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

Postoperative dislocation following total hip arthroplasty (THA) remains a significant concern with a reported incidence of 1% to 10%. The risk of dislocation is multifactorial and includes both surgeon-related (i.e. implant position, component size, surgical approach) and patient-related factors (i.e. gender, age, preoperative diagnosis, neurologic disorders). While the majority of prior investigations have focused on the importance of acetabular component positioning, recent studies have shown that approximately 60% of “dislocators” following primary THA have an acceptably aligned acetabular component. Therefore, the importance of the relationship between the spine and pelvis, and its impact on functional component position has gained increased attention. Kanawade and Dorr et al. have shown patients can be categorised into having a stiff, normal, or hypermobile pelvis based on their change in pelvic tilt when moving from the standing to seated position. The degree of change in functional position of both the acetabular and femoral components is impacted by the degree of pelvic motion each patient possesses. In the “normal” pelvis, as a patient moves from the standing to seated position the pelvis typically tilts posteriorly, thus increasing the functional anteversion of the acetabular component. However, patients with lumbar degeneration or spine pathology often have a decrease in posterior pelvic tilt in the seated position, thus potentially increasing their risk of dislocation. Bedard et al. noted an 8.3% dislocation risk in patients with a spinopelvic fusion after THA vs. 2.9% in those without. There is the potential that preoperative, dynamic imaging can be used to predict the ideal component position for each individual patient undergoing THA. However, this assumes that a patient's preoperative pelvic motion will be the same following implantation of a total hip prosthesis, and that a patient's pelvic motion will remain consistent over time postoperatively. A recent study has shown that the impact of THA on pelvic motion can be highly variable, thus potentially limiting the utility of preoperative dynamic imaging in predicting a patient's ideal component position. Future investigations must focus on preoperative factors that can be used to predict postoperative pelvic motion and how pelvic motion changes over time following implantation of a total hip arthroplasty

Introduction. One of the known mechanisms which could contribute to the failure of total hip replacements (THR) is edge contact. Failures associated with edge contact include rim damage and lysis due to altered loading and torques. Recent study on four THR patients showed that the inclusion of pelvic motions in a contact model increased the risk of edge contact in some patients. The aim of current study was to determine whether pelvic motions have the same effect on contact location for a larger patient cohort and determine the contribution of each of the pelvic rotations to this effect. Methods. Gait data was acquired from five male and five female unilateral THR patients using a ten camera Vicon system (Oxford Metrics, UK) interfaced with twin force plates (AMTI) and using a CAST marker set. All patients had good surgical outcomes, confirmed by patient-reported outcomes and were considered well-functioning, based on elective walking speed. Joint contact forces and pelvic motions were obtained from the AnyBody modelling system (AnyBody Technologies, DK). Only gait cycle regions with available force plate data were considered. A finite element model of a 32mm head on a featureless hemispherical polyethylene cup, 0.5mm radial clearance, was used to obtain the contact area from the contact force. A bespoke computational tool was used to analyse patients' gait profiles with and without pelvic motions. The risk of edge contact was measured as a “centre proximity angle” between the cup pole and centre of the contact area, and “edge proximity angle” between the cup pole and the furthest contact area point away from the pole. Pelvic tilt, drop and internal-external rotation were considered one at a time and in combinations. Results. In eight out of 10 patients, the addition of pelvic motions decreased the risk of edge contact during toe-off. There was up to 6° reduction in the proximity angles when pelvic motions were introduced to the gait cycle. In six out of 10 patients, the addition of pelvic motions resulted in an increase in the risk of edge contact during heel-strike with up to 6° increase in the proximity angles. For all patients where these effects were seen, sagittal pelvic tilt was a substantial contributor. Conclusion. The results of this study suggest that pelvic motion play an important role in contact location in THR bearings during loading phase. Both static and dynamic pelvic tilt contribute to the variability in the risk of edge contact. Further tests on larger patient cohorts are required to confirm the trends observed. The outcomes of this study suggest that pre-clinical mechanical and tribological testing of THRs should consider the role of pelvic motion. The outcomes also have implications for establishing surgical positioning safe zones, which are currently based only on risk of dislocation and severe impingement

Introduction. The functional ante-inclination (AI) of the cup after total hip arthroplasty (THA) is a key component in the combined sagittal index (CSI) to predict joint stability after THA. To accurately predict AI, we deducted a mathematic algorithm between the radiographic anteversion (RA), radiographic inclincation (RI), pelvic tilting (PT), and AI. The current study aims (1) to validate the mathematic algorithm; (2) to convert the AI limits in the CSI index (standing AI ≤ 45°, sitting AI ≥ 41°) into coronal functional safe zone (CFSZ) and explore the influences of the stand-to-sit pelvic motion (PM) and pelvic incidence (PI) on CFSZ; (3) to locate a universal cup orientation that always fulfill the AI criteria of CSI safe zone for all patients or subgroups of PM(PM ≤ 10°, 10° < PM ≤ 30°, and PM > 30°) and PI (PI≤ 41°, 41°< PI ≤ 62°, and PI >62°), respectively. Methods. A 3D printed phantom pelvic model was designed to simulate changing PT values. An acetabular cup was implanted with different RA, RI, and PT settings using robot assisted technique. We enrolled 100 consecutive patients who underwent robot assisted THA from April, 2019 to June, 2019 in our hospital. EOS images before THA and at 6-month follow-up were collected. AI angles were measured on the lateral view radiographs as the reference method. Mean absolute error (MAE), Bland-Altman analysis and linear regression were conducted to assess the accuracy of the AI algorithm for both the phantom and patient radiographic studies. The 100 patients were classified into three subgroups by PM and PI, respectively. Linear regression and ANOVA analysis were conducted to explore the relationship between the size of CFSZ, and PM and PI, respectively. Intersection of the CFSZ was conducted to identify if any universal cup orientation (RA, RI) existed for the CSI index. Results. The mathematic algorithm for calculating AI based on RI, RA, and PT is highly accurate according to the phantom and patient radiographic study. CFSZ size corresponds linearly with PM (R² = 0.638) and PI (R² = 0.129), respectively. There are significant differences in the size of CFSZ, as well as in the intersection of CFSZ and LSZ, between the subgroups of PM and PI, respectively (P<0.017). There is no universal cup orientations could be identified to fulfill the AI limits of the CSI index for all the 100 patients or any of the three subgroups, according to either PM or PI. Conclusions. The cup target orientation should be individualized. The validated algorithm between AI and RA, RI, and PT parameters can serve as the quantitative tool for patient-specific optimization of functional cup AI in different postures

Introduction. Cross table lateral (CTL) radiographs are commonly used to measure acetabular component anteversion after total hip arthroplasty (THA). CTL measurements may differ by >10 degrees from CT scan measurements, but the reasons for this discrepancy are poorly understood. We compare anteversion measurements made on CTL radiographs and CT scans to identify spinopelvic parameters predictive of inaccuracy. Methods. THA patients (n=47) with preoperative spinopelvic radiographic analysis and postoperative CT scans were retrospectively reviewed. Acetabular component anteversion was measured on post-operative CTL radiographs, and CT scans using 3D reconstructions of the pelvis. Patients were grouped by error (CTL-CT)>10° (n=11) or <10° (n=36), and spinopelvic mobility parameters were compared using t-tests. Correlation between error and mobility parameters was assessed with Pearson coefficient. Results. Patients with CTL error >10° (range 10–14) had stiffer lumbar spines with less lumbar flexion (38° vs 47°, p=0.03), greater sagittal imbalance measured by pelvic incidence-lumbar lordosis mismatch (6° vs −2°, p=0.04), more pelvic extension when seated (pelvic tilt −10° vs −2°, p=0.05), and greater change in pelvic tilt between supine and seated positions (13° vs 4°, p=0.04). The error of CTL measurements showed a positive correlation with increased CTL anteversion (r=0.5, p=0.001), standing lordosis (r=0.23, p=0.05), seated lordosis (r=0.4, p=0.01) and pelvic tilt change between supine and step-up positions (r=0.34, p=0.01). Discussion. Differences in spinopelvic mobility patterns may explain the variable accuracy of acetabular anteversion measurements on CTL radiographs. Patients with stiff spines and increased compensatory pelvic motion have less accurate measurements on CTL radiographs. Flexion of the contralateral hip is required to obtain clear CTL radiographs. In patients with a stiff lumbar spine, this movement may extend the pelvis and increase anteversion of the acetabulum on CTL views. Reliable analysis of acetabular component anteversion in this patient population may require advanced imaging with a CT scan

Introduction. In total hip arthroplasty (THA), a high radiographic inclination angle (RI) of the acetabular component has been linked to an increased dislocation rate, liner fracture, and increased wear. In contrast to version, we have more proven boundaries when it comes to a safe zone for angles of RI. Although intuitively it seems easier to achieve a target RI, most studies demonstrate a lack of accuracy and the trend towards a high RI with all surgical approaches when using a freehand technique or a mechanical guide. This is due to pelvic motion during surgery, which can be highly variable. The current study had two primary aims, each with a different primary outcome. The first aim was to determine how accurate a surgeon could obtain the target operative inclination (OI) during THA when using a cementless cup using a digital protractor. The second aim was to determine how accurate a surgeon can estimate the target OI to obtain a RI of 40° based on the patient's hip circumference as demonstrated in a previous study. Methods. In this prospective study, we included 200 consecutive patients undergoing uncemented primary THA in the lateral decubitus position using a posterior approach. Preoperatively, the surgeon determined the target OI based on the patient's hip circumference (22.5°, 25°, 27.5° or 30°). Intraoperatively, the effective OI was measured with the aid of a digital inclinometer after seating of the acetabular component. Six weeks postoperatively anteroposterior pelvic radiographs were made and two evaluators, blinded to the effective OI, measured the RI of the acetabular component. The safe zone for inclination was defined as 30°-45° of inclination. Results. The mean difference between the target OI and the effective OI of the acetabular component was −0.7° SD 1.4 (95% CI −0.9° to −0.5°). The difference between the target and effective OI was less than 1° in 108 patients (54%), less than 2° in 160 patients (80%) and less than 3° in 186 patients (93%). In 14 patients (7%) the difference was 3°-5°. The mean RI was 37.9° SD 4.7 (95% CI 37.2° to 38.5°). The mean difference between the RI and effective OI was 11.5° SD 4.7 (95% CI 10.8° to 12.1°). Overall, 188 cups (94%) were within the inclination safe zone. When analysing the RI outliers, 1 could have be avoided if a better target OI was chosen and 2 could have been avoided if the difference between the target and effective OI would have been smaller. For the remaining 9 outliers (75%) the difference between the RI and effective OI was in the upper and lower 7. th. percentile, indicating more or less than average motion of the pelvis in these patients. Discussion and Conclusions. When using a digital protractor, the mean difference between the target OI and the effective OI of the acetabular component was less than 3° in 93% and less than 5° in all patients. The use of a digital protractor allows surgeons to accurately implant the acetabular component in the desired OI in a cheap and easy way. By adjusting the target OI based on the patient's hip circumference, 94% of the acetabular components were placed within an inclination safe zone of 30°-45°. Most outliers were caused by more of less than average intraoperative pelvic motion

Background. Over 10% of total hip arthroplasty (THA) surgeries performed in England and Wales are revision procedures. 1. Malorientation of the acetabular component in THA may contribute to premature failure due to mechanisms such as edge loading and prosthetic impingement. It is known that the pelvis flexes and extends during activities of daily living (ADLs), and excessive pelvic motion can contribute to functional acetabular malorientation. Preoperative radiographs can be performed to measure changes in pelvic tilt during ADLs to identify high risk individuals and inform surgical decision making. However, radiographs require time-consuming radiation exposure, and are unable to provide truly dynamic 3-dimensional analysis. The purpose of this study was to develop and evaluate a motion capture method using inertial measurement units (IMUs). This would provide a rapid, non-invasive analysis of pelvic tilt which could be used to support surgical planning. Methods. Patients awaiting THA were fitted with a bespoke device consisting of a 3D-printed clamp which housed the IMU and positioned over the sacrum. A wide elastic belt was fitted around the patient's waist to keep the device in place. Movement data was transmitted wirelessly to a tablet computer. Pelvic tilt was measured in standing, flexed seated and step-up positions while undergoing X-rays with the IMU capturing the data in parallel. Statistical analysis included measures of correlation between the X-ray and IMU measurements. Results. Measurements from 30 patients indicated a moderate-strong correlation (R. 2. = .87; Figure 1) between IMU and radiological measures of AP pelvic tilt. Conclusions. A novel device has been developed that can suitably track pelvic movements. This could potentially be used to identify patients with large changes in pelvic tilt, and thereby inform surgical planning. For any figures or tables, please contact the authors directly

The orientation of the acetabular component is influenced by the orientation at which the surgeon implants the component and the orientation of the pelvis at the time of implantation. When operating with the patient in the lateral decubitus position, pelvic orientation can be highly variable. The goal of this study was to examine the effect of two different pelvic supports on cup orientation. In this prospective study, 200 consecutive patients undergoing uncemented primary THA in the lateral decubitus position were included. In the control group a single support over the pubic symphysis (PS) was used. In the study group, a single support over the ipsilateral anterior superior iliac spine (ASIS) was used. In every patient, the cup was inserted and the angle of the cup introducer relative to the floor (apparent operative inclination; OIa) was measured with the aid of a digital inclinometer. The radiographic inclination (RI) was measured on anteroposterior pelvic radiographs at 6 weeks postoperatively. The target zone for cup inclination was 35–45°. In both cohorts the cups were implanted close to the target OIa with an absolute difference with the OIa of 0.86° SD 0.82 in the PS cohort and 1.03° SD 0.99 in the ASIS cohort (p=0.18). The difference between the RI and OIa was higher in the PS cohort 12.2° SD 4.1 compared with 7.5° SD 3.7 in the ASIS cohort (p<0.0001) with also a bigger variance (p=0.04) in the PS cohort. The mean RI was 38.5° SD 4.4 compared with 39.2° SD 4.1 (p=0.26) respectively. There were more cups outside the RI target zone in the PS cohort compared with the ASIS cohort (respectively 26 versus 15; p<0.05). In this study the mean difference between the RI and OIa (the angle of the cup introducer during surgery) was significantly less when using a support over the ASIS compared with a support over the pubic symphysis. Apparently using a support over the ASIS causes less pelvic motion during surgery compared with a support over the pubic symphysis. This resulted in less variance and inclination outliers when using a tight target zone of 35–45°

Aims

Excessive posterior pelvic tilt (PT) may increase the risk of anterior instability after total hip arthroplasty (THA). The aim of this study was to investigate the changes in PT occurring from the preoperative supine to postoperative standing position following THA, and identify factors associated with significant changes in PT.

Aims

Spinopelvic mobility plays an important role in functional acetabular component position following total hip arthroplasty (THA). The primary aim of this study was to determine if spinopelvic hypermobility persists or resolves following THA. Our second aim was to identify patient demographic or radiological factors associated with hypermobility and resolution of hypermobility after THA.

Aims

Adverse spinal motion or balance (spine mobility) and adverse pelvic mobility, in combination, are often referred to as adverse spinopelvic mobility (SPM). A stiff lumbar spine, large posterior standing pelvic tilt, and severe sagittal spinal deformity have been identified as risk factors for increased hip instability. Adverse SPM can create functional malposition of the acetabular components and hence is an instability risk. Adverse pelvic mobility is often, but not always, associated with abnormal spinal motion parameters. Dislocation rates for dual-mobility articulations (DMAs) have been reported to be between 0% and 1.1%. The aim of this study was to determine the early survivorship from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) of patients with adverse SPM who received a DMA.

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

Aims

Pelvic tilt (PT) can significantly change the functional orientation of the acetabular component and may differ markedly between patients undergoing total hip arthroplasty (THA). Patients with stiff spines who have little change in PT are considered at high risk for instability following THA. Femoral component position also contributes to the limits of impingement-free range of motion (ROM), but has been less studied. Little is known about the impact of combined anteversion on risk of impingement with changing pelvic position.

Low back pain (LBP) is a common problem in rowers of all levels. Few studies have looked at the relationship between rowing technique, the forces generated during the rowing stroke and the kinematics of spinal motion. Of particular concern with respect to spinal injury and damage are the effects of fatigue during long rowing sessions. A technique has been developed using an electromagnetic motion system and strain gauge instrumented load cell to measure spinal and pelvic motion and force generated at the oar during rowing on an exercise rowing ergometer. Using this technique 13 elite national and international oarsmen (mean age 22.43 ± 0.02 years) from local top squad rowing teams were investigated. The test protocol comprised of a one hour rowing piece. During this session rowing stroke profiles were quantified in terms of lumbopelvic kinematics and stroke force profiles. These profiles were sampled at the start of the session and quarterly intervals during the hour piece. From this data we were able to quantify the motion of the lumbar spine and pelvis during rowing and relate this to the stroke force profile. The stroke profiles over the one hour piece were then compared to examine the effects of fatigue. This revealed marked changes and increases in the amount of spinal motion during the hour piece suggesting that to maintain stroke force profiles athletes were utilising greater ranges of spinal motion. The relevance of this with regard to low back pain however, requires further investigation

Total hip arthroplasty (THA) using minimally invasive surgeries (MIS) now become popular operative procedures. It is not easy to understand geometric information of pelvis and femur in the restricted operative fields during MIS-THA. Recently, THA in supine position comes into the limelight again to place acetabular cups in an optimum position because we can minimise the intra-operative pelvic motion during THA in supine position. To verify the usefulness of supine position, we measured the angels of acetabular trial cups intra-operatively using the CT-based navigation system. The trial cup positions were placed according to a conventional acetabular cup alignment guide. We compared the angles of acetabular trial cups between supine and lateral positions through the same MIS antero-lateral (AL) surgical approach. Thirty eight hips underwent THA in lateral position (the AL group; average age: 63.9 years old, female: 29 cases, 33 hips, male: 5 cases, 5 hips) and 40 hips underwent THA in supine position (the AL Supine group; average age: 62.2 years old, female 40 cases, 40 hips) were subjected in this study. The single surgeon (the first author) performed all surgeries. We used the Roettinger's modified Watson-Jones approach in both groups. The pelvic registration for navigation was carried out using the CT-fluoro matching procedure with VectorVision Hip (BrainLAB, Germany). After acetabular reaming, the acetabular trial cups were placed into the reamed acetabulum to be at 45 degrees of operative inclination (OI) and at 20 degrees of operative anteversion (OA) using a conventional acetabular cup alignment guide. These angles of the trial cups were measured intra-operatively using the CT-based navigation system, VectorVision Hip. After removing the acetabular trial cup, the acetabular cups were placed using the navigation system. Trilogy cups (Zimmer, USA) and AMS HA shells (JMM, Japan) were used in this study. The average angles of OI were 45.7 degrees (SD 5.5 degrees) in the AL group and 46.3 degrees (SD 4.6 degrees) in the AL Supine group. The average angles of OA were 30.0 degrees (SD 13.5 degrees) in the AL group and 23.5 degrees (SD 8.2 degrees) in the AL Supine group. The hip numbers whose errors were less than 10 degrees were 13 hips in the AL group and 26 hips in the AL Supine group, respectively. There was significant difference in hip numbers whose errors of angles were less than 10 degrees between the AL and Supine groups. The hip numbers whose errors were less than 5 degrees were 7 hips in the AL group and only 6 hips in the AL Supine group, respectively. There was no significant difference in hip numbers whose errors of angles were less than 5 degrees between the AL and Supine groups. The error values of OI were less than 10 degrees except one hip in both groups. However, the error values of 25 hips in the AL group were more than 10 degrees. In lateral position, the pelvis easily rotated when the affected lower extremity was extended, externally rotated, and adducted during the femoral preparation in the AL group, which resulted in malalignment of acetabular OA. In contrast, most hips could be set with the error values less than 10 degrees in the AL Supine position because the pelvis could be stabilised on the operative table. In addition, landmarks, such as bilateral antero-superior iliac spines and the symphysis pubis, were palpable in supine position. However, the hips with error values less than 5 degrees were only 6 out of 40 hips even though in supine position. Using MIS techniques, we can provide more stable hip joint just after surgery since the muscles surrounding hip joints can be preserved. We have to place acetabular cups in an optimum position to achieve wide range of hip motion to prevent dislocation and to provide limitation-free daily activities for patients. These data suggests that we should use more accurate guide systems for acetabular cup replacement such as navigation systems, patient specific templates, and patient specific mechanical instruments to place acetabular cups in an optimum position

Aims

Patients with abnormal spinopelvic mobility are at increased risk for instability. Measuring the change in sacral slope (ΔSS) can help determine spinopelvic mobility preoperatively. Sacral slope (SS) should decrease at least 10° to demonstrate adequate posterior pelvic tilt. There is potential for different ΔSS measurements in the same patient based on sitting posture. The purpose of this study was to determine the effect of sitting posture on the ΔSS in patients undergoing total hip arthroplasty (THA).

Aims

It is important to consider sagittal pelvic rotation when introducing the acetabular component at total hip arthroplasty (THA). The purpose of this study was to identify patients who are at risk of unfavourable pelvic mobility, which could result in poor outcomes after THA.