An instrumented distractor includes a force sensor for measuring a force exerted by opposing surfaces of adjacent vertebrae pressing against fixed and adjustable arms on the distractor. The force on the distractor may be measured by a data acquisition system (DAS). An instrumented cage including at least one force sensor may be inserted into the same intervertebral space as the distractor and a measurement of force on the cage made by the DAS. A size and shape of the instrumented cage are preferably selected such that the magnitude of force on the cage is less than or equal to the magnitude of force measured with the distractor. In some embodiments, a first DAS is included in the instrumented distractor and a second DAS is included in the insertion handle, and the two data acquisition systems may communicate measured force values with one another.
