A modular surgical patient positioner unit apparatus, system and method for adjustably positioning human joints such as knees for surgical operating procedures having a base plate to support moveably a transformable carriage that can be releasably locked in position and which adjustably supports a boot or other limb holder which can be releasably locked in position by the carriage. The base plate also is configured with a side bar located at a mid-portion of the base plate to secure to an arc clamp configured to secure the positioning system to a side rail of an operating table in a medical procedure on the left or right side of the body. An extension unit also is provided and configured to secure to a lower surface of the base plate to extend the base plate of the system, as needed, for larger limbs of patients, which extension unit is configured with a stud and post locking portions for securing to the base plate and for positioning in a medical procedure on either side of the body.