What do we know about the biomechanics of the sacroiliac joint and of sacropelvic fixation? A literature review

Sacroiliac joint dysfunction: anatomy, pathophysiology, differential diagnosis, and treatment approaches

The sacroiliac joints (SIJ) play a pivotal role in pelvic stability and load transmission. SIJ-related disorders can pose a diagnostic challenge because of complex anatomy, non-specific imaging findings, and overlapping symptomatology with other lower back conditions. Broadly, SIJ pathology can be divided into the following categories: infectious, inflammatory, degenerative, mechanical, traumatic, and neoplastic. On the spectrum of mechanical disorders is the entity of SIJ dysfunction, defined as pain localized to the SIJ due to non-inflammatory causes. This paper aims to enhance the understanding of SIJ dysfunction by exploring SIJ anatomy, pathophysiology, and differential diagnosis for SIJ pain. Etiologies, associations, and diagnostic physical examination maneuvers for SIJ dysfunction are reviewed. We will discuss the role of diagnostic imaging in SIJ dysfunction and propose imaging findings which may indicate the diagnosis. Finally, we will discuss therapeutic strategies to treat SIJ dysfunction. By delving into the complexities of SIJ anatomy and pathophysiology, this paper provides valuable discernment for the diagnosis and management of SIJ-related disorders.

Evaluating Variations in Spinopelvic Parameters from Sitting to Standing: A Comparative Analysis of 1447 Older Adults Across Age, BMI, and Gender Subgroups

Background/Objectives: Sagittal spinal alignment goals for adult spinal deformity (ASD) surgery are predominantly derived from standing radiographs, despite the biomechanical relevance of sitting posture. Existing studies on sitting alignment involve young, healthy cohorts, which poorly represent ASD patients. This study assessed posture-dependent changes in spinopelvic parameters, including pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and lumbar lordosis (LL), and examined how age, BMI, and gender influence these changes. Methods: In this retrospective cohort study, sitting and standing lateral radiographs of 1447 patients were evaluated. Spinopelvic parameters were measured, and changes (ΔPI, ΔPT, ΔSS, and ΔLL) were calculated. Multiple regression analysis was used to determine associations between these changes and age, BMI, and gender. Results: All parameters differed significantly between positions (p < 0.001); PT and PI increased in sitting (ΔPT = -19.20°; ΔPI = -4.52°), while SS and LL increased in standing (ΔSS = 14.67°; ΔLL = 18.44°). Older age correlated with increased ΔPT (p < 0.001) and ΔPI (p = 0.049) but reduced ΔLL and ΔSS (p < 0.001). Higher BMI was associated with decreased ΔPI, ΔPT, and ΔLL (p < 0.001, 0.003, and 0.025). Females showed greater ΔPT (p = 0.013) but smaller ΔPI, ΔSS, and ΔLL (p = 0.043, <0.001, and 0.001). Conclusions: Spinopelvic parameters vary significantly between sitting and standing positions, affected by age, BMI, and gender. The observed PI change suggests SIJ involvement, highlighting the need for posture-specific and demographic-adjusted alignment goals in ASD surgery to optimize outcomes.

Biomechanical Effects of Multi-segment Fixation on Lumbar Spine and Sacroiliac Joints: A Finite Element Analysis

OBJECTIVE

Spine fixation surgery affects the biomechanical environment in the sacroiliac joint (SIJ), which may lead to the SIJ pain or degeneration after surgery. The purpose of this study is to determine the impact of the number and position of fixed segments on the SIJs and provide references for surgeons to plan fixation levels and enhance surgical outcomes.

METHODS

The intact lumbar-pelvis finite element (FE) models and 11 fixation FE models with different number and position of fixed segments were developed based on CT images. A 400N follower load and 10° range of motion (ROM) of the spine were applied to the superior endplate of L1 to simulate the flexion, extension, bending and torsion motion after surgery. The peak stress on the SIJs, lumbar intervertebral discs, screws and rods were calculated to evaluate the biomechanical effects of fixation procedures.

RESULTS

With the lowermost instrumented vertebra (LIV) of L5 or S1, the peak stress on SIJs increased with the number of fixed segments increasing. The flexion motion led to the greater von Mises stress on SIJ compared with other load conditions. Compared with the intact model, peak stress on all fixed intervertebral discs was reduced in the models with less than three fixed segments, and it increased in the models with more than three fixed segments. The stress on the SIJ was extremely high in the models with all segments from L1 to L5 fixed, including L1-L5, L1-S1 and L1-S2 fixation models. The stress on the segment adjacent to the fixed segments was significant higher compared to that in the intact model. The peak stress on rods and screws also increased with the number of fixed segments increasing in the flexion, extension and bending motion, and the bending and flexion motions led to the greater von Mises stress on SIJs.

CONCLUSION

Short-term fixation (≤2 segments) did not increase the stress on the SIJs significantly, while long-term segment fixation (≥4 segments) led to greater stress on the SIJs especially when all the L1-L5 segments were fixed. Unfixed lumbar segments compensated the ROM loss of the fixed segments, and the preservation of lumbar spine mobility would reduce the risks of SIJ degeneration.

A literature review of biomechanical studies on physiological and pathological sacroiliac joints: Articular surface structure, joint motion, dysfunction and treatments

BACKGROUND

Sacroiliac joints are affected by mechanical environments; the joints are formed under mechanical stimulation, receive impact of walking between the upper and lower parts of the bodies and can be a cause of pain due to non-physiological loads. However, there are so far very few studies that reviewed biomechanics of physiological and pathological sacroiliac joints. This review article aims to describe the current sacroiliac joint biomechanics.

METHODS

Previous original papers have been summarized based on three categories: articular surface structure, sacroiliac joint motion and sacroiliac joint dysfunction and treatments.

FINDINGS

Although the articular surface morphologies vary greatly from individual to individual, many researchers have tried to classify the joints into several types. It has been suggested that the surface morphologies may not change regardless of joint dysfunction, however, the relationship between the joint structure and pain are still unclear. The range of sacroiliac joint motion is demonstrated to be less than 1 mm and there is no difference between physiological and pathological joints. The sacroiliac joint absorbs shock within the pelvis by the joint structures of pelvic morphology, ligaments and fat tissues. The morphology and motion of the sacroiliac joints may be optimized for upright bipedal walking.

INTERPRETATION

There is no doubt that pelvic mechanical environments affect pain induction and treatment; however, no one has yet provided a concrete explanation. Future research could help develop treatments based on sacroiliac joint biomechanics to support joint function.

The impact of femoral rotation on sacroiliac articulation during pregnancy. Is there evidence to support Farabeuf's hypothesis by finite element modelization?

BACKGROUND

Counter-nutation movement is deemed crucial during the management of the birth process. It is a combination of lateral ilia expansion and backward displacement of the promontory resulting from the external rotations of the femurs producing an enlargement of the pelvic inlet. However, since its description by Farabeuf, this mechanism has never been challenged and analyzed in a dynamic finite element study.

METHODS

Based on a female pelvic mesh and sacroiliac ligaments, we simulated external rotations of both femurs with imposed rotation of the two acetabulum centers. We hypothesize that lateral ilia expansion generates a sacrum movement resulting in a backward displacement of the promontory and a pelvic inlet enlargement.

RESULTS

Finite element simulation confirms our hypothesis and reveals that ilio-sacro-transverse and axile ligaments play an essential role in this mechanism. Indeed, the increase in stiffness (ranging from 500 MPa to 750 MPa) of these ligaments accentuates the counter-nutation movement and the opening of the inlet. Instead of the anatomic congruence between the ilium and the sacrum, the sacroiliac ligaments may explain the counter-nutation. After a 6° of femur rotation, the inlet area increases to 11 cm2 (141 cm2 vs. 130 cm2). This enlargement could be noteworthy in case of obstructed labor or shoulder dystocia. Moreover, the association between external rotation and flexion of the femurs could be more efficient for opening the pelvic inlet.

CONCLUSIONS

Our result did not support the original assumption of Farabeuf. By revealing how postural adjustment increases the bony birth canal, this study provides essential information for the clinical management of the delivery.

Improved Outcomes with Concurrent Instrumentation and Fusion of the Sacroiliac Joint in Patients with Long Lumbosacral Constructs

STUDY DESIGN

Retrospective Cohort Study.

OBJECTIVE

Spinal fusion, specifically constructs connected to pelvic bones, has been consistently reported as a predisposing factor to sacroiliac joint (SIJ) pain. The aim of this study is to compare SIJ outcomes in patients with constructs to the pelvis following instrumentation vs instrumentation plus fusion of the SIJ.

METHODS

Data of study subjects was extracted from a prospectively maintained database as well as retrospectively collected from records at a tertiary academic medical center in the United States between 2018 and 2020.

RESULTS

A cohort of 103 patients was divided into 2 groups: 65 in Group 1 [S2AI screw without fusion device] and 38 in Group 2 [S2AI screw with fusion device]. None of the patients in Group 2 developed postoperative SIJ pain compared to 44.6% in Group 1. Sacroiliac joint fusion occurred in all Group 2 but none of Group 1 patients. The postoperative Visual Analogue Scale (VAS) for lower extremity (LE) pain (.8 vs .5; P = .03) and postoperative Oswestry Disability Index (ODI) (18.7 vs 14.2; P < .01) were significantly higher in Group 1. The rate of distal junctional break, failure, and/or kyphosis (DJBFK) and time to DJBFK were not significantly different between the two groups, and the rate of DJBFK did not change in the presence of multiple covariates.

CONCLUSION

The SIJs carry the heavy load of long lumbosacral fusion constructs extending to the pelvis. Simultaneous SIJ instrumentation and fusion decreases the risk of disability, prevents the development of postoperative SIJ pain, and may also protect the S2AI screw from loosening and failure.

Does the anterior column realignment technique influences the stresses on posterior instrumentation in sagittal imbalance correction? A biomechanical, finite-element analysis of L5-S1 ALIF and L3-4 lateral ACR

STUDY DESIGN

Biomechanical finite-element study.

OBJECTIVE

To directly compare the biomechanical effects of two different techniques for sagittal plane correction of adult spine deformity based on the anterior longitudinal ligament (ALL) resection and use of hyperlordotic cages, namely, the anterior column realignment (ACR) in L3-4, and ALIF in L5-S1 in terms of primary stability and rod stresses using finite-element models.

METHODS

A finite-element model of the thoracolumbar spine was used to perform the analysis. Starting from this "intact" model, three further models were constructed through the insertion of spinal instrumentation, i.e., pedicle screws, rods and cages: 1) posterior instrumentation between T9 and S1 (referred to as "T9-S1"); 2) posterior instrumentation T9-S1 + Hyperlordotic (26°) ALIF cage in L5-S1 ("ALIF"); 3) posterior instrumentation T9-S1 + Hyperlordotic (30°) ACR cage in L3-4 ("ACR"). These models were studied by simulations applying, alternately, a pure moment of 7.5 Nm between the three planes of motion (flexion, extension, lateral bending, and bilateral axial rotation), uniformly distributed over the upper surface of the T9 thoracic vertebra. A total of 24 simulations were performed (6 per models).

RESULTS

All models presented a significant reduced ROM when compared to the intact model; the ROM reduction was higher both at L3-4 in the ACR model and at L5-S1 in the ALIF model. At L3-4, the ACR model had, in all cases, the lowest maximum values of Von Mises stresses on the rods, especially in flexion-extension. At L4-5, the ALIF model had the lowest stresses during flexion-extension and axial rotation, while the ACR model had the lowest stresses during lateral bending. At L5-S1, the ALIF model had, in all cases, the lowest stresses on the rods.

CONCLUSIONS

This finite-element study showed how both ACR at L3-4 and ALIF-ACR at L5-S1 are effective in restoring lumbar lordosis (LL), stabilizing the spine and reducing stress on posterior rods at the index level when compared to a simple fixation model. Interestingly, ALIF-ACR reduces rod stress even at L4-5 in flexion-extension and axial rotation, possibly due to a better distribution of LL, especially on the lower arch, while ACR reduces the stress at L4-5 in lateral bending, possibly thanks to the larger footprint of the cage that increases the area of contact with the lateral side of the endplates.

Minimally Invasive and Conservative Interventions for the Treatment of Sacroiliac Joint Pain: A Review of Recent Literature

Sacroiliac joint (SIJ) pain is responsible for approximately 15-25% of reported back pain. Patients with SIJ pain report some of the lowest quality of life scores of any chronic disease. Understanding of the physiology and pathology of the SI joint has changed dramatically over the years, and SI joint pain and injury can now be thought of in two broad categories: traumatic and atraumatic. Both categories of SI joint injury are thought to be caused by inflammation or injury of the joint capsule, ligaments, or subchondral bone in the SI joint. Treatment of SI joint pain usually involves a multi-pronged approach, utilizing both, multi-modal medical pain control and interventional pain/surgical techniques such as steroid injections, radiofrequency nerve ablation, and minimally invasive sacroiliac arthrodesis. Though conservative management through multi-modal pain control and physical therapy have their role as first line therapies, an increasing body of evidence supports the use of minimally invasive procedures, both as adjuvant treatments to conservative management and as second line therapies for patient's that fail first line treatment.

Minimally Invasive and Conservative Interventions for the Treatment of Sacroiliac Joint Pain: A Review of Recent Literature

Sacroiliac joint (SIJ) pain is responsible for approximately a third of reported back pain. Patients with SIJ pain report some of the lowest quality of life scores of any chronic disease. Understanding of the physiology and pathology of the SI joint has changed dramatically over the years, and SI joint pain and injury can now be thought of in two broad categories: traumatic and atraumatic. Both categories of SI joint injury are thought to be caused by inflammation or injury of the joint capsule, ligaments, or subchondral bone in the SI joint. Treatment of SI joint pain usually involves a multi-pronged approach, utilizing both, multi-modal medical pain control and interventional pain/surgical techniques such as steroid injections, radiofrequency nerve ablation, and minimally invasive sacroiliac arthrodesis. Though conservative management through multi-modal pain control and physical therapy have their role as first line therapies, an increasing body of evidence supports the use of minimally invasive procedures, both as adjuvant treatments to conservative management and as second line therapies for patient's that fail first line treatment.

From in vitro evaluation of a finite element model of the spine to in silico comparison of spine instrumentations

Growth-preserving spinal surgery suffer from high complications rate. A recent bipolar instrumentation using two anchoring points (thoracic and pelvic) showed lower rates, but its biomechanical behaviour has not been characterised yet. The aim of this work was to combine in vitro and in vivo data to improve and validate a finite element model (FEM) of the spine, and to apply it to compare bipolar and classical all-screws implants. Spinal segments were tested in vitro to measure range of motion (ROM). Thoracic segments were also tested with bipolar instrumentation to measure ROM and rod strain using a strain gage. A subject-specific FEM of the spine, pelvis and ribcage of an in vivo asymptomatic subject was built. Spinal segments were extracted from it to reproduce the in-vitro mechanical tests. Experimental and simulated ROM and rod strain were compared. Then, the full trunk FEM was used to compare bipolar and all-screws instrumentations. The FEM fell within 1° of the experimental corridors, and both in silico and in vitro instrumentation rods showed 0.01% maximal axial strain. Bipolar and all-screws constructs had similar maximal Von Mises stresses. This work represents a first step towards subject-specific simulation to evaluate spinal constructs for neuromuscular scoliosis in children.

Novel Interventional Techniques for Chronic Pain with Minimally Invasive Arthrodesis of the Sacroiliac Joint: (INSITE, iFuse, Tricor, Rialto, and others)

Acute and chronic pain are public health issues that clinicians have been battling for years. Opioid medications have been a treatment option for both chronic and acute pain; however, they can cause unwanted complications and are a major contributor to our present opioid epidemic. The sacroiliac (SI) joint is a common cause of both acute and chronic low back pain. It affects about 15–25% of patients with axial low back pain, and up to 40% of patients with ongoing pain following lumbar fusion. Recent advances in the treatment of SI joint pain have led to the development of a wide variety of SI joint fusion devices. These fusion devices seek to stabilize the joints themselves in order that they become immobile and, in theory, can no longer be a source for pain. This is a minimally invasive procedure aimed to address chronic pain without subjecting patients to lengthy surgery or medications, including opioids with the potential for addiction and abuse. Minimally invasive SI fusion can be performed by a lateral approach (i.e., iFuse, Tricor) or posterior approach (i.e., CornerLoc, LinQ, Rialto). The posterior approach requires the patient to be in the prone position but allows for less disruption of muscles with entry. More data are necessary to determine which fusion system may be best for a particular patient. SI fusion devices are a promising way of treating chronic lower back pain related to the SI joint. This narrative review will discuss various types of SI fusion devices, and their potential use in terms of their safety and efficacy.

International Society for the Advancement of Spine Surgery Policy 2020 Update-Minimally Invasive Surgical Sacroiliac Joint Fusion (for Chronic Sacroiliac Joint Pain): Coverage Indications, Limitations, and Medical Necessity

The index 2014 International Society for the Advancement of Spine Surgery Policy Statement-Minimally Invasive Surgical Sacroiliac Joint Fusion-was generated out of necessity to provide an International Classification of Diseases, Ninth Revision (ICD-9)-based background and emphasize tools to ensure correct diagnosis. A timely ICD-10-based 2016 update provided a granular threshold selection with improved level of evidence and a more robust and relevant database (Appendix Table A1). As procedures and treatment options have evolved, this 2020 update reviews and analyzes the expanding evidence base and provides guidance relating to differences between the lateral and dorsal surgical procedures for minimally invasive surgical sacroiliac joint fusion.

Revision by S2-alar-iliac instrumentation reduces caudal screw loosening while improving sacroiliac joint pain-a group comparison study

Lumbosacral instrumentation continues to be challenging due to complex biomechanical force distributions and poor sacral bone quality. Various techniques have therefore been established. The aim of this study was to investigate the outcome of patients treated with S2-alar-iliac (S2AI), S2-alar (S2A), and iliac (I) instrumentation as the most caudal level. Sixty patients underwent one of the 3 techniques between January 2012 and June 2017 (S2AI 18 patients, S2A 20 patients, I 22 patients). Mean age was 70.4 ± 8.5 years. Screw loosening (SL) and sacroiliac joint (SIJ) pain were evaluated during the course at 3-month and maximum follow-up (FU). All patients completed 3-month FU, the mean FU period was 2.5 ± 1.5 years (p = 0.38), and a median of 5 segments was operated on (p = 0.26), respectively. Bone mineral density (BMD), derived opportunistically from computed tomography (CT), did not significantly differ between the groups (p = 0.66), but cages were more frequently implanted in patients of the S2A group (p = 0.04). SL of sacral or iliac screws was more common in patients of the S2A and I groups compared with the S2AI group (S2AI 16.7%, S2A 55.0%, I 27.3% of patients; p = 0.03). SIJ pain was more often improved in the S2AI group not only after 3 months but also at maximum FU (S2AI 61.1%, S2A 25.0%, I 22.7% of patients showing improvement; p = 0.02). Even in shorter or mid-length lumbar or thoracolumbar constructs, S2AI might be considered superior to S2A and I instrumentation due to showing lower incidences of caudal SL and SIJ pain.