How hip implants are made

A forged hip stem is polished in acid, partly so hidden cracks stay open and show up under fluorescent dye.

stations: 9
things upstream: 47
raw sources: 22
steps from the ground: 6

The short version

A hip stem is usually forged from cobalt-chromium or titanium close to its final shape. Scale is blasted off, then the surface is machined, polished and checked for cracks with fluorescent dye. Bone-contact areas may get a plasma-sprayed coating. The cup liner is cut from consolidated polyethylene, and parts are laser marked, sealed in double pouches in a cleanroom and sterilized.

Forge near final shape

A cobalt-chromium or titanium body is pressed in forging dies to a shape only slightly bigger than the finished part. Near-net forging means little metal has to be cut away later, though stems can also be cast or milled.

Goes in
Cobalt Chromium Titanium
Blast off the scale

Forging heat leaves a scale on the metal. One patent blasts it off with fine beads hard enough to clean the surface, yet gently enough not to smear small cracks shut.
Trim and machine

CNC mills, lathes and grinders remove the flash and cut the critical features. One French plant quotes tolerances down to ±0.01mm for metal and polymer parts.
Polish in acid

In the patent's method, the part sits in an acid bath with current running through it, which strips a thin skin and leaves a bright finish. It is then cleaned ultrasonically.
Hunt cracks with dye

A fluorescent penetrant seeps into any open crack or lap and glows under inspection. Measuring machines also check dimensions against the drawing.
Spray the coating

Plasma torches spray titanium or hydroxyapatite in air or vacuum onto grit-blasted surfaces where the implant will meet bone.

Goes in
Titanium
Finish the ball

Femoral heads of metal or ceramic are ground to tight tolerances and delivered fully finished, ready for sterilizing.

Goes in
Ceramic
Cut the cup liner

Ultra-high-molecular-weight polyethylene powder is ram extruded or compression molded into solid stock, then machined into a liner. Some makers use radiation to cross-link it so it wears more slowly.

Goes in
Polyethylene
Mark, pouch and sterilize

Parts are laser marked, passivated and given a last cleaning. In a Class 8 cleanroom they go into double vacuum pouches or sealed blisters, ready for gamma or ethylene oxide sterilizing.

End of the lineHip implant

Worth repeating

Shot peening can smear a forging crack shut, hiding it from the dye that is supposed to find it.
One implant plant in Chaumont, France, turns out over 12 million implants a year.
Charnley's polyethylene cup, introduced in 1962, and his use of bone cement shaped modern hip replacement.