This section discusses the basic components of a total knee replacement.
What is a total knee replacement?
Total knee replacement usually implies replacing all three of the areas in the knee where there is a moving joint with implants comprised of metal and plastic.
If you look at the implant from the back, you can better appreciate the two articulations between femur and tibia. In a partial knee replacement the surgeon can replace just one of these articulations, so that is why we are pointing this out to you. In fact, even in a so-called total knee replacement it is quite common for the surgeon to avoid replacing the patellar implant, and patients need to specifically ask if this was done.
What materials are used in a total knee replacement?
There are different types of knee replacement -
Metal-on-plastic knee replacement
The metal-on-plastic is the most common type of knee replacement. The metals used vary from manufacturer to manufacturer but may include cobalt-chromium, titanium, zirconium or nickel. The plastic is ultra-high molecular-weight polyethylene, and it is this polyethylene part of the common total knee replacement that tends to wear, depositing microscopic particles of plastic in the joint space.
Polyethylene in knee arthroplasty: A review Chakrabarty G, Vashishtha M, Leeder D. J Clin Orthop Trauma. 2015 Jun;6(2):108-12.
Ceramic-on-plastic knee replacement
Metal-grade ceramic implants, for example of zirconia or alumina, have been widely used in hip replacements, but are less commonly used for the knee. The materials have good biocompatibility and friction characteristics, and surgical revision due to aseptic loosening, wear, or component fracture is rare, with outcomes comparable to metallic implants. The polyethylene spacer, however, is still prone to wear, although it appears to be less than with the metal-on-plastic replacements.
Ceramic implants have the advantage over metal ones in patients with metal allergies, as the ceramics are biologically inert.
Ceramic Total Knee Arthroplasty: Ready to Go? Solarino G, Piconi C, De Santis V, Piazzolla A and Moretti B. Joints. 2017 Oct 26;5(4):224-228.
Clinical outcomes of ceramic femoral prosthesis in total knee arthroplasty: a systematic review. Xiang S, Zhao Y, Li Z, Feng B and Weng X. J Orthop Surg Res. 2019 Feb 19;14(1):57.
Ceramic-on-ceramic knee replacement
Ceramic bearings (spacers) are not commonly used for knee replacements, although they are widely available for hip replacement.
Metal-on-metal knee replacement
Metal-on-metal implants may be associated with elevated levels of metal ions and adverse reactions to metal debris.
Acute Metallosis Following Total Knee Replacement – A Case Report Klontz KC, Smith WI, Jonathan CK. J Orthop Case Rep. 2014 Jan-Mar;4(1):21-3. doi: 10.13107/jocr.2250-0685.142.
High blood metal ion levels in 19 of 22 patients with metal-on-metal hinge knee replacements. Laitinen M, Nieminen J, Reito A, Pakarinen TK, Suomalainen P, Pamilo K, Parkkinen J, Lont T and Eskelinen A. J Orthop Case Rep. 2014 Jan-Mar;4(1):21-3. doi: 10.13107/jocr.2250-0685.142..
Total knee replacement surgery
The surgeon will walk through the following stages during surgery:
- Shaping the end of the femur
- Shaping the top of the tibia
- Cementing the femoral component into place
- Cementing the tibial tray into place
- Attaching the plastic spacer
- Shaping the back of the patella
- Cementing the patellar button into place
- Closing the wound in layers
Basic arthroplasty design variants
The surgeon needs to make choices for his patient of implant design from the various available options -
- Posterior-stabilised designs
- Cruciate-retaining designs
- Bicruciate-retaining designs
- Gender-specific designs
- Fixed bearing
- Mobile bearing
- Hybrid fixation
Custom implants in knee replacement
Some patients have issues that may lead the surgeon to contemplate a custom implant. The joint may be unusually small, unusually big or deformed in an unusual way.
After MRI or CT scanning, a 3-dimensional model of the knee can be produced, both digitally and printed in 3-D, allowing for manufacture of a prosthesis specifically tailored to the individual in question.
An Approach to Developing Customized Total Knee Replacement Implants. Li X, Wang C, Guo Y and Chen W. J Healthc Eng. 2017;2017:9298061. doi: 10.1155/2017/9298061.