## 3 Technique Description  
In 2006, sonography was used as a part of more complex CT-based navigation system in phantom shoulder arthroscopy to augment the standard procedure [2]. However, nowadays, this imaging modality is commonly used in numerous branches of mini-invasive surgery. Owing to the popularity of modern, high-resolution transducers and the familiarisation of orthopaedic surgeons with this imaging technique, the possible applications are growing in number. Sonography can be performed during different stages of an arthroscopic procedure or for the entire operation. It is useful in the setting of access points around the joint and provides optimal portal placement. Moreover, ultrasound facilitates the localization of the pathology and assures its complete resection. The simultaneous use of arthroscopy and ultrasound in the operative theatre allows surgeons to intra-operatively identify the vessels and nerves, especially during establishing of the arthroscopic portals. Furthermore, ultrasound navigation during orthopaedic surgery can be used to establish the access to and constant control of difficult operating areas, with a high potential for neurovascular complications, such as the sural nerve and popliteal neurovascular bundle [3]. Furthermore, an arthroscopic shaver can be placed exactly where the pathology is located (i.e., calcium deposits and osteophytes in B-mode or neovascularization in power Doppler mode) and seen via US probe navigation. [PERSON] et al., in 2004, described a special intra-articular US probe used for the quantitative assessment of articular cartilage during knee arthroscopy [4]. The idea was subsequently developed by [PERSON] et al., which resulted in the creation of a special probe to assess the cartilage degeneration during arthroscopic procedures [5; 6]. It should be emphasized, that in this case, the purpose of the ultrasound device was not an intra-operative guidance tool. These papers proved that US can be used as a quantitative measurements tool for cartilage and subchondral bones [7] [PERSON] et al. used an intra-articular probe of a higher frequency (40 MHz) to evaluate the cartilage in knee joints [8]. However, the authors emphasized that intra-operative ultrasound provided quantitative information about the cartilage integrity and thickness, which are not available when using conventional arthroscopy. Another author described the feasibility of the quantitative evaluation of the shoulder cartilage with a clinical intravascular (40 MHz) ultrasound imaging system [9]. However, [PERSON] et al. showed that USs, on the one hand, provide essential information on the integrity of the articular cartilage, and on the other hand, are a useful for monitoring arthroscopy-assisted procedures, such as retrograde drilling and bone transplantations. Thus, the quantitative assessment of the cartilage does not exclude intraoperative guidance [10]. In 2006, [PERSON] et al. first described the use of a laparoscopic US probe for the augmentation of the arthroscopic removal of the calcium deposits from the rotator cuff [11] (Figure 1). This technique was modified by [PERSON] et al., and they recommended the use of linear transducers [12; 13]. Nevertheless, this procedure is extremely difficult to accomplish without double visualisation, as calcifications are often hidden intra-tendinous from the arthroscopic spectrum of view, while being easy localized using US due to their characteristic echogenicity. [PERSON] et al. (2007) reported the use of double visualisation (arthroscopy and ultrasound) during three knee surgeries [14]. They confirmed the important role of ultrasounds and good correlation between the sonographic and arthroscopic views. However, the authors also emphasized some disadvantages: the ultrasound device was consumed a lot of-space, there was a need of more personnel movement during the surgery, the high cost of the additional equipment, there was a long learning curve of the sonography, and the potential elongation of the procedure. In another paper, [PERSON] et al.