Evaluation of total knee replacements (TKRs) started in 2014.
The total knee replacement operation
Total knee replacement is a major operation which involves resecting the damaged surfaces of the femoral condyles and the upper tibia and replacing (resurfacing) them with an artificial joint. Between the two metal implants, a plastic spacer is used. A decision is also made as to whether the patella requires resurfacing or not. Fixation of the components may involve the use of bone cement (most commonly) or the bone facing surface of the implant may allow local bone to grow into it (uncemented) and thus become secure.
The ODEP methodology is intended:
• To encourage manufacturers to monitor all the options within one of their brands using the data available
• To reduce the likelihood of “Big Data” camouflaging or obscuring a poorly performing subgroup within a brand
To emphasise that minor changes may result in unexpected consequences, ODEP rates knee replacements as a total construct, which is different to hips, where the components are rated separately. There is no mixing and matching of femoral components from one manufacturer with the tibial components and inserts from another. ODEP is however aware that there have been reports of a patella resurfacing device from one manufacturer being used with femoral and tibial components from another and this situation will be kept under review.
The criteria used for the evaluation of knee implants is demonstrated in the table at the top of the page.
Why are knee brand constructs evaluated separately?
Within a brand of knee replacement, there are many alternative variants of implant combinations that may be used. The large number of alternatives means that reviewing complete brands collectively may mean that certain types or shapes or combinations of implants may perform poorly (or better), but these differences may not be identified using a collective methodology. Therefore manufacturers have to submit data for each individual combination of knee implant to be assessed by ODEP to obtain a benchmark rating for that particular design. This means that within a brand there will be many different benchmarks. Some, presently available knee replacement brands have over 20 different options that have been evaulated, some with different ODEP ratings. Constructs have been separated into different shapes, materials and fixations into the bone.
The ODEP rating will be determined by the revision data for the newest part of the construct. Thus, if the femoral component in a construct has sufficent data for a 10A* rating, but the tibial tray from the same brand has only 3 year data to support it the rating will be either 3A, A* or 3B.
There are 3 components to a TKR with a 4th (the patella resurfacing component) being optional. They are the femoral component, the tibial component (often referred to as the tibial tray) and the tibial inserts that are held into the tibial tray. Each component may have alternative shapes, materials or fixation methods:
The Femoral Component
The femoral component is designed to resurface the curved lower end of the femur. The femoral component will articulate with the polyethylene insert on the superior surface of the tibial tray and the native patella (if it has not been resurfaced) or the patella resurfacing implant.
Shapes – There are alternative shapes of femoral component. The shape is modified to accommodate cruciate-retaining, posterior stabilised and medial pivot designs. These variants are determined by the manufacturer and depend on the specific design philosophy behind the implant.
Materials – All commercially available femoral components are made of metal and most are made of cobalt chrome Certain brands have alternative surface coatings and there are alternative alloys.
Fixation – the femoral component may be designed to be fixed using bone cement or uncemented where the bone facing surface has been treated with an additional surface coating to allow bone growth into/onto the implant.
The Tibial Component
The tibial component is implanted on the surface of the prepared upper tibia to provide stability for the knee replacement within the knee.
There are two types of tibial component, Monobloc and Modular. Modular components are more commonly used.
Monobloc – There is no tibial tray. The upper surface is shaped to articulate with the femoral implant and the underside is fixed to the tibia.
Modular – In the modular designs the tray is fixed to the tibia and plastic inserts are fitted on to its upper surface.
Shapes – the tibial component has either a stem, or a keel, or both. The plastic insert then either locks in (fixed bearing), or allows movement (mobile bearing).
Materials – the tibial component may be made of different type of metal (cobalt chrome or titanium), and alternative surface coatings may be used.
Fixation – the tibial component may be designed to be fixed using bone cement or uncemented (to allow bone growth into/onto the implant).
The Plastic (polyethylene) Insert
The insert acts as a spacer between the metal tibial and femoral components. The insert is dished (in variable amounts) to allow movement of the knee and to provide stability for the joint.
Shapes – the two main shapes available depend on whether the posterior cruciate ligament is retained. These are either cruciate retaining (CR), posterior stabilising (PS) inserts. There are alternative types of insert available, including deep dished and medial pivot inserts to match the brand philosophy. Highly constrained inserts with large central posts may be used.
There are also brands now available that spare both the anterior and posterior cruciate ligaments, with a different shape accordingly.
Materials – all inserts are made of ultra-high molecular weight polyethylene (UHWMPE. Alternative types of polyethylene are manufactured in some brands that may confer different properties (such as cross-linking or addition of anti-oxidants).
Fixation – the plastic insert is locked into the tibial component in fixed-bearing TKR systems. Mobile-bearing plastic inserts are also available that allow rotation of the component on the tibia.
Patella resurfacing (button)
Many surgeons resurface the under-surface of the patella bone (the part that is in contact with the new knee joint). The decision to resurface the patella is down to a number of factors, including the surgeon’s choice, the implant design, the degree of arthritis and symptoms.
Shapes – the resurfacing may be round or oval in shape to match the natural shape of the bone. Most are designed as an onlay prosthesis with three pegs, but there are single peg inlay alternatives available in certain brands.
Materials – most are made of plastic (polyethylene) however different polyethyelene materials are available. Metal-backed components are available.
Fixation – most are cemented but uncemented versions are available.
Standard and Patient matched Designs
Most knee replacements that are used of a standard design. Certain implants are “patient-matched” or “custom design”. Please refer to this section in ODEP Methodology.
In submitting for an ODEP rating, to ensure objective consistency:
Manufacturers must fill in the ODEP submission form in its entirety. If they have any difficulty in accomplishing this, they should contact ODEP for advice.
For Standard TKR designs:
Manufacturers will need to adhere to and certify that the following criteria are met:
• The implant must be manufactured by the said manufacturer unless clearly stated otherwise. The manufacturing process and the same standards are expected to be followed for the whole of the brand being submitted.
• The implant specifications are identical across the range, other than to accommodate increases in size.
• The material composition is identical across the range. If more than one type of plastic is used (regular, X linked and additive) then separate submissions are required.*
• The implant surface finish is identical across the range.
• The locking mechanism for a modular insert must be consistent.
• The bone facing surface is identical in all sizes.
• Separate submissions are required for cemented, uncemented and hybrid options.
• 3 D or surface layer additive technology manufacture will need separate submissions.
Ratings within brands
• ODEP will rate fixed and mobile bearing implants separately.
• ODEP will rate PS, CR and any other type of total knee separately.
• ODEP will rate cemented and uncemented implants separately.
• Separate submissions are required for a knee replacement used with and without a patella resurfacing, and for different patella resurfacing implants.*
* Where a * marks a criteria please refer to the advice under the heading “Bundling and Camouflage”
Patient Matched TKRS
• Please visit the section on ODEP methodology for the reasons behind the method ODEP uses for benchmarking these devices.
• All of the above regulations need to be fulfilled except the “Shape and size” attribute. The details in the design envelope for the device must be consistent in all other ways. Within this group there still be a need for manufacturers to submit separate submissions for CR, PS, patella / no patella options.
* Bundling and Camouflage – It is accepted that some options marketed within in a brand may not be used in sufficient numbers to qualify for a separate rating, particularly with regard to whether a patella is used at the index operation and the design of the patella. It is not ODEP’s intention to exclude these important options from the ODEP process.
Patella resurfacing – Bundling guide
An implant construct (e.g. a fixed-bearing cemented cruciate-retaining TKR) may be have sufficient numbers to receive a rating without a patella resurfacing, but there may be insufficient numbers to receive the same rating with a patella resurfacing.
When this is the case, and survivorship analysis demonstrates equally good results (albeit with insufficient numbers in one of the groups), bundling of data is permitted, as long the implant construct is identical, and a single patella resurfacing is used.
A minimum number of 100 implants must be available for each construct. In addition, at least one construct must have the minimum past the benchmark being applied for.
Bundling of different patella resurfacing implants
• Bundling of cemented and uncemented patella resurfacing components is not permitted.
• Different shapes (e.g. circular and oval shaped implants) can be bundled using the same criteria as detailed above.
As has been noted in other sections of the website, ODEP expects that data sources used by manufacturers for their submissions should come from validated sources. Registry data will always be accepted from registries which comply with the ISAR standards for a level 1 registry where compliance and completeness are over 90%. Peer reviewed publications and presentations are accepted provided they cover the areas of inquiry stipulated in the submission form.
ODEP encourages manufacturers to monitor their products carefully and in-house data that is validated (or can be validated by ODEP) is accepted in certain circumstances. The data collection must fit the submission form perfectly. If in house data is submitted from hospitals in an area of the world where there is a registry, data from that registry (however limited) must be included.
Registry data must be contemporary. If a manufacturer is using NJR data it should be no longer than 6 months old. Data extracted from other registries should be no older than the date of the registry’s annual report. ODEP keeps a record of the dates at which registries publish their annual reports.
Lapsed ratings and unacceptable submissions
After due diligence, ODEP will publish, on their website, the names of implants marked unacceptable and also those with lapsed ratings. ODEP will publish, alongside the details of these implants, any explanatory comments by the manufacturer if they are submitted in a timely manner.