The Maddox rod: Revisiting the optics.

The Maddox rod is a staple in the assessment of strabismus. However, its optics mechanism is puzzling. How is it that an orthogonal line focus is formed? Surprisingly, few texts offer an explanation and those that do either provide insufficient detail, contain misleading information or are difficult to follow. There is a prevalent misconception that multiple cylinders are required for the orthogonal line focus to appear. However, this is untrue; Maddox himself only used a single cylinder when he first described the phenomenon. Herein, we review the optics explanations provided in a sample of texts and offer an accurate explanation using the biplanar principle of astigmatic refraction.