## 1 Introduction  
The frequency of bone defect problems encountered by orthopedic surgeons is increasing. High energy trauma, open fractures, bone tumors, infections, debridement of bone for any reason, metabolic diseases and massive osteolysis are the most common causes of bone defects [1]. The main problems in dealing with bone defects are the high costs and extensive time needed for healing. Critical bone defects are difficult situations, and spontaneous healing is not expected despite adequate surgical stabilization [2]. In addition to appropriate surgical stabilization, critical bone defect treatments must include proper and functional segmental replacement regardless of the cause. In the literature, autologous bone grafts (ABG) are described as the gold standard among current treatment modalities [3]. However, due to some disadvantages, such as limited availability and donor site morbidity, the efforts to find more appropriate and optimal scaffold options continue.  
The bone scaffold is a 3D matrix that allows osteogenic cell adhesion and proliferation, as well as stimulates cells [4]. Natural and synthetic polymers, ceramic-based scaffolds,metallic, and composite scaffolds have been tried so far, but none of the used scaffolds contain all the properties of an optimal scaffold [5,6]. Moreover, synthetic scaffolds are associated with rejection, allergic reactions and failure to integrate with host tissues. As a result, tissue engineering in current technological upgrades is seminal in bone defect treatment. Presently, natural extracellular matrix composite scaffolds are of interest because these are accepted as most similar to the original tissue, and they allow cellular adhesion and proliferation. In addition, the main advantage of this scaffold type is being non-antigenic to the native tissue [7].  
For these reasons, tissue-engineered adipose substitutes have been developed that promote regeneration rather than repair [8]. The omentum has been used for defects of other organs because of its rich extracellular matrix, ability to adhere to local tissues and growth factors containment, but it is generally used in reconstructive and cardiovascular surgery as a decellularized omentum scaffold. However, in orthopedic surgery, decellularized omentum scaffolds have not been used to date [8].  
Successful bone repair is based on a combination of a suitable medium, stem cells and growth factors. Platelet-rich plasma (PRP), which has osteoinductive and osteoconductive properties, is a good candidate to be added to a scaffold [9]. It is proven to have a number of growth and differentiation factors that play important roles in the intracellular signaling pathway for cellular proliferation, osteoid and extracellular matrix formation, and also callus formation in fracture healing [10]. In addition, mesenchymal stem cells (MSCs) were shown to be involved in callus formation [11]. Both direct involvements in callus formation and inflammatory cytokines, such as transforming growth factor (TGF)-8, insulin-like growth factor (IGF), and epidermal growth factor (EGF) production, are stimulated by MSCs [12].  
Therefore, we aimed to evaluate the effects of a decellularized omentum scaffold in critical bone defects. In addition, the potential osteoinductive effects of the combination of PRP and MSCs were investigated.