Anatomical Relationship Between Anal Sphincters and the Prostate
The lower 30% of the external anal sphincter (EAS) does not exert pressure on the male prostate; the prostate sits entirely anterior and superior to the anal sphincter complex, separated by the rectoprostatic fascia (Denonvilliers' fascia). 1
Key Anatomical Principles
The anal sphincter complex and prostate occupy distinct anatomical compartments with no direct mechanical interaction:
The internal anal sphincter (IAS) provides 70-80% of resting anal canal tone but is located entirely within the anal canal, approximately 1.2 cm cephalad from the proximal margin of the EAS, with no anatomical contact with the prostate. 1, 2
The EAS extends inferiorly from the anorectal junction and wraps around the anal canal in three distinct loops (subcutaneous, superficial, and deep), each with separate innervation and function, but none extend high enough to contact the prostate. 3
The prostate gland sits anterior to the rectum at the level of the bladder neck and proximal urethra, separated from the rectal wall by the rectoprostatic fascia—a dense fibrous layer that prevents any direct pressure transmission from posterior pelvic floor structures. 4
Clinical Relevance of This Separation
Understanding this anatomical separation is critical for several clinical scenarios:
Digital rectal examination (DRE) accesses the prostate through the anterior rectal wall, not through the anal sphincters; the examining finger must pass completely through the anal canal and enter the rectal ampulla to palpate the posterior prostate surface. 5
Pelvic floor muscle contraction during sexual activity involves the voluntary EAS, puborectalis, and bulbospongiosus muscles via somatic pudendal nerve pathways (S2-S4), but these contractions do not compress the prostate—they generate ejaculatory pressure through urethral compression, not prostatic compression. 1
The IAS remains in its baseline involuntary state throughout sexual activity and does not participate in sexual function; its autonomic innervation (sympathetic tone maintenance, parasympathetic rectoanal inhibitory reflex) is functionally separate from sexual arousal pathways. 1, 6
Common Clinical Pitfall
A frequent misconception is that pelvic floor tension during sexual activity or defecation somehow "massages" or compresses the prostate:
This is anatomically impossible—the anal sphincters are located 3-5 cm inferior to the prostate apex, and the levator ani muscles (which do sit lateral to the prostate) contract in a direction that elevates the pelvic floor rather than compressing anterior structures. 5
Prostate massage during DRE requires direct anterior pressure through the rectal wall, not sphincter contraction; clinicians must distinguish between sphincter tone (which they feel as they enter the anal canal) and prostatic palpation (which occurs after advancing the finger 5-7 cm into the rectum). 5
Surgical Implications
The anatomical separation between sphincters and prostate explains why:
Radical prostatectomy does not directly damage the anal sphincters, though it may injure the nerves supplying the IAS (which originate at the posterolateral corner of the prostate) as they course inferiorly along the levator ani muscle. 7
Post-prostatectomy incontinence reflects urethral sphincter dysfunction, not anal sphincter involvement; the artificial urinary sphincter (AUS) is placed around the bulbar urethra, which is anterior and superior to the anal canal. 4
Low anterior resection of rectal cancer is most likely to damage the nerves supplying the IAS because of their intersphincteric course, but this affects fecal continence, not urinary or sexual function. 7