A reverse shoulder replacement flips the normal ball-and-socket anatomy of the shoulder. Instead of a ball sitting on top of the upper arm bone and a socket on the shoulder blade (as in a healthy joint), the implant places a metal ball on the shoulder blade and a plastic cup on the upper arm bone. On an X-ray, this swapped orientation is immediately recognizable and looks distinctly different from both a natural shoulder and a standard shoulder replacement.
How the Implant Is Arranged
A natural shoulder joint has a rounded ball (the humeral head) at the top of the upper arm bone that fits into a shallow socket on the shoulder blade. A reverse replacement literally reverses this arrangement using four main components stacked together.
On the shoulder blade side, a metal baseplate (sometimes called a metaglene) is screwed directly into the bone of the socket. Screws anchor it firmly, with one typically aimed toward the coracoid (a small bony hook near the top of the shoulder blade) and another directed into the body of the shoulder blade for stability. A polished metal half-sphere called the glenosphere then locks onto this baseplate. The glenosphere is usually made from a cobalt-chromium-molybdenum alloy or titanium, and its smooth, rounded surface is what the new “socket” will glide against.
On the upper arm side, a metal stem is inserted down into the hollow canal of the humerus (the upper arm bone), either press-fit or cemented in place. At the top of this stem sits a shallow cup lined with medical-grade polyethylene, a highly durable plastic. This cup faces upward and cradles the glenosphere, creating the reversed ball-and-socket joint.
What It Looks Like on an X-Ray
On a standard shoulder X-ray, the reversal is unmistakable. You’ll see a bright white hemisphere (the glenosphere) attached to the shoulder blade where the flat socket used to be, and a cup-shaped component sitting on top of a long metal stem in the upper arm bone. The metal parts show up as bright white, while the polyethylene liner inside the cup is radiolucent, meaning X-rays pass through it. This creates a visible gap between the glenosphere and the metal of the humeral cup, even though the joint is actually in full contact through the plastic.
Radiologists look for specific alignment markers on these images. The glenosphere should line up cleanly with the cup, and the humeral stem should sit centered within the arm bone’s canal. The angle where the cup meets the stem is set at about 155 degrees in the most common (medialized) designs, though lateralized designs use a smaller angle of 135 to 145 degrees and position the arm bone slightly farther from the body.
Why the Design Is Reversed
The reversal isn’t arbitrary. It solves a specific mechanical problem. In a healthy shoulder, the rotator cuff muscles hold the ball centered in the socket while the larger deltoid muscle lifts the arm. When the rotator cuff is severely torn or destroyed, the deltoid alone can’t generate enough leverage to raise the arm because the ball slides around without the cuff holding it in place.
Flipping the ball to the shoulder blade side shifts the shoulder’s center of rotation downward and inward. This change increases the deltoid’s mechanical advantage, giving it a longer lever arm to work with, particularly during the first 0 to 30 degrees of lifting the arm away from the body. That’s the range where a damaged rotator cuff fails most noticeably. With the reversed geometry, the deltoid can power arm elevation on its own, bypassing the need for a functioning rotator cuff entirely.
What the Shoulder Looks Like From the Outside
After surgery, the most visible external sign is an incision scar on the front or top of the shoulder, typically running several inches vertically. The scar fades over months but remains permanently visible. In the first weeks, the shoulder is noticeably swollen and bruised, and you’ll be wearing a sling continuously for two to six weeks depending on your surgeon’s protocol. Some patients use a sling with a small abduction pillow that holds the arm slightly away from the body to protect the repair.
Once healed, the shoulder’s outer appearance is close to normal. Some people notice a slightly fuller or rounder contour on the replaced side compared to their other shoulder, which comes from the glenosphere sitting slightly proud of where the natural socket was. This is subtle and usually only noticeable if you’re looking for it. The arm hangs naturally, and in clothing, there’s no visible indication of the implant.
How It Differs From a Standard Replacement
A standard (anatomic) shoulder replacement preserves the natural orientation: a metal ball replaces the humeral head, and a plastic socket replaces the glenoid. On an X-ray, the anatomy looks similar to a natural shoulder, just with metal and plastic where bone used to be. A reverse replacement is visually distinct because the large, round glenosphere on the shoulder blade side is impossible to miss, and the cup-on-a-stem shape on the arm side looks nothing like a natural humeral head.
The functional difference matters just as much as the visual one. Standard replacements work best when the rotator cuff is intact. Reverse replacements are specifically designed for people whose rotator cuff is too damaged to repair, who have complex fractures near the shoulder, or who had a previous replacement that failed. The reversed geometry is what makes the deltoid muscle capable of compensating for the missing cuff, which a standard replacement cannot do.