Spinal constructs: the vital role of implant material, structure and pore size
Without proper bone fusion, spinal constructs will eventually fail. Factors that support osseointegration and biological fixation include the material of the implant, its structure (porous / lattice), mechanical properties, surface roughness, pore size and porosity.
Spinal implants are used to treat various issues of the back and lower back. Their primary functions are to facilitate fusion between vertebrae, correct deformities and stabilize and strengthen the spine.
Spinal implants for interbody fusion are manufactured from titanium alloys (Ti) and polyetheretherketone (PEEK). Titanium alloys are lighter in weight than most other metals; they have excellent biocompatibility and currently are the most widely used metals. Titanium alloys can tightly integrate into bone and other tissues. Moreover, the rough surfaces of titanium alloy result in good osseointegration between the bone and the implant, when compared to smooth-surfaced implants, thus resulting in good clinical outcome after implant. However, titanium alloys have a higher Young’s modulus than PEEK.
In general, factors to consider when choosing a material include primary stability (biomechanical stability upon implant insertion), secondary stability (osseointegration), elasticity modulus and compressive strength of the implant as well as its radiolucency in imaging. This in turn is influenced by the material density: PEEK has a lower material density and therefore a poorer radiopacity than Ti alloys.
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Material: allowing for bone ingrowth
A titanium alloy structure with appropriate pore size and porosity provides a bio-mimic scaffold or substrate for osteocytes, allowing them to build new osteoid, promoting consecutive bone-ingrowth and consecutively improving the quantity and quality of bony fusion. By contrast, bone does not directly bond with PEEK, therefore a PEEK implant is always separated from bone by a fibrous tissue layer. [1]
Primary stability: rough surface essential
From a clinical point of view, an inadequate connection between cage and vertebral body can lead to an insufficient fusion of the segment. This in turn may provoke the fracture of the posterior stabilization system.
A rough surface allows a better interaction between bone tissue and implant than smooth-surfaced implants, as the rough cells adhere more strongly to smooth surfaces, and the ability of osteoblast proliferation and collagen synthesis is greater in surfaces with moderate roughness, and the better interaction consequently leads to an improved primary stability. [2], [3]
Secondary stability: osseointegration is key
The most important prerequisite for a long-term implant success is the stable anchoring of the implant in the bone (osseointegration). The rough surfaces of titanium alloy result in better osseointegration between the bone and the implant than smooth-surfaced implants, thus resulting in good clinical outcome after implantation. [3] Bone growth into the implant is affected by pore size and porosity.
Pore size / porosity: increasing stability
Linkage of pore and pore structure facilitates bone growth into the implant, increasing secondary stability. Studies have shown that an implant’s stability after implantation depends on the pore shape and size of the implant surface facing the host bone. [4], [5] A specific pore size and porosity of the lattice structure (characterized by open pores and non-stochastic orientations of the building unit cells) also encourages bone growth. One study investigating the effect of porosity on osteointegration found that an average void volume of 66.1% resulted in the best proportional bone-implant contact (vs. 59.2 and 46.6). [6] Other analyses have confirmed the high bone-implant contact throughout porous implants (vs. solid implants) as well as the significance of a pore size > 300μm due to enhanced new bone formation and formation of capillaries. [7], [8], [9]
Young’s modulus (elasticity module) and compression load: lattice structure close to bone
Solid titanium alloy Ti6Al4V ELI has an elasticity modulus of around 110 GPa, much higher than the 5-20 GPa of cortical bone. However, the lattice structure of the Aesculap 3D cages was shown to have a Young’s modulus far closer to that of cortical bone, improving bone tissue growth. [10] (It should also be noted that elasticity modulus values established in studies are mainly due to tests of tensile strength on solid material, while the stress on lattice-structured cages is mainly due to compression force.) The Aesculap 3D lattice structure combines a Young’s modulus more similar to bone with a high compression load, as the compression load of the 3D lattice structure has been shown to be higher than the median strength of bone as known from literature. [11] The 3D lattice structure also has a higher compressive strength than PEEK. [12]
Imaging: decreasing artifacts
Postoperative management includes imaging with either computed tomography (CT) or magnetic resonance imaging (MR) to assess the spinal canal and nerve roots. Metallic implants can cause artifacts that interferes with imaging, sometimes seriously degrading the diagnostic value. Titanium alloys produce significantly fewer artifacts than for instance stainless steel, and titanium alloy lattice structures produce a lower amount of artifacts than a solid titanium block. [13], [14]
Summary
Pore size, porosity and surface roughness all have a large impact on the primary and secondary stability of a spinal implant and hence on the success of the implantation. Implants made of titanium alloy with lattice structures of a certain pore size and porosity support both primary and secondary stability of spinal implants.
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