What Is My Knee Replacement Made Of?

People ask this more often than they ask about the operation, and it is a better question than it looks. Knowing what is going into your knee makes the rest of the conversation easier to follow, particularly the parts about how long it will last and what can go wrong.

Three parts and a bearing.

A total knee replacement is not a hinge and it does not replace the whole knee. It resurfaces the worn ends of the bones and leaves your ligaments doing the work of holding the joint together. There are three implanted parts plus the plastic bearing between them.

• The femoral component. A curved metal cap over the end of the thigh bone, shaped to reproduce the rolling and gliding of a normal knee. Almost always a cobalt-chromium-molybdenum alloy, chosen because it takes and holds a very smooth polish. Alternative surfaces exist, including oxidised zirconium and ceramic coatings, and they are used mainly where metal sensitivity is a genuine concern.
• The tibial component. A tray on the cut surface of the shin bone, usually a titanium alloy, with a stem down into the bone for stability. Some designs are a single piece of polyethylene with no metal tray at all, and these still perform well in registry data for older patients.
• The bearing. A polyethylene insert that clips into the tibial tray and forms the surface the metal femoral component runs on. This is the wear surface and it is the part that limits the life of the implant.
• The patellar button. A polyethylene disc on the back of the kneecap, used where the patellofemoral joint is worn. Whether to resurface the kneecap is a long-running debate in knee surgery; it is decided case by case.

The whole construct weighs a few hundred grams. Most people expect it to be considerably heavier, and most people cannot feel the weight of it once the swelling has settled.

Why the plastic is the interesting part.

Metal on metal would seize. Metal on bone would destroy the bone. So every joint replacement needs a bearing surface, and in the knee that is ultra-high molecular weight polyethylene, a plastic with a very long molecular chain that gives it its toughness.

Polyethylene wears. Not quickly, but over years it sheds microscopic particles, and the body's response to those particles is the mechanism behind osteolysis, which is bone resorbing around the implant and the implant loosening as a result. That is the classic long-term failure mode of a joint replacement, and it is why implant life is measured in decades rather than being indefinite.

Highly cross-linked polyethylene, introduced to reduce that wear rate, is now standard in hip replacement and widely used in the knee. Cross-linking makes the plastic more wear resistant at some cost to its mechanical toughness, and the knee loads plastic differently from the hip, which is why the change has been more gradual in knees. The practical point for a patient is that the plastic in a knee implanted today is not the plastic that produced the failure rates quoted from the 1980s.

Cement, or not.

Most knee replacements in Australia are cemented. The cement is polymethyl methacrylate, an acrylic that is mixed in theatre and sets in about ten minutes. It is not glue. It is a grout that fills the space between implant and bone and locks the two together mechanically, and it reaches full strength before you leave the operating theatre, which is why you can walk on the joint the same day.

Cementless knee components rely on bone growing into a porous or hydroxyapatite-coated surface instead. They are used selectively. Registry data is the arbiter here rather than opinion, and the Australian registry publishes revision rates by fixation type and by individual implant every year.¹

Metal allergy.

This comes up often, usually in people who react to costume jewellery. The nickel in cobalt-chromium alloy is the usual concern.

The honest position is that skin sensitivity to nickel is common, that true metal hypersensitivity causing a joint replacement to fail is rare and difficult to prove, and that the evidence linking the two is weak. Patch testing correlates poorly with implant outcomes. Where there is a clear, documented history of significant reaction, alternative bearing surfaces such as oxidised zirconium or a ceramic-coated femoral component are available and are used. Where there is a history of a rash under a watch strap, it is worth mentioning and it usually changes nothing.

MRI, airports and metal detectors.

Modern knee and hip implants are not ferromagnetic and are safe in an MRI scanner under standard conditions. Tell the radiographer what you have and where, because the metal distorts the image locally and the sequences may be adjusted for it.

Airport detectors are more variable than people expect. Some implants set them off reliably, some never do, and it depends as much on the detector as on the implant. Implant identification cards are no longer required or especially useful; simply telling the officer you have a joint replacement is the practical answer.

How you know your implant is a good one.

This is the part that Australian patients are better served on than almost anywhere else. Every joint replacement performed in Australia is recorded by the Australian Orthopaedic Association National Joint Replacement Registry, by implant, by surgeon and by hospital, with the outcome tracked over time.¹ Implants with revision rates worse than the class average are identified and reported, and that reporting has removed poorly performing implants from Australian practice on more than one occasion.

It means the choice of what goes into your knee is not a matter of preference or of who sponsored a conference. It is a choice made against published national data. If you want to know what is being used in your case and why, ask, and the answer should include what the registry says about it.

What that data implies about longevity is covered in how long does a joint replacement really last, and the same registry underlies the discussion in computer assisted and robotic surgery.

References.

1. Australian Orthopaedic Association National Joint Replacement Registry. Hip, Knee & Shoulder Arthroplasty Annual Report. Adelaide: AOA.
2. Evans JT, Walker RW, Evans JP, Blom AW, Sayers A, Whitehouse MR. How long does a knee replacement last? A systematic review and meta-analysis of case series and national registry reports with more than 15 years of follow-up. Lancet. 2019;393(10172):655-663.

This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.