Understanding spinopelvic mechanics is important for the success of total hip arthroplasty (THA). Despite significant advancements in appreciating spinopelvic balance, numerous challenges remain. It is crucial to recognize the individual variability and postoperative changes in spinopelvic parameters and their consequential impact on prosthetic component positioning to mitigate the risk of dislocation and enhance postoperative outcomes. This review describes the integration of advanced diagnostic approaches, enhanced technology, implant considerations, and surgical planning, all tailored to the unique anatomy and biomechanics of each patient. It underscores the importance of accurately predicting postoperative spinopelvic mechanics, selecting suitable imaging techniques, establishing a consistent nomenclature for spinopelvic stiffness, and considering implant-specific strategies. Furthermore, it highlights the potential of artificial intelligence to personalize care.

Cite this article: Bone Joint J 2024;106-B(11):1206–1215.

Aims. Spinopelvic pathology increases the risk for instability following total hip arthroplasty (THA), yet few studies have evaluated how pathology varies with age or sex. The aims of this study were: 1) to report differences in spinopelvic parameters with advancing age and between the sexes; and 2) to determine variation in the prevalence of THA instability risk factors with advancing age. Methods. A multicentre database with preoperative imaging for 15,830 THA patients was reviewed. Spinopelvic parameter measurements were made by experienced engineers, including anterior pelvic plane tilt (APPT), spinopelvic tilt (SPT), sacral slope (SS), lumbar lordosis (LL), and pelvic incidence (PI). Lumbar flexion (LF), sagittal spinal deformity, and hip user index (HUI) were calculated using parameter measurements. Results. With advancing age, patients demonstrate increased posterior APPT, decreased standing LL, decreased LF, higher pelvic incidence minus lumbar lordosis (PI-LL) mismatch, higher prevalence of abnormal spinopelvic mobility, and higher HUI percentage. With each decade, APPT progressed posteriorly 2.1°, LF declined 6.0°, PI-LL mismatch increased 2.9°, and spinopelvic mobility increased 3.8°. Significant differences were found between the sexes for APPT, SPT, SS, LL, and LF, but were not felt to be clinically relevant. Conclusion. With advancing age, spinopelvic biomechanics demonstrate decreased spinal mobility and increased pelvic/hip mobility. Surgeons should consider the higher prevalence of instability risk factors in elderly patients and anticipate changes evolving in spinopelvic biomechanics for young patients. Cite this article: Bone Joint J 2024;106-B(8):792–801

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. The aim of this study was to evaluate the reliability and validity of a patient-specific algorithm which we developed for predicting changes in sagittal pelvic tilt after total hip arthroplasty (THA). Methods. This retrospective study included 143 patients who underwent 171 THAs between April 2019 and October 2020 and had full-body lateral radiographs preoperatively and at one year postoperatively. We measured the pelvic incidence (PI), the sagittal vertical axis (SVA), pelvic tilt, sacral slope (SS), lumbar lordosis (LL), and thoracic kyphosis to classify patients into types A, B1, B2, B3, and C. The change of pelvic tilt was predicted according to the normal range of SVA (0 mm to 50 mm) for types A, B1, B2, and B3, and based on the absolute value of one-third of the PI-LL mismatch for type C patients. The reliability of the classification of the patients and the prediction of the change of pelvic tilt were assessed using kappa values and intraclass correlation coefficients (ICCs), respectively. Validity was assessed using the overall mean error and mean absolute error (MAE) for the prediction of the change of pelvic tilt. Results. The kappa values were 0.927 (95% confidence interval (CI) 0.861 to 0.992) and 0.945 (95% CI 0.903 to 0.988) for the inter- and intraobserver reliabilities, respectively, and the ICCs ranged from 0.919 to 0.997. The overall mean error and MAE for the prediction of the change of pelvic tilt were -0.3° (SD 3.6°) and 2.8° (SD 2.4°), respectively. The overall absolute change of pelvic tilt was 5.0° (SD 4.1°). Pre- and postoperative values and changes in pelvic tilt, SVA, SS, and LL varied significantly among the five types of patient. Conclusion. We found that the proposed algorithm was reliable and valid for predicting the standing pelvic tilt after THA. Cite this article: Bone Joint J 2024;106-B(1):19–27

Aims

The risk factors for abnormal spinopelvic mobility (SPM), defined as an anterior rotation of the spinopelvic tilt (∆SPT) ≥ 20° in a flexed-seated position, have been described. The implication of pelvic incidence (PI) is unclear, and the concept of lumbar lordosis (LL) based on anatomical limits may be erroneous. The distribution of LL, including a unusual shape in patients with a high lordosis, a low pelvic incidence, and an anteverted pelvis seems more relevant.

Aims. Symptomatic spinal stenosis is a very common problem, and decompression surgery has been shown to be superior to nonoperative treatment in selected patient groups. However, performing an instrumented fusion in addition to decompression may avoid revision and improve outcomes. The aim of the SpInOuT feasibility study was to establish whether a definitive randomized controlled trial (RCT) that accounted for the spectrum of pathology contributing to spinal stenosis, including pelvic incidence-lumbar lordosis (PI-LL) mismatch and mobile spondylolisthesis, could be conducted. Methods. As part of the SpInOuT-F study, a pilot randomized trial was carried out across five NHS hospitals. Patients were randomized to either spinal decompression alone or spinal decompression plus instrumented fusion. Patient-reported outcome measures were collected at baseline and three months. The intended sample size was 60 patients. Results. Of the 90 patients screened, 77 passed the initial screening criteria. A total of 27 patients had a PI-LL mismatch and 23 had a dynamic spondylolisthesis. Following secondary inclusion and exclusion criteria, 31 patients were eligible for the study. Six patients were randomized and one underwent surgery during the study period. Given the low number of patients recruited and randomized, it was not possible to assess completion rates, quality of life, imaging, or health economic outcomes as intended. Conclusion. This study provides a unique insight into the prevalence of dynamic spondylolisthesis and PI-LL mismatch in patients with symptomatic spinal stenosis, and demonstrates that there is a need for a definitive RCT which stratifies for these groups in order to inform surgical decision-making. Nonetheless a definitive study would need further refinement in design and implementation in order to be feasible. Cite this article: Bone Jt Open 2023;4(8):573–579

The spinopelvic alignment is often assessed via the Pelvic Incidence-Lumbar Lordosis (PI-LL) mismatch. Here we describe and validate a simplified method to evaluating the spinopelvic alignment through the L1-Pelvis angle (L1P). This method is set to reduce the operator error and make the on-film measurement more practicable. 126 standing lateral radiographs of patients presenting for Total Hip Arthroplasty were examined. Three operators were recruited to label 6 landmarks. One operator repeated the landmark selection for intra-operator analysis. We compare PI-LL mismatch obtained via the conventional method, and our simplified method where we estimate this mismatch using PI-LL = L1P - 90°. We also assess the method's reliability and repeatability. We found no significant difference (p > 0.05) between the PI-LL mismatch from the conventional method (mean 0.22° ± 13.6) compared to L1P method (mean 0.0° ± 13.1). The overall average normalised root mean square error (NRMSE) for PI-LL mismatch across all operators is 7.53% (mean -3.3° ± 6.0) and 6.5% (mean -2.9° ± 4.9) for the conventional and L1P method, respectively. In relation to intra-operator repeatability, the correlation coefficients are 0.87 for PI, 0.94 for LL, and 0.96 for L1P. NRMSE between the two measurement sets are PI: 9.96%, LL: 5.97%, and L1P: 4.41%. A similar trend is observed in the absolute error between the two sets of measurements. Results indicate an equivalence in PI-LL measurement between the methods. Reproducibility of the measurements and reliability between operators were improved. Using the L1P angle, the classification of the sagittal spinal deformity found in the literature translates to: normal L1P<100°, mild 100°<L1P<110°, and severe L1P>110°. Surgeons adopting our method should expect a small improvement in reliability and repeatability of their measurements, and a significant improvement of the assessment of the mismatch through the visualisation of the angle L1P

Adverse spinopelvic characteristics (ASC) have been associated with increased dislocation risk following primary total hip arthroplasty (THA). A stiff lumbar spine, a large posterior standing tilt when standing and severe sagittal spinal deformity have been identified as key risk factors for instability. It has been reported that the rate of dislocation in patients with such ASC may be increased and some authors have recommended the use of dual mobility bearings or robotics to reduce instability to within acceptable rates (<2%). The aims of the prospective study were to 1: Describe the true incidence of ASC in patients presenting for a THA 2. Assess whether such characteristics are associated with greater symptoms pre-THA due to the concomitant dual pathology of hip and spine and 3. Describe the early term dislocation rate with the use of ≤36mm bearings. This is an IRB-approved, two-center, multi-surgeon, prospective, consecutive, cohort study of 220 patients undergoing THA through anterolateral- (n=103; 46.8%), direct anterior- (n=104; 27.3%) or posterior- approaches (n=13; 5.9%). The mean age was 63.8±12.0 years (range: 27.7-89.0 years) and the mean BMI 28.0±5.0 kg/m. 2. (range: 19.4-44.4 kg/m. 2. ). There were 44 males (47.8%) and 48 females (52.2%). The mean follow-up was 1.6±0.5 years. Overall, 54% of femoral heads was 32 mm, and 46% was 36mm. All participants underwent lateral spinopelvic radiographs in the standing and deep-flexed seated positions were taken to determine lumbar lordosis (LL), sacral slope (SS), pelvic tilt (PT), pelvic-femoral angle (PFA) and pelvic incidence (PI) in both positions. Spinal stiffness was defined as lumbar flexion <20° when transitioning between the standing and deep-seated position; adverse standing PT was defined as >19° and adverse sagittal lumbar balance was defined as mismatch between standing PI and LL >10°. Pre-operative patient reported outcomes was measured using the Oxford Hip Score (OHS) and EuroQol Five-Dimension questionnaire (EQ-5D). Dislocation rates were prospectively recorded. Non-parametric tests were used, significance was set at p<0.05. The prevalence of PI-LL mismatch was 22.1% (43/195) and 30.4% had increased standing PT (59/194). The prevalence of lumbar stiffness was 3.5% (5/142) and these patients had all three adverse spinopelvic characteristics (5/142; 3.5%). There was no significant difference in the pre-operative OHS between patients with (20.7±7.6) and patients without adverse spinopelvic characteristics (21.6±8.7; p=0.721), nor was there for pre-operative EQ5D (0.651±0.081 vs. 0.563±0.190; p=0.295). Two patients sustained a dislocation (0.9%): One in the lateral (no ASC) and one in the posterior approaches, who also exhibited ASC pre-operatively. Sagittal lumbar imbalance, increased standing spinal tilt and spinal stiffness are not uncommon among patients undergoing THA. The presence of such characteristics is not associated with inferior pre-operative PROMs. However, when all characteristics are present, the risk of instability is increased. Patients with ASC treated with posterior approach THA may benefit from the use of advanced technology due to a high risk of dislocation. The use of such technology with the anterior or lateral approach to improve instability is to date unjustified as the rate of instability is low even amongst patients with ASCs

Mechanical failure of spine posterior fixation in the lumbar region Is suspected to occur more frequently when the sagittal balance is not properly restored. While failures at the proximal extremity have been studied in the literature, the lumbar distal junctional pathology has received less attention. The aim of this work was to investigate if the spinopelvic parameters, which characterize the sagittal balance, could predict the mechanical failure of the posterior fixation in the distal lumbar region. All the spine surgeries performed in 2017-2019 at Rizzoli Institute were retrospectively analysed to extract all cases of lumbar distal junctional pathology. All the revision surgeries performed due to the pedicle screws pull-out, or the breakage of rods or screws, or the vertebral fracture, or the degenerative disc disease, in the distal extremity, were included in the junctional (JUNCT) group. A total of 83 cases were identified as JUNCT group. All the 241 fixation surgeries which to date have not failed were included in the control (CONTROL) group. Clinical data were extracted from both groups, and the main spinopelvic parameters were assessed from sagittal standing preoperative (pre-op) and postoperative (post-op) radiographs with the software Surgimap (Nemaris). In particular, pelvic incidence (PI), sagittal vertical axis (SVA), pelvic tilt (PT), T1 pelvic angle (TPA), sacral slope (SS) and lumbar lordosis (LL) have been measured. In JUNCT, the main failure cause was the screws pull-out (45%). Spine fixation with 7 or more levels were the most common in JUNCT (52%) in contrast to CONTROL (14%). In CONTROL, PT, TPA, SS and PI-LL were inside the recommended ranges of good sagittal balance. For these parameters, statistically significant differences were observed between pre-op and post-op (p<0.0001, p=0.01, p<0.0001, p=0.004, respectively, Wilcoxon test). In JUNCT, the spinopelvic parameters were out of the ranges of the good sagittal balance and the worsening of the balance was confirmed by the increase in PT, TPA, SVA, PI-LL and by the decrease of LL (p=0.002, p=0.003, p<0.0001, p=0.001, p=0.001, respectively, paired t-test) before the revision surgery. TPA (p=0.003, Kolmogorov-Smirnov test) and SS (p=0.03, unpaired t-test) differed significantly in pre-op between JUNCT and CONTROL. In post-op, PI-LL was significantly different between JUNCT and CONTROL (p=0.04, unpaired t-test). The regression model of PT vs PI was significantly different between JUNCT and CONTROL in pre-op (p=0.01, Z-test). These results showed that failure is most common in long fused segments, likely due to long lever arms leading to implant failure. If the sagittal balance is not properly restored, after the surgery the balance is expected to worsen, eventually leading to failure: this effect was confirmed by the worsening of all the spinopelvic parameters before the revision surgery in JUNCT. Conversely, a good sagittal balance seems to avoid a revision surgery, as it is visible is CONTROL. The mismatch PI-LL after the fixation seems to confirm a good sagittal balance and predict a good correction. The linear regression of PT vs PI suggests that the spine deformity and pelvic conformation could be a predictor for the failure after a fixation

The December 2022 Spine Roundup³⁶⁰ looks at: Deep venous thrombosis prophylaxis protocol on a Level 1 trauma centre patient database; Non-specific spondylodiscitis: a new perspective for surgical treatment; Disc degeneration could be recovered after chemonucleolysis; Three-level anterior cervical discectomy and fusion versus corpectomy- anterior cervical discectomy and fusion “hybrid” procedures: how does the alignment look?; Rivaroxaban or enoxaparin for venous thromboembolism prophylaxis; Surgical site infection: when do we have to remove the implants?; Determination of a neurologic safe zone for bicortical S1 pedicle placement; Do you need to operate on unstable spine fractures in the elderly: outcomes and mortality; Degeneration to deformity: when does the patient need both?

Aims

Postoperative complication rates remain relatively high after adult spinal deformity (ASD) surgery. The extent to which modifiable patient-related factors influence complication rates in patients with ASD has not been effectively evaluated. The aim of this retrospective cohort study was to evaluate the association between modifiable patient-related factors and complications after corrective surgery for ASD.

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.

Aims. Pelvic incidence (PI) is a position-independent spinopelvic parameter traditionally used by spinal surgeons to determine spinal alignment. Its relevance to the arthroplasty surgeon in assessing patient risk for total hip arthroplasty (THA) instability preoperatively is unclear. This study was undertaken to investigate the significance of PI relative to other spinopelvic parameter risk factors for instability to help guide its clinical application. Methods. Retrospective analysis was performed of a multicentre THA database of 9,414 patients with preoperative imaging (dynamic spinopelvic radiographs and pelvic CT scans). Several spinopelvic parameter measurements were made by engineers using advanced software including sacral slope (SS), standing anterior pelvic plane tilt (APPT), spinopelvic tilt (SPT), lumbar lordosis (LL), and PI. Lumbar flexion (LF) was determined by change in LL between standing and flexed-seated lateral radiographs. Abnormal pelvic mobility was defined as ∆SPT ≥ 20° between standing and flexed-forward positions. Sagittal spinal deformity (SSD) was defined as PI-LL mismatch > 10°. Results. PI showed a positive correlation with parameters of SS, SPT, and LL (r-value range 0.468 to 0.661). Patients with a higher PI value showed higher degrees of standing LL, likely as a compensatory measure to maintain sagittal spine balance. There was a positive correlation between LL and LF such that patients with less standing LL had decreased LF (r = 0.49). Similarly, there was a positive correlation between increased SSD and decreased LF (r = 0.54). PI in isolation did not show any significant correlation with lumbar (r = 0.04) or pelvic mobility (r = 0.02). The majority of patients (range 89.4% to 94.2%) had normal lumbar and pelvic mobility regardless of the PI value. Conclusion. The PI value alone is not indicative of either spinal or pelvic mobility, and thus in isolation may not be a risk factor for THA instability. Patients with SSD had higher rates of spinopelvic stiffness, which may be the mechanism by which PI relates to THA instability risk, but further clinical studies are required. Cite this article: Bone Joint J 2022;104-B(3):352–358

Aims. This study aimed to evaluate sagittal spinopelvic alignment (SSPA) in the early stage of rapidly destructive coxopathy (RDC) compared with hip osteoarthritis (HOA), and to identify risk factors of SSPA for destruction of the femoral head within 12 months after the disease onset. Methods. This study enrolled 34 RDC patients with joint space narrowing > 2 mm within 12 months after the onset of hip pain and 25 HOA patients showing femoral head destruction. Sharp angle was measured for acetabular coverage evaluation. Femoral head collapse ratio was calculated for assessment of the extent of femoral head collapse by RDC. The following parameters of SSPA were evaluated using the whole spinopelvic radiograph: pelvic tilt (PT), sacral slope (SS), pelvic incidence (PI), sagittal vertical axis (SVA), thoracic kyphosis angle (TK), lumbar lordosis angle (LL), and PI-LL. Results. The HOA group showed higher Sharp angles compared with the RDC group. PT and PI-LL were higher in the RDC group than the HOA group. SS and LL were lower in the RDC group than the HOA group. No difference was found in PI, SVA, or TK between the groups. Femoral head collapse ratio was associated with PT, SS, SVA, LL, and PI-LL. A PI-LL > 20° and a PT > 30° correlated with greater extent of femoral head destruction by RDC. From regression analysis, SS and SVA were significantly associated with the femoral head collapse ratio within 12 months after disease onset. Conclusion. Compared with HOA, RDC in the early stage correlated with sagittal spinopelvic malalignment. SS and SVA may partially contribute to the extent of femoral head destruction by RDC within 12 months after the onset of hip pain. The present study indicates a potential role of SSPA assessment in identification of RDC patients at risk for subsequent bone destruction. Cite this article: Bone Jt Open 2022;3(1):77–84

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. Methods. This study investigated patients with preoperative posterior hypermobility, defined as a change in sacral slope (SS) from standing to sitting (ΔSS. stand-sit. ) ≥ 30°. Radiological spinopelvic parameters, including SS, pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch, anterior pelvic plane tilt (APPt), and spinopelvic tilt (SPT), were measured on preoperative imaging, and at six weeks and a minimum of one year postoperatively. The severity of bilateral hip osteoarthritis (OA) was graded using Kellgren-Lawrence criteria. Results. A total of 136 patients were identified as having preoperative spinopelvic hypermobility. At one year after THA, 95% (129/136) of patients were no longer categorized as hypermobile on standing and sitting radiographs (ΔSS. stand-sit. < 30°). Mean ΔSS. stand-sit. decreased from 36.4° (SD 5.1°) at baseline to 21.4° (SD 6.6°) at one year (p < 0.001). Mean SS. seated. increased from baseline (11.4° (SD 8.8°)) to one year after THA by 11.5° (SD 7.4°) (p < 0.001), which correlates to an 8.5° (SD 5.5°) mean decrease in seated functional cup anteversion. Contralateral hip OA was the only radiological predictor of hypermobility persisting at one year after surgery. The overall reoperation rate was 1.5%. Conclusion. Spinopelvic hypermobility was found to resolve in the majority (95%) of patients one year after THA. The increase in SS. seated. was clinically significant, suggesting that current target recommendations for the hypermobile patient (decreased anteversion and inclination) should be revisited. Cite this article: Bone Joint J 2021;103-B(12):1766–1773

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

Aims

To describe the clinical, radiological, and functional outcomes in patients with isolated congenital thoracolumbar kyphosis who were treated with three-column osteotomy by posterior-only approach.

Aims

Patients with spinal pathology who undergo total hip arthroplasty (THA) have an increased risk of dislocation and revision. The aim of this study was to determine if the use of the Hip-Spine Classification system in these patients would result in a decreased rate of postoperative dislocation in patients with spinal pathology.

Introduction. Excessive standing posterior pelvic tilt (PT), lumbar spine stiffness, low pelvic Incidence (PI), and severe sagittal spinal deformity (SSD) have been linked to increased dislocation rates. We aimed to compare the prevalence of these 4 parameters in unstable and stable primary Total Hip Arthroplasty (THA) patients. Methods. In this retrospective cohort study, 40 patients with instability following primary THA for osteoarthritis were referred for functional analysis. All patients received lateral X-rays in standing and flexed seated positions to assess functional pelvic tilt and lumbar lordosis (LL). Computed tomography scans were used to measure pelvic incidence and acetabular cup orientation. Literature thresholds for “at risk” spinopelvic parameters were standing pelvic tilt ≤ −10°, lumbar flexion (LL. stand. – LL. seated. ) ≤ 20°, PI ≤ 41°, and sagittal spinal deformity (PI – LL. stand. mismatch) ≥ 10°. The prevalence of each risk factor in the dislocation cohort was calculated and compared to a previously published cohort of 4042 stable THA patients. Results. Median supine cup inclination for the dislocating cohort was 43° (range, 26°- 58°). Median cup anteversion was 23° (range, 7° − 40°) for the dislocating cohort. 65% of the dislocating patients had socket positions within the Lewinnek safe zone (Figure 1). Standing PT (-10° v −1°), lumbar flexion (20° v 45°), and PI-LL mismatch (12° v −1°) were all significantly different (p < 0.001) in the dislocating group compared to the stable THA population (Figure 2). There was no difference in PI between the dislocating group and the stable THA population (58° v 56° respectively, p = 0.33), with the numbers available. 80% of the dislocating patients had one or more of the 3 statistically significant risk factors, compared to only 24% of the stable THA population. Conclusion. Excessive standing posterior pelvic tilt, low lumbar flexion and a severe SSD are highly prevalent in unstable THAs. Pre-op screening for these parameters may reduce the prevalence of dislocation. For any figures or tables, please contact the authors directly