Four Fascinating Facts About Kneecaps

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The patella, or kneecap, is one of the most incredible bones in your body. As a sesamoid bone, it is embedded within a tendon, where the quadriceps and patellar tendons meet. There, the rounded, triangular bone protects the knee joint and acts like a pulley, allowing the tendon to transmit more force with smoother motion.


Yes despite the kneecap's obvious usefulness today, its evolutionary history is not entirely understood.

"We know almost nothing about what the kneecap did when it first evolved, when both the tendons that held the bone and the bone itself were thinner and not as well developed," paleontologist Brian Switek wrote in his recent book Skeleton Keys. "It may have been a matter of crossing a certain threshold when individuals who just happen to have sesamoid bones at their knees were better able to cope with the stresses of running and, as luck would have it, left more offspring to carry on the trend."

The trend since spread to most four-footed animals, apart from reptiles and marsupials. Birds are also noted for their kneecaps. In the process, the patella has grown into one of the most unique and intriguing bones. Here are four fascinating facts about the kneecap:

1. Ostriches are the only animals with a double-kneecap. "We speculate that this might mean ostriches are able to extend their knees relatively faster than they would with one kneecap," says Harvard University Postdoctoral Researcher Sophie Regnault.

2. Kneecaps start out as cartilage. Unconnected from the rest of your skeleton, patellae start out as soft tissue and ossify into the hard, strong bones we all know by age three.

3. Frogs may have evolved the first kneecaps. In 2017, researchers from Argentina discovered primitive, soft kneecaps in eight species of frog. "One implication of the discovery is that kneecaps like this began to evolve in the Devonian period 400 million years ago," Andy Coghlan wrote for NewScientist.

The patella is absent.

4. Rare genetic disorders leave people without kneecaps. The most notable of these are nail patella syndrome and small patella syndrome. As its name suggests, nail patella syndrome leaves patients with underdeveloped nails and kneecaps. It's caused by a mutation to the LMX1B gene. Small patella syndrome is almost uniquely characterized by tiny or absent kneecaps and results from a mutation to the TBX4 gene. People afflicted by these disorders are able to walk without kneecaps, albeit with less strength and range of motion at the knee joint. Thankfully, both disorders are extremely rare.

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