What is expected of a modern knee replacement (State-Of- The-Art)
A modern knee replacement is expected to demonstrate a very low incidence of failures, relief from pain and a level of function that enables people to perform normal activities of daily living independently.
Likelihood of repeat surgery
The incidence of failure for an implant design can only be demonstrated by experience. By monitoring a group of patients with the same implant design for years after their surgery and recording if and when any need to have any part of their knee redone, a failure rate can be calculated. This is often called a ‘cumulative revision rate’ and is reported as a percentage at set periods of time (e.g. 1, 3, 5 and 10 years after surgery). For future surgeons and patients, it represents a likelihood of failure for the combination of factors applicable to the monitored group.
Many people consider 10 years to be a useful benchmark. Based on the experience of all knee replacements implanted in the UK over the last 17 years, the probability of failure within the first 10 years after surgery is a little over 3%. In Australia, based on knee replacements performed over the last 20 years, the probability of failure within 10 years is just over 5%. Values are similar in other countries. In other words, about 95% of patients have not required further surgery for at least 10 years.
Some knee replacement designs have been associated with higher rates of revision surgery and over the years some have been highlighted with ‘higher than expected’ numbers of failures and have been withdrawn from use. Equally some designs have been consistently reported with very low rates of failure.
Measuring knee stability is relatively straightforward. The simplest and most common method is done in the clinic. Your surgeon will ask you to lie on an examination bench, bend your leg slightly and grasp the top of your tibia (shin bone). By moving your tibia forwards and backwards against your knee, a small amount of forwards-backwards movement of the top of the shin relative to the femur (or kneecap which is more easily seen) can be observed. About 1cm of visible movement is as much as is usually desirable. If your knee is very stable knee it will be hard to see any relative movement at all.
Deciding what is acceptable is a little more difficult. All our knees have slightly different amounts of stability or, conversely, laxity: some people naturally have and can tolerate joints that might be called lax, whereas for others this would be uncomfortable. It is hard to replicate what any one individual would find normal. Joint laxity, however, is one of the more common reasons for patients needing repeat surgery.
While excessive joint laxity can be caused by patient factors, surgical factors and choice of implant design, it has been shown that some knee designs are more likely to be considered stable among groups of patients than others. It has also been shown that patients are more likely to appreciate a knee implant that feels stable even if it is perhaps more stable than their natural joint.
Most implant manufacturers offer different options for stability and claim designs have certain advanced features for stability. But very few provide evidence for how stable patients’ knees are or how infrequently patients are unhappy or even need repeat surgery because their knee is unstable.
To see whether a type of implant design you are interested in has a higher or lower incidence of failure for instability, you can search for ‘instability’ in the UK or Australian National Joint Registry Annual Reports. The Australian report separates types of knee replacement a little more than other registries for failure reasons.
The best sources of information on knee stability are in published reports of comparative studies where stability has been measured in groups of patients with specific knee designs. These can be found in full or with an overview (abstract) for free by searching the internet. You can use the implant name or type of implant, ‘knee’ and ‘stability’ in the search terms.
Range of motion
How much movement a knee replacement design can provide is measured in terms of the amount of possible flexion, or range of motion (ROM). It is a very common and simple measure that almost all comparative studies and cohort studies on knee replacement include. The goal of course is to provide as much movement or ROM as a normal healthy knee.
The most that a healthy knee can bend is a little over 150° – you might achieve in a full squat or sitting on your heels. Most activities do not require anywhere near this amount of flexion. For activities like walking, bending your knee to a right angle is enough. Some activities like tying your shoelaces and getting out of a low chair require a little more than right angles – about 110°.
Many manufacturers claim a very high range of movement based on theoretical limits of designs. The most common way this is done is to calculate the point that the back of the femur might impinge on the back of the tibial component assuming that the components are in a certain relative position when the knee is bent that far. But this is invariably not what limits flexion.
The amount that you can bend your knee after surgery will depend on a number of factors. With all things considered, patients achieve on average 110-120° flexion after their knee has healed. While some people desire more flexion for particular activities, achieving an average amount of flexion is adequate for most and for others only moderately limiting. Problems only really occur when joint becomes stiff and is unable to bend to a right angle (less than 75° is considered to be stiff) or to straighten fully.
Like instability, range of motion can be linked to patient-factors, surgical factors and choice of implant design. One of the main causes of poor flexion is inadequate physiotherapy while the knee heals, which allows scar tissues to become tight. Stiffness can also be caused by some sort of difficulty experienced in surgery or because a patient’s knee was stiff before surgery and the amount that the surgeon could correct it was limited. Nevertheless, some knee designs have been consistently shown to achieve high amounts of flexion in groups of patients, whether studied alone or when comparing groups of patients with different knee designs.
Stiffness is often treatable and is a less common reason for repeat surgery than others, so is less well reported in the registries. The best sources of information are in published study reports where range of motion has been measured in groups of patients with specific knee designs, whether reporting on one design of comparing different designs. These articles can be found in full or with an overview (abstract) for free by searching the internet. You can use the implant name or type of implant, ‘knee’ and ‘ROM’ or ‘range of motion’ in the search terms.
Pain, activity and satisfaction
Pain and activity are measured using patients’ own answers to questionnaires (PROMs). Questions relate to a variety of aspects including pain at night to when doing certain activities, your ability to do activities like climb stairs, get out of a car or walk to the shops. Some questions may even ask you how well your knee replacement allows you to do certain sports.
There has been a great deal of interest in recent years in patient satisfaction and what contributes to high levels of satisfaction. There are different views as to what is most important. Some aspects are subjective, like ‘expectations not being met’ and others are objective like ‘instability’ or ‘ability to do activities’.
As with functional aspects of stability and flexion, the best sources of information on pain, activity and satisfaction achieved with particular knee replacement designs are published cohort studies and comparative studies.
When reading about clinical achievements of knee replacement surgery, it is worth knowing what has been independently scrutinised and what has not.
The national registries are independent bodies and so can be expected to present data in an unbiased way. But presentation of ‘big data’ is rarely conclusive and is unable to account for the many contributing factors that may cause certain groups to appear better or worse than others.
Published journal articles are reviewed by independent experts in the subject matter. In this way they can usually be expected to be well-designed studies and have sufficient evidence for their conclusions. On the whole this is true and published articles from high quality journals are relied on by all discerning professionals. However, being able to read with a critical mind can be useful: there are plenty of published articles that show conflicting results or points of view to others.
Manufacturers all market their designs in a positive way. By law companies are not allowed to make false or unsubstantiated claims. But presentations of ideas are often well designed to create a good impression whether or not the ideas can really be proven. Look for claims that are substantiated and reference real clinical data, rather than use of creative images or videos or that reliance on computer or laboratory simulations.
More recently there are organisations, such as ODEP which we discuss in a later section, that are intended to tell you and clinical professionals which devices perform adequately. Their aim is to review both the quality and quantity of data supporting individual knee replacement designs. It may be reassuring to see a design you are interested in with a positive review from such an organisation.