Biomechanical treatment post-op Hyprocure

I guess what I'm trying to say here might be better explained by my adaptation of one of Kevin's pictures (below). The black lines represent a set sequence of 6 measurements of the subtalar joint axis position in the transverse plane during stance. The red line represents the 'average' of these positions over the measurement period.

 

Hi Bob while not Kevin, I´ll give my 2 cents worth and then Kevin or one of the others , "The facilitatiors" as I refer to them now, may point a new spin on things, if thats ok if not well too late I guess.

I would say your spot on here, the spartial location of the STJ axis measurements only really give us the is the patient more likely to be medially deviated or laterally deviated. It comes back to my point about forces defines axis position. The more STJ pronation force the more medial.

Again as the stent reduces the Pronation forces earlier, thru an increase in compression force (talus - calc) the axis would not deviate the same amount in a medial direction and therefore on average be more laterally deviated than before the Stent was used, but there would still be medial and lateral deviated and for that matter movement in the other 2 body planes as well.

Hope than helps some.

PS I always use the name Bob for an imaginary name but between you and Robert it would get alittle strange so Sebastian it is.

pps 1000 posts for the year and it July I need some help I think.

 

No problem Michael,

I welcome everyone's input and appreciate you working through this with me. I only specifically replied to Kevin due to the post that I quoted, but this is an open forum and it's really helpful to hear everyone's views and work out if we're all on the same page (or if I'm talking and thinking rubbish when compared with the general consensus).

1000 posts - this is pretty amazing. I only hope it doesn't reflect a poor through-put of patients? Hopefully they will be beating down your door and while your post count will go down, your bank balance will rise

 

You have as bad a case as I've seen my friend . ECT or kidnap and deprogramming are your only hope.

About the axis Bob, I'd put it thus (although I am also not Kevin)

As you say, the axis moves depending on the position of the joint. This is the "bundle of instantaneous axes".

The test described by Kevin in the initial SALRE paper is, if memory serves, to find the position of the axis with the foot in "paralell plantar position" (PPP) (please correct me if I'm wrong Kevin). This seems to make sense as this is the position of the axis when the whole foot is on the ground. Its NOT the average position.

Now, take a foot with a forefoot perpendicular to the rearfoot. Assuming all else is equal the PPP will give the position of the axis in about the middle of the bundle. Pronate the foot, the axis will move medial and vice versa.

Take the same rearfoot, the same STJ, and make the forefoot a big ole supinatus. Now the reafoot, the STJ is unchanged. But now the PPP has the foot maximally pronated. as such the PPP axial position is at the medial most point of the bundle.

In some ways one could consider that there are two things to consider. The position of the axis within its bundle at the point at which the foot is flat on the ground and the position of the entire bundle.

So to return to the arthroesis, the stent limits the range of pronation. Essentially it cuts off the medial most available axial locations so that although the bundle has not moved, its a smaller bundle.

Thats how I view it. I'm sure Kevin will put it far more clearly.

 

Thanks for the reply Robert.
Please can you point me in the direction of that paper so I can understand your post better?

 

Do what?

I think I know what you mean, but I don't think its what you said.

The stent does not reduce the pronation forces earlier. If anything it increases the pronation forces because although the axial position (average position if you prefer) moves a bit lateral it won't move as far as the COP in your supinatus foot. Thus the COP is FURTHER from the axis. Thus pronation moment is increased.

The stent provides supination moment from sinus tarsi compression earlier than it would happen otherwise and probably reduces supination moment from other structures.

It may seem pedantic (who me?) but this is the sort of thing that years of patient correction from people like Simon, kevin and Eric teaches. Terminology is key.

The stent does not necessarily reduce pronation moment. It provides supination moment. The pronation moment comes from gravity. If I jump in the air, I don't reduce gravity when I land. I just put something in the way of it (viz the ground).

The stent doesn't reduce force (in that supinatus foot), it counters it with an opposing force. Of course if the COP is not moved too far lateral but the axis is then it will reduce pronation moment. A bit. Nice force plate study there.

 

http://www.japmaonline.org/cgi/cont... k&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

Thats the 1989. I have a scanned copy if you need one, just pm me your email

and here is the 2000 paper

http://www.japmaonline.org/cgi/cont... k&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT

I'm not sure but I think the paralell plantar bit is only in the 1989 paper. I could be wrong.

If you need either, just shout.

 

Here you go Bob summer reading......

 

I´m into colours today.

Red- yes thats what I should have said- well put.

Green- in a person with a medial deviated STJ axis, I would say that most of the pronation moment would come from Ground Reaction Force (GRF) an external STJ pronation moment rather than from Gravity another external moment.

Also by laterally deviating the STJ axis with our stent, yes talus -stent-calcaneous compression provides an internal Supination moment, but if the correct foot is used a better balance between GRF external Supination and Pronation moments.

This is why my patient is looking down the barrel of alot of hurt due to the inverted position of the FF only increasing the external STJ pronation moment, with almost no external Supination moments to balance it out, so the equal and opposite reaction must come from talus -stent-calcaneous compression.

Which gets us back to the important of the correct foot being used in Hyprocure procedures in the 1st place.

 

Gravity and GRF are one and the same in this context. GRF is the ground pushing up as the body pushes down. What makes the body push down? Gravity. No gravity, no GRF. More mass for the gravity to work with, more grf.

Gravity acting the absence of GRF is falling and in free fall it exerts no rotational force on the stj.

I get what you mean, although I don't like the concept of "balance" between GRF external moments. The external moments very very rarely balance and if they did the dominance of the internal supination structures would make inversion sprain inevitable.

The internal + external moments on the pronation side ALWAYS balance with the internal and external moments on the supination side. The problem is not an imbalance between supination and pronation moments, rather it is from which tissues and in what volume such moments are being exerted. Thinking in terms of seeking a balance of external moments is a bit of a misnomer IMO. If you had it, you would'nt want it!

Love that phrase. I'm stealing that.

Agreed.

R

 

I guess I´m just getting caught up in something Eric wrote which has stuck with me .

To paraphase : It´s important to work out where the moment comes from, internal ( and what structures) or external.

It´s yours to use as much as you want, here the full version, from my old athletes coach, When we would ask whats on the training diary for today. His answer- "your looking down the barrel of awhole lotta hurt. "- never a good answer to hear.



patient notes blah blah blah

long term treatment - looking down the barrel of awhole lotta hurt

Do you think it Fits NHS guidelines ?

 

No joint of the human body has a fixed-hinge like axis like a set of door hinges. All joint axes of the human body are determined by the motion patterns between the two segments that make up the joint. There are different levels of constraint of every joint. Some joints, like the subtalar joint (STJ) are tightly constrained,. Other joints, like the talonavicular joint, are loosely constrained. You may think of the level of constraint of a joint as being how much a joint will tend to move along spatial location of joint axis even when the input force across the joint is not directed perpendicular to that joint axis location. This subject of joint constraint may deserve its own thread since it is a very important and fascinating subject.

These joint axes you speak of are often known as instantaneous axes of rotation which are the calculated joint axis spatial locations which are derived as the joint segments move from position A to position B in its rotational movement. Like you have described, Bob, when you have now measured the multiple instantaneous axes of rotation of the joint in question, you will now have what has been called a "bundle of instantaneous axes". When I describe this concept during my lectures, I use the analogy that this bundle of axes is similar to being like arrows in a quiver, each one occupying a different line in space than the other arrows within the quiver.

You are on the right track. I think once you read my papers closely, you will appreciate that STJ rotational position is different from STJ axis spatial location and that you should not confuse the two since these are two very different biomechanical parameters of the subtalar joint.

When the STJ pronation range of motion is limited by the implantation of a STJ arthroereisis (STA) implant, the STJ maximally pronated position is reset from a more internally rotated, more medially translated and more plantarflexed position pre-operatively, to a less internally rotated, less medially translated and less plantarflexed postion post-operatively. The maximally pronated STJ rotational position is of particular interest here since the STA implant is designed to limit excessive STJ pronation and the feet that are chosen for this procedure should be functioning in the STJ maximally pronated position during standing and during much of the stance phase of gait pre-operatively.

That means, the foot with a STA procedure post-operatively will still be maximally pronated during standing (because the STA implant has reset the maximally pronated position), but the STJ axis spatial location will now be positioned in a more mechanically advantageous position relative to the ground reaction forces acting on the plantar foot and to the muscle reaction forces acting on the pedal osseous structures than what was present pre-operatively.

Hope this is making better sense now. Glad to see that you are one of the podiatric surgeons that is eager to learn more about the mechanical dynamics of the human terrestrial interface organ you are often doing surgery on. I just wish more podiatric surgeons were like you.:drinks

 

It sounds like a quick review of my center of pressure paper is in order.

The STJ axis is determined by the facets on the inferior surface of the talus and the superior facets of the calcaneus. (cahil) When the talus internally rotates the STJ axis will internally rotate.

The center of pressure is the average point of force and can be considered as a single point where many forces can be considered to act. For example, ground reaction force applied to the foot is applied to the foot wherever the foot touches the ground. A weighted average can be used to create a single point for the location of ground reaction force.

The pronation moment from ground reaction force is determined by the location of the center of pressure of ground reaction force relative to the location of the STJ axis at a particular instant in time. This is a three dimensional relationship because the vector of ground reaction force is mostly vertical, but it can have a medial to lateral, or an anterior to posterior component and of course the axis has a tri planer orientation. We can greatly simplify this, without losing too much accuracy if we assume just the vertical component of ground reaction force and use the transverse plane projection of the STJ axis.

With motion of the STJ while weight bearing the talus will ab and adduct thus changing the location of the projection of the STJ axis. The location of the center of pressure under the foot can also change with the position of the STJ within its total range of motion. It is easy to confuse the position of the STJ within its range of motion with the position of the STJ axis as it is projected on to the transverse plane. They are different, but interrelated ideas.

So, when an arthroresis plug is placed into the sinus tarsi the STJ will lose some of its range of motion and not be able to internally rotate as far as it did prior to the surgery. This will make the projection of the STJ axis be more lateral than it was prior to the surgery.

Whether or not this effects pronation moment from the ground is dependent on the location of the center of pressure. This concept is very important to understand when differentiating between a successful arthroresis and unsuccessful one. If the plug changes the location of maximal pronation within its previous range of motion the STJ enough to change the location of the projection of the axis on the transverse plane without significantly altering the location of the center of pressure then procedure will have reduced the pronation moment from the ground. (Hopefully, this is part of the goal of the procedure.) On the other hand, if the plug inverts the STJ so much that there is very little force on the medial column, the location of the center of pressure will be much more lateral. So, even if the STJ axis position in the transverse plane is more lateral, the location of the center of pressure may be even farther lateral relative to the STJ axis when compared to prior to the surgery. It is the position of the center of pressure relative to the STJ axis that determines the moment from ground reaction force.

As the surgeons say in performing a triple arthrodesis: " Thou shalt not varus." Putting a too big arthroresis plug will create a rearfoot varus.

Hope this helps,

Eric

 

Just a quick one to say thanks to all. I'll read through the papers when I get a chance over the weekend. Very busy week. Saw a patient 12 months post hyprocure today - very pleased with the result, patient reports that she now has no knee or lower back pain too. I thought of this thread and laughed as I said it's probably a bonus and I can't claim credit for reducing all pains by putting a hyprocure in for a painful flexible flatfoot (did a Kidner type procedure too).
Listed a triple arthrodesis today, coincidentally, and I shalt not varus Eric - although if you are putting a patient into varus with an arthroeresis, you shouldn't be operating!

 

Bob:

The forefoot to ground alignment doesn't have to be in varus in order to get lateral column symptoms status post subtalar arthroereisis. All the surgeon needs to do is put the foot in a position where the medial colum will have relatively little plantar load on it when compared to the lateral column, and the lateral column symptoms could begin. The patient I spoke of earlier in the thread that developed lateral DMICS did not have a varus forefoot to ground relationship post-operatively but could easily get all the metatarsal heads on the ground. Aiming for about a 5 degree valgus forefoot to ground relationship when doing STA implant sizing during the STA operation should work well in most cases, theoretically.