Full-Arch surgery aims to avoid anatomic structures via strategic planning and implant angulation. That being said, there are cases where increased proximity to the neurovascular bundle is necessary to create an implant position and AP spread that will actually be successful.
Whether it’s a short implant behind the foramen or a tilted implant anterior to the foramen, here are 4 clinical warning signs that tell me I’m “too close” to the nerve.
1. Loss of an apical stop at the bottom of an osteotomy posterior to the foramen (and/or a “spongy” feeling with the perio probe).
If I’m performing a short implant posterior to the foramen, I know that I will often be in close proximity to the inferior alveolar nerve canal.
As a result, I want to ensure there is a bony apical stop to my osteotomy to confirm that I have not “popped” through the superior bony roof of the nerve canal and entered into the IAN canal itself.
The secondary clinical sign is the lack of a “spongy” feeling when the perio probe is placed into the osteotomy. You should only feel a firm bony stop.
This is important because, in theory, you might have extended through the superior roof of the IAN canal without knowing (another reason to drill slowly).
If not paying attention to your depth, you might also place your perio probe into the osteotomy and actually feel the inferior aspect of the canal floor and incorrectly assume that is your bony stop.
However, if astutely paying attention to tactile feedback, you would normally note a slight sponginess to gentle depression of the perio probe tip indicating contact with the nerve itself (picture a thick piece of spaghetti). Once confirmed, this should be avoided to prevent neurovascular damage.
Please note that I am not indicating you should poke or prod the nerve. However, if you happen to feel this instead of a bony stop, you need to pay attention to what this is telling you.
As long as I have a bony stop and no spongy feeling when checking with the perio probe, I know that it is highly unlikely (although not impossible) that I have entered the nerve canal.
2. Feeling a drop or sponginess when pulling the perio probe backward, up, and out of the osteotomy.
When placing a tilted implant anterior to the foramen, I will drag the tip of the probe backward, up, and out of the osteotomy along the posterior wall.
If I feel a “drop” or any “spongy” feeling with the probe, I know that I have likely entered either the anterior loop and/or the area of the mental foramen.
3. Seeing the mental nerve moving as it exits the foramen with the rotation of the implant during placement.
Yes, I have had this occur when stretching AP spread as far as possible via a posterior-tilted implant. Occasionally, while the implant is rotating, you can see the implant actually pull or tug the nerve (as it exits the foramen) via communication with either the foramen or anterior loop.
If you see this, obviously, your implant is in contact with the nerve and you should adjust accordingly.
4. Heavy bleeding during osteotomy creation.
Heavy bleeding during work around a neurovascular bundle is usually not a good sign.
This typically indicates you have damaged the artery that runs with the nerve, which also indicates your proximity to the nerve.
Take this as a warning sign. Adjust accordingly and proceed with caution.
“I want to stand as close to the edge as I can without going over. Out on the edge you see all kinds of things you can’t see from the center.”
Kurt Vonnegut, Player Piano
Strategically positioning implants to maximize spread is part of full-arch surgery. And, you will have cases where you have to push the limits in order to create a biomechanically favorable case. If you notice one of the warning signs above, however, you may want to hit pause and rethink your surgical approach.
Here’s to standing as close to the edge as possible, without going over.
Matt
P.S. Please note that I am not advocating placing implants “extremely” close to the nerve. These are clinical warning signs that warrant attention to help prevent that occurrence.