## 2 Results of Literature Analysis
### Content Analysis
#### 2.2.3 Constraints
If single sourcing is adopted, or if the materials are considered as leverage items rather than strategic items, then the industry may suffer from a lack of continuous supply of spare parts. Configuration  
The configuration and design of the SC structure impose a constraint on the AM-based SC. The adoption of AM into SCs can be performed in three configurations: distributed or decentralized configuration, centralized configuration, and hybrid or mixed configuration. These configurations can be based on either AM, conventional manufacturing, or both. Scenario analysis is primarily used to develop the configuration options. [PERSON] et al. [49, 63] analyzed a decentralized manufacturing scenario for spare parts. The authors tested different configurations using a central factory and distributed AM sites to identify the main differences and requirements between the levels of integration in distributed manufacturing. The authors used AM in various stages in the supply network and different configuration scenarios and mentioned that AM could support the creation of specialized central manufacturing (for developing product models) and flexible production systems closer to the client side. As mentioned earlier, different SC configurations for AM were also discussed by [PERSON] et al. [35]. The configurations focused on analyzing the safety inventory. The authors found that these SCs can be configured to use AM; such a configuration has the potential to reduce safety inventory and cut inventory-holding costs across the entire SC. SC configurations with five scenarios were also discussed by [PERSON] et al. [20]. The authors mentioned that the traditional configuration might be considered redundant. As a result, the role of the logistics service provider as the AM-based SC is focused on digital distribution, platform-based decision making on production, and the selection of the manufacturer. This creates a new type of SC configuration, which service providers should carefully assess. Similarly, [PERSON] et al. [26] proposed three SC configurations based on different locations of AM implementation and compared them based on qualitative lead time analysis. [PERSON] [43] compared SC scenarios in terms of sustainability, where CNC machining and laser additive manufacturing (LAM) were compared. The authors mentioned that factors such as material consumption, manufacturing steps, length of SC, and the swiftness of production affect the sustainability of a process. Based on this study, it can be mentioned that the SC configuration with LAM-based production can provide better sustainable gain in terms of material efficiency. Qualitative methods were also used for SC configurations. [PERSON] et al. [29] and [PERSON] et al. [27] considered the SD approach to conducting simulations to compare different configurations. [PERSON] et al. [29] used the cost and emissions from SC configurations of spare parts. The costs included transportation, manufacturing, inventory, and administration. The authors mentioned that emissions in SCs come from raw material processing, manufacturing, distribution, and use. Carbon footprint was also associated with waste, spare parts storage, and transportation. They mentioned that in terms of cost alone, SC configurations for AM may not be a good option for the industry; however, when the efficiency of energy conversion, carbon tax, and the emissions quota is provided to the industry, AM-based supply chains can be a more attractive option. [PERSON] et al. [27] analyzed spare parts management inventory policies for both AM and CM on aircraft SC and mentioned that the configuration should focus on lead time influencing inventory management. Twoother studies compared different SC configurations based on performance [31], sojourn time, and cost [19]. [PERSON] and [PERSON] [31] mention that AM is usually an option for a small number of AM-based spare parts production companies, but that would require the locations to be near the consumption centres.