Despite my unwavering respect for bone because of its durability and hardness, I must admit that teeth are equally durable and even harder. The tooth’s enamel is the hardest substance in the body and consists almost entirely of the same calcium crystal that constitutes bone. Enamel, however, has none of the collagen meshwork that give bones a bit of resiliency when suddenly stressed, so teeth are far more brittle.
Bones come in vastly more shapes and sizes than teeth and have the same collagen/calcium composition through and through. By contrast, teeth have multiple layers of differing compositions and attributes. Supporting the crown and extending to the tips of the roots, the dentin is softer and more porous than the enamel. Dentin surrounds the pulp, where a tooth’s blood vessels and nerves reside and where endodontists make their living performing root canals. Covering the dentin below the gum line is a thin layer of cementum, which is softer than both enamel and dentin. Its principal function is to support what dentists blithely call the PDL—periodontal ligament. I object to this name, because bone lovers know that ligaments span from bone to bone across joints, stabilize them, allow motion in some directions, and prevent motion in others. When I posed this flagrant misuse of the term ligament to my dentist, he lamely responded that the PDL spans from bone to tooth and stabilizes the roots in their sockets. For the most part, however, motion between tooth and bone is undesirable, so I think the PDL, which is about as thick as several sheets of paper, should be called the periodontal membrane or covering. Yet my opinion is apparently not shared with those who love teeth more than bones. (In the US alone, dentists outnumber me nearly 200,000 to one.)
Regardless of what it is called, the PDL is responsible for this essay’s title, because if teeth and bones become engaged in a shoving match, teeth win. This is because pressure on a tooth causes specialized bone-dissolving cells in the areas where the PDL is being compressed to remove bone and let the tooth move along. In its wake, specialized bone-forming cells fill the void. Voila, orthodontics is born.
Humans have been relying on the PDL to perform this bone-defying feat as far back as ancient Egypt, where mummies have been discovered with metal bands wrapped around their teeth. Archaeologists surmise that long-gone fibrous strands tied the bands together to create tooth movement and beautiful toothy grins. Primitive orthodontic appliances also appear among ancient Greek and Roman artefacts.
A major advance came in the 18th century, when Pierre Fauchard, known as the Father of Modern Orthodontia, began wiring arched metal strips to the teeth to get them moving. To achieve an even greater mechanical advantage, an 1822 invention enhanced the shoving match with external headgear, although it probably did not enhance the user’s social standing while movement was underway. An 1840 book, The Dental Art, described soldering knobs on the metal arches to which rubber bands could be attached and that greatly reduced the need for external headgear. Now, many orthodontic appliances are nearly invisible.
In addition, tooth movement can be accelerated by drilling small holes into the bone surrounding the target teeth. This weakens the bone and stimulates activity in the bone-dissolving cells. Enterprising orthodontists have also capitalized on bone’s accommodating nature to improve the bite efficiency and smile (or snarl?) of man’s best friends.
I suppose some dentists dream of taking on greater challenges in the animal realm, but I just hope they appreciate that any change stems from bone’s calm, controlled response to stress. It just moves away from those tooth-root bullies.
Should you need another conversation starter, try this. Growing evidence supports the theory that teeth developed from scales on the outside our finny ancestors’ bodies and gradually migrated inside while morphing into enamel, dentin, cementum, and the PD … layer.
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