Why are anal fissures more common in the posterior location?

Why Posterior Anal Fissures Are More Common

Posterior anal fissures predominate (approximately 90% of cases) due to the combination of relatively poor vascular perfusion in the posterior midline and the mechanical forces of defecation that concentrate trauma in this anatomical location. 1

Anatomical and Vascular Factors

The posterior midline of the anal canal represents a watershed area with inherently compromised blood supply compared to lateral positions. 1 This anatomical reality creates a vulnerable zone where:

• Internal anal sphincter hypertonia directly correlates with decreased anodermal vascular blood flow, establishing the pathophysiologic foundation for fissure development in areas already predisposed to ischemia 1
• The posterior commissure receives less robust arterial supply, making it susceptible to ischemic injury when sphincter pressures increase 1
• Local ischemia impairs wound healing once tissue injury occurs, perpetuating the fissure 2

Mechanical Trauma Distribution

The biomechanics of defecation preferentially direct traumatic forces to the posterior midline:

• Local trauma to the anoderm from passage of hard stools, diarrheal irritation, or anorectal surgery concentrates posteriorly due to the anatomical configuration of the anal canal 2
• The posterior position bears the brunt of stretching forces during bowel movements, making it the primary site of mucosal tears 3
• These mechanical factors work synergistically with the underlying vascular compromise 4

Gender-Specific Patterns

While posterior fissures dominate in both sexes, anterior fissures show notable gender differences:

• Anterior midline fissures occur in approximately 10% of women versus only 1% of men, suggesting anatomical variations related to obstetric trauma or perineal body differences 1
• The overwhelming majority of fissures in men remain posterior 1

Clinical Significance of Atypical Locations

Any fissure located laterally or multiple fissures should immediately raise suspicion for underlying pathology including inflammatory bowel disease (particularly Crohn's disease), sexually transmitted infections (HIV, syphilis, herpes), anorectal malignancy, tuberculosis, or leukemia. 1 These atypical presentations warrant:

• Thorough investigation beyond standard fissure management 1
• Consideration of endoscopy, CT, MRI, or endoanal ultrasound 1
• Mandatory exclusion of Crohn's disease, especially with recurrent presentations 5

Pathophysiologic Cascade

The development of posterior fissures follows a self-perpetuating cycle:

• Initial trauma creates a longitudinal tear in the squamous epithelium extending from the dentate line to the anal verge 1
• Sphincter spasm increases resting pressure within the anal canal, further reducing blood flow to the already ischemic posterior midline 2, 4
• Impaired healing allows progression from acute to chronic fissures, with development of sentinel tags, hypertrophied anal papillae, and visible internal sphincter muscle at the fissure base 1

Common Misconception

Contrary to widespread belief, less than 25% of patients with anal fissures actually complain of constipation, highlighting that hard stools are not universally present and the condition's complexity extends beyond simple mechanical trauma. 1