An ophthalmic surgical apparatus may include a tower extending upward from a support base, and a user-positionable arm extending from the tower to support an optical instrument attached to a distal end of the user-positionable arm. The user-positionable arm may include four members that are pivotably connected to each other. An electromagnetic brake strut may be pivotably attached in a diagonal relationship to at least two of the four members. The electromagnetic brake strut may include a rotatable lead screw having first and second lead screw end regions and a nut attached to the first pivot joint. The first lead screw end region may be threadably received into the nut. An electromagnetic clutch allows lead screw rotation when an electrical voltage is applied thereto. The electromagnetic clutch may be attached to the second pivot joint and may include a bearing that supports the second lead screw end region.