Retzius-sparing robot-assisted radical prostatectomy (RS-RARP) has been reported to exhibit better postoperative urinary continence compared to conventional RARP (C-RARP) via the anterior approach, but the reasons behind this are unknown. The early postoperative urinary incontinence and anatomical differences were compared between 51 cases each of C-RARP and RS-RARP, which were selected via propensity score matching. Dynamic-MRI was performed before and after surgery to investigate the pelvic anatomical changes under abdominal pressure. The median urine loss ratio in the early postoperative period was 11.0% and 1.0% for C-RARP and RS-RARP, respectively. Postoperative MRI revealed the anterior bladder wall after RS-RARP was fixed in a high position compared to C-RARP. Dynamic-MRI after C-RARP showed cephalocaudal compression of the bladder during abdominal pressure caused expansion of the membranous urethra and the urine flowed out. In RS-RARP cases, the rectum moved forward during abdominal pressure to compress the membranous urethra by closing it from behind, as was observed preoperatively. This was the first study using dynamic-MRI which revealed the importance of a high attachment of the anterior bladder wall for the urethral closure mechanism during abdominal pressure. RS-RARP, which can completely preserve this mechanism, was the least likely to cause stress urinary incontinence.
Research Article
Investigating The Mechanism Underlying Urinary Continence Using Dynamic-MRI After Retzius-sparing Robot-assisted Radical Prostatectomy
https://doi.org/10.21203/rs.3.rs-805834/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 10 Mar, 2022
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Robot-assisted radical prostatectomy (RARP) is the standard treatment for localized prostate cancer (PCa). However, one of the primary complaints affecting the quality of life (QOL) after RARP is urinary incontinence 1. Despite reports of RARP providing better postoperative urinary continence than conventional open or laparoscopic radical prostatectomy (RP) 2,3, its actual outcomes are not completely satisfactory 4. Galfano et al. first introduced the transperitoneal posterior approach RARP which preserves the Retzius space, aptly called Retzius-sparing RARP (RS-RARP) 5. RS-RARP has been reported to exhibit better postoperative urinary continence compared to conventional RARP (C-RARP) via the anterior approach 6,7. Several explanations were proposed for this, such as suppressing the descent bladder and maintaining a long membranous urethral length (MUL) after RS-RARP contributed to urinary continence 8, and that less bladder neck descent was observed on postoperative cystogram after RS-RARP than in those after C-RARP 9. However, no clear explanation has been given.
In this study, we compared the postoperative urinary continence of C-RARP and RS-RARP via propensity score matching. Also, we compared the anatomical changes in the pelvic region before and after each procedure via MRI. The mechanism of stress urinary incontinence (SUI), which is considered to be the main cause for urinary incontinence after RP 10, was also evaluated using dynamic-MRI for anatomical changes during abdominal pressure before and after surgery, respectively.
Patient population
This study enrolled patients with clinically localized PCa undergoing C-RARP (191 patients) and RS-RARP (51 patients) performed by a single surgeon at Kanazawa University Hospital (Japan) between February 2016 and May 2020. These study protocols were approved by the Medical Ethics Committee of Kanazawa University (approved No. 2012-027(1223)). All patients provided written informed consent, and all data were prospectively collected. All methods were performed in accordance with relevant guidelines and regulations.
Surgical technique
C-RARP was performed via a transperitoneal approach, while RS-RARP was performed similar to that described by Galfano et al 5. Nerve sparing (NS) procedures were performed depending on cancer status. Urethral catheters were removed 6–8 days postoperatively after cystographic evaluation.
Dynamic-MRI and each parameter
MRI was performed using the 1.5- or 3.0-TT MR system (Signa Premier or Signa HDx; GE Medical Systems, Waukesha, WI, USA or Ingenia, Philips Healthcare, Best, The Netherlands) with a multi-channel anterior array coil in combination with a multi-channel posterior table coil preoperatively, approximately 3 days after catheter removal after RARP. MRI was performed 30–60 min after urination, and the estimated bladder capacity in each patient was 30–100 mL based on the MRI results. After multiplanar T2-weighted axial section imaging, an adequate sagittal section was created to capture the prostatic urethra. The sagittal dynamic MRIs were acquired at resting and during the abdominal pressure phase using a fast spin-echo sequence with the following parameters: repetition time/echo time: 1,500–3,716 ms/85–104 ms, flap angle: 90°, slice thickness: 6 mm; field of view: 300 mm, and imaging matrix: 224–352 × 156–224. Figure 1 illustrates the measurements of the following: distance from the distal end of the membranous urethra to the pelvic outlet (DMU-PO) 11,12, distance from the bladder attachment to the pubic symphysis (BA-PS), the MUL, prostate length (PL), and distance from the pubic symphysis to prostate apex length (PAL) 13, and the length from the bladder neck to the pubic symphysis (BN-PS) 8.
The micturition volume (MV) and weight of urine loss (UL) in the pads were separately assessed after daily catheter removal. The UL ratio (ULR) was calculated using the formula UL / (UL + MV). The urinary continence recoveries within 6 months, defined as patient-reported use of 0 pads or one security liner per day, were evaluated.
Statistical analysis
Propensity score matching was performed between the C-RARP and the RS-RARP groups. The following ten parameters were used for matching: age, body mass index, PSA before biopsy, neoadjuvant androgen deprivation therapy, biopsy Gleason Grade Group, preoperative IPSS total score, IPSS-QOL score, International Consultation on Incontinence Questionnaire-Urinary Incontinence Short Form total score, clinical T stage on prostate MRI, and removed prostate weight. Each group comprised of 51 cases.
The categorical variables used for calculating the incidence and percentage of each factor as well as the continuous variables were summarized as their median and interquartile range. When making comparisons, the chi-square test was used for categorial variables, while the Mann-Whitney U test was used for continuous variables. Urinary continence recovery rates were calculated using the Kaplan-Meier methods, and compared between the groups by the log-rank test. All data analyses were performed using the SPSS for Windows (SPSS Inc., Chicago, IL, USA). P < 0.05 was considered statistically significant.
General characteristics. After propensity score matching of the ten variables, none were significantly different between C-RARP and RS-RARP. The number of cases undergoing NS procedure and lymph node dissection was higher in C-RARP patients. No statistically significant difference was found in console time, excluding lymph node dissection. Although the amount of blood loss was higher in RS-RARP, no cases in both groups required blood transfusion or were Grade 3 or higher in the Clavien-Dindo classification. The positive surgical margin (PSM) tended to be higher in RS-RARP, but it was not statistically significant (Table 1).
Postoperative urinary continence status. No cases of preoperative use of pads were found in both groups. The median ULR in the early postoperative period after removal of the indwelling catheter was 11.0% in C-RARP patients and significantly lower at 1.0% in RS-RARP patients (Fig. 2A). Kaplan-Meier methods showed significantly better recovery of urinary continence within 6 months in RS-RARP patients (Figs. 2B, C).
MRI findings. Among the patients reviewed in this study, 47 C-RARP and 50 RS-RARP patients who underwent MRI preoperatively and within 1 week of postoperative indwelling catheter removal had their bladder and membranous urethra anatomically examined. Preoperative MRI showed a significant difference only in BA-PS (Table 2). No change was found in BA-PS in the RS-RARP between pre- and postoperative comparisons. However, in the C-RARP, the median BA moved caudally by 13 mm, which was a significant difference (Table 2). Dynamic-MRI evaluation showed that, in both groups of patients preoperatively, the rectum moved forward and compressed the membranous urethra to close it from behind during abdominal pressure (Figs. 3A, B, E, F). Postoperative dynamic-MRI showed that the bladder was compressed cephalocaudally during abdominal pressure in the C-RARP group and that the membranous urethra was expanded, causing urine to flow out (Figs. 3C, D). Conversely, in RS-RARP patients, the rectum was observed to move forward during abdominal pressure and compressed the membranous urethra to close it from behind, as observed preoperatively (Figs. 3G, H).
As shown in this study, there have been many reports that postoperative urinary continence in RS-RARP is better than in C-RARP 6,7. This has been attributed to preservation of periurethral structures 5, higher bladder position, and longer MUL 8,9. However, these explanations made it difficult to clearly understand the reasons. Although researchers suggested that the main cause of urinary continence after RP is SUI 10, few reports examined the anatomical pelvic changes during abdominal pressure that occur after RP. In this study, dynamic-MRI was performed before and after RARP to compare the anatomical changes occurring in the pelvis during abdominal pressure. Preoperatively, the bladder was compressed caudally during abdominal pressure, and at the same time, the rectum was observed to move to compress the membranous urethra ventrally (Figs. 3B, F). A previous observation of the urethral sphincter using a urethroscope reported that the urethra moves dorsoventrally, like a shutter closing, during abdominal pressure 14. The present dynamic-MRI study suggests that pressure is applied to the bladder during abdominal pressure, and at the same time, pressure is applied dorsoventrally at the membranous area, which increases the urethral closing pressure and prevents urinary incontinence. Since the anterior surface of the bladder is widely fixed to the abdominal wall during abdominal pressure, the anterior surface of the bladder serves as a fulcrum, causing the pelvic organs to rotate as if sliding from the bottom to the front of the pelvic floor. As a result, the membranous urethra is thought to be compressed posteroventrally (Figs. 3B, F). Similar to the phenomenon observed preoperatively, dorsoventral compression of the membranous urethra was observed after RS-RARP during abdominal pressure (Fig. 3H). However, after C-RARP, it was thought that the rotational movement (to close the membranous urethra) did not occur during abdominal pressure because the anterior wall of the bladder as a fulcrum moved caudally (Fig. 3D). It would be advantageous to have the anterior surface of the bladder fixed to the abdominal wall at a high position in order for the urethra to close during abdominal pressure. In the present study, the bladder was fixed in a higher position in the RS-RARP than in the C-RARP (Table 2), which may have caused less urinary incontinence.
In women, the urethra is lined posteriorly by the vaginal wall. It is thought that during abdominal pressure, the pressure applied to the bladder and urethra also causes pressure on the vaginal wall, which in turn increases postoperative urinary continence and prevents urinary incontinence 15. Urethral hypermobility has been reported as a cause of SUI in women, and changes in pelvic anatomy during abdominal pressure have been reported in studies using dynamic-MRI 16. In women with urethral hypermobility, dynamic-MRI shows that entire pelvic organs evacuate inferiorly and anteriorly as if in a rotational motion during abdominal pressure, and clinically, the urethra rotates anteriorly from a vertical position as seen in the Q-tip test 17. In women with urethral hypermobility, the supporting structure of the vaginal wall lining the urethra is damaged, and the urethral closure pressure does not work properly, resulting in SUI 15. In men, no structure lining the urethra from the posterior aspect exists like the vaginal wall in women. A dynamic-MRI study after RP reported no evidence of urethral hypermobility with or without urinary incontinence 18. The mechanism of SUI after RP is slightly different from the mechanism of SUI associated with urethral hypermobility in women. However, they are considered to be identical in that urinary incontinence is caused by insufficient urethral closing pressure when pressure is applied distally from the bladder neck during abdominal pressure 15.
Previous reports showed that NS techniques may also exhibit a favorable effect on postoperative urinary continence 19. By performing NS, the dissection line is closer to the prostate, resulting in the preservation of the structures around the urethral sphincter, which is likely to work well for urinary continence. In the present study, few cases of NS were found in RS-RARP patients, but NS may possibly further improve postoperative urinary continence in patients who are eligible 5.
In this study, we examined the pelvic anatomy before and after RARP using MRI. On preoperative MRI at rest, no significant differences were found in the measured parameters between C-RARP and RS-RARP patients. Postoperative MRI showed no significant difference in PBN and MUL between the C-RARP and RS-RARP patients as well (Table 2). A previous report showed that the bladder neck position of RS-RARP was higher than that of C-RARP on postoperative cystography 9. In RS-RARP, the anterior bladder wall is fixed in a high position and the bladder neck is described as thin and stretched on postoperative MRI. When measuring the bladder neck via cystography, the boundary between the stretched bladder neck and the membranous urethra was difficult to identify because it depends on the amount of stored urine in the bladder. We suspect that simple cystography might cause variations in the measurement of the bladder base. Although we previously reported a slight shift of the membranous urethra to the cephalad side immediately after RP 11,12, the postoperative DMU-PO did not differ between the two groups, and the position of the membranous urethra immediately after surgery was similar in both groups (Table 2).
In transabdominal C-RARP, the peritoneum is incised, and the anterior bladder space is opened during the approach to the prostate. During reattachment after surgery, the anterior wall of the bladder is fixed more caudally than before surgery because it is moved by vesicourethral anastomosis. However, even if the peritoneum is sutured after transperitoneal RP or even if the retroperitoneal approach is used without peritoneal incision, the bladder is still strongly pulled caudally so that when the anterior bladder wall is reattached, the peritoneum is stretched and fixed more caudally than before the operation. In radical perineal prostatectomy, the position of the anterior bladder wall remains the same as preoperatively, but this is because the pelvic floor muscles are incised during the approach to the prostate; the pelvic floor muscles which support the membranous urethra from the periphery may be damaged, and urethral closing pressure may not be maintained. A previous report compared postoperative urinary continence between C-RARP and radical perineal prostatectomy and reported that early improvement was better with C-RARP 20. Therefore, we consider RS-RARP as the best reported technique to preserve urinary continence, because it does not damage the pelvic floor muscles and nor change the position of the anterior bladder wall 21. In addition, the combination of NS techniques could further improve postoperative urinary continence 19.
In this study, we compared C-RARP and RS-RARP performed by a single surgeon, and propensity score matching was performed on 191 C-RARP and 51 RS-RARP cases after the single surgeon performed more than 200 C-RARP cases. RS-RARP surgeries started in July 2017, but up to 30 cases after the start of RS-RARP, the selection was based on the following criteria: estimated prostate weight of 40 g or less, no prostate protrusion into the bladder, and no lymph node metastasis 22. C-RARP was considered to be a relatively technically stable procedure, while in contrast, for RS-RARP, the surgeon of this study was only starting to execute this and thus was in the middle of the learning curve for this procedure. Although no significant differences were found in the PSM rate, this tended to be higher in the RS-RARP group. The learning curve of RS-RARP has been reported in the past 23, and the PSM rate is expected to decrease in the future. The console time, excluding lymph node dissection, was comparable to that of the early stage of RS-RARP, and it was relatively safe to introduce without blood transfusion and complications from cases which were Clavien-Dindo classification 3 or higher. However, in RS-RARP, it was thought that case selection was more desirable in the initial stage because of the unfamiliarity with the narrow surgical field 22.
It is also important to discuss the limitations of this report. First, the present study is about short-term postoperative urinary continence and does not evaluate sexual function. Past reports have not shown that RS-RARP works well for sexual function 6,24. The present study suggests that RS-RARP works effectively only for urinary continence and that NS may be necessary to preserve sexual function. In the present dynamic-MRI evaluation, changes during abdominal pressure were evaluated, but urethral closing pressure at rest was not evaluated. Urethral closing pressure is important for preventing urinary incontinence at rest, and because of this, preserving the periurethral structures by leaving the urethra long and by sparing the nerves is important. The bladder is loosely fixed by the vascular pedicles, and after RP, there is resistance to dropping the bladder neck to the pelvic floor during urethrovesical anastomosis, which is thought to pull this cephalad 11,12. In C-RARP, the urethrovesical anastomosis is thought to be pulled cephalodorsally because the bladder vasculature is fixed from both dorsolateral sides. Conversely, in RS-RARP, the anterior bladder wall is widely fixed, and the urethrovesical anastomosis is thought to be pulled cephaloventrally as well. After RS-RARP, the urethra is pushed toward the pubic bone, and the urethral closing pressure at rest may be higher than that of C-RARP (Fig. 4). In the present study, neither urethral pressure profile nor urethral closing pressure at rest were evaluated.
To the best of our knowledge, this was the first study using dynamic-MRI which revealed the importance of the high attachment of the anterior bladder wall for the urethral closure mechanism during abdominal pressure. Among the RP techniques reported thus far, RS-RARP, which can completely preserve this mechanism, was considered to be the least likely to cause SUI.
BA-PS: bladder attachment to pubic symphysis
BN-PS: bladder neck to pubic symphysis length
C-RARP: conventional robot-assisted radical prostatectomy
DMU-PO: distal end of membranous urethra to pelvic outlet
MUL: membranous urethral length
MV: micturition volume
NS: nerve sparing
PAL: pubic symphysis to prostate apex length
PBN: pubic symphysis to bladder neck
PCa: prostate cancer
PL: prostate length
PSM: positive surgical margin
QOL: quality-of-life
RARP: robot-assisted radical prostatectomy
RP: radical prostatectomy
RS-RARP: Robot-assisted radical prostatectomy
UL: urine loss
ULR: urine loss ratio.
Author contributions:
Y.K. A.M. contributed to study concept and design. Acquisition of data was performed by S.K., H.I. and M.I. S.K. and K.S. contribute to analysis and interpretation of data. The first drafting of the manuscript was written by Y.K. and further edited by S.K. and K.I. S.K. and T.N. contributed to statistical analysis. Administrative, technical, or material support were made by K.Y. and T.G. All authors read the final manuscript and approval submission.
Financial disclosures
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Competing Interests
The authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to Yoshifumi Kadono.
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No competing interests reported.
- VideoS1.mp4
Additional Supporting Information may be found in the online version of this article. Video.S1. Dynamic-MRI showing pelvic anatomical changes during application of abdominal pressure before and after conventional and retzius-sparing robot-assisted radical prostatectomy.