This post is a bit more for the dentists in the audience than for patients, though perhaps patients who have experienced sensitivity to biting will be interested, as this will explain one of the causes.
Fossa Shallowing is a vitally important concept in dental occlusion. It's also one I use to help patients several times in the average week. And yet... In my interactions with residents at Einstein's general practice residency, as well as with the occasional dental students from Penn and Temple who I come to know, it seems that fossa shallowing is no longer being taught, at least not in the manner I encountered it. We should not lose this valuable insight into human dental occlusion.
Fossa Shallowing is, at its essence, an anthropological concept. A long time ago, our distant ancestors wore down their teeth much more than we do today. This was because of their diets, which were far more abrasive than ours are today. And surprisingly, we don't have to go all that far back to see severe wear in most peoples' teeth. These images are from the excellent journal article Masticatory Function and Malocclusion: A Clinical Perspective; Juha Varrela; Seminars in Orthodontics 2006;12:102-109. The upper two images show the dentition of a 20 to 25-year old, and the lower two images show that of a 40 to 45-year-old. These are both Finnish males born around the year 1600. This is not a time- and site-specific finding. This is how our ancestors generally were, all across the Earth, for all of our species' history.
Note the severe occlusal wear compared to today's standards. The human diet has changed dramatically. Starting in the late 1600s, the widespread availability of refined sugar and the softer, flour-based diet that came with it led to a massive increase in dental caries in the human population, and also reduced the amount by which we wear down our teeth during our lives.
Estimates of present-day average human tooth wear vary widely, but a figure of 30 microns per year for molars and 15 for premolars is reasonable. This article gives a good accounting of comparative wear of teeth and restorative materials: Yip et al, Differential Wear of Teeth and Restorative Materials: Clinical Implications, Int J Prosthodont 2004;17:350–356.
Cast Type II gold wears at the same rate as enamel. Porcelain and Zirconia wear at a much lower rate than enamel; in fact, their wear rate is almost negligible. Composite resin and modern amalgam formulations wear about twice as fast as enamel, and unfortunately, older amalgam whittles down very fast.
What does this all mean for our patients?
Many things, actually, but chief among them is this: People used to have flatter cusps as their lives went on. Now they have steeper ones.
Another way of saying it:
Ancient Humans: Younger, steeper cusps; older, flatter cusps.
Modern Humans: Younger, flatter cusps; older, steeper cusps.
In all patients who have posterior tooth occlusal restorations, the effect is accelerated compared to natural teeth. Throw in a crown or three that does not wear much at all, and we have a real mish-mash of wear effects, all in one mouth. The resulting steep, thin cusps break quite often. Also, the muscles of mastication aren't happy with all those occlusal interferences that were never there in our ancestors' mouths for the last 150,000 years.
So here's what I was taught: the technique called Fossa Shallowing. Whenever a posterior tooth requires a restoration due to caries, or especially when an existing (and worn) restoration is being replaced, look at the opposing cusps. If they protrude beyond the curve of Spee and are thin and sharp, adjust them before restoring the tooth in question. At the same time, raise or shallow out the fossa in your new restoration as compared to the worn old filling that you're taking out.
This process most certainly applies to crowns as well.
In my experience, second molars are the teeth that need Fossa Shallowing the most. They're right under the masseter, temporalis and pterygoid muscle sling, and they take a tremendous beating. When I treat mandibular second molars in particular, I constantly see that the opposing maxillary second molar has super-erupted due to wear of the mandibular restoration (so often, it's amalgam), and its cusps have thinned and sharpened. Gently re-contouring these cusps into proper form and function and then restoring the mandibular molar with a shallower fossa is vitally important to proper occlusion going forward.
Here's one example:
In this pre-op view, we note that the axial walls of the original prep have been exposed by wear of the amalgam (blue arrow), and that the disto-lingual cusp in particular has developed an excessively steep angle (black arrow).
Here's the prep, with corrections made; I've also brought the upper cusps into ideal contours by this point in the process:
And then here is the final composite resin restoration, bonded of course with Prelude SE, the finest self-etch primer there is:
Note in particular how different that disto-lingual cusp is from the pre-op view. It's much less prone to fracture, one factor in preventing the need for larger restorations like onlays and crowns. Further, the masseters, temporalii and pterygoids are relieved of the excessive stress created by cusp interferences.
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