TKA Implant Design

Solid Metal Augments

The design. Modular metal blocks attach to the tibial or femoral component to fill bone defects. They are made of cobalt-chrome, the standard and rigid material, or titanium in some systems, with a modulus closer to bone. Thicknesses are typically 5, 10, 15 and 20mm, and augments can be stacked (10mm plus 10mm for 20mm) if needed.

Indications.

• Contained bone defects (3-sided deficiency)
• Uncontained defects less than 5mm deep
• Revision TKA with mild to moderate bone loss (AORI 2A-2B)
• Correction of joint-line elevation

Trade-offs. Augments bear load immediately, are modular for intraoperative adjustment, are cost-effective compared with custom implants and are available off the shelf. But they are rigid, concentrating stress at their edges, do not integrate biologically, can subside without adequate support, and increase overall constraint, which requires stems.

Fixation. The augment is cemented to host bone and attached to the component with screws or a snap-fit. Stems are mandatory, a minimum of 50mm, to offload stress from the augment-bone interface, and thicker augments need longer stems.

Trabecular Metal (Porous Tantalum) Technology

The material. Trabecular metal is a highly porous tantalum structure with 80% porosity and pores of 400-600 micrometres that allow bone ingrowth. Its modulus of elasticity, 3 GPa, is similar to cancellous bone, and its coefficient of friction of 0.88 gives high initial stability.

Forms.

• Augments: wedges and blocks (posterior tibial, distal femoral), attached modularly to the component, cemented into the defect or press-fit for biological fixation, in various sizes and shapes (posterior slope, straight blocks)
• Cones: filling large tibial and femoral metaphyseal defects
• Sleeves: circumferential metaphyseal support
• Revision shells: acetabular, the analogous concept in the hip

Indications.

• Large uncontained bone defects (AORI 2B and 3)
• Revision TKA with poor bone quality
• Failed previous bone grafting
• A wish for biological fixation in revision

Cones. A cone is press-fitted into the remaining bone, friction giving initial stability, and bone ingrowth provides long-term fixation, with the stem passing through its centre. Sizes run from 30-50mm in diameter, in various lengths. Cones are used where metal augments are insufficient, and they need adequate press-fit stability and contact with host bone for ingrowth.

Trade-offs. Bone ingrowth, confirmed histologically, gives biological fixation; modulus matching reduces stress shielding; high friction gives excellent initial stability; and cones can fill large irregular defects. Against that, the cost is significantly higher than metal augments, the technique is sensitive, needing a good press-fit and adequate host bone, 15-year data are pending, ingrowth makes removal difficult at revision, and tantalum causes significant MRI artefact.

Results. Mid-term survival of revision TKA with cones is 85-90% at 7-10 years, and subsidence is low, less than 2mm in most series. The higher cost is offset by a reduced need for structural allograft and its donor-site morbidity.

Metaphyseal Sleeves and Cones: Advanced Defect Management

These technologies have transformed revision TKA by providing a biological solution to bone loss that was previously challenging.

Sleeves. A sleeve is a cylindrical porous metal structure that surrounds the metaphysis, giving circumferential support and load distribution, with the stem passing through its centre. It offers a large surface area for ingrowth, offloads stress from the component and restores the joint line. The metaphysis is reamed to fit, the sleeve press-fitted for initial stability, the stem inserted through it and the component cemented to the sleeve. Indications:

• AORI 3 defects
• Failed previous augments
• Circumferential bone loss
• Poor-quality metaphyseal bone

Cones. A cone is a tapered conical porous metal structure that fills a large metaphyseal defect, with the stem passing through its apex. Sizing is versatile, it fills irregular defects, fixes biologically and bears load immediately. After trial sizing, the bone is reamed to achieve a press-fit, the cone impacted, the stem passed through its centre and the component cemented to the cone. Indications:

• AORI 2B-3 defects
• Large uncontained defects
• Central defects
• As an alternative to structural allograft

Results. Sleeves have 90% survival at 5 years in complex revision. Both sleeves and cones show bone ingrowth on retrieval studies and minimal subsidence (less than 2mm) with proper technique, and their significant cost is set against the avoided morbidity of allograft.

Technique.

• Achieve an adequate press-fit: the cone or sleeve should resist manual extraction
• Stems must bypass the cone or sleeve by at least 4cm to prevent stress concentration
• Cement the component to the cone or sleeve, not to host bone
• Consider antibiotic-loaded cement in revision
• Progress weight-bearing, with 6 weeks protected, to allow ingrowth

Cruciate-Retaining Design

Philosophy. Retaining the PCL maintains normal femoral rollback and proprioception. CR is the least constrained design, and it requires an intact, functional PCL, checked for tension intraoperatively, as well as competent collaterals.

What it gains. With no intercondylar box, or a minimal one, it preserves more femoral bone stock, and it avoids the post-cam complications of patellar clunk and post fracture. Rollback comes from the patient's own PCL, there is a lower incidence of anterior knee pain, and a proprioceptive benefit is possible, though debated.

What it costs. The insert must be less conforming to allow motion, which raises contact stress on the polyethylene. PCL tension sets the flexion gap, making it less predictable, the PCL can rupture postoperatively, and CR is contraindicated when the PCL is deficient, as in inflammatory arthritis, revision or severe deformity.

In the registry. CR represents 25% of primary TKA in the AOANJRR and is most commonly used with cementless femoral fixation. Its revision rate is slightly higher than PS, a difference that is not statistically significant.

Posterior-Stabilised Design

Philosophy. A post-cam mechanism substitutes for the PCL: the tibial post engages the femoral cam, giving reliable, reproducible femoral rollback and preventing posterior sag. PS is the most commonly used design worldwide and represents 67% of primary TKA in the AOANJRR, where it is the standard choice for most surgeons, with excellent long-term survival data and a lower revision rate than CR and mobile-bearing designs.

What it gains. Without a PCL there is no PCL variability, so gap balancing is easier, and a contracted PCL can simply be resected. The insert can be more conforming, lowering contact stress and wear, and the post prevents posterior tibial subluxation. It is the standard for inflammatory arthritis and severe deformity.

What it costs. The intercondylar box that houses the cam means more femoral bone resection, requiring a 10-12mm anterior femoral resection. The post-cam adds complications of its own: patellar clunk syndrome from a fibrous nodule above the post, post wear and fracture (possible but rare with modern designs), cam-post contact stress (addressed by modern geometry) and post-cam dislocation if the components are malaligned.

How the post-cam evolved.

• IB-I (1978): the original Insall-Burstein design, a single-radius femoral cam on a flat tibial post; high contact stress and post wear established the concept but needed refinement
• IB-II (1986): improved cam geometry with gradual engagement reduced contact stress, a larger post reduced the fracture risk, and patellar clunk became less frequent
• Modern PS (2000s): a multi-radius cam for progressive engagement, jump distance optimised to 10-12mm, an HXLPE post to reduce wear, an anterior post position to reduce patellar clunk, and rollback beginning at 20-30 degrees of flexion

Constrained Condylar Knee Design

Philosophy. The CCK increases varus-valgus constraint for the ligament-deficient knee without fully constraining rotation, bridging standard PS and the rotating hinge. A taller tibial post, its increased height preventing subluxation, a deeper femoral box to house it and a tighter post-cam articulation with less coronal laxity compensate for collateral insufficiency. Because axial rotation is still allowed, less stress reaches the bone-implant interface than with a hinge, and modular systems offer a CCK insert on a standard baseplate.

Indications.

• MCL or LCL incompetence, mild to moderate
• Revision TKA with mild bone loss
• Primary TKA with severe deformity and ligament attenuation
• Failed PS with instability
• Flexion-extension gap mismatch greater than 5mm not correctable by balancing

Against the hinge. The CCK allows axial rotation, which reduces torsional stress on fixation; it needs less bone resection than a hinged prosthesis, has a lower mechanical failure rate and has better long-term survival.

Against standard PS. The extra constraint increases interface stress, the stems it requires are more invasive and remove more bone, the modular system costs more, and it may over-constrain the knee if used unnecessarily. Its constraint level, intermediate between PS and hinge, must be matched to the soft-tissue status.

Stems. Stems are mandatory, to distribute the constraint forces and protect the interface. They are at least 50mm long, press-fit by preference though cemented is acceptable, with a diameter that bypasses defects by 2 cortical diameters, and offset stems are available if needed to restore the anatomical axis.

Use. CCK is used in under 3% of primary TKA in Australian registry data but is significantly more common in revision, where survival approaches PS in properly selected cases.

Rotating Hinge Design

Philosophy. The rotating hinge gives maximum constraint with preserved axial rotation. Its femoral and tibial components are fully linked, constraining varus-valgus and anteroposterior motion completely, which eliminates the need for collaterals, while a central rotating mechanism allows axial rotation to reduce torsional stress on the stems and fixation. The hinge housing requires significant bone removal, which limits future revision options.

Indications.

• Severe ligament deficiency, both collaterals incompetent
• Massive bone loss (AORI 3)
• Tumour reconstruction with a massive segmental prosthesis
• Failed CCK with persistent instability
• Extensor mechanism disruption with allograft reconstruction
• Severe fixed deformity, greater than 30 degrees

Stems and bone. Long intramedullary stems are mandatory and critical to distribute the massive constraint forces: at least 150mm long, ideally bypassing bone defects by 2 cortical diameters, filling 70-80% of the canal, cemented or press-fit (both acceptable, cemented more common in revision), with offset stems to restore the anatomical axis if the canal is deformed. Cones or sleeves manage the bone defects, the design is available with or without a patellar tracking component, and custom implants are considered for extreme anatomy, massive bone loss or extreme deformity.

Complications.

• Aseptic loosening, 10-15% at 10 years, the highest of all designs
• Periprosthetic fracture, from the long lever arm and stress concentration at the stem tip
• Infection, from the longer operative time and more extensive dissection
• Extensor mechanism failure
• Hinge breakage, rare with modern designs

Use. Rotating hinges are used in less than 2% of primary TKA, more commonly in tumour reconstruction and complex revision, and the AOANJRR shows a higher revision rate, as expected given the severity of the underlying pathology. The hinge is the last option on the constraint ladder, reserved for cases in which lesser constraint would fail through instability or bone loss.

Conventional Ultra-High Molecular Weight Polyethylene

The material. UHMWPE is a long-chain hydrocarbon polymer with a molecular weight of 3-6 million g/mol. It has excellent biocompatibility and adequate mechanical strength, and its wear resistance is good, although its wear rate is clinically significant. How it is fabricated matters: direct compression moulding is superior to machining from bar stock.

Sterilisation. The sterilisation method is critical to long-term performance:

• Gamma in air (before 1995): 2.5-4.0 Mrad in air-permeable packaging. Free radicals reacted with oxygen, causing oxidation and embrittlement, and after the catastrophic failures of the 1990s the method was abandoned
• Ethylene oxide (1995-2000): chemical sterilisation without irradiation, so no free radicals form and there is no oxidation, at the cost of longer processing and concern about residual EtO; it is still used for some conventional polyethylene
• Gamma in inert gas (1998 to present): irradiation in nitrogen or vacuum barrier packaging sterilises without oxidation and is the standard method for conventional UHMWPE, the free radicals being quenched by post-irradiation annealing or ageing

Wear mechanisms.

• Adhesive wear: material transfer between the articulating surfaces
• Abrasive wear: third-body particles (cement, metal, bone) embedded in the polyethylene
• Fatigue wear: subsurface crack propagation from cyclic loading
• Oxidative degradation: long-term chemical breakdown in vivo

Delamination. In the knee, fatigue wear has a name and a characteristic appearance. Cracks initiate at the subsurface point of maximum shear stress, run parallel to the surface and then break out, lifting off sheets of polyethylene rather than releasing the fine particles a hip generates. It is the failure mode behind the catastrophic failures of the 1990s, and it is a knee problem rather than a hip one because the knee bearing is less conforming, so contact stress is concentrated over a smaller area.

Clinical outcomes. With modern conventional UHMWPE sterilised by gamma in inert gas, 10-15 year survival is over 90%. Osteolysis develops in 10-20% at 10-15 years, dose-dependent on wear, and revision for wear and osteolysis is the primary mode of late aseptic failure. Conventional UHMWPE served as the standard for decades but has been largely supplanted by HXLPE because of its dramatic wear reduction.

Highly Cross-Linked Polyethylene (HXLPE)

Manufacture. HXLPE is made in three steps:

1. High-dose irradiation, 5-10 Mrad against 2.5-4 Mrad for conventional polyethylene, creates more cross-links between polymer chains, increasing wear resistance. Cross-linking in an inert atmosphere prevents oxidation, but the irradiation generates free radicals that must be quenched.
2. Free-radical quenching by remelting or annealing, which trade off differently (below).
3. Final processing. Components are machined from consolidated resin or compression moulded and packaged in an inert atmosphere (nitrogen or vacuum) to prevent oxidation. The cross-linking irradiation provides sterilisation, so secondary sterilisation is either not required or uses a non-ionising method, gas plasma or EtO; gamma in inert gas was the fix for conventional UHMWPE.

Both show a similar, dramatic wear reduction, and both are in widespread use with excellent results. Remelted HXLPE's superior oxidation resistance is theoretical, since residual oxidation has not been clinically problematic for annealed material, and no difference in clinical outcomes has been demonstrated to date.

What it gains. Wear falls by 90%, a 10:1 reduction against conventional polyethylene, with emerging registry data supporting a lower osteolysis rate, and in the AOANJRR a lower revision rate than conventional polyethylene is emerging at 7-10 years. Thinner inserts become possible, allowing smaller components and less bone resection. HXLPE is now standard practice, used in over 90% of primary TKA in the AOANJRR 2023 report, and the standard of care for all ages; it represents the most significant biomaterial advance in TKA since cement fixation.

What it costs. Cross-linking lowers yield strength and fatigue strength, which makes minimum thickness critical. Manufacture costs slightly more, offset by the reduced revision burden. Long-term data are pending: only 15-year data are available, against 30+ years for conventional polyethylene.

Alternative Polyethylene Technologies

Vitamin E Stabilised Polyethylene

The rationale. Alpha-tocopherol (vitamin E) is a powerful antioxidant that quenches free radicals, allowing cross-linking without remelting or annealing. It is made in five steps:

1. Virgin UHMWPE is blended with 0.1% vitamin E by weight
2. It is consolidated into resin
3. Gamma irradiation, typically 7.5 Mrad, cross-links it
4. The vitamin E quenches the free radicals, so no remelting or annealing is needed
5. The result is cross-linked polyethylene with a crystallinity of 50-55%, the highest of the cross-linked options

Properties. Wear resistance is similar to remelted or annealed HXLPE, while mechanical properties are superior to remelted HXLPE: yield strength approaches conventional UHMWPE, and resistance to fatigue cracking and to oxidation is excellent. In theory that is the best combination of wear resistance and mechanical properties, it may allow thinner inserts with a safety margin, and its long-term oxidation resistance is potentially superior.

Clinical status. First used clinically in 2007, it has excellent 10-year THA data, with wear rates similar to HXLPE, and emerging, promising 5-year TKA data. Use is increasing in Europe and Australia, and the AOANJRR tracks it separately, with early results comparable to HXLPE. Against it are less long-term clinical data than standard HXLPE, a slightly higher cost and the theoretical concern, not realised, that the vitamin E diffuses out over time.

Minimum Thickness Requirements

Reading the table. The 8mm minimum, with 10mm as the target, is the absolute floor for conventional polyethylene and remelted HXLPE. The better mechanical properties of annealed HXLPE and vitamin E polyethylene allow 7mm, although 8-10mm is still recommended for both. Thin HXLPE inserts under 8mm have an increased risk of fracture under cyclic loading, particularly in less conforming designs, where contact stress is higher.

Why thickness matters.

• Thicker polyethylene has lower contact stress for a given conformity and is more resistant to fatigue and crack propagation
• The minimum rises for less conforming designs: flat inserts need thicker polyethylene
• Revision with bone loss may require a thinner insert to avoid oversizing the components, 8mm being the absolute minimum
• A metal-backed patella leaves thin polyethylene; it needs 6mm at minimum and 8mm ideally

For all modern TKA, a 10mm insert should be the target when possible, accepting 8mm when anatomical constraints require it.

Australian Orthopaedic Association National Joint Replacement Registry

The overall finding. In the AOANJRR 2023 report, mobile bearings have a statistically significant higher revision rate than fixed bearings, most pronounced in patients under 65, the younger patients in whom the theoretical advantages were expected to be greatest, and there is no survival advantage for a mobile bearing in any age group. The mobile-bearing revision rate is consistently higher across all age groups.

Why they were revised. Mobile bearings had higher rates of revision for pain of unknown cause, instability, insert dislocation (specific to mobile bearings) and aseptic loosening. There was no difference in infection or periprosthetic fracture.

Market share. Mobile bearings made up 20% of primary TKA in 2003, 15% in 2010 and less than 15% in 2023, still declining, while fixed bearings are over 85% and increasing. The registry finding has led most Australian surgeons to abandon the mobile bearing in favour of the fixed.

Randomised Controlled Trials: Fixed vs Mobile

Multiple RCTs have compared fixed and mobile-bearing TKA, with similar findings:

• Kim et al. (2007) - JBJS: 165 patients randomised to fixed or mobile (LCS design) bearings, minimum 10-year follow-up; no difference in clinical scores, ROM, radiolucencies or revision rate, and so no advantage to mobile bearing at 10 years
• Woolson & Northrop (2004) - JBJS: 120 patients randomised to fixed bearing or rotating platform, 5-year follow-up; no difference in clinical or radiographic outcomes, with 3 bearing spin-outs in the mobile group and none in the fixed
• MATRIX Trial (2012) - BJJ: a large multicentre UK RCT; no difference in Oxford Knee Scores or revision rates at 5 years, despite the theoretical advantages
• Biau et al. (2006) - Acta Orthop: meta-analysis pooling 9 RCTs; no clinical or radiographic differences, and no evidence supporting a mobile-bearing benefit

Polyethylene Wear: Retrieved Components

What retrievals show. Retrieved components give direct evidence of wear patterns. In a fixed bearing, wear is typically on the tibial topside articular surface and volumetric wear correlates with time in situ; HXLPE retrievals show minimal wear at short to mid term. In a mobile bearing the topside wears much as a fixed bearing does, but the backside adds a second wear component, and HXLPE reduces topside wear while affecting backside wear less.

Why the backside wears. It articulates against a metal tray harder than the femoral component and traps third-body debris in the mobile interface. Incomplete rotation (a rocking motion) causes fatigue wear, and there is micromotion even when the bearing is not intended to rotate.

The clinical correlation. Osteolysis rates are similar between fixed and mobile bearings when total wear is considered. Backside wear can lead to locking-mechanism failure, bearing dislocation and accelerated polyethylene degradation.

Cobalt-Chromium (CoCr) Alloy

Composition. Cobalt 58-70%, chromium 26-30% and molybdenum 5-7%, with carbon, nickel and iron as trace elements.

Forged or wrought CoCr is preferred over cast for its mechanical properties, and most contemporary TKA systems use forged CoCr femoral components.

Properties. A chromium oxide passive layer gives excellent corrosion resistance, and high hardness gives the articulating surface scratch resistance. CoCr has good wear resistance against UHMWPE and is biocompatible, inert with minimal ion release. Its modulus of elasticity, 210 GPa, makes it rigid, with potential for stress shielding.

Track record. CoCr has been the standard femoral component material for over 40 years, with proven durability in 30+ year data, low wear rates against HXLPE and no material-specific failure modes. It remains the gold standard.

Surface finish. Articulating surfaces are mirror-polished to a roughness of Ra less than 0.05 micrometres. The finish is critical: a rougher finish increases polyethylene wear, and scratches from third-body debris increase it significantly.

Oxidised Zirconium (Oxinium)

What it is. The base metal is a zirconium alloy (Zr-2.5Nb), whose surface thermal oxidation transforms into a 5-micrometre layer of zirconia (ZrO2) ceramic while the substrate remains metal, keeping its ductility and toughness. The process runs:

1. The zirconium alloy component is machined
2. It is thermally oxidised at 500°C in an oxygen atmosphere
3. Zirconium atoms diffuse outward and oxygen diffuses inward
4. The surface transforms to zirconia ceramic
5. A graded transition zone runs from ceramic to metal, so there is no delamination risk

Wear. Laboratory studies show a 25-50% reduction in polyethylene wear against CoCr, because the harder, smoother surface reduces adhesive and abrasive wear and its superior scratch resistance limits third-body abrasive wear; clinical RSA studies confirm reduced wear in vivo. Registry revision rates are similar to CoCr, with no survival benefit detected yet, because follow-up is insufficient for wear-related failures, and the wear reduction remains a biologically plausible benefit for longevity. The long-term randomised trial in the evidence base (Kim 2019) found no measurable difference in polyethylene wear particles or survivorship in patients under 55.

Other qualities. Oxinium is lighter, which may improve the patient's perception, biocompatible (zirconia is inert, with minimal ion release) and more MRI-compatible than CoCr. Against it are a higher cost from more complex manufacture, less long-term clinical data (20 years against 40+ for CoCr), a theoretical concern about damage to the ceramic layer that has not been realised clinically, and no proven survival benefit.

Use. Approved for TKA since 2003, it is increasingly popular in the United States and Australia, and the AOANJRR tracks it separately, with revision rates similar to CoCr to date. It may be particularly beneficial in young, high-demand patients and is considered when planning for longevity, since wear reduction is cumulative over time.

Ceramic Femoral Components

The rationale. Ceramic-on-polyethylene is established in hip arthroplasty, with ceramic heads on HXLPE liners, which raises the question of whether ceramic femoral components could reduce TKA wear further. Three materials have been investigated:

• Alumina (Al2O3): extremely hard, low friction, proven in THA
• Zirconia (ZrO2): tougher than alumina, through a transformation-toughening mechanism
• Composite ceramics: an alumina-zirconia matrix for combined properties

What happened. Limited clinical use in the 1990s and 2000s produced catastrophic ceramic fractures and no survival advantage, and most manufacturers abandoned it. Ceramic femoral components remain investigational and are not used in routine practice. Oxinium, a zirconia surface on a metal substrate, captured the ceramic benefit without the fracture risk, so although research continues on composite ceramics with improved toughness, ceramics are unlikely to replace metal components.

Metal-Backed Modular Tibial Component

The design. Separating a metal tray (the tibial baseplate) from the polyethylene insert allows intraoperative modularity and improved stress distribution. The metal-backed modular component is standard in contemporary TKA, with excellent long-term results. Trays are made of:

• Cobalt-chrome: the most common, with excellent strength
• Titanium alloy (Ti-6Al-4V): a lower, more bone-like modulus and better osseointegration for cementless use
• Compression-moulded titanium: a porous undersurface for cementless fixation

What it gains. Insert thickness can be adjusted intraoperatively, with 8, 10, 12 and 15mm options. The rigid tray distributes load more evenly to the cement and bone, a porous-coated or trabecular metal undersurface allows cementless biological fixation, and screw holes allow supplemental fixation if needed. At revision, the metal tray can be preserved and only a worn insert exchanged.

What it costs. The tray occupies space, reducing polyethylene thickness, so a minimum of 8mm of polyethylene above the tray must be ensured. Mobile-bearing versions have poly-on-metal backside wear, the component costs more than all-polyethylene, and the locking mechanism can fail, rarely with modern designs.

Fixation. Cemented fixation, with cement on the undersurface and pressurisation, is standard in primary TKA at all ages and remains the gold standard. Cementless fixation (porous-coated or trabecular metal, press-fit) gives excellent biological fixation in selected young, active patients with good bone and is increasingly used. Hybrid fixation, a cemented tibia with a cementless femur or the reverse, is rarely used and adds complexity without advantage.

Locking mechanisms. Anterior and posterior lips on the tray engage undercuts in the insert, and some designs use a screw lock, more secure but less common. Modern designs have very low dissociation rates, less than 0.1%; dissociation presents as acute instability and requires revision.

All-Polyethylene Tibial Component

The design. A single-piece polyethylene component without metal backing, with cement fixation mandatory. It has the longest track record, being the original Total Condylar design.

What it gains. With no tray, all the available bone resection space goes to polyethylene, maximising its thickness, which is the main advantage. The component can be highly conforming without backside wear concerns, has no modular junction or locking mechanism to fail, and is significantly less expensive than metal-backed.

What it costs. Thickness is selected at the bone cut and cannot be adjusted after trialling. It can only be cemented and has no screw holes for supplemental fixation. A worn insert cannot be exchanged, so the entire component must be revised, and with no metal tray to distribute load, stress is concentrated.

Indications.

• Elderly, low-demand patients
• Primary TKA with adequate bone stock
• A wish to maximise polyethylene thickness
• Cost considerations in resource-limited settings

Contraindications.

• Young, high-demand patients, who lose the option of an insert exchange
• Revision TKA, where fixation is inadequate with bone loss
• When cementless fixation is desired
• Poor bone quality, because of stress concentration

Outcomes. Long-term survival is excellent in appropriate, elderly and low-demand, patients: the AOANJRR shows revision rates similar to cemented metal-backed components in patients over 75, but higher rates in younger patients, for whom the design is less forgiving. The most common failure mode is loosening, where metal-backed components fail by wear. Its market share is declining, less than 10% of primary TKA in the AOANJRR, though some surgeons still use it for elderly patients and there is some evidence of cost-effectiveness over 75; consider it when longevity is less critical and cost is important.

Global Epidemiology

Knee osteoarthritis - the dominant indication for TKA - is one of the fastest-growing musculoskeletal burdens worldwide, which directly drives implant demand and the economics of design innovation. In the GBD-2019 analysis (Cao et al, Ageing Res Rev 2024), there were an estimated 527.8 million prevalent cases of osteoarthritis globally in 2019, with the knee responsible for roughly 63% of OA disability-adjusted life-years; the global case burden is projected to reach about 727.5 million by 2035, driven mainly by population growth and ageing. High-income North America carries the highest age-standardised rates, and women bear a disproportionate share. This rising, ageing, higher-BMI population is precisely the group in which wear-resistant bearings and durable fixation matter most.

Major Joint Registries: Side-by-Side Evidence

National registries are the highest-powered source of implant-survival evidence and broadly agree on the core messages, with some genuine practice variation between countries.

Guidance Across Major Bodies

Differential Diagnosis: The Painful or Failing TKA

Implant-design problems must be distinguished from other causes of a painful knee replacement before any revision is planned - the commonest error is exchanging a bearing for pain that is actually infective or extra-articular.

Global Practice Variation

Documentation and Consent (Globally Applicable)