Mechanical assessment of a titanium cervical spine corpectomy cage assembled with 3D-printed patient-specific endplate-conformed contact surfaces and a traditionally manufactured expandable mechanism

This study developed a new titanium cervical spine expandable corpectomy cage (ECC) that consisted of two superior/inferior patient-specific endplate-conformed contact surfaces (PECSs) fabricated through 3D printing and a standardized centrally expandable mechanism (CEM) manufactured using traditional machining. Fatigue testing was conducted to evaluate whether the components of ECC could be assembled using different manufacturing methods to meet the functional testing requirements in compliance with FDA regulations. The titanium 3D-printed superior/inferior PECSs were designed based on the CT images/CAD system to assembly with a CNC milling/precision wire cutting cuboid CEM (14 mm square cross-section) expansion driven by a set of sliders interlocked via key and keyway mechanisms for the ECC of two-segmental vertebral bodies. The ECC underwent FDA-compliant static/dynamic compression, shear, torsion, subsidence tests and finite element (FE) analysis for understanding the stress differences on endplates between PECS and flat-shaped endplate contact surface (FECS). Stiffness/yield strength were found to be 2127 ± 146 N/mm/8547 ± 1213 N for the compression test, 1158 ± 139 N/mm/ 2561 ± 114 N for the shear test, and 1.22 ± 0.32 Nm/deg/16.5 ± 0.58 Nm for the torque test. Some failure samples showed that fixation screws were fractured/loosened under the shear test. The endure limits were 1600 N, 350 N and 1.0 N*m under compression, shear and torsion cyclic load tests, respectively. The results of the static axial compression and static/dynamic compression-shear testing did not meet the acceptance criteria of ISO 23,089. The PECS with higher stiffness value showed that better subsidence resistance was achieved than FECS and was consistent with that the maximum stress value and distribution for the FECS were more harmful than those for the PECS models. This study demonstrated that mechanical testing is essential when assembling 3D-printed components with CNC-manufactured parts in multi-component medical implants, as mismatches in interface behavior may result in mechanical failure. The results of FE analysis and subsidence tests indicated that PECS can decrease stress concentration between the ECC and endplates to present better performance for subsidence resistance.