## 5 Lattice Structures in Medical Devices and Applications  
### Lattice in Bone Structures  
Some of the key reasons for using lattice structures in bone implants comprise mitigating stress shielding, facilitating drug release, reducing and removing radiographic artifacts, and promoting osseointegration for fixation. Stress shielding occurs when a bone is inadequately loaded, typically because the load is diverted by a rigid implant. This condition can result in bone resorption, ultimately leading to the implant loosening [99]. Utilizing lattices can help mitigate stress shielding by diminishing the stiffness of the implant [100]. As mentioned above, metals are still, to this day, used for spinal fusion implants. This can cause artifacts in MRI, CT, and X-ray images. Substituting solid implants with lattice structures decreases the total volume of metal, thereby eliminating the artifacts in both the cases. Osseointegration is established when there is direct contact between the bone and the implant, creating mechanical stability and forming a structural and functional connection [101]. Several factors associated with implant geometry impact osseointegration. These factors encompass the size of the pores within the structure, although there is limited consensus on the optimal size, with various sources citing anything from 100 to 1000 \(\upmu\)m [86; 102; 103; 104]. The connectivity of pores impacts the availability of vascularization, gas exchange, and the conveyance of nutrients and waste [104; 105]. Pores arranged in a structured design, as seen in the reticulating lattice shown in Figure 4, contribute to superior osseointegration compared to that of tortuous ones, while compact pores are prone to becoming occluded [106]. A lattice refers to a cellular framework consisting of solid material intertwined with pores. The cell structure can exhibit either an open or closed configuration, determined by the interconnectedness of the pores with the external environment [6]. According to [PERSON] et al., a lattice structure is characterized as a truss-like configuration with interconnected struts and nodes within three-dimensional space [107; 108]. According to [PERSON] et al., a lattice  
Figure 4: Reticulated lattice image (**A**) vs. stochastic structure image (**B**). structure is characterized as an architectural arrangement composed of an array of spatially periodic unit cells, encompassing edges and faces [5]. Open cell lattices are often used in medical applications due to their suitability for fabrication using powder bed fusion techniques. This is attributed to the absence of internal voids, which eliminates the challenge of powder entrapment during the manufacturing process [109]. Additionally, these lattices yield a less rigid structure, which proves advantageous for osseointegration and vascularization, effectively mitigating the impact of stress shielding [110]. The utilization of lattice structures in a lightweight design is common, aiming to reduce material waste and energy consumption during the manufacturing process. Effectively designing and optimizing lattice structures can lead to significant savings in energy, materials, and manufacturing time [108]. A structure with a recurring design can be termed reticulated, whereas structures with a random design are referred to as stochastic, as shown in Figure 4[111]. Stochastic structures are frequently characterized by heterogeneity, posing challenges in predicting mechanical behavior [112]. These structures may not always align with the design constraints of the printing process. Consequently, reticulating lattices are more commonly employed as implants [113]. Reticulated lattices can be established on a repeating unit known as a unit cell, which can be either surface-based or strut-based [113]. Instances of strut-based lattices encompass those derived from crystal structures like body-centered cubic (BCC) and diamond. Surface lattices, on the other hand, consist of patterns such as gyroid and Neovius sheets [114].