A modern hip replacement is built from three to four distinct materials, each chosen for a specific job. The metal stem and socket shell are typically made from titanium or cobalt-chromium alloys. The bearing surfaces, where the ball meets the socket, use some combination of ceramic, metal, or a specialized plastic called highly cross-linked polyethylene. A full implant assembly weighs roughly 145 to 240 grams depending on the design, adding two to three times the weight of the natural joint it replaces.
The Metal Framework
Two metal alloys dominate hip replacement surgery. The femoral stem, the long piece anchored into your thighbone, is most often made from a titanium alloy containing small amounts of aluminum and vanadium. This alloy is lightweight, resists corrosion, and closely matches the flexibility of natural bone, which helps prevent the implant from absorbing too much mechanical load and weakening the surrounding bone over time. Because vanadium has some toxicity concerns, newer versions substitute it with niobium or iron to improve biocompatibility.
The other workhorse is cobalt-chromium-molybdenum alloy, composed of roughly 59 to 70% cobalt, 27 to 30% chromium, and 5 to 7% molybdenum, with trace amounts of other elements. This alloy is harder and more wear-resistant than titanium, making it the standard choice for femoral heads (the ball component) and for metal-on-metal bearing surfaces. Cobalt-chromium is also commonly used for the outer shell of the acetabular cup, the socket piece that sits in your pelvis.
Ceramic Bearing Surfaces
Ceramic femoral heads have become increasingly popular because they produce far less wear debris than metal. The current generation uses a composite called zirconia-toughened alumina, which is roughly 80% alumina (aluminum oxide) blended with zirconia (zirconium oxide). Adding zirconia to the alumina base makes the ceramic significantly more resistant to fracture by slowing crack propagation, while alumina provides extreme hardness and a polished surface that glides with minimal friction.
Ceramic-on-ceramic pairings, where both the ball and the socket liner are ceramic, generate many times less wear than other combinations and dramatically reduce the risk of particle-induced bone loss around the implant. The tradeoff is a small risk of fracture and an occasional audible squeaking during movement. With the latest generation of ceramics, head fracture rates have dropped to about 0.004%, though liner fractures still occur in roughly 1 to 3% of cases. When a ceramic component does crack, patients typically notice squeaking or clicking during walking, sometimes accompanied by a feeling of instability.
Polyethylene Liners
The acetabular liner, a cup-shaped insert that sits inside the metal socket shell, is frequently made from highly cross-linked polyethylene. This is an ultra-high-molecular-weight plastic that has been exposed to radiation (around 5 megarads) and then remelted at high temperatures to increase the connections between its molecular chains. The result is a material that wears down far more slowly than standard polyethylene.
Wear matters because tiny plastic particles shed from the liner can trigger an immune response. Your body’s macrophages try to consume the debris and release inflammatory signals that gradually dissolve bone around the implant, a process called osteolysis. It’s the leading cause of implant loosening over time. Standard polyethylene liners wear at rates above the 0.2 mm per year threshold where bone loss becomes a concern. Highly cross-linked versions paired with ceramic heads wear at roughly 0.02 mm per year, well below that danger zone. This reduction in wear is one of the main reasons modern implants last longer than earlier designs.
Porous Metals for Bone Attachment
How the implant attaches to your bone is just as important as the bearing surfaces. There are two approaches, and each uses different materials.
Cementless implants have a rough, porous coating on their surface, often made from porous tantalum or 3D-printed titanium. Porous tantalum has an interconnected network of tiny pores that your bone cells can grow directly into, creating a biological lock between the implant and your skeleton. The material also has high surface energy, which encourages bone-forming cells to attach, multiply, and mineralize. Its flexibility is close to that of natural bone, reducing stress shielding, a problem where a too-stiff implant causes the surrounding bone to weaken from disuse. With advances in 3D printing, surgeons can now order custom porous implants shaped to fit an individual patient’s anatomy.
Cemented implants use polymethyl methacrylate (PMMA) bone cement to fill the gap between the implant and the bone. This isn’t a glue in the traditional sense. It’s a two-part acrylic that a surgeon mixes in the operating room: a powder containing the polymer and a radio-opaque agent (so it shows up on X-rays), and a liquid containing the monomer and an accelerator. Once combined, it hardens in minutes, locking the implant in place immediately. Cemented fixation tends to produce a heavier total assembly, around 241 grams compared to 145 grams for cementless designs.
Why Metal-on-Metal Fell Out of Favor
Metal-on-metal bearings, where both the ball and socket surface are cobalt-chromium, were once promoted for their durability. The problem is that the sliding metal surfaces shed tiny metallic particles into the surrounding tissue. Cobalt and chromium ions from those particles enter the bloodstream and can cause adverse local tissue reactions, damaging bone and soft tissue around the implant. Some patients developed painful inflammatory masses, fluid collections, or progressive bone loss without obvious symptoms until significant damage had occurred.
The FDA mandated postmarket surveillance studies of metal-on-metal total hip systems in 2011 and has issued multiple safety communications since. International regulators followed with their own alerts. While metal-on-metal implants are still on the market, their use has dropped sharply. Most surgeons now prefer ceramic-on-polyethylene or ceramic-on-ceramic pairings for new patients.
Common Bearing Combinations
Your surgeon selects a bearing combination based on your age, activity level, and anatomy. The most widely used options today are:
- Ceramic-on-polyethylene: A ceramic femoral head paired with a highly cross-linked polyethylene liner. This is the most common combination, offering very low wear rates (as low as 0.02 mm per year) with minimal fracture risk.
- Ceramic-on-ceramic: Both surfaces are ceramic. Produces the least wear of any pairing but carries a small risk of component fracture and squeaking.
- Metal-on-polyethylene: A cobalt-chromium head against a polyethylene liner. Still used but gradually giving way to ceramic heads, which scratch the liner less.
- Metal-on-metal: Rarely used for standard total hip replacements due to metal ion concerns. Still occasionally used in hip resurfacing procedures for younger, active patients.
Each combination involves the same underlying framework: a titanium or cobalt-chromium stem press-fit or cemented into the thighbone, a modular ball that snaps onto the stem’s neck, a liner inside the socket, and a metal shell anchored into the pelvis. The materials have evolved considerably over the past two decades, and the shift toward highly cross-linked polyethylene and advanced ceramics is the biggest reason implant lifespans now routinely exceed 20 years for most patients.