As the prostate gland consists of the distal part of the male urethra, it is adjacent to several anatomical structures involved in maintaining urinary continence. As a result, in cases of resection involving the surrounding area, which is the aim of radical prostatectomy, the more successful the process of resection, the greater the risk of impairing urinary continence due to unintended damage to surrounding structures. Considering the close involvement of periprostatic structures in urinary continence, in most patients, post-prostatectomy incontinence (PPI) shows a pattern of early occurrence followed by gradual recovery over time. Recovery from PPI is usually evaluated by asking the patient about incontinence pad usage, rather than obtaining objective evidence via a 24-hour pad test. This means that the reported incidence is affected by the duration of the investigation and the types of questions. To date, the incidence reported by different surgeons considerably varies, in the range of 8-77%[5], but this seems to be, in part, due to authors using different definitions. Specifically, up to the early 2000s, the most common definition of recovery from incontinence was using 0 or 1 pad per day postoperatively, and this typically included the use of “security” pads. This led to a large number of patients being classified as having recovered from incontinence; however, major differences in quality of life were observed between patients who still used pads in daily living and those who did not, so this classification was criticized for only making surgeons’ outcomes appear more favorable. Indeed, one study found that among 1,616 patients who were followed up for a mean of 50.7 months (range, 12-216 months) after open retropubic prostatectomy, 1,459 (90.3%) reported experiencing urinary leakage, which clearly shows the enormous effect of the definition of the prevalence of the condition [6]. With regard to the appropriate definition of PPI, Ahlering et al. divided 500 patients who had undergone RARP into groups with daily pad usages of 0 and 1, and reported a significant difference in quality of life between the groups (1.16 vs. 3.41, p<0.0005), arguing, on this basis, that recovery from PPI should be defined as using 0 pads per day [7]. Likewise, the definitions of survival, continence, and potency (SCP) proposed in 2012 by world-leading surgeons as objective indicators to evaluate the outcomes of radical prostatectomy also consider 0 pad usage to be the standard definition for recovery from PPI [8]. However, different authors are still using their own definitions in studies published since then; thus, some caution is required when interpreting individual studies.

Although we still cannot claim to fully understand the mechanisms of urinary continence in males, alongside advances in radical prostatectomy techniques and increased numbers of operations, a large amount of related anatomical knowledge has been accumulated. In the 1980s, on the basis of the classical anatomical theory, the urogenital diaphragm, lying flat below the prostate gland, was thought to be a key structure in urinary continence by acting as the urethral sphincter; however, at the end of the 1990s, human and cadaver studies showed that the structure previously identified as the urogenital diaphragm did not exist [9,10].

In terms of our present understanding of the mechanisms of male urinary continence, a review published in European Urology in 2010 provides a representative collection of results, and this review was updated in 2016 with the latest research [11,12].

Currently, male urinary continence is understood to be achieved by the combined actions of multiple anatomical structures surrounding the prostate gland; below, we summarize the major constituent muscular structures in this urethral sphincter complex and their roles.

(1) The smooth muscle sphincter (lissosphincter) consists of two layers (inner longitudinal and outer circular) and is innervated by the autonomous nervous system. It forms a spongy structure below the urethral mucosa, and external contraction completely cuts off the flow of urine. (2) The stratified sphincter (rhabdosphincter; the posterior part forms the median fibrous raphe [MFR] with no muscle layer) is responsible for the slow-twitch, passive control. It forms a cylindrical shape that originates from the prostate apex and attaches to the deep transverse perineal muscles. In the transverse crosssection, the muscle in the stratified sphincter is distributed in an omega shape; posteriorly, there is no muscle, but instead, forming the MFR are dense fibrous tissues. The MFR forms a posterior support complex by connecting to the central tendon posteriorly and Denonvilliers’ fascia superiorly. (3) The puboperinealis muscle is responsible for fast-twitch, active control. It forms the medial part of the levator ani muscle. In the coronary view of magnetic resonance imaging, the puboperinealis muscle appears as two teardrop shapes running bilaterally, lateral to the urethra. As it attaches to the perineal body posterior to the urethra, the puboperinealis muscle ultimately forms a structure that supports the urethra [13].

On the basis of the understanding of the urethral sphincter complex described earlier, widespread efforts have been made to mitigate PPI by identifying and preserving this complex during surgery. By reviewing the current literatures, the techniques available during radical prostatectomy can be identified [14]. Conceptually, these techniques can be summarized as preservation of the internal/external sphincters and reconstruction of anterior/posterior support structures.

Considering that problems with incontinence can have a major effect on quality of life, functional recovery is a primary objective. The 1-year recovery rate is reported to be >90% due to surgical measures based on the improved anatomical knowledge discussed earlier, and of those patients who were still showing incontinence after the first postoperative year, approximately half recover their PPI within 3 years postoperatively [28]. Conversely, the recovery rates from postoperative erectile dysfunction reported in several papers do not exceed 10-29%. Although several causes have been identified, the low recovery rate from erectile dysfunction is, unlike incontinence, largely due to the lack of consistent criteria for recovery. Approximately half of the reports on this topic up to 2009 used self-reports of successful sexual relations by the patients as evaluation criteria for recovery; 37% of the reports did not even indicate the criteria for potency recovery [29], and those that did state the criteria for recovery used very different criteria. Even among studies that used the International Index of Erectile Function (IIEF) 5, which is the most commonly used questionnaire for erectile function, different cutoff scores of 21, 17, and 15 were used. Moreover, many researchers, whether using the established IIEF or a less established questionnaire, only select some questions to use in the assessment. Among the various criteria proposed to date, erection sufficient for intercourse (ESI) is one of the most commonly used definitions and has been used most commonly in reports on robotic surgery; however, objectivity is difficult to achieve because this is not a quantifiable index.

In recent reports, the second leading cause of the low recovery rate from erectile dysfunction is that unlike urinary incontinence, preoperative function cannot easily be assessed postoperatively in all patients. In other words, age, preoperative function measured with the IIEF-5, and the extent of intraoperative NVB sparing are well-proven independent factors that directly affect recovery of erectile function. However, because these variables differ among individuals, consistent postoperative improvements are difficult to identify. Thus, to control for these variables, most reports on erectile dysfunction have only included a subset of patients such as those aged <60 years; those with normal preoperative erectile function, defined as an IEF-5 score of at least 22; and those with complete bilateral preservation of the NVB during surgery. However, the patients satisfying these conditions typically only account for <10% of all patients [30].

The third factor that affects the recovery rate from erectile dysfunction is the use of phosphodiesterase (PDE)-5 inhibitors. Although it is true that the release of sildenafil in 1997 revolutionized the treatment of erectile dysfunction, various types of PDE-5 inhibitor are available, and as they have become established as the primary treatment for erectile dysfunction, postoperative improvements tended to be considered in erectile function as recovery irrespective of whether or not the patient is taking PDE-5 inhibitors. Salonia et al. criticized this trend while arguing that the effects of PDE-5 inhibitors and spontaneous recovery of erectile function should be differentiated [31].

To resolve the issues discussed earlier, in 2012, authorities in the field of prostate surgery proposed the SCP system, which can consistently describe functional and oncological outcomes after radical prostatectomy. In this system, S is classified as S1 if the prostate-specific antigen (PSA) level is ≥0.2 ng/mL; C is classified as C0 in cases of 0 pad usage, as mentioned in the discussion of incontinence; and P is classified as P0 for scores of ≥18 in the Sexual Health Inventory for Men (SHIM) without medication, and P1 for SHIM scores of ≥18 while using a PDE-5 inhibitor [8]. This raised expectations that it would be possible, by limiting subjective assessment factors that depend on the researcher, to quantitatively evaluate erectile function, but still only few reports used this system. Nevertheless, the system is important insofar as it provides criteria for erectile function that can be perceived objectively in the future, and cutoff values based on SHIM, in particular, provide a direction for future studies in the field.

Although the development of urinary incontinence can be explained by intraoperative injury to various structures affecting urinary continence, postprostatectomy erectile dysfunction has more simple causes, and the most important factor is injury to the cavernosal nerves. These nerves are the most important structures in maintaining healthy erectile function; owing to their anatomical relationship with the prostate, running in the 3 and 9 o’clock directions from the apex of the prostate, and along the posterolateral aspect of the prostate body, injury during radical prostatectomy is inevitable, and this results in postoperative erectile dysfunction. Mechanisms for this nerve injury include unintended physical damage to the nerve itself during surgery, but the extent of nerve injury is reported to differ, and neuropraxia, in particular, is known as a representative mechanism of nerve injury capable of recovery and rehabilitation [32]. Owing to continual efforts to minimize intraoperative nerve injury, when complete resection is not oncologically necessary in patients with preserved preoperative erectile function, bilateral nerve-sparing surgery has become a major trend in modern surgery. Minimizing unintended intraoperative damage to the NBV is considered the most important principle in preserving erectile function.

Meanwhile, fibrotic remodeling, cavernous smooth muscle apoptosis, and ischemic nerve injury can also have negative effects on the recovery of erectile function, and the use of various regenerative materials has recently been reported as another potential strategy to restrict the inflammatory response and promote regeneration.