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Knee X-rays and arthritis

Early arthritis can easily be missed on X-ray. This article discusses several of the reasons why.

Knee patients with painful arthritic and creaking knees frequently feel confused when their X-ray report fails to mention any problem with the joint surface and their doctor dismisses the diagnosis of arthritis. But doctors often forget to specify to the radiographer (the X-ray technician) how they want the X-ray to be taken and the potentially serious arthritic changes frequently get missed.

You see, arthritis first destroys the hyaline cartilage covering the part of the bone within a joint - the white shiny 'gristle' - and generally only then does the destruction progress to the bone itself. The hyaline cartilage shows poorly, if at all, on a routine knee X-ray - doctors are used to interpreting its presence or absence by measuring the gap between the bony surfaces of the joint.

If you are lying on your back on an X-ray table, with the knees out straight, the X-ray may show a perfectly acceptable amount of 'gap'. Stand up, taking your weight with the legs bent to 30 degrees, however, and in an arthritic knee a completely different picture emerges, with the gap in many cases completely obliterated by the destruction of the hyaline cartilage. In this position, also, the kneecap may appear normal, although there may be severe destruction of the joint surface.

Arthritis does not only affect the long bones of the knee. Much of the suffering of knee patients is due to arthritis behind the kneecap, causing pain and crunching noises on descending stairs. Again an X-ray of the knee lying down will miss most of the changes due to arthritis in this area. A special 'skyline' view is necessary, taking an X-ray picture of the 'horizon' of the joint, with the knee bent to 45 degrees.

Usually, at least three views are taken at different angles to show the joint as the kneecap slides during joint movement.

A report in the medical journal, 'The Knee' (July 2000), highlights widely varying practice amongst orthopaedic surgeons in the UK when ordering X-rays, with the conclusion that from an arthritis perspective the knee joint is rarely adequately assessed by this means. This lack of comparable X-ray views between patients of orthopaedic surgeons also makes scientific comparison of X-rays from different hospitals an impossible task.

What would a well taken X-ray show in an arthritic knee?

A number of features might be evident:

  • reduced joint space where the hyaline cartilage (which is invisible or poorly visible in an X-ray) is worn away. Three millimetre or less of joint space is almost certainly indicative of hyaline cartilage loss.
  • bony outgrowths called 'osteophytes' ('bone mushrooms') - an attempt by the distressed bone to 'splint' the damaged joint
  • bony hardening, called 'sclerosis' under the areas of affected joint cartilage, indicating that there is harmful change in the bone as well as its cartilage covering
  • bone cysts - i.e. discrete areas where the substance of the bone is dissolving away

So if you have been suspecting that your knees are arthritic but have been dismissed by your doctor after a set of X-rays which did not attend to the details discussed, then you might be justified in challenging the relevancy of the views which were taken. The earlier that the arthritic process is recognised in knees, the more effective the surgical intervention, and total knee replacement may be delayed by several years, if not altogether.

Updated: 18 Apr, 2013
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