The tangential treasure trove hidden in one peer-reviewed research article

I mentioned in my last post that I am only working 20 hours/week. So, what am I *doing* for those 20 fabulous hours? Reading research articles! PubMed, GoogleScholar, and ILL all day, accompanied by several cups of delicious (i.e. not from a crusty lab coffeepot) coffee.

A little background: This past Fall I was only TAing part-time (~13 hrs/week) and was in rather uncomfortable financial straits after missing my thesis deadline and unexpectedly needing to pay for an additional 3-credit semester. I was fortunate to get an internship with an amazing start-up company that is developing patient education/health communication apps for medical professionals. They are basically taking those standard health info pamphlets that doctor’s offices and student health centers hand out and turning them into interactive iPad apps full of really brilliant 3-D animations and other nifty tools to allow clinicians to more effectively communicate with patients. My role is to go through the medical research literature and pull out data on outcomes (success rates, side effects, etc) for different treatment routes for various medical conditions.

I have extensive experience with the diving-into-the-literature-and-pulling-out-important-bits part of the work, but hadn’t done much with clinical research literature. It’s been a great learning opportunity. Reading about such a broad range of conditions and treatments is giving me a great overview of what conditions and treatments are really lacking in research, as well as where the more challenging problems exist in various medical fields.

On occasion, the frustration of a sparsely researched condition combined with the slight tedium of searching through mounds of articles can be frustrating. I usually deal with this by going off-the-clock for a few minutes and allowing myself to do fun things, like browsing interesting surgical images, or wandering off down the literature trail of an interesting tidbit.

This second boredom-busting method led to the inspiration for this post. I started off in a typical article about the effectiveness of taping for flexor tendon pulley tears in rock climbers, and ended up learning fabulous facts and theories about bat (the flying mammal kind) toe flexor tendon adaptations!

This journey started as I was skimming through Impact of Taping After Finger Flexor Tendon Pulley Ruptures in Rock Climbers, by Isabelle Schoffl et al., in which the authors bravely discuss gruesome finger injuries without letting even a hit of discomfort or disgust creep into their writing. (In contrast, there’s this Reddit thread with a lovely video in which you can hear someone’s flexor tendon pulley tearing with an audible snap. Ugh…)

Here’s a nice illustration (from climbing blog Crux Crush) of what this ‘snap’ means anatomically and what Schoffl et al. were looking at preventing/correcting via athletic taping techniques:

bowstring

The athletic tape typical wraps around the phalanxes (finger bones), applying pressure over the tendon and (hopefully) preventing bowstringing where the tendon moves away from the phalanx. Usually the flexor tendon sheaths (A1 – A5) do this job – here they are all torn up (A2, A3 in this figure).

Here’s a view of a proximal flexor tendon pulley injury (A2) as it would look for a person trying to flex their finger (from this article in the Indian Journal of Plastic Surgery):

ijps_2013_46_3_458_121931_f4

Eeesh. I’m usually not too squeamish, but hand injuries make me a little squirmy.

Anyhow, the article by Schoffl et al. was focused on testing new and existing finger-taping techniques commonly used by climbers as both preventative measures and post-pulley-tear reinforcement. This part was actually really cool – force diagrams, finger-flexor resistance contraptions, and ultrasound-based tendon-to-bone distance measurement techniques galore! So nifty!

BUT…the really random, unexpected bit occurred in the 2nd of these two sentences:

“The friction between tendon and tendon sheath is maximal over the pulley and especially over the pulley edge (Zhao et al., 2000). This was first observed on chiropterans (Quinn & Baumel, 1993), which are able to use the friction between pulley and tendon in order to dangle without muscular activity.”

Wait, we just went from talking about ouch-painfully-rubbing-tendons to BAT FEET?!? Awesome! And totally unexpected. So, off I went into the Quinn & Baumel article in the Journal of Morphology** to learn more. It turns out that many bats have a cool ratchet system in place between their flexor tendons and the surrounding tendon sheath. The tendon has small bumps, or tubercles, on it right near the base of the digit on the sole/palm side. The corresponding tendon sheath has little ridges, which interact with the tubercles to allow the bat to lock its digits into a flexed position and then hang using its hook-like toes without having to maintain a strong isometric contraction in its flexor muscles. The bat’s body weight holds the ratchet mechanism in place, allowing the bat to eat, groom, and even sleep in the upside-down hanging position. There is a really cool detailed description with magnified images of the tubercles and corresponding ridges here (page 2).

It makes makes me extremely happy (and just a little amused) that the authors of the article on finger taping in human rock climbers decided to stick in a citation related to bat morphology. They could totally have skipped that second sentence, leaving us with the initial statement about high friction over the pulley edges, and left out the slightly tangential link to the bat tendon-locking mechanism paper. However, the little snippet and resulting swerve into the literature of bats and tendon locking mechanisms was perfectly delightful. Not only did the authors succeed in mentioning bats, which are just plain amazing and should be mentioned *whenever possible*, but they also provided the reader with a possible prompt to think about the idea of tendon-pulley friction in a wider context. Personally, this article totally made my day by sending me off into a lunch-hour literature search!

**For any climbers out there, the article by Quinn and Baumel has some fascinating ‘cited-by’ articles in Google Scholar. The citing articles talk about everything from finger taping techniques to estimations of the hanging force produced by tendon-pulley friction in humans. A heads up: there are also lots of slightly jarring cadaver-finger testing images.

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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!

Biomechanics article discussion post #1: “Safety factor of the proximal femur during gait: A population-based finite element study”

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!

Title:
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)

Abstract:
“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.”

Posting goals for 2015

As you may have noticed, I’m not particularly systematic as far as this blog goes. The name about sums it up – I post about an assortment of things on a fairly erratic schedule. My goals in having a blog are 1) to share my experiences in the hopes that someone else might find something useful/interesting, 2) to connect with others with similar interests, and 3) to give myself a fun creative outlet.

However, I would like to add more direction to my writing time as I start the new year. I have two aims that I’ll try to work towards over the next 12 months:

1) Write with purpose. Start posts with a general outline of where they are going and WHY I am writing them. I’m great at aimless rambling, but that’s not always the most satisfying thing to read.

2) Add in some biomechanics! I’ve been getting plenty of science and engineering-focused writing in with my thesis, but now that that is done I’d like to have some sort of writing assignment in order to encourage myself to keep on top of the literature. My goal for now is to write about a recent biomechanics paper each week over a series of 1 – 7 posts. I’ll be focusing on orthopaedic biomechanics but might branch out if I see a particularly *AWESOME* comparative/soft tissue/sports biomechanics paper or whatnot. I’ll try to keep it interesting and not bore anyone to tears…and I’ll clearly mark those posts so any non-biomech folks who don’t want to hear about femur fractures or catastrophic implant failures can skip them 😉 However, I will also try to keep my writing relatively jargon-free (or at least jargon-defined) so that my posts are accessible rather than prickly and hostile to non-biomechanics readers.

Happy (late) New Year!

Wobbly toes

Yesterday I participated in a couple fellow grad students’ study as a research subject.  They were looking at some different treadmill gait biomechanics, which meant I got to get all markered up (full body gait, so like 200 shiny balls stuck on with toupee tape, whee!) and stroll along on a treadmill at different paces. I also succeeded in kicking off my medial ankle marker at least every other trial and doing really helpful stuff like forgetting to swing my arms normally and reaching up to scratch my nose instead.  Oops…

Anyhow, the other students did end up getting some good data out of me and I ended up benefiting too…and not just cause I got to hear about their cool research & go for a nice brisk stumble on the ‘mill 😉 While doing the slow pace walking trials I realized that my balance on my right foot is still really weak – not enough to make me actually fall over or wobble when walking but enough that I had to focus much more during the slow trials to stay steady during the slow roll through stance on my right foot.  Apparently even though I can run normally now and my right foot feels pretty good overall I still have some lingering weakness/proprioception issues.  Since I don’t usually walk freakishly slowly on a treadmill I never would have noticed – and now I can work to fix that issue!

Lucky for me a recent RunnersWorld had a great series of exercises for big toe strength and single-leg balance.  So now every thesis writing break involves at least 30 seconds of marching around my apartment or cubicle like a very dedicated weirdo runner 😉

Walking like a lizard

Walking like a lizard

I spotted this article via a link on twitter and the first thing I thought of was how insanely frustrating it must have been to get good force plate data off a lizard.  I’ve done force plate collections with little kids and it’s crazy how many trials it takes til you get one stumpy little leg stepping on one force plate cleanly (two steps on one plate = no good as far as gait data).  Add in 3 more legs, a ground-skimming belly, and a dragging tail and that’d be enough to drive any biomechanist bonkers!

Pretty cool science though and an enjoyable look at evolutionary biomechanics 🙂