Biomechanics article discussion post #2

Ok, I may have gotten a little distracted since Post #1. I ended up realizing that I would need to dive into a bunch of other articles to really do a proper discussion if the first article, and as the post turned into a full on literature review it got *really* out of hand. Oops…

Anyhow, I don’t really want to turn this into a full blown literature review. Instead, I’m going to highlight a few interesting tidbits from the article.

First, what were the objectives? The authors (Taddei et al.) describe work by others that demonstrates that 1) bone strength can be modeled based on CT data (density, geometry) and 2) that safety factor for accidental overloading scenarios, such as falls, decreases with age and decreasing bone mineral density (BMD), but may be preserved for frequent activities such as walking. The second point makes sense, since bones will adapt better to frequent stress – your skeleton can’t predict a fall, so it isn’t necessarily going to grow in such a way as to be well-adapted to withstand those forces. In contrast, if you walk frequently your bones will add strength in regions that are stressed by the forces of walking, and it becomes well adapted to bearing those loads.

Taddei et al. combine CT-based finite element modeling and personalized loading condition estimates in order to 1) determine the safety factor of the femur for two common activities – level walking and stair climbing and 2) determine whether this safety factor is related to age, volumetric bone mineral density, and gender.

Highlights from the methods:
The researchers used CT scans from 200 participants, ~25 – 85 years in age, to build the finite element models of the femurs. Basically, the CT scans provided the geometry and bone mineral density. The geometry was broken down into tetrahedral element volumes in the finite element model, and the bone mineral density at each location throughout the femur was translated into a specific elastic modulus for each of the small tetrahedral elements. This part caught my eye because I’m curious how automated that process was – did they have some cool process for translating between the CT scans and the finite element model or did some tragic figure have to go through the density measurements, figure out which elements each one corresponded to, and assign the elastic modulus by hand? *shudder*

The researchers then did motion capture with human subjects performing the level walking and stair climbing activities. These subjects were not the same subjects who’s femurs were scanned. Because the motion capture subjects didn’t necessarily possess the same femur characteristics as those shown by the CT-based model femurs, the researchers actually went through the biomechanics data and calculated how the joint centers and contact forces would shift for different femoral characteristics. This provided a spectrum of possible loading conditions, which were then be applied to the finite element models.

In total, there were 78 level walking loading conditions and 50 stair climbing conditions – so 128 total loading conditions. This means that for each modeled femur, there were 128 possible safety factors based on the safety factor for each loading condition. The safety factor was defined as the ratio between the maximum principal strain seen in the finite element model of the femur during the simulated loading and a set limit strain associated with femur neck fracture onset (around 1% in tension and 0.7% in compression).

The fact that the authors used a limit strain that had been specifically validated for the femoral neck makes me curious as to whether that limit strain varies for different parts of the skeleton. Or for different bone mineral densities… Might have to wander back over to Google scholar/PubMed after this!

Highlights from the results:
The major result from this paper was that the safety factor, for flat walking and stair climbing in men and women through the tested age and bone mineral density ranges, never went under one, and was around 5 on average. If the safety factor had dropped down to one that would actually be quite concerning since that would give an extremely high probability of femoral neck fracture and there’d be femurs snapping all over the place…

However, the authors DID see safety factors that were within the range where spontaneous fracture could occur – in the femurs modeled after older, low bone mineral density (osteoporotic) women. The safety factor for flat walking and stair climbing declined with age in both men and women, but was much better preserved than the safety factor for a sideways fall, which makes sense in terms of bone adaptation to physiological loading. The results were consistent with what would be expected based on which population most commonly experiences femoral neck fractures (elderly women with delicate bones). It looked from these results like these particularly high-risk osteoporotic women were in danger of spontaneously fracturing a hip during walking or stair climbing. I’d been talking to my mother (a physical therapist) recently about this because I’d always been under the impression that hip fractures occurred when people stumbled and fell. However, she explained that it was often the hip fracture that occurred first, causing the fall. How awful would it be to be walking along and feel your bone snap underneath you after a step? I’m going to go chug some milk and sit in the sun now. And then go for a long, femoral neck loading walk…

*Interesting side note – people have tried to come up with training protocols to pre-hab for side falling and to build up bone strength in the areas that would be stressed by a sideways fall. One protocol that I vaguely recall had people holding themselves up in a side plank and then repeatedly thunking down to the floor hip first. Ouch…but probably worthwhile if it lowers the risk of hip fracture!

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