Glenoid baseplate orientation in reverse shoulder arthroplasty (RSA) influences clinical outcomes, complications, and failure rates. Novel technologies have been produced to decrease performance heterogeneity of low and high-volume surgeons. This study aimed to determine novice and experienced shoulder surgeon's ability to accurately characterise glenoid component orientation in an intra-operative scenario. Glenoid baseplates were implanted in eight fresh frozen cadavers by novice surgical trainees. Glenoid baseplate version, inclination, augment rotation, and superior-inferior centre of rotation (COR) offset were then measured using in-person visual assessments by novice and experienced shoulder surgeons immediately after implantation. Glenoid orientation parameters were then measured using 3D CT scans with digitally reconstructed radiographs (DRRs) by two independent observers. Bland-Altman plots were produced to determine the accuracy of glenoid orientation using standard intraoperative assessment compared to postoperative 3D CT scan results. Visual assessment of glenoid baseplate orientation showed “poor” to “fair” correlation to 3D CT DRR measurements for both novice and experienced surgeon groups for all measured parameters. There was a clinically relevant, large discrepancy between intra-operative visual assessments and 3D CT DRR measurements for all parameters. Errors in visual assessment of up to 19.2 degrees of inclination and 8mm supero-inferior COR offset occurred. Experienced surgeons had greater measurement error than novices for all measured parameters. Intra-operative measurement errors in glenoid placement may reach unacceptable clinical limits. Kinesthetic input during implantation likely improves orientation understanding and has implications for hands-on learning

Introduction. Model-based radiostereometric analysis (MBRSA) allows the in vivo measurement of implant loosening (i.e. migration) from a host bone by acquiring a pair of biplanar radiographs of the patient's implant over time. Focusing on total knee replacement patients, the accuracy of MBRSA in calculating tibial baseplate migration depends on the accuracy in registering a 3D model onto the biplanar radiographs; thus, the shape of the baseplate and its orientation relative to the imaging planes is pertinent. Conventionally, the baseplate coordinate system is aligned with the laboratory coordinate system, however, this reference orientation is unnecessary and may hide unique baseplate features resulting in less accurate registration (Figure 1). Therefore, the primary objective of this study was to determine the optimal baseplate orientation for improving accuracy during MBRSA, and an acceptable range of orientations for clinical use. A second objective was to demonstrate that a custom knee positioning guide repeatably oriented the baseplate within the acceptable range of orientations. Materials and Methods. A tibia phantom consisting of a baseplate rigidly fixed to a sawbone was placed in 24 orientations (combination of six rotations about X (i.e. knee flexion) and four rotations about Z (i.e. hip abduction)) with three pairs of radiographs acquired at each orientation. The radiographs were processed in MBRSA software, and the mean maximum total point motion (MTPM), an indicator of bias error during model registration, was plotted as a function of the two rotations to determine the optimal orientation and a range of acceptable orientations (Figure 2). A custom knee positioning guide was manufactured with the goal of orienting the baseplate close to the optimal orientation and within the acceptable range of orientations (Figure 3). Ten independent pairs of biplanar radiographs were acquired by repeatedly placing a knee model in the knee positioning guide, and the images were processed in MBRSA software to determine the baseplate orientation. Results and Discussion. Results showed an 85% decrease in bias error between the reference orientation (i.e. no rotation) and the optimal orientation (10° rotation about X and 5° rotation about Z). An acceptable range of orientations from 5° − 20° rotation about an axis perpendicular to the sagittal imaging plane and from 5° − 15° rotation about an axis perpendicular to the coronal imaging plane was defined as these orientations decreased the bias error by more than 50%. Additionally, the custom knee positioning guide controlled the mean orientation ± one standard deviation within the acceptable range of orientations. Conclusions. The accuracy of MBRSA is significantly improved if the tibial baseplate is placed in the range of acceptable orientations as opposed to the conventional reference orientation. A custom knee positioning guide can be used during a clinical study to repeatably position the patient's knee within the range of acceptable orientations. For any figures or tables, please contact the authors directly

Superiorly eroded glenoids in cuff tear arthropathy represent a surgical challenge for reconstruction. The bone loss orientation and severity may influence glenoid component fixation. This computed-tomography study quantifies both the degree of erosion and orientation in superiorly eroded Favard E2 glenoids. We hypothesized that the erosion in E2 glenoids does not occur purely superiorly, rather, it is oriented in a predictable posterosuperior orientation with a largely semicircular line of erosion. Three-dimensional reconstructions of 40 shoulders with E2 glenoids (28 female, 12 male patients) at a mean age of 74 years (range, 56–88 years) were created from computed-tomography images. Point coordinates were extracted from each construct to analyze the morphologic structure. The anatomical location of the supra- and infraglenoid tubercle guided the creation of a superoinferior axis, against which the orientation angle of the erosion was measured. The direction and, thus, orientation of erosion was calculated as a vector. By placing ten point coordinates along the line of erosion and creating a circle of best fit, the radius of the circle was placed orthogonally against a chord that resulted by connecting the two outermost points along the line of erosion. To quantify the extent of curvature of the line of erosion between the paleo- and neoglenoid, the length of the radius of the circle of best fit was calculated. Individual values were compared against the mean of circle radii. The area of bony erosion (neoglenoid), was calculated as a percentage of the total glenoid area (neoglenoid + paleoglenoid). The severity of the erosion was categorized as mild (0% to 33%), moderate (34% to 66%), and severe erosion (>66%). The mean orientation angle between the vector of bony erosion and the superoinferior axis of the glenoid was 47° ± 17° (range, 14° – 74°) located in the posterosuperior quadrant of the glenoid, resulting in the average erosion being directed between the 10 and 11 o'clock position (right shoulder). In 63% of E2 cases, the line of erosion separating the paleo- and neoglenoids was more curved than the average of all bony erosions in the cohort. The mean surface area of the neoglenoid was 636 ± 247 mm2(range, 233 – 1,333 mm2) and of the paleoglenoid 311 ± 165 mm2(range, 123 – 820 mm2), revealing that, on average, the neoglenoids consume 67% of the total glenoid surface. The extent of erosion of the total cohort was subdivided into one mild (2%), 14 moderate (35%) and 25 severe (62%) cases. Using a clock-face for orientation, the average orientation of type E2 glenoid defects was directed between the 10 and 11 o'clock position in a right shoulder, corresponding to the posterosuperior glenoid quadrant. Surgeons managing patients with E2 type glenoids should be aware that a superiorly described glenoid erosion is oriented in the posterosuperior quadrant on the glenoid clock-face when viewed intra-operatively. Additionally, the line of erosion in 63% of E2 glenoids is substantially curved, having a significant effect on bone removal techniques when using commercially available augments for defect reconstruction

Introduction. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. Methods. Cup orientation in 50 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 184 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. 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 inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Supine tilt-adjusted Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p< .0001). Supine tilt-adjusted Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p<.01). Alt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 30.6 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 +/− 7.8 degrees of tilt-adjusted operative anteversion and 42.4 +/− 13.5 degrees of operative inclination (Figure 1). Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup mal-position and its associated complications. For figures and tables, please contact authors directly

Introduction. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. Methods. Cup orientation in 21 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 115 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. 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 inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p < .001). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p=.01). Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 28.9 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 +/− 7.8 degrees of tilt-adjusted operative anteversion and 42.4 +/− 13.5 degrees of operative inclination. Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup malposition and its associated complications

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. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago. 1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. METHODS. Cup orientation in 30 hips revisedin 27patients for recurrent instability was measured using CT. These hips were compared to a group of 115 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. 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. 2. Both absolute cup position relative to the APP and tilt-adjusted cup position. 3. were calculated. RESULTS. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p < 0.001). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p < 0.01). Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 21.8 or more than 42.6 degrees or operative inclination of less than 30.6 or more than 55.9 degrees or both. The center of the “safe zone” is 32.2 ± 10.4 degrees of tilt-adjusted operative anteversion and 45.3 ± 8.7 degrees of operative inclination (Figure 1). CONCLUSIONS. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination. Improved methods of accurately placing the acetabular component placement may reduce the incidence of cup malposition and its associated complications

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

BACKGROUND. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation. PURPOSE. We assessed the orientation of acetabular components revised due to recurrent instability and compared the results to a series of stable hip replacements. METHODS. Cup orientation in 21 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 115 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. RESULTS. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p=.01). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p<.001). Operative inclination was not significantly different between the control and dislocating groups. Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 28.9 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 ± 7.8 degrees of tilt-adjusted operative anteversion and 42.4 ± 13.5 degrees of operative inclination. CONCLUSION. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination

Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement. Bilateral THA was performed on three fresh frozen cadavers using oversized (jumbo) offset head center revision acetabular cups with an anterior recess (60, 62 and 66 mm diameter) and tapered wedge primary stems through a posterior approach. A 2mm diameter flexible stainless steel cable was inserted into the psoas tendon sheath between the muscle and the surrounding membrane to identify the location of the psoas muscle radiographically. CT scans of each cadaver were imported in an imaging software. The acetabular shells, cables as well as pelvis were segmented to create separate solid models of each. The offset head center shell was virtually replaced with an equivalent diameter hemispherical shell by overlaying the outer shell surfaces of both designs and keeping the faces of shells parallel. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion on psoas impingement, we virtually varied the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles for both shell designs. The CT analysis revealed that the original orientation (inclination/anteversion) of the shells implanted in 3 cadavers were as follows: Left1: 44.7°/23.3°, Right1: 41.7°/33.8°, Left2: 40/17, Right2: 31.7/23.5, Left3: 33/2908, Right3: 46.7/6.3. For the offset center shells, the shell to cable distance in all the above cases were positive indicating that there was clearance between the shells and psoas. For the hemispherical shells, in 3 out of 6 cases, the distance was negative indicating impingement of psoas. With the virtual implantation of both shell designs at orientations 40°/10°, 40°/20°, 40°/30° we found that greater anteversion helped decrease psoas impingement in both shell designs. When we analyzed the influence of inclination angle on psoas impingement by comparing wire distances for three orientations (30°/20°, 40°/20°, 50°/20°), we found that the effect was less pronounced. Further analysis comparing the offset head center shell to the conventional hemispherical shell revealed that the offset design was favored (greater clearance between the shell and the wire) in 17 out of 18 cases when the effect of anteversion was considered and in 15 out of 18 cases when the effect of inclinations was considered. Our results indicate that psoas impingement is related to both cup position and implant geometry. For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect. An offset head center cup with an anterior recess was effective in reducing psoas impingement in comparison to a conventional hemispherical geometry. In conclusion, adequate anteversion is important to avoid psoas impingement with jumbo acetabular shells and an implant with an anterior recess may further mitigate the risk of psoas impingement

Introduction. Proper acetabular component orientation is an important part of successful total hip replacement surgery. Poorly positioned implants can lead to early complications, such as dislocation. Mal-positioned acetabular components can also generate increase wear debris due to edge loading which can cause pre-mature loosening. It is essential to be able to measure post-operative implant orientation accurately to assure that implants are positioned properly. It is difficult and potentially inaccurate to manually measure implant orientation on a post-op radiograph. This is particularly true for the immediate post-op radiograph where the patient is not as well aligned relative to the x-ray beam. However, the best time to determine if an acetabular component is mal-aligned is immediately following surgery so the patient could be taken back to the OR for immediate revision. Taking post-op CT scans is expensive and subjects the patient to increased radiation exposure, so using CT post-operatively is not done routinely. With the increased use of robotics and computer navigation at surgery there are often pre-op CT scans for total hip replacement patients. Current radiological tools do not take advantage of this pre-op CT scan for assessment of acetabular component orientation. A new software module for Mimics medical imaging software (Materialise, Leuven, Belgium) is able to overlay 3D CT data onto radiographs. We used this x-ray module to see if we could measure acetabular component orientation using the pre-op CT scan and the routine post-op x-ray that is taken immediately following total hip arthroplasty at our institution. Methods. From a prior study, we had pre-op, and post-op CT scans of a group of twenty patients who received a total hip replacement. The post-op scan was used to measure the actual acetabular component orientation, both inclination and anteversion (Figure 1). We then measured component orientation using only the pre-op CT scan and the initial post-op x-ray using the Mimics x-ray module. We created a 3D model of the pelvis from the pre-op CT using Mimics. Then, the x-ray module was used to import the post-op radiograph into the Mimics file. Using the software, the x-ray was registered to the pre-op 3D pelvis. A 3D .stl file of the acetabular component used at surgery was then imported into the Mimics file and also registered according to the post-op radiograph (Figures 2 and 3). Once the cup and pelvis were both registered to the post-op radiograph, they were exported as .stl files and the acetabular anteversion and inclination were measured using the same method we used for the post-op scan. We then compared the results of our measurements from the post-op 3D reconstruction to the 2D overlay method to determine the accuracy of this new measurement technique. Results. The average error for anteversion and inclination was 1.5±1.5 and −0.8±1.6 degrees respectively. Maximum error for anteversion and inclination was 5.7 and −5.0 degrees respectively. Conclusion. The x-ray module could be a powerful tool in the assessment of post-operative orientation of the acetabular component in total hip arthroplasty

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

Statement of Purpose. Mechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint replacements as studies have reported increases in local tissue reactions. 1. Two surgical factors that may effect taper seating mechanics are seating load magnitude and orientation. In this study 12/14 modular taper junctions were seated over a range of loads and loading orientations. The goals of this study were to assess the effects of load magnitude and orientation on seating load-displacement mechanics and to correlate these to the pull-off load. Methods. Ti6Al4V 12/14 tapers and CoCrMo heads were tested axially at four seating load levels (n=5): 1-, 2-, 4- and 8- kN. Three orientation groups were tested at 4 kN (n=5), 0°, 10° and 20°. The load-displacement behavior during testing was captured using data acquisition methods and two non-contact eddy current sensors fixed to the neck, targeting head-neck relative motion (Micro-Epsilon). Loads were ramped (200 N/s) with a servohydraulic system from 0 N to peak load and held for 5s (Instron). Off-axis test samples were oriented in an angled fixture. Displacement and load data were recorded in LabView. Seating displacement was the distance traveled between 50 N and thepeak load. Axial tensile pull-off loads (5 mm/min) were applied until the locking ability of taper junctions failed. Statistical analysis was performed using ANOVA test (P<0.05). Results. Axial tests. Seating load-displacement behavior at different seating loads (Fig. 1) show consistent characteristic behavior. Displacements rise parabolically to the peak load reflecting elastic deformation and rigid-body motion. Unloading is elastic and the y-intercept of the unloading curves reflects seating displacement. Displacement and work of seating for the axial tests (Fig. 2a and 2b, respectively) show that both increase with seating magnitude. Displacement increases approximately linearly, while work increases parabolically with seating load. All groups are significantly different (P<0.05). Pull-off loads (Fig. 3) increase linearly with seating load. Pull-off loads are approximately 44% of the seating load. All loads are statistically different (P<0.05). Off-axis tests. Measured seating displacements were comprised of rigid-body and elastic motion. A stiffness-correction method removed elastic motion. 2. Displacement, work and pull-off at the different seating load orientations were not significantly affected (P>0.05, data not shown). Discussion. Static seating load magnitude and orientation effects on seating mechanics and pull-off loads simulating surgical assembly were quantitatively studied in an instrumented seating test method. Increased seating loads increased work of seating, seating displacement and pull-off load. Such seating plots may assist in better understanding of the design, material, and surgical factors associated with taper locking mechanics. Seating orientation to 20° offset did not significantly affect seating mechanics or pull-off loads. Pull-off loads were about 50% of the seating loads for all cases which is consistent with other work. 3. . Conclusions. Increased seating load magnitude increased seating displacement, work and pull-off loads in 12/14 tapers. Load orientation had no significant effect. For figures/tables, please contact authors directly.

Introduction. Most of studies on Total Hip Arthroplasty (THA) are focused on acetabular cup orientation. Even though the literature suggests that femoral anteversion and combined anteversion have a clinical impact on THA stability, there are not many reports on these parameters. Combined anteversion can be considered morphologically as the addition of anatomical acetabular and femoral anteversions (Anatomical Combined Anatomical Anteversion ACA). It is also possible to evaluate the Combined Functional Anteversion (CFA) generated by the relative functional position of femoral and acetabular implants while standing. This preliminary study is focused on the comparison of the anatomical and functional data in asymptomatic THA patients. Material and methods. 50 asymptomatic unilateral THA patients (21 short stems and 29 standard stems) have been enrolled. All patients underwent an EOS low dose evaluation in standing position. SterEOS software was used for the 3D measurements of cup and femur orientation. Cup anatomical anteversion (CAA) was computed as the cup anteversion in axial plane perpendicular to the Anterior Pelvic Plane. Femoral anatomical anteversion (FAA) was computed as the angle between the femoral neck axis and the posterior femoral condyles in a plane perpendicular to femoral mechanical axis. Functional anteversions for the cup (CFA) and femur (FFA) were measured in the horizontal axial patient plane in standing position. Both anatomical and functional cumulative anteversions were calculated as a sum. All 3D measures were evaluated and compared for the repeatability and reproducibility. Statistical analysis used Mann-Whitney U-test considering the non-normal distribution of data and the short number of patients (<30 for each group). Results. Functional cumulative anteversion was significantly higher than anatomical cumulative anteversion for all groups (p<0.05). No significant difference could be noted between the cases according to the use of short or standard stems. Conclusion. This study shows the difference of functional implant orientation as compared to the anatomical measurements. This preliminary study has limitations. First the limited sample of patients. Then this series only includes asymptomatic subjects. Nevertheless, this work focused on the feasibility of the measurements shows the potential interest of a functional analysis of cumulated anteversion. Standing position influences the relative position of THA implants according to the frontal and sagittal orientation of the pelvis. The relevance of these functional measurements in instability cases must be demonstrated, especially in patients with anterior subluxation in standing position which is potentially associated with pelvic adaptative extension. Further studies are needed for the feasibility of measurements on EOS images in sitting position and their analysis in case of instability. For any figures or tables, please contact authors directly

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

Consideration of biomechanical aspects during computer assisted orthopaedic surgery (CAOS) is recommendable in order to obtain satisfactory long-term results in total hip arthroplasty (THA). In addition to the absolute value of the hip joint resultant force R the pre- and post-operative orientation of R is an important aspect in the context of the development of a planning module for computer-assisted THA and furthermore for planning of acetabular orientation in periacetabular osteotomy interventions. It is possible to estimate the orientation of hip joint resultant force R for individual patients based on geometrical and anthropometrical parameters. The aim of this study was to examine how far the choice of the mathematical model influences the computational results for the orientation of R in the frontal plane. A further aspect was the comparison of the results with in-vivo data published in the open access OrthoLoad database (www.orthoload.com). Our comparative study included the 2D-models suggested by Pauwels, Blumentritt and Debrunner as well as the 3D-model suggested by Iglič and three patient datasets from the Orthoload database. As computation of R according to each model relies on standardized X-ray imaging, three anterior-posterior (a.p.) digitally reconstructed radiographs (DRRs) were generated from CT data (x21_x21, x8_x8, x12_x12). The orientation of R was expressed in terms of the angle δ for these three patient individual datasets. The angle δ is defined as the angle between the longitudinal axis and R. The computation results were also compared with in vivo telemetric measurement data from the OrthoLoad database. The following data were used to evaluate R in the frontal plane: the highest load peak of the single leg stance (static conditions) of three patients (EBL, HSR, KWR) respectively in the same manner for planar gait (dynamic conditions) of one patient (KWR). The mean value of the orientation of R under static conditions in single leg stance was calculated in order to get a reference value. For the orientation of R under dynamic conditions δ was calculated by using only the highest peak of three cycles (heel strike to toe off) determined in one single patient (among the three patients involved in the measurements under static conditions) of the database. The following values of δ were obtained:. Pauwels: 18.26°/20.34°/17.31° (x21_x21/x8_x8/x12_x12) Debrunner: 12.37°/14.30°/12.59° Blumentritt: 5.18°/6.52°/6.14° Iglič: 9.24°/9.01°/9.20°. OrthoLoad database (in-vivo): 28.41°/17.08°/13.32°-static (EBL/HSR/KWR) 16.44°-dynamic (KWR). The differences in the computational results appear to depend more on the hip model than on the variability of patient-specific geometrical and anthropometrical parameters. The results obtained with in-vivo measurement data are best approximated by using Pauwels' model. The mean values of Pauwels (18.64°), Debrunner (13.09°) and Iglič (9.15°) are a little bit more vertically orientated than the mean value of the static in-vivo results (19.60°). Only Pauwels' model result has a larger angle δ than the in-vivo dynamic result (KWR = 16.44°). By comparing the in-vivo values obtained under dynamic conditions, i.e. gait, (16.44°) with the static in-vivo values of the same patient (13.32°), it could be recognized that the static values are a little bit more vertically orientated than the dynamic result. But both are in the same range as the mathematical models. The computational biomechanical hip models try to approximate the physiological conditions of the hip joint and the OrthoLoad database represents the physiological reconstructed (artificial) hip joint. Therefore, we think our validation approach is useful for a comparison of the biomechanical computation models. In contrast, Blumentritt's model outcomes have the largest deviation from the other models as well as from the in-vivo data (static and dynamic conditions). Blumentritt used the weight bearing surface as a reference. He defined it being perpendicular to the longitudinal axis [3]. He postulated that a valid and optimal orientation of R is approximately perpendicular on the weight bearing surface respectively parallel to the longitudinal axis. This approach for validation is questionable because the results show that in the three included and analysed DDR's the orientation is in the mean value 5.95° to the longitudinal axis. It can be concluded that Blumentritt's model assumptions have to be carefully reviewed due to the deviations from in-vivo measurement data. Among the limitations of our study is the fact that the OrthoLoad database offers only a small number of patient datasets. There is only one dataset for the direct comparison of static (single leg stance) and dynamic (free planar gait) in-vivo measurement data of the same patient included. Furthermore, the individual anatomic geometry data of the patients included in the database are not revealed. Additionally, a source of errors could be an inaccuracy during the data acquisition from the DRR. Further research seems to be recommendable in the context of implementing a biomechanical hip model in a planning module for computer-assisted THA or periacetabular osteotomy interventions, respectively. Sensitivity analyses and parameter studies for different mathematical models using a multi-body-simulation system are objectives of our ongoing work

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:. Wear, wear-associated osteolysis, and instability are the most common reasons for revision total hip arthroplasty. These failures have been shown to be associated with acetabular component malpositioning. However, optimal acetabular component orientation on a patient-specific basis is currently unknown. The current study uses CT to assess acetabular orientation in a group of unstable hips as compared to a control group of stable hips. Methods:. Our institutional database of CT studies performed in the region of the hip beginning in February of 1998 (41,975 CT studies) was compared against our institutional database of revision total hip arthroplasties beginning in August of 2003 (2262 Revision THA) to identify CT studies of any hip treated for recurrent instability by revision of the acetabular component. Twenty hips in 20 patients with suitable CT studies were identified for the study group. Our control group consisted of 99 hips in 93 patients who had CT studies either for computer-assisted surgery on the contralateral side or for assessment of osteolysis. Using the CT data, the AP plane (APP) was defined, supine pelvic tilt was measured, and cup orientation was calculated by fitting a best fit plane to 6 points on the rim of the acetabular component. Cup orientation was calculated in degrees of operative anteversion and operative inclination according to the definitions of Murray. Both absolute cup position relative to the APP and tilt-adjusted cup position. 1. were calculated. Results:. The study group of 20 hips treated for instability showed a mean operative anteversion of 30.3 degrees (SD 17.6, range 1.0 to 58.1), a mean operative inclination of 35.9 degrees (SD 8.4, range 25.1 to 55.9), and a mean tilt-adjusted operative anteversion of 29.7 (SD 14.2, range 1.8 to 53). The control group of 99 hips showed a mean operative anteversion of 30.5 degrees (SD 10.7, range −1.9 to 57.5), a mean operative inclination of 37.7 degrees (SD 8.0, range 18.4 to 68.1), and a mean tilt-adjusted operative anteversion of 26.7 (SD 10.8, range −0.2 to 47.3). Most interestingly. all of the hips treated for instability had an operative anteversion of either 22.9 degrees or less or 38.67 degrees or more of tilt-adjusted operative inclination of either 30.5 degrees or less or 55.9 degrees or more, or both. The center of the safe zone in this study is 30.7 of tilt-adjusted operative anteversion and 43.2 degrees of operative inclination (Figure 1). There was no discernable safe zone in the non tilt-adjusted group. Discussion and Conclusion:. Most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. The hip dislocation safe zone appears to be narrower in operative anteversion than in operative inclination. Improved methods of improving the accuracy and reliability of acetabular component placement may reduce the incidence of cup malposition and its associated complications

The success of total hip replacement (THR) is closely linked to the positioning of the acetabular component. Malalignment increases rates of dislocation, impingement, acetabular migration, pelvic osteolysis, leg length discrepancy and polyethylene wear. Many surgeons orientate the cup in the same anteversion and inclination as the inherent anatomy of the acetabulum. The transverse acetabular ligament and acetabular rim can be used as a reference points for orientating the cup this way. Low rates of dislocation have been reported using this technique. Detailed understanding of the anatomy and orientation of the acetabulum in arthritic hips is therefore very important. The aim of this study was to describe the anteversion and inclination of the inherent acetabulum in arthritic hips and to identify the number that fall out with the ‘safe zone’ of acetabular position described by Lewinnek et al. (anteversion 15°±10°; inclination 40°±10°). A series of 65 hips, all with symptomatic osteoarthritis undergoing THR were investigated. Patients with developmental dysplastia of hip (DDH) were excluded. All patients had a navigated THR as part of their normal clinical treatment. A posterior approach to the hip was used. A commercially available non image based computer navigation system (Orthopilot BBraun Aesculap, Tuttlingen, Germany) was used. Rigid bodies (using active trackers) were attached to pelvis and femur. Anterior pelvic plane was registered using the two anterior superior iliac spines and pubic symphysis. The femoral head dislocated and removed and the labrum and soft tissue were excised to clear floor and rim of the acetabulum. Inner size of the empty acetabulum was sized with cup trials and appropriately size trial fixed with a computer tracker was then aligned in the orientation of the natural acetabulum as defined by the acetabular rim ignoring any osteophytes. The inclination and anteversion were calculated by the software. Surgery then proceeded with guidance of the computer navigation system. The computer software defines the anatomical values of orientation, to allow comparison with radiographs these were converted to radiological values as described by Murray et al. The acetabular inclination in all hips was also measured on pre-operative anteroposterior pelvic radiographs. This was done using digital radiographs analysed with the PACS system (Kodak, Carestream PACS Client, version 10.0). Acetabular inclination was measured using as the angle between a line passing through the superior and inferior rim of the acetabulum and a line parallel to the pelvis as identified by the tear drops, using the method described by Atkinson et al. All patients were Caucasian and had primary osteoarthritis. There were 29 males and 36 females. The average age was 68 years (SD 8). Mean anteversion was 9.3° (SD 10.3°). Anteversion for males was significantly lower than females with a mean difference of −5.5° (95%CI −10.5°,−0.5°) p = 0.033 but there was no significant difference in the number falling outside the “safe zone”. Mean inclination was 50.4° (SD 7.4°). There was no significant difference between males and females with respect to inclination angle or the number that fell outside the “safe zone”. Overall 69% of patients had a combined inclination and anteversion of the native acetabulum that fell outside the “safe zone” of Lewinnek. Mean acetabular inclination falls out with the ‘safe zone’. This trend has been seen in a recent study of arthritic hips using CT scans which found that the average angle of inclination in both males and females was greater than the upper limit of the safe zone. This study using CT also demonstrated a statistically significant 5.5° difference between males and females in terms of anteversion. This is the same as the figure we have found in our work. Inherent acetabular orientation in arthritic hips falls out with the safe zone defined by Lewinnek in 69% of cases. When using the natural acetabular orientation as a guide for positioning implants it should therefore not be assumed this will fall with in the safe zone although the validity of safe zones itself is questionable. Variation between patients must be taken into account and the difference between males and females, particularly in terms of anteversion, should also be considered

Introduction. The success of total hip replacement (THR) is closely linked to the positioning of the acetabular component. Malalignment increases rates of dislocation, impingement, acetabular migration, pelvic osteolysis, leg length discrepancy and polyethylene wear. Many surgeons orientate the cup to inherent anatomy of the acetabulum. Detailed understanding of the anatomy and orientation of the acetabulum in arthritic hips is therefore very important. The aim of this study was to describe the anteversion and inclination of the inherent acetabulum in arthritic hips and to identify the number that fall out with the ‘safe zone’ of acetabular position described by Lewinnek et al. (anteversion 15°±10°; inclination 40°±10°). Materials and Methods. A series of 65 hips all with symptomatic osteoarthritis undergoing THR were investigated. Patients with dysplastic hips were excluded. All patients had a navigated THR as part of their normal clinical treatment. A commercially available non image based computer navigation system (Orthopilot BBraun Aesculap, Tuttlingen, Germany) was used. Anterior pelvic plane was registered using the two anterior superior iliac spines and pubic symphysis. Inner size of the empty acetabulum was sized with cup trials and appropriately size trial fixed with a computer tracker was then aligned in the orientation of the natural acetabulum as defined by the acetabular rim ignoring any osteophytes. The inclination and anteversion were calculated by the software. The acetabular inclination in all hips was also measured on pre-operative anteroposterior pelvic digital radiographs. Acetabular inclination was measured using as the angle between a line passing through the superior and inferior rim of the acetabulum and a line parallel to the pelvis as identified by the tear drops, using the method described by Atkinson et al. Results. All patients were Caucasian and had primary osteoarthritis. There were 29 males and 36 females. The average age was 68 years (SD 8). The inclination was 50.4(SD 7.4) and 58.8(SD 5.7) on navigation and radiographs respectively. The anteversion was 9.3(SD 10.3) on navigation. Anteversion for males was significantly lower than females with a mean difference of −5.5° (95% CI −10.5°, −0.5°) with a p value of 0.033. There was no significant difference with respect to inclination. Overall 69% of patients had a combined inclination and anteversion of the native acetabulum that fell outside the “safe zone” of Lewinnek. Conclusions. Inherent acetabular orientation in arthritic hips falls out with the safe zone defined by Lewinnek in 69% of cases. When using the natural acetabular orientation as a guide for positioning implants it should therefore not be assumed this will fall with in the safe zone although the validity of safe zones itself is questionable. Variation between patients must be taken into account. The difference between males and females, particularly in terms of anteversion, should also be considered