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sphincter as a rhabdosphincter. This term suggests
that the external sphincter is an exclusively striated
muscle [3, 7].
From histomorphological investigations it is clear
that there is a smooth muscular sheet (dot in Fig. 2.3.2)
under the external striated muscle layer (star in
Fig. 2.3.2). This smooth muscular part of the external
sphincter is likely to ensure continence at rest after
resection of the internal vesical sphincter during radical
prostatectomy or transurethral resection. Histological
and functional observations support the notion
that the smooth muscular part of the external
urethral sphincter is mainly responsible for continence
at rest while the striated part provides reflex
continence during stress [9].
Several authors promote the notion that continence
at rest is also mediated by the striated urethral
sphincter. Indeed, some studies have demonstrated
that the striated external sphincter is made up of two
different striated muscle fibre types, the slow and the
fast twitch fibres [10]. The 'fast twitch' fibres are
thought to compensate for sudden abdominal pressure
increases. In males the proportion of 'fast twitch
fibres' is 65%, whilst in females it is 13%. The 'slow
twitch fibres' contribute to sustained urethral pressure
during filling [11]. In an effort to explain postoperative
urine continence, investigators hypothesised
that under certain conditions (i.e. in the absence of
smooth muscular vesical sphincter), the fibre type becomes
interchangeable. The switch to slow twitch fibres
should ensure continuous continence. Some have
postulated a threefold innervation of the striated urethral
musculature by somatic, sympathetic and parasympathetic
nerve fibres [12]. Nevertheless, the concept
of triple innervation of the urethral sphincter
must be viewed with caution.
During radical prostatectomy attention in surgical
dissection of the sphincter is focused on the integrity
of the urethral sphincter. Bladder neck-sparing prostatectomy
also involves the vesical sphincter, which continues
to be a subject of heated debate in the literature.
2.3.2 Vesical Sphincter
Former assumptions that the vesical sphincter could
be formed out of muscular slings from the detrusor
are contrary to current opinion. There is overall
agreement on the existence of a distinct muscular
structure but no consensus on the composition of the
Chapter 2.3 J.-U. Stolzenburg et al.
muscle. Some authors regard the continuation of the
detrusor lamellae to be the origin of all urethral musculature
[13, 14]. Others favour the concept of a floor
panel when describing the vesical sphincter [15, 16].
This panel is thought to consist ventrally of the detrusor
muscle und dorsally of trigonal musculature. In
our own study we showed that in a strictly transverse
dissection this structure really appears to be composed
of two different muscles (Fig. 2.3.3b), but the
bladder neck is not positioned in a totally perpendicular
plane. The physiological position of the entire
bladder neck has an oblique direction. Changing the
histological investigation plane accordingly, we were
able to identify a distinct muscle surrounding in a circular
manner the bladder neck, which is called internal
or vesical sphincter (musculus sphincter vesicae)
[17]. Lamellae of the detrusor do not participate in the
formation of this muscle (Fig. 2.3.3c, d).
2.3.3 Urethral Muscles
2.3.3 and Radical Prostatectomy
A further point of controversy is the craniocaudal extension
of the external sphincter over the prostate and
bladder. Dorschner’s groups and others advocate that
the urethral sphincter is ventrally more strongly developed.
Furthermore, the apex of the prostate is ventrally
overlapped by the striated muscle fibres of the external
urethral sphincter. In contrast, Oelrich et al. and
Myers et al. described a vertically orientated sphincter
muscle system, from the base of the bladder to the bulb
of the penis [18, 19]. Interestingly, Dorschner et al.
found two vertically orientated muscles in the urethral
muscular sheet, the ventrolateral longitudinal muscle
and the dorsal longitudinal muscle ( 2.3.4) [20–22].
The interpretation of the various longitudinally orientated
muscular structures remains unclear.
During radical prostatectomy we are able to identify
and preserve three muscular structures. The ringshaped
vesical (internal) sphincter can be preserved
during bladder neck-sparing radical prostatectomy.
However, this is not always possible due to anatomic
variation in the shape of the prostate (e.g. large middle
lobe). The most important component responsible for
postoperative urinary continence is the circularly orientated,
horseshoe-shaped urethral sphincter. The
main difficulty for the surgeon is the inability to identify
intraoperatively the exact border of the projection
of the urethral sphincter, overlapping approximately