When selecting a soft toric lens power, what is the correct order to determine the power?

When selecting a soft toric lens power, what is the correct order to determine the power?

• A. Sphere, axis, cylinder
• B. Axis, sphere, cylinder
• C. Axis, cylinder, sphere
• D. Cylinder, axis, sphere

Answer: (C)

Toric lenses work by having different powers in two principal meridians, with the axis marking the meridian that has no cylinder and the perpendicular meridian carrying the cylindrical power. Locking in the orientation first is essential because the cylinder’s effect depends on its direction. Once the axis is set, you determine how much cylindrical power is needed to correct the astigmatism. Only after axis and cylinder are known can you pick the sphere to achieve the overall refractive goal, since the power in the perpendicular meridian is the sphere plus the cylinder. Choosing sphere first or fixing axis later would risk misaligning or miscalculating the two principal powers. So the order is axis, cylinder, sphere.

Toric lenses work by having different powers in two principal meridians, with the axis marking the meridian that has no cylinder and the perpendicular meridian carrying the cylindrical power. Locking in the orientation first is essential because the cylinder’s effect depends on its direction. Once the axis is set, you determine how much cylindrical power is needed to correct the astigmatism. Only after axis and cylinder are known can you pick the sphere to achieve the overall refractive goal, since the power in the perpendicular meridian is the sphere plus the cylinder. Choosing sphere first or fixing axis later would risk misaligning or miscalculating the two principal powers. So the order is axis, cylinder, sphere.