## 4 Results
### Total Hip Arthroplasty  
Six studies reported on the application of AR in THA [19; 22; 30; 31; 32; 33], and among those, one reported on hip resurfacing (HR) [22], and two were on clinical THA [19; 31]. [PERSON] et al. [22] explored the feasibility of an AR system in HR using optical see-through headsets during femoral preparation to simplify the procedure of locating and drilling the guide hole. The position and orientation of the drilled holes on the sawbones were compared with the preoperative plan, and the mean errors were approximately 2 mm and 2\({}^{\circ}\). Similarly, [PERSON] et al. [30] reported in a preclinical study the results of an AR solution based on intraoperative planning for the easy and accurate placement of acetabular components during THA. The system is based on two C-arm X-ray images that are combined with 3D AR visualization that simplifies impactor and cup placement and provides a real-time red-green-blue-depth (RGBD) data overlay. The authors reported errors in translation, anteversion, and abduction of 2 mm, 1\({}^{\circ}\), and 0.5\({}^{\circ}\), respectively. [PERSON] et al. [32] compared the accuracy of component placement, procedure time, radiation dose, and usability of a novel 3D AR guidance system compared with standard fluoroscopic guidance for acetabular component placement using a radiopaque foam pelvis ([PERSON], Pacific Research Laboratories, Vashon Island, WA) in a supine position to simulate the direct anterior approach. Fluoroscopic and 3D-images were obtained using a motorized C-arm scanner with cone-beam CT capability (Siemens Medical Solutions USA, Malvern, PA) and optical data were coregistered to create the AR environment from an RGBD camera (Intel Corporation, Santa Clara, CA). The authors reported that the AR technique was associated with significantly increased accuracy for target inclination (\(p\) = 0.01) and anteversion (\(p\) = 0.02), and increased precision for target anteversion (\(p\) < 001). In addition, the authors reported that the AR technique was faster (mean \(\pm\) SD, 1.8 \(\pm\) 0.25 vs. 3.9 \(\pm\) 1.6 min; \(p\) < 0.01), and that it was considered easier to use by the participants. Moreover, no difference was documented in terms of the radiation dose to the patients between the two techniques (\(p\) = 0.48), while radiation exposure to the surgical team could be eliminated with the AR technique. Regarding the clinical application, [PERSON] et al. [19; 31] reported in a pilot study the development of an acetabular cup measurement device (AR-HIP system) to investigate the accuracy of the acetabular placement angles in 56 primary THAs compared with a conventional goniometer. The AR-HIP system was based on a preoperative CT scan and allowed the surgeon to view an acetabular cup image superimposed on the real surgical field through the display of a smartphone that showed inclination and anteversion angles. postoperatively, a second CT scan was required to calculate the placement angles and the difference with either the intraoperative measurements obtained with the goniometer or the AR system. The authors reported significantly better accuracy on the acetabular cup anteversion when the AR system was used (mean difference 4.1\({}^{\circ}\), 95% confidence interval, 3.0-5.2; \(p\) < 0.0001). Subsequently, a randomized controlled trial was performed (41 hips) by the same authors [31] on acetabular cup placement using the AR marker-based portable navigation system and the conventional free-hand technique. Preoperative CT images were used for planning. The evaluation of inclination and anteversion was performed radiographically after surgery and at 3-month follow-up with a CT scan. The authors reported no difference in acetabular anteversion accuracy and no clinically important difference in acetabular inclination between the AR-based navigation system and the conventional technique. Finally, [PERSON] et al.