Are you struggling with knee pain, or an ongoing issue with your knees? If so, sooner or later, you may find yourself being referred for a medical scan.
But which type? Most of us can name the four main scans we use to investigate knee problems: X-ray, MRI, ultrasound and CT. But what they are, and why you’d have one rather than another, isn’t always so obvious. With that in mind, here’s a brief guide to the four key kinds.
X-rays have been with us since 1895. The “ray” bit is radiation; as the radiation passes through your body (on its way to a sensor or sheet of photographic film), it’s absorbed by different tissues at different rates. This is why denser objects – such as bone – appear white, while soft tissues appear in darker greys. Where knees are concerned, as you may have guessed, that makes X-rays very good at showing up problems with bones. Breaks and fractures appear as little dark patches. The outline of dislocations, or abnormal alignments, becomes more obvious. Bone spurs (osteophytes) stand out from the crowd of other tissues. X-rays are also very useful for advanced osteoarthritis. The scan will show us the extent of the damage within your knee, and the positions of bones within the joint, so we can decide whether you would benefit from a knee joint replacement.
Read on: A brief guide to knee fractures
MRIs, as you may know, use magnetic fields and radio waves to create images – the name stands for magnetic resonance imaging. A combination of powerful magnets and radio frequencies causes the atoms in your body to temporarily realign (these processes are totally harmless); when the radio waves are switched off, the atoms go back to their original alignment. The resulting signals can be converted into a very detailed picture of your knee, not just the bones and general structure but the soft tissues too (cartilage, ligaments, tendons, muscles). That’s why MRIs are so useful in so many different scenarios. They give us a full picture of your joint, helping us to analyse and diagnose damage, swelling, infections, pain and weakness – including issues we may not be able to explain in other ways.
Read on: MRI simplified: how to get scanned and seen in a fraction of the time
CT stands for computerised axial tomography; some people refer to it as a CAT scan. CT scans also use X-rays to generate images. The difference is that they do this by taking multiple pictures from multiple angles. The device itself looks like a large donut. Unlike a full-body MRI, the CT scan will be set to just one part of your body (the knee and / or leg). The resulting images – which can number in the dozens or even hundreds – are then fed into a computer, which uses them to create a cross-section of your knee joint. This is very useful for examining the bones around your knee, for seeing how fractures are healing, and to assess the overall alignment of your knee joint and leg. That’s why we use CT scans for knee replacement with robotic surgery: the computer produces a 3D model of your knee, and its alignment, so it can pre-plan the operation and tailor it precisely to your joint.
Read on: Knee replacement surgery with robotic technology
Unlike the other scans, ultrasound (the name is the giveaway) uses high-frequency sound waves to create the imagery inside your knee. The major difference here is that the imagery is dynamic. Think of a pregnancy ultrasound – you see a live video-feed of the little person inside, with their movements in real time. Translated to the knee context, this means we can gently move your leg around and see what’s happening inside the joint as we do so. For instance, we might spot a little piece of soft tissue like a tendon catching on the bone. Or we might use it to examine a cystic swelling, or some inflammation within the soft tissues, or some kind of clicking that’s going on at the back of your knee. Ultrasound is also useful for guided injections into the knee, helping us to deliver long-lasting steroids for pain relief into very specific parts of the knee joint.
Read on: The Baker’s Cyst – what is it, and will it go away?
Are you worried about an issue with your knees, or in need of further investigation. If so, we can get you seen, scanned and treated very quickly. Click here to chat to one of the Chris Bailey Orthopaedics team members, or to book an appointment online.