outcome following surgery, a midurethral sling is often found to be located too proximally. Therefore a sling that is found to be located under the proximal urethra may not necessarily be a bladder neck sling, but could be a midurethral sling.

Multicompartment 3D imaging including dynamic functional assessment is very useful in determining the type of sling that was inserted in such patients. As described above, the intrapelvic course of the sling can be tracked by manipulating the 3D data volume. The sling can be examined in the three different data volumes obtained with transperineal ultrasound, 180° endovaginal scan with the 8848, and/or 360° scan, and hence the diagnosis obtained through one probe can be confirmed through the other. Rendered volumes can be used to track the sling better.

As detailed in the previous section, dynamic functional assessment helps to distinguish slings based on elasticity and deformability. It is also possible to distinguish different types of materials, with the previous-generation intravaginal slingplasty being much less echogenic than the TVT [6]. Because the I-STOP sling is less deformable, it appears fatter and wider than TVT or Monarc slings. Also since the SPARC sling carries a central suture that prevents pre-tensioning [17], it generally seems flatter and wider than TVT sling [6].

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