Sphere-on-Sphere Glenohumeral Shoulder Model

Musculoskeletal research model for exploring how glenohumeral joint congruence influences humeral head translations, rotator cuff loading, and dynamic stability using Force-Dependent Kinematics (FDK).
Published paper (Journal of Biomechanics, 2025)

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Table of Contents

1. About the Project
2. Key Features
3. Scientific Background
4. Repository Layout
5. Installation & Prerequisites
6. How to Run a Simulation
7. Parameterization
8. Output & Post-Processing
9. Typical Use Cases
10. Validation & Interpretation Notes
11. Roadmap
12. Contributing
13. Citation
14. License
15. Acknowledgments

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About the Project

This repository contains an AnyBody Modeling System implementation of a sphere-on-sphere glenohumeral (GH) joint formulation coupled with an FDK (Force-Dependent Kinematics) solution strategy. The approach enables the humeral head to translate relative to the glenoid in response to the balance of muscle, passive, and joint reaction forces—rather than prescribing a fixed center of rotation.

The model was developed to investigate how altering joint congruence (relative radii of the humeral head and glenoid representation) affects:

• Humeral head translations (inferosuperior, anteroposterior)
• Rotator cuff muscle force demands (e.g., supraspinatus, infraspinatus, subscapularis)
• Joint stabilization strategies during abduction task

NOTE: This README summarizes the published work without reproducing copyrighted figures or verbatim text. Please consult the linked open-access article for full methodological and result details.

Key Features

• Sphere-on-sphere analytic GH joint geometry with configurable radii (rh, rg).
• FDK-controlled translational DOFs (2 directions) of the humeral head allowing force-driven gliding.
• Parameterized morphotype inputs to explore anatomical variation (radii, insertion adjustments, offsets).
• Abduction moment arm evaluation utility (EvaluateAbductionMomentArm.any, etc.).
• Output scripting for extracting muscle forces, joint translations, and reaction loads.

Scientific Background

Shoulder stability in vivo at mid-range elevation primarily depends on the coordinated production of rotator cuff forces counteracting destabilizing detloid forces. Reducing congruence (e.g., increasing mismatch between humeral and glenoid radii) can:

1. Increase humeral head translations.
2. Elevate required compressive and balancing muscle forces to maintain centering.
3. Alter directionality of resultant joint reaction vectors.

This model represents the glenohumeral joint as a connecting rod rather than a traditional ball-and-socket. The published simulations reported systematic increases in both humeral head displacement and rotator cuff force magnitudes when congruence decreased—qualitatively matching experimental imaging trends.

Repository Layout

├── GH_2spheres.main.any        # Main entry script (load this in AnyBody)
├── Model/                      # Core model & customization scripts
│   ├── Jnt2spheres.any         # Sphere-on-sphere joint + FDK & passive forces
│   ├── Evaluate*MomentArm*.any # Moment arm evaluation utilities
│   ├── ArmMuscleList.any       # Muscle definitions / groupings
│   ├── BodyModelConfiguration.any
│   ├── ScalingCustom_muscle_insertion.any
│   └── ...
├── Input/                      # Morphotype, STL surface, picked point data
│   ├── InputVariables.any      # User-editable parameters (radii, muscle F0, springs)
│   ├── humerus/, scapula/      # Geometries & point clouds
│   └── dumbbell.stl            # External load example
└── README.md                   # This file

Installation & Prerequisites

Requirements

• AnyBody Modeling System (version supporting FDK constraints; AMS 7.x or later recommended).
• (Optional) Python (for any pre-processing sphere fitting scripts referenced by comments).

Getting the Model

Clone or download the repository into your AnyBody workspace directory.

git clone https://github.com/AnyBody/sphere-on-sphere_shoulder_model.git

Opening in AnyBody

1. Start AnyBody Modeling System.
2. Open GH_2spheres.main.any.
3. Adjust parameters in Input/InputVariables.any as needed.
4. Load the model to compile kinematics and muscle architecture.

How to Run a Simulation

1. Open GH_2spheres.main.any.
2. (Optional) Select a study (e.g., abduction) if multiple are defined.
3. Run: Kinematics → Inverse Dynamics (FDK will iterate internally on allowed DOFs).
4. Export desired outputs via AnyOutputFile.any or add custom AnyOutputFile statements.

Evaluating Moment Arms

Scripts such as EvaluateAbductionMomentArm.any can be included or loaded after the main file to compute functional muscle moment arm in specific planes.

Parameterization

Central user-editable inputs live in Input/InputVariables.any.

MorphotypeParameters

Parameter	Description	Example Default
rh	Humeral head sphere radius (m)	0.0236
rg	Glenoid sphere radius (m)	0.0316
glenoid_offset	Translational offset of glenoid sphere center {x,y,z}	{0,0,0}
CoordoSubscap12/34/56	Adjusted insertion coordinates for subscapularis fiber sets	(dataset)
AcromionScalingFactor	Local adjustment for acromial geometry	0.0
SupraHumScalingFactor	Local humeral head adjustment impacting supraspinatus path	0.0

MusclesPersonalization

F0<Muscle><FiberIndex> parameters define maximal isometric force (N) for distinct fiber bundles (deltoid anterior/lateral/posterior, supraspinatus, subscapularis, infraspinatus). Modify to reflect subject-specific strength or scaling experiments.

FDK Passive Elastic Terms

Parameter	Axis (Scapular Frame)	Interpretation
kML	Mediolateral	Linear spring resisting medial (+) / lateral (−) translation
kIS	Inferosuperior	Spring resisting superior (+) / inferior (−) glide
kAP	Anteroposterior	Spring resisting anterior (+) / posterior (−) translation

Set to zero for unconstrained translational compliance (muscles + joint contact only) or tune to approximate passive capsuloligamentous restraint curves (future enhancement: nonlinear polynomial—commented in Jnt2spheres.any).

Joint Mechanics (Model/Jnt2spheres.any)

Key constructs:

• AnyKinEqSimpleDriver GH_contact: maintains constant center separation = rg - rh (geometric congruence constraint).
• AnyKinLinear GHLin: measures 3D vector between sphere centers.
• GH_fdk: FDK driver enabling force-dependent solution for two translational components (MeasureOrganizer = {1,2}) while constraining distance.

Output & Post-Processing

Add or modify AnyOutputFile blocks (see Model/AnyOutputFile.any if present) to extract:

• Muscle forces per time step (Fm)
• Joint reaction forces and moments
• Humeral head translation (GHLin.Pos) in scapular frame

Example snippet (conceptual):

AnyOutputFile GHTranslations = {
	FileName = "results/GH_translations.txt";
	AnyFloat ML = Main.HumanModel.BodyModel.Right.ShoulderArm.Jnt.GHLin.Pos[0];
	AnyFloat IS = Main.HumanModel.BodyModel.Right.ShoulderArm.Jnt.GHLin.Pos[1];
	AnyFloat AP = Main.HumanModel.BodyModel.Right.ShoulderArm.Jnt.GHLin.Pos[2];
};

For batch studies (e.g., sweeping rg/rh ratios), create a Python or AnyScript macro that edits InputVariables.any and relaunches analyses.

Typical Use Cases

• Sensitivity analysis of joint congruence on muscle force requirements.
• Investigating rotator cuff compensation strategies under altered passive stiffness.
• Educational visualization of GH joint translation under load.
• Preclinical exploration of implant geometry simplifications (sphere-on-sphere prototypes).

Validation & Interpretation Notes

• Ensure radii values remain within anatomical plausibility (consult imaging-based morphometric studies).
• Excessive humeral head translation may signal insufficient stiffness or unrealistic muscle strength scaling.
• Comparisons to in vivo fluoroscopy or dynamic MRI should consider coordinate frame alignment (scapular reference).
• Passive restraint currently modeled as linear; nonlinear capsular behavior can be reintroduced (commented polynomial in code).

Roadmap

• Add nonlinear passive restraint option (restore polynomial terms).
• Provide automated batch script for congruence sweeps.
• Add validation dataset & reference kinematic envelopes.
• Include example post-processing Jupyter notebook (plots: translation vs elevation, cuff force profiles).
• Add images/diagrams from open-access components of the publication (with proper attribution) or recreated schematics.
• Parameter documentation auto-generation.

Contributing

Contributions are welcome:

1. Fork the repo
2. Create a feature branch (git checkout -b feature/YourFeature)
3. Commit changes (git commit -m "feat: add X")
4. Push (git push origin feature/YourFeature)
5. Open a Pull Request

Please open issues for: bugs, documentation gaps, feature proposals, validation data suggestions.

Citation

If you use this model, cite the associated paper:

Peixoto M, Soyeux D, Tétreault P, Begon M, Hagemeister N. Effect of congruence variations on a musculoskeletal model considering humeral head displacements. Journal of Biomechanics. 2025;190:112885. doi:10.1016/j.jbiomech.2025.112885

BibTeX:

@article{Peixoto2025SphereOnSphereGH,
	title   = {Effect of congruence variations on a musculoskeletal model considering humeral head displacements},
	author  = {Peixoto, M. and Soyeux, D. and T{\'e}treault, P. and Begon, M. and Hagemeister, N.},
	journal = {Journal of Biomechanics},
	year    = {2025},
	volume  = {190},
	pages   = {112885},
	doi     = {10.1016/j.jbiomech.2025.112885}
}

License

Refer to the repository's license (add a LICENSE file if absent). If the intention is open academic reuse, consider a permissive license (e.g., MIT, Apache-2.0) or Creative Commons for data-only content. The linked paper is released under a Creative Commons BY-NC 4.0 license (non-commercial) which does NOT automatically apply to this code unless explicitly stated.

Acknowledgments

• Original research team (see publication authors).
• AnyBody Technology for the modeling framework.
• Prior morphometric and statistical shape modeling studies informing radii defaults.

Adding Figures

To enrich this README:

1. Create a docs/ folder.
2. Add schematic images you have rights to (e.g., recreated diagrams or open-access figures with attribution).
3. Reference them with standard Markdown: ![Conceptual GH Joint](docs/gh_joint_concept.png)

Avoid embedding copyrighted publisher figures directly unless license terms allow redistribution in repository form.

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