Referred Knee Pain

Let's talk today about the concept of referred knee pain, that is, pain that seems to be coming from the affected knee joint, but in reality is coming from somewhere else. Many times in my practice, I would see a patient that complained of knee pain, only to find, after a thorough workup, that there was no observable knee pathology. These cases were invariably frustrating for both the patient and for me. If you're the patient, all you know is that your knee hurts and here's this guy telling you that it isn't your knee. What are you supposed to think?

The first principle is that all joint pain, like water, flows "downhill." In other words, the cause of the referred pain has to come from ABOVE the affected joint. An injured ankle generally doesn't refer pain up to the knee, unless you've also injured that knee. With this principle in mind, let's consider a few other problems that are known to mimic knee pain.

Back problems, especially herniated discs, can refer pain to the knee on the side of the herniation. This is most common at the L3-4 or L4-5 levels in the lumbar spine. That means that the disc that lies between the 3rd and 4th lumbar vertebrae has squished out its jelly-like core (the "nucleus pulposis"), like toothpaste out of a tube, into the spinal canal, or the foramen ("window" in Latin) where the nerve roots emerge. This herniated disc material applies pressure on the spinal cord, or a specific nerve root, on its way to the knee. You would then feel the pain in the knee joint, because the nerves going into the joint are irritated or compressed, even though there's no pathology in the knee joint.

In the same way, a tendinitis, an inflammation of the long muscle in front of your thigh, the rectus femoris, (Figure 1-E) can present with knee pain. A doctor can often differentiate this from true knee pain by examining the upper attachment of the muscle tendon at the "point" of the hip (the anterior inferior iliac spine). Its next door neighbor, the Sartorius muscle (Figure 1-D), is attached just above it, at the anterior superior iliac spine, and it, too, can refer pain to the inner (medial) side of the knee joint below.

The hip joint, too, can refer pain to the knee. In fact, one of the most common errors made by Family Practicioners, Internists, Chiropractors and even some orthopaedists, is to fail to evaluate and x-ray the hip joint, when a patient comes in for a knee problem. It was always a favorite Orthopaedic Board Exam question, too.

I can't tell you how many times I found unsuspected arthritis of the hip, when a patient came in complaining of knee pain. They were always skeptical when I recommended x-rays of the hip joint in addition to those of the knee (probably figured I was "padding" their bill), but they were always amazed when I solved their problem, by identifying and then treating hip disease that everyone else had missed.

The most common reason that it had been missed is "Tunnel Vision." It's very common for everyone, including the doctor, to focus on the apparent site of pain, and fail to consider the other possible sources of those symptoms. So, if you are ever in to see the doctor for your painful knee, be sure he or she at least considers, and preferably examines, your hip and your back, especially if your knee exam and x-rays don't readily disclose the reason for your pain. If they don't, you haven't had a complete evaluation of your knee. If they don't look, they won't find the cause in those few cases where the pain is referred.

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