Types of total knee replacements
Partial Knee Replacements
The knee has 3 articulating ‘compartments’: (1) a medial articulation between the femur (thighbone) and tibia (shin bone), (2) a lateral articulation between the femur and tibia, and (3) an articulation between the end of the femur and the kneecap. Based on a guiding principal of orthopaedic surgery of saving natural bone and tissue where possible, surgeons can replace just one or two of the compartments.
Replacement of one compartment is called unicondylar knee replacement (medial or lateral compartment) or patellofemoral knee replacement. Two components can be replaced at the same time or on separate occasions. If the two parts are joined together in one component it is called a bicompartmental knee replacement.
Partial knee replacement is only suitable for people with limited disease that affects one or two of the compartments. The ligaments inside the knee need to be in very good condition. This situation is less common than arthritis affecting the whole joint. Surgery is also technically more difficult because components must be accurately placed in small spaces without disrupting any surrounding tissues.
In theory, a successful partial knee replacement will restore function sooner and feel more normal than a total knee replacement because more of the natural joint and stabilising tissues are maintained. Another advantage is that it may delay the need for total knee replacement. In case of failure of a partial knee replacement, conversion to a total knee replacement is relatively straightforward.
Minimally Stabilised Knees
Minimally stabilised knees are sometimes called cruciate retaining knees because they usually attempt to retain a functioning posterior cruciate ligament (PCL). Technically this is a more difficult operation. There is also some uncertainty over whether the PCL remains functioning: published literature shows that it is often damaged during surgery without the surgeon being aware. When the PCL is retained and functioning, characteristics of the ACL and menisci are still missing. So to achieve adequate stability, this type of surgery usually requires the surgeon to tighten the joint capsule more than normal to achieve stability. Studies have shown that even when surgery is considered successful, these knees are more likely to show symptoms of instability when tested on the clinician’s examination table. Nevertheless, this type of knee replacement has become much more common and is widely used worldwide.
Posterior Stabilised Knees
Until recently a ‘posterior stabilised knee’ was the most commonly used type of knee replacement around the world. This design stabilises the knee using an upright post at its centre. The upper (femoral) component contacts the post towards the end of flexion or extension. The post blocks the femur from moving too far backwards in the extended knee and from moving too far forwards in a flexed knee. It also provides a centre of rotation for the knee’s natural twisting action. Because the post is in the centre of the knee, it means the plastic tibial bearing can be symmetric.
This concept was first used in the 1970s and its basis was that the cruciate ligaments (ACL and PCL) work like a 4-bar link hinge. Unfortunately the idea was based on incomplete review of historical work that had already concluded that the concept is flawed. Ligaments could only work this way if they were rigid structures; they are not rigid and only work in tension.
Over the years this type of knee design has worked reasonably well for patients. But national registry data and comparative studies show that current patient expectations – for extremely low failure rates, feeling of normality, and return to activity – are not consistently being met with this type of device.
Medially Stabilised Knees
Very few designs of TKR around the world have included a medially stabilised knee design. The first design with this concept was implanted in the early 1990s. It was developed by the same surgeon and engineer team that had developed the first successful condylar knee in 1969 and was a 4th iteration of that original design. It was unique in that it included a ball-and-socket articulation on the medial side of the knee and a roller-in-trough articulation on the lateral side. The idea for this functionally asymmetric design was that it would mimic the natural combination of freedom of movement and stability of a natural healthy knee. The asymmetry also made it possible to position the groove that guides the kneecap towards the outside of the knee – like the normal knee.
The design was based on hands-on experience and study of how the knee works. But it was a series of works in the late 1990s using MRI and other imaging methods that essentially demonstrated this to more closely replicate natural knee movement. The ball and socket articulation stabilises the medial side taking into account the removed meniscus and ligaments. The rolling lateral side allows freedom of movement, and the asymmetry allows natural movement of the patella.
The design was distinctly different to the established designs and was not quickly taken up. More recently most implant manufacturers are working towards this concept. But rather than introducing bold step change, the major companies are evolving their designs with increasing bias to stabilising the medial side and with marketing claims of more normality. A selection of smaller companies has developed versions of medially stabilised knee in the last decade, each with small differences. The original design and an evolved version from the concept originators is being used more widely than before and demonstrating excellent outcomes for patients.
Hinged knee designs are still used, but they are reserved for severely damaged knees that don’t have sufficient stability in the joint capsule. This amount of instability can be caused by trauma, progression of disease, gross deformity or difficult revision knee surgery. For patients that require a hinged knee, the welcome relief from pain and return to a modest level of mobility warrants that this concept should remain available.
Custom Knee Implants
Custom knee implants may be required for patients with particular deformities or where the amount of bone required for a standard implant is diseased or not present. Healthcare providers make special provisions for occasional use of these devices because they cost more and take longer to design and manufacture. These implants are usually based on an existing standard design with modifications to surfaces that are used to fix the implant within the joint.
With the digital age and advances in plastic and metal ‘printing’ technologies, producing implants and surgical instruments customised for individual patients has become more cost-effective. And a small number of companies are also now marketing custom knee implants for standard use.
The concept is similar to a tailor offering a semi-bespoke service where customers select an off-the-peg clothing item and are then offered certain adjustments to achieve a better fit.
Custom knee implants are based on a standard implant design that may be a ‘posterior stabilised knee’ or a ‘minimally stabilised knee’ design. Using a 3D scan of the patients’ joint, adjustments to the size and the profile of the part of the implant that is fixed to bone are made prior to manufacture. The goal of this is to achieve a near-perfect fit for the individual’s bone interface.
Marketing claims suggest that a custom knee may feel more normal to the patient. A well-fitting implant is appealing, but custom designs do not work any differently to standard knee implants. And while patients can perceive differences in movement and stability between knee designs, there is no evidence to confirm that patients with a customised fit would perceive any difference to a standard implant. Nevertheless the price for this technology is high: custom implants cost 2-3 times more than standard implants.
It is worth noting that some news articles have promoted custom knee surgery where the implant is not customised at all. It is more common nowadays that customised instruments are used. Using a 3D scan of a patients’ knee, custom plastic 3D-printed guides can be produced to assist the surgeon in positioning the implant during surgery according to a surgical plan created on a computer screen. Although this may be referred to as ‘custom’ surgery, in fact all surgery is customised whether standard or 3D-printed instruments are used: surgeons always need to fit implants correctly to individual patients’ anatomy and unique arthritic condition.