What is porosity? Porosity refers to the percentage of an object that is not solid. An example would be bread, a sponge, or your bones. Porosity allows for the optimization of strength vs weight.
One technique to achieve porosity in orthopedic Implants is through the use of lattice structures. Lattices are regular, 3d-dimensional, repeating structures. Lattices seen in nature are honeycombs, or our bones. 3D manufacture (design and printing) allows for the creation of porous lattices in orthopedic Implants.
Porous lattice structures are becoming increasingly more common in orthopedic implants with a range of potential benefits from improved osseointegration to material properties which may mimic native bone. While traditional manufacturing techniques have historically limited the ability of implant manufactures to incorporate complex porous lattice structures, the rise of additive manufacturing has opened new horizons. As implant manufacturers begin to adopt additive manufacturing as a mainstay of production, we are poised to see a significant utilization of porous lattice structures in orthopedic implants.
The ability to obtain early osseointegration is of utmost importance in numerous orthopedic, maxillofacial, and dental procedures and can mean the difference between a success and failure. Porous lattice structures are able to offer an increased potential of osseointegration by providing a scaffold to encourage cell ongrowth and ingrowth into the pore spaces (1, 2). It has been documented that initial matrix mineralization leading to osseointegration occurs more rapidly in porous lattice microstructure surfaces as compared to smooth or roughened surfaces (1). In addition to allowing for a scaffold for cellular integration, porous lattice structures significantly increase the surface area of the implant bone interface, allowing for an increased potential for cellular ingrowth and osseointegration (3).
Use of lattice structures is also beneficial as they can allow for specific structural properties of an implant such as matched stiffness and modulus to the surrounding tissue and bone. Popular biocompatible metals, such as titanium, stainless steel, and cobalt-chromium, which are widely used in modern orthopedic implants, are known to be significantly stiffer than natural bone (4). This mismatch in stiffness leads to a phenomenon known as “stress shielding.” Bone is a dynamic and alive, it responds to the environment around it. Bone mass is increased with exercise and lost with disuse. Astronauts experience bone loss due to prolonged stays in zero gravity.
Due to the difference in modulus between the metallic implant and bone, the surrounding bone no longer experiences physiological loading, thus leading to bone resorption and eventual implant loosening and failure (4). In an attempt to limit stress shielding, the integration of lattice structures with the desired mechanical properties are being explored in orthopedic implants. The use of open lattice structures in prototype total hip and knee arthroplasty implants created with 3d printing has demonstrated promising results, with the ability to create implants from titanium alloys and cobalt chromium with desirable modulus that approaches that of native bone (5).
Lattice Design and Optimization:
The generation of lattice structures optimized for the purpose of osseointegration has become an area of increasing interest. Small variations in lattice characteristics can have a significant impact on the structural properties (6). Lattice structures are currently largely either manually or computer (automatic) generated. The creation of a manually generated lattice structure utilizes repeating units of geometric structures while a computer-generated lattice is created from advanced algorithms that can be used to integrate desirable characteristics (6). In addition to manually created repeating cells and computer-generated microstructures, the idea of using the microstructure of native bone as a lattice structure has been approached (7). In this process, the cancellous structure of bone as visualized on micro-CT scan data was used to model a repeating lattice structure which could be incorporated into an implant (7). While in the past, the process of manufacturing a complex lattice structure that mimics cancellous bone would have been deemed nearly impossible, recent advancements in additive manufacturing have made the ability to create this complex lattice structure a reality.
In an attempt to determine the lattice structure optimized for osseointegration, numerous studies have examined how lattice structure influences osseointegration with varying results. One study that compared three levels of porosity and pore size (180um/30%, 300um/30%, and 180um/40%) found that increasing pore size and porosity lead to increased osseointegration in a rabbit model as compared to a smaller pore size and less porosity (2). Another study, also in a rabbit model, which examined the effect of pore size (300 um, 600 um, 900 um) with a fixed porosity of 65%, similarly found that increasing pore size had improved osseointegration as compared to a smaller pore size (8). While the two previously highlighted studies found an increased propensity for osseointegration in a lattice with a larger pore size, a different group found that when comparing a pore size of 300-400 um, 400-500 um, and 500-700 um, in a goat model, osseointegration was most readily seen in the group with the smallest pore size of 300-400 um (9). While it remains unclear what the ideal porosity and pore size for specific areas of osseointegration are, it is important to consider that that these are just two or many factors which can have effect on osseointegration. Further research is warranted to determine the optimal lattice shape, size, and porosity for osseointegration and implant performance.
The integration of lattice structures into orthopedic implants is promising, with the ability to improve osseointegration as well as create desirable structural properties such as a matched modulus to that of host bone to prevent stress shielding. While many of these designs are being integrated into modern orthopedic implants, further work will lead to improved lattice structure design for superior implant performance. The use of additive design and manufacturing can be harnessed to integrate these complex design elements into modern implants, to improve clinical outcomes.
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