## 1 Introduction  
The flexibility of all steps of product development and design is always desirable. To achieve this, automation must be implemented in all possible steps of product development and design. Height level of automation can be achieved using different types of software for design, numerical analysis, optimization and manufacturing.  
Today, software for computer aided design (CAD), numerical analysis, simulations and optimization are introduced in all steps of product development and design, with a goal to reduce development time, and to increase the quality of developed products. The integration of this software, together with skeleton-based generative modeling methods [1] and algorithms for optimization, as well as systems for knowledge-based engineering (KBE), can be developed. KBE systems have integrated tools for parametric CAD modeling, numerical analysis using finite element modeling (FEM) and structural optimization of various designs [2; 3].  
Structural optimization is commonly used in various fields, such as: fiber composites [4], printable structures [5], car body [6], adaptive structures [7], and especially with integration with FEM [8]. Structural optimization, in correlation with optimization variables, can be divided into topology optimization [9], shape optimization [10] and size optimization [11]. Looking to the process of product development and design, it can be noticed that topology optimization accrues in the conceptual phase, shape optimization in the phase of choice of components, and size optimization in detailed design phase.  
The parametric description of physical objects, using dimensional, geometrical, physical and functional parameters, is a basis for KBE system development. Knowledge of the KBE system can be represented as knowledge about functional principles of product, and how those principles effect the manufacturing process of that product. Elements of knowledge in KBE systems can be algorithms, parameters, functions and rules.  
KBE systems and structural optimization are widely used for the development and optimization of fixation devices and implants [12; 13; 14]. External fixation device is a medical device which is used to immobilize bone fraction using pins, which goes inside and through bone, and which is externally connected to external mounting. There are a lot of design, biomechanical and manufacturing constrains which must be fulfilled in the design of external fixation device. The most important constrains are the safety of usage and practical application to the bone. The external fixation device, which is analyzed in this paper, is a unilateral, biplanar external modular fixation device with half pins (Schanz screws) [15; 16]. The concept of developed KBE system for structural dimensional optimization of external fixation device is shown in Figure 1.  
Using sensors inside structural analysis, displacements are measured and imported into an optimization model as constrains. In this way, displacements and stress are controlled on important places on an external fixation device. The data of design parameters are imported into the CAD model after every iteration of the optimization process, and then forwarded to the optimization model. Moreover, the other design parameters that are needed to calculate the volume used material for external fixation device are imported into the optimization model. Using these parameters, volume (as cost function) can be calculated. The described process represents one iteration of the developed KBE system. The iteration process is repeated until one of the constrains is reached, and converges of results are obtained. The initial CAD/FEM model is verified using experimental verification on the real design of the external fixation device (Figure 1).  
Figure 1: Concept of developed KBE system.