Cadaver study of STJ neutral

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Assessment of subtalar joint neutral position: a cadaveric study.
Chen YX, Yu GR, Mei J, Zhou JQ, Wang W.
Chin Med J (Engl). 2008 Apr 20;121(8):735-9.

 

10 degrees eversion= isn't that maximal pronation by Root's observations? Nice study.

 

Good point.

 

Dear All

There are a couple of things in this paper that are not clear.

Assuming the neutral position was as ISB protocol (diagram - reference neutral 1R-3R)

1) What was the total possible surface area of STJ that was covered by the pressure film?

2) Were the positions of interest measured (diagram Max Contact 1N-3N) as individual displacements or as a triplanar motion that included all three possible combinations. EG when in the closed kinetic chain ie weight bearing the STJ joint does not have to evert 10dgs to achieve 20dgs of dorsiflexion.

How would the different methods, if used, affect the total contact areas?

3) It appears the position of neutral used was a nominal position as set out by the ISB. (diagram - Reference neutral 1R-3R)

How did the palpated position of neutral compare to this for each cohort and as a mean for the test population?

Area of potential error

1) The fuji film indicates perpendicular force and on the curved surface the force measured is a function of the sine of the angle of the applied force. This may have been lower than the mimimum threshold of 0.5MPa. (diagram 1N)
The authors only touch on this area of error. (pge 736 specimin loading)

Areas of limitations

1) We do not no the discreet force / area distribution so it is not possible to know the mechanical effect of the contact areas without the magnitude of discreet applied forces. (Diagram 1N 2N 1EV) and (diagram - contact areas A, B, C, D)

1a) In Diagram - contact areas, (copied from Chen's paper page 737) if the discreet force applied to each area on 'A' (in the horizontal plane) were equal, in terms of moments about the STJ axis, then this may result in joint stability. If these forces are then distorted as seen in diagrams 'B,C,D' then this may result in joint instability.

1b) The same may be said about these forces when considered in the frontal plane. Even tho the contact area may be greater in dia 1EV the centre of force may be translated laterally of the STJ axis and increase moments about the STJ axis. These moments (Mcof) need to be balance by some other opposing force from a soft tissue perhaps (opposing force = of). So the Mof + Mcof = zero. Clealy then although there is greater apparent congruence the joint is unstable and require other forces for stability. This could lead to pathology potentialy.

Therefore it is possible that the congruency (where congruency means magnitude of surface area contact) is greater than the results show but are not measured due to losses in data input. Or less congruency but with optimum force distribution can result in better joint stability or a combination of both.





Your thoughts

Cheers Dave

 

please e-mail full paper.

 

Big assumption...which model of STJ neutral did they use as the reference or starting point? I assume it probably was the ISB one ....but thats not the one that we traditionally use associated with talonavicular cogruity (or whatever you use) ...

 

Anybody have a reference of why the position of the joint where maximum area of joint contact is seen is relevant. Are tissue stress tolerances exceeded when there is 85% of maximal contact? For all joints? So many ignored assumptions.

Cheers,

Eric

 

Craig

The ISB protocol is the one they refer to in the paper but do not actually asy that is the one they used. However there is a written description of the neutral position and extrapolating from the diagrams in the paper of the block used to position the STJ I assumed the neutral reference was as my diagrams show in my last post.

I've attached the paper in PDF