Because what better way to attract new readers than with the riveting topic of those tiny numbers on the handles of instruments such as hatchets and hoes?! This is one of those things people will tell you might turn up on boards, so you should know it. Is it commonly asked on boards? I don't know. I don't remember seeing it on my board exam, but I took that quite some time ago.
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...
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.
A dentist and educator, sharing resources and experiences with her colleagues and students.
Side note - whenever I refer to Sturdevant or Summitt, I'm citing one of these two electronic textbooks:
- Heymann, Harold, et al. Sturdevant's Art and Science of Operative Dentistry. 6th ed., Mosby, 2012.
- Hilton, Thomas J. Summitt's Fundamentals of Operative Dentistry: A Contemporary Approach. 4th ed., Quintessence, 2013.