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Biomechanical Impact of Footprint Mismatch and Implant Positioning in Cervical Disk Replacement: A Finite Element
Ming-Kai Hsieh1, Fu-Cheng Kao1, Yu-Cheng Wang2
1Department of Orthopaedic Surgery, Spine Section, Bone and Joint Research Center, Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Taoyuan, Taiwan.
Background And Objectives:
A footprint mismatch between the cervical disk replacement implant and vertebral endplates is common; however, debate remains regarding the optimal implant position when a footprint-undersized device is unavoidable. Our purpose is to evaluate the biomechanical impact of a footprint-undersized ball-and-socket cervical disk replacement on spinal kinematics, facet joint stress, endplate contact stress, and adjacent disk stress in different sagittal position.
Methods:
A validated C4-C7 finite element model was developed from computed tomography data. A Prodisc® C Vivo prosthesis was implanted at C5-C6 in 2 configurations: (1) footprint-matched and (2) mismatched using undersized implants positioned anteriorly, centrally, and posteriorly. Physiological follower loads (98 N) and pure moments (2 Nm) were applied to simulate flexion, extension, lateral bending, and axial rotation. Ranges of motion, implant-endplate contact stress, core stress, facet joint stress, and adjacent disk stress were compared.
Results:
The mismatched-anterior model reduced C5-C6 ranges of motion by 22% in flexion but increased facet joint stress by 166% during extension and adjacent disk stress by up to 148% in flexion. Implant-endplate contact stress increased by up to 277% in the mismatched-anterior model, suggesting a higher risk of subsidence. The mismatched-central model exhibited lower increases in core stress, while the mismatched-posterior model preserved comparable flexion and extension motions, indicating that both models demonstrated relatively more favorable biomechanical performance.
Conclusion:
Undersized cervical disk implants significantly alter segmental biomechanics, especially when placed anteriorly, increasing the risk of subsidence, facet overload, and adjacent segment degeneration.
