Is it part of a scapula or a pelvis?

As I've learned first-hand during my time in human osteology this semester, identifying bone fragments can be a very tricky process. It is easy to identify the differences between a radius and a fibula or a scapula and a pelvis when you have the whole bones in front of you, but if you only have a handful of broken pieces the task becomes exponentially more difficult. In reviewing the dirty & dusty collection available to me in preparation for my final exam next week, I've been focusing on some of the errors I consistently made and thought "What better way to remind myself than to write about what I've learned?" Today I'll cover one problem that has fooled me over and over again; telling the difference between scapula and pelvis fragments.

Thinking about the whole bones, telling the difference between a scapula (or shoulder blade) and os coxae (pelvis) seems easy. The scapula is relatively thin, flat, and triangular while the pelvis is thicker, curved, and has a huge hole for the attachment of the femur (called the acetabulum). All of these traits are good to keep in mind, but they are not enough by themselves to help you if you come across a few tricky fragments. More often than not, I've confused part of the scapula as part of the pelvis rather than the other way around, and only attention to a few specific details can help sort out which bone is which.

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The superior & lateral aspect of the scapula (looking at the scapula as if you were looking straight at someone with x-ray specs and could see their shoulder-blade). The three pertinent parts of the bone to this discussion are marked. Image from Wikipedia.


Prior to taking human osteology, I had thought of the scapula as being flat. It looked flat in all the illustrations I had seen it in, and I could definitely see why it was called the "shoulder blade." Looking at an actual bone, however, it was clear that the scapula was far more complex than I had previously thought. Running across the spine near the superior (top) end is the scapular spine, a raised surface that terminates in a bit of bone called the acromion on the lateral side (the side furthest from the midline of the body). If you look at the scapula in lateral view (use this website, eskeletons, select the scapula and then lateral view), there will be another hook-like process coming off of the scapula; this is called the coracoid process. Between the two will be a vertically-oriented flattened surface in the shape of a tear drop. This is the glenoid fossa, a key marker of the scapula and the place where the humerus (your upper arm bone) articulates.

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The inferior & posterior portion of the pelvis, containing the ischium, part of the acetabulum, and obturator foramen. You're looking at the pelvis as if someone had turned perpendicular to you facing right and you could see the part of the pelvis just behind and below their femur. Image from Wikipedia.


Most of the fragments that I have been tested on have been from the superior and lateral part of the scapula, or (in other words) from the top of the scapula closest to the midline of the body that preserves the coracoid process, glenoid fossa, and (sometimes) the acromion. The problem is that this area can look a lot like a portion of the pelvis, the smoothed curvature of the coracoid process coming off the scapula in proximity to the glenoid fossa (an articular surface) making it seem like I'm looking at the acetabulum and the big hole between the pubis and ischium, called the obturator foramen. (As a side note, I've always found it amusing how much a human hip resembles that of a saurishchian dinosaur when viewed laterally. I'm sure that I could come up with a good April Fool's post for next year noting the similarities and proposing that humans evolved from something like Nanotyrannus).

The curvature of the coracoid definitely threw me off during earlier tests, but when I looked at the other aspects of the fragments of the os coxae and scapula together, there were more differences than similarities. You'll recall that the humerus articulates with the scapula at the glenoid fossa, a relatively small, shallow depression. On the pelvis, however, the femur articulates with the acetabulum, a deep cavity that has a smoothed area called the "lunate surface" around about 3/4 of the joint. More telling, however, is that the area around the coracoid is relatively thin and smooth with the pelvis is much thicker and rougher, with a large tuberosity on the ischium. The angle of the ischium (the more posterior hip bone that helps form the obturator foramen) isn't as sharp as that of the coracoid, either, and is another tell-tale sign that you've got part of a pelvis and not a scapula.

Unfortunately I still don't have a camera so I can't take pictures of the fragments to show you precisely what I mean (I will update this post with photos as soon as I have the ability to add them), but the similarities between the superior & lateral part of the scapula and the posterior & inferior (to the back and behind) aspect of the pelvis can easily confuse. I've modified a few images from Gray's Anatomy to try to give you a feel for what I'm talking about, but if you'd like to do some more exploring I definitely recommend checking out eskeletons.

[Many thanks to my friend Melanie for helping to set me straight on this problem.]

Next time: Is it a radius or a fibula?

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Besides processes, it can also help to look at the orientation of the bone mineral itself. This often gives clues that a flat piece of bone may not be so flat if the bone fibers are radiating out.

C'mon, can't a guy have a little fun? I couldn't very well say that we evolved from Tyrannosaurus now, could I?