A hip replacement is made of four main parts: a metal stem, a metal cup, a ball, and a plastic or ceramic liner. Each component uses different materials chosen for strength, durability, and compatibility with your body. The specific combination depends on your age, activity level, and surgeon’s preference, but most modern implants rely on titanium alloys, cobalt-chromium alloys, advanced plastics, and ceramics.
The Four Parts of a Hip Replacement
A total hip replacement recreates the ball-and-socket joint where your thighbone meets your pelvis. It has four distinct pieces that work together:
- The stem is a long, tapered piece that fits down into the hollow center of your thighbone. It anchors the entire implant.
- The cup (acetabular shell) is a half-sphere that gets pressed into your pelvis, replacing the socket side of the joint.
- The ball (femoral head) sits on top of the stem and acts as the new head of your thighbone.
- The liner snaps into the cup and serves as your new cartilage, providing a smooth surface for the ball to glide against.
Metals Used in the Stem and Cup
The stem and cup bear your full body weight with every step, so they need to be extremely strong without irritating surrounding tissue. Titanium alloy is the dominant choice for both. It’s lightweight, resists corrosion, and bonds well with living bone over time. Some cups use tantalum, another metal with excellent bone-bonding properties.
The ball is typically made from a cobalt-chromium alloy. This metal is harder than titanium and can be polished to a very smooth finish, which reduces friction where the ball meets the liner. So the standard setup is a cobalt-chromium ball sitting on top of a titanium stem, pressed into a titanium cup.
What the Liner Is Made Of
The liner is arguably the most important material choice because it’s the surface that experiences the most friction and wear over decades. Most liners today are made from highly cross-linked polyethylene, a medical-grade plastic that has been treated with radiation to create stronger bonds between its molecules. This process dramatically reduces the rate at which tiny plastic particles flake off during movement compared to the conventional polyethylene used in older implants.
The trade-off with cross-linking is that it can make the plastic slightly more brittle. To counter this, newer “second generation” liners are infused with vitamin E, which acts as an antioxidant. These vitamin E liners resist breakdown from oxidation while maintaining strong wear resistance and mechanical strength. Early results over the first decade have been encouraging.
Ceramic as an Alternative
Instead of a metal ball and plastic liner, some implants use ceramic components. Ceramic balls and liners have been used since the 1970s, and modern versions are far more advanced than those early designs. The most common ceramic today is zirconia-toughened alumina, which blends two types of ceramic to get the best of both: alumina’s extreme hardness and zirconia’s resistance to cracking.
Ceramic surfaces produce very little wear debris and are essentially scratch-resistant, making them appealing for younger, more active patients who will put more demand on the joint over a longer lifetime. A ceramic ball can also be paired with a polyethylene liner rather than a ceramic liner, giving surgeons flexibility. The main drawback is a small risk of fracture, though modern ceramics have made this rare, and some patients notice a squeaking sound from ceramic-on-ceramic joints.
Why Metal-on-Metal Fell Out of Favor
Older hip replacements sometimes used a metal ball against a metal cup with no separate liner. These metal-on-metal designs seemed promising because metal is extremely durable, but they created a serious problem. Every time the joint moved, microscopic metal particles shed into the surrounding tissue. Cobalt and chromium ions entered the bloodstream and, in some patients, triggered painful inflammatory reactions that damaged bone and soft tissue around the implant.
The FDA has flagged these complications, and international joint registries have documented higher-than-expected failure rates for metal-on-metal designs. Many patients needed revision surgery to replace the failed implant. As a result, metal-on-metal bearings have been largely abandoned. Today’s implants almost always use ceramic or polyethylene as at least one of the two surfaces that slide against each other.
How the Implant Attaches to Bone
The materials don’t stop at the implant itself. How the implant is fixed inside your bone matters just as much, and there are two approaches.
Cemented fixation uses a fast-setting acrylic material that fills the gap between the implant and the bone, locking everything in place immediately. This works well in older patients whose bone may be thinner or softer.
Cementless fixation relies on the implant’s surface texture to encourage your bone to grow directly into it. The outside of the stem or cup is roughened or coated with a porous material. Some implants add a coating of hydroxyapatite, a mineral similar to what’s naturally found in bone, to speed up this bonding process. Studies show that bone successfully grows into porous cementless implants about 90% of the time over a 10-year period. Some newer implants use 3D-printed titanium that mimics the spongy, lattice-like structure of natural bone, which helps create an even more secure bond.
How Long These Materials Last
Modern materials have pushed implant lifespans well beyond what earlier generations achieved. Data from the Finnish joint registry, one of the largest and longest-running tracking systems, found that 58% of hip replacements were still functioning after 25 years. That number continues to improve as cross-linked polyethylene, better ceramics, and improved manufacturing become standard. For most people receiving a hip replacement today, the implant will likely last the rest of their life, particularly if they’re over 60 at the time of surgery. Younger patients face a higher chance of eventually needing a revision simply because they’ll use the joint for more years and typically at higher activity levels.