A knee replacement is made of two kinds of metal and a tough medical plastic. A polished cobalt-chromium cap covers the end of your thigh bone, a metal tray sits on top of your shin bone, and a plastic insert between them works as your new cartilage.[1] This guide explains each part using the Stryker Triathlon knee that Dr Vishal Pai uses, what the Australian joint registry shows about it, and how much a new knee weighs.
At a glance
- A knee replacement is mostly cobalt-chromium metal and a tough medical plastic, with either a cobalt-chromium or a titanium tray on your shin bone.
- The implant is held either with bone cement or by your bone growing into a porous titanium surface, and so far the two versions of the Triathlon knee have similar registry results at 5 years.
- A new knee adds roughly 300 g to your leg, about three-quarters of the weight of a full can of soft drink.
- Tell your surgeon about any metal or nickel allergy before surgery.
On this page
What are the parts of a knee replacement?
Which metals are in the knee Dr Vishal Pai uses?
What is the plastic made of, and does it wear out?
Is the knee cemented, or does the bone grow into it?
How long does the Triathlon knee last?
How much does a knee replacement weigh?
What are the parts of a knee replacement?
A total knee replacement has three or four parts.
- The femoral component, a curved metal cap on the end of your thigh bone (femur).
- The tibial component, a flat metal tray on top of your shin bone (tibia).
- The plastic insert, which locks into the tray and acts as the new cartilage.
- A plastic button on the back of your kneecap (patella), which Dr Vishal Pai uses routinely.
A partial knee replacement uses similar materials but replaces only the worn side of the knee.
Which metals are in the knee Dr Vishal Pai uses?
Dr Vishal Pai uses the Stryker Triathlon total knee in its cruciate-retaining (CR) version, which keeps your posterior cruciate ligament.
The thigh-bone part is a cobalt-chromium alloy, a hard metal that can be polished very smooth so it glides on the plastic with little wear. The shin-bone tray depends on how the knee is fixed. A cemented tray is also cobalt-chromium. The cementless tray, called Tritanium, is 3D-printed titanium alloy with a porous, sponge-like surface that your bone grows into.[2]
Each metal is chosen for its job, so neither is better overall. Some knees use a thigh-bone part with a special coated or ceramic-like surface instead of plain cobalt-chromium. One large US registry study found similar revision rates for these after about 4 years,[3] but in the Australian registry they were revised more often, with 5.8 compared with 4.4 in 100 revised by 10 years. A revision is a further operation to change or add a part.[4]
What is the plastic made of, and does it wear out?
The insert is ultra-high-molecular-weight polyethylene, a very tough medical plastic. Over many years, tiny wear particles can loosen an implant, so highly cross-linked polyethylene was developed, a treated plastic designed to wear less.
Across all brands in the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR), knees with cross-linked plastic had fewer revisions after the first few months. The difference was largest in people under 65, with 7.0 compared with 9.9 in 100 revised by 15 years, and smaller at 65 and over, with 4.1 compared with 5.2 in 100.[4] Not every registry agrees, and the UK registry found no overall difference.[5]
About 9 in 10 Triathlon CR knees in Australia use cross-linked plastic. Within the Triathlon, the registry found no difference between the two plastics, with 4.7 in 100 cross-linked and 4.4 in 100 conventional knees revised by 15 years.[4] In the cementless Triathlon, wear of the cross-linked plastic was tiny and stopped increasing after 5 years, while conventional plastic kept wearing through to 10 years. People rated their knees the same with either plastic.[6]
Triathlon CR knees revised, by type of plastic
Cumulative percentage revised, Triathlon CR knees for osteoarthritis in Australia
- Cross-linked plastic
- Conventional plastic
Show the figures as a table
| Plastic | 1 yr | 5 yrs | 10 yrs | 15 yrs | 20 yrs |
|---|---|---|---|---|---|
| Cross-linked (148,588 knees) | 0.8% | 2.3% | 3.4% | 4.7% | – |
| Conventional (12,052 knees) | 0.7% | 2.3% | 3.4% | 4.4% | 6.8% |
Is the knee cemented, or does the bone grow into it?
Bone cement is an acrylic that sets hard within minutes and grips the implant straight away. A cementless implant is pressed firmly into place, and over the following weeks your bone grows into its porous surface. Dr Vishal Pai uses both and chooses mainly by your bone quality. This is his own practice, and surgeons differ.
Across trials comparing the two methods, revision rates and knee function were similar at an average of about 8 years.[7] With the Triathlon itself, the cementless titanium tray settles slightly over the first year or two and then stays stable. Over 5 years, its movement in the bone was not significantly different from the cemented tray, and knee scores were similar.[8]
The 2026 registry report includes 50,367 Triathlon knees with the titanium tray. By 5 years, 2.4 in 100 had been revised, compared with 2.3 in 100 cemented Triathlon CR knees.[4] Dr Vishal Pai routinely resurfaces the kneecap. In people with osteoarthritis who also had the kneecap resurfaced, 1.8 in 100 cementless Triathlon knees had been revised by 5 years, compared with 4.4 in 100 when the kneecap was left alone.[4]
These are early results. The registry does not yet report 10-year figures for the titanium tray, and across all brands, cementless knees have a slightly higher revision rate than cemented knees, with 5.3 compared with 4.5 in 100 revised by 10 years.[4]
Early results for the cementless titanium tray
Cumulative percentage of Triathlon CR knees revised in Australia
- Cemented tray (98,083 knees)
- Cementless titanium tray (50,367 knees)
Show the figures as a table
| Tray | 1 yr | 3 yrs | 5 yrs | 10 yrs |
|---|---|---|---|---|
| Cemented | 0.7% | 1.7% | 2.3% | 3.3% |
| Cementless titanium | 1.0% | 1.9% | 2.4% | – |
| Cementless titanium, kneecap resurfaced (osteoarthritis) | 0.8% | 1.4% | 1.8% | – |
| Cementless titanium, kneecap not resurfaced (osteoarthritis) | 1.8% | 3.6% | 4.4% | – |
How long does the Triathlon knee last?
Of more than 160,000 Triathlon CR knees with the standard tray done for osteoarthritis in Australia, 2.3 in 100 had been revised by 5 years, 3.4 by 10 years, 4.7 by 15 years and 7.0 by 20 years.[4] Across all modern total knee replacements for osteoarthritis, the figures were 4.6 in 100 at 10 years, 6.2 at 15 years and 8.0 at 20 years.[4]
These are averages for large groups that mix many surgeons and patients, not a prediction for you. Younger people have higher revision rates,[9] and infection, stiffness and instability can lead to further surgery as well as wear of the materials. Our guide on how long a knee replacement lasts covers this in more detail.
How much does a knee replacement weigh?
A cemented knee replacement makes your knee about 270 to 335 g heavier than before, around three-quarters of the weight of a full can of soft drink.[10,11] This is the weight of the metal, plastic and cement, minus the bone and tissue removed during surgery.[10]
Your thigh ends up about 3% heavier.[11] Most people stop noticing the weight as the knee recovers.
What if you have a metal or nickel allergy?
Cobalt-chromium contains a small amount of nickel, and nickel skin allergy is common. A knee replacement does not appear to cause a new nickel allergy in people who did not already have one. Whether an existing allergy affects results is uncertain, because the evidence conflicts.[12] In people with a known nickel allergy, standard cobalt-chromium and nickel-free implants had similar results, with 94 and 98 in 100 free of revision and no difference in how people rated their knee at 1 year, although only a small number in that group had the standard implant.[13]
Nickel-free options exist, so tell your surgeon about any metal allergy. Allergy is a rare cause of a painful knee replacement and is considered only after more common causes have been ruled out.
Summary
A knee replacement is made of polished cobalt-chromium, a cobalt-chromium or porous titanium tray and a tough medical plastic, held with bone cement or by bone growing into the implant. In the Triathlon knee that Dr Vishal Pai uses, the cemented and cementless versions have similar registry results so far, although across all brands cementless knees are revised slightly more often. In a trial with this knee, cross-linked plastic showed very low wear. The implant adds roughly 300 g to your knee, and a nickel allergy is worth mentioning before surgery. Materials are only one part of a good result, alongside careful surgery and rehabilitation.
If you would like to discuss how this applies to your knee, you can book a consultation with Dr Vishal Pai.
References
Show all 14 referencesHide references
- Munazzam SW et al. 2025. A narrative review of advancements in knee arthroplasty: analyzing diverse prosthetic materials and their implications. Journal of Clinical Orthopaedics and Trauma.
- Bendich I et al. 2022. The impact of component design and fixation on stress shielding after modern total knee arthroplasty. Journal of Arthroplasty.
- Gowd AK et al. 2023. Oxidized zirconium versus cobalt chromium for primary TKA: no difference in midterm revision rates from the American Joint Replacement Registry. Clinical Orthopaedics and Related Research.
- Smith PN et al. 2026. Hip, Knee and Shoulder Arthroplasty: 2026 Annual Report. Australian Orthopaedic Association National Joint Replacement Registry.
- Partridge T et al. 2019. Conventional versus highly cross-linked polyethylene in primary total knee replacement: a comparison of revision rates using data from the National Joint Registry for England, Wales, and Northern Ireland. Journal of Bone and Joint Surgery (American).
- de Ridder R et al. 2026. Ten-year insert wear and the migration of uncemented tibial and femoral components with either conventional polyethylene or sequentially annealed highly crosslinked polyethylene: a blinded randomized controlled trial using radiostereometric analysis. Bone and Joint Journal.
- Prasad A et al. 2020. Cemented vs. cementless fixation in primary total knee arthroplasty: a systematic review and meta-analysis. EFORT Open Reviews.
- van der Lelij TJN et al. 2023. Continued stabilization of a cementless 3D-printed total knee arthroplasty. Journal of Bone and Joint Surgery (American).
- Rashed SS et al. 2021. The impact of the largest national joint registry on current knee replacement longevity estimates: an analysis and review of knee prosthesis brand and fixation technique. Journal of Arthroplasty.
- Gibon E et al. 2014. Doctor, what does my knee arthroplasty weigh? Journal of Arthroplasty.
- Rossi R et al. 2026. Does implant weight matter? The effect of total knee arthroplasty on thigh mass distribution and estimated hip loading. Journal of Experimental Orthopaedics.
- Peacock C et al. 2022. The effect of nickel hypersensitivity on the outcome of total knee arthroplasty and the value of skin patch testing: a systematic review. Arthroplasty.
- Siljander B et al. 2023. A comparison of clinical outcomes after total knee arthroplasty in patients with preoperative nickel allergy receiving cobalt-chromium or nickel-free implant. Journal of Arthroplasty.
- Hui C et al. 2011. Five-year comparison of oxidized zirconium and cobalt-chromium femoral components in total knee arthroplasty: a randomized controlled trial. Journal of Bone and Joint Surgery (American).
Frequently Asked Questions
Is a titanium knee replacement better than a cobalt-chrome one?
Not in general. The Triathlon knee combines both metals, with cobalt-chromium for the smooth thigh-bone surface and either cobalt-chromium or porous titanium for the shin-bone tray. By 5 years, 2.3 in 100 cemented and 2.4 in 100 titanium-tray Triathlon knees had been revised.[4] A nickel allergy is the main reason a surgeon might choose a nickel-free implant.
Will I feel the difference between materials?
Probably not. When people had a different thigh-bone metal in each knee, there were no differences in pain, function or X-rays at 5 years. Asked which knee they preferred, 44% had no preference and 38% chose the standard cobalt-chromium knee.[14] How your knee feels depends more on how it settles after surgery and on your rehabilitation.
Can I have an MRI scan after a knee replacement?
Yes, in most cases. The metals used are not magnetic in the way iron is, so an MRI of another part of your body is generally possible. Tell the radiology staff you have a knee replacement, because images close to the implant can be distorted and another type of scan is sometimes chosen to look at the knee itself.
Will my knee replacement set off airport security?
It may. Walk-through metal detectors and hand-held wands often pick up a knee replacement, and body scanners can show it as an area to check. You do not need a special card or letter. Simply tell security staff before screening that you have a joint replacement.
Want to know what would go into your knee?
Dr Vishal Pai performs total and partial knee replacement and can talk you through the implant materials and fixation options. You can arrange a consultation to discuss what may suit your circumstances.
Written by Dr Vishal Pai
Vishal is an experienced knee surgeon based in Bayside, Melbourne. He consults at Sandringham Orthopaedic Specialists and operates at The Avenue Hospital and Calvary Holmesglen Private Hospital. His practice focuses on sports knee surgery, including ACL reconstruction and meniscal repair, as well as robotic knee replacement.