Shoulder replacements are made from a combination of metal alloys, medical-grade plastic, and sometimes bone cement. The metal parts are typically cobalt-chromium-molybdenum or titanium alloy, while the bearing surface that allows smooth movement is made from ultra-high-molecular-weight polyethylene, a dense, wear-resistant plastic. These materials work together to recreate the ball-and-socket mechanics of a natural shoulder joint.
The Two Metal Alloys
Two metal alloys dominate shoulder implant manufacturing. Cobalt-chromium-molybdenum (roughly 59-65% cobalt, 27-30% chromium, and 5-7% molybdenum) is the harder of the two. Its strength and resistance to wear make it ideal for the ball component that needs to glide against plastic thousands of times a day. The alloy holds up well under repetitive friction, which is why it’s used for the articulating surfaces that bear the most mechanical stress.
Titanium alloy (titanium combined with aluminum and vanadium) is softer and less wear-resistant, but it has a different advantage: bone bonds to it more readily. This makes titanium the preferred choice for stems and base plates, the parts that anchor into your bone and need to stay fixed for decades. Many implant stems have a textured or porous titanium coating that encourages your bone to grow directly into the surface, locking the implant in place without cement.
The Plastic Bearing Surface
The component that allows the joint to move smoothly is made from ultra-high-molecular-weight polyethylene, a specialized plastic used across hip, knee, and shoulder replacements. It’s far denser than everyday plastic, engineered to withstand millions of loading cycles inside the body. The main concern with this material has always been wear. Over years of use, tiny plastic particles shed from the surface and can trigger an inflammatory response that loosens the implant from the bone.
To address this, manufacturers now use highly cross-linked polyethylene, which has additional chemical bonds between its molecular chains that make it significantly more durable. In lab testing, highly cross-linked liners produce less than half the wear debris of conventional polyethylene. One study found wear rates dropped from about 84 cubic millimeters per million cycles with standard polyethylene to roughly 37 cubic millimeters with the cross-linked version.
A newer refinement adds vitamin E during manufacturing. Vitamin E acts as an antioxidant, preventing the plastic from degrading over time due to oxidation, a process that makes conventional polyethylene brittle. In shoulder-specific testing, vitamin E-enhanced polyethylene showed a pristine wear rate of about 0.8 milligrams per million cycles compared to 7.0 milligrams for highly cross-linked polyethylene without vitamin E. That’s roughly a ninefold reduction under ideal conditions. Under harsher abrasive conditions (which better simulate real-world use with scratched metal surfaces), the vitamin E version still cut wear by more than half compared to standard polyethylene.
How the Parts Fit Together
In a standard (anatomic) total shoulder replacement, the implant mirrors your natural anatomy. A metal ball attaches to a stem that fits down into the hollow center of your upper arm bone. On the socket side, a polyethylene cup is fixed to the shoulder blade. The metal ball rotates inside the plastic cup, recreating the original joint mechanics.
A reverse shoulder replacement flips this arrangement. A metal half-sphere called a glenosphere attaches to the socket side (shoulder blade), while a concave polyethylene liner sits on top of the arm bone side. This reversal changes the biomechanics so the deltoid muscle can compensate for a damaged rotator cuff. The materials are the same metals and plastics, just configured differently. A typical reverse replacement includes five distinct pieces: a base plate screwed into the shoulder blade, the metal glenosphere that snaps onto it, a stem inserted into the arm bone, a metal cup on top of the stem, and a polyethylene liner that cups over the glenosphere.
How Implants Attach to Bone
There are two ways a shoulder implant stays anchored: bone cement or biological ingrowth.
Bone cement is a two-part acrylic material called polymethylmethacrylate. It comes as a powder and a liquid that the surgeon mixes during the operation. When combined, the mixture is paste-like for a few minutes, allowing it to be packed around the implant stem inside the bone canal. It then hardens in place, creating a mechanical bond. The powder also contains a radio-opacifier, a substance that makes the cement visible on X-rays so doctors can check the bond over time. Cemented fixation provides immediate stability, which is particularly useful in patients with softer bone.
Cementless (press-fit) implants rely on your bone growing directly into the implant surface over weeks to months. To encourage this, manufacturers create porous coatings on the parts that contact bone. One approach sinters tiny metal spheres (250-355 micrometers each) onto the implant surface, building up a rough, sponge-like layer with 30-40% porosity. Bone cells migrate into these pores and mineralize, essentially welding the implant to your skeleton. Porous tantalum is another material used for this purpose. Its three-dimensional structure closely mimics the architecture of natural spongy bone, and studies show progressive bone mineralization developing inside porous tantalum sections over the first 2 to 12 weeks after implantation.
Surface Coatings and Finishing
Beyond the bulk materials, implants often receive specialized surface treatments. Titanium plasma spray coats the bone-contacting surfaces with a rough, porous layer of pure titanium about 300 micrometers thick. This roughness gives bone cells something to grip as they grow in.
On the articulating surfaces where metal contacts plastic, the goal is the opposite: extreme smoothness. Titanium nitride is a ceramic coating applied in a thin layer (about 5.5 micrometers) that creates a golden-yellow, ultra-smooth surface. This coating reduces friction and wear against the polyethylene, and it’s also used for patients with metal allergies because it acts as a barrier between the underlying alloy and surrounding tissue.
Some cobalt-chromium components receive porous coatings on one side (where they meet bone) and polished finishes on the other (where they meet plastic), combining both strategies in a single piece. The interplay of these surface treatments is as important as the core material in determining how long the implant lasts and how well it integrates with your body.