Something that every astronaut is familiar with is the effects of near-zero gravity on bone density. Without Earth’s gravity constantly working against them even as we stand still and lie down, every bone in an astronaut’s body slowly starts to weaken and degrade. The same thing happens to their muscles, but muscles are easier to keep up with just some basic exercises
Thinning bones can take a lot longer to regain their old density, and because they may eventually reach the point where they become dangerously thin, NASA doesn’t allow astronauts to spend more than a few months away from the surface.
The thing about bones is that they don’t stop growing even after your last growth spurt ends. Special cells called osteoblasts are constantly producing more of the calcium lattices bones are made of all throughout your body, but to keep them from creating too much your body also has osteoclasts which break these lattices down and reabsorb them. For the most part these two cell types are evenly matched, giving your bones a stable density, but certain things can change this equilibrium.
For instance, much like with muscle tissue, a consistent amount of stress can cause your bones to grow somewhat larger and denser. Your bones aren’t nearly as dynamic as your muscles are, so you aren’t going to notice any dramatic changes over the course of a few weeks or months, but your shins may become slightly less prone to breaking if someone consistently kicks them for several years.
On the other hand, you’re very likely to see your bones lose their size and strength as you enter your old age, even if it doesn’t go as far as osteoporosis. And if your teeth are removed, your jawbone will also shrink: without your teeth around to stimulate the bone they’re attached to, the osteoclasts will break down the apparently unneeded lattices. This leads to the vertical wrinkles around the mouth you see on some people, since the lips are falling inwards without any bone to keep them out.
Since dental implants can integrate directly with your bone in a process called osseointegration, they can stimulate your jaw and keep it at full strength the same way your normal teeth once did, but then they do need some bone to anchor into in the first place. If it’s been some years since your natural teeth first came out, you may not have enough bone to support an implant, but a dental surgeon can fix this by performing a bone graft.
A bone graft is a simple procedure, although it may demand a few months of recovery. The surgeon will take a small piece of bone from elsewhere in your body, such as along the base of your jaw or off the side of your hip, and attach it to the part of the jaw where the implant will grow. Once there, the osteoblasts will knit the two pieces together and the dental surgeon can install the implant. Artificial bone can work as well, but living bone is best.
So the reason why bone grafts are a part of the dental implant process is because your bones are a living, changing part of your body and your jaw tends to shrink when your teeth aren’t present. As such, it’s best to get dental implants straight away after you remove your natural teeth, but even if it’s been years since that point you can still get an implant with the help of a bone graft.
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