Background. Trust in the validity of a measurement tool is critical to its function in both clinical and educational settings. Acetabular cup malposition within total hip arthroplasty (THA) can lead to increased dislocation rates, impingement and increased wear as a result of edge loading. We have developed a THA simulator incorporating a foam/Sawbone pelvis model with a modified Microsoft HoloLens® augmented reality (AR) headset. We aimed to measure the trueness, precision, reliability and reproducibility of this platform for translating spatial measurements of acetabular cup orientation to angular values before developing it as a training tool. Methods. A MicronTracker® stereoscopic camera was integrated onto a HoloLens® AR system. Trueness and precision values were obtained through comparison of the AR system measurements to a gold-standard motion capture system”s (OptiTrack®) measurements for acetabular cup orientation on a benchtop trainer, in six clinically relevant pairs of anteversion and inclination angles. Four surgeons performed these six orientations, and repeated each orientation twice. Pearson”s coefficients and Bland-Altman plots were computed to assess correlation and agreement between the AR and Motion Capture systems. Intraclass correlation coefficients (ICC) were calculated to evaluate the degree of repeatability and reproducibility of the AR system by comparing repeated tasks and between surgeons, respectively. Results. The trueness of the AR system was 0.24° (95% CI limit 0.92°) for inclination and 0.90° (95% CI limit 1.8°) for anteversion. Precision was 0.46° for inclination and 0.91° for anteversion. There was significant correlation between the two methods for both inclination (r = 0.996, p<0.001) and anteversion (r = 0.974, p<0.001). Repeatability for the AR system was 0.995 for inclination and 0.989 for anteversion. Reproducibility for the AR system was 0.999 for inclination and 0.995 for anteversion. Conclusion. Measurements obtained from the enhanced HoloLens® AR system were accurate and precise in regards to determining angular measurements of acetabular cup orientation. They exceeded those of currently used methods of cup angle determination such as CT and computer-assisted navigation. Measurements obtained were also highly repeatable and reproducible, therefore this platform is accurately validated for use in a THA training simulator

Background. Complications such as dislocations, impingement and early wear following total hip arthroplasty (THA) increase with acetabular cup implant malorientation. These errors are more common with low-volume centres or in novice hands. Currently, this skill is most commonly taught during real surgery with an expert trainer, but simulated training may offer a safer and more accessible solution. This study investigated if a novel MicronTracker® enhanced Microsoft HoloLens® augmented reality (EAR) headset was as effective as one-on-one expert surgeon (ES) training for teaching novice surgeons hip cup orientation skill. Methods. Twenty-four medical students were randomly assigned to EAR or ES training groups. Participants used a modified sawbone/foam pelvis model for hip cup orientation simulation. A validated EAR headset measured the orientation of acetabular cup implants and displayed this in the participant”s field of view. The system calculated the difference between planned and achieved orientation as a solid-angle error. Six different inclination and anteversion combinations, related to hypothetical patient-specific anatomy, were used as target orientations. Learning curves were measured over four sessions, each one week apart. Error in orientations of non-taught angles and during a concealed pelvic tilt were measured to assess translation of skills. A post-test questionnaire was used for qualitative analysis of procedure understanding and participant experience. Results. Novice surgeons of similar experience in both groups performed with a similar error prior to training (ES: 15.7°±6.9°, EAR: 14.2°±7.1°, p>0.05). During training, EAR participants were guided to significantly better orientation errors than ES (ES: 6.0°±3.4°, EAR: 1.1°±0.9°, p<0.001). After four training sessions, the orientation error in both groups significantly reduced (ES: 15.7°±6.9° to 8.2°±4.6°, p<0.001; EAR: 14.2°±7.0° to 9.6°±5.7°, p<0.001). Participants in both groups achieved the same levels of orientation accuracy in non-taught angles and when the pelvis was tilted (p>0.05). In post-training evaluation, participants expressed a preference towards ES rather than EAR for learning orientation skills and related visuospatial and procedure-specific skills. 79% of participants indicated EAR simulator training and ES in combination would be their preferred training method. Discussion. A novel head-mounted EAR platform delivered training to novice surgeons more accurately than an expert surgeon. Both EAR and ES enabled novices to acquire and retain skills on a learning curve to orientate the implant. These skills were translated to non-taught orientations and in the presence of a pelvic tilt. Conclusions. Augmented-reality simulators may be a feasible and valid method for teaching novice surgeon”s visuospatial skills for THA on a learning curve, to compliment traditional intraoperative training

Introduction. In total hip arthroplasty, a high radiographic inclination angle (RI) of the acetabular component has been linked to short- and long-term complications. There are several factors that lead to RI outliers including cup version, pelvic orientation and angle of the cup introducer relative to the floor. The primary aim of this study was to analyse what increases the risk of having a cup with an RI outside the target zone when controlling cup orientation with a digital inclinometer. Methods. In this prospective study, we included 200 consecutive patients undergoing uncemented primary THA in the lateral decubitus position using a posterior approach. Preoperatively, the surgeon determined the target intraoperative inclination (IOI. target. ). The intra-operative inclination of the cup (IOI. cup. ) was measured with the aid of a digital inclinometer after seating of the acetabular component. Anteroposterior pelvic radiographs were made to measure the RI of the acetabular component. The target zones were defined as 30°-45° and 35°-45° of RI. The operative inclination relative to the sagittal plane of the pelvis (OI. math. ) was calculated based on the radiographic inclination and anteversion angle. The difference between two outcome measures was expressed as Δ. Results. The mean RI was 37.9° SD 4.7, there were 12 cases with RI outside the 30°– 45° zone (6%) and 53 outliers (26.5%) with RI outside the 35°-45° zone. The mean absolute ΔIOI. cup. -IOI. target. was 1.2° SD 1.0. The absolute ΔIOI. cup. -IOI. target. was less than 1° in 108 patients (54%), less than 2° in 160 patients (80%), less than 3° in 186 patients (93%), and in 14 patients (7%) the difference was 3°-5°. The mean pelvic motion (ΔOI. math. -IOI. cup. ) was 8.8° SD 3.9 (95% CI 8.2° to 9.3°). The absolute deviation from the mean ΔOI. math. -IOI. cup. , which corresponds with the amount of pelvic motion, was significantly higher in RI outliers compared with non-outliers for both the 30°-45° and 35°-45° inclination zone (7.4° SD 3.3 vs 2.8° SD 2.1 and 4.7° SD 2.8 vs 2.5° SD 2.0 respectively) (p<0.0001). A linear regression analysis demonstrated a strong correlation between ΔOI. math. -IOI. cup. and the RI of the cup (r. 2. =0.70; P<0.0001). A multiple regression was run to predict ΔOI. math. -IOI. cup. from gender, BMI, side and hip circumference. These variables statistically significantly predicted ΔOI. math. -OIa. cup. , F(4, 195) = 19,435, p<0.0001, R2 = 0.285, but only side (p=0.04) and hip circumference (p<0.0001) added statistically significantly to the prediction. Discussion and Conclusion. When using a digital inclinometer 94% of cups had a RI within a 30°-45° zone and 73.5% of cups within a 35°-45° zone using a predefined IOI. target. based on the patient's hip circumference. The difference between the IOI. target. and the IOI. cup. of the acetabular component was less than 3° in 93% and less than 5° in all patients signifying that the surgeons were able to implant the cup close to their chosen intra-operative orientation. Deviation from the mean ΔOI. math. -IOI. cup. was significantly bigger in the RI outliers indicating that RI outliers were caused by more or less than deviation of the sagittal plane of the pelvis at time of cup impaction

Introduction. Acetabular component orientation is an important determinant of outcome following total hip arthroplasty (THA). Although surgeons aim to achieve optimal cup orientation, many studies demonstrate their inability to consistently achieve this. Factors that contribute are pelvic orientation and the surgeon's ability to correctly orient the cup at implantation. The goal of this study was to determine the accuracy with which surgeons can achieve cup orientation angles. Methods. In this in vitro study using a calibrated left and right sawbone hemipelvis model, participants (n=10) were asked to place a cup mounted on its introducer giving different targets. Measurements of cup orientation were made using a stereophotogrammetry protocol to measure radiographic inclination and operative anteversion (OA). A digital inclinometer was used to measure the intra-operative inclination (IOI) which is the angle of the cup introducer relative to the floor. First, the participant stated his or her preferred IOI and OA and positioned the cup accordingly. Second, the participant had to position the cup parallel to the anteversion of the transverse acetabular ligament (TAL). Third, the participant had to position the cup at IOI angles of 35°, 40° and 45°. Fourth, the participant used the mechanical alignment guide (45° of IOI and 30° of OA) to orient the cup. Each task was analysed separately and subgroup analysis included left versus right side and hip surgeons versus non-hip surgeons. Results. For the first task, hip surgeons preferred smaller IOI and larger OA than non-hip surgeons, but there was no significant difference in accuracy between both groups. When aiming for TAL, both surgeon groups performed similar, but accuracy on the non-dominant side was significantly better compared with the dominant side (mean deviation 0.6° SD 2.4 versus −2.6° SD 2.3) (p=0.004). When aiming for a specific IOI target of 35°, 40° or 45°, non-hip surgeons outperformed hip surgeons (mean deviation form target IOI 1.9° SD 2.7 versus −3.1° SD 3.8) (p<0.0001) with less variance (p=0.03). Contrary to version, accuracy on the dominant side was significantly better compared with the non-dominant side (mean deviation −0.4° SD 3.4 versus −2.1° SD 4.8). When using a mechanical guide, surgeons performed similar (0.6° SD 1.2 versus −0.4° SD 2.1 for inclination p=0.11 and −0.5° SD 2.6 versus −1.8° SD 3.3 for version p=0.22) and these values did not differ significantly from the actual IOI and OA of the mechanical guide. When using a mechanical guide, there was no difference in accuracy between the dominant and non-dominant side. Conclusion. There was no difference in accuracy between hip surgeons and non-hip surgeons when they aimed for their preferred IOI and OA or used a mechanical guide. When aiming for a specific IOI target, non-hip surgeons outperformed hip surgeons. Hip surgeons overestimate IOI and underestimate OA, presumably because this helps to achieve the desired radiographic cup orientation. Regarding accuracy, the non-dominant side was better for version and the dominant side for inclination. When aiming for a specific IOI and OA target, using a mechanical guide is significantly better than freehand cup orientation

Abstract. Objectives. Accurate orientation of the acetabular component during a total hip replacement is critical for optimising patient function, increasing the longevity of components, and reducing the risk of complications. This study aimed to determine the validity of a novel VR platform (AescularVR) in assessing acetabular component orientation in a simulated model used in surgical training. Methods. The AescularVR platform was developed using the HTC Vive® VR system hardware, including wireless trackers attached to the surgical instruments and pelvic sawbone. Following calibration, data on the relative position of both trackers are used to determine the acetabular cup orientation (version and inclination). The acetabular cup was manually implanted across a range of orientations representative of those expected intra-operatively. Simultaneous readings from the Vicon® optical motion capture system were used as the ‘gold standard’ for comparison. Correlation and agreement between these two methods was determined using Bland-Altman plots, Pearson's correlation co-efficient, and linear regression modelling. Results. A total of 55 separate orientation readings were obtained. The mean average difference in acetabular cup version and inclination between the Vicon and VR systems was 3.4° (95% CI: −3–9.9°), and −0.005° (95% CI: −4.5–4.5°) respectively. Strong positive correlations were demonstrated between the Vicon and VR systems in both acetabular cup version (Pearson's R = 0.92, 99% CI: 0.84–0.96, p<0.001), and inclination (Pearson's R = 0.94, 99% CI: 0.88–0.97, p<0.001). Using linear regression modelling, the adjusted R. 2. for acetabular version was 0.84, and 0.88 for acetabular inclination. Conclusion. The results of this study indicate that the AescularVR platform is highly accurate and reliable in determining acetabular component orientation in a simulated environment. The AescularVR platform is an adaptable tracking system, which may be modified for use in a range of simulated surgical training and educational purposes, particularly in orthopaedic surgery. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Abstract. Objectives. The importance of cup position on the performance of total hip replacements (THR) has been demonstrated in in vitro hip simulator tests and clinically. However, how cup position changes during gait has not been considered and may affect failure scenarios. The aim of this study was to assess dynamic cup version using gait data. Methods. Pelvic movement data for walking for 39 unilateral THR patients was acquired (Leeds Biomedical Research Centre). Patient's elected walking speed was used to group patients into high- and low-functioning (mean speed, 1.36(SD 0.09)ms. −1. and 0.85(SD 0.08)ms. −1. respectively). A computational algorithm (Python3.7) was developed to calculate cup version during gait cycle. Inputs were pelvic angles and initial cup orientation (assumed to be 45° inclination and 7° version, anterior pelvic plane was parallel to radiological frontal plane). Outputs were cup version angles during a gait cycle (101 measurements/cycle). Minimum, maximum and average cup version during gait cycle were measured for each patient. Two-sample t-test (p=0.05) was used to compare groups. Results. Over a gait cycle the mean minimum, maximum and average version angles for the high-functioning group were −4.5(SD 4.4)°, 5.0(SD 4.3)°, 9.5(SD 4.0)° and for low-functioning group 2.0(SD 3.7)°, 6.2(SD 2.9)°, 8.1(SD 3.2)°. There were no significant differences for the minimum, maximum and average version angles between the two groups. Conclusions. The study shows that dynamic acetabular cup version changes substantially during gait and this must be considered clinically and in pre-clinical testing. There was no significant difference between the two groups; however, dynamic cup version was more negative in high-functioning compared to low-functioning patients. Further studies on a larger cohort are required to determine whether patients’ profiles can be stratified to provide enhanced inputs for pre-clinical THR testing. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

The current, most popular recommendation for cup orientation, namely the Lewinnek box, dates back to the 70's, that is to say at the stone age of hip arthroplasty. Although Lewinnek's recommendations have been associated with a reduction of dislocation, some complications, either impingement or edge loading related, have not been eliminated. Early dislocations are becoming very rare and most of them probably occur in “outlier” patients with atypical pelvic/hip kinematics. Because singular problems usually need singular treatments, those patients need a more specific personalised planning of the treatment rather than a basic systematic application of Lewinnek recommendations. We aim in this review to define the potential impacts that the spine-hip relations (SHRs) have on hip arthroplasty. We highlight how recent improvements in hip implants technology and knowledge about SHRs can substantially modify the planning of a THR, and make the « Lewinnek recommendations » not relevant anymore. We propose a new classification of the SHRs with specific treatment recommendations for hip arthroplasty whose goal is to help at establishing a personalized planning of a THR. This new classification gives a rationale to optimize the short and long-term patient's outcomes by improving stability and reducing edge loading. We believe this new concept could be beneficial for clinical and research purposes

Positioning of the hip resurfacing is crucial for its long term survival and is critical in young patients for some reasons; manly increase the wear in the components and change the load distribution. THR have increased in the last years, mainly in young patients between 45 to 59 years old. The resurfacing solution is indicated for young patients with good bone quality. A long term solution is required for these patients to prevent hip revision. The resurfacing prosthesis Birmingham Hip Resurfacing (BHR) was analyzed in the present study by in vitro experimental studies. This gives indications for surgeons when placing the acetabular cup. One synthetic left model of composite femur (Sawbones®, model 3403), which replicates the cadaveric femur, and four composite pelvic bones (Sawbones®, model 3405), were used to fix the commercial models of Hip resurfacing (Birmingham model). The resurfacing size was chosen according to the head size of femurs with 48 mm head diameter and a cup with 58 mm. They were introduced by an experimented surgeon with instrumental of prosthesis. The cup is a press fit system and the hip component was cemented using bone cement Simplex, Stryker Corp. The acetabular cup was analyzed in 4 orientations; in anteverion with 15º and 20°; and in inclination 40 and 45°. Combinations of these were also considered. The experimental set-up was applied according to a system previously established by Ramos et al. (2013) in the anatomic position. The femur rotates distally and the Pelvic moves vertically as model changes, such that the same boundary conditions are satisfied. This system allows compensating motions of the acetabular cup orientation. A vertical load of 1700 N was applied on all cases, which have resulted in joint reaction force of 2.4 kN. The femur and iliac bone was instrumented with rosettes. 5 repetitions at each position were conducted. When the femur was instrumented with three rosettes in medial, anterior and posterior aspect, the maximum strain magnitude was observed in the medial aspect of femur with a minimum principal strain of −2070µε for 45° inclination and 20° of anterversion. The pubic region was found most critical region after instrumenting the Iliac bone with four rosettes, with a minimum principal strain around −2500µε (rosette 1), for the 45° inclination and 20° of anterversion. We have observed the great influence of the inclination on the strain distribution, changing its magnitude from compression to traction in different bone regions. The minimum principal strain is more critical in medial aspect of the femur and the influence of strain is about 7% when orientation and inclination change. The maximum influence was observed in the anterior aspect, where the anteversion presents a significant influence. The results show the interaction between inclination and anterversion in all aspects, being observed lower values in lower angles. The orientation of the acetabular cup significantly influences the strain distribution on the iliac surface. Besides, as anterversion increases, more strains are induced, mainly in the region of iliac body (rosette 3)