STAPLE: Shared Tools for Automatic Personalised Lower Extremity modelling

Table of contents

• What is STAPLE?
• What can I do with STAPLE?
• Requirements
• Installation
• How to use the STAPLE toolbox
  • Workflow to generate subject-specific lower limb models
  • Before using STAPLE: data preparation
  • Quick start guide: provided workflows
  • Detailed steps to setup a STAPLE workflow
  • Algorithms for bone morphological analysis
  • Datasets available for testing
  • STAPLE variables and conventions
• Troubleshooting your workflow
• Does STAPLE work only with OpenSim?
• What are the differences between the STAPLE toolbox, NMSBuilder and the MAP Client for generating OpenSim models?
• Current limitations
• How to contribute
• Code of conduct
• License
• Acknowledgements

What is STAPLE?

STAPLE, acronym for Shared Tools for Automatic Personalised Lower Extremity modelling, is a MATLAB toolbox that enables researchers in the biomechanical field to create models of the lower extremity from subject-specific bone geometries with minimum effort in negligible processing time. In most cases, that can be done just changing the input data in one of the provided workflow and running a MATLAB script.

STAPLE requires three-dimensional bone geometries as an input. These geometries are normally surface models segmented from medical images like magnetic resonance imaging (MRI) or computed tomography (CT) scans. STAPLE performs morphological analyses on the provided bone geometries and defines reference systems used to create models of entire legs or few joints, depending on the available data or the research intent. Currently the toolbox creates kinematic and kinetic skeletal models but will soon be extended with complete musculoskeletal capabilities.

The STAPLE toolbox and some of its applications are described in the following open access publication (also available as preprint):

@article{Modenese2021auto,
  title={Automatic Generation of Personalized Skeletal Models of the Lower Limb from Three-Dimensional Bone Geometries},
  author={Luca Modenese and Jean-Baptiste Renault},
  journal={Journal of Biomechanics},
  volume = {116},
  pages = {110186},
  year={2021},
  doi={https://doi.org/10.1016/j.jbiomech.2020.110186},
  url = {http://www.sciencedirect.com/science/article/pii/S0021929020306102},
  keywords = {Anatomical coordinate system, Lower limb, Skeletal model, Musculoskeletal model, Kinematics, Three-dimensional bone model, Surface fitting, 3D imaging}
}

The models, simulations and results of the publication can be fully reproduced using the scripts and datasets available in this GitHub repository.

What can I do with STAPLE?

• Creating complete skeletal models of the lower limb from segmented bone geometries that include the following bodies: pelvis, femur, tibia and fibula, talus, calcaneus (including foot bones) and phalanges connected by joints.

• Creating partial skeletal models of the lower limb that are a subset of a complete lower limb model and include any meaningful combination of bones listed for the complete model connected by joints. For example, models of hip, knee and ankle joints can be created as individual models.

• Extracting the articular surfaces of the lower limb joints: some of the algorithms included in STAPLE can identify the articular surfaces of the lower limb joints and export them for you. These surfaces can be used to define models of articular mechanics including contact.

• Transformations to local reference systems: STAPLE performs morphological analyses using the medical images reference system where the bone geometries are segmented, but then it can transform both the bone geometries and model objects to standard segment-based biomechanical reference systems. This is an important feature if you are using STAPLE in OpenSim, as joint reactions are expressed in body reference systems. See the provided example of this functionality.

• Basic identification of bony landmarks: certain bony landmarks can be easily identified following the morphological analysis of the bone surfaces. These landmarks are intended as first guess for registration with gait analysis data.

• Merging subject-specific and generic models: STAPLE includes some basic utilities to merge partial subject-specific skeletal models (obtainable from localized MRI scans) with generic musculoskeletal models. See the provided advanced examples.

Requirements

In order to use the STAPLE toolbox you will need:

1. MATLAB R2018b or more recent installed in your machine with the following toolboxes:
   • Curve Fitting Toolbox
   • Statistics and Machine Learning Toolbox
2. OpenSim 4.1 installed in your machine.
3. the OpenSim 4.1 API for MATLAB correctly setup and working. Please refer to the OpenSim documentation for instructions about installation.

Installation

Once you have MATLAB and OpenSim installed you will have to:

1. download the latest version of STAPLE from the SimTK project page.
2. add the STAPLE folder, normally locate in msk-STAPLE\STAPLE to your MATLAB path.

Note: you can run the provided examples to evaluate the toolbox without adding STAPLE to the MATLAB path.

Optional: if you want to contribute to the development of the toolbox, you can clone the GitHub repository using Git and running this command on the terminal:

git clone https://github.com/modenaxe/msk-STAPLE

How to use the STAPLE toolbox

Workflow to generate subject-specific lower limb models

The figure below presents a common workflow for the generation of subject-specific (or patient-specific) lower limb models. The operations handled by the STAPLE toolbox are grouped within the red box.

Before using STAPLE: data preparation

The typical STAPLE workflow consists in:

1. segmenting bone geometries from medical images, normally computed tomography (CT) or magnetic resonance imaging (MRI) scans. This step is not performed using in STAPLE but using third-party segmentation software, of which you can find a list at this link. The bone geometries are normally exported as surface models in STL format.

2. improving the quality of the segmentated bone geometries, normally running filters on the surface models to improve their quality and topology. Also in this case, there are several options to process the geometries and a list of software is available at at this link.

3. renaming bone geometries: the surface meshes are renamed following the typical names of standard OpenSim models, presented in the table below. Individual bones are also grouped at this stage, for example the surface meshes of tibia and fibula can be joined using the flatten mesh layers filter in MeshLab. NOTE: This is actually an optional step, but if not performed then you will not be able to use the provided workflows straight away.

STAPLE Input Name	(Grouped) Segmented Bone Geometries
pelvis_no_sacrum	right iliacus, left iliacus
femur_r	right femur
femur_l	left femur
tibia_r	right tibia, right fibula
tibia_l	left tibia, left fibula
patella_r	right patella
patella_l	left patella
talus_r	right talus
talus_l	left talus
calcn_r	right calcaneus, right foot bones excluded phalanges
calcn_l	left calcaneus, left foot bones excluded phalanges
toes_r	right foot phalanges
toes_l	left foot phalanges

4. convert geometries to MATLAB triangulations (optional): this step is suggested to reduce the size of files and increase speed of input reading if you are processing a dataset more than once, for example while figuring out the best algorithms to apply to your bone geometries. STAPLE can also read stl file in input.

5. store bone geometries and saved them in folders named conveniently. This step is especially important for batch processing. The examples of the provided workflows are organised as follows:

study_folder
        |- dataset_1_folder
                        |- tri
                            |- pelvis_no_sacrum.mat
                            |- femur_r.mat
                            |- etc.
							
                        |- stl
                            |- pelvis_no_sacrum.stl
                            |- femur_r.stl
                            |- etc.
							
        |- dataset_2_folder
                        |- tri
                            |- pelvis_no_sacrum.mat
                            |- femur_r.mat
                            |- etc.
							
                        |- stl
                            |- pelvis_no_sacrum.stl
                            |- femur_r.stl
                            |- etc.

where:

• study_folder is the main folder of the current study
• dataset_1_folder is where the bone geometries for the first partecipant data are stored
• dataset_2_folder is where the bone geometries for the second partecipant data are stored, and so on.

At this point you should be able to use one of the available workflows or implement your own based on the instructions below. If a provided example demonstrates a use similar to your intended one, you can use it as starting point.

Quick start guide: provided workflows

The examples scripts provide implementations of common workflows that can be adapted and run with minimum effort for processing new datasets. In most cases, adapting a script will be as easy as modifying the input folders or dataset names. More examples will be added in time.

Script name	Workflow	Description and notes
Example_create_kinetic_models.m	Full workflow	Creates ipsilateral kinetic models. Same models and datasets from the paper of Modenese et al. (2020).
Example_full_leg_left.m	Ipsilateral workflow (left side)	Creates models of the lefs lower limb. Batch processing.
Example_full_leg_right.m	Ipsilateral workflow (right side)	Creates models of the right lower limb. Batch processing.
Example_bilateral_model.m	Bilateral workflow	Merges ipsilateral models. Batch processing.
Example_hip_model.m	Partial model workflow	Creates a hip joint model.
Example_knee_model.m	Partial model workflow	Creates a knee joint model.
Example_ankle_model.m	Partial model workflow	Partial model Creates an ankle joint model.
Example_extract_tibiofem_artic_surf.m	Articular surface extraction workflow	Extracts hip and tibiofemoral articular surfaces.
Example_extract_ankle_artic_surf.m	Articular surface extraction workflow	Extracts talocrural and subtalar articular surfaces.
Example_use_Kai2014.m	Partial model workflow (no foot)	Uses Kai2014 algorithms to process low-quality bone geometries

Detailed steps to setup a STAPLE workflow

This is a checklist for setting up a functioning workflow using STAPLE:

• ensure that have your data prepared as described in this previous section.
• define the dataset(s) to process.
• define a cell array with names of bones to process. The same names will be used for the rigid bodies.
• define a body side if not evident from bone names. Otherwise STAPLE can take care of this in most processing function using the function inferBodySideFromAnatomicStruct.m.
• decide the joint definitions (joint_defs variable in the examples), meaning how incomplete joint reference systems will be completed. You can refer to the functions jointDefinitions_auto2020.m and jointDefinitions_Modenese2018.m as two examples.
• use createTriGeomSet.m for creating a set of MATLAB triangulation objects (TriGeomSet structure).
• use writeModelGeometriesFolder.m to write the visualization geometry files for your model from the TriGeomSet structure. You can specify the format (obj preferred, as more compact) and the level of subsampling of the original surface models (30% by default, usually required or OpenSim will struggle to visualize them).
• use initializeOpenSimModel.m to start building the OpenSim model.
• use addBodiesFromTriGeomBoneSet to create bodies corresponding to the fields of the TriGeomSet structure. These bodies will be added to the OpenSim model, but are not yet connected by joints. If you print the model at this stage all bodies will be connected to Ground with free joints. NOTE: At this stage the assigned segment mass properties are calculated from the bone geometries using a bone density of 1.42 g/cm^3 as in Dumas et al. (2015). If you are building a kinetic model, you need to use assignMassPropsToSegments.m to update the inertial properties after creating the model joints (see below).
• use processTriGeomBoneSet.m to process the bone geometries using the available algorithms and compute body coordinate systems (CS), joint coordinate systems (JCS) and bone landmarks (BL). This step does not rely on the OpenSim API and consists of a morphological analysis of the bone shapes performed using the algorithms available in STAPLE and listed in the Table below. This step could be run before initializing the OpenSim model, if you prefer to keep the morphological analysis clearly separated from the modelling operations.

STAPLE Input Geometry	Joint Coordinate Systems	Algorithms
pelvis	ground-pelvis	STAPLE-Pelvis Kai-Pelvis
femur	hip child	GIBOC-Femur Kai-Femur
	knee parent	Kai-Femur GIBOC-Spheres GIBOC-Ellipsoids GIBOC-Cylinder
tibia	knee child	Kai-Tibia GIBOC-Ellipse GIBOC-Plateau GIBOC-Centroids
	ankle parent	coordinate system assembled based on joint_defs
patella	TBA	TBA
talus	ankle child	STAPLE-Talus
	subtalar parent	STAPLE-Talus
calcn	subtalar child	uses parent JCS
	mtp parent	STAPLE-Foot
toes	mtp child	uses parent JCS

• use createOpenSimModelJoints.m to complete the definitions of the joints connecting the OpenSim rigid bodies and add them to the initialised model. This is the step where more adjustments can be made. Joints are created using:

• the information from the JCS reference systems identified by processTriGeomBoneSet.m during the morphological analysis
• parameters defined in the getJointParams.m function: joint name, parent and child body names, coordinates, order of rotations, etc. Currently we provide a standard joint definition in the main STAPLE folder and an example of a customization of joint definition in the advanced examples.
• instructions on how to finalise incomplete JCS given in the script indicated by joint_defs. Currently we provide two options for completing the JCS:
  1. Modenese2018 based on a previous publication of Modenese et al. (2018) that does not rely on anatomical axes calculated from the tibia bone.
  2. a default approach named auto2020, which connect bones using as much information as possible from the bone morphological analysis.

• (optional) use assignMassPropsToSegments.m to update the mass properties of the segment using the the actual anthropometry of the subject that you are modelling. Segment masses and inertias of the lower limb are scaled from those of the standard gait2392 OpenSim model, which are identical to those of the more recent Rajagopal full body model. NOTE: this feature is still very basic in its implementation and will be further developed.
• (optional) use addBoneLandmarksAsMarkers.m to add to the OpenSim models the bony landmarks identified automatically during the morphological analyses.
• finalise the OpenSim model using the osimModel.finalizeConnections() API method.
• print the OpenSim model using the osimModel.print(model_path\model_name.osim) API method.
• use the obtained model for your biomechanical analyses.

Algorithms for bone morphological analysis

STAPLE collects some algorithms described in the literature and others that we have developed ad hoc. The following table lists the algorithms currently available in this repository.

Family of Algorithms	Bones of interest	Reference publication
Kai2014	pelvis femur tibia	Kai et al. (2014)
GIBOC	femur tibia patella	Renault et al. (2018)
STAPLE	pelvis talus foot bones	Modenese and Renault (2020)

Please note that STAPLE toolbox includes a minimal version of GIBOC, renamed GIBOC-core. You can download or inspect the original GIBOC-knee toolbox published by Renault et al. (2018) at this link.

Datasets available for testing

Datasets of bone geometries available in the "datasets_folder" directory for testing and development purposes are listed in the following table. Details describing the data and, when specified, its license, are included in each dataset folders.

Dataset	Gender	Age	Height	Mass	Mesh Quality	Reference publication	Notes
ICL-MRI	M	38	1.80	87	Fair	Modenese et al. (2020)
JIA-MRI	M	14	1.74	76.5	Low	Montefiori et al. 2019a
JIA-ANKLE-MRI	M	N/A	N/A	N/A	Low	Montefiori et al. 2019b	Data from supplementary materials.
LHDL-CT	F	78	1.71	64	Very Good	Viceconti et al. (2008)
MCC22	F	72	1.60	66.3	Very Low	Montefiori et al. (2021)	Data from this link. Bilateral.
TLEM2	M	85	N/A	45	Fair	Carbone et al. (2015)	Released with the TLEM2 musculoskeletal model. Bones have local ISB reference systems. STAPLE cannot create a full lower limb (yet).
TLEM2-MRI	M	85	N/A	45	Fair	Carbone et al. (2015)	Geometries from the TLEM2 specimen's MRI scans. Bilateral.
TLEM2-CT	M	85	N/A	45	Good	Carbone et al. (2015)	Geometries from the TLEM2 specimen's CT scans. Bilateral.
VAKHUM-CT	M	N/A	N/A	N/A	Fair	Van Sint Jan (2006)	Bones from two CT scans. STAPLE cannot create a full lower limb (yet).

STAPLE variables and conventions

These notes are provided to offer a minimal guidance to anyone that will investigate the STAPLE code in details:

• Reference system conventions: the output reference systems of all STAPLE scripts have axes consistent with conventions of the International Society of Biomechanics, but internally this is not always the case. In many algorithms, technical reference systems mutuated from GIBOC-core are used. These reference systems are defined according to the following convention:

  • X directed in anterior-posterior direction, pointing posteriorly,
  • Y directed in medio-lateral direction, pointing laterally for the right leg,
  • Z directed in proximal-distal direction, pointing cranially.

• When an error or some unexplained interruption of the scripts happens during a morphological analysis, it is always possible to enable debug_plots and reconstruct step-by-step what the algorithm of interest is doing. As a general guidelines (not always respected), the colors of points, surfaces etc. were generally decided following the following convention:

  • red: medial
  • blue: lateral
  • green: not compartimentalised anatomical structures (basically the rest).

Body Coordinate Systems (BCS)

When running a morphological analysis, the body coordinate system (BCS) associated with a bone geometry is returned.

The BCS is a MATLAB structure with the following fields:

1. CenterVol [3x1] vector: the geometrical centroid of the geometry
2. InertiaMatrix [3x3] matrix: the inertia matrix calculated from the bone geometry
3. Origin [3x1] vector: the origin of the body coordinate system
4. V [3x3] matrix: the pose matrix of the body coordinate system
5. AuxCSInfo [MATLAB structure]: a variable including geometrical parameters obtained from the morphological analysis.

Joint Coordinate Systems (JCS)

When running a morphological analysis, the joint coordinate system (JCS) associated with the analysed geometry is also returned, together with the BCS.

The JCS is a MATLAB structure with the following fields:

1. joint name: the name of the joint if more than one are stored in the variable.
2. Origin [3x1] vector: the origin of the joint in the bone being analysed
3. V [3x3] matrix: the pose matrix of the joint
4. parent_location [1x3] vector: the origin of the parent coordinate system in OpenSim
5. parent_orientation [1x3] vector: three angles describing the pose of the parent reference system in OpenSim, obtained by matrix decomposition with XYZ rotation order.
6. child_location [1x3] vector: the origin of the child coordinate system in OpenSim
7. child_orientation [1x3] vector: three angles describing the pose of the parent reference system in OpenSim, obtained by matrix decomposition with XYZ rotation order.

Note that the algorithms used on a single bone normally cannot define a joint completely, but require a joint definition file that "merges" the information from all the articular surfaces of the joint. The available parameters computed by the morphological analyses are shown in the following table. The missing parameters are obtained by the user-provided joint definition.

Bone geometry	joint name	parent_location	parent_orientation	child_location	child_orientation
pelvis	ground_pelvis			Yes	Yes
	hip		Yes
femur	hip			Yes	Yes
	knee	Yes	Yes
	patellofemoral	Yes	Yes
tibia	knee				Yes
	ankle		Yes
patella	patellofemoral			Yes	Yes
talus	ankle			Yes	Yes
	subtalar	Yes	Yes
foot	mtp	Yes	Yes

Bone Landmarks

Bone landmarks are stored in MATLAB structures as [3x1] vectors. Each field name of the structure corresponds to the name of a landmark.

Bone geometry	Landmarks
pelvis	RASI/LASI: anterior superior iliac spine RPSI/LPSI: posterior superior iliac spine SYMP: pubic symphysis
femur	RKNE/LKNE: lateral femoral epicondyle RMFC/LMFC: medial femoral epicondyle RTRO/LTRO: greater trochanter
tibia	RTTB/LTTB: tibial tuberosity RHFB/LHFB: head of fibula RANK/LANK: lateral malleolus RMMA/LMMA: medial malleolus RLM/LLM: most distal point of lateral malleolus
patella	RLOW/LLOW: most distal point of patella
talus	None
foot	RHEE/LHEE: heel RD5M/LD5M: distal point of 5th metatarsal bone RD5MPROX/LD5MPROX: proximal point of 5th metatarsal bone RD1M/LD1M: distal point of first metatarsal bone R1MGROUND/L1MGROUND: distal point of first metatarsal bone on foot sole R5MGROUND/L5MGROUND: distal point of 5th metatarsal bone on foot sole RHEEGROUND/LHEEGROUND: calcaneus most distal point on foot sole

Troubleshooting your workflow

It can happen that you have issues processing some of your datasets. It could be a bug, but it could also be an issue related to the input data. Before informing us as suggested in the contributing guidelines, please go through the following troubleshooting checklist first:

• ensure that the entire STAPLE folder is on your MATLAB path
• if you are using one of the workflows from the examples, ensure that you are using the correct names for the bone geometries, e.g. pelvis_no_sacrum, femur_r, tibia_r, etc.
• ensure that the quality of your bone surface geometries is sufficient for running your selected algorithm. The GIBOC algorithms, in particular, require relatively good quality surface meshes. You can have an idea of what we mean by "good mesh" consulting the table that describes the datasets provided with STAPLE. If necessary, use the filters and tools available on software like MeshLab to improve your dataset.
• if possible, try running alternative algorithms in order to establish if your bone mesh is processable in the first place or if you have encounter a proper bug in the algorithm. You can specify the processing algorithms for each bone as input to the processTriGeomBoneSet.m function. For bad quality meshes, we recommend using the STAPLE algorithm at the pelvis and Kai2014 algorithms for femur and tibia. Keep an eye on issue #78 as we will develop an example on how to do this.
• verify that processing of your dataset is not failing because of the current limitations of the STAPLE toolbox.

Does STAPLE work only with OpenSim?

The algorithms collected in the STAPLE toolbox were proposed in publications that did not have modelling focus, and can be applied in broader contexts, such as reference system definition for orthopaedics applications or modelling in other platforms. Similarly, the outputs of a STAPLE workflow, e.g. from processTriGeomBoneSet.m, include all the necessary information to create a kinematic or kinetic model in any biomechanical modelling workflow. All our modelling examples, however, rely on OpenSim.

What are the differences between the STAPLE toolbox, NMSBuilder and the MAP Client for generating OpenSim models?

Some of the functions of the STAPLE toolbox overlap with those of NMSBuilder and of the statistical shape modelling package MAP Client, part of the Musculoskeletal Atlas Project (MAP). The following table outlines the differences between the out-of-the-box functionalities of STAPLE, NMSBuilder and the publicly available version of the MAP Client and its plugins. If you are a Map Client user and want this table to be more specific or spot any inaccuracy, we encourage you to contribute to it.

Functionality	STAPLE toolbox	NMSBuilder	MAP Client	Notes
Platform used for development	Matlab 2018b or higher.	User Interface	Python 2.7	Python 2.7 has been discontinued by developers since January 2020 (see details here). sThe OpenSim 4.x distribution support MATLAB and Python 3.x, while OpenSim 3.x supports Python 2.7.
Can I generate bone geometries?	No	No	Yes	Both STAPLE and NMSBuilder can create models from bone geometries generated from any source, including segmentation of medical images and statistical shape modelling frameworks like the MAP Client, but require them as user-provided inputs.
Can I generate OpenSim models?	Yes	Yes	Yes	Both STAPLE and NMSBuilder can create models for OpenSim v4.1, the MAP Client for OpenSim v3.3.
Can I generate full OpenSim models?	Yes	Yes	No	The models generated with the public version of the MAP Client do not include the bones below the tibia and their joints.
Can I generate partial models?	Yes	Yes	Yes
Can I merge a personalized and a generic model?	Yes	No	No	Ad hoc STAPLE functions are available for merging models. See the advanced example provided on this topic.
Can I batch process OpenSim models?	Yes	No	Yes	NMSBuilder is currently lacking a programming interface.
Can I include muscles in the generated models?	No	Yes	Yes	STAPLE has a muscle generation pipeline in development, as previewed in our published paper, but it has not yet been released.

Current limitations

• The STAPLE toolbox is still in strong development, so some key documentation might be missing. Please refer to the examples included the main STAPLE repository for now.
• STAPLE cannot create models from bones reconstructed from images that have more than one reference system (see provided VAKHUM_CT dataset as an example). This is due to the fact that each STAPLE morphological analysis is local to the bone where it is performed and therefore the only information about the relative position of the bones comes from the medical scans resting pose. It is possible to implement workarounds to this issue, e.g. estimating the missing joint parameters using regression equations, generic models or other approaches.
• The lower limb models currently include a patella rigidly attached to the tibia. An articulated patellofemoral joint is under development.

How to contribute

We welcome any contribution from the biomechanical and open source community, in any form. You can report a bug, submit code implementing a new feature or fixing an issue or share your ideas about new functionalities that you would like to see included in STAPLE. See below few tips for contributing:

• To report a bug, or anomalous behaviour of the toolbox, please open an issue on this page. Ideally, if you could make the issue reproducile with some data that you can share with us.
• To contributing to the project with new code please use a standard GitHub workflow:
  1. fork this repository
  2. create your own branch, where you make your modifications and improvements
  3. once you are happy with the new feature that you have implemented you can create a pull request
  4. we will review your code and potentially include it in the main repository.
• To propose feature requests, please open an issue on this page, label it as feature request using the Labels panel on the right and describe your desired new feature. We will review the proposal regularly but work on them depending on the planned development.

Code of conduct

Please refer to the code of conduct. In brief, be a nice person.

License

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Uses beyond those permitted by the license must be discussed with the authors.

Acknowledgements

Luca Modenese was supported by an Imperial College Research Fellowship granted by Imperial College London and by an Academy of Medical Sciences Springboard Grant [SBF004\1056] supported by the Academy of Medical Sciences, the British Heart Foundation, Diabetes UK, the Global Challenges Research Fund, the Government Department for Business, Energy and Industrial Strategy and the Wellcome Trust.