Why is there a slight overjet in the normal bite?

[dentalguy]

Ok, I thought I'd start a new thread to ask this question as the other one was a bit waylaid with my mistaken assumption that a normal bite involved a genuine overjet.

In reading and being told what the situation really is I now know that the average overjet in a normal occlusion is 2mm, and not what I'd previously thought. This is from the edge of the upper incisors back to the front surface of the lower incisors, and of course requires teeth of the right thickness and at the correct inclination. But my next and possibly equally dumb question is why is there this slight overjet? I mean if it's the average in the 'normal' population it must be there for a reason.

I'm wondering if it's something to do with the fact that if you lean your head forwards – or lift your head when you're lying on your back – your lower jaw slides forwards a bit? Given teeth are meant to overlap vertically, if you didn't have this horizontal gap the lower incisors would surely slide into and catch on the upper incisors. Wouldn't this pressure cause the teeth to move over time?

Or is there some other reason?

[Audra]

The slight overlap (of about 2mm) is because the incisors are supposed to act like scissors (hence the name incisors) so you can "cut" through food when you take a bite.

[dentalguy]

Hi Audra, only getting back to this now.

I suppose the thing I'm asking is why there is any horizontal gap at all in the normal bite. I mean surely all that's required - based on what I know so far - is for the upper incisors to just be ahead of the lowers to allow the vertical overlap to occure (the scisors type action you mention).

What I'm asking is is there something I've missed or don't know about that's behind most people having upper incisors more anterior than their lowers and then some? Why the slight horizontal gap that must exist if the point of the lower edge of the upper incisors is 2mm beyond the labial surface of the lower incisors?

Surely if the incisors acted just like scissors they would shear right past each other without this horizontal gap?

Finally, I'm not convinced about this whole analogy with scissors (not having a go at you here Audra, just the point in general ). I think incisor just shares the same Latin root - cut - as scissors and it's simply that function that the word describes not the slightly modified function found in scissors; does the confusion arise because they 'coincidentally' share the same root?

Take me, I can cut food the very best and my incisors meet edge to edge; indeed, only the two central incisors on the upper and lower jaws actually meet at all.

[UGHBRACES]

I notice that when i go to bite through something with my front teeth, i jet my lower jaw forward so that its in line with my upper teeth. But as i bite through, my lower jaw gradually jets back and into its natural position by the time i finish biting through. This makes for a tearing AND cutting motion all at the same time which would obviously be ideal for those tough raws meats our ancestors ate. Also, an edge to edge bite would have a harder time completing the tear through tough meat. Whereas, when you have the proper scissor like bit the meat will more easily finish the complete tear at the end.

Kind of like, say you use scissors to cut paper. Well if you stop cutting at the point where the scissor blades meet edge to edge, the paper won't be completely cut through. But once you compress the scissors more until there is an overlap in the cutters, the paper will slice right through with ease.

I am having a hard time finding the right words to explain what i mean lol, so hopefully you can make sense out of it.