“Fundamental discovery” overblown
Jul 18th 2005Wallabynerd alert
“Sacrificial bonds and hidden length dissipate energy as mineralized fibrils separate during bone fracture” — Nature Materials (online July 17 2005, doi:10.1038/nmat1428)
(Technical stuff in the full article…)
Fantner et al. are in the “sacrificial bond” camp, which supposes that there’s some sort of “sacrificial bond” between … well, between either mineral crystallites and collagen, collagen and collagen, collagen and other matrix molecules, crystallites and other matrix molecules, …. etc. No real idea of what this “sacrificial bond” is supposed to be, and it’s not a chemist’s notion — it’s just supposed to be weak enough to give when bone is deformed or stressed, but strong enough to hold bone together the rest of the time. (Call it the bone physiologists’ version of epicycles, or the missing link.)
Sacrificial
bonds are additional, weak but reformable, bonds that break before
the strong bonds that hold the structure together break. The energy
that is needed to break the weak sacrificial bonds (and subsequently
stretch the molecules) increases the total energy needed to fracture
the material, thereby increasing the toughness of the material.
This paper presents “evidence” that mineralized collagen fibers in compact bone are held together by non-collagen matrix material, using some rather cool AFM measurements of just how much force is required to pull two fibers (or rather tiny plates of bone) apart. This suggests that the “sacrificial bonds” are therefore located in this “glue”, and that they depend in some way on calcium ion (compared a buffer with 40 mM Ca++, 110 mM Na+ to one with 150 mM Na+…. has no one told them about ionic strength effects for electrostatic interactions?). The total energy required to separate the two plates was greater if they were given longer to “heal” back together when placed in contact between pulls (up to about 10 seconds, with n.s.d. after that — questionable, their graph makes this look a bit shaky).
Then they generalize this into a “fundamental discovery” about the nature of bone fracture — that it’s based on this “glue” having limited yield strength. Lots of ballpark guesstimates about how much of this glue might be present, how many of the “glue fibers” might be required to match the bulk yield point of bone, etc.
Problem: As a quick look at almost any picture of compact bone will show (see, for instance, these slides), it’s not made up of tiny little squares of bone held together by glue; the fibrils are twisted and woven together, then arranged into concentric layers surrounding osteocytes. Result: Stress is dissipated, not concentrated; pulling adjacent fibers apart by 2-3 microns (as they had to do in the AFM measurements) is difficult to do when the fibers in question are surrounded by other fibers. And
This is not to say that the immediate mechanism of bone fracture *isn’t* fibers pulling apart. It’s just that this paper isn’t grounds to shout “eureka”… no real attempt to identify or locate the “sacrificial bonds”, no attempt to prove that the non-mineralized fibrils seen connecting mineralized fibers are not collagen, no attempt to see whether the apparent dependence on calcium ion is really an ionic strength effect, and graphs of three points to make a line.
Finally, Hansma’s good for pompous quotes: “Our paper is the beginning research” on how bone works at the molecular level. No, no it really isn’t; it’s something our lab’s been working on for several years now.
As Hansma also said, “a paper on bone is published every six minutes”. This is one of them.