Most people have their knee X-ray described to them in about fifteen seconds, in language built for a referral letter rather than for a person. This is the longer version. None of it will let you diagnose yourself, and that is not the point. The point is that you can follow the conversation in the room instead of nodding through it.
Why an X-ray and not an MRI.
An X-ray is the correct first test for suspected arthritis, and an MRI usually is not. That surprises people, because an MRI sounds like the better scan. It is a better scan for soft tissue: a meniscal tear, a ligament, a chondral defect. For arthritis it is usually the wrong instrument, because arthritis is a structural problem of the joint under load, and an MRI is taken lying down with the joint unloaded.
An MRI in an arthritic knee also has a specific way of misleading. It will find a degenerate meniscal tear in a large proportion of middle-aged knees, including knees that do not hurt, and that finding then gets treated as the diagnosis when the arthritis underneath it is the actual problem. That mistake sends people to arthroscopy that will not help them.
Why it has to be taken standing up.
This is the single most common problem with the films that arrive with a referral. A knee X-ray taken lying down lets the joint surfaces separate slightly, so the gap between the bones looks wider than it is in life. The same knee X-rayed standing, with your weight through it, can look a full grade worse, and the standing film is the honest one.
The most useful single view is a standing posteroanterior film taken with the knee bent about 45 degrees, sometimes called a Rosenberg view. Bending the knee brings the part of the joint surface that actually carries load in mid-flexion into the beam, which is where wear tends to start.1 A straight-leg standing film can look normal in a knee that has clear loss further back. A lateral view and a skyline view of the kneecap complete the set.
The four things being looked at.
Radiographic osteoarthritis has been described in the same four features since the 1950s, and the grading system built on them is still the one in use.2
- Joint space narrowing. The gap between the bone ends on an X-ray is not empty. It is cartilage, which does not show up on plain film. So the width of that gap is an indirect measure of how much cartilage is left. Bone touching bone on a standing film means the cartilage in that compartment is essentially gone.
- Osteophytes. The spurs of extra bone that form at the edges of a worn joint. They are the joint trying to spread load over a wider area. They are also what makes an arthritic knee feel and look knobbly.
- Subchondral sclerosis. The bone immediately under the worn surface gets denser, and shows up whiter on the film. It is bone responding to being loaded without its cushion.
- Subchondral cysts. Small dark spaces in that same dense bone. They generally indicate more advanced change.
Which part of the knee.
The knee is three joints in one, and where the wear is matters more than how much there is. The medial compartment is the inside of the knee and is by far the most commonly affected. The lateral compartment is the outside. The patellofemoral compartment is where the kneecap runs on the front of the thigh bone, and wear there produces a different pattern of symptoms: pain on stairs, pain getting out of a chair, pain sitting with the knee bent.
This is the distinction that decides whether a partial knee replacement is even on the table. Wear confined to one compartment can sometimes be treated by resurfacing that compartment alone. Wear in two or three cannot.
Alignment.
A standing film also shows which way the leg is pointing. A knee worn on the inside tends to collapse into varus, which is bow-legged. A knee worn on the outside tends to collapse into valgus, which is knock-kneed. This matters because it is self-reinforcing: the more the leg bows, the more load goes through the side that is already worn.
It also matters for planning. Correcting alignment is part of what a knee replacement does, and how much correction is appropriate is one of the things robotic planning is used to work out.
The grade, and why it is less useful than it sounds.
The Kellgren and Lawrence system grades a knee from 0 to 4 on those four features. Grade 0 is normal, grade 2 is definite osteophytes with possible narrowing, grade 4 is marked narrowing with severe sclerosis and definite deformity.2 If a report mentions a grade, that is the scale being used.
Here is the part worth understanding. The grade on your film and the amount of trouble your knee gives you correlate far more weakly than anyone expects. A systematic review of the literature found substantial discordance in both directions: a large proportion of people with clear radiographic osteoarthritis have no knee pain, and a substantial proportion of people with painful knees have unremarkable films.3
So the X-ray does not decide anything on its own. It establishes what the structure is doing. What you can and cannot do, and how much the knee is costing you, comes from the history and the examination. A decision to operate needs both to agree, which is the subject of what to try before surgery.
What the X-ray will not show.
It will not show cartilage, meniscus, ligaments or synovium directly. It will not show inflammation. It will not show whether your pain is actually coming from your hip or your back, which is common enough to be worth ruling out and is covered in where is your knee pain coming from. And it will not show early cartilage damage at all, which is why a normal X-ray in a young painful knee is a reason to look further rather than a reason to stop.
Practical.
Bring the images, not only the report. Radiology practices in Queensland almost all provide online access now, and the access code on the report is usually enough. A report tells you what one radiologist wrote; the images let the surgeon look at the thing itself, compare it with any older films you have, and plan from it. Older films are genuinely valuable, because the rate at which a knee is changing is information that a single snapshot cannot give.
References.
- Rosenberg TD, Paulos LE, Parker RD, Coward DB, Scott SM. The forty-five-degree posteroanterior flexion weight-bearing radiograph of the knee. J Bone Joint Surg Am. 1988;70(10):1479-1483.
- Kellgren JH, Lawrence JS. Radiological assessment of osteo-arthrosis. Ann Rheum Dis. 1957;16(4):494-502.
- Bedson J, Croft PR. The discordance between clinical and radiographic knee osteoarthritis: a systematic search and summary of the literature. BMC Musculoskelet Disord. 2008;9:116.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.