The Convergent Talus: Episode 4 - Charcot Marie Tooth Disease

Key Clinical Takeaways

• Charcot-Marie-Tooth (CMT) Disease as a Prototype for the Convergent Talus
  CMT exemplifies the biomechanical and neuromuscular imbalances that create a convergent talus—with progressive cavovarus deformity, adduction of the foot, and subtalar joint supination. Recognizing early myotendinous dysfunction, particularly tibialis anterior weakness and peroneus brevis paralysis, is essential for preventing chronic ankle varus and instability.

• Importance of Addressing Myotendinous Imbalance in Surgical Planning
  Effective correction requires comprehensive tendon balancing, not just bony realignment. Procedures such as tibialis posterior transfer through the interosseous membrane and peroneus longus-to-brevis transfer restore equilibrium between antagonistic tendons, mitigating progressive deformity and improving long-term alignment.

• Radiographic and Procedural Precision for Durable Outcomes
  Using Meary’s angle to locate the deformity apex guides targeted osteotomies (e.g., modified Cole, dorsiflexory wedge, or naviculocuneiform correction). Surgeons should avoid distal overcorrections and instead focus on restoring midfoot-to-rearfoot alignment and preserving ankle joint function through procedures like gastroc recession or supermalleolar osteotomy when necessary.

Transcript

Welcome back to Podiatry Today Podcasts, where we bring you the latest in foot and ankle medicine and surgery from leaders in the field. Today, we're continuing our series with Dr. Visser on the convergent talus, this time focusing in on Charcot-Marie tooth disease. Dr. Visser, as you know, was the president of the St. Louis Podiatric Medical Society and a past president of the Missouri Podiatric Medical Association. Among his many accolades, he served as an examiner of the American Board of Podiatric Surgery and served on the Missouri State Board of Podiatric Medicine over several governorships. He's been a residency director for 40 years, most currently, at SSM-Depaul Foot and Ankle Reconstructive and Trauma Surgical Program in St. Louis, where he has trained at present over 114 residents. Dr. Visser, welcome back, and we're so excited to get into this next topic.

And this strikes me that there's another uncommon podiatric condition that could pop up that might also apply. What do you think about Charcot-Marie tooth disease as it relates to the convergent talus? Well, I would say that Charcot-Marie tooth disease is probably the prototype of the convergent talus. As we know with CMT and Charcot-Marie tooth, it affects the forefoot, it affects the midfoot, and it affects the rear foot, but it also can affect the ankle, again depending on the relative overall chronicity.

And the big issue is the myotendinous structures. Myotendinous structures are very important in CMT. Sometimes we get the idea that, you know, you have to treat it with a dorsiflexory wedge osteotomy or you have to do a triple arthrodesis in severe cases because there's a lack of understanding of what the neuromuscular imbalance that is being created and creating a nonplantargrade type of foot. When we talk about the forefoot, we talk about intrinsic atrophies, one of the very earliest things that can happen in CMT. And then we get what we call extensor substitution phenomena, and that will lead to hammering of the digits.

Interesting, hallux malleus, is something that can be a very early subtle change in the CMT foot and basically what happens in this particular instance is when we talk about the biomechanics and this goes with Dr. Root in his book explained that in dorsiflexion basically when we do have swing phase what happens the tibialis anterior tendon, which inserts in the area of the first met cuneiform, will pull up and maximally dorsiflex the first metatarsal. What does this do? This locks up the first metatarsal phalangeal joint. The first metatarsal will be dorsiflexed as much as it'll go, and the proximal phalanx will be locked in that dorsiflexed position.

This is a so-called rigid beam effect. And then what happens, the first, the extensor longus tendon and muscle, then fires. And because the metatarsal phalangeal joint is locked, it will cause retrograde firing back at the ankle joint. In other words, all its force of dorsiflexion and swing will occur at the ankle joint.

Can you tell us a little bit about what to expect in the early stages here?

In the case of CMT, one of the earliest neuromuscular units to be paralyzed is the tibialis anterior. And when the tibialis anterior is either paralyzed or weakened, what happens? And no longer is able to dorsiflex the first metatarsal and lock the metatarsal phalangeal joint, and thus we do not form our so-called rigid beam effect. So now, in swing phase, as the foot is dorsiflexing, where is all the force of the EHL going to go? It's going to go at the IP joint and the MP joint, and we're going to get a hallux malleus. So one of the earliest changes that we can see in CMT as a hallux malleu. The other then comes to the midfoot area, and in the area of the midfoot, we have the fact that the peroneus longus is basically strong. The tibialis anterior, as we described, is weak.

And as a result of the weakness, what happens? It has a force in plantarflexing the first metatarsal, and thus changes in the midfoot, and we use Meary's angle here again, relationship of the bisection of the talus to the first metatarsal, we see that that bisection often will be at the first met-cuneiform. So oftentimes you've seen in CMT where the patient has a Dwyer osteotomy and they have a dorsiflexory wedge osteotomy of the first metatarsal. This is not very complete. You have to think the whole process through. And remember, this is a progressive disease state. It is progressive. And if you do something shortchanged like that, it's not going to correct the patient long term.

The patient can literally be corrected long term with basically osteotomy and soft tissue balance. The tibialis posterior tendon - this particular myotendinous structure is its antagonist, the peroneus brevis, is completely and oftentimes very early involved and paralyzed. And what does this do? This produces an adduction and a cavovarus attitude to the subtalar joint, and there's where convergence of the talus occurs here, and it creates close kinetic chain supination. Now, depending how long this has been present, depends on how far and proximal Meary's angle will go. As it hangs around, Meary's angle, because of this imbalance the TP and the brevis tendon and the subtalar position, it will begin to actually bisect in the area of the navicular medial cuneiform joint area. And in that particular instance,what is very key to treating CMT is to deal with the tibialis posterior tendon.

Oftentimes is not because people are not very well trained or don't feel very comfortable in performing a tibialis posterior transfer. So basically, what you have to do in this particular instance where we have Meary's angle at the NC joint, we need to transfer the tibialis posterior through the interosseous membrane to the dorsum of the foot and perform a non -phasic transfer of the tibialis posterior tendon. We also, from mild tendinous balancing, need to take the peroneus longus tendon, which is very strong in overpowering the TA. And basically, we want to transfer it to the brevis. This will remove the deforming force on the first ray, and it now will replace some of the weakness of the peroneus brevis and given an abduction type of force. After completion of the tibialis posterior transfer through the interosseous membrane, the tendon is basically placed in the area of the intermediate cuneiform.

At this point, then, the platform of the foot needs to be assessed and addressed. This we do by viewing Meary's angle on a lateral view. If the apex of the deformity is at the first met-cuneiform joint, in that essence, we will then do a dorsiflexory wedge osteotomy of the first metatarsal, or in some instances, a first met met-cuneiform osteotomy if we feel there is a significant planterflexion of the first metatarsal.

However, in a large number of these cases, the apex is proximal.It's at the navicular met-cunieform joint, and by doing a more distal, it can create what we call a Z midfoot deformity. So, in this essence, we like to consider to do a modified Cole osteotomy. And by modified, I mean it is a tapering wedge beginning medial to lateral. It's a dorsiflexory wedge that involves primarily the three cuneiforms and the navicular in its joint. As we know, normally on these types of cavus deformity, the medial and central rays are more involved in plantarflexion than the lateral rays. So, the wedging is done at that particular level. Laterally, then we do have to address this, but usually it's through a separate incision and it's a cuboid osteotomy, in other words, not really a wedging unless it's significant. And in that case, then what we do is we shift the distal fragment downward to dorsiflex it without significant amounts of dorsiflexion.

In this way, we will have a platform from medial to lateral that allows the midfoot to be basically aligned with the rear foot without any significant protuberances distally in the metatarsal heads.

What about more proximal considerations?

We have to address what's going on at the ankle because when we take a look atthis foot on a lateral view, we see a high calcaneal pitch somewhere around 35 degrees. We see a bullet hole sign because the tail is dorsiflex and maximally abducted in that particular instance. The gastroc to Silfverskiold important to differentiate that so the approach then to deal with this would be again start with the rear foot probably a gastroc recession address the heel varus Dwyer osteotomy probably with a modification of a superior shift of the tuberosity. Actually, it's a Mitchell modification in that situation.

You then want to transfer the peroneus longus to the peroneus brevis in the area of the midfoot. And then you need to transfer the tibialis posterior tendon through the interosseous membrane to the dorsum of the foot, the so-called Watkins procedure, to balance the rearfoot to the midfoot. Now, that's a lot of procedures done, and most often the forefoot procedures will be held in a second operation. What I like to do is basically by the time the sutures are being removed and the incisions are healed at that time, yet the patient is still in a non-weight -bearing state. I then go ahead and do the forefoot procedures, which include a Jones sling with an IP fusion to deal with the hallux malleus deformity, created the way we described it. Also, remember, there is some intrinsic atrophy that contributes to the hallux malleus also. Effect of the abductor hallucis, the flexor hallucis brevis, the adductor hallucis involved in this situation can also contribute to the hallux malleus.

That's a lot to consider there. What else might have to come next?

Then because of intrinsic loss, we consider then a Jones or Hibbs procedure to basically get the deforming force of the long extensor, which was created by extensor substitution, and consider usually Labmbrinudi type of a fusion in that instance. And in that particular way, by understanding the pathomechanics and the biomechanics of the foot and this prototype disease state that occurs, we can actually correct this foot without subjecting the patient to a fusion.

Another thing is at times Meary's may get as far back as the talonavicular joint. It may meet at that point. In that particular instance, again, any area in the midfoot area needs to be addressed at the apex of the deformity. If you do not address it at the apex of deformity, you will get basically a swan neck issue and you're not addressing it at the appropriate area. If it's at that area, you probably want to consider a talar beak operation where you basically reserve the neck blood supply to the subtalar joint and you would consider a talonavicular fusion at that point where you've locked the underlying navicular into the whole top of the talus that has a notch connected to it, or you can consider a Lambrinudi fusion triple arthrodesis.

Again, it's rare for it Meary’s ever to get to the TN joint. It usually gets back proximal to the NC joint, yet I always see a lot of DFWOs done in the CMT without anything being done to the tibialis posterior tendon. It's just never really addressed in those particular situations, and it leads to a failure of the potential operation.

Are there any other radiographic considerations here?

Another thing to remember is that Meary's angle will never go more proximal than the TN joint. And the reason being is the gastro-soleus muscle group is not affected. It's never affected. So you'll never see Meary's angle go further proximal than the TN joint. Now, the other thing that can happen is with the subtalar varus and the divergent or the convergence of the talus can be significant to the point that the talus is intrinsically placed into a varus position in the ankle mortise.

So it will impinge the planter medial portion of the tibia and create, obviously, a form of a localized osteoarthritis and a chronic talar instability issue. That's an instance where you have to even consider supermalleolar osteotomy. Equinus, the peroneus longus, the brevis relationship, and the tibialis posterior to the brevis relationship. And you can consider at the forefoot, the extensor to the extensor tendon overpowering the flexor tendons due to intrinsic loss.

Thank you so much, Dr. Visser, for yet another deep dive into this really complex topic. I know there's a lot more to say, but we'll save it for the next episode. In the meantime, our audience can continue to listen to episodes on podiatrytoday.com, and your favorite podcast platforms.