Page 454 - IJB-9-5

International Journal of Bioprinting 3D-printed oblique lumbar interbody cage

1. Introduction printing, which can create a porous structure on a dense
titanium body [9-15] .
Lumbar interbody fusion is an effective and common
treatment of spine-related diseases. Alongside the The objective of this study was to design an anatomical
development of minimally invasive surgery, the technique contour OLIF cage based on the anatomical surface
of oblique lateral lumbar interbody fusion (OLIF) has also morphology of patients with osteoporosis. We aim to design
recently been developed, which reduces the risk of wound an optimal structure with lattices structure to enhance
infection, minimizes muscle/soft tissue cutting, and involves bone ingrowth and embedded screw fixation to increase
a shorter operation time [1,2] . During surgery, a larger-area the stability. This OLIF cage was fabricated via metal 3D
cage can be expected to provide greater spinal stability, printing and tested using the fatigue biomechanical test to
but the resulting vertebral instability can lead to cage confirm whether its characteristic is in compliance with the
subsidence [1,2] . In particular, patients with osteoporosis have criteria set by U.S. Food and Drug Administration (FDA).
unstable spines and are at a higher risk of cage subsidence
when managed with an OLIF cage alone [2,3] . 2. Materials and methods
Combining the posterior pedicle screw fixations with 2.1. Finite element model generation and validation
the OLIF cage can reduce surgical complications and A previously validated finite element (FE) model was
prevent cage subsidence [2,3] . However, the use of pedicle used in our mechanical simulation analysis, and a brief
[16]
screws increases surgical burden and causes additional description of the generation method is given below . A
wounds when being implanted into the spine bodies. 70-year-old female without severe bone spurs, fractures,
One approach to enhance OLIF cage stability is through and scoliosis was selected as a volunteer, and computed
additional anterior fixation with a lateral bone plate. tomography (CT) scans were performed using a 0.625-mm
Nevertheless, protrusion of the lateral bone plate was prone interval to reconstruct lumbar vertebrae mock-ups of the L2–
to surrounding tissue wear and damage as the clinical L5 lumbar vertebrae. The solid model includes cortical bone,
complication [4,5] . Therefore, embedding screws within the cancellous bone, endplates, intervertebral discs (nucleus
OLIF cage can directly fix the cage and the vertebral body and annulus fibrosus), and facet joints (with a 1-mm gap).
through surgical insertion. The ligaments associated with L2 to L5 were constructed
according to their anatomical location (Figure 1) .
[17]
Some studies have pointed out that the contact area
between the cage and the endplate can affect subsidence , The lumbar spine solid model was exported to the
[6]
and the contour of the superior/inferior surface of the cage ANSYS Workbench (ANSYS Workbench v18.2, ANSYS
was related to the morphology of the endplate. The point Inc., PA, USA) for simulation. Mesh generation was
contact at the surface of the endplate easily causes stress performed using quadratic ten-node tetrahedral structural
concentration and endplate damage, resulting in a higher solid elements, and the mesh model accommodated a total
[16]
risk of subsidence. In recent years, an anatomical titanium of 689,810 elements and 1,022,598 nodes (Figure 1) .
alloy cage that conforms to the physiological appearance The material properties of cortex, cancellous bone,
of a patient-specific endplate was proposed to reduce endplates, and intervertebral discs were linear elastic and
stress concentration . However, the clinical application isotropic and were adopted from the literature (Table 1) [14,18] .
[7]
of anatomical titanium alloy cage is limited because of its All ligaments used the hyperelastic Ogden third-order
complicated design and time-consuming manufacturing formula for material properties [14,18] . The facet joint used
processes. a contact element with a friction coefficient of 0.2 to
simulate the sliding mechanism of the joint surfaces. It was
Nevertheless, a structure that assists bone ingrowth is validated that this FE model is reliable when it reaches a
needed to achieve effective bone fusion between the cage reduced order model (ROM) for L3–L4 variation within
[8]
and the endplate . Replacing the surface of the cage with a 20% of that reported by Yamamoto et al. [19-21] .
porous (lattice) biomimetic microstructure can effectively
promote the differentiation and growth of osteocytes 2.2. OLIF cage design
attached to the pore structure [9,10] . It is well known that To simulate the morphology of vertebral endplates with
pore design with 60%–70% porosity and pore size under osteoporosis in elderly patients, the average morphological
800 μm, manufactured using AM, provide biologically subsidence of L2–L5 endplates in 20 elderly patients with
active and mechanically stable surfaces for implant fixation osteoporosis was obtained [16,22] . Endplate subsidence was
to bone [10-12] . However, a complex structure with a high- calculated at 25%, 50%, and 75% of the endplate length
precision hybrid design of the cage cannot be fabricated in both the coronal and sagittal planes (Figure 1), and the
by the traditional mechanical cutting process, and thus, FE model was modified to represent the endplate concave
there is a need to employ metal three-dimensional (3D) characteristics.

Volume 9 Issue 5 (2023) 446 https://doi.org/10.18063/ijb.772