Sphincter Complex
Components
The sphincter complex defines the surgical anal canal and consists of:
Internal Sphincter
- Continuation of the circular muscle layer of the muscularis propria
- Involuntary smooth muscle
- Separated from external sphincter by the intersphincteric plane
Intersphincteric Plane
- Continuation of the longitudinal muscle layer of the muscularis propria
- Separates internal from external sphincter
- Defines the inferior extent of the anal verge at MRI (inferior intersphincteric groove)
External Sphincter
- Composed of:
- Distal fibers of the levator ani muscle
- Puborectalis muscle
- Small external sphincter muscles
- Voluntary striated muscle
- Maintains continence
Puborectalis Muscle
- Arises from the inferior aspect of the pubic symphysis
- Extends on either side of the rectum and fuses posteriorly
- Creates the anorectal angle
- Top of puborectalis = anorectal ring = top of surgical anal canal
- On sagittal T2WI: Plane defined by a line from anorectal angle to inferior pubic symphysis
- On coronal T2WI: Thickened muscle on either side of the rectum
Surgical Anal Canal
- Extends from the anorectal ring (superior) to the anal verge (inferior)
- The anal verge at MRI = inferior-most intersphincteric groove (most reproducible landmark)
- Length is variable: 2.5-5.5 cm (depends on gender and body habitus)
- Contains:
- Upper portion: anatomic rectum (above dentate line)
- Lower portion: anatomic anal canal (below dentate line)
Significance for T Staging
See Rectal Cancer T Staging for the controversy regarding sphincter involvement and T4b classification.
| Involvement | Surgical Implication |
|---|---|
| Tumor confined to mucosa/internal sphincter | Sphincter-sparing surgery possible |
| External sphincter involvement | Abdominoperineal resection required |
| Levator ani involvement | Extralevator abdominoperineal resection required |
MRF Relationship
- The MRF terminates at the top of the puborectalis muscle
- For low rectal tumors extending into the anal canal, MRF involvement should be assessed only for the component above the puborectalis
- Below the puborectalis, the surgical plane deviates from the MRF (variable CRM)