Landmarks

This page has been updated, but the original page can be found here.

Landmarks are like virtual markers, whose positions are computed automatically by Visual3D based on the positions of the actual markers, segments, or equations, according to rules which you define.

Landmarks provide a powerful method of representing significant locations within space or locations defined by the model. Landmarks are created by either specifying a location in a segment coordinate system or by transforming markers or other landmarks into a significant location.

Marker Set for Examples

The following examples use the markers related to the left shank illustrated below.

1. LMK: Left medial knee
2. LLK: Left lateral knee
3. LSK: Left shank
4. LMA: Left medial ankle
5. LLA: Left lateral ankle
6. LTUB: Left tibial tuberosity (shown projecting a shadow onto the plane)
7. LKJC: Left knee joint centre (landmark)

General Options

The landmark dialog can be broken into 3 sections:

1. Define Orientation
2. Offset
3. Calibration Only

1. Define Orientation Using

Landmarks can be defined relative to a target, along a line, on a plane, or as a projection onto a line or a plane. Landmarks can be defined in lab space, or in a segment coordinate system.

2. Landmark Offset from Start Point (Reference) or Segment Origin

Any time a landmark can be defined using Offsets in three directions, a user has the option to define the offsets manually (X/Y/Z or ML/AP/AXIAL) or they can use the "Offset to Existing Calibration Target or Landmark" option.

2a. Offset to Existing Calibration Target or Landmark

The Offset to Existing Calibration Target or Landmark option allows users to get the location of a target (or landmark) relative to a plane or segment coordinate system. The offsets are populated automatically. This option can be used any time all three offsets can be specified, but it is only described as a special type for Type 4 since this is a crucial example.

2b. Offset Using the Following ML/AP/AXIAL Offsets

Offsets should be specified in meters unless "Offset by Percent" is checked. Offsets can be specified as Expressions.

3. Calibration Only Landmark

When a landmark is set to "Calibration Only" it means that the landmark will only exist in the calibration file. When this option is not checked, it means that the landmark will be created in the associated motion files.

If a landmark is created in the associated motion files, it can be used to track a segment or for other calculations.

Landmark Types

Although there are many different ways to define a landmark, all of these methods result in a "virtual marker" whose position is calculated by Visual3D throughout motion trials.

Type 1: In Reference Coordinate System

The simplest landmark is a landmark which is defined in a segment coordinate system. There are many variations of this, but a landmark can be defined in LAB space, a landmark can be defined in a segment coordinate system, or a landmark can be defined by a 4x4Rotation Signal . The offsets define where the landmark is created relative to the coordinate system.

Type 1a: In LAB Space

A landmark defined in LAB space.

Sample Landmark Definition (click to expand)

1. Create Relative_Axes: Click Landmarks button Click Add New Landmark button Create Landmark: Relative_Axes    Landmark Name: Relative_Axes    Define Orientation Using:    Existing Segment: Lab Offset Using the Following ML/AP/AXIAL Offsets:    X: LAJC::X    Y: LAJC::Y    Z: 0.0 Do NOT Check: Offset by Percent (1.0 = 100%)

Example Using Starting Point (click to expand)

Landmarks can be created as an offset from the specified marker in a segment coordinate system or in the LAB coordinate system. vRLK that is created from RLK as an offset in the x direction in the LAB coordinate system.

Other Examples:

• Virtual Lab Tutorial: The LAB_O, LAB_X, and LAB_Y landmarks in the virtual lab tutorial are defined in LAB space, where the offsets refer to the distance the landmarks are created in meters.
• The following two examples describe how to define landmarks that represent stair corners. The position of targets placed on the steps are calculated from dynamic trials is used to define the landmarks.
  • Using Motion Trial Example 1
  • Using Motion Trial Example2

Type 1b: In a Segment Coordinate System

Landmarks can be created relative to a segment (in the segment coordinate system).

Sample Landmark Definition (click to expand)

1. Create Relative_Segment: Click Landmarks button Click Add New Landmark button Create Landmark: Relative_Segment    Landmark Name: Relative_Segment    Define Orientation Using:    Existing Segment: Left Shank Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.0    Y: 0.0    Z: -1 Check: Offset by Percent (1.0 = 100%)

Other Examples:

• Create a landmark relative to the thigh segment using ML, AP, and AXIAL offsets as percents.
• Creating a landmark at the end of a segment that is tracked with the segment.
• Hip Joint Centers are often defined using regression equations which locate the hip joint centers in the pelvis coordinate system:
  • Bell and Brand Regression Equations
  • Hip Landmark from Custom Regression Equation
• A Knee Alignment Device (KAD) defines the joint centers by defining landmarks in the KAD segment coordinate system
• Create a Landmark Mimicking contralateral segment. This example uses several different types of landmarks, but the end result is a landmark created in the foot coordinate system.

Type 1c: Relative to a 4x4 Rotation Matrix

Sample Landmark Definition (click to expand)

1. Create RTH_Origin: This will create a Landmark at the position of the Rotation signal RTH4x4 Click Landmarks button Click Add New Landmark button Create Landmark: RTH_Origin    Landmark Name: RTH_Origin    Define Orientation Using:    Existing Coordinate System: RTH4X4 Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.0    Y: 0.0    Z: 0.0 Check: Offset by Percent (1.0 = 100%)

Type 2: On a Line

A landmark can be defined along a line.

Sample Landmark Definition (click to expand)

1. Create Point_Line: Click Landmarks button Click Add New Landmark button Create Landmark: Point_Line    Landmark Name: Point_Line    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK Offset Using the Following AXIAL Offset: 0.5 Check: Offset by Percent (1.0 = 100%)

Create landmark as a midpoint (click to expand)

The landmark RKnee is a point that is halfway between the lateral and medial knee markers RLK and RMK. The point on a line can be defined as a distance in percentage or distance in meters. When Offset by Percent is checked, the distance is defined as a percentage. Note: When using a Point on a Line landmark, only the AXIAL offset is used.

Type 3: On a Plane

Type 3a: On a Plane using manual offsets

Landmarks can be created relative to a plane or technical coordinate system.

Sample Landmark Definition (click to expand)

1. Create LAND: Click Landmarks button Click Add New Landmark button Create Landmark: LAND    Landmark Name: LAND    Define Orientation Using:    Starting Point: P1    Ending Point: P2    Lateral Object: P3 Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.01    Y: 0.02    Z: 0.01 Do NOT Check: Offset by Percent (1.0 = 100%)

A right handed orthogonal coordinate system is defined by the three points (P1, P2, P3). The origin of the coordinate system is at the point P1. The frontal plane of the coordinate system is defined by the three points. The positive AP direction is into the screen.

Other Examples:

• Digitized Landmarks are stored and tracked relative to tracking targets
• Functional Landmarks (Joints) are stored and tracked relative to tracking targets
• Muscle insertion points are stored and tracked relative to tracking targets

Type 3b: On a Plane using target/landmark offsets

Create a landmark relative to three targets. The offset of the landmark is defined using the location of a target relative to the three targets during the static trial.

Landmark Definition (click to expand)

1. Create Relative_Tracking: Click Landmarks button Click Add New Landmark button Create Landmark: Relative_Tracking    Landmark Name: Relative_Tracking    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Lateral Object: LSK Offset to Existing Calibration Target or Landmark: LKJC Do NOT Check: Offset by Percent (1.0 = 100%)

Example 1 (click to expand)

To define a landmark as a point on a plane defined by 3 points Select the Targets and/or Landmarks radio button Select three targets and/or landmarks for the Reference/Axial Starting Point, Axial Ending Point and lateral object. The landmark/target reference offset is also selected. The RKnee calibration only landmark is the point that is being tracked by 3 target markers on the thigh RTH1, RTH2, and RTH3.

Other Examples:

• Create a Landmark Using two standing trials
  • Example 1
  • Example 2
• Example using a reference location
• Example defining a segment endpoint relative to 3 tracking targets

Type 4: Project on a Line

A target or landmark may be projected onto a line defined by two points.

Landmark Definition (click to expand)

1. Create Project_Line: Click Landmarks button Click Add New Landmark button Create Landmark: Project_Line    Landmark Name: Project_Line    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Project From: LTUB Do NOT Check: Offset by Percent (1.0 = 100%)

Project the heel target onto the line defined by the mediolateral ankle targets (click to expand)

Point projection onto a line can be used at the shoulder to project a lateral target on to a vertical line passing through the acromium or it can be used at the knee to project lateral or medial targets on a functional axis. The RAnkleCenter is the point that is created by projecting the RHL heel marker onto the line created from the lateral and medial ankle markers RLA and RMA.

caption

Other Examples:

• Project onto a line

Type 5: Project on a Plane

A point can be projected onto a plane defined by three points.

Landmark Definition (click to expand)

1. Create Project_Plane: Click Landmarks button Click Add New Landmark button Create Landmark: Project_Plane    Landmark Name: Project_Plane    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Lateral object: LMK    Project From: LTUB Do NOT Check: Offset by Percent (1.0 = 100%)

Example: Project the iliac crest target onto the plane defined by the R/LASIS and RPSIS (click to expand)

By projecting multiple landmarks onto the same plane you can build segment coordinates systems in a specific plane. The RPPinPlane is the point that is created by projecting the RPP marker on the illiac crest onto the plane created from the right and left ASIS's (RASI,LASI) and the right PSIS RPS marker.

Other Examples:

• Project the hip joint onto a plane passing through the GT's
• Example 2: Project onto a principal plane of the lab

Digitized Landmarks

Digitized landmarks are defined using a digitized pointer and again result in a "virtual marker" being computed by Visual3D relative to the positions of other, existing markers. The goal of digitized landmarks is to allow users to track anatomical landmarks without having to place markers on that location. See instructions for using a digitizing pointer to learn more.

Functional Landmarks

Functional landmarks indicate the locations of functional joints. Calculating functional joint positions requires movement of one segment relative to another segment. The chosen algorithm then searches for a point (or for a one degree of freedom joint, an axis) that is stationary relative to the 2 segments (or 2 sets of markers).

Visual3D's algorithms for computing functional joint positions are adapted from:

[Schwartz MH, Rozumalski A (2005) A new method for estimating joint parameters from motion data. Journal of Biomechanics, 38, 107-116]

and from

[Jensen E, Lugade V, Crenshaw J, Miller E, Kaufman K (2016) A principal component analysis approach to correcting the knee flexion axis during gait. Journal of Biomechanics, in press]

See the in-depth explanation of functional joints to learn more.