Page 419 - IJB-9-3

International Journal of Bioprinting 3DP PILF cage for osteoporotic

1. Introduction design for bone ingrowth. The cage structure was generated
by integrating finite element (FE) analysis and weight
Posterior lumbar interbody fusion is a common, effective topology optimization (WTO) under different lumbar
treatment for spinal degeneration and instability that restores spine activity ratios. The CS-type cage superior/inferior
intervertebral height and stabilizes the spinal column [1,2] . surfaces were generated based on the statistical results
However, subsidence is one of the potential clinical from osteoporosis patient endplate surface morphology.
complications of interbody cages and a major cause of The CS-type biomechanical behaviors and plate (P-type)
intervertebral disc height restoration failure. Biomechanically, cages under different daily activities were compared by
intervertebral cage subsidence is associated with the stress performing non-linear FE analysis. The lattice was designed
transfer capability influenced by the endplate/cage interfacial in the CS-type cage internal cavity and manufactured using
contact area and the stress-shielding effect degree . metal 3D printing to conduct in vitro biomechanical tests.
[3]
Although the lumbar cage superior and interior surface
morphology has been proposed for redesign with sufficient 2. Materials and methods
endplate-cage contact area to improve stress transfer and 2.1. Statistical analysis of endplate morphology of
reduce cage subsidence risk factors [3,4] , morphology changes elderly osteoporosis patients
in the lumbar spine and intervertebral discs were found to
be more significant for osteoporotic patients [5-7] . Serious The lumbar spine computed tomography (CT; Gold Seal
endplate concave curved surfaces, that is, intervertebral Optimal CT660 scanner, GE Healthcare, Chicago, IL,
disc mid heights were higher in the osteoporosis and USA) images from 20 elderly osteoporosis patients over
osteopenia patients compared to those of normal subjects. the age of 65 years from August 2018 to December 2018
[14]
This is unfavorable for stress transfer through the lumbar were collected to measure the endplate morphology .
cage surfaces. However, in the current posterior cage All lumbar spine images were obtained using a human
trial program, in accordance with the ethical standards
surface design, no suitable superior/interior surface
designs were found based on the endplate morphology for approved by the Institutional Review Board of the Tri-
osteoporosis statistical results. Services General Hospital, Taipei, Taiwan (approval
number: 2-107-05-109). The image interval was 0.625 mm
The stress-shielding effect caused by the cage must to maintain clearer image quality for easy measurement of
also be avoided to reduce the subsidence risk factor. lumbar endplate morphology.
Accordingly, many authors have promoted metal-free
The lumbar spine mean Hounsfield units (HU) value
material with the aim of reducing segmental stiffness, such was measured as the criterion for determining osteoporosis
as polyether ether ketone (PEEK), ceramics, or composite according to the methodology defined by Patel et al. . The HU
[5]
material (COMBO cage composed of metal and PEEK) to value range between 70 (about bone density = 0.648 g/cm )
3
prevent obvious stress-shielding effects [3,8] . However, the and 120 (about bone density = 0.833 g/cm ) was used as
3
interbody fusion mechanism of these materials was not the acceptance criterion for judging osteoporosis in this
found to be more prominent than that of titanium alloys . study. Severe osteoporosis with HU value below 70 was
[3]
Making a metal cage with sufficient strength and stress- excluded because vertebral interbody fusion surgery was
shielding effect reduction through structural optimization not recommended for patients by the clinician . To ensure
[14]
must be improved in the redesign process.
lumbar spine image measurement accuracy, older adults
Using metal additive manufacturing (AM, also known as who had lumbar fractures with lumbar implantation,
3D printing) techniques to create a lattice design on the implant vertebroplasty, kyphoplasty, endplate fracture, kyphosis,
surface or internal cavity has been proven in many studies to Kümmell’s disease, or other lumbar surgeries, and who had
induce and promote better osseointegration [9-13] . The titanium spinal cancer, cancer with spinal metastasis or bone cortex
cage is expected to have appropriate structural strength and hyperplasia were excluded from the study .
[14]
bone growth ability when the lattice design concept can be The superior/inferior endplate morphologies were
used in the internal cage cavity. However, this complex, high- measured from the second (L2) to the fifth (L5) osteoporotic
precision hybrid design must be achieved using the metal AM lumbar vertebrae. After the 3D lumbar spine image was
technique, which has great potential to create a porous (lattice) reconstructed using reverse engineering software (Creo
structure in a dense titanium body [9-13] .
Parametric v2.0, PTC, Needham, MA, USA), the curved
In this study, we designed a posterior lumbar interbody surface endplate position variations were measured at
fusion cage for the osteoporosis patient with appropriate 25%, 50%, and 75% of the endplate length in the coronal
structural strength and superior/inferior curved surface and sagittal planes (Figure 1). The amount of each
(CS-type) design for stress transfer as well as internal lattice measurement position was counted to obtain the average

Volume 9 Issue 3 (2023) 411 https://doi.org/10.18063/ijb.697