Shoulder MRI
- The shoulder is a golf ball balanced on a tee — the rotator cuff and labrum are what stop it from rolling off.
- The rotator cuff is four muscles whose tendons fuse over the humeral head; supraspinatus is the one that tears most and the one you check first.
- Tears come in two flavors: partial-thickness (frayed but still spanning) and full-thickness (a hole the fluid leaks through).
- The labrum is the rubbery rim that deepens the socket; tearing it (often after a dislocation) is what causes that "my shoulder feels loose" complaint.
- Fluid-sensitive sequences are your friend: torn tissue lights up bright because it fills with fluid, like a crack filling with water.
The shoulder is the body's most mobile joint, and it pays for that freedom by being spectacularly unstable. Anatomically, it's a golf ball (the humeral head) sitting on a tee (the shallow glenoid). What keeps the ball from rolling off isn't the bony socket — it's the soft tissue scaffolding around it. Shoulder MRI exists to interrogate that scaffolding, because most of it is invisible on a radiograph.
If you're shaky on what makes tissue bright or dark, it's worth a two-minute detour through MRI basics: T1, T2 and weighting before reading on. The one-liner: on fluid-sensitive sequences, water glows white, and damaged tissue tends to fill with water.
The rotator cuff: four muscles, one job
The rotator cuff is four muscles whose tendons blend into a hood over the top of the humeral head. The names are a mouthful, so here's the cheat sheet:
| Muscle | Where its tendon sits | Main job |
|---|---|---|
| Supraspinatus | On top | Starts lifting the arm out to the side; the usual tear victim |
| Infraspinatus | Behind | Rotates the arm outward |
| Teres minor | Behind, below infraspinatus | Also rotates outward |
| Subscapularis | In front | Rotates the arm inward |
Think of them as four guy-wires anchoring a tent pole from different directions. As long as all four pull evenly, the pole (your humeral head) stays centered. Cut or fray one wire and the pole drifts.
Supraspinatus gets the most attention because it runs through a tight space under the bony arch of the acromion, where it gets pinched and worn down over the years. By the time someone's in the scanner for shoulder pain, this is the tendon I'm scrutinizing first.
Partial versus full-thickness tears
A normal tendon is a uniform dark band — dense, organized fibers with no water to brighten them. A tear disrupts that and lets fluid seep in, which lights up bright on fluid-sensitive images. The key question is how far the bright signal goes.
- Partial-thickness tear: the bright signal involves part of the tendon's depth but doesn't go all the way through. The tendon is frayed but still spans the gap — like a rope that's lost some strands but hasn't snapped.
- Full-thickness tear: the fluid signal crosses the entire thickness of the tendon, top to bottom. Now there's a true hole, and in a big tear the torn ends pull apart and retract like a snapped rubber band recoiling toward the muscle.
A full-thickness tear is not the same as a complete tear. "Full-thickness" means fluid crosses the whole depth at one spot — but the tendon may still be partly attached front-to-back. A complete tear means the whole tendon is detached. Precision here changes whether the surgeon operates.
When a tear has been there a long time, the orphaned muscle stops working and slowly turns to fat — fatty atrophy. That matters because a muscle that's already marbled with fat may not recover even after a perfect repair. So I always glance at the muscle bellies, not just the tendon.
The labrum: the rim that deepens the cup
The labrum is a rubbery ring of cartilage circling the glenoid, deepening the shallow socket and giving the ligaments a place to anchor. Picture the rubber gasket around a jar lid — it adds grip and seal without adding much bone.
The classic labral injury follows a dislocation. When the humeral head pops out the front, it commonly shears off the labrum at the front-lower part of the rim. The patient is then left with a shoulder that feels loose and keeps wanting to dislocate again.
The labrum has normal variants that mimic tears — small gaps and recesses where the labrum naturally isn't attached. Calling one of these a tear is a classic trap. This is exactly why labral questions often get an MR arthrogram, where contrast injected into the joint outlines the rim and helps separate a real tear from normal anatomy.
Why some shoulders get contrast injected
A plain MRI is great for the rotator cuff. But the labrum is small and hugs the bone, so fluid doesn't always reach the spots you care about. In an MR arthrogram, dilute contrast is injected into the joint first, distending the capsule and pushing bright fluid into any labral defect — like pumping dye into a crack to make it obvious. If the clinical question is instability or a labral tear, this is often the study of choice.
Reading a shoulder MRI is really two searches in one: trace each rotator cuff tendon looking for bright fluid crossing it, then run the rim of the labrum looking for a defect. Cuff for pain and weakness; labrum for instability.
For another joint where MRI turns invisible soft tissue into the whole diagnosis, see Knee MRI: internal derangement — the logic of "torn tissue fills with fluid and lights up" carries straight across.