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Measuring and improving the outcome of total knee replacement surgery

Posted Feb 15 2012 5:46am

This article presents an overview of the statistics of total knee replacement surgery in the UK. This will be of help to patients contemplating surgery who would like to know how long knee replacements last and what they should expect.

According to data from the UK National Joint Registry, almost 80,000 total knee replacements are performed every year. This underestimates the true number because not all patients are entered onto the database. (Table 1)

The demand for total knee replacements increases every year and is set to significantly outstrip the demand for total hip replacements in the future. In fact, it has been estimated that the number of total knee replacements performed will increase by 200% in the next ten years. This increased demand is due to several factors. Firstly, more knee replacements are performed in women than men for all ages. There is a clear link between obesityExcess accumulation of fat in the body. and osteoarthritisA disease mainly of the large joints of the body, as a result of wear and tear of the surface cartilage. of the knee and obesity is more prevalent in women than men. Secondly, the cases of meniscal surgery to the knee following injury has increased which, in turn, increases the future risk of osteoarthritis. Osteoarthritis of the hip is very rare in a number of different ethnic groups whereas osteoarthritis of the knee is not. In patients from the Arab states and the Indian subcontinent osteoarthritis of the hip is very rare, but that is not the case for the knee and the reasons for this are not clear.

Table 1

National Joint Registry UK - Summary of annual statistics (England and Wales)

NHS 2011 Year to date: 2012
Total completed ops 111,584 4,160
Hip procedures 54,013 2,024
Knee procedures 57,211 2,124
Ankle procedures 360 12
NJR consent rate 90% 92%
Independent 2011 Year to date: 2012
Total completed ops 49,211 2,379
Hip procedures 23,908 1,096
Knee procedures 25,182 1,282
Ankle procedures 121 1
NJR consent rate 95% 96%

The goals of knee replacement surgery are to relieve pain, improve mobility, achieve a stable joint and improve function. When successful, patients regain their independence, can return to work and are able to give up taking any medication for pain. This is clearly achieved in the vast majority of patients

Historically we measured this in a very crude way. Surgeons published outcome data based on when the knee failed and further surgery was undertaken to revise the joint to a new one.

The time of the revision operation is defined as the time of “ failure” of the implant. This of course does not reflect the time at which the artificial joint was starting to fail and the patient first started getting problems with the joint.

Many of these studies are either undertaken by surgeons who helped develop different types of total knee replacement, or by very experienced surgeons. In both groups one might expect the results to be skewed to suggest a better than average result. To this end countries in Scandinavia, in particular Sweden, developed National Joint Registries, ultimately capturing nearly all the total joint replacements performed in those country, by all orthopaedic surgeons of all grades of experience, wether they worked in a teaching hospital or a district general hospital. Annual reports are then produced from the data collected. Other countries have since followed their example, particularly Australia, and all are now available on the internet for patients, and surgeons, to review.

These independent national studies are very valuable; they have identified failing implants at an early stage, highlight reasons for failure and ultimately can identify surgeons and hospitals with poorer outcomes.

This may not reflect that the knee replacement operation was badly performed as it may be that the patients have other problems or that there is associated co-morbidity that is more prevalent in one area of the country, or the world, than another. Therefore, when looking at these databases it is important to remember that the results may not always translate to another country population.

Several years ago the UK started a similar national database, which is now the biggest in the world. Now at least 96% of all total hip and total knee replacements that are performed in the NHS and private units are identified and entered onto this national database. All patients have to complete a signed consent form agreeing to be entered onto the database. The type of operation, the need for surgery, the type of artificial joint (implant), the surgeon and the hospital are logged. However, apart from the patient’s age and sex the patient’s identity is anonymous.

If the knee replacement operation has to be redone for any reason this will be defined as a failure of the procedure. The reason for this is then documented and logged.

There are a number of reasons why a further operation may be required. Apart from infectionInvasion by organisms that may be harmful, for example bacteria or parasites. (1%), where the joint may have to be removed completely, there are other “early” reasons for failure. These include ongoing pain in the knee, poor function and an unstable joint. These are often due to failure in surgical technique, not implanting the implant in the correct position, or using an implant that is inappropriately sized for that patient’s anatomy. Occasionally, early failure can be due to a poorly designed implant or problems in the manufacturing process, which damage the materials and lead to early failure of the artificial bearing.

Measuring and improving the outcome of total knee replacement surgery

Several years ago the Swedish reviewers found that one particular type of total knee replacement was being revised much more than others. The problem was a fault in the manufacturing process of that implant. This resulted in very early wear and loosening of that implant and further surgery was required to revise it. Ultimately here the manufacturer was found to be at fault. Late failure, ten years later or longer, is frequently due to the fact that the bearing has worn out, often due to the polyethylene wearing.

All of these studies document the reasons why total knee replacements fail and ultimately the operation has to be redone. What is clear from all these studies is that some total knee replacements seem to perform and last longer than others.
As the number of patients in the database increases over time it will also become clear that some units and surgeons will have better results than others. In some cases the units results may not be as good as others because they deal with more complicated cases. Therefore all data must be carefully presented and reviewed to allow for this fact.

It is clear from these studies that if we can put the implant in more accurately then the knee will feel better, have a better range of movement, and will feel more stable, achieving all the goals of a successful operation with a very satisfied patient.

The one problem with all the National Studies is that they do not reflect the patient’s satisfaction with their surgery, and how it has impacted and improved their quality of life. Several scoring indices have been developed to address this. These include scoring systems looking at functional outcome scores for the joint replacement and including scoring for pain relief.

The biggest study of its kind is in the UK, and is the patient recorded outcome study (PROMS). In this study patients are asked to fill in a questionnaire after their surgery that then generates scores for several elements and includes variables that attempt to measure an improvement in not only knee and hip function, but how this has improved their quality of life.

PROMS results

The figure on PROMS results (please click to zoom in) demonstrates a clear difference between the successful outcomes following a total hip replacement compared to a total knee replacement, in all categories that are reviewed.

I think that the reasons for this are due to the points that I have made earlier on why knee replacements are revised and the mechanical complexity of this joint compared to the simpler mechanics in the hip joint.

Total knee replacement is a very successful operation, and the worldwide demand for this procedure is significant, and much greater than that for hip surgery. The designs of the implants that we now use are much better, as our knowledge of how the normal knee works has improved, so has the engineering of these artificial joints. Our understanding of how to implant these more accurately has also improved significantly, and as we are doing ever greater numbers every year so does our experience. However we are a long way from having the perfect joint for every person’s individual mechanics. For the patient, the consumer of this, I think the more they understand about the risk and benefit of the procedure, the more able they are to make a considered choice as to whether to have surgery or not. Key questions for their surgeon are: what implant do you use and what are the results, in particular how long has it been in use, and how many operations does that surgeon perform each year?

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