3D-printed porous titanium versus PEEK interbody cages in lumbar fusion: biomechanical rationale, clinical evidence, and emerging indications.

Lumbar interbody fusion is a cornerstone treatment for degenerative spinal conditions. PEEK cages offer an elastic modulus approximating cancellous bone and radiolucency, yet their bioinert surface can result in fibrous encapsulation rather than direct bone apposition. 3D-printed porous titanium (3D-pTi) cages address these limitations through an engineered porous architecture that reduces effective stiffness to near-cancellous bone levels while enabling bone ingrowth throughout the implant volume. This narrative review synthesizes evidence spanning in vitro surface science, large-animal models, human retrieval histology, randomized trials, cohort studies, and meta-analyses. A rat tibial experiment demonstrated that surface topography accounted for substantially more variance in fixation strength than surface chemistry under the specific conditions tested (∼72 μm pore size, 8-week endpoint); whether this relative weighting extrapolates to the human lumbar interbody environment has not been established. Large-animal studies show that 3D-pTi cages without bone graft can outperform grafted PEEK cages, and human retrieval histology confirms lamellar bone in direct contact with the titanium scaffold without fibrous interposition. A randomized trial terminated at interim analysis reported 6-month fusion rates of 100% for 3D-pTi versus 0% for PEEK, though small sample size limits generalizability. Meta-analytic evidence consistently suggests accelerated early fusion with 3D-pTi, though between-group differences narrow by 12 months and no network meta-analysis pairwise comparison reached statistical significance. The clinical significance of accelerated fusion for long-term patient-reported outcomes remains unestablished-meta-analyses report no significant between-group differences in pain or disability scores. Regression analyses indicate that cage surface area and shape, rather than material identity, independently predict subsidence. Titanium-coated PEEK improves early fusion over uncoated PEEK but cannot achieve three-dimensional bone ingrowth. PEEK retains specific advantages: radiolucency, a well-characterized safety record, and greater ease of revision when cage removal is anticipated. For most clinical scenarios, current evidence does not mandate one material; cage selection should be individualized.