Point-on-the-plane mandible model muscle attachment sites

[iman]

Hello AnyBody,

I would like to ask if there is any specific reason that in the POP mandible model, the muscle attachment points to the bones are not located on the bones surface?

As depicted in the following, for some muscles it is located inside the bone (e.g. left temporalis muscle for the skull) and for some muscles it is attached to a point far from the bone surface (e.g. LateralPterygoids).

Best regards,
Iman

[Bjorn (Bjørn Keller Engelund)]

Hi @iman

These things tend to happen across models. It is hard to align and scale stl surfaces to fit all geometrical points and since muscles are line segments at their ends thinks might seem a bit off. Since the mandible model is quite old and only a demonstration model, some of the stl drawings or settings might have changed over the years making the aligned slightly off. But it is merely cosmetic. The nodes are still placed accordingly and the model performs as intended.

Best regards,
Bjørn
AnyBody Technology

[iman]

Dear Bjørn,

Thank you for your reply.
I scaled this model to a target patient and because the source model has this problem (muscle attachment points offset), the scaled model has the same problem.
Therfore, I think the attachment points should be corrected manually after the scaling.

The question in this case is: are the results still valid (as the validated pop mandible model)? Or do I need to perform a calibration study other than those already defined for this model (CalibrationMandibleStudy)?

Best regards,
Iman

[Bjorn (Bjørn Keller Engelund)]

Hi @iman

As soon as you start scaling any model it is wise to do some sanity checks to see if it behaves as intended. The level and work to put into this is up to you to define. If you start altering muscle attachement locations I would also look at the calibration routine since this model uses the 3-element muscle models. These are prone to errors if the parameters are not carefully calibrated.

Best regards,
Bjørn

[iman]

Dear Bjørn,

As you know, this mandible model includes a calibration study for the tendons
and it calibrates the tendon length at an interincisal separation of 12 mm.
I have adjusted the parameters to have this interincisal separation of 12 mm for the scaled model as well.

Question 1: As far as I know AnyBodyCalibrationStudy takes care about the calibration of all tendons length. Or do I need to perform some manual calibration as well?

Question 2: When I check the Fp (parallel elastic force) for the muscle elements of the scaled mandible model, it is as follows:

Should all of the Fp forces be initially zero as mentioned in the Lesson 7: Calibration, to make sure the model is calibrated correctly!?
However, the magnitude and pattern of the curves is the same for the Original AnyBody POP mandible model (before applying any changes or scaling).

Question 3: For this particular mandible model do you have any suggestion for the sanity check?

Best regards,
Iman

[Bjorn (Bjørn Keller Engelund)]

Hi @iman

1. Yes the study takes care of all the tendons. You don't need to add anything. The thing you should be aware of is what postures are used to calibrate. The postures should represent the postures where the muscle is at it's optimal length. It is probably not necessary to change these but new literature might have arise since the model was developed.
2. The Fp should not necessarily be zero initially. It depends on the posture. Some muscles might have passive forces in the posture your model starts in. From a physiological perspective there is probably not a lot since it would not make sense to have large passive forces in anatomically neutral postures, but smaller might occur somewhere.
3. I think your approach with inspecting the passive forces of the muscles is a good check. Another check could be to look at what maximal forces your muscles obtain after calibration. This might could reveal if the strength of a known large and strong muscle is suddenly to low or vise-versa. Such a scenario could indicate that the calibration postures did not set that muscle it its optimal position.

Best regards,
Bjørn