A full-thickness tear is a complete disruption of a tendon from one side to the other, creating a gap that goes all the way through the tissue. In the rotator cuff, where these tears are most common, this means the tear extends from the joint side of the tendon to the outer (bursal) side. What this looks like depends on whether you’re looking at an MRI, an ultrasound, or the outside of your body, but in each case there are distinctive signs that set a full-thickness tear apart from a partial one.
What It Looks Like on MRI
On an MRI, a full-thickness rotator cuff tear shows up as a bright streak of fluid cutting completely through the dark band of the tendon. Normal tendon appears as a low-signal (dark) strip on most MRI sequences, so when fluid fills the gap, it creates a high-signal (bright white) line that runs from the bottom surface of the tendon to the top. This bright fluid signal passing all the way through is the hallmark that separates a full-thickness tear from a partial tear, where the bright signal only penetrates partway.
Radiologists also look at how far the torn tendon edge has pulled back from its original attachment point on the bone. This retraction is graded on a scale originally described by Patte: grade I means the tendon has barely pulled away, grade II means it has retracted back over the top of the humeral head, and grade III means it has pulled all the way to the socket rim or beyond. Greater retraction generally means a worse prognosis and a more complex repair.
Some full-thickness tears also show a horizontal split within the tendon itself, separating it into upper and lower layers like pages of a book peeling apart. These delaminated tears can be tricky because the layers may retract different amounts, creating a more complex picture on imaging. Fluid filling the space between the layers, along with thickened tissue, can reduce the tendon’s ability to heal.
MRI catches full-thickness tears with about 91% sensitivity and 97% specificity, making it the most reliable non-surgical tool for confirming them. Partial-thickness tears are harder to spot, with sensitivity dropping to around 80%.
What It Looks Like on Ultrasound
On ultrasound, a normal tendon appears as a bright, fibrous band with a characteristic texture. A full-thickness tear replaces that bright tissue with a dark (hypoechoic) or completely black (anechoic) gap where the tendon fibers have separated. The sonographer can measure both the length and width of the defect by scanning along and across the tendon.
Two indirect signs help confirm the diagnosis. The first is the “cartilage interface sign,” where the bone surface beneath the torn tendon appears unusually bright. This happens because the missing tendon tissue allows the ultrasound beam to reach the underlying cartilage with less interference, creating an abnormally sharp, bright reflection. The second is a visible sagging of the tissue layer (bursa) that normally sits on top of the tendon, which droops into the gap left by the tear.
What It Looks Like From the Outside
Full-thickness tendon tears don’t always cause visible changes on the surface of the body, but when they do, the signs are distinctive. In the quadriceps tendon above the kneecap, a complete tear creates a visible depression or dent just above the knee, sometimes called the “gap sign.” The person typically can’t straighten the knee against gravity or lift the leg while keeping it straight.
In the biceps tendon, a full-thickness rupture produces the well-known “Popeye deformity,” where the muscle bunches up in the middle of the upper arm because it’s no longer anchored at one end. For rotator cuff tears, the external signs are subtler. You won’t usually see a visible gap, but over time, chronic tears can cause noticeable muscle wasting on the back of the shoulder blade as the disconnected muscles shrink.
How Chronic Tears Change the Muscle
When a full-thickness tear goes unrepaired for months or years, the muscle attached to the torn tendon gradually fills with fat. This process is graded on a five-point scale. At grade 0, the muscle looks completely normal. At grade 1, a few streaks of fat appear. By grade 2, there’s significant fat but still more muscle than fat. Grade 3 means equal amounts of fat and muscle, and grade 4 means fat has overtaken muscle entirely.
This fatty replacement is visible on both MRI and CT scans, and it matters because it’s largely irreversible. Even if the tendon is surgically reattached, a muscle that has reached grade 3 or 4 fatty infiltration may never regain full strength. This is one reason early detection of full-thickness tears matters: the longer the tendon stays retracted, the more the muscle degrades.
Tear Size and What You Actually Feel
Not every full-thickness tear causes pain. Studies comparing symptomatic and asymptomatic people with confirmed full-thickness rotator cuff tears found that many people walk around with complete tears and have no idea. The factor most strongly linked to symptoms was tear size in the medial-to-lateral direction (how far the tear extends from the bone toward the middle of the tendon). Tears wider than 3 cm in that dimension carried four times the odds of causing symptoms compared to smaller tears.
Fatty degeneration beyond grade 1 in the muscles around the tear also correlated with symptoms. So on imaging, a small full-thickness tear with healthy-looking muscle might belong to someone who feels fine, while a large tear with significant muscle fatty change is far more likely to be causing weakness and pain. The visual size of the tear on MRI or ultrasound is not just an academic detail; it directly informs whether repair is recommended and how likely that repair is to hold.
Full-Thickness vs. Partial: The Key Difference
The critical visual distinction is straightforward. A partial-thickness tear shows damage on one surface of the tendon (either the joint side or the bursal side) without extending all the way through. On MRI, you’ll see a bright signal that penetrates partway into the dark tendon band but doesn’t cross to the other side. On ultrasound, the dark defect occupies only part of the tendon’s thickness.
A full-thickness tear, by contrast, creates a complete communication between the joint space below and the bursal space above. Fluid can flow freely through the gap, which is why it lights up so clearly on MRI. This complete disruption is what defines it, regardless of how wide or narrow the tear is. A full-thickness tear can be as small as a pinhole or span the entire width of the tendon. What makes it “full thickness” is depth, not width.