There’s a lot of stuff going on physiologically when you put a constant force on a tooth, but that mostly involves gradually breaking down and rebuilding - or “remodeling” - the tooth socket walls. The periodontal ligament (PDL) fibers don’t do much themselves during tooth movement other than compress or relax and resist displacement of the tooth from the original center of the tooth socket. It’s actually a bit of a problem, because their natural rate of turnover (replacement of old fibers with new fibers) is slow and because tooth movement doesn’t seem to accelerate the rate of turnover for many of the PDL fiber components. That means that the fibers that were stretched during tooth movement will remain stretched and continue to apply a force that would return the tooth to its original position. This is one of the primary reasons for relapse following orthodontic treatment (in addition to continuous forces applied by the cheeks, lips, tongue, and jaws), and it’s why retainers are almost always recommended following orthodontic treatment.

For an idea of the timescale over which PDL and gingival fibers experience turnover, consider that the biological half-life of collagen fibers is about 9 weeks and that the half-life for elastin fibers is over 70 years! Meanwhile, bone can begin remodeling in as little as a week following force application to a tooth.

Interestingly, it’s the most accessible fibers around the neck of the tooth (just under the surface of the gums) that seem to provide the bulk of the relapse force discussed above, while the deeper fibers seem to remodel into more relaxed conformations within several months. So one method to reduce relapse - particularly for previously severely rotated teeth - is to sever those “supracrestal fibers” by cutting straight down through the gingival sulcus (gum pocket) to about the level of bone with either a scalpel or a laser. This is called a circumferential supracrestal gingival fiberotomy.

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