Consider this passage from the late Robert Heaney*: "Although bone mass is certainly the most extensively studied of the fragility factors, low bone mass is not the whole of the osteoporosis story and may not even be its most important component (despite frequent assertions to the contrary). If one could magically normalize bone mass in everyone, would one eliminate osteoporotic fractures? The best answer that can be given today is ‘no.’"
Assume (as you should**) that Heaney is correct. What else, besides intrinsic bone conditions, might lead to fragility fractures of the hip (that is, those caused by low energy mechanisms)?
Patients that sustain hip fractures after low energy trauma (e.g. a fall from a standing height) often have more than just low bone mass to blame for their injury (though they have that too, don’t forget). These patients typically have an increased propensity to fall, are often unable to protect themselves when they do fall and lack sufficient soft tissue around the bones to absorb some of the shock.
Even though a fall from a standing height is considered a “low energy” trauma (for example, compared to a motor vehicle collision) there is more than enough energy generated from fall to break even a normal hip.
Consider this passage from Lotz and Hayes***: “We conducted an in vitro investigation of the loads and energies needed to fracture the proximal part of the femur in twelve fresh cadavera under loading conditions simulating one particular type of fall. The fracture loads ranged from 778 to 4,040 newtons and the work to fracture, from 5 to 51 joules.....the measured work to fracture for the isolated femur was an order of magnitude smaller than estimates of the energy available during a typical fall (about 450 joules), suggesting that energy absorbed during falling and impact, rather than bone strength, may be the dominant factors in the biomechanics of fracture of the hip.”
That is, the energy available during a typical fall is ~10 times greater than the energy needed to break the hip. Yet most falls do not result in fracture. Indeed, falls that do not result in fracture are ~10 times more common that those that do, according to the paper by Grisso et al.***
One way to think about this is that if there is no energy absorption, a hip fracture is likely independent of bone density. Or to put in Heaney’s context, if one could magically normalize bone mass in everyone, one would not eliminate osteoporotic fractures: if the normal means of energy absorption were absent (e.g. cachexia, sarcopenia, poor reflexes, weakness, etc) an ordinary fall could break the bone, even if the bone were entirely normal.
Another thing to think about is that even if most falls do not result in fracture, a propensity for falling (i.e. more total falls) should increase the risk of fracture overall. (It’s like playing Russian Roulette with more bullets loaded.)
And indeed conditions that promote/allow falling create a risk of fracture, says the paper by Grisso et al ***: “A number of factors that have been identified as risk factors for falls are also associated with hip fracture, including lower-limb dysfunction, neurologic conditions, barbiturate use, and visual impairment.”
Additional Points to Consider
And that leaves one more thing to think about: is osteoporosis really a good name for the condition?
Consider this passage from Cummings and Nevitt **** “We propose that four conditions must be satisfied in order for a fall to cause a hip fracture: (a) the faller must be oriented to impact near the hip**; (b) protective responses must fail; (c) local soft tissues must absorb less energy than necessary to prevent fracture, and (d) the residual energy of the fall applied to the proximal femur must exceed its strength. All of these events become more likely with aging and lead to an exponential rise in the risk of hip fracture with advancing age. This model also suggests that a combination of measurements of neuromuscular function and of bone strength may be the most accurate approach to assessing the risk of hip fracture.”
If “bone strength” is only part of the risk of fracture, and thus bone mass is only a sub-part of the issue (for mass is necessary for strength but not sufficient), why does the diagnosis name, osteoporosis, center on “porous bone”? Might it be that some entities might want you to think of the fragility fracture problem as a porous bone problem? And if you wonder how the name of the disease might affect your thinking, consult the paper offering this passage: **** “Even well-formed medical terms may skew thinking about treatment.”
* Heaney RP Bone Mass, Bone Loss, and Osteoporosis Prophylaxis. Annals Internal medicine 15 February 1998 128: 313-314. https://pubmed.ncbi.nlm.nih.gov/9471936/
** Validated Prediction of Imminent Risk of Fracture for Older Adults https://pubmed.ncbi.nlm.nih.gov/32181621/
*** (The Use of
Quantitative Computed Tomography to Estimate Risk of Fracture of the Hip From
Falls JBJS: Jun 1990 - Volume 72 - Issue 5 - p 689-700) https://pubmed.ncbi.nlm.nih.gov/2355030/
*** Risk Factors for Falls as a Cause of Hip Fracture in Women N Engl J Med 1991; 324:1326-1331 https://www.nejm.org/doi/full/10.1056/NEJM199105093241905
*** This criterion, that the faller must be oriented
to impact near the hip, is not likely true. A twisting motion or other indirect
force seems to be more than adequate to break the bone.
*** Hypothesis: The Causes of Hip Fracture https://pubmed.ncbi.nlm.nih.gov/2738306
*** In the Beginning was the Word JBJS: February 2006 - Volume 88 - Issue 2 - p 442-445 https://pubmed.ncbi.nlm.nih.gov/16452759/
Peer reviewed by Jaimo Ahn, MD PhD
Need to review and revise the footnote numbering.