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Cohort studies

When designs are new or not widely available, data may not yet be available in the national registers. In this case smaller studies will be of interest. It is common for individual surgeons or small groups of surgeons monitor their patients and publish their results in clinical journals. These ‘cohort’ studies provide early information on new designs and whether or not they are achieving acceptable success rates compared to established implants.

It is usual for these studies to focus on the rate of revision and serve to demonstrate that a device does not have any particular issues and is not experiencing a higher than expected number of failures. The information is presented in the same way as in the registry reports – a percentage is given with a level of confidence based on the total number and ability to account for all the patients (lower and upper percent values usually in brackets and often called ‘95% confidence interval’). Usefully, these study reports usually also describe the reasons that knees within the study had failed and how they might relate to characteristics of the design.

If you are interested in a new device, search on the internet for the device name and ‘revision’. You may find that some data has been published. If not, you could contact the company directly or a surgeon known to use the design. They may not be in a position to provide a specific revision rate, but they will be able to say whether or not they are aware of any higher than expected failure rates or if there have been any issues with a new design.

As well as reporting on success rates, cohort studies will also report on a range of other measures that indicate how well a design works when compared to the state-of-the-art. This will include the results of questionnaires completed by the patients (PROMs) and clinical assessment of patients’ range of motion. Although you may not be sufficiently familiar with what scores are considered good or excellent, the authors’ conclusions usually include how well they think the device performs.

Sometimes cohort studies include further analysis on some or all patients in a group. These may be done to find out whether a particular design intent is actually achieved in patients. One example might be to measure whether the components of a knee design move in the same way as a normal healthy knee. Further analyses may also be used to test the ability of a design to perform to a higher level in patients who are capable of certain activities of interest and have not experienced any other issues during their recovery. Some newer designs that are aimed at more active or demanding people will feature in studies like this.

Cohort studies can be very useful to understand how well knee replacement designs actually work. While they are limited in their ability to confirm that there would not be any more or fewer incidences of failure in a large population, they provide far more information than population studies on patients’ experience of particular knee designs or types of knee design.


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