# How many pounds of pressure does it take to break the average human bone?

Just curious.

My Kung Fu instructor once told me it takes about 40 lbs to pop a knee out. Not sure how accurate this is, he was pissed most of the time :smack:

It depends upon the bone and it’s situation. I thought I had this information in my bachelor’s thesis, but apparently, although I looked up the rupture modulus of bone and such, it didn’t maske it into the final draft. Nevertheless, an arm bone supported on both ends and struck in the middle will have a different breaking force than a rib in a standing person.
Also, figures for rupture modulus are often given for dry bone, rather than for realistic wet bones. I don’t have any values, though. Sorry.

Thaks for the replies so far.

I don’t remember what figure was finally reached, but they measure the force required to break a leg bone on and episode of Mythbusters one time.

Pounds are units of force, not pressure.

Pressure is in force per unit area such as psi (pounds per square inch) or psf (pounds per square foot.)

I recall a soccer player who broke his femur (!) running into another player. He must have hit the sweet spot.

According to Wikipedia tensile strength is 130 MPa. And according to Some School in Wisconson the compressive strength is 205 MPa.

It should also be noted that how the bone is being loaded plays a very important role, sometimes more important than how much force or pressure is actually applied. For instance, bones break a lot easier when subjected to torsion than to compressive or sheer forces, and produce much nastier breaks. Of course, only certain bones are able to be subjected to torsion, pretty much just the leg and arm bones (I guess fingers and toes, too.). It would be mighty hard to break a rib bone with torsion. And if you CAN do it, then I think you’ve got bigger problems than a broken rib. :eek:

My whole problem is with the “average human bone” concept. The human body has 200-odd bones, most of them in matched sets of 2, but not all of them, and not all of them constructed similarly. (Notably, teeth are a lot harder than other bones.) I guess the numbers provided for tensile strength and compressive strength are probably your best answer, but in the real world the answer would have to be, “depends on the bone.”

Perhaps this question needs to be somewhat rephrased.

What is the average minimal applied force psi needed to achieve a clean break in each the following skeletal parts (assume no parts are clothing protected)

Skull

Jaw

Spinal column

Ribs

Hand (randomly)

Fingers

Leg

Foot (randomly)

Toes

If we assume values above are applicable for dry bones, would more, less or about the same force (on average) be needed to break living endoskeletal human bone?

I’m very sorry to say I don’t know, but I do want to weigh in on another variable.

You need to specify the rough age of the person whose bones you are breaking. I don’t mean twenty-three versus twenty-four, but infant/child, teenager, adult in the 20’s or 30’s, adult in midline, older adult.

This is why. Human skeletons start out (fetally) as all cartilage, and gradually turn to bone. In newborns they are extremely elastic but fairly fragile. It is nearly impossible to break a newborn’s ribs doing CPR, but you can break a rib in place simply by pressing on it hard with a finger. Very old people have brittle ribs which fracture over and over during CPR - all those of us who have tried to resuscitate someone over 80 remember the horrible feeling of the crunch.

It takes decades of life for the elasticity to go away. I once autopsied a nineteen year old who had gotten killed by his ex-best friend. (They lost their friendship over a girl they both wanted.) They met on the street at night when the ex-best friend had two other guys with him he had to impress. The E-BF, who was big and muscular, lifted the guy up over his head, imitating a WWF body slam, and threw him to the ground. This hurt the guy (minor aches and pains), but didn’t kill him. The E-BF then stomped once hard on his chest with his Timberland boot. The guy gasped, lifted his head, looked funny, and collapsed. Everyone got really worried and they did the right thing, which was call 911. Obviously, he couldn’t be saved.

At autopsy, there was a rounded mass of blood in the top of his right lung that, if you squinted at it just right, bore a vague resemblance to the shape of the heel of a Timberland boot. A major artery running out of the arch of the aorta towards the collarbone had snapped. Just popped; the end of it looked like a pants cuff. Most of the blood in his body had left through the popped end. This would have been under the hindermost part of the ball of his foot. Across the trachea on the inside was a distinct patch of red with narrowing but widely separated edges. This also suggested the outline of the front of a Timberland boot.

But none of the guy’s ribs broke.

His ex-friend stomped him so hard he burst his artery and yet the 19 year old’s ribs were so elastic that they bounced to that blow and gave without breaking.

As a matter of fact, if they hadn’t been so elastic, they might have saved his life. What doesn’t break may transmit more force to the next deepest tissue.

While I was rereading bouv’s answer, I suddenly recalled a word for a material which has different breaking strengths depending on whether you load it with pressure slow or fast.

Viscoelasticity.

I googled “viscoelasticity bone” and came up with a whole lot of sites exploring the different stress and strain failure rates of bone. That is, they’re answering your question in too much detail. They are saying, not, one hundred pounds of pressure will break the wet human tibia, but for example, it is possible that something like seventy-five pounds of pressure loaded over ten milliseconds will break the human tibia when applied from the side (tension, the usual image we have of a leg bone breaking); one hundred and fifty when loaded over ten seconds; six hundred when loaded in ten milliseconds from the top (in compression, like the legs of a weight lifter bringing a weight up over his head); one thousand when loaded in ten seconds from the top.

And many other variables.

Do not use the numbers I just put in, I created them and they are as an example only. I actually couldn’t figure out the pounds per square inch from the many charts; they confused me.

I will put the link here, but you have to bring it up in PDF to read it, because it’s a set of notes displaying slides from a presentation, and if you bring it up as HTML, you don’t see any of the illustrations. So you’re better off to google “viscoelasticity bone” (it’s the third link down when I try it) or to put in the name of the lecture, “Mechanical properties of Cortical and Cancellous Bone”. link

Several hits down is a Cornell site which also seems useful though I stopped skimming after two or three pages. [cornell link](http://www.mse.cornell.edu/courses/engri119/ Class_Notes/bone_structure__growth.htm - 16k -)

The problem is all these helpful people want to answer your question in TOO much detail. It’s almost like the evolution questions that cmosdes asked in GQ today, and Blake was pounding the answers into the ground with a stake - define your terms. You have to know what you mean by “break” and “bone”.

Hajario brought up another important variable to further confuse the issue. A bullet of a given mass will shatter a bone when it hits it where a fall onto a large flat surface may not, even given the same impact velocity. The two impacts can be defined as having the same mass times velocity squared, but wounding energy is unfortunately proportional to mv2 divided by (area)x(time). Area is the area over which the energy is applied (the smaller, the more devastating, as in the case of the bullet) and time is the amount of microseconds it takes the energy to transfer. If you roll going down hill, you spread out the energy and increase the time, so the same mass times velocity squared may not break your bones as if a fall with the identical kinetic energy all happened in an instant.

Uh - last line of my answer - if you roll going down hill, you spread out the area and increase the time.

It makes no sense in the equation Wounding Energy = k (mv2)/(AT) to spread out the “energy”.

California fruitcake style en -ur- geeee!

This concept is known as impulse in physics.

You’re still not using these words correctly.

As was stated before, “force” is expressed in pounds or newtons.

“Pressure” is expressed in psi (pounds per square inch) or pascals.

Force is not the same thing as pressure, and psi is not a unit of force.

In addition, assuming we are talking about forces, it depends on where on the bone the force is being applied, and if the bone is restrained in some manner.

Robby. That’s why people smarter than me are correctly intuiting the proper phrasing of the question, while gently correcting my mistakes. My ego appreciates it.

While I am just fascinated with multisyllabic physics concepts such as viscoelasticity and the endless ENDLESS endless permutations of the question: ("Do you mean an adult or child’s bone? There’s no such things as “average bones.” Is this from a bullet or a fall? Are we taking about pressure or forces? – I mean, yeesh, people.) Y’all are seriously overthinking this.

I’m shocked no one’s said, “Under the simplest scenario, this-and-this would produce broken bones with this amount of force (or pressure), but any change in these variables would require MORE or less force or more speed or it may not produce a break as described.” Y’know. Something simple.

Think force (I guess that’s right) applied during, I dunno, karate or judo, unarmed hand to hand combat between adults, maybe include some bone breaks due to kicks or flipped falls, go from there.

Thanks for your thoughtful replies gabriela. I especially smiled at this insight:

You’re tellin’ me.

:eek: I didn’t think it was that bad.

This is the sort of question you can always make more complicated. I did an honours degree in biomechanical engineering before going on to fix fractures in the emergency room.

To answer your question simply, you could probably fracture a kneecap with an impact force of 6.92 kN. You could probably fracture a human female pelvis with an impact force of 7 kN as well. You could probably break most bones with an impact force of 20 kN. Simple, no?

Uh, no. One way to see how much force it takes to break a bone is to get a cadaveric bone, apply a speciifc load in a specific way, and see when it breaks. You get a number. People differ. You could get 20 cadaveric kneecaps, find the average and standard deviation, and get a rough idea… The first number comes from a University of Michigan abstract: http://www.obl.msu.edu/resabs/pjmacken.htm – where they did just that, and found gross kneecap fracture in cadaver kneecaps with forces between 3 and 10 kN. The study was looking to see if other factors apart from patellar geometry (kneecap shape) influenced the breaking point. The study found that cadaveric age, size or sex did not influence the result. It stands to reason the number must be pretty useful… except lots of other in vivo things would affect the number.

The second study, from the “Etheridge annals” is looking at cadaver specimens of female pelvises to see the force needed for fracture when loaded to approximate side impact automobile collisions. The “5th percentile” pelvis (in terms of bone mineral density) could barely withstand a load of 5.77 kN, but not more. Again, this number must be pretty useful… and at least this study looked at bone mineral density and tissue thickness to get a better number. Lots of other studies use finite element analysis to approximate a number.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15771279&dopt=Citation

Except that life is more complicated than hitting a dry, aged specimen with a hammer. There is good evidence that cadaveric specimens might require more force than an in vivo bone (due to age or dryness). There is excellent evidence that bone periosteum and tissue shield bone from fracture at much higher loads that bone itself. There is enormous variation in bone biomechanics from person to person. Age, disease, genetics play a role independent of periosteal tissue thickness, bone mineral density, bone architecture and geometry, etc.

Some bones fracture only if a force is applied at a certain angle or direction, the spine is very resistant to parallel loads since its function is to support the body. Bones also fracture in different (if predictable) ways. A fracture may be at a microscopic level – bone breaks and heals itself all the time, Wolff’s law suggests that bone grows in response to mechanical stress so as to produce an anatomical structure best able to resist the applied stress. For example, should a fracture of a weight-bearing long bone heal with an angulation, each step that the patient subsequently took would result in a bending stress with compression on the concave side at the angulation and tension on the convex side. Rather than progressively weaken the bone structure at this site, such repeated mechanical stress results in a “remodeling,” with new bone growth on the concave side and bone resorption on the convex side. If the patient is young enough, the bone will ultimately through this process grow straight.

Or a fracture may be due to repeated stresses; stress fractures are due to the number of cycles of load, and are very different from breaking due to a one off shot in the solar plexus.

And viscoelasticity, Young’s modulus, creep, fatigue and other engineering parameters only confound the issue. The periosteum of bone helps resist fracture, and subcutaneous tissues help absorb loads so that it is tough to know how much of it actually gets to the bone… and this would depend a great deal on the fine details of the load (direction, angle, time, changes with time, etc.)

I’d settle for the second paragraph and not sweat the details. It is pretty easier to make biomechanics hopelessly complicated. For all that, I treat fractures all the time. And know what type of fractures to expect if someone falls rollerblading, is hit by a car or punches the wall. Neverthless, there might be 20 different ways to break an ankle – many of which are treated differently.

Dr_Paprika, Wow. That looks like you did a lot of work to come up with that figure. I really appreciate cite, the thought and effort behind it, as well as the acknowledgements of the variables that could affect the final number. Thanks a lot.

Well, I’m satisfied–! That is, barring someone else coming along and saying, “Not so fast!” and coming up with a whole new set of figures.

One last question, though. KN are kiloNewtons, right? How do you convert that to psi?