HAVE you ever looked in the mirror and noticed that your lower front teeth have become more crowded, tilted or overlapped than they used to be?
As we get older, most people begin to notice these changes.
There is the wisdom tooth phenomenon about late eruption causing crowding, which is not entirely true as research has shown that they do not exert enough physical force to push front teeth out of alignment which proves this to be a myth.
We now know that people who do not have wisdom teeth at birth experience adult anterior crowding at an almost identical rate as the ones who have them.
Hence, what are the real reasons for this problem that a majority of adults are encountering?
The real culprit is quite a fascinating, lifelong interaction between evolutionary biology, biomechanical forces and microscopic fluid physics that takes place deep inside the mouth: mesial drift as explained by the thixotropic theory of periodontal ligament function.
Our teeth are in perpetual equilibrium and change every single day.
The teeth move slowly not always at a constant rate along the dental arch.
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What happens is when we chew, the teeth slowly drift towards the midline of the dental arch — mesial drift (sometimes called medial drift).
The mechanism is that our teeth press vertically into the food and rub slightly against each other at their side-by-side contact points.
The friction of teeth over years of chewing causes minor wear on the enamel surfaces where adjacent teeth touch.
This is known as interproximal wear.
If nothing else, this wear would continue and make small gaps between all of our teeth, so food would be trapped and the stability of our bite would be threatened.
To avoid this, the human body employs an evolutionary safety mechanism to protect our bite’s integrity: a forward-directed force vector that encourages the posterior (back) teeth to move forward.
This fills in the gaps and keeps our bite in a tight, continuous chewing position.
While this is a very good way to keep our bite from breaking down over time, it has a secondary effect.
The teeth keep moving forward and eventually reach the front part of the mouth and because the jaw arch naturally narrows towards the front, and because the lower jawbone (the mandible) slightly contracts as a natural part of the human aging process, the teeth suddenly find themselves with less physical space.
As such, with a narrowing arch and an unstoppable forward force, front teeth begin to compete for room.
When they are not able to stand perfectly straight, they are forced to rotate, overlap and bunch up in late-stage adult crowding.
To understand how this forward movement can take place without disrupting tooth roots or our jaw anatomy, we must look closely into a microscopic level.
Teeth do not rest directly against the hard alveolar bone of the jaw, every tooth is held in its socket by a complex and highly specialised shock-absorbing cushion called the Periodontal Ligament (PDL).
The PDL is a thick band of collagen fibre bundles with fluid-rich blood vessels, nerves and ground matter.
For more than 100 years, scientists have been debating whether this small ligament space, which is less than 0,2mm wide, could be able to withstand the huge mechanical force of chewing by human jaw muscles, which can generate up to 150 pounds of pressure.
One of the simplest and most elegant explanations for this remarkable accomplishment is the Thixotropic Theory of the PDL.
Thixotropy is a physical property of some fluids and gels which are thick (or very viscous or semi-solid) in a static state, but which quickly becomes fluid, thin and free-flowing in the presence of mechanical stress, agitation or shear.
The PDL has a dual state of nature that takes place in two stages: The static phase (at rest): When you are not chewing, talking or swallowing, the fluid and fibre network of the PDL is very solid, firm and viscous.
It is a solid gel structure that holds your teeth in place in the jawbone and tolerates small changes of orientation from day to day.
The dynamic phase (under function): At the exact moment your teeth strike each other during chewing or swallowing, the sudden stress from mechanical shear triggers an immediate physical change.
The PDL’s firm gel-like structure instantly begins to fluidise.
The material becomes thinner so that fluid can flow freely and collagen fibres can be rotated.
This short transition to a liquid-like state allows the tooth to slightly displace and sink safely within its socket, absorbing the impact without fracturing the brittle tooth structure or the surrounding bone.
The PDL matrix immediately resets back to its firm, protective gel state when the chewing pressure is released.
However this fantastic shock-absorbing system has an architectural vulnerability.
The brief microscopic fluid state that occurs thousands of times a day during mastication provides a temporary window of vulnerability due to the fact that the PDL is fluidised to handle the large vertical forces of chewing, it is easily affected by small, but persistent horizontal forces on the teeth at the same time.
The gentle resting pressure of your tongue, the resting tension on lips and cheeks and ultimately the forward biological vectors of mesial drift all work on this fluid state.
These tiny pressures over thousands of chewing cycles a day and months and years to come, slowly but surely drive the teeth forward through the liquefied PDL.
It is a slow architectural change driven by fluid mechanics that results in the crowded smiles we see in the mirror as we age.
Why this matters for oral health
The fact that the changes in adult tooth alignment are caused by the fluid physics of the PDL and natural biological drift completely changes our way of thinking about the whole human oral care process.
Anterior crowding is often overlooked as a superficial issue that a person might like to look good on the outside of their teeth and keep the straight teeth they grew up with.
However, it is a public health issue and teeth get crowded, rotated and overlapped so tightly, they leave behind deep and unyielding gaps and that often cannot be cleaned with a toothbrush or floss.
Then these microscopic spaced harbour virulent oral biofilm and plaque, increasing the risk of dental caries (cavities) and chronic periodontal disease.
The gum tissue surrounding crowded teeth is also prone to chronic inflammation as it is very difficult to remove bacteria from twisted interproximal spaces.
Years prior, dentists believed that patients only had to wear a retainer for a couple of years after their braces came off until the teeth “settled”.
Now that we know that mesial drift and thixotropic displacement are continuous, dental professionals know that orthodontic retention must be lifelong.
Or by using removable clear plastic retainers at night or by attaching a permanent, thin lingual wire to the back of the front teeth, you can actually hold the teeth down for good against mesial drift.
