Dislocations following total hip arthroplasty in patients with restricted spinopelvic movement: a narrative review

Introduction

Total hip arthroplasty (THA) is widely regarded as one of the most successful procedures in orthopaedic surgery, yet instability remains a leading cause of early failure and revision (1). In recent years, attention has shifted from a purely “hip-focused” view of dislocation to a broader perspective that considers the dynamic interaction between the spine, pelvis, and hip. This so-called “hip-spine relationship” has emerged as a key factor in understanding why some patients experience instability despite apparently “well-positioned” components (2).

The concept of spinopelvic mobility describes how pelvic tilt changes between functional positions such as standing and sitting, and how these changes modify the effective orientation of the acetabular component (2-4). When the pelvis rotates posteriorly in sitting, functional acetabular anteversion increases and the hip can flex without posterior impingement. Conversely, when the pelvis tilts anteriorly in standing, functional anteversion decreases, contributing to stability in extension. Abnormal spinopelvic patterns—such as reduced mobility, sagittal imbalance, or excessive motion—can therefore shift the functional cup position outside traditional “safe zones” and predispose to dislocation (3,4).

Epidemiological data suggest that a substantial proportion of patients undergoing THA present with spinopelvic risk factors, including lumbar fusion, degenerative spinal disease, or sagittal malalignment (5). These conditions alter the normal coordination between lumbar spine and pelvis and may profoundly influence the functional position of the cup during daily activities. Recent reviews and narrative syntheses have emphasized the importance of integrating hip and spine assessment, but there is still no unified framework that translates spinopelvic parameters into practical guidance for acetabular component positioning (6,7).

Given the growing recognition of abnormal spinopelvic mobility as a major contributor to instability after THA, there is a need for a structured narrative review that not only summarizes the available evidence, but also clarifies the underlying biomechanical mechanisms and proposes clinically applicable strategies. Therefore, the aims of this review are to:

• describe the main patterns of spinopelvic mobility observed in patients undergoing THA;
• analyze the association between these patterns and postoperative dislocation; and
• outline practical recommendations for functional assessment and individualized acetabular component positioning.

Despite increasing awareness of the hip–spine relationship, current literature remains fragmented, and no unified framework exists to translate spinopelvic parameters into practical, reproducible intraoperative targets. This review aims to address this gap by synthesizing biomechanical concepts, diagnostic strategies, and clinical implications into a comprehensive and clinically applicable overview. We present this article in accordance with the SANRA (Scale for the Assessment of Narrative Review Articles) reporting checklist (available at https://asj.amegroups.com/article/view/10.21037/asj-25-81/rc), which provide methodological standards for ensuring clarity of rationale, transparency of the literature search, critical appraisal of evidence, and coherent presentation.

Methods

Study design

The review was structured to: (I) justify the clinical relevance of the topic; (II) define clear objectives; (III) transparently describe the literature search; (IV) present an evidence-based synthesis; and (V) discuss limitations inherent to the available data.

SANRA compliance

This narrative review was conducted in accordance with all six items of the SANRA (Scale for the Assessment of Narrative Review Articles) guidelines. Specifically, we ensured clear justification of the importance of the topic, transparent description of the literature search process, appropriate referencing, scientifically sound argumentation, and high-quality presentation. The literature search strategy is reported in Table S1 and Table 1.

Literature search strategy

A structured literature search was performed across four major databases: PubMed/MEDLINE, Embase, Web of Science, and the Cochrane Library, from their inception until June 30, 2024.

The search strategy combined controlled vocabulary (MeSH/Emtree) and free-text terms related to:

• spinopelvic mobility, spinopelvic motion;
• pelvic tilt, sacral slope, lumbar lordosis;
• hip-spine relationship;
• THA, THA dislocation, prosthetic instability;
• functional acetabular orientation.

Boolean operators (“AND”, “OR”) were used to combine terms. Search filters were limited to English language and human subjects.

A summary of the complete search strings is provided in Table S1.

Eligibility criteria

Studies were considered eligible if they met the following criteria:

Inclusion criteria:

• Clinical, radiographic, biomechanical, or imaging studies evaluating spinopelvic mobility in the context of primary or revision THA.
• Studies reporting quantitative parameters such as sacral slope (SS), pelvic tilt (PT), pelvic incidence-lumbar lordosis (PI-LL) mismatch, lumbar flexion, ΔSS, or functional acetabular anteversion.
• Studies analyzing or reporting outcomes related to dislocation, instability, impingement, or component positioning.
• Articles published in English.

Exclusion criteria:

• Case reports, technical notes, expert opinion pieces, instructional course lectures, and conference abstracts.
• Studies with fewer than 20 patients, to minimize unstable estimates due to small sample sizes.
• Articles focusing exclusively on spinal surgery without analyzing hip or acetabular consequences.
• Studies not providing extractable spinopelvic parameters.

Study selection

The initial search yielded 152 records.

After removing duplicates and screening titles and abstracts, 74 articles were selected for full-text evaluation.

Following application of the eligibility criteria, 30 studies were included in the final synthesis.

A PRISMA-style flow structure was not used, as this is not a systematic review, but selection steps were explicitly documented according to SANRA principles of transparency.

Data extraction

For each included article, the following data were extracted:

• Study design and population characteristics;
• Definitions of spinopelvic parameters (SS, PT, PI-LL, ΔSS, lumbar flexion, anterior pelvic tilt);
• Imaging modality used [standing/sitting radiographs, EOS, computed tomography (CT)];
• Classification of mobility patterns (e.g., stiffness, hypermobility, sagittal imbalance);
• Acetabular component position and functional anteversion;
• Incidence or mechanism of dislocation or instability;
• Proposed preventive strategies (dual mobility, component orientation).

Data extraction was performed manually by two independent reviewers to ensure accuracy and reduce interpretation bias.

Synthesis of evidence

Given the heterogeneity of methodologies, imaging protocols, and definitions of spinopelvic parameters across studies, the evidence was synthesized narratively rather than through meta-analysis.

The synthesis was structured into:

• Spinopelvic mobility patterns and their biomechanical implications;
• Radiographic and functional parameters associated with instability;
• Relationship between spinal pathology and THA outcomes;
• Diagnostic modalities;
• Preventive and surgical strategies.

This structure aligns with SANRA recommendations for logical and clinically relevant presentation.

Results

A total of 30 studies were included. The evidence converged into six major thematic domains:

• patterns of spinopelvic mobility associated with instability;
• biomechanical parameters influencing functional acetabular orientation;
• the impact of spinal pathology and deformity surgery;
• functional cup position and limitations of static safe zones;
• diagnostic modalities for assessing spinopelvic mobility;
• preventive and surgical strategies for high-risk patients.

Spinopelvic mobility patterns associated with instability

The foundational work of Lewinnek et al. first highlighted component malposition as a source of dislocation and introduced a “safe zone” for cup orientation (8). Subsequent clinical evidence demonstrated that instability frequently occurs even when components fall within this static safe zone, indicating the influence of dynamic factors beyond anatomy alone.

Heckmann et al. reported that late dislocation often results from spinopelvic imbalance, characterized by abnormal pelvic tilt and inadequate compensatory lumbar motion (9). Bedard et al. similarly showed that patients undergoing THA after spinal deformity surgery had markedly elevated dislocation rates, underscoring the role of altered spine–pelvis kinematics (10).

Kanawade et al. quantified functional changes in cup orientation between standing and sitting and demonstrated that postural shifts significantly modify effective inclination and anteversion during daily activities (11). Figure 1 shows the biomechanical changes in acetabular orientation from standing to sitting.

Figure 1 Functional acetabular anteversion in a healthy subject with normal lumbosacral mobility: the degree of functional anteversion increases when seated compared to standing.

Kalichman et al. documented the high prevalence of spondylolysis and spondylolisthesis—conditions capable of altering lumbar flexibility and pelvic rotation (12).

Stefl et al. classified spinopelvic mobility into stiff, normal, and hypermobile patterns, each associated with different risks of impingement and instability (13). McKnight et al. confirmed that reduced lumbar flexion and insufficient posterior pelvic rotation produce patterns predisposing to posterior impingement, whereas excessive posterior rotation increases functional anteversion and anterior instability (14).

Tezuka et al. further demonstrated that instability arises when functional pelvic mechanics deviate markedly from expected physiological ranges, especially in stiffer spines (15).

Biomechanical parameters modulating functional acetabular orientation

EOS-based analyses by Lazennec et al. showed that pelvic incidence, pelvic tilt, lumbar lordosis, and sacral slope dynamically interact to determine functional acetabular anteversion and inclination across standing, sitting, and squatting (16,17).

Figure 2 summarizes the anatomical reference planes relevant for evaluating pelvic tilt, sacral slope, and functional cup anteversion.

Figure 2 The three primary reference planes used to evaluate anatomical anteversion include: (A) the anatomical plane, which is perpendicular to the support surface, (B) the weight-bearing plane in an upright posture, and (C) the weight-bearing plane while seated.

Kaiser et al. contributed data demonstrating that high-risk biomechanics such as limited ΔSS or excessive posterior tilt predict instability patterns that benefit from enhanced implant constraints (18).

Smith et al. demonstrated that robotic-assisted THA improves precision in cup placement relative to planned orientation, reducing the number of outliers and potentially improving functional alignment (19).

Haffer et al. synthesized biomechanical concepts into an integrative framework linking mobility patterns to THA outcomes and clinical decision-making (20).

Lazennec et al. further delineated the anatomical hip–spine relationship and showed how sagittal alignment directly affects cup position during postural transitions (21). In subsequent CT-based studies, they confirmed large variations in anteversion when comparing supine to simulated standing or sitting (22).

Kim et al. showed that pelvic incidence alone does not predict cup orientation, highlighting the importance of multi-parameter functional assessment (23).

Lembeck et al. demonstrated that unaccounted pelvic tilt introduces systematic errors in navigation-guided implantation (24), while Lazennec et al. emphasized functional imaging as essential to understanding true in-vivo orientation (25).

Wan et al. confirmed discrepancies between planned and achieved cup position when pelvic tilt changes intra- or post-operatively (26).

Impact of spinal pathology and deformity surgery

Farfan’s seminal anatomical study offered mechanistic insight into degenerative spondylolisthesis and its impact on pelvic mechanics (27). Yves et al. expanded these concepts into a THA-specific paradigm, showing how spinal disease modifies acetabular functional orientation (28).

DelSole et al. provided direct clinical evidence that as sagittal deformity severity increases, the risk of placing the cup outside the safe zone increases proportionally leading to higher instability and revision rates (29).

Wera et al. emphasized that deformity, stiffness, and lumbar fusion significantly alter mobility patterns and must be recognized as structural drivers of instability after THA (30).

Functional cup position and re-evaluation of safe zones

Findings across the studies consistently indicated that static safe zones are insufficient predictors of stability. Postural changes induce large and clinically relevant variations in functional anteversion and inclination (11,13,22).

Functional safe zones, which incorporate ΔSS, PT, and spinopelvic class, more accurately identify patients at risk than any static metric (15,20,21,25).

Diagnostic modalities for assessing spinopelvic mobility

Standing and sitting lateral radiographs were the most commonly used tools for classifying mobility patterns (11-15,29). EOS imaging offered superior three-dimensional assessment with high reproducibility (16,17,21). Figure 3 illustrates the radiographic acquisition of standing and seated lateral views used to assess sagittal parameters.

Figure 3 Lateral views (derived from real standard X-rays) for assessing spinopelvic mobility in the (A) standing, (B) upright seated, and (C) fixed seated positions in an asymptomatic individual. APPt, anterior plane pelvic tilt; LL, lumbar lordosis angle; PFA, proximal femoral angle; PI, pelvic incidence; sPT, sagittal pelvic tilt; SS, sacral slope.

CT imaging proved essential for quantifying functional anteversion but insufficient alone when used supine (22). Figure 4 provides representative measurements comparing asymptomatic subjects and patients with degenerative hip disease.

Figure 4 Measurements based on real standard lateral X-rays for radiographic spinopelvic parameters in the (A) standing, (B) upright seated, and (C) fixed seated positions. These measurements assess spinopelvic mobility in a volunteer without hip osteoarthritis (A-C) and a patient with hip osteoarthritis prior to total hip arthroplasty (A1-C1). APPt, anterior plane pelvic tilt; LL, lumbar lordosis angle; PFA, proximal femoral angle; PI, pelvic incidence; sPT, sagittal pelvic tilt; SS, sacral slope.

Navigation and robotic systems improved precision but remained sensitive to pelvic tilt variability (19,24,26).

Preventive and surgical strategies

Dual-mobility constructs demonstrated reduced instability in patients with stiffness, imbalance, or high-risk spinopelvic patterns (18,30).

Robotic assistance improved placement accuracy but must be interpreted within the context of functional, not static, alignment (19).

Integrated frameworks (Haffer et al.) recommended:

• Preoperative functional imaging;
• Mobility-pattern classification;
• Individualized cup positioning;
• Selective dual-mobility use;
• Considering navigation/robotics where available (20).

Discussion

Understanding the complex interplay between the spine, pelvis, and hip has become essential for interpreting instability after THA. While Lewinnek’s historical safe zone 40°±10° inclination and 15°±10° anteversion laid the foundation for acetabular positioning (8), it is now well recognized that such static parameters fail to account for dynamic functional changes occurring during everyday postures. Patients do not load their hip prostheses in a single, fixed sagittal alignment; instead, spinopelvic motion continuously alters acetabular orientation. This concept, increasingly supported across the literature, reframes THA biomechanics from a static model to a functional, posture-dependent one (21).

Lazennec et al. were the first to demonstrate that acetabular orientation changes significantly depending on whether imaging is obtained in standing, sitting, or supine positions (17). In their cohort of 84 individuals, standing posture showed forward pelvic rotation associated with increased lumbar lordosis, raising the sacral slope and modifying acetabular inclination. In contrast, sitting posture induced posterior pelvic rotation, decreased sacral slope, and increased acetabular anteversion movements essential to allow physiological hip flexion without impingement (22). These posture-dependent variations underscore the dynamic nature of the pelvis and challenge the historical assumption that supine radiographs alone can guide component placement.

Acetabular anteversion in particular increases by 17–20° in the seated position (23), reflecting a functional adaptation that accommodates femoral flexion. Navigation and imaging studies have quantified this relationship, showing that for every 1° decrease in sacral slope, acetabular anteversion increases by approximately 0.7° (24-26). This correlation builds the mechanical foundation for understanding why variations in spinopelvic mobility directly influence edge loading, impingement, and dislocation risk. Figure 1 illustrates these functional variations in cup anteversion across postures.

The mechanical pathway leading to dislocation becomes evident when considering patients whose pelvis cannot posteriorly rotate during sitting. In individuals with normal mobility, posterior pelvic tilt decreases sacral slope by roughly 21°, elevating functional anteversion and preventing anterior impingement during flexion. However, patients with fixed anterior pelvic tilt—due to spinal fusion, stiff degenerative spine, or sagittal imbalance retain a “standing alignment” even when seated. The acetabular component remains relatively anterior, obliging the femur to flex excessively to achieve sitting posture, thereby increasing anterior impingement and predisposing to posterior dislocation (14). When lumbar lordosis remains uncorrected and sacral slope cannot decrease appropriately, the hip must compensate for lost spinal motion, often beyond a safe biomechanical range.

Spinopelvic imbalance results from restricted lumbosacral motion, forcing the hip into compensatory movements that increase instability risk. Factors associated with reduced spinal flexibility include degenerative disc disease, facet arthropathy, spondylolisthesis, and age-related stiffening (12,27). Although some authors proposed that most primary THA dislocations are spinopelvic in origin, evidence continues to show that component malposition remains the most common cause (22). Nonetheless, spinopelvic abnormalities appear disproportionately represented in late dislocations. Heckmann et al. found that 90% of late dislocations occurred in patients with abnormal spinopelvic mechanics, often compounded by subtle component malalignment or soft-tissue laxity (9). In contrast, early dislocations rarely exhibit such abnormalities, confirming that surgeon-related technical factors predominate early failures while functional biomechanics increasingly influence later failures.

Five pathological patterns consistently emerge across the literature:

• Spinal imbalance PI-LL mismatch >10°, often with increased kyphosis (2).
• Hypermobility sacral slope change (ΔSS) >30°, increasing excessive functional anteversion.
• Stiffness ΔSS <10°, preventing appropriate adjustment of acetabular functional position.
• Fixed anterior pelvic tilt inability to increase anteversion during sitting, high risk for posterior dislocation.
• Degenerative changes progressive kyphosis, loss of lumbar lordosis, and altered pelvic tilt (12, 27).

These patterns have direct clinical implications for cup positioning and should be recognized preoperatively.

Accurate identification of these abnormalities relies on functional imaging. Lateral standing and seated radiographs remain the most accessible tools and allow reliable assessment of pelvic tilt, sacral slope, and functional anteversion (Figure 1). EOS imaging provides simultaneous biplanar acquisition with reduced radiation exposure and higher reproducibility, making it the most accurate modality for evaluating sagittal alignment and spinopelvic kinematics (16,17). Studies show the two methods yield comparable angular measurements, though EOS exhibits superior interobserver reliability and improves measurement consistency (18,20). Figure 4 provides examples of radiographic measurements in asymptomatic and osteoarthritic patients.

The responses to key questions highlight how deeply spinopelvic mechanics influence THA stability:

• Influence on dislocation risk. Reduced spinopelvic mobility prevents the increase in functional anteversion that protects against posterior dislocation. Stiff or fused spines produce functional cup retroversion during flexion, increasing femoroacetabular conflict (6,25).
• Biomechanical changes involved. Loss of sacral slope reduction during sitting, lack of pelvic posterior rotation, abnormal lumbar–pelvic compensatory pathways, and excessive hip flexion produce anterior impingement and posterior instability (13). Hypermobile pelvises, conversely, may increase the risk of anterior instability due to excessive increases in anteversion (14).
• Diagnostic accuracy of imaging tools. EOS and functional lateral radiographs both provide clinically useful data, though EOS offers superior three-dimensional detail and lower radiation. However, accessibility remains limited (16,17).

While the literature strongly supports the role of spinopelvic evaluation, variability across radiographic protocols, definitions (e.g., ΔSS thresholds), and posture-dependent measurement techniques limits the comparability of individual studies. As recommended by SANRA, it is important to acknowledge methodological heterogeneity, potential publication bias, and the predominance of retrospective designs. The absence of standardized imaging guidelines across centers prevents meta-analytic synthesis and increases the risk of bias. Furthermore, patient-level factors such as activity level, sagittal alignment compensation strategies, and soft-tissue tensioning remain insufficiently studied in high-quality prospective cohorts. Limited accessibility to advanced imaging (EOS), the inconsistent integration of functional imaging into preoperative planning, and the lack of large-scale validation for newer classification systems remain obstacles requiring further research.

Despite these limitations, the overall strength of evidence is substantial. Across 30 studies, there is consistent agreement that spinopelvic mechanics critically influence acetabular functional orientation, that sagittal deformities and spinal stiffness increase dislocation risk, and that traditional static safe zones inadequately predict stability. This convergence of findings strongly supports the integration of spinopelvic assessment into routine THA planning.

Clinical implications

Surgeons should routinely assess standing and sitting spinopelvic parameters, including sacral slope, pelvic tilt, lumbar lordosis, PI-LL mismatch, and ΔSS. Patients with stiff or fused spines should receive modified acetabular targets, often with increased anteversion or use of dual mobility constructs (18,30). Conversely, hypermobile patients require more conservative adjustments. Functional rather than static orientation should guide cup positioning.

Practical recommendations

• Obtain standing AND sitting lateral radiographs preoperatively.
• Calculate ΔSS and PI-LL to classify mobility and sagittal alignment.
• Identify stiff patterns before planning cup version.
• Consider dual mobility or constrained constructs in high-risk patterns.
• Avoid relying exclusively on supine radiographs or traditional safe zones.

Strength of evidence

Despite heterogeneity, findings across multiple imaging modalities, populations, and surgical approaches consistently indicate that functional spinopelvic assessment is crucial for preventing instability.

Collectively, these data reaffirm that THA stability is not determined by acetabular orientation alone, but by the dynamic interaction between spine, pelvis, and hip across functional postures. Incorporating functional imaging, mobility classification, and personalized acetabular positioning strategies can meaningfully reduce instability risk particularly in patients with spinal deformity, degenerative disease, or altered sagittal alignment.

Practical preoperative algorithm for spinopelvic assessment

Based on the integrated evidence across biomechanical and clinical studies, a practical preoperative algorithm can support surgeons in identifying high-risk patients and optimizing acetabular positioning. This workflow includes:

• Standing and seated lateral radiographs to quantify pelvic tilt, sacral slope, and functional anteversion.
• Calculation of ΔSS to classify mobility into stiff (<10°), normal (10–30°), or hypermobile (>30°) patterns.
• Assessment of sagittal alignment parameters (PI-LL mismatch, C7–SVA) to detect spinal imbalance.
• Functional cup targeting, adjusting anteversion and inclination according to mobility class rather than fixed safe zones.
• Selective adoption of dual-mobility constructs for patients with severe stiffness, prior spinal fusion, or combined abnormal parameters.
• Optional use of navigation or robotic systems, interpreted within the framework of functional not static alignment.

This structured approach improves risk identification and supports reproducible cup positioning tailored to dynamic pelvic behavior.

Limitations

This narrative review has several limitations inherent to its methodology. The included studies display heterogeneity in imaging protocols, measurement techniques, and definitions of spinopelvic parameters, which limits direct comparability. Publication bias may favor reporting of significant associations, potentially overestimating the role of specific biomechanical variables. Access to advanced imaging such as EOS is not universal, reducing the generalizability of findings to all clinical settings. Furthermore, long-term outcomes of emerging functional alignment strategies remain insufficiently documented. Finally, patient-specific factors such as age, activity level, and comorbidities were variably addressed across studies, leaving residual confounding. Despite these limitations, the convergence of biomechanical and clinical evidence supports the conclusions presented.

Conclusions

This study highlights the critical role of spinopelvic mobility in predisposing patients to dislocation after THA. There is strong evidence that patients with spinopelvic stiffness, fixed anterior pelvic tilt, or spinal degeneration have a significantly increased risk of postoperative instability—even when components are placed within the traditional “safe zone”. Preoperative assessment of spinopelvic dynamics, through weight-bearing radiographs and advanced imaging such as EOS, is essential for proper surgical planning. The use of dual-mobility implants, personalized component positioning, and tailored surgical strategies can markedly improve joint stability and functional outcomes in at-risk patients.

However, several areas remain insufficiently explored, including the identification of precise biomechanical cut-off values and the algorithmic personalization of surgical treatment based on individual spinopelvic profiles. The future integration of more accurate diagnostic tools and advanced robotic technologies could offer increasingly effective predictive and therapeutic solutions. Further prospective studies are needed to validate these findings and develop standardized clinical guidelines for managing patients with spinopelvic dysfunction undergoing THA.

Acknowledgments

The authors declare limited use of an artificial intelligence (AI) based tool (ChatGPT, OpenAI) exclusively to assist with English language editing (grammar and style). No content generation, interpretation of evidence, data extraction, or scientific writing was performed by AI. All intellectual and scientific content was conceived, written, and approved by the authors.

Footnote

Reporting Checklist: The authors have completed the SANRA reporting checklist. Available at https://asj.amegroups.com/article/view/10.21037/asj-25-81/rc

Peer Review File: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-81/prf

Funding: None.

Conflicts of Interest: The authors have completed the ICMJE uniform disclosure form (available at https://asj.amegroups.com/article/view/10.21037/asj-25-81/coif). B.Z. serves as an unpaid editorial board member of AME Surgical Journal from January 2025 to December 2026. D.A. is the founder and equity holder of a medical device start-up developing knee prosthesis designs and related technologies. This company has no commercial interest, no intellectual property, and no financial relationship related to the topic of the present manuscript, and no royalties or payments have been received in relation to this work. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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