## 5 Discussion  
The application of AR in modern orthopaedic surgery and TJA is progressively finding wider approval with promising results in multiple settings. Indeed, it has been widely used in spine surgery, considering the complexity of the anatomical district and the risk of iatrogenic injury during instrumentation [10; 38; 39; 40; 41; 42; 43; 44]; favorable outcomes have also been reported in complex corrective osteotomies and trauma surgery, increasing accuracy while reducing radiation exposure and surgery time [44; 45; 46]. The correct implantation of prosthetic components in TJA is a key element for success in terms of the functional outcomes, patient's recovery and rehabilitation, and implant survivorship [47]. For those reasons, different technological innovations have been developed and introduced including robotic surgery [48], 3-D printed patient-specific instrumentation (PSI) [49], navigation devices [3], and, finally, AR [50]. To date, there are several different AR systems that have been applied in preclinical and clinical studies [26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]; however, their application in TJA has not been thoroughly investigated yet. Despite that, a few studies have reported the potential benefits of this technology in terms of the reduced radiationexposure of patients and OR staff, reduced surgical time, and improved accuracy of surgical performance. In addition, studies have reported that, at least in Sawbones, AR navigation provides more accurate results in terms of implant positioning when compared to the conventional free-hand technique in total hip [19; 22; 23; 24; 25; 30; 31] and knee [26; 27; 28; 29] arthroplasty. Regarding the application in THA, according to the current literature, we can state that AR is able to provide several advantages, particularly with regards to increased accuracy in acetabular component positioning. [PERSON] et al. [32] compared an AR system with standard fluoroscopic guidance on pelvic sawbones reporting increased accuracy of the placement angles. Despite that, the study is limited by its application on sawbones, and clinically relevant differences with conventional techniques have not yet been reported to support this technique in daily practice. Similarly, [PERSON] et al. [19; 31] reported promising results when an AR system was used to improve the quality of the cup positioning considering that the conventional tools may not recognize determinant elements such as the pelvic tilt or the pelvic movement related to the retractors, leading to potential version errors. In spite of that, even though there was an absence of reported complications, there was no evaluation of the clinical outcomes, forcing us to suggest that additional studies are necessary to assess the cost-effectiveness and risk-effectiveness of the technique before its introduction in daily practice. This technology has also been explored in HR by [PERSON] et al. [22] to evaluate the accuracy of the guide hole along the axis of the femoral neck in a sawbones model. The authors reported an error of 2 mm and 2\({}^{\circ}\) for position and direction compared to what was planned, providing good accuracy according to the [PERSON] et al. criteria [51]. Nevertheless, despite the promising results, the biggest obstacle to its application in a realistic setting would be to identify suitable image processing algorithms to segment the target from the surrounding surgical scene in order to correctly register it, limiting its dissemination to date. Similar results have been described in TKA surgery. [PERSON] et al. [29], reported on the AR-KNEE system and suggested that it may provide reliable accuracy for the coronal, sagittal, and rotational alignment when used for tibial and femoral resection and that it may become a useful alternative to conventional navigation devices. In addition, the authors reported promising results in an experimental setting when analyzing the error in distal femoral resection (<1.0\({}^{\circ}\)) in both the coronal and sagittal planes.