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Board Review Pathomechanics

Equinus - Overview of Pathomechanics

Marc Benard, DPM

Marc A Benard, DPM provides an overview of the equinus condition, including its etiology, clinical presentation, radiographic parameters, compensatory mechanisms and treatment options, and its influence in gait.

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Goals and Objectives
  1. Understand the clinical and radiographic aspects of equinus.
  2. Recognize the difference between open and closed chain kinetics relative to equinus.
  3. Understand the compensatory mechanisms utilized by patients with the condition.
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  • Author
  • Marc Benard, DPM

    Executive Director, American Board of Podiatric Medicine
    Co-director, Baja Project for Crippled Children
    Hermosa Beach, CA

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  • Lecture Transcript
  • Marc Bernard: This is a review of the equinus including an overview of its pathomechanics as well as a review of common compensatory mechanisms. I’m Dr. Marc Bernard, the Executive Director of the American Board of Podiatric Medicine, as well as co-Director of the Baja Project for Cripple Children. There are no disclosures with respect to this lecture or lecture content. Among the important learning objectives included in this presentation are an understanding of the clinical and radiographic aspects of equinus, recognizing the difference between open and closed-chain kinetics relative to equinus which is a very important aspect of the understanding of biomechanics in general. And finally, understanding the compensatory mechanisms utilized by patients with the condition. It’s important to understand this condition because it has significant implications in the cascade of events pathomechanically and biomechanically, that occurred during the gait cycle in the foot. And is often in fact overlooked by the practitioner or misdiagnosed altogether. And failure to recognize or accurately assess it may and often does compromise the treatment and that include both non-surgical as well as surgical care. Before we go into equinus specifically, let’s take a look at some initial concepts, what enables heel off in the normal gait cycle? Well, there are two parts to this. Part one is momentum. Part two would be muscle contracture. Momentum is important because the body utilizes momentum for any motion because it limits the expenditure of energy. When you look at a patient for instance with cerebral palsy who is propulsive and they are constantly contracting their muscles, there’s an enormous use and waste of energy. So the smooth integration of momentum along with muscle contracture is what creates effective gait. Included in heel off, we need to look at what the non-weight bearing or contralateral limb is doing. Well, the contralateral limb is swinging. As it does so the trunk moves forward, carrying the weight bearing thigh and leg forward with it. So the weight bearing thigh and leg is rotating over the planted foot. Not only is it moving over the planted foot in the sagittal plane, the swing phase limb as it comes through is causing the stance phase limb to externally rotate. The talus, likewise, is taken with the stance phase limb in an external or abducted position because the talus has no degree of freedom in the transverse plane. It’s locked in the ankle mortise. It has a degree of freedom in the sagittal plane and a very small degree of freedom in the frontal plane but not on the transverse plane. If we then look at what the calcaneus is doing, the calcaneus is on the ground being held there by frictional forces and ground reactive force. So as the talus is moving externally over the calcaneus, the subtalar joint is resupinating in response to that motion. Part two, are the muscle activity in connection with heel off is shown on the right. If you look in the upper right hand corner, as the leg moves forward or rotates forward on the planted foot as a result of the origin of the gastroc on the femoral condyles and the soleus on the proximal aspect of the tibia. And their common insertion point via the tendo-achilles unto the calcaneus, tension will build in both muscles as the leg rotates forward over the planted foot. Eventually, as can be shown on the lower right hand illustration, when the center of gravity is sufficiently forward, continued gastrocsoleus contracture, plus the preexisting momentum combined to lift the heel off the ground.

    [05:00]

    Note the insertion point of the tendo-achilles relative to both the ankle joint and the subtalar joint. It is below both joints. It’s important to be aware of this because while the learner is typically aware that the gastroc and soleus have a major effect on the activity of the ankle joint, they are less aware of the effect of these muscles on the subtalar joint, not on the sagittal plane but in the frontal plane. We will discuss this later. It’s important to understand and be able to define equinus in both open and closed-chain terms and be able to conceptualize it. In opened-chain, for example on the examining table, the equinus relates to a lack of adequate dorsiflexion of the foot on the leg at the ankle joint. However, in closed-chain, in stance or more importantly during gait, it relates to a lack of anterior rotation of the leg on the foot prior to heel lift. I repeat that. A lack of adequate anterior rotation of the leg on the foot prior to heel lift. The reason the two of these concepts need to be put together and understand in that context are that the lack of joint range of motion is more easily perceived by an evaluation of the joint directly, and that would be in opened-chain. In closed chain however, you can’t see the joint motion as well and what will happen is that you need to interpret it in terms of what the patient is doing to compensate for the lack of adequate rotation of the leg on the foot prior to heel lift. So by understanding what it means, you can better interpret it in terms of clinical assessment and we’ll go through that later in the lecture. Clinically, there are three distinct types of equinus: Soft tissue equinus, functional or positional equinus, and osseus equinus. We will cover all three. Within the realm of soft tissue equinus, by far the most common is gastrocnemius tightness. This clinical entity can be assessed by checking in opened-chain the foot dorsiflexion on the leg with the knee both flexed and extended. If the foot dorsiflexes adequately on the leg with the knee flexed i.e. when the gastrocnemius is on slack, and then comparatively when a lack of foot dorsiflexion occurs on the leg with the knee extended. And anatomically we can determine that it’s the gastrocnemius which originates above the knee and inserts into the heel as the factor that’s causing this. In long standing conditions where the foot is maintained in this position, the soleus can contract secondarily. And then we have both the gastroc and the soleus being tight, and we have a gastrocsoleus equinus. In this scenario there would be little to no difference between the relative amount of dorsiflexion of the foot to the leg whether the knee was flexed or whether the knee is extended. This would be far less common but you need to be aware of it. You need to make sure that you clinically assess it in all patients. Another type which I won’t dwell upon because we’re not going to cover it in this particular lecture is a neurologic equinus. What this would mean would be an upper moto-neuron condition in which there’s hypertonicity or spasticity causing inappropriate firing of the gastrocsoleus muscles relative to where the trunk and the leg is over the planted foot. And in this scenario, an inappropriate stretch of that muscle tendon complex will cause contracture and early heel off or other compensatory mechanisms due to the upper motor-neuron condition and not necessarily because of an innate tightness of the gastrocnemius or the gastrocsoleus complex.

    [10:01]

    Now let’s take a look at functional or positional equinus. This is sometimes referred to as pseudo equinus in some of the literature. In this condition there is no limitation of ankle-joint range of motion in opened-chain. This phenomenon is caused because the forefoot is plantar flexed on the hind foot and loads prematurely, as seen with a cavus foot type, be it rigid or occasionally semi rigid. Now if you remember that equinus in closed-chain is lack of adequate anterior rotation of the leg on the foot, in this situation the prematural loading of the forefoot on the hind foot due to the cavus foot type result in the talus being positioned more horizontal relative to its normal declination angle. If you will, the talus is rocked more horizontally and its trochlear surface is rotated more horizontally than would be normally the case if the foot were not in this shape, if the forefoot were not accessibly plantar flexed. So when the talus is in this position, the end point of motion as the leg comes over the planted foot is reached too soon in the gait cycle. What does too soon mean? It means that the superstructure is not adequately forward over the planted foot to achieve heel off in its normal point within the cycle. The end point is reached too soon and either the heel comes up early or patient may engage a different compensatory mechanism and the heel won’t come up early but there will be some other manifestation in gait that will tip you off to the fact that there is a problem going on relative to the so-called equinus. Another common type of equinus condition is an osseus equinus. In this situation, there is a bone block present that doesn’t allow anterior migration of the tibia on the planted foot. As you can see in the image on the upper right hand corner, there is osteophytic lipping anteriorly as well as narrowing of the joint space and flattening of the trochlear surface. All of these combined to limit total range of motion. As a result of this, anterior motion of the superstructure with the planted foot can’t occur in a normal fashion. In the image on the lower right, we don’t see narrowing of the joint space but as a result of adaptative change of the talar trochlear surface you see some flattening. As a result of the flattened ark of the trochlear surface, the amount of distance that the tibia needs to travel over the planted foot is extended before heel off can occur. So this is yet another less common aspect of osseus equinus. Let’s return to soft tissue equinus because that’s the most common equinus that most clinicians encounter. Let’s review its influence on the adjacent joints. The most common joints affected would be the subtalar joint, the talonavicular joint, occasionally the calcaneal cuboid joint, the naviculocuneiform joint, and occasionally the cuboid fourth and fifth metatarsal articulations. So looking at this in sequence relative to the gait cycle, most commonly we see subtalar joint pronation. Why? Because the lack of ankle joint dorsiflexion, and again, we’re talking about anterior migration of the leg on the planted foot, creates a demand from the subtalar joint for that sagittal plane motion. Recalling that the subtalar joint access is oblique to the three cardinal body planes, we can get some sagittal plane motion via pronation of that joint. Understanding that that motion has additional consequence of funnel and transverse plane which is not what is really needed but this is the only way that that joint can accommodate it.

    [15:08]

    So it’s a double edge sword. You can get the motion that you do want but you also incur motion that’s otherwise unneeded. So as the subtalar joint pronates, the talus is driven plantarly and medially. Why? This occurs so that the leg can rotate forward. But as the lower leg follows the talar position, it is rotated medially on the planted foot. Recall that the talus has a free range of motion in the sagittal plane but is otherwise captive in the ankle mortise in the transverse plane. There’s very little independent range of motion in the frontal plane. So understand that when the talus plantar flexes immediately deviates as a result of subtalar pronation, it is internally rotated and brings the leg with it. Looking at the talonavicular joint and occasionally the calcaneal cuboid joint, the motions around these joints are primarily transverse and frontal plane as a consequence of the talus rotating medially with the leg. And moving forward, the cuboid fourth and fifth metatarsals also subluxate in the sagittal plane as the mid foot is driven plantarly as a result of subtalar joint pronation and midtarsal joint resupination due to a ground reactive force. Let’s take a look at the common radiographic views that we encounter. On the right we have a weight-bearing lateral. Now these are static images and not dynamic which is what occurs in gaits. However, in long standing equinus conditions we start to see cardinal changes that are pronatory in nature as a result of the influence of the soft tissue equinus. That would be lowered calcaneal inclination, anterior break of the cyma line, often decreased talar declination of what we see here is a significant superimposition of the navicular and the cuboid which is consistent with flattening of the medial longitudinal arch. Again, this is from long term adaptative change to a hyperpronatory state induced by, among other things, soft tissue equinus. If we look to the image on the left we’re looking at a DP image which shows kite’s angle is increased. We’ve got the talus medially deviated. There is “uncovering of the talar head by the navicular” which more appropriately refer to is that the talus is moving on the navicular, not the other way around. And you need to appreciate that because the heel is captive on the ground as is the forefoot. So ground-reactive force is holding the heel and the forefoot on the ground. What is moving is the talus driven medially by subtalar joint range of motion and internal rotation of the leg. So it’s the talus moving medially on the navicular. That’s what’s uncovering the talus. It’s not the other way around. Again, you need to think in terms of closed-chained function rather than open-chained function. Often there is lateral column subluxation as it’s shown in the upper left clinical photo. Where this is occurring is predominantly at the cuboid fourth-fifth metatarsal articulations. Again, it’s another manifestation of the need to create anterior rotation of the leg on the planted foot. So patients who have a range of motion that’s adequate in the fifth metatarsal and occasionally the fourth ray will utilize that sagittal motion to achieve the desired effect of leg over the planted foot. Obviously if you look at the lower right, this is a normal patient in whom the sagittal adaptation does not occur because there’s adequate range of motion for the leg itself to move over the planted foot. So when you see patients that have this “peel off effect,” as though the foot is being peeled off the supporting surface, you know that it’s an adaptation to achieve sagittal plane motion at the expense of stability in the mid foot.

    [20:18]

    We discussed some contents to our mechanisms innate to the foot itself. However there are other areas that the body can utilize another mechanism, I should say, that allow the body to compensate. One of the things that is common is that there is a decrease in the stride length. When the stride length is decreased, it lessens the need for anterior rotation of the leg on the foot. That’s why you see senior citizens for example on a plus range of motion; they tend to take shorter strides. Thereby becoming less propulsive but also decreasing the demand of the leg to move over the planted foot. We already discussed subtalar joint pronation which is innate to the foot itself, and the sagittal plane motion available within the foot via the subtalar and mid foot subluxation. Another external area of compensation would be genuflexion. This would be utilized by the body to lessen the tension on the gastrocnemius. Recall that the gastrocnemius originates above the knee and inserts via the tendo-achilles into the calcaneus. So if the knee doesn’t fully extend, then it puts that tight soft tissue structure on some slack, allowing the leg to move over the planted foot. Less common than genuflexion is genu recurvatum. This is most often noted when present in teenage or preteen girls in which you will see the knee going into a recurvatum position. This is compensatory and that the lower leg angle to the planted foot is more posterior than it would normally be at heel contact. So if you will, the lower leg is teethed posteriorly, allowing the leg to move over the planted foot and utilize that range of motion to allow the superstructure to pass over the planted foot. And then finally there’s external rotation of the leg also perceived as an increase in the angle of gait. Essentially what this mechanism does is takes the ankle out of the equation altogether or partially so that instead of the leg rotating over the planted foot, the leg rotates over the medial arch as the feet are abducted to the plane of progression. In essence we’re talking about a patient who is becoming less propulsive. It’s an apropulsive gait type. Here we see an illustration of external rotation of the leg or hyperpronation of the foot in either case utilizing transverse plane compensation. As you can see, the foot and ankle are abducted to the line of progression. And as I previously said, this decreases the demand for range of motion on the ankle joint itself, however, at the expense of propulsivity and at the expense of the foot that can resupinate normally. Here we see a clinical photo that depicts genu recurvatum. As you can see by the red lines that bisect the thigh and the leg, we’ve got a position that is beyond straight. As you can appreciate, the lower leg is posterior to the position that it normally would be in relative to the planted foot. Also notice the position of the swing face limb so that the superstructure is able to be carried over the body without creating an early heel off. And again, this is due to the fact that the tibia and fibula are beginning more posterior than they normally would at heel contact. Let’s review some of the treatment modalities for an equinus condition. Certainly orthotic management via a foot orthosis, which is most common, or an ankle foot orthosis, is one of the most effective treatments for this.

    [25:05]

    Especially when incorporating a rear foot post on an orthotic that is slightly elevated. The purpose of this is to in elevating the heel, rock the talus plantar-ward relative to the leg thus creating space for the leg to move over the planted foot. Other shoe modifications can be beneficial such as a rocker bottom sole where there is a fixed equinus, or a rocker bottom where there is a soft tissue equinus that is not responding to physical therapy. Speaking of physical therapy, in many instances, a well-applied stretching program can be a benefit to relax the gastroc or gastrocsoleus compex, and thereby allowing the leg to move over the planted foot more easily. If none of these modalities are available or effective I should say, then one could consider surgery. There are variety of surgical techniques that could be used inclusive but not limited to gastrocnemius recession or partial recession, percutaneous tendo-achilles lengthening, open tendo-achilles lengthening, and some other more osseus-driven kinds of procedures like displacement calcaneal osteotomy and so forth. But it’s the combination of these treatment modalities together that need to be in the armamentarium of the treating podiatrist, so that by one combination of procedures of another, we get functional restoration of the foot and ankle. Here’s an example of an orthodic-based accommodation for the equinus condition which is a heel lift. Now usually heel lifts are used for a limb length discrepancy. However, they can also be used for patients with equinus especially with an osseus equinus or even a forefoot or pseudoequinus. In this case, what you’re doing is you’re bringing the ground up to the heel and allowing the trochlear surface of the talus to function in its existing position in space, but it decreases the demand of the leg to move over the planted foot. Again, depending upon the extent of the equinus condition, it would determine the height of the heel lift or the rear foot post. Embellishing treatment through shoe modification. When you do use this modality, you want to avoid rigid-soled shoes that lack any other modifications that address the equinus because the rigid-sole shoes will force the foot into a pronated position typically. We know that a pronated position maintains a lower calcaneal inclination and eventually the origin to insertion of the gastrocsoleus will shorten and it just becomes a vicious cycle. So if one is going to use a sole that’s more rigid than the shoe, you need to add some other modality such as a heel lift. That heel lift could be either external on the shoe itself or via an insert in the heel area of the shoe or some combination of the two. Again, not to beat this to death, but the intent of this is to allow the talus rotate into a more plantar-flexed position so that the leg can move over it and take the superstructure with it at the appropriate point in the gait cycle. As previously stated, you want to create some slack in the gastrocsoleus complex.

    [30:00]

    Here we have two examples of a forefoot rocker which you can see outlined in red. What this allows for is for the foot to roll over the sole of the shoe, thus decreasing the demand on the ankle joint itself. Thank you for your attention to this presentation. This concludes the overview of the equinus. It is hoped that you have some orientation to the condition, its ideology, its closed-chain manifestations, its compensations, and various modalities and its treatment. You are directed to look into this further for greater detail in future lectures.