Jackson Cross Cylinder Technique

Overview

This lecture covers the Jackson Cross Cylinder (JCC) technique, its structure, and its clinical application in detecting, refining, and correcting astigmatism during subjective refraction.

Structure and Optics of the JCC

• The JCC is a sphero-cylindrical lens: sphere = half cylinder power, with opposite sign.
• It is marked by red for minus cylinder axis and white for plus cylinder axis, with the handle at 45° to the axes.
• Cylinder power acts perpendicular to the axis marking (e.g., axis at 180° gives power at 90°).
• Power cross form: two equal but opposite cylinders at perpendicular axes.
• Prescription form: cylinder and sphere components can be written based on the JCC setup.

Functions of the JCC

• Used to detect the presence of astigmatism.
• Used to refine the axis of the correcting cylinder.
• Used to refine the power of the correcting cylinder.

Clinical Application and Mechanics

• When placed over the eye, JCC alters the interval between Sturms' conoid focal lines (interval of astigmatism).
• Decreasing the interval improves clarity; increasing it results in blur.
• The circle of least confusion remains on the retina during JCC testing.

Steps: Detecting and Refining Astigmatism

• To detect astigmatism, place JCC in principal (90°/180°) and oblique (45°/135°) meridians after best sphere correction.
• No preference means no astigmatism; preference suggests astigmatism is present.
• To refine axis: place JCC handle parallel to cylinder axis; patient preference determines axis rotation direction.
• Minus cylinders: rotate axis toward red (minus) JCC marker when preferred.
• Plus cylinders: rotate axis toward white (plus) JCC marker when preferred.
• Adjust axis incrementally until no further preference.

Steps: Refining Cylinder Power

• Place JCC with axis parallel to cylinder axis in the frame.
• Flip JCC to test for clarity preference; add or subtract cylinder power on the preferred meridian.
• Adjust sphere simultaneously, adding half of the JCC cylinder (with opposite sign) to maintain the circle of least confusion on the retina.

Example of Clinical Calculation

• Algebraically add the JCC power to the correcting cylinder on the preferred meridian.
• Adjust the sphere by adding half the JCC cylinder (with the opposite sign).

Key Terms & Definitions

• JCC (Jackson Cross Cylinder) — A lens combining equal but opposite cylindrical powers, used in subjective refraction for astigmatism.
• Sturm's Conoid — The interval between two focal lines created by astigmatic lenses.
• Circle of Least Confusion — The location within Sturm’s conoid where blur is minimized.
• Principal Meridian — The meridians with maximum and minimum powers in astigmatism correction.

Action Items / Next Steps

• Review the concept of Sturm’s Conoid for a better understanding.
• Practice using the JCC to refine both axis and power in clinical refraction.
• Adjust both cylinder and sphere according to patient responses and JCC findings.