Instrumentation refers to the special devices surgeons sometimes use when fusing bones together. Another term for these devices is hardware. Instrumentation is used to get the best possible results from the surgery.
In the case of neck surgery, for example, a surgeon may implant a special man-made device to hold the neck bones in place. In most cases, this device will be sturdier than natural bone alone. Instrumentation can help the area heal correctly, which can speed recovery time and make the results of the surgery last.