Page 188 - IJB-10-4

International Journal of Bioprinting Effects of structure on the interbody cage

Figure 6. (A) Weight loss rate and (B) change curves of distinct degradation phases of each set of cages.

per week. We believe that this velocity change is related Consequently, these dissimilar properties can induce a
to the shedding of HA particles. The proportion of PCL stress-shielding phenomenon, ultimately resulting in the
in the surface layer of the initial state fusion apparatus is collapse of bone tissues surrounding the implantation
larger, so the degradation rate is faster. As the degradation site. 29,32 The setting of the foramen can adjust the elastic
time increases, HA is exposed and the percentage of PCL modulus to optimize the mechanical match between the
on the surface decreases, leading to a slower degradation cage and the surrounding bone tissues, providing stable
rate. With further degradation of PCL, the HA attached support for spinal fusion. 30
to the surface is shed, and the percentage of PCL on the The compressive strength of the cages is taken as
surface increases again, accelerating the degradation again. the value of compressive stress corresponding to 10%
By comparing the change in weight loss rate for different strain, and the compressive modulus is the value of the
structural features, we found that a larger aperture size slope of the elastic phase of the compressive curve. The
and more intersecting layers of beams translates to a compressive stress–strain curves of distinct degradation
bigger weight loss rate at each stage, which implies a faster phases of each set of cages are illustrated in Figure 7.
degradation rate. This variation may be attributed to the From the picture, it can be observed that the compression
difference in the internal aperture size and penetration deformation process is separated into three stages: elastic
characteristics of the cages. In addition, a larger internal
pore size contributes to accelerated exchange rate of the deformation, plastic yielding, and collapse densification
soaking solution between the inside of the cages and the stage because the fusion apparatus is of a polymer
outside environment, resulting in a faster rate of dissolution porous structure. The initial stage is characterized as
of the degraded small-molecule oligomers and a faster rate the elastic deformation phase, wherein the enclosure
of HA loss. undergoes minimal compressive force. During this
phase, the stress and strain exhibit a linear relationship,
3.4. Changes in mechanical characteristics before and the cage demonstrates elastic behavior, indicating
and after degradation of the cages that its deformation is reversible. The second stage is
The mechanical properties of the cage are an important the plastic-yielding stage. The compressive force on the
indicator of whether it can be employed in spinal fusion cage steadily develops throughout this stage, and when it
surgery. In general, the mechanical properties of cages exceeds the elastic limit, plastic deformation begins, and
are strongly influenced by their material composition. 18,22 the stress–strain curve enters a flat stage. As the external
However, the influence of meso-structural features on load continues to increase, the multi-layered beam-
the mechanical properties of the cage cannot be ignored. constructed pore walls of the cage begin to topple, leading
The contact region between the interbody fusion cage and to the gradual collapse of the porous structural units. This
the vertebral body consists entirely of cancellous bone. process results in the lateral expansion of the macroscopic
Typically, the spinal interbody fusion cages employed structure, the densification of the cage, and a sharp
in clinical practice are predominantly solid or box-type increase in deformation resistance, which align with the
devices, exhibiting mechanical and biological properties law proposed by Gibson et al. By comparing the stress–
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that significantly differ from the natural bone tissues. strain curves of cages with varied structural properties,

Volume 10 Issue 4 (2024) 180 doi: 10.36922/ijb.1996