1 Introduction

Dislocation and instability in total hip arthroplasty (THA) continue to be one of the annoying troubles among surgeons to date. Until recent times it was primarily focused on maintaining offset, limb length, and securing acetabular cup position in the safe zones described by Lewinnek and Callanan.^1,2 Ranawat and most authors reassured surgeons to consider the combined anteversion angle (CAVA, includes both version of acetabulum and femur), since both these parameters are hard to ascertain radiographically, this area remains overlooked in literature.^3,4 It should be emphasized that patients who had suboptimal acetabular cup orientations (based on the Lewinnek safe zone) had high metal wear and were further prone to contract soft tissue interactions and effects in the hip.^5,6 However, wear would not always arise in patients with inadequate cup orientation; in fact, some patients with ideal component orientation also developed wear, suggesting that the best component orientation may vary from patient to patient and is probably multifactorial.^7,8

Since Abdel et al.’s⁹ milestone article, many authors have raised objections to the validity of Lewinnek's safe zone (LSZ), since a substantial proportion of hips that are unstable have cup orientations within the safe zone in the supine position, and a large number of patients outside this zone show no signs of hip instability.^10,11 The sagittal plane relationship between the hip and spine prompts a variation in posture, which in turn alters the pelvic position and affects cup orientation.¹² Cup orientation influences joint kinematics, promoting the risk of impingement and thus pain, wear, and instability.¹³ Similarly, patients with loss of Lumbar lordosis (LL), low back pain (LBP), and thoracic kyphosis (TK) likely to develop decline in balance, and greater postural instability¹⁴ by decrease in movement at lumbar spine and lumbar extension strength, impaired proprioception, and abnormal posture.¹⁵

Patients with spinal disorders (like the degenerative lumbar spine and post spinal arthrodesis) are known to have more risk of dislocation and have a "narrower safe zone" for optimal placement of the cup thereby stating that cup orientation has no universal safe zone¹⁶ and the distinct participation of the spine (lumbar spine) and hip (femur and acetabulum) in the movement of body must be taken together. The pelvis serves as an interlink between the two influencing the acetabular orientation (i.e. hip flexion/extension) and the sagittal balance/lumbar lordosis (i.e. spine flexion/extension).¹⁶ Importantly, there is general agreement about the contrast in individuals with a stiff pelvis or hip who need extra lumbosacral range of motion to perform sitting and standing (spine users). In contrast, patients with stiff spines need compensatory motion at the hip, knee, and even ankle to accomplish the same (hip users).^17,18 Moreover, any release of contracture that had a fixed flexion deformity at the hip producing an improved range of motion (ROM) is likely to alleviate the demand in the lumbar spine and can decrease spinal pain. After THA and return of flexion at the hip, spinopelvic characteristics prone to change/normalize,¹⁶ Thereby creating interest in seeking consensus in spinopelvic mobility, hip-spine relationship, and functional zone of cup placement.

However, most of the subjects undergoing THA do not have a history of spine surgery, even though they have some spine involvement, like sagittal imbalance or stiffness.¹⁹ Heckmann et al.²⁰ in their study stated, about 90 % of the late dislocations in hips (>1 year after THR) were having imbalance of spinopelvic complex without previous symptoms.^21–23 Also, significant deviations in both static and dynamic spinopelvic traits occur in healthy individuals who are devoid of any hip or spinal pathologies.¹⁶ To comprehend and treat these issues, preoperative planning must include functional changes in the hip joint and the interaction of the spine, pelvis, and hip throughout joint motion.²⁴

It should be kept in mind that the hip joint is a single axle in a gear frame of spine-pelvis-hip-complex which are linked such that some move along together and some in opposite directions.

²⁵. On appropriate function and synergic coordination, both the anatomical and functional position of the implants become identical. The lack of this may put the entire complex at risk by negatively impacting the functional position of the acetabular component.²⁶

Other than dislocation, the difference between the operative positioning and functional positioning of the acetabular cup results in a significant risk of anterior or posterior impingement lately, due to the more retroversion or anteversion of the cup in relation to the functional safe zone in positions of either sitting or standing, ambulation, recumbent respectively.²⁷ Depending on the subject, this may manifest as discomfort, pain, or a restricted range of motion during these activities, or, even worse, as molecular accumulation like metallosis, which can cause osteolysis.²⁷ High contact point pressures and joint reaction force may occur due to suboptimal coverage of the femoral head resulting in edge loading and eventual wear which clinically presents as pain with instability, aseptic loosening, and a precocious revision scenario.²⁷

Regardless of much evidence indicating the influence of spinopelvic alignment in hip arthroplasty, currently, a clear solidarity is not yet established for functional cup orientation concerning Age, BMI, Spine stiffness and hypermobility, order and timing of Spine surgery and THA, Grade of Osteoarthritis, in the setting of existing Ankylosing spondylitis (AS) and relationship of native femoral retroversion with spine. In consequence, as expected in the evolving field, the literature contains a variable amount of conflicting and confusing terminology and results prompting the present probe for critical analysis and review.

The formidable aim of this study is to critically analyze the published literature to simplify and better understand the terms and association of spine-pelvis-hip complex with its diverse clinical distribution and to emphasize its relevance in hip arthroplasty thereby creating consensus and providing guidance and consistency in further research.

2 Methods

This study was performed and formulated upon preferred reporting items for systematic reviews and meta-analysis (PRISMA) updated guidelines and meta-analysis of observation of studies in epidemiology (MOOSE).

Information sources and databases searched: An extensive literature search was done in COCHRANE, PUBMED (MEDLINE), SCOPUS, SCIENCE DIRECT, EMBASE, WEB OF SCIENCE, and LILACS. The used search engines were Google Scholar, ERIC (Education Resources Information Centre, CORE, and VLRC (Virtual Learning Resources Centre). The search terms and descriptors used are shown in Table 1, 'AND', 'OR', and 'NOT' were used appropriately between phrases for the search. The Inclusion criteria were; 1. Articles that assessed spinopelvic parameters (SS, SPT, PI, and acetabular cup orientation) in patients undergoing THA. 2. Articles that assessed balance in spinopelvic complex after THA. 3. Randomized control trial. 4. Systematic review. 5. Meta-analysis. 6. Review article. 7. Clinical trials/original research article. 8. Articles after 2015. Exclusion criteria were; 1. Those articles which indirectly measured the spinopelvic parameters. 2. Articles lacking one or more appropriate parameters. 3. Articles published more than 10 years ago. 4. Expert opinion.

The literature search and screening using the title and abstracts were done independently by three authors and the fourth author addressed any disparity. Duplicates and those articles, at this point, were deemed unfit for review were eliminated. All the articles were reviewed for eligibility and quality checking. After a full-text review of the remaining articles, those articles that met the inclusion criteria were included in the study. The flow of the review selection and synthesis is shown in Fig. 1.

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Fig. 1 PRISMA Flow diagram depicting the steps of processing in the study.

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3 Definition of terms and parameters

To better understand, the nomenclature of the common terms has to be defined and standardized such that it contributes to better communication as well. Here an attempt is made to concisely elucidate with description, the angles and parameters to achieve the same and tabulated in Table 2 and illustrated in Fig. 2.

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Fig. 2 Illustration of the spinopelvic parameters with appropriate landmarks. Inset A shows the angles SS and SPT linked and reciprocating each other with a constant spatial area (grey) PI between them. [LL – Lumbar lordosis (black reference lines), SS - Sacral slope (yellow), SPT- Spinopelvic Tilt (red), PI – Pelvic incidence (red and black), FPP – Functional pelvic plane (green)].

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4 Results

Based on the PRISMA guidelines a total of 218 studies were selected and retrieved initially through the search engines in the domains mentioned earlier. After removing duplicates and those studies not meeting the inclusion criteria 11 articles were found to be eligible for full-text review and analysis. Five articles^33–37 were removed as these studies did not analyze the cup version directly. One study was excluded as it did not take into account the acetabular cup inclination and sacral slope³⁸ and another study was also excluded as it reported only femoral anteversion³⁹ as a major parameter and did not mention the corresponding acetabular inclination and version. Finally, at the end of the selection process, four articles were included (Fig. 1) for systematic review evaluation, out of these, one was an observational study,⁴⁰ 2 were assessment studies^41,42 and one was a comparative study.⁴³ The characteristics of the articles are listed in Table 3 and the summary of the data extracted from each article is depicted in Tables 4 and 5.

Spine, pelvis, and hip parameters such as Pelvic Incidence (PI), Sacral Slope (SS), and Spinopelvic tilt (SPT), including acetabular orientation were assessed postoperatively in all the included studies (shown and listed in Table 6 including other study details). Hendryk H et al.⁴⁰ estimated mean cup inclination and version rather than ante Inclination (AI). Hence, the mean of sum (μΣ) calculated for Ante Inclination in standing (AI STAND) is 29.7 ± 12.29 and sitting (AI SIT) is 34.69 ± 12.96 was for 242 patients and consequently correlated with corresponding 242 patients of SPT-STAND and SPT-SIT which is 21.46 ± 8.42 and 18.37 ± 12.46 respectively. While the correlation of parameters SS and SPT in standing and sitting as well as SS, SPT, and PI were done among all the 439 patients, which is represented in Figs. 3, 4 and 8. The change Δ in parameters AI, SS, and SPT from standing to sitting posture are 4.99, 2.4, and 1.66 respectively were calculated from the overall sum as shown in Tables 4 and 5

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Fig. 3 Bar diagram showing the correlation between AI and SPT in standing and sitting positions.

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Fig. 4 Shows the Correlation between the Sacral Slope (SS) with SPT in positions of standing and sitting.

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5 Discussion

Precise acetabular cup placement in THA is crucial as even the slightest of errors may affect the clinical outcome awfully with potential complications ranging from decreased ROM, Psoas and bone impingement, edge loading, accelerated component wear, leg length discrepancy to instability and dislocation.⁴⁴ Though the proposed method of “safe zone” by Lewinnek¹ and “combined anteversion” by Ranawat³ are popular, it doesn't account for the postural change, especially from standing to sitting¹¹ and vice versa (as it was described using static 2D images⁴⁵), which affects the coverage of the prosthetic head contributed by the spine-pelvis-hip complex.

For thorough evaluation of the hip joint, the combined mobility of the pelvis and femur with respect to the spine should be studied because, as the patient moves the acetabulum moves with the pelvis in the sagittal plane so is the acetabular cup asserting its not static during postural movements and this change in cup position is called the “functional cup position”.¹¹ Hence, by our understanding the hemisphere of the acetabulum over the sphere of the head are in motion against one another with a same ‘center of rotation’ but with a varying ‘axis of rotation’ based on posture (Fig. 5). Applying this to the traditional LSZ, Tezuka et al. stated that 14.2 % of cups within the LSZ were literally outside the functional zone following THA while Abdel et al.¹³ proclaimed 58 % of the dislocated THAs had the cup within the LSZ (with 84 % inside the safe zone of inclination and 69 % lying inside the safe zone of anteversion) which made Dorr and Callaghan in 2019 to proclaim the “death” of LSZ.⁴⁶

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Fig. 5 Illustrates the differential axis of rotation (A and B) of the hemisphere and sphere, with the direction of rotation A1 and B1 (shown in arrows) respectively, upon the same Center of rotation (COR), represented in vector form (5a), where ‘Bx’ and ‘By’ are the variation in axeses of the sphere given in blue. Figure (5b) shows the bicoxofemoral axis (A) through which the pelvis tilts, and the rotational axis of the femoral head (B)in the given position in the 3D model.

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Thus, it is evident that the change in the spatial cup position due to movements in sitting and standing or spinopelvic imbalance doesn't always keep the cup in the LSZ and thereby becomes vulnerable to set the aforementioned complications in series. Thus, in recent day THA, achieving functional cup orientation and considering lumbopelvic kinematics is more pertinent and indispensable.⁴⁷ Though the notion was started 40years ago by Offierski and MacNab, the understanding is still primitive and cryptic thus warranting conspicuous review for more granular understanding⁴⁸

Normally, in a standing position the lumbar spine is in physiological lordosis making the pelvis to tilt anteriorly, by which the acetabulum is relatively closed and covers the femoral head (Fig. 6A with inset). While in a relaxed seated position, the lumbar lordosis is obliterated and the pelvis tilts posteriorly (≈20°) anteverting the acetabulum thereby opening the acetabulum anteriorly (Fig. 6B with inset) 0.8° for each degree of pelvic tilt, approximately.¹¹ Hence the transition from standing to sitting causes tilting of the sacrum (with pelvis) posteriorly by which the acetabulum anteverts (by 15°–20°) and hip flexes,⁴⁹ as illustrated in dotted lines in Fig. 6B with arrows. If movements at any of these sequences are restricted, then there must be compensation at the others. About 75 % of the movement in the sagittal plane is contributed by the hip and when the hip ROM (especially flexion) is restricted, they compensate with increased PT and eventual increase in LL¹⁸ and once the available PT is usedup in its ability to compensate (for PI-LL mismatch), the body resorts to pelvic shift and knee flexion to accommodate further compensation.⁵⁰ Comparably, in spinal stiffness of any cause (degeneration, ankylosis, or LSF), the decreased spinopelvic motion, i.e. the reduced ability to tilt the pelvis posteriorly is compensated with increased femoral anteversion.¹⁸ As a corollary fate because of decreased cup motion (due to the inability to retrovert the pelvis) and the resulting increased femoral motion, the cup position potentially falls outside the functional safe zone despite being placed within the LSZ,¹¹ thus resulting in posterior over-coverage, anterior edge loading, increased wear, instability, and anterior dislocation.

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Fig. 6 Demonstrates the pelvic rhythm in standing (6A) and sitting (6B) posture in the normal (Flexible & balanced) spine. Where the available lumbar lordosis tilts the pelvis anteriorly, anteverting the acetabulum thereby opening the acetabulum anteriorly. While the decrease in lordosis on sitting retroverts the pelvis producing a more posterior coverage, creating a posteriorly tilted functional pelvic plane (FPP) from a more vertical FPP.

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Therefore, in the above setting, the spinopelvic rhythm can be categorized into 4 types, 1. Flexible and balanced with normal motion; which allows for the above compensatory motions at the spine and hip. 2. Rigid and balanced; due to degeneration or ankylosed spine or fusion surgeries, these patients demonstrate a posterior tilt on standing while more anterior tilt on sitting due to altered sitting biomechanics where hip flexion is substituted by flexion of the spine thereby creating a reverse spinopelvic rhythm⁴⁹ (illustrated in Fig. 7A and B), in which the pelvis cannot be anteverted hence are at greater risk of anterior impingement, posterior edge loading and consequently posterior dislocation. 3. Flexible and unbalanced; in which due to compensatory changes causes more pelvic retroversion thereby functionally anteverting the acetabulum producing a higher risk of posterior impingement, anterior edge loading, and subsequent anterior dislocation. 4. Rigid and unbalanced; similarly causes functional anteversion and anterior dislocation.⁵¹

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Fig. 7 Illustrates the reverse movement of the pelvis from standing (7A) to sitting posture (7B) in the rigid spine, with the decreased hip flexion (displayed as the dotted line of the femur and the 3D femur at the same level) on sitting is compensated by flexion at the spine (shown in dots & arrows), thereby the decrease in SPT is compensated by the increase in SS.

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Fig. 8 Shows the inverse relationship between SS and SPT, maintaining a constant pelvic incidence (PI) in both standing and sitting.

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Thereby it necessitates to define a functional safe zone by assessing (both preoperatively and postoperatively) the relationship between the sagittal balance of the spine and the motion of the pelvis based on distinct anatomic parameters. As proposed by Legaye et al.,³⁰ pelvic incidence, sacral slope, and spinopelvic tilt are widely accepted as the fundamental parameters acting as a keystone in sagittal spinopelvic balance and determining the mechanical energy expenditure to sustain energy-efficient posture both in normal and diseased states described by the ‘cone of economy’ concept by jean Dubosset.⁵²

Moreover, the pelvis's functional motion in relation to postural variations poses an issue for the surgeon in establishing an optimal cup position as the pelvis is static intraoperatively, creating a significant difference between operative inclination and version with that of radiographic inclination and version as the average PT changes 25°–30° from supine to standing position and hence the acetabular component position.⁵³ Thus, Kanawade et al. described a method of sagittal cup position by measuring ante-inclination (AI) for functional cup position and in the evaluation of impingement risk, as the changes in inclination and anteversion are complementary with respect to pelvic tilt in either standing or sitting.⁵⁴ The orientation of the cup positioned intraoperatively should be based on the anticipated ante-inclination on sitting which can be assessed preoperatively based on pelvic stiffness.¹³

Known the PI, SS, and SPT from Table 2, and based on the properties of right-angled triangles (depicted in inset A of Fig. 2), a simple arithmetic configuration derives the pelvic incidence (PI) as the algebraic sum of the spinopelvic tilt (SPT) and the sacral slope (SS), PI = SPT + SS.^13,55,56 Where PI depicts the sagittal pelvic orientation in relation to the innate hip joints, it is a constant parameter decided at the end of skeletal maturity,^30,57 that does not change with the position of the spine or pelvis.¹³ That makes it obvious that the parameters SPT and SS are inversely related variables, so and whenever one increases, the other must inevitably decrease.⁵⁶ The sacral slope (SS) is an integral factor in alignment and dictates the proportions of lumbar lordosis (LL). The higher the SS, the deeper the LL, and vice-versa therefore not only affecting the PT directly but also the entire sagittal alignment of the spine including posture.⁵⁸ While SPT individually represents the rotational profile of the pelvis over the femoral head,¹³ prompts the standing anteverted pelvis with a high SS and low SPT. While in sitting, the pelvis retroverts and generates a lower SS and high SPT,⁵⁵ illustrated in the insets of Fig. 6A–B which demonstrates the change in parameters from standing to sitting.

To be precise, from standing to sitting, the pelvis tilts more posteriorly; the mean SPT increases from standing to sitting by 20°–40°.⁵⁴ Lembeck et al.⁵⁹ established that a sagittal pelvic tilt change of 10° will change the acetabular version by ≈ 7° and inclination by ≈ 3°, and the posterior pelvic tilt naturally creates a functional anteversion and anterior pelvic tilt produces a functional retroversion.⁴⁵ While the ante-inclination changes by 1° for each 1° change in pelvic tilt.⁵⁴ There is a wide arc of movement in the pelvis ranging from 5° to 70° in stiff and hypermobile pelvices respectively from standing to sitting.⁶⁰ Also, Ishidaet et al. demonstrated a mean increase of 8.2° in SPT post-THA.²⁸

In our analysis, the pooled results among 242 postoperative THAs (Tables 4 and 5), the mean of AI-STAND and AI-SIT of 29.7 and 34.69 to that of mean SPT-STAND (21.46) and SPT-SIT (18.37) generates an ΔAI of 4.99 and ΔSPT of 3.09 displaying a 1.61° change of AI for every 1° of SPT.

Indistinguishably from the studies and demonstrations by Tan.TH et al.,⁵⁵ Bhosale.S et al.⁵⁶ and Chavarria.JC et al.,¹³ the collective breakdown of our assessment in 439 patients from various studies shows the mean ΔSS from standing to sitting as 2.4° and ΔSPT as 1.66° whereby approximately maintaining the PI as a constant parameter with reciprocal accommodations in SS and SPT as illustrated in Fig. 8.

Thus, the appreciation of coherence in the spine-pelvis-hip complex and its rational movements in normal and pathologic states associated with the property of compensatory mechanisms understandably improved our knowledge greatly in search of precision and longevity, mandating special attention to consider the spinopelvic motion by rigorous error-free evaluation through physical examination based on patients' predominant symptoms with appropriate radiographs or other methods in THA. Though this research sums up and outlines many facts with statistics, further studies are needed to narrow down the formula for the execution of intraoperative cup placement in the functional safe zone.

Limitations of the study are that the quality and the inherent biases of the respective article selected drives the strength of this analysis as most of the studies are retrospective, observational, and case-control studies. Also, the patients analyzed in the study are postoperative THA patients and thus cannot be compared with normal subjects.

6 Conclusion

The results of the various above studies from the heterogenous pool of population, it is certain that we interpret there is a measured alteration in the inclination and version in proportions with respect to change in pelvic tilt which may be synchronous with the normal spinopelvic rhythm or might exhibit asynchrony based on the affection on spine. Thus, it is crucial to evaluate the spine to identify the stiff and the stuck patterns to determine the native functional acetabular orientation which can help in the precise placement of the cup in the appropriate functional inclination and version thereby reducing the risk of prosthetic impingement and instability due to the interplay within the spine-pelvis-hip complex.

Ethical statement

As this Manuscript is a systematic literature review the ethical clearance/statement is NOT APPLICABLE.

Funding statement

I Dr. RAM SUDHAN.S truly and with best of my knowledge affirm that, this original research or any of its authors were not been funded by any sponsors or corporates.

Guardian/patients consent

As the submitted manuscript is a systematic literature review the guardian/patients’ consent is NOT APPLICABLE

CRediT authorship contribution statement

S. Ram Sudhan: Writing – original draft, preparation, Literature search, Validation. Sibin Surendran: Writing. Naveen P. Gopinath: Figure designing and drafting, Writing – original draft. Jijulal C U: Literature search, resources, software. V.V. Muhammed Fazil: Software, Validation. P. Gopinathan: Conceptualization, Methodology, Supervision. K.V. Nikhil: Data curation and visualization.