Little Ossa Coxae

My husband soon-to-be and I just moved into our recently renovated new flat and we have found ourselves overcome by the interiors frenzy commonly experienced by couples our age with a predisposition for upcycled G-plan furniture. I just about lost my shoe when we stumbled upon this butterfly specimen display at a vintage shop in our area. Real butterflies! Carefully pinned in a delicate display box with typewritten labels! Swoon. 

Now that this piece is hanging prominently in our living room, I can’t look at it without comparing butterfly architecture to pelvic girdle variation in humans. Because, the correlation is totally obvious to everybody, right? OK, so maybe a gentle explanation is required for our guests to see these butterflies as little ossa coxae.

Os coxae is the Latin for hip bone, plural ossa coxae. This term is from the Latin os meaning bone and coxa, a Latin term that is likely to have come from the Sanskrit word kaksha referring to the hip.

The flat human hip bones are each formed by three separate bones that are connected in utero and childhood by cartilage: the ilium, ischium, and pubis. These bones later ossify, or fuse together, at their junction deep in the acetabulum, aka the hip socket.

How fascinating, the etymology of the hips! They have another name: the innominate bones. Although they’re composed of the three bones mentioned previously that Galen had named in the second century AD, the entire flat hip bone was not named. Thus the hip bone became termed as ‘innominate’ by Vesalius, who himself later referred to the hip bones as the os coxae. 

The entire lexicon of anatomical terms is robust with this kind of intrigue. If it turns you on then get stuck into this: Medical Meanings: A Glossary of Word Origins by William S. Haubrich.

The pelvic girdle is formed in the fusion of these three bones of the ossa coxae, as they articulate with the sacrum and unite the inferior limbs at the hip (femurs) to the axial skeleton (sacrum). Generally speaking, the bone mass is composed of two plates of compact bone with an internal core of spongy bone. This composition is variable with the thickness of the flat hip bones. In the gluteal fossae, for example, where the gluteal muscles reside within the postero-lateral aspect of the pelvic girdle, the inner spongy bone is absent and there exists only a thin plate of compact bone.

Consider now the largest and most superior aspect of the hip bones, the wings of the ossa coxae, known to anatomists as the ilia. Each ilium is divisible into its body and its ala, or wing. The wing of each ilium forms an S-shaped crest that is easily palpable through the skin, and if you place your hands on your hips now, you can liken these bony prominences to the outer edges of butterfly wings. The distinction between the body and the ala is indicated on the top surface by a curvature called the arcuate line, and on the external surface by the margin of the acetabulum. These crests form attachment sites for the muscular walls of the trunk.

As we move inward (medially) from these easily palpable edges of the ilia toward their attachment to the sacrum, here is where I start to see the butterfly connection. The ilia articulate with the sacrum at the eponymous “sacro-iliac” joint, as wings join with the body of the butterfly.

The SI joint mobility is known as nutation and counternutation, and this range is designed to prioritise stability and attenuate forces into the lower limbs. Think of the butterfly fluttering its wings: it needs stability to translate its forces into propulsion. Another apt analogy comes to mind here – one cannot fire a cannon from a canoe! The two primary movements of the SIJ occur in the sagittal plane when the sacrum moves relative to the iliac bones.

Nutation occurs as the sacrum moves moves anteriorly and inferiorly, or forward and down, while the coccyx moves posteriorly relative to the ilium. Practically speaking, slight nutation is the natural tendency of the pelvis during walking and free standing. The subtle forward tilt of the sacrum supports the natural lordic curve of the lower back and enhances normal movement as it facilitates the hips to move freely as needed for walking. The wedge-shaped sacrum opposes this motion; opposition is aided by the ridges and depression of the articular surfaces, the friction coefficient of the joint surface, and the integrity of the strong ligaments and assorted myofascial structures acting on the joint.

Counternutation occurs relative to the ilium as the sacrum moves posteriorly and superiorly, or back and up, while the coccyx moves anteriorly. This action is opposed by the posterior sacroiliac ligament that is supported by the multifidus muscle. Here is a beautiful article by Doug Keller talking about the keystone role of the sacrum in backbending and the distribution of forces for safe bending biomechanics in Yoga.

The range of motion associated with human sacroiliac nutation and counternutation is commonly approximated not to exceed 4 degrees. All of this might at first seem quite complex, but it is nothing compared to the sophistication of butterfly aerodynamics! Here’s a glimpse from the exquisite book, Bio-mechanisms of Swimming and Flying: Fluid Dynamics, Biomimetic Robotsedited by Naomi Kato, Shinji Kamimura:

I just adore those equations exploring things like flapping and feathering angles of butterflies! It has become a well-known meme in the physics of flight that how butterflies fly is theoretically impossible. Biotensegrity embraces the chimerical design of nature that lets minute adaptations defy laws codified by humans.

Getting back to humans now, there are 35 muscles that attach to the sacrum or ossa coxae which mainly provide stability to the joint rather than producing movements. Interestingly, the ligaments mooring the sacrum to oppose nutation and counternutation are the strongest in the human body. These are:

Anterior sacroiliac

Interosseus sacroiliac

Posterior (Dorsal) sacroiliac



This joining station must allow symmetrical movement as well as some independent movement, and due to its role as the singular interchange between the lower appendicular and axial skeletal systems, must supply stability adequate for holding the movement together as an integrated organism negotiating its particular routes of forward progress.

The type of forward progress required, including but not limited to reproductive, ambulatory, or flying progress, will inform the evolution of the ilia and all its neighbouring structures. For an excellent discussion of sacroiliac joint force and form closure factors, you can get technical here. My focus today is on how appreciating the aesthetics of similar shapes helps us better understand the reflection of this gorgeous variation in natural form.

If you had to pick a human joint with which to compare the wing-to-body junction of the butterfly or bird, the more obvious correlation would be the shoulder, as we liken our human arms to wings more readily than likening our legs to wings. But I’m working on the similarity of shapes here, each time I walk past the butterfly display, and inspiration strikes in these moments of comparison. So today we consider the pelvis as a butterfly, and here is an interesting array of pelvic variations arising in nature (thanks to the physiopedia): 

android pelvis

typically found in the male, with a wedge-shaped inlet and narrow anterior aspect

anthropoid pelvis

the anteroposterior diameter equals or exceeds the transverse diameter.

assimilation pelvis

one in which the ilia articulate with the vertebral column higher (high assimilation pelvis) or lower (low assimilation pelvis) than normal, the number of lumbar vertebrae being correspondingly decreased or increased.

beaked pelvis

– one with the pelvic bones laterally compressed and their anterior junction pushed forward.

brachypellic pelvis

– a short oval type of pelvis, in which the transverse diameter exceeds the anteroposterior diameter by 1 to 3 cm.

contracted pelvis

– one showing a decrease of 1.5 to 2 cm in an important diameter; when all dimensions are proportionately diminished, it is a generally contracted pelvis.

cordate pelvis

– heart-shaped!

dolichopellic pelvis

– long, oval pelvis with the anteroposterior diameter greater than the transverse diameter.

flat pelvis

– one in which the anteroposterior dimension is abnormally reduced.

frozen pelvis

– a condition of the pelvis in which the adnexa and uterus are fixed, due to infection or carcinoma.

funnel pelvis

– one with a normal inlet but a greatly narrowed outlet.

gynecoid pelvis

– the normal female pelvis: rounded, oval with well-rounded anterior and posterior segments.

infantile pelvis

– a generally contracted pelvis with an oval shape, a high sacrum, and inclination of the walls; called also juvenile pelvis.

pelvis justo major

– an unusually large gynecoid pelvis with all dimensions increased.

pelvis justo minor

– a small gynecoid pelvis with all dimensions symmetrically reduced.

juvenile pelvis

– infantile

kyphotic pelvis

– a deformed pelvis marked by increase of the conjugate diameter at the brim with decrease of the transverse diameter at the outlet.

lesser pelvis

– pelvis minor.

Nägele’s pelvis

– one contracted in an oblique diameter, with complete ankylosis of the sacroiliac synchondrosis on one side and imperfect development of the sacrum and coxa on the same side.

Otto pelvis

– one in which the acetabulum is depressed, accompanied by protrusion of the femoral head into the pelvis.

platypellic pelvis (platypelloid pelvis)

– one shortened in the anteroposterior aspect, with a flattened, transverse oval shape.

rachitic pelvis

– one distorted as a result of rickets.

renal pelvis

– the funnel-shaped expansion of the upper end of the ureter into which the renal calices open; it is usually within the renal sinus, but under certain conditions a large part of it may be outside the kidney (extrarenal pelvis).

scoliotic pelvis

– one deformed as a result of scoliosis.

split pelvis

– one with a congenital separation at the pubic symphysis

spondylolisthetic pelvis

– one in which the last, or rarely the fourth or third, lumbar vertebra is dislocated in front of the sacrum, more or less occluding the pelvic brim.

true pelvis

– pelvis minor.

The variation in osteology is an awing testament to the functional range of movement possible amongst species, and the interplay of forces that cause adaptation to structural anatomy. Comparative anatomy gives us us a fascinating perspective from which to consider our own bodies, especially when the imagination takes you further than you’d have gone simply studying in the normal way. A big takeaway for anyone practicing Yoga is that we must all appreciate the natural limitations that nature bestows upon our asana by way of our bone structure.

Not everyone can give birth to a 1o pound baby thanks to the specifics of their pelvic girdle; nor will some Yogis be able to get their eka pada sirsasana – the leg might never go behind the head due to their trochanteric length and angle. Our backbending might indeed be limited by something described decades ago as the pelvifemoral angle. These relationships are best explored with a balance of Abhyasa and vairagya.

The next time you feel butterflies in your stomach, maybe it is just your pelvis having a few friends over.

Here are some of my references:
Classification of the Normal Variation in the Sagittal Alignment of the Human Lumbar Spine and Pelvis in the Standing Position: Roussouly, Pierre MD*; Gollogly, Sohrab MD*; Berthonnaud, Eric PhD†; Dimnet, Johanes PhD†

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