Normal dentin has apatite crystals in the intertubular dentin and odontoblastic processes in the tubular lumen.

Fusayama reported that carious dentin consists of two distinct layers – an outer layer and an inner layer. Sturdevant refers to the outer layer as infected dentin and the inner layer as affected dentin.

Infected dentin is the outer layer of dentin, meaning it’s closer to the DEJ. It is the softest type of dentin. It is infected with bacteria and other damaging things such as acids and enzymes. We must remove infected dentin because it cannot be repaired through remineralization, and we must also remove the bacterial infection.

Affected dentin is the inner layer because it is usually closer to the pulp. It’s harder than infected dentin, but softer than normal dentin. It is softer because of the demineralization – minerals lost from the intertubular dentin have formed precipitates in the tubular lumen. It does not need to be removed because if the tooth remains vital, affected dentin can be remineralized.

As a caries lesion advances, the bacteria are not at the front. It’s like a marching band in a parade – there are usually some folks who carry a banner ahead of the band. They’re part of the band, but they don’t make any music. I love marching bands, so I hate to drag them down in a rather weak caries analogy, but…

Let's say we don't want live music in our parade, just like we don't want decay in our teeth. (I said it was a weak analogy.) If we take away the musicians, we can keep music from being played as the parade marches down the street. We can leave the folks with the banner in the parade because since they don’t have instruments, they can’t make music.

If this marching band were a caries lesion, the musicians would be infected dentin carrying instruments, ahem, bacteria. The folks with the banner would be affected dentin – part of the band (or caries lesion), actually the front of it – but not making any music.

If that’s not confusing enough, the lagniappe included in this post is a brief discussion of the difference between affect and effect. Affect means to change. Effect is what happens as a result of that change. So the caries process affects dentin, and the effects are infected and affected dentin. Ah, words.

​This next edit is a rather extreme example, but if you create too much of an undercut in your prep, and the wall is too convergent, you'll have unsupported enamel. See the cut rods highlighted in orange? They are not supported by dentin and can easily fracture away.

I really love Dr. Mark Winkler's analogy using carpet to explain this whole concept. I'll see if I can illustrate it here. Think of the carpet backing as the DEJ - the dentinoenamel junction - and think of the carpet fibers as the enamel rods. Where the cavosurface angle is greater than 90 degrees - in green - there is no unsupported enamel, and all of those fibers, even the cut fibers, can remain firmly attached to the backing. In the purple highlighted area, those fibers have been cut away from the backing and can easily be vacuumed right up.

Just a few creepy images from the world of dentistry to haunt your Halloween!

• Illuminated drop cap from an article about dental treatment in medieval England (via medievalists.net)
• George Washington's dentures - and some myths and facts about them (via mountvernon.org)
• Creepy-cool spider and web RPD (via Instagram user @stomstav26)
• Frightening illustration from Textbook of Exodontia published in 1927 (via AbeBooks.com)
• An abandoned dentist's office in Detroit, Michigan (via imgur.com)
• "Phantom head" with spaces for extracted teeth from the recent exhibition "Teeth" at the Wellcome Collection in London (via nytimes.com)

Personally, I find it easiest to understand information arranged in a visual format. Graphics such as charts and diagrams are convenient for referencing at a glance. I used to refer to this particular diagram of the structures numbed by the various nerve blocks all the time. ​

When I was a student, I remember thinking things made perfect sense as the presenter explained them during lecture. Then I would go to review my notes and think, "What in the world is this, and why didn't I take better notes?" If your notes sometimes don't cut it either, remember to consult your textbook (or books) required for the course. I still have access to my digital library of 115 e-books that I purchased as a dental student, and while I'm thankful for how portable they are in this format, I do miss paging through a traditional textbook. Summitt goes into greater detail on the G.V. Black formula for numbering hand instruments, and I think the accompanying figures are easier to understand than what I found in Sturdevant.

But remember - there are other resources out there. I found this great Instrument Reference Guide from Hu-Friedy, available for download as a PDF from their website. It includes a lot of great information about instrument care, sterilization, and sharpening, but on the very last page, it breaks down Black's formula for describing instruments.

So in my mind, to make things easier...

• add a decimal point to the first number, then the first two numbers are width and length in millimeters – 1.5mm wide, 8mm long
• and the centigrade circle is just a 360° circle cut into 100 degrees instead, so that number is just a percentage of 360. (That 12 tells me it's 12% x 360 = 43, so this is about a 45° angle.)

The last number in the sequence is always the angle of the entire blade. The instrument is positioned on the centigrade circle so that this number is always less than 50, and looking at the centigrade circle below, you can understand why - a straight instrument would essentially measure 0 or 50.
The additional number that gets placed between the first and second numbers (whew, this is awfully complicated!) is always the angle of the cutting edge only. The instrument is positioned so that this number is always greater than 50. I think the whole >50, <50 thing helps to keep from confusing the two - in my mind, the extra number is always the big number.