Shoulder Replacement Implants and Materials: Metals, Polyethylene, Fixation and Wear

A shoulder replacement is built from two materials working as a pair: a hard metal ball, usually a cobalt-chromium or titanium alloy, and a smooth plastic (polyethylene) surface for it to glide against, chosen so the joint moves with a fraction of the friction of bone grinding on bare bone. The metal parts anchor into bone and the plastic forms the low-friction bearing, and the whole system is designed to move quietly and hold for many years¹.

When my surgeon first said the word “reverse”, I pictured something far more exotic than what he then put in my hand: a dull metal ball, a cupped plastic insert, and a couple of anchoring pieces that looked more like engineering than medicine. What I could not find written down anywhere was the plain account of what the thing is actually made of, why a metal ball needs a plastic partner, and which parts wear. This is that account, written from my own reverse shoulder replacement outward. If you want the whole picture first, start with the shoulder replacement overview and come back to the materials here.

What is a shoulder replacement made of?

Every shoulder replacement combines a metal component, usually a cobalt-chromium or titanium alloy, with a polyethylene (a tough medical-grade plastic) bearing surface, so that a smooth metal ball turns against smooth plastic rather than worn bone. Metal gives the strength to anchor into bone and take load; polyethylene gives a low-friction surface that a hard ball can slide across for years².

The principle is the same one used across joint replacement: pairing a hard, polished metal against a softer plastic produces far less friction and wear than metal on metal or bone on bone. In the shoulder that means the arthritic surfaces, which had ground themselves smooth in my own case, are removed and rebuilt as a metal-and-plastic joint that glides. The parts are made to close tolerances and finished to a mirror surface, because the smoother the metal ball, the slower the plastic wears.

The socket side: polyethylene, and the reverse flip

In an anatomic total replacement the ball is metal and the socket is resurfaced with polyethylene, so metal turns against plastic; in a reverse replacement the same two materials swap sides, with a metal ball fixed to the socket and the polyethylene cup sitting on the arm bone. The materials do not change between the two operations, only which side of the joint each one is fixed to³.

That flip is the whole point of the reverse design. Switching the metal ball onto the socket side lets the powerful deltoid muscle lift the arm when the rotator cuff can no longer do it, which is exactly why I had a reverse rather than a standard total. The reasoning behind choosing one layout over the other, rather than the materials themselves, is set out in anatomic versus reverse.

Stemmed and stemless humeral components

The part that replaces the ball can be a stemmed implant, whose metal shaft is seated down the inside of the arm bone, or a shorter stemless design fixed to the cut bone surface at the top; stemless implants are increasingly used in suitable bone because they can spare bone stock. The choice turns on the quality of the bone the surgeon finds, not on age alone⁴.

A stem gives reliable fixation, especially in softer or fractured bone, which is why it remains the default in many cases. A stemless design keeps more of the patient’s own bone, which can matter years later if the joint ever needs revising, since there is more to work with. The trade-offs and who each suits are covered in stemless shoulder replacement.

How the implant is fixed: cement or press-fit

Implants are held in one of two ways: bone cement, which grips the component immediately, or a press-fit surface, which is textured or coated so that bone grows into it over the following weeks and holds it biologically. Both are in routine use, and neither is simply better; the humeral stem is often press-fit and the plastic socket often cemented, but the combination is a surgical judgement⁵.

Cement suits softer, older, or fractured bone and gives a joint you can move confidently from the start. A press-fit implant relies on living bone knitting into the surface, so it depends on decent bone quality and on the early movement limits your surgeon sets to protect it while it takes hold. Mine came with clear instructions about what not to push in the first weeks, and I understood them better once I knew the fixation was still bedding in.

Wear, loosening and why the material matters over time

The materials wear slowly rather than fail suddenly: the polyethylene surface thins over years, and in anatomic replacements the socket component can loosen, both of which are among the leading reasons a shoulder eventually returns to surgery. Even so, pooled registry and study data put overall survival at around 90% still in place at 10 years, with anatomic total replacements commonly quoted at roughly 90% to 95%⁵.

This is where the material choice quietly earns its keep. A polished metal ball on a well-fixed plastic socket wears predictably and slowly, but nothing lasts forever: after the first decade the risk of needing a revision is roughly 1% a year, so a joint put in at a younger age is more likely to need redoing in a lifetime. The honest long-term picture is in how long does a shoulder replacement last, and wear and loosening sit alongside the other named risks in shoulder replacement risks and complications.

Does the material change the operation or the recovery?

The implant material has very little effect on how the recovery feels or how long it takes: the sling weeks, the staged physiotherapy, and the 6 to 12 months of gradual gains are driven by soft tissues healing and muscles relearning the arm, not by which alloy the ball is made from. What the materials and fixation do change is the fine detail of early restrictions, especially with a press-fit implant that needs bone to grow into it¹.

I remember being faintly disappointed that the impressive-looking piece of engineering my surgeon had shown me made no difference at all to the tedious, one-handed reality of the first month in a sling. The metal and plastic do the joint’s job; the slow work of getting an arm back is done by rehabilitation and time, whichever design is fitted.