**6. Processing**

Several bone grafts storage ways are used all over the world. The two most widely accepted and used are the deep-frozen (−80°C) and lyophilisation ones.

Extremely low temperatures (<−80°C) are achieved by using temperature-controlled freezers equipped with power generators, just in case of lack of electric power cut, and alarms triggered by the system when the temperature increases. In these special freezers, a bone tissue can be maintained for up to 5 years. It should be noted that very low temperatures do not have a role in the sterilization of bone tissue.

Proposed and diffused by the Tissue Bank of USA Navy in 1951, the lyophilisation process became the technique in which bone is washed, centrifuged, decellularised, chemically degreased and subjected to physical processes of cleaning and sterilisation, and this way, only the protein-mineral matrix still remains in the graft, which is later frozen and then dehydrated. All these processes decrease antigenicity, tumour cell transmissions and inactivate prions [28, 35]. The lyophilisation is an important method of processing and storage for musculoskeletal tissues that allows not only the use of human bones, tendons and fascia (allogeneic) but also and mainly the bovine (xenogenic) [27, 31, 33, 39]. Currently, in major health centres, different types of bone grafts are available for reconstructive orthopaedic surgery: frozen and lyophilised autologous, (allogeneic) and lyophilised bovine grafts (xenogeneic) [2].

The frozen human graft (allogeneic) was the most used and widely accepted but the number of Tissue Banks in our country and in other developing countries is not enough to overcome the huge demand as well as the number of donors [47]. In the same way, as mentioned, there is a risk of transmission of diseases and tumour cells [32, 34, 41, 53]. The vast majority of USA Tissue Banks produce lyophilised human grafts (allogeneic); however, there are few studies on its use or production. Therefore, lyophilised human grafts are still not widespread used among us [8, 56].

The bovine bone has a chemical composition, porosity, size, shape and biological behaviour similar to its human counterpart, although controversial bovine grafts is commonly used in dentistry surgeries and reconstructions [14]. It provides structural support, osteoconduction and a high content of calcium and phosphorus; essential factors for the newly formed bone tissue [40]. For these reasons, its use is growing in orthopaedic surgeries as reported by Prof. Galia [14] and by his co-workers in his research group, Rosito et al. [45, 46], Henning et al. [23] and Diesel et al. [9].

The most important issue is whether the graft has been processed and stored according to the standards of Associations of Tissue Banks and by national and international health authorities.
