To side a clavicle, you need to identify three things: which end is medial (toward the chest) and which is lateral (toward the shoulder), which surface faces up versus down, and which curve faces forward. Once you orient all three correctly, the side the bone belongs to becomes clear. The whole process takes about 30 seconds once you know what to look for.
Step 1: Identify the Two Ends
The clavicle has two ends that look noticeably different from each other. The medial (sternal) end is the one closest to the center of your chest. It’s bulky, rounded, and has a large oval articular surface where it connects to the top of the breastbone at the sternoclavicular joint. The lateral (acromial) end is the one that reaches toward the shoulder. It’s flatter, thinner, and broader, with a smaller articular facet where it meets the acromion of the scapula at the acromioclavicular joint.
If you’re holding a clavicle and one end looks like a thick, rounded knob while the other looks like a flattened paddle, the knob goes toward the sternum and the flat end goes toward the shoulder.
Step 2: Find the Rough Inferior Surface
The top (superior) surface of the clavicle is relatively smooth. The bottom (inferior) surface is rougher, with distinct grooves and raised areas where muscles and ligaments attach. This contrast is your key to orienting the bone vertically.
On the underside near the lateral end, you can feel two important landmarks. The conoid tubercle is a small, raised bump located about 4.5 cm from the far lateral tip of the bone, sitting slightly toward the back edge. The trapezoid line is a low, rough ridge running along the inferior surface about 3 cm from the lateral tip, just in front of the conoid tubercle. Both of these are attachment points for the strong ligaments connecting the clavicle to the coracoid process of the scapula below. On the underside near the medial end, there’s a roughened area (sometimes called the costal tuberosity) and a shallow groove for a muscle that runs beneath the bone.
If you run your thumb along one surface and it feels smooth, that surface faces up. If it feels bumpy and textured, that surface faces down.
Step 3: Orient the S-Shaped Curve
Viewed from above, the clavicle has a distinctive S-shape (sometimes called sigmoid). The medial two-thirds of the bone curves so that its convexity faces forward. The lateral one-third curves the opposite way, with its convexity facing backward (its concavity faces forward). Think of it this way: the inner curve near the chest bows toward the front of the body, while the outer curve near the shoulder bows toward the back.
This curvature is the most reliable single feature for siding. Once you’ve identified the sternal end (bulky, round) and placed it toward the midline, the forward-facing convex curve of the medial shaft tells you which side of the body the bone belongs to.
Putting It All Together
Hold the clavicle horizontally in front of you with the bulky, rounded end pointing toward the midline of your body and the flat end pointing outward. Rotate it so the smooth surface faces up and the rough, ridged surface faces down. Now check the S-curve: the medial portion should bow forward (toward you) and the lateral portion should bow slightly backward (away from you).
If the bulky end sits to your left when properly oriented this way, it’s a left clavicle. If it sits to your right, it’s a right clavicle. You can double-check by confirming that the conoid tubercle on the underside of the lateral end points toward the back, since the coracoid process it connects to sits behind and below the acromial end.
Quick Reference Checklist
- Sternal end: bulky, round, with a large articular surface. Points toward the midline.
- Acromial end: flat and broad, with a small facet. Points toward the shoulder.
- Superior surface: smooth. Faces up.
- Inferior surface: rough, with the conoid tubercle and trapezoid line near the lateral end. Faces down.
- Medial curve: convex anteriorly (bows forward).
- Lateral curve: concave anteriorly (bows backward).
Common Mistakes When Siding
The most frequent error is confusing the two ends, especially in bones that are damaged or come from younger individuals where the ends haven’t fully developed. The clavicle is one of the first bones in the body to begin hardening, with primary ossification starting as early as the fifth week of embryonic development. But the secondary growth center at the sternal end doesn’t finish fusing until the mid-20s, so in younger specimens that medial end may look less distinctly bulky than expected.
Another common pitfall is relying on the curve alone without checking the inferior surface. Some clavicles have a subtle S-curve that’s hard to read by eye, particularly in smaller or more gracile individuals. In those cases, feeling for the conoid tubercle on the underside of the lateral third is more reliable. It consistently sits on the posterior aspect of the inferior surface, giving you both a vertical and a front-to-back reference point in one landmark.
If you’re still uncertain, place the bone against your own body. Rest the rounded end against your sternum and the flat end against your shoulder, with the smooth side up. If it sits comfortably and the curves follow the natural contour of your chest and shoulder, you’ve matched the correct side.