Re: PSI needed to break the weakest bone in the body?

Dear Hannah,

According to BBC science and Nature ( http://www.bbc.co.uk/science/humanbody/body/factfiles/leg/leg_and_foot.shtml),

"Your leg bones are the longest and strongest bones in your body. When you stand or walk, all the weight of your upper body rests on them. Each leg is made up of four bones. The three long bones are your femur, your tibia and your fibula. The fourth bone is your small patella, which is better known as the kneecap.

"Your femur, or thighbone, is the largest bone in your body. The head of your femur fits into your hip socket and the bottom end connects to your knee. The two bones beneath your knee that make up your shin are your tibia and fibula. Your upper and lower leg are connected by a hinge joint. Your patella, or kneecap, rests on the front of your femur.

"The bones of your leg have roughened patches on their surfaces where muscles are attached. When your muscles contract, they pull the bone they're attached to, making your leg move."

This is confirmed by a book on forensic Anthropology ( http://www-personal.une.edu.au/~pbrown3/skeleton.pdf), which took its sources from,
Warwick, R. and Williams, P. 1973. Gray's Anatomy, 35th edition. Longman.
Bass, W. 1971. Human Osteology: A laboratory and field manual of the human skeleton. Missouri Archaeological Society.

The strength of a bone is proportional to its size as all bones are made of the same material. Therefore the femur (thigh bone) being the longest is also the strongest and the tiny ossicles found in the inner ear (the stapes being the smallest, are the weakest. The strongest material in the human body, however, is tooth enamel (Ross et al. p245).

The force needed to break a human femur is about 1700 PSI or over 1 million kilograms per square meter (according to emedicine http://emedicine.medscape.com) which agrees with numerous other sources around the internet.

Note that it is the force per unit area that is quoted so it depends on how much the force is spread out. A sharp edge would need a much smaller force than a blunt one. Assuming we can scale the force down to the size of the stapes which is about 3mm wide, with the typical femur width of 3cm , this would mean a pressure of 170 PSI (100,000 kilograms per square meter) would be enough to 'break' the stapes. Assuming the stapes to be about 10 square mm in area (about 0.00001 square meters) the force needed would be about 1 N (0.2 pounds), which is a very small force indeed.

One newton is equivalent to the force of a small apple when dropped. Luckily our stapes is encased inside our skull and is very rarely broken, even in head injuries. Using similar estimations it would require a force of a thousand Newtons to break a femur, thats about the weight of a small car. Please look at the websites quoted, some of them are very interesting and I enjoyed looking through them very much.

I hope this clears up your confusion!

Sean Hunt

More websites: http://www.biomedtown.org/biomed_town/LHDL/Reception/collection/DestructiveTesting
http://www.biomedtown.org/biomed_town/LHDL/Reception/collection/BendingOfTheDiaphysisOfTheFe mur
http://www.bmj.sk/2008/10909-01.pdf
http://en.wikipedia.org/wiki/Bone_fracture
http://www.merckmanuals.com/professional/sec21/ch309/ch309b.html
http://video.about.com/orthopedics/Fractures-2.htm

References:
Ross, Michael H., Kaye, G.I. and Pawlina, W. (2003) Histology: a text and atlas, 4th ed., Philadelphia; London: Lippincott Williams & Wilkins
Warwick, R. and Williams, P. 1973. Gray's Anatomy, 35th edition. Longman.
Bass, W. 1971. Human Osteology: A laboratory and field manual of the human skeleton. Missouri Archaeological Society.