You have two questions regarding posterior foraminotomy. One is surgical technique and one has to do with indications.
Foraminotomy is performed for foraminal stenosis, the compression of a nerve root in the foramen from a herniated disc or bone spur. If the source of compression is a herniated disc, this disc fragment must be in the foramen and not under the cord in the front of the canal. If the fragment to too far forward, manipulation of the spinal cord to remove the fragment is necessary which in my opinion, puts the spinal cord at risk for injury.
If the compression is from a bone spur, there are two potential origins of the compressive spur; the facet or the uncovertebral joint. If the spur originates from the facet, then the foraminotomy will generally be successful as this procedure removes about 1/2 of the medial facet and obliterates this spur. If the spur however originates from the uncovertebral joint (which is more common), the bone spur cannot be removed completely and there are more failures from this procedure in this case. The reason is that the uncovertebral joint is in the anterior half of the spine and this procedure is posteriorly located which limits access to the spur.
The procedure can be performed open or through a tube. I use the tube but any surgeon who performs this procedure without a tube will most likely have the same results. The endoscope is a small camera attached to the end of a wand. I use a microscope as there is more working room to see the global picture but there is nothing wrong with using an endoscope. Either technique should elicit the same results.
The key to this procedure is to make sure you have the correct diagnosis. I just saw a patient who had this procedure performed at another institution and had increasing pain after the procedure. The reason was that his pain originated from an arthritic facet and had nothing to do with his foraminal stenosis. Make sure the correct pain generator is identified (see section on diagnostic nerve and facet blocks).