Today on the Doc On The Run Podcast, we’re talking about three different ways to determine limb length discrepancies in a runner.
Today’s episode actually comes from one of the podcast listeners who was listening to the Doc On The Run Podcast and then sent in a great question. This comes from Sandra who asked, “Is there a best way to ascertain if a person has a leg length discrepancy? MRI, measuring, what is it?” and so that is a great question.
There are lots of ways to figure out whether or not you have a limb length discrepancy and a limb length discrepancy just means that one leg is a little bit longer than the other. That can happen two different ways. One way is if you have some kind of injury that makes you stand in a way that effectively creates a limb length discrepancy. Like, for example, if you stood with one leg on the curb and the other in the street, in the gutter, well, you would have to compensate or stand differently to hold yourself there.
Sometimes you get an injury, you can compensate and get what we call a functional limb length discrepancy, meaning that you’re holding yourself in a position that makes one leg kind of longer or shorter than the other. But what I think Sandra’s actually talking about here is a structural limb length discrepancy. And that means that one bone is actually longer than another or positioned in a way that makes it longer than another, and that can happen.
When you get a limb length discrepancy, the reason it’s important to measure is to figure out what to do about it. So, if you have an inaccurate assessment, like you think, “Just a little bit longer,” so you add just a little bit of a heel lift, well, it may not be enough of a heel lift to actually help relieve your symptoms, and it might be too much of a heel lift. In which case, you start getting different symptoms often on the other leg, the other foot that wasn’t even bothering you before.
So this is a great question. The first thing we’ll talk about is the old school method of just measuring. When we do that, we take a tape measure and we actually measure the limb length, which is usually done by putting the tape measure on the front of your hips at what we call the anterior superior iliac spine. It’s a little pointy prominence on the front of your hip bones that you can easily feel and find. So we hold the tape measure there, and then we measure to the ground. In most cases, we measure to this specific bump where you can actually feel on the inside of your ankle, this lump of bone right here. We find the end of the tibia right there, we mark it on the skin, and we measure to that mark on the skin.
Now that is how we typically measure it. Now, that oftentimes can be pretty accurate. I always like to double check it because there’s such a huge amount of error that can happen when you do that because if you measure yourself, it’s really hard to do. You can’t reach your feet, so you have to have somebody help you. So what most people do is they take a tape measure, they hold it on that bump on the hip, and then the person down on the ground is actually holding and measuring to this point on the end of your tibia or the inside of your ankle.
Now, if the person pulling the tape down your ankle actually tugs on it a little bit, that could move it a couple of millimeters or more. If you have a limb length discrepancy that’s only two millimeters and you pull on the tape and you are two millimeters off, then you could either two millimeters or make it appear that you don’t have a limb link discrepancy at all.
Most recently, I saw someone who actually was having problems. She had always had problems on one side, so I actually measured her. What I came up with was a four millimeter limb length discrepancy. But I told her, I said, “Look, there’s a lot of room for error when you’re doing with this with a tape measure. So I think that we should double check it with another imaging study to actually get a more accurate assessment and see if I’m right or not.” I told her, I said, “We could go on with this if you want, but that’s not what I would personally do.” But that’s the first way to do it, just old school tape measure, measuring the actual length of the two limbs and then comparing them.
The second way is full body x-rays. And some chiropractors do this because they obviously do lots of x-rays of your spine. They can actually do full body x-rays where you effectively get life-sized x-rays of your body. Then they can very effectively measure from the ground all the way up to where your hip joints are, the positioning of your hip, even the top of the hips to actually see if you have some tilting of the hips due to back problems, hip problems, or anything else. That’s another way to do it that is actually better than just measuring with a tape measure because you’re actually getting to measure for a structural limb length discrepancy by looking at life-sized x-rays.
The third way is my preferred way, and this is called a CT scanogram. A CT is a CAT scan, and a CAT scan is just a fancy way to do a whole bunch of x-rays. And most often this is done when we’re looking for very specific problems in specific bones like a bone cyst or a bone tumor or a crack in the bone, a fracture that’s very subtle. CTs are great for imaging bone because they use basically x-rays to do them with a computer helping.
But a CT scanogram is basically a very, very low radiation version of a CT scan that just actually does more or less a full body x-ray. So we basically just do from the feet all the way up to the hips. We get a full length measurement, and then we can individually, with a computer program, actually measure the length of the tibia, the length of the femur, the length of the entire extremity, all of that stuff pretty accurately.
Now, the reason I’m bringing that up is that patient who I had measured with the tape measure and I came up with a four millimeter limb length discrepancy, well, we got a CT scanogram. It turns out it was exactly four millimeters, and her tibia on one side was four millimeters longer than the other tibia. Her femurs, the thigh bones, were actually the exact same length.
So all of her structural limb length discrepancy was actually in the tibia or the shin bone, but that accounts for four millimeters of trouble. And so, then we can make a decision on how much limb lift to put under her on her shorter side to help compensate from that and remove some of her symptoms and correct for that link discrepancy. But those are the three ways you can determine a limb length discrepancy in a runner.