If you go to your doctor or an orthopaedic surgeon after you hurt your knee, you might want to skip xrays and go straight for an MRI. You might feel fairly confident that you didn’t break a bone, in which case the xray might be “negative.” Most orthopedic surgeons prefer to start with xrays before ordering a more expensive and time-consuming test like an MRI.
The reason many patients want to skip x-rays is pretty straightforward. X-rays show bones, but they don’t show tendons, ligaments, articular cartilage, the meniscus and other soft tissue structures in the knee. If you think you suffered a soft tissue injury, you might think an MRI is the appropriate first step.
Here are some reasons that orthopedic surgeons prefer to start with x-rays. These tests, plus the patient’s history and physical exam, can be very helpful in making a diagnosis.
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An xray can show if you suffered a fracture.
X-rays provide very good images of bony anatomy. They can demonstrate fractures of almost any bone in the body. Occasionally a fracture can be hard to see on x-rays, especially if it’s lined up perfectly (nondisplaced). Occasionally, a CT scan is necessary for a complex fracture around the joint. An MRI can reveal an early stress fracture. X-rays, however, are generally the best test to start looking for a broken bone.
An xray reveals possible bone injuries in children.
In adults, traumatic forces and excessive stress can cause ligament and tendon injury. These soft-tissue structures are usually the weak links around the bones and joints. In kids, however, the bones, and especially the growth plates of the bones, are often most susceptible to injury. For example, an adult might injure his medial collateral ligament (MCL) when he is hit on the outside of his knee playing football. A 12-year-old athlete might fracture through the growth plate at the end of the femur if he has that same mechanism of injury. X-rays can demonstrate these bony injuries.
An xray can demonstrate some tendon and ligament injuries.
While ligaments and tendons are not directly visible on x-rays, secondary signs of an injury can show up on them. A good example of this idea is seen in some patients with ACL injuries. On x-ray, the surgeon might see a Segond fracture, or an avulsion of a very small piece of bone off the lateral tibial plateau just below the joint line on the outside of the knee. If an orthopedic surgeon sees that Segond fracture on the x-rays of a patient who suffered a traumatic knee injury, that sign almost always suggests that the ACL is injured. Assuming the history and physical exam confirm that x-ray finding, an MRI could be unnecessary.
An xray can show if you have degenerative changes, like arthritis.
While x-rays don’t show the articular cartilage that lines the ends of the bones, they can suggest more advanced degenerative changes. When the articular cartilage breaks down over time, the joint space between the bones narrows and spurs develop on the ends of the bones. X-rays can accurately demonstrate these degenerative changes. Knowing if a patient has degenerative knee arthritis can be important for planning treatment, even for a soft-tissue injury like a meniscus tear.
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From a cost and convenience standpoint, it’s understandable why patients might want to skip x-rays and get an MRI immediately. If you have knee pain, or if you suffered a knee injury, you might consider seeing an orthopedic surgeon to discuss necessary steps to diagnose and treat your injury.
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