What does posterior midline mean?

The posterior midline is the back of the anal canal — the side closest to the tailbone. If you imagine the anus as a clock face, the posterior midline is at the six o'clock position. This is where the majority of anal fissures occur.

Why do most fissures happen at the back?

The posterior midline has the poorest blood supply of any part of the anal canal. Blood vessels that supply the area are arranged in a way that leaves the posterior midline relatively less vascularised. This means tissue in this area is more vulnerable to injury and slower to heal when damaged.

Does the location of my fissure matter for treatment?

For initial treatment, the approach is similar regardless of location — stool management, sitz baths, and topical treatments if prescribed. However, fissures outside the posterior midline (anterior in women, or lateral) are sometimes investigated more thoroughly to rule out underlying conditions. The posterior midline is the expected location and does not usually raise additional concerns.

What does it mean if my fissure is not at the posterior midline?

A fissure that is not in the typical posterior position is sometimes called atypical. This does not necessarily mean something is wrong, but it may prompt your clinician to consider other factors — such as Crohn's disease, previous surgery, or pelvic floor involvement — that could explain the unusual location.

Posterior fissure: why this location

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.