At a glance
If you have been told your fissure is at the posterior midline, this is the most common location for anal fissures. It is not a sign of anything unusual. This guide explains why this location is so common and what it means for your care.
The anatomy
The anal canal is a short tube surrounded by two rings of muscle (the internal and external sphincters). The tissue lining the canal receives its blood supply from arteries that come in from specific directions.
The critical detail: the posterior midline — the back of the anal canal — receives less blood supply than the rest of the circumference. The arteries supply the sides and front more generously, leaving the posterior midline as the least well-vascularised area.
This matters because:
- Less blood supply means the tissue is more fragile — more susceptible to tearing
- Once torn, it heals more slowly — the blood supply needed for repair is limited
- The sphincter muscle adds pressure — the internal sphincter wraps around the anal canal and can compress the posterior area, further reducing blood flow
What this means in practice
Most fissures are posterior
Roughly 80 to 90 per cent of anal fissures occur at the posterior midline. In men, the figure is even higher. In women, there is a somewhat higher proportion of anterior (front) fissures due to the anatomy of the female pelvis.
The cycle of poor healing
The posterior midline’s limited blood supply creates a self-reinforcing problem:
- A hard stool tears the tissue at the weakest point (the posterior midline)
- The tear triggers sphincter spasm as a protective response
- The spasm further reduces blood flow to the area
- Reduced blood flow impairs healing
- The unhealed tear is vulnerable to re-injury at the next bowel movement
This cycle — tear, spasm, poor blood flow, failed healing, re-tear — is the fundamental mechanism of chronic fissures.
Treatment targets the cycle
Understanding this cycle explains why fissure treatments work the way they do:
- Stool management breaks the re-injury component
- Topical treatments (GTN, diltiazem) relax the sphincter, which reduces spasm and improves blood flow
- Sitz baths increase local blood flow through warmth
- Botox temporarily paralyses the sphincter to break persistent spasm
- LIS surgery permanently reduces sphincter tension
Every effective treatment addresses one or more components of the cycle rather than the fissure itself.
When location raises questions
Posterior midline: straightforward
A fissure at the posterior midline in an otherwise healthy person is the standard presentation. It does not typically require investigation beyond a clinical examination.
Anterior midline: common in women
An anterior fissure in a woman is a well-recognised variant related to the anatomy of the female pelvis. It may be investigated further but is not inherently concerning.
Lateral or multiple fissures: further investigation
Fissures that are:
- On the side of the anal canal (lateral position)
- Present in multiple locations simultaneously
- Recurring in unusual positions
These patterns may prompt further investigation. They can be associated with conditions such as Crohn’s disease, infections, or other inflammatory conditions. This does not mean these conditions are present — it means they should be considered and ruled out.
The practical takeaway
Knowing that your fissure is at the posterior midline is mildly reassuring — it is the expected location and the one that responds most predictably to standard treatment. It also helps explain why the fissure occurred and why the healing process involves addressing blood flow and sphincter tension rather than simply waiting for a wound to close.