Medicine
Artificial Joints (Joint Replacement Prostheses)
Quick fact
The first modern hip replacement was performed in 1960 by Sir John Charnley, and his design—using a metal femoral head and a polyethylene socket—is still the basis for most hip implants today.
Why this is interesting
Your knee bends over a million times a year—what happens when the cushioning wears out? Artificial joints have helped millions walk again, but how do they actually work?
Read the full explanation
Understanding Artificial Joints (Joint Replacement Prostheses)
Think of your natural joint as a ball-and-socket hinge with smooth cartilage as a cushion. Arthritis or injury can erode this cartilage, causing bone-on-bone friction and pain. An artificial joint (prosthesis) replaces these damaged surfaces with man-made materials. For a hip replacement, the surgeon removes the damaged femoral head (top of the thighbone) and fits a metal stem with a smooth ball into the bone. The worn acetabulum (socket in the pelvis) is lined with a durable plastic or ceramic cup. The new ball rotates inside the cup, mimicking natural motion. Knee replacements work similarly: the ends of the thighbone and shinbone are resurfaced with metal components, and a plastic spacer acts as the cartilage. These implants are fixed to the bone using either bone cement (like a strong glue) or a rough coating that allows bone to grow into it (cementless fixation). The goal is to create a stable, pain-free joint that allows everyday activities like walking, sitting, and climbing stairs.
A deeper explanation
The success of artificial joints hinges on material science and biomechanics. The bearing surface—where artificial parts rub together—must endure millions of cycles with minimal wear. Common pairings are cobalt-chromium alloy on ultra-high-molecular-weight polyethylene (metal-on-plastic), ceramic-on-ceramic, or ceramic-on-polyethylene. Ceramic is extremely hard and produces less wear debris, but can fracture; metal is strong but may cause allergic reactions in some patients. Polyethylene debris can cause inflammation and bone loss (osteolysis), leading to implant loosening. To combat this, modern designs use highly cross-linked polyethylene, which is more resistant to wear. The stem and cup must transfer load to the surrounding bone without stress shielding, a phenomenon where the implant takes too much load and the adjacent bone weakens. Cementless implants rely on a porous surface that encourages bone ingrowth, creating a biological fixation. The surgical procedure requires precise alignment—if the components are off by even a few degrees, the joint can dislocate or wear prematurely. Despite these complexities, over 90% of hip and knee replacements last 15–20 years, making them one of the most successful surgical interventions in modern medicine.