Non-invasive quantification of lower limb mechanical alignment in flexion

Area of Science:

• Orthopedic biomechanics
• Medical imaging and navigation
• Surgical planning technologies

Background:

• Non-invasive navigation systems offer potential for determining mechanical femorotibial alignment (MFTA) in extension.
• Evaluating the precision and accuracy of these systems, particularly with varying flexion and stress, is crucial for clinical application.
• The method of attaching optical trackers can significantly impact measurement reliability.

Purpose of the Study:

• To assess the precision and accuracy of an image-free navigation system for multiple knee kinematic measurements.
• To compare the effectiveness of fabric versus rubber straps for optical tracker fixation.
• To evaluate MFTA measurement accuracy under varying degrees of knee flexion and coronal stress.

Main Methods:

• Seventy-two registrations were performed on 6 cadaveric knees using an image-free navigation system.
• Mechanical femorotibial alignment (MFTA) was measured with fabric straps, bone screws, and rubber straps under no stress, varus, and valgus stress at varying flexion angles (0-60°).
• Precision and agreement were analyzed using intraclass correlation coefficients, repeatability coefficients, and limits of agreement (LOA), with ≤ 3° considered acceptable.

Main Results:

• Fabric straps and screws demonstrated high precision (repeatability coefficient ≤ 2°) for MFTA measurement in extension.
• Precision decreased with increased knee flexion (>50°) and with the application of varus/valgus stress, particularly beyond 40° flexion.
• Rubber straps showed lower precision and wider limits of agreement compared to fabric straps, especially at higher flexion angles and under stress.

Conclusions:

• Non-invasive systems can achieve acceptable precision and accuracy for knee kinematics in early flexion, but accuracy diminishes with greater flexion.
• Passive trackers should not be mounted with rubber strapping due to reduced measurement reliability.
• This technology holds promise for clinical applications like deformity assessment, surgical planning, and post-operative evaluation.