Knee caps, why no elbow caps?

Hmmmm.....  Where to start.

Okay, sessamoid bones are round bones that form under ligaments due to repeated motions.  The patella (or knee cap) is the only sessamoid bone that all people (except your mother-in-law, apparently - I assume she was born without them) have.  Some people also form sessamoid bones in their knuckles or other joints if they use the joint a lot - these little bones always form under ligaments - the knee cap is just a great big sessamoid bone.  The constant pressure from the ligament moving over the area causes a tiny bit of bone to enlarge in order to protect the tissues underneath from the repetitive motion.  So the knee cap develops during the course of your life under the ligament that connects your femur (thigh bone) to your tibia (shin bone) when you walk.

What does the knee cap do?

It holds the bones stable in the knee when the knee bends and straightens.  The knee is a modified hinge joint that has slight rotation.  The reason it has slight rotation, is because the femur does not sit very "deep" on top of the tibia.  This gives it greater movability, but also makes the joint less stable and more likely to move side to side.  The anterior and posterior cruciate ligaments form an X-shape inside the knee to hold them stable, and the collateral ligaments on the sides support the joint on the side.  The patella helps support it in the front, so your femur is less likely to displace and cause owies.  (yes, owies is the technical term)

The elbow is also a hinge joint (and a pivot joint - two joints in one!), but the humerus sits "deep" in the ulna, so there is less side to side and rotation motion (read - none) in that hinge joint.  Why no elbow cap?  There's no need for it there.  The elbow is stable enough.  There is a bursa (fluid-filled sack) under the ligament on the elbow, but no extra bone, because the elbow bending doesn't need the guidance that the knee needs.  Also, there is less repetitive motion in the area because you don't walk on your hands (or do you?).  Though if you bent your elbow as often as you bent your knee and put as much weight on it, you might form a sessamoid bone there.  Then you would have an "elbow cap" - not likely.

It's all about architecture, Viv.

That little bony point on your elbow is part of your ulna and is called the olecranon.  Free trivia.

Did that answer your question?  Pictures would greatly clarify this.

My orthopedic doctor said, if we could find a way to resurface the bones/joint in that area there would be less need for knee replacements.  Right now, those lovely ligaments are catching on something in my knee that causes a ping--like a rubberband-- a flash of pain, and numbness--I don't move when that happens because I don't think my leg would hold me and then goes away. I'm suppose to see the doctor about this next week. this is the knee he operated on and is now acting up again.  Sometimes stretching the muscles help. Gah.

I wonder if you have a bone spur, Sia.  Yeah, do take it easy on that knee.  Best to avoid further injury, as much as possible.  Does not sound fun.

Maybe this will help.  The knee hinge would be like a tea cup sitting on a saucer, where the elbow hinge is more like a wrench wrapping around a big bolt.  That's what I mean by "deep".  The elbow hinge "holds on" better.  The knee hinge is just sorta sitting there waiting for something to knock it loose in any direction.  The knee cap keeps the tea cup from sliding off the saucer as easily.

Jamie we have more questions below.

Wow! You make the human body sound so cool.I like the tea cup and wrench analogy, very visual and immediately graspable.

So why do some people's knee caps stick out more than others. Am I correct in assuming the more one uses their legs the larger their knee caps get? Is there a maximum size of the knee cap?

As a person with a recent serious owie to the knee, this is fascinating! (I think I chipped the patella, but being allergic to doctors, I'm living with the weird pains). And I want to know what some people have such sharp elbows. Honest to goodness, my hubs could cut glass with his pointy little elbows. I've been injured more than once by the things!

I would assume so, Vivian.  I wonder if there is a world record for largest knee cap?   Knobby knees are more due to lack of adipose (fat) tissue, so the knee caps stick out more - there is less cushion for the pushin'... erm, knees.

Bone adapts to stress.  If you look at the skeleton of an adult it has ridges and bumps and grooves and points, while a newborn baby has a smooth skeleton.  When tension is put on a bone, it adapts by growing, so all those ridges, etc develop because muscles are pulling on the bones.  Why would you have a pointier elbow?  Your tendons (attach muscles to bones) are pulling on that bone there, so I'd hazard to guess that hubs has a narrow tendon that connects his triceps muscle to his elbow in back.  (It can be hazardous to guess, though.)

My anatomy and physiology students are always fascinated with the differences we see in internal structures between individuals.  You don't expect everyone to look the same on the outside, so why would everyone look the same on the inside?  Variety is good.

Wanda and Pat have further questions. Though we've moved on to include Achilles tendons.

I've been searching for the world's largest knee cap to no avail.

I don't even want to imagine what my bones look like, probably not pretty. So if I get fatter, I'll have smoother knees? Excellent! I'll have that extra helping of pasta tonight.

Excellent conjecture, Jamie. It's as good as anything I've come up with. And I never guessed we'd differ on the inside! My bones must look like I've been rode hard and put away wet.  The knees especially. No rude conjecture there. I'm just a born klutz with a faulty sense of balance and depth perception.

OK, can muscles be "dented"? And do they grow back into place? That arm I banged a few months ago still has a "dent", as if the muscle or adipose got squished up on either side, leaving the bone feeling exposed. And are there nerves in all that bone/tendon/muscle structure?

Muscle tissue heals slowly if at all.  It depends on if the muscle tissue died when it was injured or if it just atrophied (shrank).  If it atrophied, it can come back with use of the muscle, if the tissue actually died, it probably will never be the same - though over time it will likely improve.  And you must have banged it pretty hard if it dented it to the bone.  Ouch!  Sounds like the tissue got smashed between a rock and a hard place (bone).

There are lots of nerves in bones and muscles.  Fewer nerves in tendons.  (You mean pain sensory nerves, I'm presuming.)

Great answer! And yes, I did indeed bang it hard enough to dent it to the bone. I'm going to go with "atrophied" for now and see if some light weights and hand exercises improve it any. Thank you!

Jamie, you are a treasure! My knee pain is caused by the posterior and anterior cruciate ligaments being stretched by a couple of falls. I don't think they're torn at least I hope not. But if I step wrong there's a bad twinge and a kind of burn and I have to wait for that to subside before walking again.

Now can you tell me if my Achilles tendon which I did tear and it has healed but it still bothers me if I walk too much. Will it ever be all better with no owies???

Ligaments, tendons and cartilage all heal incredibly slow and never completely recover from injury because they don't have a blood supply.  That's why joints give people so many problems.  They're composed of those 3 tissues.

Now that you've lost so much weight, does it still bother you, Wanda?  Losing weight should make it feel at least better, in any case.  There's less strain on it and it's probably weakened from that old football injury.  It was a football injury, wasn't it?

I could say that it was an old football injury, given to me by an old football player??? LOL It doesn't bother me as much as it did before I lost this much weight but still doesn't act up from tiime to time.

Actually I stepped into a cement garbage can holder when I worked with the Oregon Parks System and forced my big toe all the way back. It created a horrible burning sensation along the foot and then when I got home I couldn't walk at all on that foot. Took forever for it to get to where I could walk unaided. Imagine spending the rest of a summer season, like about 4 months, in a small kiosk taking money, handing out pamphlets, etc, WITH a pair of crutches! LOL I was a site to behold.

Nice to have our own anatomy prof here to answer our questions. Now about that fairly large bump sticking out at the bottom of my knee, under where they hit you with a little hammer to see if you have reflexes. Why do I have a bump there? It's hard like bone, etc. I fell on that knee twice after my back surgery, due to foot drop. Or an old football injury, same old football player. LOL

Ouch!

As my knees hurt I wondered why I think I found out why.Over work!!I never fell and hurt them but I would work 16 hours on hard brick floors.