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Because biological limbs are connected to our skeletons, we don't notice that they weigh a lot! As technology develops, scientists have designed lighter, more functional prostheses and the latest can even use the skeleton like a biological limb does!

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If you’ve ever given someone a piggyback ride, you know that even tiny humans can feel heavy. But we don’t notice the weight of our own bodies.

And there’s a song that explains why:. The knee bone is connected to the thigh bone. The thigh bone is connected to the hip bone… and so on.

Prosthetic limbs generally feel much heavier than biological ones, because they aren’t connected to your skeleton. Nowadays, though, engineers are trying to use the body’s natural bony support system to make prosthetics even better. Your bones are connected to each other with ligaments.

And these ligaments have mechanoreceptors that help you feel how your joints are moving. But they aren’t able to sense how heavy things are. That job goes to the Golgi tendon organs, which are mechanoreceptors in the tendons connecting all your muscles to bones.

Because your leg is connected to your skeleton, your muscles don’t have to provide that much force to hold things in place. So you don’t really notice how how heavy your leg is — even though just one of them makes up about 16% of your total weight. Most prosthetic devices are attached with a socket held in place with a harness or suction.

Harnesses limit the range of motion, and the suction of the socket activates the Ruffini corpuscles, which are mechanoreceptors that are sensitive to stretching skin. That’s why prosthetic limbs can feel like they’re hanging onto the body, instead of connected to it. And if you had a prosthetic leg that was as heavy as a biological leg, it might feel like walking around with like a toddler hanging on to you.

So scientists have had to design lighter prostheses, sometimes at the cost of function. But as a different solution, they’ve been working on a way to connect prosthetic limbs to the skeleton, using a technique called osseointegration. Osseointegration is where bones grow into foreign materials.

Humans have actually been doing this for a long time, like in 600 CE when the Mayans used mother of pearl to make teeth that integrated into the jawbone. But it wasn’t until 1965 that Per-Ingvar Brånemark used osseointegration to put titanium dental implants in the first human patient. Titanium is super biocompatible.

When there’s oxygen around, it makes an oxide film that prevents lots of other chemical reactions — so it doesn’t, like, break down inside your body. At the same time, it activates the immune system some, so the cells that are normally responsible for bone resorption slow down, and new bone grows around the titanium fixture. These titanium implants are made with lots of nooks and crannies that bone cells can grow into, so they stay secure for decades.

Since dental implants became a thing, lots of researchers have applied osseointegration to prosthetic limbs too. All patients have to do is clamp their prosthesis onto the titanium implant, and they’re good to go. Because there’s no harness, there’s more range of motion.

And because there’s no suction socket, the prosthesis doesn’t feel like it’s pulling on your skin. There is one drawback: a metal rod sticking out of your skin is an open wound that you have to take care of, or it can get badly infected. Even though this technique is still experimental, nearly 1000 patients have undergone this surgery for their prosthetic legs, arms, and even thumbs.

Researchers are working on ways to address the risks, and the technology can only improve from here. And we’ll keep learning along with you here on SciShow! If you’re curious about other ways engineers are working with human bodies, check out our episode about human interface technologies!

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