The aim of this study was the analysis of the mechanical behaviour of a partially porous lumbar custom-made cage by means of a subject-specific finite element analysis (FEA). The cage, made of Ti6Al4V ELI alloy, was produced via electron beam melting (EBM) process and surgically implanted in a female subject, 50 years old. The novelty of this study was the customized design of the cage and of its internal structure, which is impossible to obtain with the traditional production techniques. The 3D model of the spine was obtained from the computed tomography (CT) of the patient. Moreover, high-resolution industrial CT was also used to reconstruct a 3D model of the cage, with its real (as-produced) features, such as superficial roughness, morphology of the bulk and of the porous structure. The workflow was divided in several steps: the main finite element analyses were non-linear and quasi-static regarding: the rhombic dodecahedron (RD) unit cell of the porous structure; the device; the whole L4-L5 motion segment with the implanted cage. Stress distribution was calculated under compression load for all models. For the RD unit cell, the maximum stress appeared at the connected cross nodes, where notch effect was present. For the cage subjected to a load of 1 kN, the porous structure did not present any functional failure. For the whole biomechanical system subjected to a physiological load of 360 N, the calculated stress in the bone was smaller than its yield strength value. On the axial view, a zone with higher compressive stresses was present on the L5 vertebral body. This was due to the contact stress between the cage and the vertebra. From the comparison between FE results and the CT images of the spine, bone remodelling was supposed, with the formation of new bone. Graphical abstract Workflow showing the phases of the research.