a method and device for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient , comprising inserting a distractor into the intervertebral space . the vertebral endplate depth is measured by moving a posterior rod on the distractor longitudinally to a position at the posterior margin of the vertebral endplate and moving an anterior rod on the distractor to a position at the anterior margin of the vertebral endplate . the force applied during distraction is measured using at least one transducer mounted on the distractor and the intervertebral space is distracted by rotating the posterior and anterior rods on the distractor until the measured force reaches a predetermined value . the posterior height of the intervertebral space is measured based on the amount that the posterior rod is rotated and the anterior height of the intervertebral space is measured based on the amount that the anterior rod is rotated . the intervertebral angle is measured based on the angular orientation of two pivotally connected handles on the distractor .