Assessment of the Strain Field Induced Under Load in Transforaminal Lumbar Interbody Fusion Cage Printed in Additive Manufacturing

Transforaminal lumbar interbody fusion (TLIF) cages are used to reduce compression on the neural tissue of the spine and to restore the intervertebral space. Today, these medical devices can be advantageously obtained by additive manufacturing and consist of one or more lattice structures, which allow for a substantial improvement of the osseointegration process. Due to the small sizes and, above all, the fine level of detail of the lattice structure, traditional investigation methods applied for evaluating the mechanical behavior of such components are difficult to employ. A promising alternative is noncontact optical measurement techniques. This work aims to evaluate the feasibility of digital image correlation (DIC) to study the mechanical response of small and complex 3-D medical protheses, specifically of the TLIF cages structures. A cage was loaded statically, using two dedicated coupling elements, to reproduce the physiological mechanical behavior of the lumbar area of the spine. Both the displacement and the strain fields were measured on three regions of the sample. Load-deflection curves, computed by DIC, have been compared with those obtained by a testing machine, and a good agreement was observed. The results highlighted a severe overstress of the lattice structure, which eventually might lead to local failure of the trabeculae, causing injury to the vertebrae, the bones, and the connective tissues of the spine.