Not all knee replacement implants are designed identically, and differences between designs can influence how a prosthesis functions over time. An implant typically consists of three main components: a metal cap for the femur (thighbone), a metal plate with a plastic spacer for the tibia (shinbone), and often a plastic button for the patella (kneecap). Materials, design principles, and fixation methods vary between systems.
Your orthopaedic surgeon evaluates your anatomy, activity level, bone quality, age, and weight to recommend implant specifications designed to support stability and joint function.
Implant Types by Material
The metal parts of a knee implant are most commonly made from a cobalt-chromium alloy, chosen for its durability and smooth surface. Titanium-based components are available as an alternative for patients with known metal sensitivities, and ceramic-coated options are sometimes used with the aim of further reducing wear over time.
The plastic spacer that sits between the metal parts has also improved over the years. Newer plastic formulations are designed to wear more slowly than earlier versions, which may support longer implant life. Your surgeon selects the specific materials and component sizes based on your bone quality, activity level, and any known sensitivities.
Implant Types by Design
Several implant designs exist, each aiming to recreate natural knee movement in a slightly different way.
- Cruciate-retaining designs preserve one of the knee’s original stabilising ligaments, which suits patients with a healthy, well-balanced ligament
- Posterior-stabilised designs use a built-in mechanical feature to provide stability when the original ligament is damaged or removed during surgery
- Medial-pivot designs aim to mimic the way a natural knee moves, with one side staying more stable while the other rotates
- Rotating platform designs allow the plastic insert to rotate slightly, which may suit patients with higher activity levels
Many patients are aware of the differences in how each design distributes movement and stress on the knee. There is no single “best” design for everyone. Your surgeon recommends the option best suited to your ligament condition, activity level, and anatomy.
Implant Types by Coverage: Partial or Total
Unicompartmental (partial) knee replacement addresses arthritis confined to one compartment of the knee, most commonly the medial (inner) compartment. These smaller implants resurface only the damaged portion, preserving healthy bone, cartilage, and ligaments in unaffected areas.
Partial knee implants consist of a small femoral cap and tibial component, available in both fixed-bearing and mobile-bearing designs. The procedure involves smaller incisions, less bone removal, and typically faster recovery compared to total knee replacement. Patients report their partial replacement often feels more like a natural knee.
Candidacy for partial replacement requires intact cruciate ligaments, arthritis limited to one compartment, correctable deformity, and adequate range of motion. The preserved compartments may develop arthritis later, potentially requiring conversion to total knee replacement.
Implant Types by Fixation Method
Implants are secured to bone in one of two ways: with bone cement, which sets quickly and allows most patients to bear weight immediately, or with a cementless technique, where the implant surface is designed to allow natural bone to grow into it over the following weeks to months.
Some surgeons use a combination of both approaches within the same knee, cementing one component while using the cementless technique for another. The choice depends on bone quality and surgical judgement, and all three approaches have long-term data supporting their use.
Regardless of fixation method, some surgeons use robotic-assisted or computer-guided tools to help position components precisely. This is a separate consideration from implant design and fixation method, and is discussed in more detail in our guide on robotic knee replacement benefits and risks.
Sizing and Fit: Matching the Implant to You
Knee implant manufacturers offer extensive size ranges to match patient anatomy. Component sizing affects coverage of the cut bone surfaces; undersized implants may sink into softer bone, while oversized components overhang and irritate soft tissues.
Gender-specific implants address anatomical differences observed between male and female knees. These designs feature narrower profiles, adjusted angles, and modified shapes to better match typical female knee anatomy, and some studies show these designs reduce femoral overhang.
However, most randomised trials comparing gender-specific and standard implants have found no meaningful difference in pain, movement, or satisfaction, so many surgeons prioritise correct sizing over gender-specific design.
Did You Know? The human knee bends and straightens thousands of times daily. Modern polyethylene formulations are tested in laboratory simulators that replicate decades of this cyclical loading, helping predict long-term wear performance before clinical use.
When to Seek Professional Help
- Knee pain that persists despite adequate trial of non-surgical treatments
- Difficulty walking distances you previously managed comfortably
- Night pain that disrupts sleep despite positioning changes
- Knee stiffness limiting daily activities like climbing stairs or rising from chairs
- X-ray evidence of bone-on-bone arthritis in one or more knee compartments
- Failure to respond to injections, physical therapy, and activity modification
Commonly Asked Questions
How long do knee replacement implants last?
Modern knee implants demonstrate good long-term survival rates in registry data. Implant longevity depends on patient factors (weight, activity level, bone quality), surgical technique, and implant design. Younger, more active patients may face higher revision rates due to longer expected lifespan and greater cumulative wear.
Will I set off metal detectors with a knee replacement?
Knee implants frequently trigger airport security scanners. Carrying a medical card identifying your implant may expedite screening, though security personnel typically use alternative screening methods for patients with joint replacements. The metal poses no safety concern during screening.
Can I kneel after knee replacement?
Many patients can kneel after recovery, though some find it uncomfortable due to residual numbness around the incision or pressure on the prosthesis. Using padding and kneeling on the operated knee (rather than placing weight on the kneecap) often improves comfort. Kneeling ability varies considerably between individuals.
What activities should I avoid with a knee replacement?
High-impact activities like running, jumping sports, and heavy manual labour accelerate wear and increase fracture risk around implants. Low-impact activities, such as walking, swimming, cycling, and golf, are generally encouraged. Your surgeon provides specific guidance based on your implant type, fixation method, and overall health.
Are newer implant designs better than older ones?
Established implant designs with long track records offer predictable outcomes supported by extensive clinical data. Newer designs may offer theoretical advantages but lack comparable follow-up. Many surgeons favour implants with proven survival rates over novel technologies until sufficient long-term evidence accumulates.
Next Steps
No single implant suits every patient. Implant design, fixation method, and component sizing must be matched to your bone quality, ligament integrity, and activity level. A surgeon’s familiarity with a particular implant system is a clinically relevant factor in achieving reliable outcomes. If non-surgical treatments have not adequately controlled your knee arthritis symptoms, a formal orthopaedic assessment is the appropriate next step.
If you are experiencing persistent knee pain, bone-on-bone arthritis on imaging, or difficulty with daily activities such as walking or climbing stairs that has not responded to non-surgical treatments, an accredited orthopaedic surgeon can evaluate your condition and advise on whether knee replacement, and which implant approach, is appropriate for you.
