Skip to content
Material World
Object

Joint replacement

Three materials in one device, each chosen against a different failure — and it fails from the debris rather than the load.

A total hip replacement is a stem in the femur, a head on the stem, and a cup in the pelvis. Almost every version uses more than one material, because the three components fail in different ways.

The stem is usually titanium, chosen for a stiffness closer to bone's: a stiff implant carries load the bone should carry, the bone resorbs, and the implant loosens. The head is cobalt-chromium or ceramic, chosen because it must not wear. The cup is usually cross-linked polyethylene, chosen as the sacrificial bearing surface.

What ends a joint replacement is usually not mechanical failure. It is the wear particles: microscopic polyethylene debris provokes an immune response that resorbs the bone around the implant, and the implant loosens in bone that has been dissolved from under it.

History

John Charnley's low-friction arthroplasty in the early 1960s is the origin of the modern device, and its history is a materials history. His first bearing was PTFE, chosen for its friction, and it wore catastrophically — he revised several hundred patients. Ultra-high-molecular-weight polyethylene replaced it and has been the standard bearing ever since.

Metal-on-metal resurfacing in the 2000s is the counter-example. It promised lower wear volumes and delivered cobalt and chromium ions and particulate instead, causing adverse local tissue reactions in a significant minority; the devices were largely withdrawn and are the subject of long-running litigation. A material's biocompatibility is a property of the whole system, not of the alloy.

Cultural significance

Around two million hip and knee replacements are performed a year, and it is among the most reliably life-changing operations in medicine — the outcome is usually the return of walking without pain.

It is also the clearest everyday case of materials being chosen against a specification a person's body writes: warm, wet, chloride-rich, loaded a million times a year, and unforgiving of anything that sheds.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is made of

  • Ti-6Al-4V — alloy · the stem, chosen for a stiffness closer to bone's than any other implant metal
  • Cobalt-chromium alloy — alloy · the bearing head, chosen because it must not wear
  • Polyethylene — material · the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device

is made using

  • PMMA — material · as bone cement, which is what fixes a cemented stem into the femur

is used in

  • Medical devices — industry · around two million hip and knee replacements a year

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Joint replacement back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Joint replacement → is made of (the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Joint replacement → is made using (as bone cement, which is what fixes a cemented stem into the femur) → PMMA → is produced by (free-radical polymerisation of methyl methacrylate, cast between glass for optical sheet or in bulk for moulding granules) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Joint replacement → is made of (the stem, chosen for a stiffness closer to bone's than any other implant metal) → Ti-6Al-4V → is produced by (melted under vacuum or inert gas, because molten titanium reacts with essentially every crucible material and with air) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Joint replacement → is made of (the bearing head, chosen because it must not wear) → Cobalt-chromium alloy → is produced by (chromium into cobalt, vacuum melted because the alloy is used in bodies and inclusions are failure sites) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Joint replacement → is made of (the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is produced by (electrolysis of potassium bifluoride, which is molten and conducts — there is no chemical oxidant strong enough to displace fluorine from a compound, so electricity is the only route and always has been) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
  • Joint replacement → is made of (the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (into polyethylene, and via ethylene dichloride into PVC — the two highest-tonnage plastics between them) → Ethylene → is produced by (the principal product, and the largest-tonnage organic chemical made anywhere) → Steam cracking → takes as input (the European and Asian cracker feed; North American crackers mostly run on ethane from natural gas instead, and make a different product slate as a result) → Naphtha

These are the most distinct paths back. Joint replacement can be traced through others besides.

Downstream — what it becomes