Is there an easy way to calculate RCSP?-student question again

Nice
I suppose it is the relationship between movement and force that mean orthoses based on morphological relationship were largely successful. I guess it is because that relationship is not perfectly linear that we have sought a better explanation of treatment success and failures.

 

At the moment it is based totally on clinical experience and anecdote. How do you think it should be done?

 

Force is for linear physics. Moment (Force times distance) is for rotational physics.
Linear: Force = mass x acceleration.
Rotational: Moment = moment of inertia x angular acceleration.
More precisely it is net force and net moment that cause accelerations.

So, a force can cause an angular acceleration only if it has a moment arm (distance)

Not only does force have a magnitude and direction, it also has a point of application. A group of forces (e.g. force distributed over the entire plantar surface of the foot) can be averaged to a single point called the center of pressure.

For the foot, rotational motion or moments are changed by changing the location (the point of application) of the force.

You don't have to know the exact point of location of the force to change it. But you do have to decide which direction you want to move it. You also have to decide how hard you want to try to change it. The location of the force is changed by placing wedges under the foot. The bigger the wedge, the bigger the change in the location of force. You decide if the wedge is big enough by observing if symptoms resolve or other problems develop.

I've placed a wedge large enough under my heel where I could feel that my peroneal muscles were becoming fatigued. This happened in my feet with medially deviated STJ axes.

Hope this helps,

Eric

 

Hey Lawrence,

C'mon... we both know this is what many people believe and do on a daily basis...

As I said to pod on sea:

 

Hi Kevin,

You are far more articulate (not to mention knowledgeable) than me of course, so thanks for the correction. Consider it rephrased.

Thanks

Ian

 

I reserve the right to ask inane questions!!
Anyone ever done any repeatabilty on clinical assessment of the spatial location of the STJ axis?

If this is one of the prime determinants of STJ pronation and supination, anyone ever used its reltive position to dictate the amount of wedging?

 

Ian:

The point I was trying to make is that forces can create motion but motion can create forces also. Therefore, to say that injuries are not related to motion or lack of motion would not be correct. The mechanics of the foot and lower extremity are best understood by appreciating the continual interplay between the constantly changing forces and changing motions, each affecting the other.

 

The idea that 10 degrees of varus correction with a Blake orthosis will produce 1 degree of motion is indeed something I have heard Rich Blake say during his lectures. However, I believe this is misleading and not always true. Feet with laterally deviated STJ axes will respond much more (i.e. invert at rearfoot) to varus rearfoot wedging than feet with medially deviated STJ axes. Therefore, I think one should be careful in making a claim that a certain number of degrees of varus correction will always produce a certain number of degrees inversion motion of the foot since this certainly is not true.

 

I have been contacted a few times in the past by people who were going to do this - don't know if they did.

I can tell from my experience with students it probably the most unreliable of all the clinical tests we use; which is why I moved away from "trying to find the axis" to "trying to find where we do and do not want the orthotic to push"

Just putting together a study at the moment on STJ axis position and comfort from "arch supports", so will have to include a reliability component in that.

 

I thought his paper used the ratio of 5 to 1... does he lecture differently?

The amount of correction in an inverted device is a measure of how much tri-planar force you wish to apply. There is a point- and it is different on each person- where the effect of the force changes from an inversion force, to a lateral shifting force. It most be related to axis location and other forces which resist the supinatory force (obviously...)

This brings me to another question for Kevin...
I understand that you often prescribe some inversion to you orthoses in combination with the medial heel skive.
Do you apply the skive before or after the inversion?
Example- We generally apply a standard skive which is 15 degrees (ie- 15 degrees to the supporting surface). If we inverted 10 degrees (nice round figure) before we applied this, we have a combination of the inversion, and its effect, and ther skive would be still 15 degrees.
But this would be different of the skive was apllied before the inversion. If you did this, then the skive is also inverted, and the direction of the force applied would change slightly.
The skive would have an would be more inverted and have an inclination which perhaps may be useful if you wanted some more transverse plane control?
Is this something that you take into consideration?

 

An undergrad at Plymouth did this some years ago, I have the thesis at home (I'm surfing North Devon at the moment) The intra-rater ICC's were pretty good, Inter not so good. Experience of the testing procedure improved reliability. I'll post ICC's next week when I get back, if you're really interested.

Interesting. I guess one of the reasons is that the spatial location changes throughout gait (which is why your mates MOSI correction is a bit off the mark IMO). I suppose you could ask STJ position when? and use this to determine. So assuming you know which tissue being stressed and when this stress is likely to be pathologic during gait you might be able to correlate this with a certain level of wedging. You'd need to build a model.

Kevin, I can't send e-mails from my VW, I can only receive. In answer to your question- it's under review.

 

Hey Kevin,

Point taken. I think I am guilty here of misinterpreting something a read a while ago and I have said it enough times between then and now for it to be the truth (in my head at least). I will be more careful with my wording of this in future - apologies to pod on sea for the misleading info.

Ian

 

:good: Thanks Kevin.

There are two things which come to my mind viz a viz force and movement.

The first is Kevins work on residual moments, for me one of the most productive and useful elements of SALRE but it rarely gets a mention . Often, in tissue stress it is the residual moments which are harmful and understanding that these only start when motion stops (as I understand it) is something I've found very helpful.

The other is something which Dave Smith has tried to explain to me many times, the role of inertia. If we include inertia into rotational force considerations they A: Become horribly horribly complex and B: Always balance, even when a system is in motion. If we apply this to, say, a force equation for pathological tissue stress at the point of maximal pronation then the velocity of pronation (movement) becomes really rather important!

Just thinking out loud.

Regards
Robert

 

Simon:

Dave sent me your reply. Hope you are having fun in the sun. "Catch a wave and you're sitting on top of the world."

 

Craig:

The positive cast should be balanced inverted before the medial heel skive is skived into the cast. Balancing the positive cast inverted is a standard modification in those patients receiving a medial heel skive since balancing the cast inverted will increase the medial longitudinal arch height of the orthosis which also aids in shifting the orthosis reaction force medial in the midfoot to increase the external subtalar supination moments from the orthoses. Hope this helps.

 

Ian:

Keep up the fine work and congratulations on your outstanding knowledge in these subjects.

 

Robert:

Yes, of course Dave is correct in that the inertial component of the system in question is always something that should be considered and certainly will be more important if the segment has greater mass or the segment is being accelerated more rapidly. However, often times the inertial component of the foot segments during the stance phase of gait can be essentially ignored with only a slight loss of accuracy since the foot is of low mass relative to the mass of the rest of the body and the accelerations of the foot segments are normally low during stance phase.

Because of the added complexity of systems in motion, you will note, Robert, that I have always used static analyses in my papers and books in order to simplify the concepts for the non-engineer clinicians that I am trying to educate. Static analyses of the foot and lower extremity joints are much simpler to understand since the inertial components of the foot and lower extremity segments may be completely ignored. The reason that I chose static analyses is that I believed that once the clinician completely understood the concepts of my static analyses of the foot and lower extremity, then they would be better able to understand the more complex dynamics of the foot and lower extremity as time went on.

And by the way Robert, I am very impressed with your considerable increase in knowledge since you first started contributing here to Podiatry Arena. In addition, I am delighted about your decision to start writing independently on biomechanics topics for the podiatrists of your country. I am looking forward to more valuable contributions from you in the coming years in our understanding of foot and lower extremity biomechanics.

 

Perhaps you are all making assumptions here.

Maybe the correct answer (and the one the professor is looking for) is:
"No, there isn't and why would you anyway?"