Failure Mechanisms of Zero-Profile Cervical Cages
Numerical and experimental study of screw-bone interface failure and windshield wiper effect in zero-profile ACDF implants.
Anterior cervical discectomy and fusion (ACDF) is one of the most common surgical procedures for treating cervical disc herniation. Traditional cages rely on anterior fixation plates, which can cause complications such as dysphagia and adjacent segment degeneration. Zero-profile cages were developed to mitigate these issues: they are anchored directly to the vertebral bodies via screws, eliminating the need for an anterior plate.
While this design reduces implant profile and anterior soft-tissue irritation, it shifts the mechanical demands to the screw-bone interface. Under cyclic physiological loading, micro-movements at this interface can enlarge the bone tunnel through a ratcheting mechanism known as the windshield wiper effect, potentially causing premature screw loosening — particularly in the early stages after implantation. Current testing standards, designed for traditional cages evaluated in isolation, do not adequately capture this failure mode.
This project aims to identify and quantify the failure mechanisms of zero-profile cervical cages through an integrated numerical-experimental approach:
- Geometric and constitutive modeling of the full cage-screw-vertebra system in 3D, incorporating bone and implant material properties;
- Experimental validation via pull-out and primary stability tests at the LEBm (Laboratório de Engenharia Biomecânica) on bone substitute specimens, to calibrate the computational screw-bone interface model;
- Fatigue simulation under cyclic compressive, shear, and torsional loads, characterizing micro-movements at the interface and the onset of the windshield wiper effect;
- Identification of critical loading scenarios and comparison with failure modes of traditional cages;
- Normative proposal: development of testing guidelines and protocols that account for direct vertebral fixation, addressing a gap in current regulatory standards for zero-profile devices.
This project is conducted in direct collaboration with the LEBm, which holds ANVISA accreditation for mechanical testing of medical devices and prostheses, and with national orthopedic industry partners.
Image: 3D render of a zero-profile cervical interbody cage (C2C Spiker System) showing the PEEK body and integrated anchoring screws. © MJ Surgical / C2C Spine System (mjsurgical.com).