Reliability of Achilles Tendon Moment Arm Measured In Vivo Using Freehand Three-Dimensional Ultrasound

Area of Science:

• Biomechanics
• Musculoskeletal Ultrasound

Background:

• The Achilles tendon moment arm (ATMA) is crucial for understanding ankle biomechanics.
• Accurate in vivo measurement of the ATMA is essential for clinical and research applications.

Purpose of the Study:

• To assess the reliability of freehand 3D ultrasound (3DUS) for measuring the in vivo human Achilles tendon moment arm.
• To compare measurements using straight versus curved tendon models and evaluate the impact of ankle position.

Main Methods:

• Sixteen healthy adults underwent repeated 3DUS scans of the Achilles tendon and surrounding bony landmarks.
• 3D reconstructions were created from segmented ultrasound images to calculate the ATMA using straight and curved tendon lines-of-action.
• Measurements were taken across a range of ankle positions from maximal dorsiflexion to maximal plantar flexion.

Main Results:

• Freehand 3DUS demonstrated high reliability for ATMA measurement, both within and between sessions (ICC > 0.92, CV < 2.5%).
• The ATMA significantly increased from maximal dorsiflexion (~41 mm) to maximal plantar flexion (~48 mm).
• Accounting for tendon curvature resulted in a small, clinically insignificant overestimation (< 2 mm) of the ATMA.

Conclusions:

• Freehand 3D ultrasound is a reliable method for in vivo Achilles tendon moment arm measurement.
• Ankle plantar flexion increases the Achilles tendon moment arm, a finding consistent across both measurement models.
• The effect of tendon curvature on ATMA measurement is minimal and can be disregarded for practical purposes.