Introduction. The direct anterior approach (DAA) for total hip arthroplasty continues to gain popularity. Consequently, more procedures are being performed with the patient supine. The approach often utilizes a special leg positioner to assist with femoral exposure. Although the supine position may seem to allow for a more reproducible pelvic position at the time of cup implantation, there is limited evidence as to the effects on pelvic tilt with such leg positioners. Furthermore, the DAA has led to increased popularity of specific softwares, ie. Radlink or JointPoint, that facilitate the intra-op analysis of component position from fluoroscopy images. The aim of this study was to assess the difference in cup orientation measurements between intra-op fluoroscopy and post-op CT. Methods. A consecutive series of 48 DAA THAs were performed by a single surgeon in June/July 2018. All patients received OPS. TM. pre-operative planning (Corin, UK), and the cases were performed with the patient supine on the operating table with the PURIST leg positioning system (IOT, Texas, USA). To account for variation in pelvic tilt on the table, a fluoroscopy image of the hemi-pelvis was taken prior to cup impaction, and the c-arm rotated to match the shape of the obturator foramen on the supine AP Xray. The final cup was then imaged using fluoroscopy, and the radiographic cup orientation measured manually using Radlink GPS software (Radlink, California, USA). Post-operatively, each patient received a low dose CT scan to measure the radiographic cup orientation in reference to the supine coronal plane. Results. Mean cup orientation from intra-op fluoro was 38° inclination (32° to 43°) and 24° anteversion (20° to 28°). Mean cup orientation from post-op CT was 40° inclination (29° to 47°) and 30° anteversion (22° to 38°). Cups were, on average, 6° more anteverted and 2° more inclined on post-op CT than intra-op. These differences were statistically significant, p<0.001. All 48 cups were more anteverted on CT than intra-op. There was no statistical difference between pre- and post-op supine pelvic tilt (4.1° and 5.1° respectively, p = 0.41). Discussion. We found significant differences in cup orientation measurements performed from intra-op fluoro to those from post-op CT. This is an important finding given the attempts to adjust for pelvic tilt during the procedure. We theorise two sources of error contributing to the measurement differences. Firstly, the under-compensation for the anterior pelvic tilt on the table. Although the c-arm was rotated to match the obturator foramen from the pre-op imaging, we believe the manual matching technique utilised in the Radlink software carries large potential errors. This would have consistently led to an under-appreciation of the adjustment angle required. Secondly, the manual nature of defining the cup ellipse on the fluoro image has previously been shown to underestimate the degree of cup anteversion. These combined errors would have consistently led to the under-measurement of cup anteversion seen intra-operatively. In conclusion, we highlight the risk of over-anteversion of the acetabular cup when using 2D measurements, given the manual inputs required to determine a result

Introduction. The Intellijoint HIP system is a mini-optical navigation system designed to intraoperatively assist with cup orientation, leg length and offset in total hip replacement (THR). As with any imageless navigation system, acquiring the pelvic reference frame intraoperatively requires assumptions. The system does however have the ability to define the native acetabular orientation intra-operatively by registering 3-points along the bony rim. In conjunction with a pre-operative CT scan, the authors hypothesised that this native acetabular plane could be used as an intraoperative reference to achieve a planned patient-specific cup orientation. Method. Thirty-eight THR patients received preoperative OPS. TM. dynamic planning (Optimized Ortho, Sydney). On the pre-operative 3D model of each patient's acetabulum, a 3-point plane was defined by selecting recognisable features on the bony rim. The difference in inclination and anteversion angles between this native 3-point reference plane and the desired optimal orientation was pre-operatively calculated, and reported to the surgeon as “adjustment angles”. Intraoperatively, the surgeon tried to register the same 3-points on the bony rim. Knowing the intraoperative native acetabular orientation, the surgeon applied the pre-calculated adjustment angles to achieve the planned patient specific cup orientation. All patients received a post-operative CT scan at one-week and the deviation between planned and achieved cup orientation was measured. Additionally, the cup orientation that would have been achieved if the standard Intellijoint pelvic acquisition was performed was retrospectively determined. Results. The absolute mean inclination deviation from plan of the 3-point rim method was 5.6° (0.0° to 16.7°). The absolute mean anteversion deviation from plan of the 3-point rim method was 2.7° (0.1° to 9.5°). This constituted 90% within 10° of the desired patient-specific target. All anteversion measurements were within 10°, with 90% within 5°. The retrospective analysis on what would have been achieved if the standard pelvic acquisition was used, showed that the absolute mean inclination deviation from plan would have been 4.0° (0.0° to 14.2°) and the absolute mean anteversion deviation would have been 6.7° (0.1° to 24.1°). Only 74% of cups would have been within 10° of the desired target. Conclusions. Using the native acetabular orientation as a reference plane for delivering a patient-specific cup orientation showed promising preliminary results. Errors in inclination were significantly larger than anteversion. More points on the rim could reduce this error

Introduction. Cup malposition in hip arthroplasty and hip resurfacing is associated with instability, accelerated wear, and the need for revision. The current study assesses the validity of intraoperative assessment using a specialized software to analyze intraoperative radiographs. Methods. Cup orientation as measured on intraoperative radiography using the RadLink Galileo Positioning System was assessed in 10 patients. These radiographs were measured by personnel trained to support the system. The results were compared to cup orientation measured by CT. Cup orientation on CT was measured by first identifying the Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was then calculated. The same definition of cup orientation was used for both methodologies. Results. As compared to direct measurement using CT, the intraoperative radiograph system underestimated anteversion by an average of 8.0 degrees and overestimated cup inclination by 2.9 degrees. The radiographic measurement error in anteversion ranged from −27.4 to +4.0. degrees and for inclination ranged from −2.0 to +5.3 degrees. Conclusion. The use of an intraoperative radiological assessment system is relatively reliable in estimating the inclination of the acetabular component. Anteversion of the acetabular component is extremely poorly assessed by the system

Introduction. The limited field of view with less-invasive hip approaches for total hip arthroplasty can make a reliable cup positioning more challenging. The aim of this study was to evaluate the accuracy of cup placement between the traditional transgluteal approach and the anterior approach in a routine setting. Objectives. We asked if the (1) accuracy, (2) precision, and (3) number of outliers of the prosthetic cup orientation differed between three study groups: the anterior approach in supine position, the anterior approach in lateral decubitus position, and the transgluteal approach in lateral decubitus position. Methods. In a retrospective comparative study we compared the inclination and anteversion of the cup after total hip arthroplasty (THA) in a consecutive series of 325 patients (350 hips). The transgluteal approach group consisted of 67 hips operated in lateral decubitus position; the anterior approach in supine position consisted of 127 operated and the anterior approach in lateral decubitus position consisted of 156 hips. The aim of the cup orientation was Lewinnek's safe zone defined by an inclination of 40±10° and an anteversion of 15°±10°. The postoperative cup orientation was determined using a validated computer-assisted method based on statistical shape modeling. This method allows the virtual creation of an accurate three-dimensional pelvic model for each individual patient based on the two-dimensional anteroposterior pelvic radiograph. The inclination and anteversion was then calculated relative to the anterior pelvic plane – a natural reference plane for the calculation of inclination and anteversion. Accuracy was defined as the difference from the cup orientation to Lewinnek's target value. Precision was defined as the standard deviation of the two angles. Outliers were characterized by an anteversion or inclination angle outside of Lewinnek's safe zone. Results. (1) The accuracy of the anterior approach in supine position did not differ compared to the transgluteal approach, but differed to the anterior approach in supine position for inclination (p=0.882; p<0.001) (Figure 1) and anteversion (p = 0.014; p<0.001) (Figure 2). (2) The precision of the anterior approach in supine position was significantly higher compared to the transgluteal approach (p<0.001) and the anterior approach in lateral decubitus position for anteversion (p<0.001 for both groups) and inclination (p<0.001 for both groups) (Figure 3). (3) There was a significantly reduced number of outliers for the anterior approach in supine position compared to the anterior approach in lateral position (p=0.001) but not in comparison to the transgluteal approach (p=0.999) (Figure 2). Conclusions. The anterior approach in supine position results in a more precise placement of the prosthetic cup both for inclination and anteversion. Cup placement with less-invasive approaches does not lead to a higher variability of cup placement despite the more limited surgical field of view. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Introduction. The resultant cup orientation depends upon the orientation of the pelvis at impaction. No studies to date have assessed whether patient-position during total hip arthroplasty (THA) has an effect on cup orientation. This study aims to 1) Determine the difference in pelvic position that occurs between surgery and radiographic, supine, post-operative assessment; 2) Examine how the difference in pelvic position influences subsequent cup orientation and 3) Establish whether pelvic orientation, and thereafter cup orientation, differences exist between THAs performed in the supine versus the lateral decubitus positions. Patients/Materials & Methods. This is a retrospective, multi-surgeon, single-centre, consecutive series. 321 THAs who had intra-operative, post-cup impaction, AP pelvic radiograph, in the operative position were included; 167 were performed with the patient supine (anterior approach), whilst 154 were performed in the lateral decubitus (posterior approach). Cup inclination/anteversion was measured from intra- and post-operative radiographs and the difference (Δ) was determined. Change in pelvic position (tilt, rotation, obliquity) between surgery and post-operatively was calculated from Δinclination/anteversion using the Levenberg-Marquardt algorithm. Results. The mean post-operative inclination and anteversion were 40°±8 and 23°±9 respectively. 74 had either Δ. inclination. and/or Δ. anteversion. ±10° (21%). Intra-operatively (compared to post-operative), the pelvis was on average 4°±10 anteriorly tilted; 1°±10 internally rotated and 1°±5 adducted. Having a Δ. inclination. and/or Δ. anteversion. ±10° was associated with an odds ratio of 3.5 in having a cup orientation outside the target. A greater proportion of cases had Δ. inclination. and/or Δ. anteversion. > ±10° amongst the hips operated in the lateral decubitus (54/153) compared to the supine position (8/167) (p<0.001). The pelvis was significantly more anteriorly tilted (p<0.001) and the hemi-pelvis was significantly more internally rotated intra-operatively in the lateral position (p=0.04) compared to supine. Discussion. Pelvic movement is significantly less in supine position, which leads to more consistent cup orientation. Significant differences in pelvic tilt and rotation were seen in the lateral position, illustrating the difficulties for surgeons to consistently position the pelvis

The goal of this study was to validate accuracy and reproducibility of a new 2D/3D reconstruction-based program called “HipRecon” for determining cup orientation after THA. “HipRecon” uses a statistical shape model based 2D/3D deformable registration technique that can reconstruct a patient-specific 3D model from a single standard AP pelvic X-ray radiograph. Required inputs include a digital radiograph, the pixel size, and the film-to-source distance. No specific calibration of the X-ray, or a CAD (computer-assisted design) model of the implant, or a CT-scan of the patient is required. Cup orientation is then calculated with respect to the anterior pelvic plane that is derived from the reconstructed 3D-model. The validation study was conducted on datasets of 29 patients (31 hips). Among them, there were 15 males and 14 females. Each dataset has one post-operative X-ray radiograph and one post-operative CT-scan. The post-operative CT scan for each patient was used to establish the ground truth for the cup orientation. Radiographs with deep centering (7 radiographs), or of pelvises with fractures (2 radiographs), or with both (1 radiograph), or of non-hemispherely shaped cup (1 radiograph) were assessed separately from the radiographs without above mentioned phenomena (18 radiographs) to estimate a potential influence on the 2D/3D reconstruction accuracy. To make the description easier, we denote those radiographs with above mentioned phenomena as non-normal cases and those without as normal cases. The cup anteversions and inclinations that were calculated by “HipRecon” were compared to the associated ground truth. To validate the reproducibility and the reliability, one observer conducted twice measurements for each dataset using “HipRecon”. The mean accuracy for the normal cases was 0.4° ± 1.8° (−2.6° to 3.3°) for inclination and 0.6° ± 1.5° (−2.0° to 3.9°) for anteversion, and the mean accuracy for the non-normal cases was 2.3° ± 2.4° (−2.1° to 6.3°) for inclination and 0.1° ± 2.8° (−4.6° to 5.1°) for anteversion. Comparing the measurement from the normal radiographs to those from the non-normal radiographs using the Mann-Whitney U-test, we found a significant difference in measuring cup inclination (p = 0.01) but not in measuring cup anteversion (p = 0.3). Bland-Altman analysis of those measurements from the normal cases indicated that no systematical error was detected for “HipRecon,” as the mean of the measurement pairs were spread evenly and randomly for both inclination and anteversion. “HipRecon” showed a very good reproducibility for both parameters with an intraclass correlation coefficient (ICC) for inclination of 0.98 (95% Confidence Limits (CL): 0.96–0.99) and for anteversion of 0.96 (95% CL: 0.91–0.98). Accurate assessment of the acetabular cup orientation is important for evaluation of outcome after THA, but the inability to measure acetabular cup orientation accurately limits one's ability to determine optimal cup orientations, to assess new treatment methods of improving acetabular cup orientation in surgery, and to correlate the acetabular cup orientation to osteolysis, wear, and instability. In this study, we showed that “HipRecon” was an accurate, consistent, and reproducible technique to measure cup orientation from post-operative X-ray radiographs. Furthermore, our experimental results indicated that the best results were achieved with the radiographs of non-fractured pelvises that included the anterior superior iliac spines and the cranial part of the non-fractured pelvis. Thus, it is recommended that these landmarks should be included in the radiograph whenever the 2D/3D reconstruction-based method will be used

Introduction. Studies show that cup malpositioning using conventional techniques occurs in 50 to 74% of cases defined. Assessment of the utility of improved methods of placing acetabular components depends upon the accuracy of the method of measuring component positioning postoperatively. The current study reports on our preliminary experience assessing the accuracy of EOS images and application specific software to assess cup orientation as compared to CT. Methods. Eighteen patients with eighteen unilateral THA had pre-operative EOS images were obtained for preoperative assessment of leg-length difference and standing pelvic tilt. All of these patients also had preoperative CT imaging for surgical navigation of cup placement. This allows us to compare cup orientation as measured by CT to cup orientation as measured using the EOS images. Application specific software modules were developed to measure cup orientation using both CT and EOS images (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). Using CT, cup orientation was determined by identifying Anterior Pelvic Plane coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module allows for creation of a plane parallel with the opening plane of the acetabulum and subsequent calculation of plane orientation in the AP Plane coordinate space according to Murray's definitions of operative anteversion and operative inclination. Using EOS DICOM images, spatial information from the images were used to reconstruct the fan beam projection model. Each image pair is positioned inside this projection model. Anterior Pelvic Plane coordinate points are digitized on each image and back-projected to the fan beam source. Corresponding beams are then used to compute the 3D intersection points defining the 3D position and orientation of the Anterior Pelvic Plane. Ellipses with adjustable radii were then used to define the cup border in each EOS image. By respecting the fan beam projection model, 3D planes defining the projected normal of the ellipse in each image are computed. 3D implant normal was estimated by determining 3D plane intersection lines for each image pair. Implant center points are defined by using the back-projected and intersected ellipse center beams in the image pairs (Figure 1). Results. The results are shown in Figure 2. The mean anteversion error was −0.9 degrees (SD 4.1, range −6.9 to 10.3). The mean inclination error was 1.8 (SD 2.1, range −2.9 to 8.6). All three cups with errors greater than 7 degrees were in cups with 40 or more degrees of anteversion. Discussion and Conclusion. The current study, while very preliminary, demonstrates the potential that EOS images can be used to measure cup orientation with a reasonable degree of accuracy. Accurate determination of cup orientation appears to be more challenging in cups with higher anteversion

A 10° deviation from the ideal cup orientation in Metal on Metal (MoM) bearing couples leads to increased wear and the subsequent risk of early revision surgery. We assessed the accuracy of orthopaedic trainees and consultants in achieving optimal acetabular cup orientation. 49 trainees and 18 consultants were asked to orientate an acetabular component to 40° inclination and 20° anteversion in 3 consecutive pelvic models:. osteoarthritic (OA),. OA with anterior pelvic tilt,. OA with soft tissue cladding, the task most realistic of a surgical scenario. The trainee group experience in performing hip arthroplasty procedures ranged from novice to expert (> 100 procedures performed). Performance was measured using an image based navigation system. Average angular error in all tasks was less than 10°, but the range in anteversion or inclination was up to 65°. Eighteen percent of trainees were +/− 10° of the target orientation in Station A, 29% in B and 2% in C. Forty four percent of consultants achieved the safe zone in A, 16% in B and 0% in C. There was no significant difference in accuracy between the two groups in any of the tasks (p> 0.01). There was no correlation between experience and angular accuracy. We have been unable to demonstrate trainees have the ability to achieve the optimal cup orientation in a clinically relevant safe zone. A similar range of error is found in experienced surgeons. Focused training or intra-operative computer assistance may provide the solution to improving accuracy in this core orthopaedic skill

The radiographic analysis of over 5000 metal on metal (MoM) hips using Ein Bild Roentgen Analyse (EBRA) software have been recently published in an attempt to determine the influence of cup orientation on bearing function. The validation of this software relies one study, conducted in a phantom pelvis without a femoral head in situ. Three dimensional computed tomographic (3D-CT) has been shown to be more accurate for hip and knee arthroplasty than plain radiographs for measurements of component orientation and position. The accuracy of EBRA when compared to 3D-CT for MoM hips specifically is unclear. We measured the cup orientation of 96 large diameter MoM hips using EBRA analysis of plain radiographs and compared this to 3D-CT. All measurements were made using the radiographic definition of cup orientation. The mean difference in version between the two imaging modalities was 8°; with wide limits of agreement of −21.2° and +5.6°. Three retroverted cups were not detected by EBRA. The mean difference in inclination values was 0.3°, but there was up to 9°difference between imaging modalities. When measured by 3D CT, 64% of hips were within a 10° safe zone around 45° inclination and 20° version, compared to only 24% when measured by EBRA (Fishers Exact test, p< 0.0001). The measurement of cup orientation of MoM hips using EBRA software is insufficiently accurate, particularly for the assessment of cup version. The cup rim is obscured by the large diameter femoral head on plain radiographs. Research studies using EBRA analysis for version have limited value if accuracy of more than 20 degrees is required to draw conclusions. This software may not be suitable to measure the performance of a device or surgeon. The limitations of EBRA can be overcome, if 3D-CT with an extended Hounsfield scale for data capture is used

This matched cohort study aims to (a) assess differences in spinopelvic characteristics of patients having sustained a dislocation following THA and a control THA group without dislocation; (b) identify spinopelvic characteristics associated with risk of dislocation and; (c) propose an algorithm to define the optimum cup orientation for minimizing dislocation risk. Fifty patients with a history of THA dislocation (29 posterior-, 21 anterior dislocations) were matched for age, gender, body mass index, index diagnosis, and femoral head size with 100 controls. All patients were reviewed and underwent detailed quasi-static radiographic evaluations of the coronal- (offset; center-of-rotation; cup inclination/anteversion) and sagittal- reconstructions (pelvic tilt, pelvic incidence, lumbar lordosis, pelvic-femoral-angle, cup ante-inclination). The spinopelvic balance (PI-LL), combined sagittal index (CSI= Pelvic-femoral-angle + Cup Anteinclination) and Hip-User-Index were determined. sagittal index (CSI= Pelvic-femoral-angle + Cup Anteinclination) and Hip-User-Index were determined. Parameters were compared between the two groups (2-group analysis) and between controls and per direction of dislocation (3-group analysis). There were marginal coronal differences between the groups. Sagittal parameters (lumbar-lordosis, pelvic-tilt, CSI, PI-LL and Hip-User-Index) differed significantly. PI-LL (>10°) and standing pelvic tilt (>18°) were the strongest predictors of dislocation risk (sensistivity:70%/specificity:70%). All hips with a standing CSI<195° dislocated posteriorly and all with CSI>260° dislocated anteriorly. A CSI between 200–245° was associated with significantly reduced risk of dislocation (OR:6; 95%CI:2.5–15.0; p<0.001). In patients with unbalanced and/or rigid lumbar spine, standing CSI of 215–245° was associated with significantly reduced dislocation risk (OR:10; 95%CI:3.2–29.8; p<0.001). PI-LL and standing pelvic-tilt determined from pre-operative, standing, lateral spinopelvic radiograph can be useful screening tools, alerting surgeons of patients at increased dislocation risk. Measurement of the pelvic-femoral angle pre-operatively provides valuable information to determine the optimum, cup orientation associated with reduced dislocation risk by aiming for a standing CSI of 200–245°

Introduction. Cup malposition in hip arthroplasty and hip resurfacing is associated with instability, accelerated wear, and the need for revision. A recent study measuring cup orientation on conventional radiodiographs demonstrated an incidence of cup malpositioning of 50% according to the safe zone that they defined 1,2. A prior study of 105 conventionally placed cups using CT demonstrated a cup malpositioning incidence of 74%3. The current study similarly assesses the variation in cup position using conventional techniques as measured by CT. Methods. CT studies of 123 hips in 119 patients with total hip arthroplasties performed using conventional techniques were used for this study. The indications for the CT studies were for CT-based surgical navigation of the contralateral side or for assessment of periprosthetic osteolysis. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination. Since these studies including images through the femoral condyles, femoral anteversion could be measured on these hips as well (Osirix v5.6, Pixmeo SARL, Bernex, Switzerland). Results. Cup orientation for the 123 hips is shown in Figure 1. Operative anteversion averaged 29.7 degrees with a standard deviation of 12.2 and a range of −24.4 to 57.5. Operative inclination averaged 37.5 degrees with a standard deviation of 7.7 and a range of 18.4 to 68.2. Femoral anteversion averaged 21.1 degrees with a standard deviation of 14.0 and a range of −20.5 to 60.9. Using 25 degrees of operative anteversion and 45 degrees of operative inclination as the center of a safe zone for example, 78 of 123 (63%) were more than 10 degrees off in either anteversion and inclination and 23 of 123 (19%) were more than 10 degrees off in both anteversion and inclination. Discussion and Conclusion. Most conventionally placed acetabular components are malpositioned. While the incidence of cup malorientation using conventional techniques is quite high, the incidence in our series appears to be lower than that reported by Saxler et al. It is curious that most experienced surgeons who perform total hip arthroplasty using conventional methods of cup alignment believe that their accuracy quite good. Yet, multiple objective studies of cup alignment demonstrate that accuracy is quite poor. Since cup malposition is so closely associated with instability, impingement, wear, bearing fracture, osteolysis and loosening, questions remain as to how conventional methods of cup alignment remain an acceptable standard of care in our field

Introduction. Appropriate acetabular cup orientation is an important factor in reducing instability and maximising the performance of the bearing after Total Hip Arthroplasty (THA). However, postoperative analyses of two large cohorts in the US have shown that more than half of cups are malorientated. In addition, there is no consensus as to what inclination and anteversion angles should be targeted, with contemporary literature suggesting that the orientation should be customised for each individual patient. The aim of this study was to measure the accuracy of a novel patient specific instrumentation system in a consecutive series of 22 acetabular cups, each with a customised orientation. Methodology. Twenty-two consecutive total hip replacement patients were sent for Trinity Optimized Positioning System (OPS) acetabular planning (Optimized Ortho, Sydney). The Trinity OPS planning is a preoperative, dynamic analysis of each patient performing a deep flexion and full extension activity. The software calculates the dynamic force at the hip to be replaced and plots the bearing contact patch as it traces across the articulating surface. The software modelled multiple cup orientations and the alignment which best centralised the load was chosen by the surgeon from the preoperative reports. Once the target orientations had been determined, a unique patient specific guide was 3D printed and used intra-operatively with a laser guided system to achieve the planned alignment, Fig 1. All patients received a post-operative CT scan at 3 months and the radiographic cup inclination and anteversion was measured. The study was ethically approved by The Avenue Hospital Human Research Ethics Committee, Trial Number 176. Results. The mean planned radiographic inclination, reference to the Anterior Pelvic Plane (APP), was 42.8° (range 36.2° – 50.1°). The mean planned radiographic anteversion, reference to the APP, was 28.3° (range 19.4° – 37.0°). Only 23% of the planned orientations fell within Lewinnek's “safe zone”, taking into consideration that that this safe zone is not comparable to the coronal plane of radiographs. However, all 22 cups were planned within a range of 40° ± 10° of inclination and 25° ± 10° of anteversion, when referenced to the coronal plane when supine. The mean inclination difference between the planned and achieved orientations was −1.3° (range −7.6° – 9.2°). The mean anteversion difference was 1.2° (range −5.3° – 7.0°). The mean absolute difference was 4.2° for inclination (range 0.4° – 9.2°) and 3.6° for anteversion (range 0.6° – 7.0°). All 22 cups were within ±10° of their intended target orientation, Fig 2. All 22 cups were within the range of 40° ± 10° of inclination and 25° ± 10° of anteversion, when reference to the coronal plane when supine, Fig 3. Conclusions. These are the early results of a new technology for planning and delivering a customised acetabular cup orientation. We expect further improvements in accuracy with current developments. However, the results suggest that Trinity OPS is a simple way to achieve a patient-specific cup orientation, with accuracy comparable to imageless navigation

INTRODUCTION. Cup orientation of total hip arthroplasty (THA) is critical for dislocation, range of motion, polyethylene wear, pelvic osteolysis, and component migration. But, substantial error under manual technique has been reported specially in revision THA due to a bone loss and poor anatomical landmark. We have used three kinds of navigation systems for cup positioning in primary and revision THA. OBJECTIVES. The purpose of this study is to evaluate the accuracy of navigation in revision THAs. METHODS. Since 2005, consecutive 24 revision THAs were performed with volumetric post-operative CT scan images to measure three dimensional positionings of cups. We implanted cementless hemispherical cups in 14 hips using fluoro-based navigation (FN) system (Stealth Station Tria), in 5 hips using a CT-based navigation (CTN) system (VectorVision CT Hip 3.1) and in 5 hips using fluoro-CT-based navigation (FCTN) system (VectorVision CT Hip 3.5). For all the patients, volumetric post-operative CT scan was performed to measure 3D cup orientation. Using 3D image-processing software (JMM, Japan) we converted all data to radiographic angles to compare different navigation system. RESULTS. The difference from target angles of anteversion was 5.6 ± 4.9 degrees. The absolute value of difference from target angles of inclination was 6.5 ± 3.8 degrees. The system accuracy was 4.2±2.9 in inclination and 4.7 ± 4.1. Accuracy of three navigation system was not significantly different. No postoperative dislocation was observed in this series. No complication related to navigation system was observed, either. CONCLUSION. Cup malpositioning can easily occur with a conventional aligment guide especially in MIS THA due to complexity of a operation. In revision THAs, high incidence of malpositioning of cups and post operative dislocation were reported. We previously reported that navigation system improved the accuracy of acetabular component orientation in MIS THA comparing to manual technique. Conventional CT-based navigation system in revision THAs had two problems. First, it was difficult to get accurate surface registration because a presence of a bone loss prevented surface registration. Secondly, an artifact of CT images due to previous implants during a preoperative planning. For surgical approach (removal of implants), damages of bone easily occurred before surface registration. Damages of bone changed the landmarks for surface registration. These problems might lead the error of CT-based navigation system. On the other hand, FN and FCTN system does not need surface registration around acetabulum. This feature is great advantage to revision THA. Therefore, FN and FCTN system have theoritically would be friendlier for revision THA than CTN system. We also reported FCTN system showed superior accuracy than CTN system [2]. In the present study, there were no significant differences between three kinds of navigation systems. But, our volume of patients was too small to draw differences between systems. We need to continue this study to get more cases. However, in conclusion, an application of navigation system in revision THAs was effective and safe procedure to eliminates postoperative complications such as a malpositioning and post-operative dislocation,

Obtaining consistently an optimal cup orientation in THA is vital to obtain adequate head coverage and maximum impingement free range of motion and thus reduce the incidence of polyethylene wear, cup loosening, and dislocation rates associated with a limited range of motion. It is clear that THA instability, the most frequent cause of early failure, is a complex problem related to a wide range of causes. However cup orientation is one of the surgeon dependant potentially modifiable variables that continue to have an important influence due to the lack of reliable means of assuring an adequate orientation of the components, particularly the cup anteversion. Standard mechanical guides like Muller’s have been shown to be inaccurate and imprecise. Not surprisingly, dislocation is the most frequent short term complication after a THA. Acetabular cup orientation is a key factor determining joint stability and one of the most important ones under the surgeons’ control. An in vitro study was used to determine the precision, reproducibility and ease of use of a new mechanical guide in comparison to a standard mechanical guide Müllers. The new guide (Gravity Assisted Navigation System) consists of a simple to use navigation tool. It uses the constant direction of the force of gravity identified by two bulls’ eye levels providing real time intraoperative augmented reality thus controlling the orientation of the pelvis. Visualisation of the guide from a single perspective is enough to determine in real time, the orientation of the cup in abduction and anteversion. By using anatomic repairs within the pelvis its flexion/extension is taken into consideration. As part of an invitro study, 310 press-fit acetabular cups were impacted into a plastic model of a pelvis by 5 surgeons (Power 90%, Type I error 5%), The orientation obtained was measured with respect to a fixed reference of 15° of anteversion and 45° of abduction. Results: an average of 10.4° anteversion ,(Range 3°to 21°, Standard of Deviation 5.0°) for Müller s guide and of 0.4° anteversion (Range 1° to 3°, Standard of Deviation 0.7°) for the new guide and an average of −4.7° abduction (Range 7° to −11°, Standard of Deviation 2.3°) for Müllers guide and 0.3° abduction (Range 0° to 3°, Standard of Deviation 0.5°) for the new guide. The average time required for the orientation of the cups was similar with both guides. (6 seconds for Mullers guide and 5 seconds for the new guide) The precision and reproducibility of the cup orientation obtained with the new guide were significantly better than those obtained with Müllers guide (p< 0.00001). The results obtained with with the new mechanical guide are encouraging. The in vitro results are encouraging, the high precision and accuracy are comparable to results obtained by computer assisted navigation systems in similar studies

Cup orientation of total hip arthroplasty (THA) is critical for dislocation, range of motion, polyethylene wear, pelvic osteolysis, and component migration. But, substantial error under manual technique has been reported. Therefore, various navigation systems were introduced to reduce outliers. CT based navigation (CTN) was reported to reduce outliers in cup orientation. Recently, a noble technique, fluoroscopy-CT-based navigation (FCTN), has recently been developed using 2D-3D matching technique. Because of much less registration points, FCTN might be friendly to MIS THA and cases with sever bone deformity. Between October 2006 and April 2008, 33 THAs were performed through MIS approach with navigations. We prospectively randomized those into two groups, CTN and FCTN groups. We implanted cementless hemispherical cups in 18 hips using CTN (VectorVision CT Hip 3.1) and in 15 hips using FCTN (VectorVision CT Hip 3.5). For all the patients, volumetric post-operative CT scan was performed to measure 3D cup orientation. using 3D image-processing software (JMM, Japan). The difference from target angles of anteversion was 2.7 ± 2.4 degrees in FCTN group, and 12.1 ± 5.7 degrees in CTN group (p < 0.001). The absolute value of difference from target angles of inclination was 2.7 ± 2.4 degrees in FCTN group, and 6.5 ± 4.5 degrees in CTN group (p = 0.006). FCTN does not need surface registration around acetabulum, which is great advantage to MIS THA. Our study clearly showed that FCTN significantly improved a cup orientation to CTN

Aims: Postoperative dislocation of the implant is still an unsolved problem in THR. Malposition of the socket is reported to be one major risk factor. The aim of this study was to investigate the inßuence of cup orientation on the probability of postoperative prosthetic dislocation. Methods: The orientation of the socket was studied in 137 patients with postoperative dislocations, and compared with a control group (n=342). The EBRA-method (Ein-Bild- Roentgen-Analyse) was used to determine anteversion and inclination angles of the cups. A previous study showed that the precision of the method for measuring these parameters was ± 1degree (SD 0.33). Results: Mean values for the orientation of the prosthetic cup in the control group was 44¡ for inclination and 15¡ for anteversion. Patients with anterior dislocations after primary hip replacements showed signiþcant differences of the mean inclination (48¡, Mann-Whitney-Test: p=0.045) and anteversion angle (17¡, p=0.015). For posterior dislocations, mean inclination was 42¡ (p=0.277) and anteversion 11¡ (p=0.002) respectively. After revision hip arthroplasty, only patients with posterior dislocation of the implant showed signiþcant differences of the cup orientation (inclination: 40¡, p=0.03; anteversion:12¡,p=0.017). Conclusions: The results demonstrate the importance of accurate positioning of the socket to minimise the risk of subsequent dislocation. An anteversion of 15¡ and an inclination of 44–45¡ has the lowest at risk for dislocation

INTRODUCTION. Poor acetabular cup orientation in total hip arthroplasty (THA) can cause dislocation and impingement, and lead to osteolysis (Little et al., 2009) and inflammatory soft tissue reactions (Haan et al., 2008). While the intrinsic accuracy of cup positioning in navigation is reported as low as 1° (Parratte et al., 2009), a large anterior pelvic tilt may lead to an offset of the same magnitude in the final cup anteversion (Wolf et al., 2005). The objectives of this study are to demonstrate feasibility of a new, non-invasive radiographic tool for accurate preoperative determination of a patient's specific pelvis angle, and intraoperative and postoperative assessment of the acetabular cup orientation with respect to boney landmarks. METHODS. The methodology stitches multiple radiographic views around the pelvis using a multi-planar radiography setup (Amiri et al., 2011) and reconstructs the reference boney landmarks and the acetabular cup in three dimensions using previously developed algorithms and software (Amiri et al., 2012). To validate the methodology, a Sawbone model of the pelvis and femur was implanted with a standard cementless metal-on-polyethylene THA, and was tracked and digitized by an Optotrak motion tracking system. Five radiographic views were acquired at the pubic tubercle (PT) and anterior-superior iliac spine (ASIS) levels (Views 1 to 5 in Fig 1). Imaging and analysis were repeated 10 times. Custom software (Joint 3D) was used to reconstruct the right and left PT and ASIS by fitting spheres to the corresponding pairs of images (Fig 1). The three-dimensional pose of the acetabular cup was reconstructed in the software by solving a back-projection equation of the elliptical shadow of the cup opening. Accuracies were measured as mean differences from the digitized references. A sample of the reconstructed graphical output for the anterior pelvic plane (APP) and the cup, in comparison to the digitized reference, is shown in Fig 2. Repeatability was estimated as standard deviation of the measures for the reconstructed locations of the boney landmarks and the APP (known as a standard reference plane for cup placement). RESULTS. Accuracy for the pelvis pose angles was <1.6°, with SD <0.8° (Fig 3). Pelvic tilt was the most accurate with accuracy of 0.1° and SD=0.4°. For the acetabular cup, accuracy was 2.5° or better, with SD <0.2°. Accuracies in the cup operative anteversion and inclination were 2.4° and 0.6°, with SD=0.4° and 0.9°, respectively. DISCUSSION. The measured accuracies were within an acceptable range, according to previous studies that recommended a 5° cut-off error for acetabular anteversion. The method shows accuracy and radiation dose advantages over current radiographic, fluoroscopic and computed tomography methods. These results suggest that the proposed method is feasible for assessing cup placement with reference to the functional and anatomical references. CONCLUSION. Use of this technique could improve acetabular cup placement and reduce the incidence of instability, wear and loosening, by providing tools to incorporate the individual's pelvic pose in preoperative planning of the surgery, and by serving as an accurate and reliable tool for intraoperative and postoperative assessment of the acetabular cup position

Optimal alignment of the acetabular cup component is crucial for good outcome of total hip arthroplasty [THA]. Increased accuracy of implant positioning may improve clinical outcome. To achieve this, patient specific instrumentation was developed. A patient-specific guide manufactured by 3D printing was designed to aid in positioning of the cup component with a pre-operatively defined anteversion and inclination angle. The guide fits perfectly on the acetabular rim. An alignment K-wire in a pre-operatively planned orientation is used as visual reference during cup implantation. Accuracy of the device was tested on 6 cadaveric specimens. During the experiment, cadavers were positioned for a THA procedure using a posterolateral approach. A normal-sized incision was made and approach used as in the conventional surgical procedure. The PSI was subsequently fitted onto the acetabular rim and secured into its unique position due to its patient specific design. The metallic pin was placed in a drill hole of the PSI. Post-operative CT image data of each acetabulum with the placed pin were transferred to Mimics and the 3D model was registered to the pre-operative one. The anteversion and inclination of the placed pin was calculated and compared to the pre-operatively planned orientation. The absolute difference in degrees was evaluated. A secondary test was carried out to assess the error during impaction while observing the alignment K-wire as a visual reference. In a laboratory setting, error during impaction with a visual reference of the K-wire was measured. Deviation from planning showed to be on average 1.04° for anteversion and 2.19° for inclination. By visually aligning the impactor with this alignment K-wire, the surgeon may achieve cup placement as pre-operatively planned. The effect of the visual alignment itself was also evaluated in a separate test-rig showing minimal deviations in the same range. The alignment validation test resulted in an average deviation of 1.2° for inclination and 1.4° for anteversion between the metallic alignment K-wire used as visual reference and the metallic K-wire impacted by the test subjects. The inter-user variability was 0.9° and 0.8° for anteversion and inclination respectively. The intra-user variability was 1.6° and 1.0° for anteversion and inclination respectively. Tests per test subject were conducted in a consecutive manner. We investigated the accuracy of two factors affecting accuracy in the cup insertion with PSI, i.e. accuracies of the errors of bony fitting and cup impaction. Since the accuracy of the major contributing factors to the overall accuracy of PSI for cup insertion with linear visual reference of a metallic K-wire was within the acceptable range of 2 to 3 degrees, we state that the PSI we have designed assists to achieve the preoperatively planned orientation of the cup and as such leads to the reduction of outliers in cup orientation. This acetabular cup orientation guide can transfer the pre-operative plan to the operating room

INTRODUCTION. Dislocation is one of the most frequent complications in total hip arthroplasty (THA), affecting an estimated 1% to 5% of THA patients. Malposition of the acetabular cup is thought to be a likely contributor. As the field searches for solutions, new experimental methods can help engineers, scientists, and surgeons better understand the problem as well as evaluate novel techniques and products. OBJECTIVES. Create a laboratory simulation to assess patient positioning and pelvic motion during THA. Apply this simulation to assess (1) variation in patient positioning; (2) various methods to identify the pelvic plane via palpated anatomic landmarks. METHODS. A patient surrogate was developed to recreate patient-like modality, palpation, and motion, especially focusing on the spine's influence on pelvic flexion and rotation. Five different registration methods were evaluated (3 supine, 2 lateral decubitus). An ASIS-to-ASIS measurement was always used in calculations. The other axes measured were: 1) supine/trunk; 2) supine/ASIS-to-Pubis; 3) supine/neutral femoral axis; 4) LD/spine; and 5) LD/trunk. Three infrared LED markers were attached to the iliac spine of the surrogate's pelvis and monitored with an Optotrak Certus motion-tracking camera (Northern Digital). A second sensor was mounted to the top of a patient positioner (Innomed) to measure the orientation of the pelvis relative to the positioner. A third sensor was mounted to a set of calipers, which were aligned with anatomic landmarks during registration. To compare results from registration methods, a reference orientation of the pelvis was recorded by digitizing landmarks comprising the anterior pelvic plane (APP). The APP is the plane created by three points: the left ASIS, right ASIS, and midpoint of pubic tubercles. Theoretical pelvic orientation was calculated using these digitized points. The vectors generated from the gross anatomic registration steps were used to calculate the measured orientation of the pelvis compared to theoretical. The rotation, or error, matrix between theoretical and measured pelvic orientations was computed and then projected on an APP coordinate system to translate the error matrix to cup inclination and version. RESULTS. Inter- and intra-operator variability was good for most registration methods. The error in cup orientation when compared to the Lewinnek zone is promising. Of the 92 registrations, 91 (99%) were within the Lewinnek abduction range (30°–50°), 80 (87%) were within the Lewinnek version range (5°–25°), and 79 (85%) were within the range for both. When only considering the supine trunk and ASIS-pubis registrations, all 37 calculated cup orientations were within the Lewinnek zone. CONCLUSIONS. By aligning an instrument with rigid body markers along two vectors, operators were able to create a patient coordinate system that translated to error of cup inclination and version of only a few degrees from the theoretical target. The laboratory simulation developed in this study will aid scientists and engineers in evaluating novel patient positioning solutions for THA. While further research with more operators and perhaps cadaveric tissue is warranted to confirm these results, there is promise that a simple and intuitive patient registration method may reduce variation in cup placement during THA

The purpose of this preliminary study was to evaluate the feasibility and accuracy of HipAlign (OrthAlign, Inc., USA) system for cup orientation in total hip arthroplasty (THA). The subjects of this study were 5 hips that underwent primary cementless THA via a posterior approach in the lateral decubitus position. Evaluation 1; after reaming acetabular bone, a trial cup was placed in the reamed acetabulum in an aimed alignment using HipAlign. Then, the trial cup alignment was measured using HipAlign and CT-based navigation system in the radiographic definition. Evaluation 2; a cementless cup was placed in the reamed acetabular in an aimed alignment using CT-based navigation and cup alignment was measured using both methods. After operation, we measured the cup alignment using postoperative CT in each patient. In the results, the average cup inclination measured with HipAlign was around 5 degrees of true cup inclination angles. The average cup anteversion with HipAlign tended to be larger than that with CT-based navigation or postoperative CT in both evaluations. That is because there is a difference in the pelvic sagittal tilt between the lateral position and supine position. In conclusion, this study suggests that guiding cup alignment with the use of HipAlign is feasible through a posterior approach and the mean cup inclination measured with HipAlign showed an acceptable level of accuracy, but the mean cup anteversion is not reliable. We need a further modification for pelvic registration to improve the accuracy of cup anteversion