A high-precision 3D scanning system acts as the foundational geometric tool for calculating Rotating Midfoot Moment (RMM) by automating the measurement of the lever arm involved in foot mechanics. By creating a complete digital model of the foot, the system identifies the "Navicular Moment Arm" (NMA)—the precise horizontal distance between the navicular point and the foot’s centerline—which serves as the critical multiplier for determining pronation forces.
The system converts complex foot anatomy into precise mathematical data. By combining the geometrically derived Navicular Moment Arm (NMA) with the subject's weight distribution, the system provides an objective calculation of RMM that reveals how the midfoot handles torque under weight-bearing conditions.
The Mechanics of Digital Measurement
Capturing the Complete Foot Structure
To calculate moments accurately, you first need a high-fidelity representation of the object.
A high-precision 3D scanning system captures multi-dimensional images of the foot. Unlike simple footprint scanners, this system records both the plantar (bottom) and dorsal (top) surfaces. This dual-surface capture is essential for locating anatomical landmarks that are not visible from the bottom alone.
Automating the Reference Axis
In biomechanics, every moment (torque) requires an axis of rotation.
The scanning software automatically processes the 3D model to generate a foot centerline. This line is defined geometrically by connecting the second toe to the heel. This centerline serves as the standardized axis against which lateral or medial deviation is measured.
Defining the Navicular Moment Arm (NMA)
Once the axis is established, the system determines the length of the lever arm.
The software identifies the navicular point on the digital model. It then calculates the exact horizontal distance from this navicular point to the previously established centerline. This specific distance is defined as the Navicular Moment Arm (NMA).
From Geometry to Biomechanics
Integrating Weight Distribution
Geometry alone does not create torque; force is required.
To move from a simple distance measurement (NMA) to a biomechanical metric (RMM), the system combines the NMA data with the distribution of the subject's weight. The weight acts as the force applied to the lever arm identified by the scanner.
Quantifying Pronation Tendency
The final output of this process is the Rotating Midfoot Moment (RMM).
The RMM is a core physical quantity that reflects the torque acting on the midfoot. This value directly correlates to the pronation tendency of the foot under weight-bearing conditions. A larger moment typically indicates a greater tendency for the arch to collapse or the foot to roll inward.
Understanding the Trade-offs
Dependence on Landmark Detection
The accuracy of the RMM calculation relies entirely on the system's ability to identify the navicular point correctly.
If the scan of the dorsal surface is obstructed or low-resolution, the system may miscalculate the horizontal distance to the centerline. This geometric error would cascade into the final RMM calculation, leading to incorrect assumptions about pronation.
The Sensitivity of the Centerline
The system uses a standardized definition for the centerline (second toe to heel).
While effective for the vast majority of subjects, this geometric standard assumes a relatively normal forefoot alignment. In cases of severe foot deformity or toe deviation, the standard "second toe to heel" axis might not perfectly represent the functional axis of the foot, potentially skewing the NMA measurement.
Making the Right Choice for Your Goal
When utilizing 3D scanning for biomechanical analysis, consider how these metrics align with your objectives:
- If your primary focus is Clinical Diagnosis: Rely on the Navicular Moment Arm (NMA) measurement to objectively quantify the severity of midfoot collapse before assessing motion.
- If your primary focus is Custom Footwear or Orthotics: Use the Rotating Midfoot Moment (RMM) to understand the rotational forces the support device must counteract to stabilize the foot.
Accurate RMM calculation transforms subjective observation of pronation into a quantifiable physical metric based on precise 3D geometry.
Summary Table:
| Metric | Definition | Role in Biomechanics |
|---|---|---|
| NMA | Navicular Moment Arm | The horizontal distance between the navicular point and the foot centerline. |
| Foot Centerline | Axis of Rotation | Standardized line connecting the second toe to the heel for torque calculation. |
| RMM | Rotating Midfoot Moment | The torque (NMA × Weight) reflecting pronation tendency under weight-bearing. |
| 3D Scanning | Geometric Foundational Tool | Captures plantar and dorsal surfaces to identify critical anatomical landmarks. |
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References
- Tomoko Yamashita, Shingo Ata. Evaluation of Hallux Valgus Using Rotational Moment of Midfoot Measured by a Three-dimensional Foot Scanner: a Cross-sectional Observational Study. DOI: 10.14326/abe.12.154
This article is also based on technical information from 3515 Knowledge Base .
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