New paper: double and triple variants of fibularis tertius in humans

I have a new paper out:

Bas, A., Kay, K., Labovitz, J., and Wedel, M.J. 2024. New double and multiple variants of fibularis tertius. Extremitas 11: 111-118.

This is a straight human anatomy paper, with a dual origin. But first let me tell you a little about the fibularis tertius muscle.

Fibularis tertius 

Fibularis tertius (FT), also known as peroneus tertius, is a muscle that originates in the front of the calf, below the knee, and gives rise to a tendon that crosses the ankle and inserts on the lateral (pinky toe side) of the top of the foot, usually on the base of the fifth metatarsal (or sometimes the fourth). FT sits next to a muscle with a similar appearance, extensor digitorum longus (EDL), that sends tendons to the second through fifth toes (the big toe gets its own separate muscle, extensor hallucis longus). The appearance of these tendons varies a lot from person to person, but if you raise your toes you’ll probably see at least some of the EDL tendons popping out on the upper side of your foot, as shown in the photo up top, especially if you wiggle your toes up and down.

FT usually attaches to the top of the fifth metatarsal, about halfway between your ankle and your pinky toe, so it’s not a toe-wiggling muscle; its function is to evert the foot (tilt the lateral border of the foot upward). The two other major foot evertors are fibularis longus and fibularis brevis, both of which come down out of the lateral compartment of the calf, sending their tendons behind the lateral malleolus (the big bony bump on the outside of the ankle). These muscles are opposed by tibialis anterior and tibialis posterior, which come down on the medial border of the foot and serve to invert the foot (tilt the medial border of the foot upward). Imagine sitting with both legs straight out in front of you, and tilting your feet toward each other, so that their soles are touching — that’s foot inversion. Tilting the feet in the opposite direction is foot eversion. Most of us can invert our feet a LOT farther than we can evert them, because the ability to face the feet inward was extremely important for our tree-climbing ancestors. Now that we’re down on the ground, our wildly-mobile ankle and foot joints mostly just make us susceptible to sprains.

Frequency of fibularis tertius in various primates, including humans. Kay et al. (2021: fig. 4).

FT is a highly variable muscle in humans and other primates. The muscle is clearly present more often in great apes than in other primates, and some folks have speculated that it’s precisely because we need more eversion than inversion for walking on two feet. (Personally I think the evolutionary scenario is probably more complicated, but that’s a story for another day.)

Dissection-based studies consistently find FT present in about 90% of people. In contrast, palpation-based studies are all over the map — some find FT in 80-85% of people, some in only about 40% of people. What’s going on here? Dissection is the gold standard; FT is a decent-sized muscle and there’s simply no way to miss it in dissection. Without dissection, relying only on palpation (i.e., searching for the FT tendon by touching the feet of living people), it’s possible to miss the FT tendon, which doesn’t stand out prominently in everyone. You can try to see if you have FT by getting your EDL tendons to stand out, then everting your foot. If you see the FT tendon, great, you’ve got it. If you don’t see the FT tendon, it might mean that you’re in the 10% that lack the muscle, or it might just mean that you have a small FT tendon that doesn’t stand out prominently amidst the other tendons and connective tissues on the top and side of your foot.

(So if palpation-based studies so often overlook FT, why do people still do them? In short, because they’re easy and inexpensive. Large-scale dissection studies are only possible at the biggest and best-funded anatomy labs, which tend to be at medical schools in wealthy countries. In contrast, almost anyone anywhere can do a palpation study, which only requires a few dozen participants who will consent to having their feet touched. And the very best palpation studies find FT present almost as commonly as dissection studies, so there’s probably a lot of variation in the training and expertise of the people doing the palpating.)

Distinguishing fibularis tertius from extensor digitorum longus

FT doesn’t just sit next to EDL in the front of the calf, it is sometimes attached to EDL or even blended with it. Some sources even say that FT is just a tendinous slip of EDL. Krammer et al. (1979) argued that FT is almost always distinct from EDL in having its own muscle belly, although the two muscles typically lie adjacent to each other and can be mistaken for a single muscle without careful dissection. That matches my experience — almost every time I’ve seen an FT that looked continuous with EDL, a little poking and spreading with sharp-tipped iris scissors revealed that there was in fact a narrow fascial plane separating the two muscles. 

But sometimes the two muscles really are blended into one muscle belly, which sends tendons to both the toes and the fifth metatarsal, or there’s an otherwise normal EDL that only sends tendons to digits 2-4, and an otherwise normal FT that sends tendons to the pinky toe and the fifth metatarsal. How should we parse that complexity?

(Quick aside: this is a methodological question. The body does what it does and seems untroubled by muscles and tendons that sometimes merge or split and recombine. Anatomical terminology is to some extent a doomed attempt to impose neat, well-defined categories on a complex and seemingly chaotic continuum of form. Still, words are tools, so we might as well have the best ones we can.)

Origin and insertion sites of fibularis tertius. The densely colored polygons connect the FT attachments at its origins and insertions and the translucent polygon shows the course of the muscle in a diagrammatic fashion. Bas et al. (2024: fig. 1).

Here’s the tack we decided on for the new paper: muscle bellies can originate from multiple surfaces and give rise to multiple tendons, so at their most complex they are almost maximally ambiguous. Tendons usually insert at just one place (although they can also split and recombine…), so they’re usually less ambiguous. We recommend that any tendon that comes out of the anterior calf and inserts on the dorsal surface of the 2nd-5th toes is referred to as an EDL tendon, regardless of what muscle belly it originated from, and any tendon from the anterior calf inserting on metatarsal 4 or 5 is likewise referred to as an FT tendon, regardless of its origin. Then work back up the calf and name the muscle bellies for the tendons that they give rise to: EDL, FT, or a blended EDL+FT. 

This distinction might seem pointless and fiddly, but as you’re about to see, we had to make some kind of call in the face of the new variants.

The new variants

Subject 1 in close-up, showing the insertions of the proximal FT tendon on the base of the fourth metatarsal, and of the distal FT tendon on the base of the fifth metatarsal. Bas et al. (2024: fig. 3).

In the fall of 2022 one of our gross anatomy students, Andrew Bas, found an interesting variant, in which the FT muscle was completely duplicated, with two separate FT muscle bellies, each of which gave rise to a separate FT tendon. The only previous report we could find of a completely doubled FT muscle and tendon was in Le Double (1897), but that report was short on details and didn’t illustrate the double FT. Andrew’s case became our Subject 1.

Subject 2 with the FT and EDL tendons fanned out, showing the relationships of tendons and muscle bellies. Extensor hallucis longus (EHL) was visible in the gap between the proximal and distal muscles bellies where they originated from the fibula. Bas et al. (2024: fig. 5).

Another variant was turned up by one of our anatomy TAs, Kaelen Kay, who found a triple FT tendon. That was also interesting, because although doubling of the FT tendon had been reported in the biomedical literature, we didn’t know of any cases of a triple FT tendon. Later we did find a couple reports of FT muscles with three tendons, but in both cases at least one of the tendons inserted on a toe — by our criteria, those ‘triple’ FTs were really a blend of EDL and FT. This is where we needed to decide how to identify various muscles and their tendons, just to describe the complexity of the FT and EDL in this person. The labels in the above figure reflect our thoughts on how these muscles and tendons should be named: EDL for anything going to toes, FT for anything going to metatarsals. Kaelen’s case became our Subject 2.

At that point it made sense to combine forces. Separately, Andrew’s double FT muscle and Kaelen’s triple FT tendon would each perhaps have been a little thin for an anatomical case report. But the first detailed description of a double FT muscle, plus the first report of a triple FT tendon (that actually inserted where FT tendons go), plus good photos of both, seemed like a solid basis for a paper. We sent the manuscript to my friend and colleague, Jonathan Labovitz, DPM, who made so many useful suggestions that we brought him on as an author.

When it came time to submit our completed manuscript to journals, we struggled a bit to find a home for it. The journals we submitted to first didn’t object to our reasoning or our conclusions, they just didn’t find the paper noteworthy enough. A lot of the usual OA outlets I would have turned to were ruled out because they don’t take case reports. Ultimately we submitted to Extremitas, a student-run peer-reviewed journal of lower limb medicine produced by WesternU’s College of Podiatric Medicine, and we had a great experience there. The new issue of Extremitas just dropped yesterday, with our paper on FT and two other student-led papers on anatomical variants from our cadaver lab. (All of the issues of Extremitas are freely available at the bottom of this page; the new one will be up soon if it’s not already.)

I’m proud of Andrew and Kaelen for spotting these variants in the dissecting lab, cleaning them up, getting good photos, doing the library work, writing the descriptions and discussion, but most of all for tirelessly pushing the project forward until the paper was done and out. I’m fascinated and inspired by the fact that we all have little anatomical quirks like this lurking under the hood, most of which our bodies compensate for so seamlessly that we never suspect them, and I’m happy to have helped get two more of them into the literature.

References

• Kay, K., Estes, A., and Wedel, M. 2021. Peroneus tertius: a review of its evolution, variability, and clinical significance. American Podiatric Medical Association Annual Scientific Meeting, Program.
• Krammer, E.B., Lischka, M.F., & Gruber, H. 1979. Gross anatomy and evolutionary significance of the human peroneus III. Anatomy and Embryology 155(3): 291–302.
• Le Double, A.F. 1897. Traité des Variations du Système Musculaire de l’homme et de Leur Signification an Point de l’anthropologie Zoologique, Vol. 2. Paris: Schleicher Frères.