Computational modelling and additive manufacturing to enhance the design process of lumbosacral interbody fusion implants

Lumbosacral (L5-S1) interbody fusion is a widely applied procedure for the treatment of an array of degenerative spinal conditions, deformities and traumas. Despite the dramatical increase in spinal fusion surgeries over the past two decades, complications such as pseudoarthrosis and subsidence are still frequent. The aim of this research was to enhance the design process of fusion implants by exploring different additively manufactured (AM) and computational modelling methods. This study involved both engineering and clinical studies. First, a combined approach of fused ilament fabrication (FFF) and finite element (FE) modelling was proposed to characterise the influence of material and infill parameters on the performance of a low-density porous implant. AM polyether-ether-ketone (PEEK) was included as a candidate material for implant fabrication. Second, the influence of implant width on clinical outcomes was explored assessing biomechanical stability and subsidence following minimally invasive transforaminal lumbosacral interbody fusion (MI-TLIF). A FE model of an L5-S1 segment instrumented with different implant sizes was developed. Results indicated that the larger implant could be safely implanted without additional complications and reduced risk of subsidence. Third, a statistical shape modelling framework was implemented to a set of L5-S1 vertebral endplates to describe variability in terms of endplate shape and intervertebral alignment, by computing a mean endplate shape along with its deformation models. The results of this study showed that variation in endplate morphology is described by changes in scale, eccentricity and curvature, which can be gender- and disease-specific. As all females with spondylolisthesis fell within the same shape cluster, a pilot study was conducted to design a lumbosacral MI-TLIF implant tailored for this subgroup. Topology optimisation was implemented to increase the hollow space for bone graft material, whilst affording implant stability. This work provides the basis for future investigation of materials and population-based design as alternative solutions for lumbosacral fusion implants