Contact Lens Fitting: FEB and SAM Rules

Key Concepts

• FEB: Flatter, add Plus
• SAM: Steeper, add Minus
• Objective: Achieve optimal fit for contact lenses to maintain visual acuity

FEB and SAM Rules

• If fit is steeper than optimal, add minus power (SAM)
• If fit is flatter than optimal, add plus power (FEB)
• Reason: To prevent visual acuity issues

Tear Lens Concept

• Tear lens: Formed by tears between contact lens and cornea
• Impact on visual acuity:
  • Steeper fit: More space at center, acts as a convex lens, causes myopia
  • Flatter fit: Less space at center, acts as a concave lens, causes hypermetropia

Visual Acuity Impact

• Steeper fit: Rays focus in front of the retina (myopia)
• Flatter fit: Rays focus behind the retina (hypermetropia)
• Requires adjustment in contact lens power to correct visual errors

Practical Application of FEB and SAM Rules

• Steeper fit:
  • Increase base curve radius
  • Decrease power of contact lens prescription
• Flatter fit:
  • Decrease base curve radius
  • Increase power of contact lens prescription

Situational Use

• Durable fit: Steeper fit may be used, especially for rigid gas permeable lenses
• Patient comfort: May choose flatter fit for comfort in some cases

Conclusion

• Adjust fitting parameters and prescription power to ensure visual acuity is unaffected

Tips

• Study and understand the relationship between contact lens fit and visual acuity
• Apply FEB and SAM rules effectively for different scenarios

-- Stay with smart Optometry and study Optometry smartly