Can we identify abnormal pelvic tilt using pre-THA anteroposterior pelvic radiographs?

BACKGROUND: Patients with increased pelvic tilt (PT) are at risk for instability following total hip arthroplasty (THA). Identification of increased PT using anteroposterior (AP) pelvic radiographs could avoid additional spinopelvic radiographs. This study aimed to (1) describe which AP pelvic parameters most accurately estimate sagittal PT, and (2) determine thresholds for these parameters that can identify patients with increased PT. METHODS: This was a retrospective, consecutive, cohort study in a tertiary referral hospital on 225 patients (age: 66 ± 12 years-old; 52% female) listed for THA. Patients underwent pre-operative standing AP pelvic radiographs to measure distance- and angular- based parameters from several anatomical landmarks. Sagittal PT was measured on a standing lateral spinopelvic radiograph and considered high when ≥ 20°. RESULTS: No AP pelvic parameters correlated strongly with sagittal PT. Ratio between horizontal and vertical diameter of the pelvic foramen (C/D ratio) (rho - 0.341; p < 0.001); and vertical distance between trans-SIJ and trans-ASIS line (SITA) (rho 0.307; p < 0.001) correlated moderately with sagittal PT. Sacro-femoral-pubic (SFP) angle < 60° had highest sensitivity (85%), but lowest specificity (52%) to differentiate between patients with and without increased PT. If SITA > 62 mm, C/D ratio < 0.5 and SFP < 60°, specificity increased (88%), but sensitivity was low (49%). CONCLUSION: In the absence of computerized models, AP pelvic parameters cannot accurately predict sagittal PT. However, an SFP < 60° should alert a hip surgeon that a patient may have an increased PT, and would benefit from additional lateral spinopelvic imaging prior to THA. LEVEL OF EVIDENCE: Level II, diagnostic study.

Mobility of the lumbo-pelvic-hip complex (spinopelvic mobility) and sagittal spinal alignment - implications for surgeons performing hip arthroplasty.

INTRODUCTION: The optimal positioning of the hip prosthesis components is influenced by the mobility and balance of the spine. The present study classifies patients with pathology of the spino-pelvic-hip complex, showing possible methods of preventing hip dislocations after arthroplasty. HYPOTHESIS: Hip-Spine Classification helps arthroplasty surgeons to implant components in more patient-specific position. MATERIALS AND METHODS: The group of 100 patients treated with total hip arthroplasty. Antero-posterior (AP) X-rays of the pelvis in a standing position, lateral spine (standing and sitting) and AP of the pelvis (supine after the procedure) were analyzed. We analyzed a change in sacral tilt value when changing from standing to sitting (∆SS), Pelvic Incidence (PI), Lumbar Lordosis (LL) Mismatch, sagittal lumbar pelvic balance (standing position). Patients were classified according to the Hip-Spine Classification. Postoperatively, the inclination and anteversion of the implanted acetabular component were measured. RESULTS: In our study 1 A was diagnosed in 61% of all cases, 1B in 18%, 2 A in 16%, 2B in 5%. 50 out of 61 (82%) in group 1 A were placed within the Levinnek "safe zone". In 1B, 2 A, 2B, the position of the acetabular component was influenced by both the spinopelvic mobility and sagittal spinal balance. The mean inclination was 43.35° and the anteversion was 17.4°. CONCLUSIONS: Categorizing patients according to Hip-Spine Classification one can identify possible consequences the patients at risk. Pathology of the spino-pelvic-hipcomplex can lead to destabilization or dislocation of hip after surgery even though implanted according to Lewinnek's indications. Our findings suggest that Lewinnek safe zone should be abandoned in favor of the concept of functional safe zones.

Increased Dislocation Rates After Total Hip Arthroplasty in Patients With Prior Isolated Sacroiliac Joint Arthrodesis.

BACKGROUND: While stiffness of the lumbosacral spine is a known predictor of instability following total hip arthroplasty (THA), little is known about the medical- and surgical-related outcomes following THA in patients who have prior isolated sacroiliac (SI) joint arthrodesis. METHODS: 197 patients who had a history of isolated SI joint arthrodesis who subsequently underwent elective primary THA for a diagnosis of osteoarthritis (THA-SI) from 2015 to 2021 were identified in a national administrative database. Using propensity score matching and logistic regression analyses, this cohort was compared to two groups of patients: patients who did not have any history of lumbar or SI arthrodesis and patients undergoing primary THA who had a history of lumbar arthrodesis without extension into the SI joint (THA-LF). RESULTS: The THA-SI group had a significantly higher incidence of dislocation (odds ratio 2.06, 95% confidence interval 1.04-4.04, P = .037) with no increased incidence of medical complications or other surgical complications when compared to patients without a history of SI or lumbar arthrodesis. There were no significant differences in any complications in THA-SI patients when compared to THA-LF patients. CONCLUSION: Patients who had prior isolated SI joint arthrodesis undergoing primary THA demonstrated a two-fold increased incidence of dislocation when compared to those who did not have prior SI arthrodesis, although the risk of complications in this population was similar to that observed in patients who had prior isolated lumbar spine arthrodesis.

Variability of pre-operative functional pelvic tilt in total hip arthroplasty patients.

PURPOSE: Pelvic tilt (PT) is important to consider when planning total hip arthroplasty (THA) due to its dynamic impact on acetabular orientation. The degree of sagittal pelvic rotation varies during functional activities and can be difficult to measure without proper imaging. The purpose of this study was to evaluate PT variation in the supine, standing, and seated positions. METHODS: A multi-centre cross-sectional study was performed that included 358 THA patients who had preo-perative PT measured from supine CT scan and standing and upright seated lateral radiographs. Supine, standing, and seated PT and associated changes between functional positions were evaluated. Anterior PT was assigned a positive value. RESULTS: In the supine position, mean PT was 4° (range, -35° to 20°), where 23% had posterior PT and 69% anterior PT. In the standing position, mean PT was 1° (range, -23° to 29°), where 40% had posterior PT and 54% anterior PT. In the seated position, mean PT was -18° (range, -43° to 47°), where 95% had posterior PT and 4% anterior PT. From standing to seated, the pelvis rotated posteriorly in 97% of cases (maximum 60°) with 16% of cases considered stiff (change ≤ 10°) and 18% of cases considered hypermobile (change ≥ 30°). CONCLUSION: Patients undergoing THA have marked PT variation in the supine, standing, and seated positions. There was wide variability in PT change from standing to seated, with 16% of patients considered stiff and 18% considered hypermobile. Functional imaging should be performed on patients prior to THA to allow for more accurate planning.

Functional implant positioning in total hip arthroplasty and the role of robotic-arm assistance.

INTRODUCTION: Accurate implant positioning, tailored to the phenotype and unique biomechanics of each patient is the single most important objective in achieving stability in THA and maximise range of motion. The spine-pelvis-hip construct functions as a single unit adapting to postural changes. It is widely accepted in the literature that no universaltarget exists and variations in spinopelvic mobility mandate adjustments to the surgical plan; thus bringing to the fore the concept of personalised, functional component positioning. METHODS: This manuscript aims to outline the challenges posed by spinopelvic imbalance and present a reproducible, stepwise approach to achieve functional-component positioning. We also present the one-year functional outcomes and Patient Reported Outcome Measures of a prospective cohort operated with this technique. RESULTS AND CONCLUSION: Robotic-arm assisted Total Hip Arthroplasty has facilitated enhanced planning based on the patient's phenotype and evidence suggests it results in more reproducible and accurate implant positioning. Preservation of offset, avoiding leg-length discrepancy, accurate restoration of the centre of rotation and accomplishing the combinedversion target are very important parameters in Total Hip Arthroplasty that affect post-operative implant longevity, patient satisfaction and clinical outcomes.

The effect of lumbar spinal fusion on native acetabular anteinclination in standing position.

PURPOSE: The complex and dynamic spinopelvic interplay is not well understood. The aims of the present study were to investigate the following: (1) whether native acetabular anteinclination (AI) in standing position changes following lumbar spinal fusion (LSF); (2) potential correlations between AI change (ΔAI) and several spinopelvic parameters such as the change in lumbar lordosis (ΔLL), pelvic tilt (ΔPT), and anterior pelvic plane angle (ΔaPP). METHODS: A total of 485 patients (Males: 262, Females: 223) with an average age of 64 ± 13 years who underwent a primary LSF were identified from our institutional database. The difference (Δ) between pre-and postoperative acetabular anteinclination (AI), lumbar lordosis (LL), anterior pelvic plane angle (aPP), sacral slope (SS), and pelvic tilt (PT) were measured on a standing lateral radiograph (EOS®) and compared to find the effect of LSF on the lumbopelvic geometry. RESULTS: Following LSF, the average absolute ΔAI was 5.4 ± 4 (0 to 26)°, ΔLL: 5.5 ± 4 (0 to 27)°, ΔaPP: 5.4 ± 4 (0 to 38)°, ΔPT: 7 ± 5 (0 to 33)° and ΔSS: 5.3 ± 4 (0 to 33)°. No significant differences were observed between LSF levels. A ΔAI ≥ 10° was observed in 66 (13.6%) and ΔAI ≥ 20° in 5 (1%) patients. The Pearson correlation demonstrated a strong negative correlation of ΔAI with ΔLL (r = 0.72, p < .001). CONCLUSION: Clinical decision-making should consider the relationship between native anteinclination and lumbar lordosis to reduce the risk of functional acetabular component malalignment in patients with concomitant hip and spine pathology. LEVEL OF EVIDENCE: Retrospective case-control study, Level III.

The Top Three Burning Questions in Total Hip Arthroplasty.

Total hip arthroplasty (THA) for end-stage osteoarthritis is one of the most effective surgical treatments in medicine. Impressive outcomes have been well documented in the literature with patients gaining ambulation and recovery of hip joint function. Nevertheless, there are still debatable issues and controversies that the orthopedic community has not been able to provide a definitive answer for. This review is focused on the current three most debatable issues surrounding the THA procedure: (1) new cutting-edge technology, (2) spinopelvic mobility, and (3) fast-track protocols. The scope of the herein narrative review is to analyze the debatable issues surrounding the three aforementioned topics and conclude the best contemporary clinical approaches regarding each issue.

Variations in lower limb alignments indicate pelvic tilt after total hip arthroplasty.

OBJECTIVE: We sought to correlate various spinopelvic and lower limb alignments, and to examine the current spinopelvic theories on a Chinese cohort. METHODS: We retrospectively reviewed 166 patients undergoing THA. Among them, 138 patients with unilateral THA met the inclusion criteria. Sagittal alignments and cup orientations were measured on standing and sitting lateral EOS images. Patients were categorized into two groups with a scoring system for lumbar spine degeneration. Patients' demographics including age, sex, lumbar spine degeneration and radiographic measurements were studied. RESULTS: PT, SS, LL and TK differed significantly between standing and sitting within each group except for TK in degenerative group (32.8 ± 13.9 vs. 32.9 ± 14.2, p = 0.905). Compared with degenerative spine group, non-degenerative spine patients have great pelvic mobility (ΔPT, -24.4 ± 12.5° vs. -17.6 ± 10.7, p = 0.0008), greater lumbar mobility (ΔLL, -34.8 ± 15.2 vs. -21.7 ± 12.2, p = < 0.0001) and compensatory cup orientation changes (ΔRA, -15.5 ± 11.1 vs. -12.0 ± 8.4, p = 0.00920; ΔRI, -10.8 ± 11.5 vs. -5.6 ± 7.5, p = 0.0055). Standing PT and ankle dorsiflexion angle correlated positively (R2 = 0.236, p = 0.005). CONCLUSION: THA patients in this cohort showed a spinopelvic motion paradigm similar to that from previous studies on Caucasians. Ankle dorsiflexion indicate greater posterior pelvic tilt on standing. Surgeons should beware of risks of instability in patients with lower limb compensations. ADVANCES IN KNOWLEDGE: This study provides new insights into the clinical relevance of lower limb alignments to spinopelvic motion after THA in a relatively young Chinese population.

Does spinopelvic motion change after total hip arthroplasty?

PURPOSE: Spinopelvic motion plays an important role in functional acetabular cup position after total hip arthroplasty (THA). Sacral slope (SS) has been a useful surrogate for spinopelvic motion. The present study aimed to investigate statistical characteristics of spinopelvic motion before and after THA using changes in SS in supine, standing, and sitting positions. METHODS: A total of 76 patients (88 hips) were assessed. To classify spinopelvic mobility, defined as a change in SS from standing to sitting position (ΔSSstand/sit), 10° ≤ ΔSSstand/sit ≤ 30°, ΔSSstand/sit < 10°, and ΔSSstand/sit > 30° were considered normal, stiff, and hypermobile, respectively. RESULTS: Over ± 7° changes in SS between before and one year after THA were observed in 39 (44.3%) hips in the sitting position, 19 (21.6%) hips in the supine position, seven (7.9%) in the standing position. Percentages of hips with stiff spinopelvic mobility (11.4% vs. 22.7%) and hypermobile spinopelvic mobility (23.9% vs. 12.5%) between before THA and one year after THA were significantly different (p = 0.034 and p = 0.016, McNemar's test). At one year after THA, 40.0% (4/10) of hips with stiff spinopelvic mobility and 57.1% (12/21) of hips with hypermobile spinopelvic mobility shifted to normal spinopelvic mobility. CONCLUSIONS: Change in SS between before THA and one year after THA had a high inter-subject variability especially in the sitting position. In addition, there was a distinct shift to normal spinopelvic mobility postoperatively in hips with stiff and hypermobile spinopelvic mobility pre-operatively.

How Does Spinopelvic Mobility and Sagittal Functional Cup Orientation Affect Patient-Reported Outcome 1 Year after THA?-A Prospective Diagnostic Cohort Study.

BACKGROUND: This prospective cohort study aimed to characterize how spinopelvic characteristics change post-total hip arthroplasty (THA) and determine how patient-reported outcome measures are associated with 1) individual spinopelvic mobility and 2) functional sagittal cup orientation post-THA. METHODS: One hundred consecutive patients who received unilateral THAs for end-stage hip osteoarthritis, without spinal pathology were studied. Preoperatively and postoperatively, patients underwent clinical and radiographic evaluations. Patient-reported outcomes were collected using the hip disability and osteoarthritis outcome score - physical function shortform (HOOS-PS). Radiographic parameters measured from standing and relaxed-seated radiographs, included the lumbar lordosis angle, pelvic tilt, pelvic femoral angle and cup orientation in the coronal (inclination/anteversion) and sagittal (anteinclination) planes. Spinopelvic mobility was characterized (ΔPT: "stiff" [<10°], "normal" [10°-30°], and "hypermobile" [>30°]). RESULTS: Preoperative spinopelvic characteristics were not associated with HOOS-PS. Post-THA, the spinopelvic characteristics changed, with less patients having spinopelvic hypermobility (7%) compared with preop (14%). Postoperatively, patients with spinopelvic hypermobility showed significantly worse HOOS-PS scores (21 ± 17 vs 21 ± 22 vs 41 ± 23; ANOVA P = .037). Sagittal but not coronal cup orientation was associated with postoperative spinopelvic characteristics. Cup anteinclination was less in the patients with postoperative spinopelvic hypermobility (27 ± 7° vs 36 ± 8° vs 36 ± 10°; ANOVA: P = .035). CONCLUSION: We hypothesize that spinopelvic hypermobility is secondary to impingement and reduced hip flexion; to achieve a seated position, impinging hips require more posterior pelvic tilt. Patients with spinopelvic hypermobility are likely impinging secondary to the low cup anteinclination (sagittal malorientation despite optimum coronal orientation) and thus have lower HOOS-PS compared. Sagittal assessments are thus important to adequately study hip mechanics. LEVEL OF EVIDENCE: Level II, diagnostic study.

The Effect of a Degenerative Spine and Adverse Pelvic Mobility on Prosthetic Impingement in Patients Undergoing Total Hip Arthroplasty.

BACKGROUND: Despite the placement of acetabular components in the traditional "safe-zone", dislocations and all parts of the instability spectrum, including impingement, continue to be an issue. Recent research has established the importance of a degenerative spine and adverse pelvic mobility on functional acetabular orientation. The purpose of this study is to quantify the clinical consequences of a degenerative spine and adverse pelvic mobility on prosthetic impingement in patients undergoing total hip arthroplasty. METHODS: Between January 2018 and December 2019, a series of 1592 patients undergoing total hip arthroplasty had functional lateral radiographs and a computed tomography scan taken. Two spinal parameters and 2 pelvic mobility parameters were investigated for their association with impingement. Each patient was evaluated for anterior and posterior impingement, at all orientations within a traditional supine safe zone and a patient-specific functional safe zone. RESULTS: Patients with limited lumbar flexion (stiff spine), higher pelvic incidence-lumbar lordosis mismatch (sagittal imbalance), and more anterior pelvic mobility from stand to flexed-seated, exhibit increased anterior impingement. Patients with larger posterior pelvic mobility from supine-to-stand exhibited increased posterior impingement. Impingement was reduced 3-fold when the target cup orientation was tailored to a patient's functional safe zone rather than a generic target. Six percent of patients showed unavoidable impingement even with an optimized functional cup orientation. CONCLUSION: Our results support growing evidence that patients with a degenerative spine and adverse pelvic mobility are likely to have unfavorable functional cup orientations, resulting in prosthetic impingement. Preoperative functional radiographic screening is recommended to assess the likelihood of a patient experiencing impingement due to their unique spinopelvic mobility.

The Hip-Spine Relationship in Total Hip Arthroplasty: How to Execute the Plan.

BACKGROUND: Patients with spinopelvic pathology, including lumbar spine stiffness and sagittal spinal deformity, are at increased risk for postoperative complications, including instability, dislocation, and revision after total hip arthroplasty (THA). Recent evidence has suggested that the Lewinnek safe zone should no longer be considered an appropriate target for all patients, especially those with spinopelvic pathology, as the safe zone is a dynamic rather than static target. There are 2 distinct issues for arthroplasty surgeons to consider: lumbar spinal stiffness and sagittal spinal deformity, each of which has its own management. METHODS: In order to manage patients with spinopelvic pathology undergoing THA, a basic understanding of spinopelvic parameters, including sagittal balance, sacral slope, and anterior pelvic plane, is essential. Techniques outlined in this manuscript describe a systematic preoperative work-up and intraoperative management of acetabular component positioning according to patient-specific spinopelvic parameters, ensuring optimal component placement and a reduced risk for impingement, instability, and poor postoperative outcomes. RESULTS: Evaluation of each patient's spinopelvic parameters informs patient classification according to the Hip-Spine Classification for THA. Patient classification is determined by the presence of spinal stiffness and spinal deformity, with corresponding scoring and classification into one of the 4 categories used to determine risk for postoperative dislocation, define patient-specific cup positioning, and create their functional safe zone. CONCLUSION: A simple 2-step preoperative assessment with measurements of the anterior pelvic plane and the sacral slope on standing and seated lateral X-rays will identify patients at high risk for postoperative dislocation due to spinal deformity and/or stiffness. Accounting for spinopelvic pathology and adhering to the Hip-Spine Classification guidelines for acetabular component positioning can help reduce the burden of instability and revisions in this complex patient population.

Prevalence of Risk Factors for Adverse Spinopelvic Mobility Among Patients Undergoing Total Hip Arthroplasty.

BACKGROUND: Patients with adverse spinopelvic mobility have higher complication rates following total hip arthroplasty (THA). Risk factors include a stiff lumbar spine, standing posterior pelvic tilt ≤ -10°, and a severe sagittal spinal deformity (pelvic incidence minus lumbar lordosis mismatch ≥20°). The purpose of this study is to define the spinopelvic risk factors and quantify the prevalence of risk factors for pathologic spinopelvic mobility. METHODS: A retrospective cohort analysis from January 2014 to February 2020 was performed on a multicenter series of 9414 primary THAs by 168 surgeons, all with preoperative spinopelvic measurements in the supine, standing, and flex-seated positions. All patients were included. The prevalence of adverse spinopelvic mobility and frequency of each spinopelvic risk factor was calculated. RESULTS: The cohort was 52% female, 48% male, with an average age of 65 years. Thirteen percent of patients exhibited adverse spinopelvic mobility and 17% had one or more of the 3 risk factors. Adverse mobility was found in 35% of patients with at least 1 risk factor, 47% with at least 2 risk factors, and 57% with all 3 risk factors. CONCLUSION: Forty-six percent of patients had spinopelvic pathology driven by one or more of the risk factors. Number of risk factors present and risk of adverse spinopelvic mobility were positively correlated, with 57% of patients with all 3 risk factors exhibiting adverse spinopelvic mobility. Although this study defines the prevalence of these risk factors in this highly selected cohort, it does not report incidence in a general THA population. LEVEL OF EVIDENCE: Prognostic Level IV.

Letter to the Editor concerning "Impact of spinal alignment and stiffness on impingement after total hip arthroplasty: a radiographic study of pre­ and post­operative spinopelvic alignment" by Hagiwara S, et al. (Eur Spine J. 2020; doi.org/10.1007/s00586-020-06589-z).

PURPOSE: Dislocation is one of the remaining challenges after total hip arthroplasty. The spinopelvic mobility is considered to be the key to solve this problem and is of interest both to arthroplasty and spine surgeons. The purpose of this letter is to discuss the spinopelvic mobility and spinal stiffness described in the paper titled "Impact of spinal alignment and stiffness on impingement after total hip arthroplasty: a radiographic study of pre­ and post­operative spinopelvic alignment." by Hagiwara S, et al. METHODS: Examining the consistency between this paper and previously published papers on spinopelvic mobility. RESULTS: In this article, radiographic clearance of anterior impingement was defined as adding of femoral shaft angle and sacral slope (SS), and that of posterior impingement as adding SS and femoral shaft angle subtracting 90º in the sitting position. The impingement itself and other factors for dislocation including implant design, implant orientation, extra-prosthetic impingement and their mobilities are not considered in this parameter, and it is better if the validity of this parameter is shown. The term "rigid spine" and "spinal stiffness" are used in the manuscript. When THA candidates are evaluated, they are categorized according to the flexibility and/or sagittal balance. It would be better if the definition was described in the text and the clearance for impingement was shown to be affected by spinal stiffness. CONCLUSION: The conclusions and titles are overstated from the results, but this paper is highly valuable in reminding spinal surgeons of the importance of spinopelvic alignment and mobility in THA.

How Can Patients With Mobile Hips and Stiff Lumbar Spines Be Identified Prior to Total Hip Arthroplasty? A Prospective, Diagnostic Cohort Study.

BACKGROUND: Patients with reduced lumbar spine mobility are at higher risk of dislocation following total hip arthroplasty (THA). Therefore our study aimed to (1) define the optimal protocol for identifying patients with mobile hips and stiff lumbar spines and (2) determine clinical and standing radiographic parameters predicting these patients. METHODS: A cohort of 113 patients with end-stage hip osteoarthritis awaiting THA was prospectively studied. Clinical data, patient-reported outcome measures, and spinopelvic radiographs were assessed with the patient in the standing, "relaxed-seated," and "deep-flexed seated" position. A "hip user index" was calculated quantifying the percentage of sagittal hip movement compared to overall movement between the standing and deep-flexed seated position. RESULTS: Radiographs in the relaxed-seated position had an accuracy of 56% (95% confidence interval 46-65) to detect patients with stiff lumbar spines, compared to a detected rate of 100% in the deep-flexed seated position. A standing pelvic tilt of ≥19° was the only predictor for being a hip user with a sensitivity of 90% and specificity of 71% (area under the curve 0.83). Patients with a standing pelvic tilt ≥19° and an unbalanced spine with a flatback deformity had a 30× fold relative risk (95% confidence interval 4-226, P < .001) of being a hip user. CONCLUSION: Patients awaiting THA and having combined high hip and reduced lumbar spine mobility can be screened for with lateral standing radiographs of the spinopelvic complex. Hip user verification should be done utilizing radiographs in the deep-flexed seated position due to a higher accuracy compared to relaxed-seated radiographs. LEVEL OF EVIDENCE: Level II, diagnostic study.

A Preoperative Workup of a "Hip-Spine" Total Hip Arthroplasty Patient: A Simplified Approach to a Complex Problem.

BACKGROUND: A large body of evidence has confirmed that patients with spinal deformity, lumbar fusion, and abnormal spinopelvic mobility are at significantly increased risk for instability, dislocation, and revision after total hip arthroplasty (THA). METHODS: Achieving a stable construct in patients with pre-existing spine disease requires an understanding of basic spinopelvic parameters and the compensatory mechanisms associated with abnormal spinopelvic motion. Indicated patients with concomitant hip-spine pathology should be assessed for (1) the presence of spinal deformity and (2) the presence of spinal stiffness before undergoing THA. Preoperative imaging should include a standing anteroposterior pelvis x-ray, as well as two lateral spinopelvic radiographs in the standing and seated position. RESULTS: Based on the presence of spinal deformity and/or spinal stiffness, patients may be categorized as one of the four groups of the "Hip-Spine Classification in THA." A series of illustrative case examples is provided. CONCLUSION: A simple three-step assessment with minimal measurements will effectively identify the complex "hip-spine" THA patient at high risk for postoperative instability. Adhering to group-specific recommendations for acetabular cup position can help to further reduce the burden of instability and related revisions in this challenging population.

The Current Knowledge on Spinopelvic Mobility.

Recent studies may suggest that our conventional knowledge of risk factors for dislocation may need rethinking. Previous studies have demonstrated a large majority of total hip arthroplasty instability with acetabular cups implanted in safe zones. Recently discovered spinopelvic motion is a coordinated biomechanical relationship among acetabular anteversion, pelvic tilt, and lumbar lordosis. Classification includes normal, hypermobile, stiff, stuck standing, stuck sitting, and fused. Normal spinopelvic motion from standing to sitting occurs with hip flexion, posterior sacral tilt, and decreased lumbar lordosis to accommodate a flexed femur and prevent impingement and dislocation. Acetabular cup implantation ideally is adapted based on spinopelvic interactions. This may lower the rate of impingement and subsequent dislocation. These new biomechanical interactions may provide a better understanding of the safe zones of anteversion and inclination.

Spinopelvic Motion: A Simplified Approach to a Complex Subject.

Knowledge of the relationship between the hip and spine is essential in the effort to minimize instability and improve outcomes following total hip arthroplasty (THA). A detailed yet straightforward preoperative imaging workup can provide valuable information on pelvic positioning, which may be helpful for optimum placement of the acetabular cup. For a streamlined preoperative assessment of THA candidates, classification systems with a capacity for providing a more personalized approach to performance of THA have been introduced. Familiarity with these systems and their clinical application is important in the effort to optimize component placement and reduce the risk of instability. Looking ahead, the principles of the hip-spine relationship are being integrated using emerging innovative technologies, promising further streamlining of the evaluation process.

In silico analysis of the patient-specific acetabular cup anteversion safe zone.

INTRODUCTION: Various computer-assisted surgical systems claim to improve the accuracy of cup placement in total hip arthroplasties after assessing spinopelvic mobility to prevent prosthetic impingement. However, no study has yet analyzed the extent of the patient-specific cup anteversion safe zones. HYPOTHESIS: We hypothesized that most patients have a safe zone >10 °, except those with abnormal spinopelvic mobility, who have a much narrower safe zone. MATERIALS AND METHODS: We simulated the risks of prosthetic impingement using the planned cup anteversion. The consecutive cohort included 341 patients who underwent total hip arthroplasty. Our primary endpoint was the patient-specific impingement-free zone for cup anteversion, which was then divided into four subgroups: 0 °, 1 ° to 5 °, 6 ° to 10 °, and >10 °. This data was then secondarily analyzed for abnormal spinopelvic mobility (the difference in the spinopelvic tilt [ΔSPT] from a standing to a flexed seated position >20 °). RESULTS: The mean anteversion safe zone was 22.8 ° with 82.4% (281/341) of patients with a zone strictly >10 °. The mean safe zone was 8.9 ° (+/- 9 °) in patients with an ΔSPT ≥20 ° (18.2%), with 37.1% of these patients having a zone of 0 °, 16.13% a zone between 1 ° and 5 °, 8.06% a zone between 6 ° and 10 ° and 38.71% a zone >10 °. The mean safe zone was 25.9 ° (+/- 9 °) in patients with an ΔSPT <20 ° (81.8%), and the proportion of cases in each zone was 2.51%, 1.08%, 4.3%, and 92.11%, respectively (p < 0.001). CONCLUSION: The safe zone for anteversion appears to be fairly wide in most patients. However, identifying patients at risk of abnormal spinopelvic mobility seems necessary to identify the two-thirds of patients with a narrow safe zone. LEVEL OF EVIDENCE: IV; retrospective study.

"Stuck in the middle": the missing lumbosacral link in total hip arthroplasty.

INTRODUCTION: Spinopelvic mobility drives functional acetabular position, influencing dislocation risk after total hip arthroplasty (THA). Patients have been described as "stuck sitting" or "stuck standing" based on pelvic tilt (PT). We hypothesised that some patients are "stuck in the middle," meaning their PT changes minimally from sitting to standing - increasing their risk of dislocation. METHODS: We reviewed 195 patients with standing and sitting whole body radiographs prior to THA. Standing anterior pelvic plane tilt (APPT) and standing and sitting sacral slope (SS) were measured and used to calculate sitting APPT. Normal standing and sitting were defined as APPT >-10° and <-20°, respectively. Spinal stiffness was classified as <10° change in sacral slope between sitting and standing. Patients were categorised as: (A) able to fully sit and stand; (B) "stuck sitting" - able to fully sit; unable to fully stand; (C) "stuck standing" - able to fully stand; unable to fully sit; or (D) "stuck in the middle" - unable to sit or stand fully. RESULTS: 84 patients could sit and stand normally (A), 22 patients were stuck sitting (B), 76 patients were stuck standing (C), and 13 patients were stuck in the middle (D). While 111 patients (56.9%) were considered stuck, only 58 patients (29.7%) met criteria for spinal stiffness. DISCUSSION: We identified a subset of patients with stiff spines and abnormal PT in both sitting and standing, including 37.1% of patients who would be classified as "stuck sitting" based only on standing radiographs. Placing acetabular components in less than anatomic anteversion in these patients may increase posterior dislocation risk.