Joint replacement implants and orthopaedic technology

What an artificial joint is made of

A joint replacement recreates a smooth, low-friction surface where cartilage has worn away. Most implants combine hard-wearing materials: a metal alloy (often titanium or cobalt-chromium) or ceramic for the load-bearing parts, paired with a tough, very smooth plastic called polyethylene as the gliding surface. In a hip, a metal or ceramic ball moves in a cup lined with polyethylene or ceramic; in a knee, metal caps on the bone ends slide on a plastic spacer. The materials are chosen to be strong, biologically well tolerated, and slippery enough to move millions of times. Modern polyethylene is specially treated to resist wear, one of the key advances that has helped implants last longer.

How implants stay attached

An artificial joint has to bond firmly to living bone. There are two main approaches. Cemented implants are fixed with a fast-setting bone cement that grips the implant and the bone immediately, giving a solid hold from day one. Uncemented implants have a specially textured or coated surface that bone gradually grows into over weeks, locking the implant in place biologically. Sometimes a combination is used. The choice depends on the patient's age, bone quality and the surgeon's judgement, and both approaches have excellent long-term results. The engineering challenge is to spread load smoothly so the bone stays healthy and the implant does not loosen — a major reason implants are carefully shaped and sized to each person.

Computer-guided and robotic surgery

Getting an implant in exactly the right position matters for how well it works and how long it lasts. Newer technologies help surgeons achieve this precision. Computer navigation uses sensors to track instruments and bones in real time, giving the surgeon live feedback on alignment. Robotic-assisted systems go further: from a scan or measurements taken during surgery, they help plan the exact cuts and can guide or constrain the surgical tools so the implant is placed to plan. These tools do not replace the surgeon — the surgeon remains in control — but they can improve accuracy and consistency. Evidence on whether they improve long-term outcomes is still developing, and good results are achieved with conventional techniques too.

Personalisation and newer designs

Implants come in a range of shapes and sizes so the surgeon can match the person's anatomy, and pre-operative planning software helps select the right components and position. Some centres use patient-specific instruments or, occasionally, custom implants designed from a person's own scans for complex cases. Implant designs continue to evolve — for example, coatings that encourage bone growth or resist infection, and bearing materials engineered to wear less. Not every new design proves better, however: history includes examples, such as certain metal-on-metal hips, that were withdrawn after real-world data showed problems. This is exactly why careful, long-term monitoring of every implant matters as much as the innovation itself.

Tracking what works: registries and regulation

The UK is a world leader in tracking joint replacements. The National Joint Registry records details of hip, knee and other replacements across the country, following how implants perform over many years and flagging any that fail earlier than expected. This real-world evidence guides surgeons toward implants with proven track records and can trigger alerts if a device underperforms. Medical devices are also regulated for safety, and the Medicines and Healthcare products Regulatory Agency (MHRA) can issue safety notices and recalls. Together, registries and regulation mean that the choice of implant is informed by decades of outcomes from real patients — a safety net that turns individual operations into collective learning.