Cervical Disc Arthroplasties Fail to Maintain Physiological Kinematics Under Lateral Eccentric Loads

Area of Science:

• Biomechanics
• Spinal Surgery
• Orthopedic Prosthetics

Background:

• Cervical disc arthroplasties (CDA) aim to restore spinal function but may alter biomechanics.
• The instantaneous helical axis (IHA) is a key parameter for analyzing spinal unit kinematics.
• Limited research exists on IHA changes in all three motion dimensions after CDA.

Purpose of the Study:

• To investigate the impact of two different CDA prostheses on the instantaneous helical axis (IHA) of human cervical spinal units.
• To analyze changes in IHA orientation and position under various loading conditions.
• To identify kinematic alterations that could inform future prosthesis design.

Main Methods:

• In vitro biomechanical analysis of ten human C4-5 functional spinal units.
• Testing in intact conditions (IC) and after CDA with two prosthesis types.
• Application of axial rotation, lateral bending, and flexion/extension with eccentric preloads.
• Analysis of IHA orientation and resting position (IHA0).

Main Results:

• CDA altered IHA kinematics compared to intact conditions, particularly under lateral preloads.
• The shift and orientation of the IHA0-position and IHA orientation reversed direction after CDA compared to IC under lateral loading.
• Flexion/extension also showed altered IHA orientation post-CDA.

Conclusions:

• Current CDA prostheses do not fully restore physiological IHA characteristics under lateral loading.
• Findings highlight a need for improved prosthesis design to optimize cervical spine kinematics.
• This study provides new insights into biomechanical performance for better CDA development.