## 5 Lattice Structures in Medical Devices and Applications
### Topological Design of Cellular Scaffolds in Spinal and Other Orthopedic Implants  
The design of cellular structures in orthopedic implants can vary significantly to replicate the characteristics of the natural bone ECM [89]. One of the most significant features in this design is the permeability of a porous scaffold, which plays a crucial role in facilitating blood flow through the implant. Adequate blood flow is essential for delivering nutrients and removing waste, processes that are vital for tissue regeneration and the overall success of osseointegration. Pore shape is the key factor influencing permeability and the implant's performance. For example, altering the pore shape from spherical to cylindrical can enhance the permeability of the scaffold, promoting better blood circulation and improving nutrient flow [89]. However, this design modification comes with disadvantages. Increasing permeability by altering the pore shapes often leads to a reduction in the implant's stiffness [135]. Reduced stiffness may compromise the structural integrity of the implant, making it less able to support mechanical loads, which is a concern, particularly in weight-bearing applications like spinal fusion. In addition to pore shape, the porosity and pore size of the scaffold significantly impact the degree of bone ingrowth [129,135]. The size and distribution of pores determine how deeply bone tissue can infiltrate the scaffold and how much new bone can form within the implant. According to the literature, the optimal pore size for bone ingrowth has been reported to range from 50 to 1500 \(\upmu\)m, with corresponding porosity levels between 40% and 70% [129; 130]. These ranges reflect the diversity of materials and structures used in different implants, as well as variations in the specific anatomical requirements of the implant site. While smaller pore sizes may promote faster cell attachment and initial bone growth, larger pores are generally more favorable for long-term vascularization and deeper tissue integration, as shown in Figure 5.  
However, increasing the pore size can negatively affect the mechanical properties of the scaffold. For example, doubling the pore size has been shown to reduce the elastic modulus of the scaffold by over threefold [88; 89]. This reduction in stiffness can weaken the scaffold's ability to bear physiological loads, highlighting the challenge of balancing mechanical strength with biological functionality. The ideal pore size and structure for achieving both biocompatibility and mechanical strength remains a topic of ongoing debate in the literature, as different materials and scaffold designs, whether stochastic or reticulated, exhibit varying responses to changes in pore size and shape [129]. Another aspect of the cellular structure design is the interplay between pore interconnectivity and scaffold architecture. A highly interconnected network of pores promotes not only bone ingrowth, but also vascularization, as the interconnected channels allow for the formation of new blood vessels [88]. This is critical for the long-term success of the implant, as adequate blood supply ensures continued bone remodeling and maintenance. However, balancing pore interconnectivity with sufficient mechanical stability remains a significant design challenge, as higher interconnectivity typically results in decreased stiffness. In conclusion, the topological design of cellular structures in orthopedic implants is a complex and finely tuned process. Designers and researchers must carefully consider factors, such as pore shape, size, porosity, and interconnectivity, to optimize both the biological and mechanical performances of the scaffold. While advances in 3 DP and biomaterials have allowed for greater customization and control over these variables, further research is necessary to fully understand the ideal design parameters for different clinical applications.