Guido LaPorta, DPM, MS discusses the biomechanics of an equinus deformity and the impact on treatment programs. Dr LaPorta reviews what angles are important to measure when accessing an equinus deformity and how these angles can help in determining the appropriate medical or surgical intervention that is best for that patient.
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Release Date: 03/16/2018 Expiration Date: 12/31/2018
Guido LaPorta, DPM, MS
Director Podiatric Medical Education
Community Medical Center
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Male Speaker: Today, I have a two-part presentation. Second part will be tomorrow morning but I'm going to talk about biomechanics and the implication that it has in our treatment programs. Those are my disclosures. They have nothing to do with this topic. And the goals and objectives here are to understand the effect of equinus on foot function, how to perform a comprehensive clinical exam, and how to describe the effect of the osseus framework on equinus. I was trained in residency in California. My attendings were [indecipherable] [00:45] Scarlato. I got equinus to deaths. And I can tell you that we successfully eradicated all equinus in the Bay Area in the two years that I was there. I really think that equinus is the most devastating biomechanical disorder that affects the human foot. In fact, I would go so far as to say that in my opinion it's just as important to screen kids for equinus as it is for scoliosis because the effects it has on function are devastating. And we'll talk a little bit about that today. When I was out there, we paid great attention to the torque within the Achilles tendon and we use this knowledge in order to affect a slide lengthening of the gastrocnemius tendon only. And this was done by cutting 2/3 of the tendon medially, proximal, and 1/3 of the tendon laterally, distally, and we got a pretty good lengthening and they were very successful. But I would point out in my observation is that there are ways to lengthen the posterior muscle group without cutting the tendon. Controlled lengthening I think is very important. And tomorrow we'll talk about the Baumann procedure and how it does that. But what effect does the Achilles tendon have on the foot? And if you look at the relationship of the attachment of the Achilles tendon to the axes of the various joints, you will notice that the greatest vector for the pull of the Achilles tendon is the oblique axis of the midtarsal joint. And consequently, the greatest effect that you will see clinically is collapse of the midtarsal joint. There is very little dorsiflexion that can be gotten from the subtalar joint. But the subtalar joint must pronate in order to unlock the midtarsal joint. The deformity is purely an oblique axis deformity. If we look at the gait cycle, we get some idea of when this gastrocsoleus complex is active. And if you look at phasic activity, you see the synergistic action of these two muscles. And by that I mean, the gastroc and the soleus have different functions pretty much but they summate so that they work together. Jacquelin Perry was the first to describe the effects of a tight heel cord on function. There's still an argument as to whether or not we should call this equinus. As long as we have a good definition of what we're talking about I think we can accurately cause this an equinus deformity and I'll mention that in a minute. But of all of the functions of the gastrocnemius, let us just say that its most important some focus is to literally lift the heel into propulsion. Now having said that, there's only one way it can do that. And the way it does that is that the soleus helps it by decelerating the tibia, allowing the need to re-extend and as soon as that occurs, the gastroc is in a perfect position to cause a forceful lifting of the heel into propulsion. Scarlato in his compendium in the early '70s talked about equinus deformity and he describe it as a lack of 10 degrees of ankle joint dorsiflexion when the subtalar joint was neutral.
He said that it could occur because of either muscle tightness or osseous abnormality. And he described three forms, uncompensated, partially compensated, or fully compensated. Uncompensated equinus is when there is an insufficient range of ankle dorsiflexion or pronation in the subtalar joint to allow the calcaneus to contact the ground. This probably represents less than 1% of all equinus but it's true equinus. Patients should walk on the balls of their foot and the heels could never touch the ground. In partially compensated equinus, there is sufficient ankle dorsiflexion to get the heel on the ground but not enough dorsiflexion to get the necessary 10 degrees which is necessary for normal gait. And in fully compensated equinus, you have sufficient ankle joint dorsiflexion and/or subtalar and midtarsal joint pronation to not only let the heel hit the ground but also to get a 10-degree excursion of the leg over the weightbearing foot. So in this regard, if we use this scheme, we can call this equinus but I think what you need to do is use the adjectives partially or fully compensated. There are many ways equinus can compensate. Patients can externally rotate their limb. Patients can walk in a knee flexion or a knee hyperextension. They can have an early heel off, the so-called bouncy gait. They can pronate the subtalar or midtarsal joint. They can shorten their strides so they don't need to get as much out of their ankle. And they can have proximal compensations. And here you see two effects of the equinus foot. On your left, you'll notice that the side opposite the equinus foot has a shorter stride length. And this serves to require less motion out of the equinus ankle. On your right, you see an example of a normal stride length or a longer stride length but the only way this can be accomplished in an equinus foot is to have excessive rotation of the hip joint. Consequently that is responsible for a lot of more proximal symptoms in these equinus patients. Additionally, you may see the situation where in order to clear the ground during swing, the patient has an exaggerated elevation of the thighs so that they can clear the ground and this can cause more proximal symptoms in and around the knee and in the anterior thigh. Externally rotating the foot has a very beneficial function in people with equinus. The reason is that if you look at the normal foot and you compare the pivot point for propulsion relative to the flexion axis of the ankle, you'll notice that in the rectus foot, the distance between those two points is quite significant whereas if that patient externally rotates the foot, the distance between the flexion axis and the rotation point becomes much less. So it is much easier for that patient to walk. Additionally, they can have proximal compensation and this can occur in anyone of a number of ways. There can be a backward tilt, there can be a forward lean, there can be a recurvatum of the knee, or an exaggerated lordosis. All of which can be compensatory mechanisms for equinus. So the only thing I'm going to talk about today is peritalar subluxation, flat feet. Why do I call it peritalar subluxation? Well, I had a conversation once with Ted Hansen in Seattle and I asked him that very same thing. Why do you call this peritalar subluxation? And he said very simple reason. Number one, he really thinks it is. But number two, insurance's don't pay for flat feet but they do pay for subluxations.
So he calls this a peritalar subluxation. If you have an equinus deformity, it's a dorsolateral peritalar subluxation. So here's a foot and let's look at this and by all x-ray criteria, this is a peritalar subluxation. This foot is at least hyperpronated and is tending towards flatness. And if you look at all of those angles you see the cuboid, the abduction angle, the talocalcaneal angle and I measure the talo-second metatarsal angle. I think the paper done by Graham shows that this is a very reproducible angle and I think it means much more than measuring a talo-first metatarsal angle given the fact that the first metatarsal has a range of motion that can really skew your measurements. So this is a foot tendoning towards flatness. But look, lo and behold, clinically, this foot has a valgus heel if you look at this foot on the hindfoot alignment view and you draw a few angles, you can tell a few things. First, the ankle is at 90 degrees to the tibia so there is no contribution of the ankle joint to any valgus positioning of the foot. That's called the lateral distal tibial angle. I wanted to point this out and Paley was the first one I hear really talk about this happening clinically. I always knew that if you had a varus foot that it could compensate by going into a pronation physician and you could do that very easily because the ground reactive vector is lateral and that tends to pronate the foot. But I always thought it was impossible for the subtalar joint to supinate in order to decelerate a pronation motion. I thought that the musculature wasn't strong enough and that the ground reactive force would overpower that. But he showed me that I was wrong. In this particular case if you look at this patient has clinically a valgus heel. But if you bisect the heel in the tibia, you notice one thing, they're parallel to each other. There's no valgus heel here and they're one centimeter apart. That's the definition of normal. So when I look an x-ray like this and I see a heel that's parallel to a bisection of the tibia and about a centimeter lateral, that's perfect. That's my goal. Alright. So, my description of having a valgus heel has to be altered. But, look at this, and this is my point about what we do clinically. I have outlined the soft tissue on this x-ray and I have bisected the soft tissue and that if you bisect that clinically is in valgus. So, much of the valgus we see is a soft tissue picture and has very little correlation with where the heel really is. And if you happened to have an adult with some swelling around the posterior tib, they even look like they're in more valgus. Okay. So I think we have to be careful about what we call valgus and what really is happening in the foot. Now, on the lateral view, if you take nothing from this lecture but this I'll be happy. Most of us including me thought that the ankle joint on a lateral view was at right angles to the shaft of the tibia, it is not. In fact, the normal is absolutely 80 degrees. There is a 10-degree upslope of the ankle joint to the shaft of the tibia. If you were to look at the knee, you would notice that they knee joint is angled back 10 degrees. It's not perpendicular to the shaft of the tibia. Why is that? That's because in single support during stance, when the opposite limb is in swing phase and your knee is flexed 10 degrees because it never really extends fully during gait, those joints become parallel to the ground.
That's the way the body is built to provide stability under the stress of single-leg support. So the distal tibial articular surface must be less than 90 degrees and the ideal number is 80. Another point, the line bisecting the tibia passes through the lateral process of the talus. If the lateral process to the talus is distal to that line, you have a significant abnormality within the ankle joint. So these are all things that I think I look at when I'm trying to evaluate a foot to see not only what type of pathology is present but if I'm going to do something invasive, what's the best thing to do. So if you take this particular patient who has an 80-degree anterior distal tibial angle and you dorsiflex them 10 degrees, they do it with ease. They do it with ease. There is no impingement anteriorly. But if you take the same patient and they have a 90-degree anterior distal tibial angle, and you try to dorsiflex them 10 degrees, the front of the ankle impinges. Okay. Two points from that. Number one, anterior impingement exostoses is not a diagnosis, it's a sign. And it's a sign that the anterior distal tibial angle isn't normal. Okay. Second thing, if you have an equinus patient and they have a 90-degree angle of the distal tibial, you can stretch them for the next 20 years. They will not get any additional dorsiflexion when they're walking. So the only way you can treat them is with a heel lift. Talk to Dr. Schoenhaus about things you can do to an orthotic for patients who have an osseous equinus. This is an osseous equinus. It's called tibial procurvatum. In these patients, your orthotic is the mainstay of treatment. The third point is if I lengthen the posterior muscle growth, I will get excellent dorsiflexion on the table and not one degree more when the patient is walking. The reason for that is that their osseus alignment will not allow more dorsiflexion without impinging in front of the ankle. Another angle, anybody measure this? I do it all the time. I get criticized for doing this. You treat in the x-ray and I treat in the patient. Well if the x-rays aren't important, don't take them, okay? But I think they are important especially if you classify and do these the same way all the time. Here's a patient who's pronated, has peritalar subluxation and their tibial talar angle is a 130. The normal is 118, alright. I'll give you 120 if you want but the normal is 118. There is this 130. So they have a 12-degree deformity in the position of the talus, okay? Now, if you happen to measure zero degrees dorsiflexion during your clinical exam, or even a couple of degrees of plantarflexion as their maximum amount of dorsiflexion, so they have 10 to 12 degrees of equinus and this angle measures 12 degrees, that means that all of the deformity is in the ankle joint. It's a true ankle joint contracture. If however, the amount of the tibiotalar angle is less than the amount of equinus that you're measuring clinically, you have a deformity somewhere else. You need to go find that deformity before you decide what your treatment is. I'm not saying that all of these abnormalities need to be treated. I'm just saying that you and I can't make the decision whether or not we should until we know what's there.
And consequently this exam becomes very important. So if the amount of talar deformity equals the amount of measurable deformity in the ankle or equinus then the deformity is purely in the ankle joint, it's purely contracture, and it's all soft tissue. Here's the lateral talocalcaneal angle. Lateral talocalcaneal angle, the normal range for this angle is 50 to 58 degrees. In this pronated foot, peritalar subluxation foot, this angle measures 58 degrees, purely normal. And therefore, if you bring this patient to surgery and do an arthroereisis to lift up the talus, you're overcorrecting the foot. Absolutely overcorrecting the foot. And you heard Dr. Schoenhaus say, don't do that. In fact, if you're going to air, air to the valgus side not the varus side. There's another important angle, how long does it takes three minutes to measure all these angles. It's not a big deal, alright. But it gives you a ton of information. I think I screwed up a little. Okay, Meary's angle. Alright, here's Meary's angle, very important, what should this be? Zero to two degrees. So when you measure 22 degrees here and you have a normal lateral talocalcaneal angle, you know your whole hindfoot collapsed. Alright, it wasn't a talar plantarflexion, it was a hindfoot plantarflexion. This is through midtarsal joint compensation. You can see these relationships and how they pan out and we look at this and where is the biggest deformity here? The biggest deformity is in the hindfoot, okay? So we have a tendency to say that the forefoot is dorsiflexed down the rearfoot. Uh-uh. The rearfoot is plantarflexed on the forefoot. There's only four degrees of deformity in the forefoot, hardly anything. There's much more 16 degrees of deformity in the hindfoot. There's much more deformity in the hindfoot. Alright, what's the prime cause of that? Tight heel cord. Alright, so here's our friend and I'm going to shock you with this but I measure equinus on x-ray, alright. I certainly do the clinical exam but I don't trust it. And what I found is that if I have a normal 80-degree angle of the distal tibia and I take an x-ray and measure the amount of available dorsiflexion at the ankle, most patients get 20 degrees, or more, okay. So I propose that if equinus is the issue, and you're trying to decide whether or not a posterior muscle group lengthening should be done or whether you can handle it with orthosis, I think you should get an accurate measurement of somebody's true ankle joint range of motion. And that can only be done on an x-ray. Okay, I'm going to stop there. Are there any questions about equinus and excuse the expression "flatfoot"? Lateral or dorsolateral peritalar subluxation. Yes, no? Okay. So I'll see you tomorrow morning and we'll talk about true equinus. I think we're done, are we not? Okay. Have a good evening.