Hi all,
Members do not see these Ads. Sign Up.
I have a question in regards to subtalar arthroereisis and tibia vara.
Many podiatrist advocate the use of a subtalar implant to reduce excess motion at the subtalar joint to about 5 degrees(?) pronation.
If a patient with tibia vara undergoes the procedure, is it possible that the device can limit the calcaneal eversion necessary to compensate for a tibia vara, even if it allows the normal degress of "normal" pronation.
Also, how will these devices effect a forefoot valgus deformity.. will it create more strain on the forefoot?
Finally, on a little different issue...
In feet with a rearfoot/tibia vara, the foot is inverted relative to the ground while in subtalar neutral, yes?
If tibia vara is a big cause of pronation in feet and if a forefoot valgus often occurs alongside the tibia/rearfoot vara as the medial foot reaches toward the ground from an inverted position, then why would a medial forefoot post which allows the medial column to contact the ground sooner in the cycle, not be a good idea, ASSUMING that it does not adversely effect the windlass as Mr. Kirby has written about.
Thanks,
Adam
-
-
Yes. I've done EMED pressure plate analysis on patients after an arthroresis. I've seen very high lateral forefoot pressures.
Another question is why do we care about the motion of pronation. The motion of pronation doesn't hurt. It's the stopping of the motion that hurts.
There are many interpretations of medial forefoot post. It appears you are describing a varus wedge under the forefoot. A varus forefoot wedge will tend to increase ground reaction force under the medial forefoot. This medial shift of force may not be far enough to create enough supination moment to change the position of the STJ. In feet with medially positioned STJ axes, force under the first met head is still on the pronation side of the STJ axis.
Eric -
"If a patient with tibia vara undergoes the procedure, is it possible that the device can limit the calcaneal eversion necessary to compensate for a tibia vara, even if it allows the normal degress of "normal" pronation."
Yes, absolutely.
"Also, how will these devices effect a forefoot valgus deformity.. will it create more strain on the forefoot?"
I have not performed a STJ Arthroereisis on a patient with a forefoot valgus. It would be unusual for a patient with a FF Valgus to provide the criteria for an arthroereisis procedure.
Steve -
So these lateral forefoot pressures would be due to lack of eversion range of motion or lateral deviation of the subtalar axis.. ie overcorrection?
Regarding varus wedging, it took me a moment to understand but your saying that in a foot with a medial deviated subtalar axis the posting under the medial column would cause further pronation.. I was speaking more about it preventing the foot ( with a "normal" subtalar axis) from overpronating to find the ground whendue to the foot being inverted from a tibia/rearfoot varus.. meaning it would act as a "stopper" to keep the forefoot in the same plane as the inverted heel.
Maybe it doesn't matter if the bisection of heel is inverted relative to the ground, but I was under the impression that the forefoot and the heel "like' to be on the same plane for optimum function.. hence the "comensated vs .uncompensated rearfoot varus." ?
Thanks for the thoughts. :) -
So if a foot does not have the heel eversion necesary to compensate for a tibia/rear vara,(perhaps due to an overcorrected arthroereisis, then what would the result be in terms of pathology... I guess I am asking how does the pathology of an uncompensated rearfoot varus present clinically? -
Eric -
"I guess I am asking how does the pathology of an uncompensated rearfoot varus present clinically?"
Pain usually.
You can get the engineering answer from the BioMech gurus here.
As far as in a clinical setting, feet find a way.
The problem with the STJ arthroereisis is the opposite. Many of these patients have quite a FF varus preoperatively; when you limit the STJ pronation they cannot easily get the first ray on the ground. At that point you can either get an orthotic to bring the ground up to the first ray or you can try and surgically bring the first ray down (not easy).
If we could just get rid of gravity we wouldn't have all these concerns.
Steve -
If they are not in the same plane, the body weight will transmit unevenly through the foot during midstance and also create a moment about the stj axis unevenly... or in an effort to reach the ground evenly, the foot will be subject to torsional tissue stress.. which may result in a forefoot varus or valgus deformity...
specifically the stj may have to overpronate at early midstance, which causes a supination issue at late midstance and possible heavy tissue stress at whatever mechanism is used to stop the pronation as you mentioned..
So a rearfoot/tibia varus creates a situation where the foot and subtalar joint must compensate to feel the ground, which alters the timing of the gait cycle and creates soft tissue changes due the planar misalignment of the forefoot to rearfoot...
and thats why this is bad.. :) -
This is what I was asking actually... If the patient has trouble getting the medial column to the ground, why not bring the ground up under the first met... regardless of whether that creates a pron/sup moment.
I assume if you use an orthotic to bring the ground up, the positing will be somewhere around the navicular? -
Eric -
In some of our earlier discussions we were talking about position and not moment. My point was that even if you change the moment from ground reaction force (with a forefoot varus wedge), you may not change the position.
If you try to support the first ray at the navicular, you will lose your support after heel off in gait.
Eric -
To lead you through the questioning of your assumptions...
The forefoot is more of a line than a plane. And a line isn't even that good of a descriptor. The metatarsal move indepnedently from each other.
Eric -
Hi
In the cases I've done where we have substantially limited rear foot pronation in patents with a large FF varus, they ambulate with the first ray off the ground. Not very good to see in the office. The angle of gait is decreased and the push off is obviously abbreviated. These patients need an orthotic badly. In time some displace the implant and continue to pronate, which is why we try and evaluate preoperatively for this.
This is not a foot that responds well to STJA procedures. I think when the MBA was first introduced, because it was a relatively simple procedure, it was over performed and criteria was lacking. Patients with semirigid RF deformities, fixed forefoot varus, talipes equinus - all were getting MBAs and nothing else. Not good. Not all surgeons who were performing implants felt comfortable performing achilles lengthenings, medial column fusions, etc...
Again, I'm not a biomechanical expert; but I would suggest that a patient with an "uncompensated forefoot varus" is a rare bird. Like I said, the foot finds a way, especially on a planet with gravity and McDonald's.
Steve
Loading...
- Similar Threads - Subtalar Arthroereisis Tibia
-
- Replies:
- 0
- Views:
- 4,209
-
- Replies:
- 79
- Views:
- 41,823
-
- Replies:
- 0
- Views:
- 4,171
-
- Replies:
- 22
- Views:
- 11,540
-
- Replies:
- 14
- Views:
- 16,449
-
- Replies:
- 1
- Views:
- 633
-
- Replies:
- 1
- Views:
- 959