P. Pallav, DDS, PhD

There is a very strong influence of the shape of the occlusal surfaces on the wear, which occurs at restorations in these places.

What we say on this page is based on an assessment of what happens between two molars during chewing. The backgrounds are treated in depth in the pages behind this one.

Wear Rate vs. Erosive Activity

We assume that the wear rate is proportional to the pressure and the shear rate at which the food moves over the surface when all other circumstances (composition of the food, restorative, etc.) are constant. The multiplication of these two we have named the erosive activity.

When two materials would be compared, their wear rates are likely to differ. This means that these respond differently to the same amount of erosive activity. What is important, is that when the erosive activity would for example be doubled, the wear rates of both materials become twice as great.

To increase the readability, we often speak of the wear rate where we mean erosive activity. Of course wear rate then refers to that, which would occur if the surface would have been made of an arbitrary wearing restorative.

The Inter Occlusal Space

There is a rather complex relationship between the shape of the space between the occlusal surfaces and how much CFA wear occurs in which spot. The extra pages deal with the fluid mechanics that describe the flow of the food as it is compressed and forced out sideways.

Clearly most CFA wear occurs during the last tenth of a millimeter before the teeth come together at centric occlusion. This means that when we speak of for example the height at some arbitrary point in the inter occlusal space, this is shortly before occlusion.

Non Time-Linear Wear

It is important to understand the difference between wear and wear rate. With wear is meant height loss and it may be expressed in micrometers (µm) or millimeters (1mm = 1000µm).
The wear rate is the pace at which wear develops in time and it can be expressed in such units as millimeter per month (mm/mo) or per year (/yr).

The wear rate is generally not constant, but it nearly always decreases with time. As we will see later, the wear rate actually varies with the height (wear!) and that is what causes it to vary with time. This is also why wear resistant materials, which wear less, appear to have a more constant wear rate.

Overall Parameters

There are two main factors, which have a strong influence on the wear rate.

The height is the distance between the occlusal surfaces. It varies from zero at contacts to a few tenths of a millimeter at fissures, and the average height will be somewhere in between these, calculated as the rms-value (root of the mean square).

The width is the (lateral) width of the space between the molars, optionally corrected for the waviness. It is difficult to say to where this should be measured exactly. The intersection with the dotted lines, a little bit outside the necking part could be an example. More to the outside is more work in a simulation and contributes increasingly less to accuracy.

Both the width and the height have a strong effect on the wear. The wear rate varies with W^2.5 and 1/h³ (W^2.5/h³ looks too unclear), which means that 10% wider molars or 10% less average height will give a 25% or 30% greater average wear rate respectively. This is because in both cases the average pressure on, and ~ sliding speed of the food is greater.

Restorations in first molars tend to wear faster than those in second molars, which wear faster than second premolars, etc., because of their width 'measured' in this way.

Vertical Asymmetry

Because with most people the overhang of the buccal cusps of the upper molars 'locks off' the inter occlusal space (above) better than the oral cusps of the lower molars, more food will flow out on the oral side and more wear will occur on the restoration in the lower molar. The point of zero horizontal speed of the food will be more to the buccal side.

Channel

If you take a look at a wax bite you will see that most of the inter occlusal space forms a network of channels around and between the occlusal contacts.

Because of this there are numerous places where a cross-section in the appropriate direction looks either like this or up-side down.

Here we find that the wear at the center of the restored fissure is about twice as great as near the margin, a well known phenomenon in clinical studies. This arises because the food follows the way of the least resistance, therefore more food will flow at the center, causing a greater wear rate there.

Earlier we were in fact looking along the direction of the (main) flow when we found the wear rate to vary with 1/h³. This picture is square to the direction of flow and the wear rate varies with h². Rather than more wear, there will be a 20% smaller wear rate where there is 10% less height.

Two Molar System

In reality the upward motion of the lower molar is not fully vertical. Looking from the front, there is a part in the middle, which is higher than the two parts at the medial and lateral sides, which are much more shallow.

Outlet

At the side parts the speed of the food is the greatest. However the pressure is much less because towards the outside boundary of the inter occlusal space the pressure decays all the way to zero, the static pressure of the environment. Altogether the wear rate here is likely to be up to, say twice as great as in the middle part.

These side parts are structured as rows of channels (above) running in parallel, which work as the outlet of the system. The oral row is formed by the oral fissures (+restorations) of both the upper and the lower molar and the cusps opposing these. The vestibular rows similarly.

Pressure Chamber

In the middle part the horizontal speed of the food is much less because the height is greater, but also because the point of zero horizontal speed must be somewhere here.

The middle part, with its greater height and lesser flow rate (of the food) works as a pressure balance between the rows of channels, which make up the outlet. It not only balances channels within a row, but also between the oral and vestibular rows. Because of this the wear rate in all the outlet channels will be in proportion to the square of their height (h²).

Various Outlet Channels

The picture shows the development of wear in a two-molar system where one third of the outlet channels (above) has an average size, in one third the height is enlarged by 20%, and in one third the height is 20% downsized.

Here we see the effect of redirection. Less food flows out of the smaller channels and more out of the larger channels.

Because of this, the larger channels wear at an even greater rate, the initial 20% relative difference between the channels increases with time. After 10 years some 70% more wear has developed at the greater channels!

For the outlet channels we can say that wear strongly tends to increase any height variations existing among these.

This is all within a single two-molar system. This mechanism of distribution of wear may also work, but to a little extent between adjacent systems, such as sixes in line with neighboring fives and sevens.

Systems With Different Outlet Channels

When different independent two-molars systems are compared, that is when all channels are 20% larger or smaller the differences are reversed. The picture shows three identical systems, the only difference is the vertical size of all the outlet channels.

When all channels are greater, no redirection occurs and the main effect of the greater height is that the food flows out easier, with less shearing and less wear.

The Unchanged Middle Part

Here too there are numerous situations, where wear of a restoration gradually changes the food flow, similar to larger and smaller channels in a two-molar system (above).

Although we varied the height of all the outlet channels, we did not vary any channels in the middle part. Nevertheless there is a great change in wear here, caused by changing the outlet channels.

As said, the pressure in the middle part is always greater than at the outlet channels.

This is because the pressure builds up from equal to the static pressure of the environment starting at the outlet channels.
Further towards the middle, more upstream the speed of the food decreases and consequently less (extra) pressure build-up occurs.

This means that the pressure at which the food slides in the middle part is determined by the outlet channels. Therefore, changing the average height of the outlet channels has an even stronger effect on the wear of an arbitrary channel in the middle part.

Protective Effect of Cavity Walls

It has been suggested that a decreasing wear rate could be caused by a protective effect of the increasingly exposed cavity walls. This is a nice idea, but it becomes nonsense when you try to imagine what happens, like we did when we considered simulating that too.

The protective effect occurs when the direction of the flow of the food is square enough to the cavity wall. It is negligible when the flow is parallel.

Indeed the inter occlusal space looks like a random network, but it's not random. If you take a look at a wax bite you will need to look very hard to find places where the flow is not perfectly parallel to the cavity walls.

It looks like this effect has been found in search for a decreasing wear rate, because cavity walls may increase the wear rate too.

Of course, we didn't take it into account.