Ok, finally getting around to writing about something *not* related to how badly my calf is behaving.
This will be my first biomechanics research focused post for 2015 – 3 days late but I’ll call this the week one post. I’ve been reading Cat’s Paws and Catapults, by Steven Vogel, and recently read through a section on the difference between safety factors, or the ratio of the strength of a structure to the maximum load that it is expected to bear/designed for, for engineered versus biological structures.
In engineering the use of safety factors is ubiquitous. If you have a critical part that needs to hold a 5 pound load without failing, you aren’t going to build a part that can hold exactly 5 pounds and no more – a small variation in the load could be catastrophic. Instead, you use a safety factor. For example, you might apply a safety factor of 2.0, designing the part to hold up to 10 pounds before failure. The part is now over-engineered, and may cost more to manufacture. However, if an unexpected overloading occurs, the design with the safety factor is better prepared to handle the adverse loading event without giving way. This is especially worthwhile if you’re building something like an elevator, bridge, airplane, or ski lift, where parts snapping apart all over the place is frowned upon (and leads to far too much terrified screaming).
Biological structure safety factors are much less clear cut than those for most engineered structures. They haven’t been neatly designed by an engineer, so no nice notes on how the appropriate safety factor was determined. In addition, it may be difficult to measure the expected or maximum physiological loading conditions. Individual variation complicates things as well. Lastly, there is a cost associated with increasing safety factor, just as there is in an engineered structure – thicker bones, for example, require more bone material (minerals, collagen, etc) and a heavier skeleton with increased energy costs when moving that skeleton.
The Vogel text mentioned that tendons and bones generally have a safety factor between 2 to 6. This made me curious about what work has been done to estimate safety factors in human lower limb bones and how that work has been applied to predict bone failure (fracture). I did some quick googling and found a recent article that examined safety factors in the femoral neck during gait, and also looked at how this safety factor might change with osteoporosis. The researchers used a mix of biomechanics data collection (to get muscle force magnitudes and directions) and finite element analysis (FEA) to model the proximal femur/femoral neck under the experimentally measured loading conditions. Cool stuff!
Since this post is already getting a bit long-winded, I’ll put the abstract here and will (hopefully) move on to discussing the actual article tomorrow!
European Society of Biomechanics S.M. Perren Award 2014: Safety factor of the proximal femur during gait: A population-based finite element study
Fulvia Taddeia, Ilaria Palmadoria, William R. Taylorb, Markus O. Hellerc, Barbara Bordinia, Aldo Tonia, Enrico Schileod (doi:10.1016/j.jbiomech.2014.08.030)
“It has been suggested that the mechanical competence of the proximal femur is preserved with respect to physiological loading conditions rather than accidental overloading, but the consequences of this adaptation for fracture risk in the elderly remain unclear. The goal of the present study was to analyse the safety factor of the human femur in the two most frequent daily activities, level walking and stair climbing, and to understand the dependence, if any, of this safety factor on age, volumetric bone mineral density (vBMD), and gender.
To this aim, a finite element study was performed on 200 subjects (116 women and 84 men), spanning a large range of age (23–84 years) and vBMD levels (T-score from 0 to −3.59). For the first time, finite element models that included a subject-specific description of the anatomy and mineral density distribution of each bone were coupled with a personalisation of the loads acting on the proximal femur during movement, including the action of the muscles and their variability across the population.
The results demonstrate that the human proximal femur is characterised by a high safety factor (on average five, never reaching fracture threshold), even in the presence of advanced age and low mineral content. These results corroborate the hypothesis that the relationship between loading and mechanical competence is generally preserved in the elderly population for the most frequent motor activities, walking and stair climbing. Interestingly, a decrease of the safety factor was observed with increasing lifespan and reduced mineral content in women but not in men.”