by turning knob 21 knob 21 is connected via pin 33 to both gear shaft 25 and spline gear 24 gear shaft 25 extends axially beyond spline gear 24 this extended portion has shaft gear teeth 38 which engage clutch mechanism 40 which controls the amount of force required to turn knob 21 the knob 21 may be moved between a retracted position fig1 and an extended position fig1 in the retracted position spline gear teeth 27 engage arc gear teeth 37 such that movement of knob 21 will move arc gear member 12 in the extended position spline gear teeth 27 do not engage arc gear teeth 37 and arc gear member 12 is free to slide in an arc shaped path as limited by upper plate 22 lower plate 23 and roller 35 located within slot 36 this feature would generally only be used preoperatively or intraoperatively to correctly position the arc gear member 12 in both the retracted and extended positions shaft gear teeth 38 engage clutch mechanism 40 the combination of collar 29 upper plate 22 lower plate 23 with these elements being held together by plate fasteners 28 and collar fasteners 28a form the body of drive means 13 spline gear 24 is slideably mounted within collar 29 upon being moved.