This is a very relevant question. The ideal patient has a soft disc prolapse. It is even useful in some cases of recurrent herniation following previous standard surgery as the access route through the foramen will be free of scarring. Likewise, visa versa – if there is a recurrence following an endoscopic procedure we may elect to use the standard open keyhole microdiscectomy.
Spinal canal stenosis from degenerative disease can be treated though the more narrow and the more the narrowing affects the central canal the more difficult it becomes. Similarly, combinations of marked canal stenosis plus disc herniation is difficult with this technique.
Finally, some levels are more challenging than others and some prolapses can migrate up or down the spinal canal to positions where they are difficult to access with this method. We can tell all this from the MRI scan. The most difficult is an L5/S1 disc prolapse which has migrated up the spinal canal a few millimeters.
Each case needs to be judged on its own merits and offered whichever method suits that case best. With the correct case selection both keyhole open microdiscectomy and endoscopic discectomy have excellent results accordingly. With the wrong case selection things are unlikely to go well. For most cases, either method will give an excellent outcome…Excellent, is OK but we are after the best.