In many cases spine surgery is performed in 2 stages a few days, or a week apart. The reasons are many and varied. Minimally invasive surgery is performed through small incisions with very little disruption to the tissues. The advantage is far greater success rates. But the disadvantage is that the surgeon relies on radiology and computer navigation to get the job done rather than direct vision. As a result, when we know that we have to use multiple approaches in a patient's surgical reconstruction we will often want to get a scan or assess the clinical result of the first approach, before proceeding to the second approach. Contrary to patient's beliefs 2 stage surgery is safer and involves less anaesthetics than performing the whole lot in one stage due to turn-around time between stages.
Other reasons are not necessarily medical – financially, patients are out-of-pocket less money with staged surgery. Medicare will only pay for single level disc replacement surgery so if you require two or more then these have to happen a minimum of 28 days apart in order to comply with Medicare billing legislation.