Influence of sagittal balance on spinal lumbar loads: a numerical approach

Sagittal spinopelvic alignment in ambulatory persons with cerebral palsy

PURPOSE

This study aimed to describe the spinopelvic alignment of a cohort of young ambulatory individuals with cerebral palsy (CP) and compare it to published spinopelvic alignment data for the typically developing adolescents.

METHODS

Thirty-seven adolescents (18 females) with CP at GMFCS I-III were included in this retrospective case series. Lumbar lordosis and pelvic incidence were measured, and their mismatch was calculated. A model that calculates predicted lumbar lordosis based on pelvic incidence in normative data was utilized to calculate a predicted lumbar lordosis in this cohort with cerebral palsy.

RESULTS

At imaging, ages were mean and standard deviation 13.5 ± 3.0 years. Pelvic incidence was 46.2° ± 12.9°, pelvic tilt was 2.8° ± 9.4°, sacral slope was 43.6° ± 10.8°, and measured lumbar lordosis was 59.4° ± 11.6°. There were no differences in pelvic incidence or lumbar lordosis among the GMFCS levels; however, pelvic incidence was higher in females. Pelvic incidence-lumbar lordosis mismatch greater than 10° was found in 67% of the cohort. Mean predicted lumbar lordosis based on the model was 54.7° ± 8.5°, averaging 8° less than measured lordosis.

CONCLUSION

PI-LL mismatch was identified in 67% of this cohort of ambulatory adolescents with CP, in part due to greater lordosis than predicted by a model based on data from adolescents without CP. The implications of this finding, such as the correlation between sagittal spinopelvic alignment and quality of life in this population, should be assessed further in ambulatory patients with cerebral palsy.

LEVEL OF EVIDENCE

Level IV-retrospective cohort study and literature comparison.

Comprehensive assessment of global spinal sagittal alignment and related normal spinal loads in a healthy population

Abnormal postoperative global sagittal alignment (GSA) is associated with an increased risk of mechanical complications after spinal surgery. Typical assessment of sagittal alignment relies on a few selected measures, disregarding global complexity and variability of the sagittal curvature. The normative range of spinal loads associated with GSA has not yet been considered in clinical evaluation. The study objectives were to develop a new GSA assessment method that holistically describes the inherent relationships within GSA and to estimate the related spinal loads. Vertebral endplates were annotated on radiographs of 85 non-pathological subjects. A Principal Component Analysis (PCA) was performed to derive a Statistical Shape Model (SSM). Associations between identified GSA variability modes and conventional alignment measures were assessed. Simulations of respective Shape Modes (SMs) were performed using an established musculoskeletal AnyBody model to estimate normal variation in cervico-thoraco-lumbar loads. The first six principal components explained 97.96% of GSA variance. The SSM provides the normative range of GSA and a visual representation of the main variability modes. Normal variation relative to the population mean in identified alignment features was found to influence spinal loads, e.g. the lower bound of the second shape mode (SM2-2σ) corresponds to an increase in L4L5-compression by 378.64 N (67.86%). Six unique alignment features were sufficient to describe GSA almost entirely, demonstrating the value of the proposed method for an objective and comprehensive analysis of GSA. The influence of these features on spinal loads provides a normative biomechanical reference, eventually guiding surgical planning of deformity correction in the future.

Risk factors for mechanical complications after fusion extension surgery for lumbar adjacent segment disease

PURPOSE

Adjacent segment disease (ASD) is a common complication in fusion surgery. In the event of solid segmental fusion, previous implants can be removed or preserved during fusion extension for ASD. To compare the surgical outcomes of patients with and without implants and analyzes the risk factors for postoperative mechanical complications.

METHODS

Patients who underwent fusion extension for lumbar ASD from 2011 to 2019 with a minimum 2 year follow-up were retrospectively reviewed. Spinopelvic parameters were measured preoperatively and postoperatively. Clinical outcomes and surgical complications were compared between groups with implants preserved and removed. Risk factors for mechanical complications, including clinical, surgical, and radiographic factors were analyzed.

RESULTS

Sixty-nine patients (mean age, 69.9 ± 6.9 years) were included. The mean numbers of initial and extended fused segments were 2.8 ± 0.7 and 2.7 ± 0.7, respectively. Previous implants were removed in 43 patients (R group) and preserved in 26 patients (P group). Both groups showed an improvement in clinical outcomes without between-group differences. The operation time was significantly longer in R group (260 vs 207 min, p < 0.001). Mechanical complications occurred in 13 patients (12 in R group and 1 in P group) and reoperation was needed in 3 patients (R group). Implant removal, index fusion surgery including L5-S1, and postoperative sagittal malalignment were risk factors for mechanical complications.

CONCLUSION

Implant removal was a risk factor for mechanical complications. Index fusion surgery including L5-S1 and postoperative sagittal malalignment were also risk factors for mechanical complications.

Thoracolumbar/Lumbar Degenerative Kyphosis-The Importance of Thoracolumbar Junction in Sagittal Alignment and Balance

PURPOSE

To conduct a more comprehensive study of sagittal alignment in patients with thoracolumbar/lumbar (TL/L) degenerative kyphosis.

METHODS

A total of 133 consecutive patients from September 2016 to March 2019 with degenerative spinal kyphosis were enrolled. These patients were divided into different types according to sagittal alignment, including thoracolumbar junctional kyphosis (TLJK). Then, we divided the patients with TLJK into two groups: the Sagittal Balance group (C7-SVA < 50 mm) and the Sagittal Imbalance group (C7-SVA ≥ 50 mm). The sagittal parameters of each type or group were compared and correlation analysis was conducted.

RESULTS

Thoracolumbar/lumbar degenerative kyphosis consists of four types: Type I, lumbar kyphosis; Type II, degenerative flat back; Type III, thoracolumbar junctional kyphosis; and Type IV, global kyphosis. According to different sagittal alignments, Type III can further be divided into three subtypes: IIIA, with smooth kyphosis of thoracic and upper lumbar; IIIB, like a clasp knife, with a flat thoracic and lumbar angle; and IIIC, with bigger thoracic kyphosis and lumbar lordosis. The thoracolumbar kyphosis angle (°) of the three subtypes were -23.61 ± 5.37, -25.40 ± 7.71, and -40.01 ± 8.40, respectively. Lumbar lordosis was correlated with thoracic kyphosis (IIIA, r = -0.600, p = 0.005; IIIB, r = -0.312, p = 0.046; IIIC, r = -0.657, p = 0.015), and correlated with sacral slope (IIIA, r = 0.537, p = 0.015; IIIB, r = 0.654, p = 0.000; IIIC, r = 0.578, p = 0.039). All spinopelvic parameters were compared between the Sagittal Balance group and the Sagittal Imbalance group, and only the thoracolumbar kyphosis angle showed statistical difference (t = -2.247, p = 0.028).

CONCLUSIONS

The common characteristics of thoracolumbar junctional kyphosis were found to be a bigger thoracolumbar junctional angle and vertex of kyphosis located in the thoracolumbar junction (T10-L2). Despite TLJK, a change in the thoracic angle was still important to maintain sagittal balance. The thoracolumbar junction plays an important role in sagittal alignment and balance.

Biomechanical responses of human lumbar spine and pelvis according to the Roussouly classification

Background

Few studies have analyzed the different biomechanical properties of the lumbar with various morphological parameters, which play an important role in injury and degeneration. This study aims to preliminarily investigate biomechanical characteristics of the spine with different sagittal alignment morphotypes by using finite element (FE) simulation and in-vitro testing.

Methods

According to the lumbar-pelvic radiographic parameters of the Chinese population, the parametric FE models (L1-S1-pelvis) of Roussouly’s type (1–4) were validated and developed based on the in-vitro biomechanical testing. A pure moment of 7.5 Nm was applied in the three anatomical planes to simulate the physiological activities of flexion, extension, left-right lateral bending and left-right axial rotation.

Results

The sagittal configuration of four Roussouly’s type models had a strong effect on the biomechanical responses in flexion and extension. The apex of the lumbar lordosis is a critical position where the segment has the lowest range of motion among all the models. In flexion-extension, type 3 and 4 models with a good lordosis shape had a more uniform rotation distribution at each motor function segment, however, type 1 and 2 models with a straighter spine had a larger proportion of rotation at the L5-S1 level. In addition, type 1 and 2 models had higher intradiscal pressures (IDPs) at the L4-5 segment in flexion, while type 4 model had larger matrix and fiber stresses at the L5-S1 segment in extension.

Conclusion

The well-marched lordotic type 3 lumbar had greater stability, however, a straighter spine (type 1 and 2) had poor balance and load-bearing capacity. The hypolordotic type 4 model showed larger annulus fiber stress. Therefore, the sagittal alignment of Roussouly’s type models had different kinetic and biomechanical responses under various loading conditions, leading to different clinical manifestations of the lumbar disease.

Influence of Lumbar Lordosis on Posterior Rod Strain in Long-Segment Construct During Biomechanical Loading: A Cadaveric Study

Objective

The lordotic shape of the lumbar spine differs substantially between individuals. Measuring and recording strain during spinal biomechanical tests is an effective method to infer stresses on spinal implants and predict failure mechanisms. The geometry of the spine may have a significant effect on the resultant force distribution, thereby directly affecting rod strain.

Methods

Seven fresh-frozen cadaveric specimens (T12-sacrum) underwent standard (7.5 Nm) nondestructive sagittal plane tests: flexion and extension. The conditions tested were intact and pedicle screws and rods (PSR) at L1-sacrum. The posterior right rod was instrumented with strain gauges between L3–4 (index level) and the L5–S1 pedicle screw. All specimens underwent lateral radiographs before testing. Lordotic angles encompassing different levels (L5–S1, L4–S1, L3–S1, L2–S1, and L1–S1) were measured and compared with rod strain. Data were analyzed using Pearson correlation analyses.

Results

Strong positive correlations were observed between lordosis and posterior rod strain across different conditions. The L3–S1 lordotic angle in the unloaded intact condition correlated with peak rod strain at L3–4 with PSR during flexion (R=0.76, p=0.04). The same angle in the unloaded PSR condition correlated with peak strain in the PSR condition during extension (R=-0.79, p=0.04). The unloaded intact L2–S1 lordotic angle was significantly correlated with rod strain at L3–4 in the PSR condition during flexion (R=0.85, p=0.02) and extension (R=-0.85, p=0.02) and with rod strain at L5–S1 in the PSR condition during flexion (R=0.84, p=0.04).

Conclusion

Lordosis measured on intact and instrumented conditions has strong positive correlations with posterior rod strain in cadaveric testing.

Computational modelling of the scoliotic spine: A literature review

Scoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed.

Load Distribution in the Lumbar Spine During Modeled Compression Depends on Lordosis

Excessive or incorrect loading of lumbar spinal structures is commonly assumed as one of the factors to accelerate degenerative processes, which may lead to lower back pain. Accordingly, the mechanics of the spine under medical conditions, such as scoliosis or spondylolisthesis, is well-investigated. Treatments via both conventional therapy and surgical methods alike aim at restoring a "healthy" (or at least pain-free) load distribution. Yet, surprisingly little is known about the inter-subject variability of load bearings within a "healthy" lumbar spine. Hence, we utilized computer tomography data from 28 trauma-room patients, whose lumbar spines showed no visible sign of degeneration, to construct simplified multi-body simulation models. The subject-specific geometries, measured by the corresponding lumbar lordosis (LL) between the endplates of vertebra L1 and the sacrum, served as ceteris paribus condition in a standardized forward dynamic compression procedure. Further, the influence of stimulating muscles from the M. multifidus group was assessed. For the range of available LL from 28 to 66°, changes in compressive and shear forces, bending moments, as well as facet joint forces between adjacent vertebrae were calculated. While compressive forces tended to decrease with increasing LL, facet forces were tendentiously increasing. Shear forces decreased between more cranial vertebrae and increased between more caudal ones, while bending moments remained constant. Our results suggest that there exist significant, LL-dependent variations in the loading of "healthy" spinal structures, which should be considered when striving for individually appropriate therapeutic measures.

Estimation and assessment of sagittal spinal curvature and thoracic muscle morphometry in different postures

Spine models are typically developed from supine clinical imaging data, and hence clearly do not fully reflect postures that replicate subjects' clinical symptoms. Our objectives were to develop a method to: (i) estimate the subject-specific sagittal curvature of the whole spine in different postures from limited imaging data, (ii) obtain muscle lines-of-action in different postures and analyze the effect of posture on muscle fascicle length, and (iii) correct for cosine between the magnetic resonance imaging (MRI) scan plane and dominant fiber line-of-action for muscle parameters (cross-sectional area (CSA) and position). The thoracic spines of six healthy volunteers were scanned in four postures (supine, standing, flexion, and sitting) in an upright MRI. Geometry of the sagittal spine was approximated with a circular spline. A pipeline was developed to estimate spine geometry in different postures and was validated. The lines-of-action for two muscles, erector spinae (ES) and transversospinalis (TS) were obtained for every posture and hence muscle fascicle lengths were computed. A correction factor based on published literature was then computed and applied to the muscle parameters. The maximum registration error between the estimated spine geometry and MRI data was small (average RMSE∼1.2%). The muscle fascicle length increased (up to 20%) in flexion when compared to erect postures. The correction factor reduced muscle parameters (∼5% for ES and ∼25% for TS) when compared to raw MRI data. The proposed pipeline is a preliminary step in subject-specific modeling. Direction cosines of muscles could be used while improving the inputs of spine models.

Computational Challenges in Tissue Engineering for the Spine

This paper deals with a brief review of the recent developments in computational modelling applied to innovative treatments of spine diseases. Additionally, it provides a perspective on the research directions expected for the forthcoming years. The spine is composed of distinct and complex tissues that require specific modelling approaches. With the advent of additive manufacturing and increasing computational power, patient-specific treatments have moved from being a research trend to a reality in clinical practice, but there are many issues to be addressed before such approaches become universal. Here, it is identified that the major setback resides in validation of these computational techniques prior to approval by regulatory agencies. Nevertheless, there are very promising indicators in terms of optimised scaffold modelling for both disc arthroplasty and vertebroplasty, powered by a decisive contribution from imaging methods.

Sagittal spinopelvic changes after posterior spinal fusion in adolescent idiopathic scoliosis

PURPOSE

This article examines if longer posterior spinal fusions with instrumentation (PSFI) into the lumbar spine (L3/4) alter spinopelvic parameters compared with selective fusions to T12/L1/L2 in adolescent idiopathic scoliosis (AIS) patients.

METHODS

We analysed radiographs of 84 AIS patients, 58 (69%) females and 26 (31%) males, who underwent PSFI at an mean age of 15 years ± 2.5 years, range 10 years to 21 years, between 1st January 2007 and 31st December 2014. Radiographic parameters were measured pre- and post-operatively at most recent follow-up (range 2 years to 8.2 years): pelvic incidence (PI), lumbar lordosis (LL, L1-S1 and L4-S1), sagittal vertical alignment (SVA), scoliosis angle and proximal junctional kyphosis (PJK). PI-LL was calculated. Data was analysed using t-tests or Wilcoxon rank-sum tests.

RESULTS

In total, 32 patients underwent a selective fusion with lowest instrumented vertebra (LIV) T12-L2, and 52 patients underwent a fusion with LIV L3-L4. In both groups, scoliosis angle was significantly corrected at follow-up (p < 0.005).Pre-operatively, both groups had similar LL (L1-S1) and PI-LL. Post-operatively, LL increased in the L3-4 fusion group (p < 0.005) but did not change in the selective fusion group (p = 0.116). This change in LL in the L3-4 fusion group affected the post-operative PI-LL (T12-L2 fusion -4.9° versus L3-4 fusion -13.6°, p = 0.002). No differences were seen in PI, SVA or LL L4-S1 between groups. Radiographic PJK occurred in seven of the L3-4 patients with and without PJK (noPJK -8.8° versus PJK -25.8°, p = 0.026).

CONCLUSIONS

In patients who underwent a fusion ending at L3 or L4, LL was increased. This altered the PI-LL relationship, and appeared to increase the risk of PJK.

LEVEL OF EVIDENCE

III.

Biomechanical evaluation of anterior and posterior lumbar surgical approaches on the adjacent segment: a finite element analysis

The purpose of this study was to use models of spine to compare range of motion and intradiscal pressure of adjacent segments performing anterior and/or posterior lumbar surgical approaches and predict potential risk of adjacent segment degeneration. A previously validated finite element model of the intact L1-S1 segments was used. Three different anterior and one posterior surgical fixation approaches for tuberculosis were performed in L3-L5. Three different anterior surgical models were constructed according to the anterior approaches involving debridement, bone graft with or without titanium mesh, and internal fixation with different number of screws and rods. The posterior surgical approach involved transforaminal lumbar interbody debridement, bone graft, and internal fixation. Range of motion and intradiscal pressure of segments adjacent to the fusion were assessed, and biomechanical influences were compared. Intradiscal pressure and range of motion of the adjacent L2/3 and L5/S1 increased during different physiological movements after anterior and/or posterior surgical approaches as compared to baseline values. Comparison between the biomechanical values assessed after different anterior surgical approaches yielded no significant difference. After anterior and posterior surgical approaches were performed on the same model, there were no significant differences in intradiscal pressure and range of motion of the adjacent L2/3 and L5/S1. Anterior and/or posterior lumbar surgical approaches increased range of motion and intradiscal pressure in L2/3 and L5/S1, suggesting each lumbar surgical approach assessed has the potential risk of adjacent segment degeneration. However, there were no significant differences between the biomechanical measurements across the different surgical approaches evaluated.

Low lordosis is a common finding in young lumbar disc herniation patients

Purpose

The sagittal alignment of the lumbar spine and pelvis can be classified into several subtypes. It has been suggested that the risk of developing certain pathologies, such as a lumbar disc herniation (LDH) is affected by spinal sagittal profiles. The main aim of this study was to investigate the sagittal profile in young patients surgically treated for a lumbar disc herniation and if a discectomy would alter the sagittal parameters.

Methods

Sixteen active young patients (mean age 18.3 ± 3.2 SD) with a lumbar disc herniation having a discectomy were included. A classification according to Roussouly of the sagittal parameters was made by two senior spinal surgeons, both pre-operatively and post-operatively on radiographs. The distribution of sagittal parameters and spinopelvic profiles were analysed and compared to a previous established healthy normal population.

Results

This series of active young patients with LDH exhibited a low lumbar lordosis dominance, with Roussouly sagittal profiles type 1 and type 2 accounting for more than 75% of the examined patients. An analysis of the erect radiographs revealed no significant changes in the post-operative sagittal profile.

Conclusions

This study showed that sagittal spinal alignment according to Roussouly in a young population with LDH is skewed compared with a normal population cohort. Furthermore, the lack of post-operative correction is suggestive of a non-ephemeral response to a LDH. Roussouly type 2 spinal sagittal profile may be a risk factor in young individuals suffering a disc herniation.

Spinopelvic sagittal alignment of patients with transfemoral amputation

PURPOSE

This study aims to describe the spinopelvic sagittal alignment in transfemoral amputees (TFAs) from a radiologic study of the spine with a postural approach to better understand the high prevalence of low back pain (LBP) in this population.

METHODS

TFAs underwent X-rays with 3-D reconstructions of the full spine and pelvis. Sagittal parameters were analyzed and compared to the literature. Differences between TFAs with and without LBP were also observed.

RESULTS

Twelve subjects have been prospectively included (TFA-LBP group (n = 5) and TFA-NoP group (n = 7)). Four of the five subjects of the TFA-LBP group and two of the seven in TFAs-NoP group had an imbalanced sagittal posture, especially regarding the T9-tilt, significantly higher in the TFA-LBP group than in the TFA-NoP (p = 0.046). Eight subjects (6 TFA-NoP and 2 TFA-LBP) had abnormal low value of thoracic kyphosis (TK). Moreover, the mean angle of TK in the TFA-NoP group was lower than in the TFA-LBP group (p = 0.0511).

CONCLUSION

In the considered sample, TFAs often present a sagittal imbalance. A low TK angle seems to be associated with the absence of LBP. It can be hypothesized that this compensatory mechanism of the sagittal imbalance is the most accessible in this population. This study emphasizes the importance of considering the sagittal balance of the pelvis and the spine in patients with a TFA to better understand the high prevalence of LBP in this population. It should be completed by the analysis of the spinopelvic balance and the lower limbs in 3D. These slides can be retrieved under Electronic Supplementary Material.

Dependence of lumbar loads on spinopelvic sagittal alignment: An evaluation based on musculoskeletal modeling

Still little is known about how spinopelvic alignment affects spinal load distribution. Musculoskeletal modeling can potentially help to discover associations between spine alignment and risk factors of spinal disorders (e.g. disc herniation, vertebral fracture, spondylolisthesis, low back pain). The present study exploited the AnyBody full-body musculoskeletal model to assess the relation between lumbar loads and spinopelvic alignment in the sagittal plane. The model was evaluated in the standing position. The simulated postures were set using spinopelvic parameters gleaned from the literature and characterizing the healthy adult population. The parameters were: sagittal vertical axis, Roussouly lumbar type, sacral slope, and pelvic incidence. A total of 2772 configurations were simulated based on the following measurements: compression force and anterior shear at levels L4L5 and L5S1; multifidus, longissimus spinae, and rectus abdominis muscle forces. Changes in global sagittal alignment, lumbar typology, and sacral inclination, but not in pelvic incidence, were found to affect intervertebral loads in the lumbar spine and spinal muscle activation. Considering these changes would be advantageous for clinical evaluation, due to the recognized relation between altered loads and risk of disc herniation, vertebral fracture, spondylolisthesis, and low back pain. Musculoskeletal modeling proved to be a valuable biomechanical tool to non-invasively investigate the relation between internal loads and anatomical parameters.

Case-matched comparative analysis of spinal deformity correction in arthrogryposis multiplex congenita versus adolescent idiopathic scoliosis

OBJECTIVEAs scoliosis in arthrogryposis multiplex congenita (AMC) is unusual and the number of cases reviewed in previous studies is also relatively small, no previous study exists that has directly compared the results of spinal deformity correction between AMC and adolescent idiopathic scoliosis (AIS) patients. The aim of this study was to compare the radiographic and clinical outcomes of surgical correction of spinal deformity associated with AMC versus AIS.METHODSTwenty-four adolescents with AMC were matched with 48 AIS patients in terms of Cobb angle of main curve, curve pattern, sex, age at surgery, Risser grade, and length of follow-up. Patients in both groups underwent posterior-only spinal correction and fusion procedures. The surgical outcomes and complications were analyzed and compared between the 2 groups.RESULTSIn comparison to the AIS group, the AMC group had a significantly longer mean operation time (5.6 vs 4.4 hours, p = 0.002), more blood loss (1620 ± 250 ml vs 840 ± 260 ml, p < 0.001), and more fusion levels (14.1 ± 2.3 levels vs 12.4 ± 2.5 levels, p = 0.007) as well as a lower correction rate (44.3% ± 11.1% vs 70.8% ± 12.4%, p < 0.001) and a higher rate of loss of correction (5.0% ± 3.1% vs 2.1% ± 1.9%, p < 0.001). Nine patients in the AMC group had preoperative pelvic obliquity, which was corrected from a mean of 14.2° ± 8.4° to a mean of 4.3° ± 3.2° (p < 0.001) after the surgery. The thoracic lordosis and sagittal vertical axis were significantly improved in the AMC group. Notably, however, the AMC group was found to have higher rates of screw malpositioning (15.9% vs 9.5%, p = 0.002) and complications (8/24 [33.3%] vs 4/48 [8.3%], p = 0.016) as compared to the AIS group.CONCLUSIONSCorrection of AMC-associated scoliosis tends to require a longer operating time and involve more fusion levels but results in less correction, more blood loss, and more complications, in comparison with AIS. In addition, more attention should be paid to pelvic obliquity and sagittal hyperlordosis in AMC patients.

Dynamic analysis of forces in the lumbar spine during bag carrying

OBJECTIVE

The intervertebral disc supports axial and shear forces generated during tasks such as lifting and carrying weights. The objective of this study was to determine the forces in the lumbar spine of workers carrying a bag on the head, on the shoulder and on the anterior part of the trunk.

METHODS

Kinematic measurements were recorded for 10 subjects carrying bags of 10, 20 and 25 kg on each of the three aforementioned positions. A simple dynamic model implemented in a custom program was then developed to determine the lumbar forces using the accelerations and positions obtained from the kinematic analysis.

RESULTS

The analyses yielded a maximum compressive force of 2338.4 ± 422 N when a 25-kg bag was carried on the anterior part of the trunk.

CONCLUSION

Carrying bags on the anterior part of the trunk generated higher lumbar forces compared to those developed by carrying the bag on the head or on the shoulder. Force levels suggest that this activity represents a moderate risk for the subjects. However, future biomechanical models should be developed to analyze the cumulative effect in the discs when longer periods of time are spent in this activity.

The effect of anterior longitudinal ligament resection on lordosis correction during minimally invasive lateral lumbar interbody fusion: Biomechanical and radiographic feasibility of an integrated spacer/plate interbody reconstruction device

BACKGROUND

Lateral lumbar interbody fusion is powerful for correcting degenerative conditions, yet sagittal correction remains limited by anterior longitudinal ligament tethering. Although lordosis has been restored via ligament release, biomechanical consequences remain unknown. Investigators examined radiographic and biomechanical of ligament release for restoration of lumbar lordosis.

METHODS

Six fresh-frozen human cadaveric spines (L3-S1) were tested: (Miller et al., 1988) intact; (Battie et al., 1995) 8mm spacer with intact anterior longitudinal ligament; (Cho et al., 2013) 8mm spacer without intact ligament following ligament resection; (Galbusera et al., 2013) 13mm lateral lumbar interbody fusion; (Goldstein et al., 2001) integrated 13mm spacer. Focal lordosis and range of motion were assessed by applying pure moments in flexion-extension, lateral bending, and axial rotation.

FINDINGS

Cadaveric radiographs showed significant improvement in lordosis correction following ligament resection (P<0.05). The 8mm spacer with ligament construct provided greatest stability relative to intact (P>0.05) but did little to restore lordosis. Ligament release significantly destabilized the spine relative to intact in all modes and 8mm with ligament in lateral bending and axial rotation (P<0.05). Integrated lateral lumbar interbody fusion following ligament resection did not significantly differ from intact or from 8mm with ligament in all testing modes (P>0.05).

INTERPRETATION

Lordosis corrected by lateral lumbar interbody fusion can be improved by anterior longitudinal ligament resection, but significant construct instability and potential implant migration/dislodgment may result. This study shows that an added integrated lateral fixation system can significantly improve construct stability. Long-term multicenter studies are needed.

Lumbar Lordosis of Spinal Stenosis Patients during Intraoperative Prone Positioning

Background

To evaluate the effect of spondylolisthesis on lumbar lordosis on the OSI (Jackson; Orthopaedic Systems Inc.) frame. Restoration of lumbar lordosis is important for maintaining sagittal balance. Physiologic lumbar lordosis has to be gained by intraoperative prone positioning with a hip extension and posterior instrumentation technique. There are some debates about changing lumbar lordosis on the OSI frame after an intraoperative prone position. We evaluated the effect of spondylolisthesis on lumbar lordosis after an intraoperative prone position.

Methods

Sixty-seven patients, who underwent spinal fusion at the Department of Orthopaedic Surgery of Gwangmyeong Sungae Hospital between May 2007 and February 2012, were included in this study. The study compared lumbar lordosis on preoperative upright, intraoperative prone and postoperative upright lateral X-rays between the simple stenosis (SS) group and spondylolisthesis group. The average age of patients was 67.86 years old. The average preoperative lordosis was 43.5° (± 14.9°), average intraoperative lordosis was 48.8° (± 13.2°), average postoperative lordosis was 46.5° (± 16.1°) and the average change on the frame was 5.3° (± 10.6°).

Results

Among all patients, 24 patients were diagnosed with simple spinal stenosis, 43 patients with spondylolisthesis (29 degenerative spondylolisthesis and 14 isthmic spondylolisthesis). Between the SS group and spondylolisthesis group, preoperative lordosis, intraoperative lordosis and postoperative lordosis were significantly larger in the spondylolisthesis group. The ratio of patients with increased lordosis on the OSI frame compared to preoperative lordosis was significantly higher in the spondylolisthesis group. The risk of increased lordosis on frame was significantly higher in the spondylolisthesis group (odds ratio, 3.325; 95% confidence interval, 1.101 to 10.039; p = 0.033).

Conclusions

Intraoperative lumbar lordosis on the OSI frame with a prone position was larger in the SS patients than the spondylolisthesis patients, which also produced a larger postoperative lordosis angle after posterior spinal fusion surgery. An increase in lumbar lordosis on the OSI frame should be considered during posterior spinal fusion surgery, especially in spondylolisthesis patients.

A computational model to describe the regional interlamellar shear of the annulus fibrosus

Interlamellar shear may play an important role in the homeostasis and degeneration of the intervertebral disk. Accurately modeling the shear behavior of the interlamellar compartment would enhance the study of its mechanobiology. In this study, physical experiments were utilized to describe interlamellar shear and define a constitutive model, which was implemented into a finite element analysis. Ovine annulus fibrosus (AF) specimens from three locations within the intervertebral disk (lateral, outer anterior, and inner anterior) were subjected to in vitro mechanical shear testing. The local shear stress-stretch relationship was described for the lamellae and across the interlamellar layer of the AF. A hyperelastic constitutive model was defined for interlamellar and lamellar materials at each location tested. The constitutive models were incorporated into a finite element model of a block of AF, which modeled the interlamellar and lamellar layers using a continuum description. The global shear behavior of the AF was compared between the finite element model and physical experiments. The shear moduli at the initial and final regions of the stress-strain curve were greater within the lamellae than across the interlamellar layer. The difference between interlamellar and lamellar shear was greater at the outer anterior AF than at the inner anterior region. The finite element model was shown to accurately predict the global shear behavior or the AF. Future studies incorporating finite element analysis of the interlamellar compartment may be useful for predicting its physiological mechanical behavior to inform the study of its mechanobiology.

Numerical evaluation of the correlation between the normal variation in the sagittal alignment of the lumbar spine and the spinal loads

We present a numerical approach to reproduce various patterns of spino-pelvic organization. We wanted to predict the spinal loads in two static conditions (standing and holding a weight in the hands) based on parameters describing the shape of the lumbar spine: type following Roussouly classification, sacral slope, apex, inflection point and lumbar lordosis. Four hundred eighty finite element models including trunk muscles and representing the entire range of normal variability were created. The models predicted that, in the case of a moderate external load of 50 N, a lordotic and well balanced spine (e.g., type 3) could reduce the muscle activation in comparison with a more lordotic (type 4) spine, with negligible differences compared to a more straight spine (type 2). However, such a sagittal configuration was not correlated with a minimization of the loading state in the intervertebral discs, especially regarding anteroposterior shear loads. In the standing posture without any additional load, a less lordotic and more vertical spine (e.g., types 1 and 2) was sufficient to ensure a condition of minimal spinal loads. Despite a number of limitations, inverse statics numerical models of spine biomechanics including trunk muscles appear to be a promising tool to fill the knowledge gap between the clinical observations of the correlations between the spino-pelvic organization and the consequent spinal disorders.

Pelvic incidence-lumbar lordosis mismatch results in increased segmental joint loads in the unfused and fused lumbar spine

PURPOSE

Symptomatic adjacent segment disease (ASD) has been reported to occur in up to 27 % of lumbar fusion patients. A previous study identified patients at risk according to the difference of pelvic incidence and lordosis. Patients with a difference between pelvic incidence and lumbar lordosis >15° have been found to have a 20 times higher risk for ASD. Therefore, it was the aim of the present study to investigate forces acting on the adjacent segment in relation to pelvic incidence-lumbar lordosis (PILL) mismatch as a measure of spino-pelvic alignment using rigid body modeling to decipher the underlying forces as potential contributors to degeneration of the adjacent segment.

METHODS

Sagittal configurations of 81 subjects were reconstructed in a musculoskeletal simulation environment. Lumbar spine height was normalized, and body and segmental mass properties were kept constant throughout the population to isolate the effect of sagittal alignment. A uniform forward/backward flexion movement (0°-30°-0°) was simulated for all subjects. Intervertebral joint loads at lumbar level L3-L4 and L4-L5 were determined before and after simulated fusion.

RESULTS

In the unfused state, an approximately linear relationship between sagittal alignment and intervertebral loads could be established (shear: 0° flexion r = 0.36, p < 0.001, 30° flexion r = 0.48, p < 0.001; compression: 0° flexion r = 0.29, p < 0.01, 30° flexion r = 0.40, p < 0.001). Additionally, shear changes during the transition from upright to 30° flexed posture were on average 32 % higher at level L3-L4 and 14 % higher at level L4-L5 in alignments that were clinically observed to be prone to ASD. Simulated fusion affected shear forces at the level L3-L4 by 15 % (L4-L5 fusion) and 23 % (L4-S1 fusion) more for alignments at risk for ASD.

CONCLUSION

Higher adjacent segment shear forces in alignments at risk for ASD already prior to fusion provide a mechanistic explanation for the clinically observed correlation between PILL mismatch and rate of adjacent segment degeneration.

The effect of age on sagittal plane profile of the lumbar spine according to standing, supine, and various sitting positions

Background

The sagittal alignment of the spine changes depending on body posture and degenerative changes. This study aimed to observe changes in sagittal alignment of the lumbar spine with different positions (standing, supine, and various sitting postures) and to verify the effect of aging on lumbar sagittal alignment.

Methods

Whole-spine lateral radiographs were obtained for young volunteers (25.4 ± 2.3 years) and elderly volunteers (66.7 ± 1.7 years). Radiographs were obtained in standing, supine, and sitting (30°, 60°, and 90°) positions respectively. We compared the radiological changes in the lordotic and segmental angles in different body positions and at different ages. Upper and lower lumbar lordosis were defined according to differences in anatomical sagittal mobility and kinematic behavior.

Results

Lumbar lordosis was greater in a standing position (52.79° and 53.90° in young and old groups, respectively) and tended to decrease as position changed from supine to sitting. Compared with the younger group, the older group showed significantly more lumbar lordosis in supine and 60° and 90° sitting positions (P = 0.043, 0.002, 0.011). Upper lumbar lordosis in the younger group changed dynamically in all changed positions compared with the old group (P = 0.019). Lower lumbar lordosis showed a decreasing pattern in both age groups, significantly changing as position changed from 30° to 60° (P = 0.007, 0.007).

Conclusions

Lumbar lordosis decreases as position changes from standing to 90°sitting. The upper lumbar spine is more flexible in individuals in their twenties compared to those in their sixties. Changes in lumbar lordosis were concentrated in the lower lumbar region in the older group in sitting positions.