Search for: All records

Abstract PurposeTo assess the effect of various pelvic fixation techniques and number of rods on biomechanics of the proximal junction of long thoracolumbar posterior instrumented fusions. MethodsA validated spinopelvic finite-element (FE) model was instrumented with L5–S1 ALIF and one of the following 9 posterior instrumentation configurations: (A) one traditional iliac screw bilaterally (“2 Iliac/2 Rods”); (B) T10 to S1 (“Sacral Only”); (C) unilateral traditional iliac screw (“1 Iliac/2 Rods”); (D) one traditional iliac screw bilaterally with one midline accessory rod (“2 Iliac/3 rods”); (E) S2AI screws connected directly to the midline rods (“2 S2AI/2 Rods”); and two traditional iliac screws bilaterally with two lateral accessory rods connected to the main rods at varying locations (F1: T10–11, F2: T11–12, F3: T12–L1, F4: L1–2) (“4 Iliac/4 Rods”). Range of motions (ROM) at T10–S1 and T9–T10 were recorded and compared between models. The T9–T10 intradiscal pressures and stresses of the T9–10 disc’s annulus in addition to the von Mises stresses of the T9 and T10 vertebral bodies were recorded and compared. ResultsFor T10–S1 ROM, 4 iliac/4 rods had lowest ROM in flexion and extension, while 2 S2AI/2 rods showed lowest ROM in rotation. Constructs with 3 or 4 rods had lower stresses on the primary rods compared to 2-rod constructs. At the proximal adjacent disc (T9–10), 4 iliac/4 rods showed lowest ROM, lowest intradiscal pressures, and lowest annular stress in all directions (most pronounced in flexion–extension). Under flexion and extension, 4 iliac/4 rods also showed the lowest von Mises stresses on the T10 vertebral body but the highest stresses on the T9 vertebral body. ConclusionsDual iliac screws with 4 rods across the lumbosacral junction and extending to the thoracolumbar junction demonstrated the lowest T10–S1 ROM, the lowest adjacent segment disc (T9–T10) ROM, intradiscal pressures, and annular stresses, and the lowest UIV stresses, albeit with the highest UIV + 1 stresses. Additional studies are needed to confirm whether these biomechanical findings dictate clinical outcomes and effect rates of proximal junctional kyphosis and failure. 

Abstract PurposeTo evaluate proximal junctional biomechanics of a MLSS relative to traditional pedicle screw fixation at the proximal extent of T10-pelvis posterior instrumentation constructs (T10-p PSF). MethodsA previously validated three-dimensional osseoligamentous spinopelvic finite element (FE) model was used to compare proximal junctional range-of-motion (ROM), vertebral body stresses, and discal biomechanics between two groups: (1) T10-p with a T10-11 MLSS (“T10-11 MLSS”) and (2) T10-p with a traditional T10 pedicle screw (“Traditional T10-PS”). ResultsThe T10-11 MLSS had a 5% decrease in T9 cortical bone stress compared to Traditional T10-PS. Conversely, the T10 and T11 bone stresses increased by 46% and 98%, respectively, with T10-11 MLSS compared to Traditional T10-PS. Annular stresses and intradiscal pressures (IDP) were similar at T9-T10 between constructs. At the T10-11 disc, T10-11 MLSS decreased annular stresses by 29% and IDP by 48% compared to Traditional T10-PS. Adjacent ROM (T8-9 & T9-10) were similar between T10-11 MLSS and Traditional T10-PS. T10-11 MLSS had 39% greater ROM at T10-11 and 23% less ROM at T11-12 compared to Traditional T10-PS. ConclusionsIn this FE analysis, a T10-11 MLSS at the proximal extent of T10-pelvis posterior instrumentation resulted in increased T10 and T11 cortical bone stresses, decreased discal annular stress and IDP and increased ROM at T10-11, and no change in ROM at the adjacent level. Given the complex and multifactorial nature of proximal junctional kyphosis, these results require additional biomechanical and clinical evaluations to determine the clinical utility of MLSS on the proximal junctions of thoracolumbar posterior instrumented fusions. 

Abstract PurposeTo develop and validate a finite element (FE) model of a sacral pedicle subtraction osteotomy (S1-PSO) and to compare biomechanical properties of various multi-rod configurations to stabilize S1-PSOs. MethodsA previously validated FE spinopelvic model was used to develop a 30° PSO at the sacrum. Five multi-rod techniques spanning the S1-PSO were made using 4 iliac screws and a variety of primary rods (PR) and accessory rods (AR; lateral: Lat-AR or medial: Med-AR). All constructs, except one, utilized a horizontal rod (HR) connecting the iliac bolts to which PRs and Med-ARs were connected. Lat-ARs were connected to proximal iliac bolts. The simulation was performed in two steps with the acetabula fixed. For each model, PSO ROM and maximum stress on the PRs, ARs, and HRs were recorded and compared. The maximum stress on the L5–S1 disc and the PSO forces were captured and compared. ResultsHighest PSO ROMs were observed for 4-Rods (HR + 2 Med-AR). Constructs consisting of 5-Rods (HR + 2 Lat-ARs + 1 Med-AR) and 6-Rods (HR + 2 Lat-AR + 2 Med-AR) had the lowest PSO ROM. The least stress on the primary rods was seen with 6-Rods, followed by 5-Rods and 4-Rods (HR + 2 Lat-ARs). Lowest PSO forces and lowest L5–S1 disc stresses were observed for 4-Rod (Lat-AR), 5-Rod, and 6-Rod constructs, while 4-Rods (HR + Med-AR) had the highest. ConclusionIn this first FE analysis of an S1-PSO, the 4-Rod construct (HR + Med-AR) created the least rigid environment and highest PSO forces anteriorly. While 5- and 6-Rods created the stiffest constructs and lowest stresses on the primary rods, it also jeopardized load transfer to the anterior column, which may not be favorable for healing anteriorly. A balance between the construct’s rigidity and anterior load sharing is essential. 

Study DesignFinite element (FE) study. ObjectivePedicle subtraction osteotomy (PSO) is a surgical method to correct sagittal plane deformities. In this study, we aimed to investigate the biomechanical effects of lumbar disc degeneration on the instrumentation following PSO and assess the effects of using interbody spacers adjacent to the PSO level in a long instrumented spinal construct. MethodsA spinopelvic model (T10-pelvis) with PSO at the L3 level was used to generate 3 different simplified grades of degenerated lumbar discs (mild (Pfirrmann grade III), moderate (Pfirrmann grade IV), and severe (Pfirrmann grade V)). Instrumentation included eighteen pedicle screws and bilateral primary rods. To investigate the effect of interbody spacers, the model with normal disc height was modified to accommodate 2 interbody spacers adjacent to the PSO level through a lateral approach. For the models, the rods’ stress distribution, PSO site force values, and the spine range of motion (ROM) were recorded. ResultsThe mildly, moderately, and severely degenerated models indicated approximately 10%, 26%, and 40% decrease in flexion/extension motion, respectively. Supplementing the instrumented spinopelvic PSO model using interbody spacers reduced the ROM by 22%, 21%, 4%, and 11% in flexion, extension, lateral bending, and axial rotation, respectively. The FE results illustrated lower von Mises stress on the rods and higher forces at the PSO site at higher degeneration grades and while using the interbody spacers. ConclusionsLarger and less degenerated discs adjacent to the PSO site may warrant consideration for interbody cage instrumentation to decrease the risk of rod fracture and PSO site non-union. 

Background:Previous studies have examined the effect of whole body (WB) parameters on anterior cruciate ligament (ACL) strain and loads, as well as knee joint kinetics and kinematics. However, articular cartilage damage occurs in relation to ACL failure, and the effect of WB parameters on ACL strain and articular cartilage biomechanics during dynamic tasks is unclear. Purposes:(1) To investigate the effect of WB parameters on ACL strain, as well as articular cartilage stress and contact force, during a single-leg cross drop (SLCD) and single-leg drop (SLD). (2) To identify WB parameters predictive of high ACL strain during these tasks. Study Design:Descriptive laboratory study. Methods:Three-dimensional motion analysis data from 14 physically active men and women were recorded during an SLCD and SLD. OpenSim was used to obtain their kinematics, kinetics, and muscle forces for the WB model. Using these data in kinetically driven finite element simulations of the knee joint produced outputs of ACL strains and articular cartilage stresses and contact forces. Spearman correlation coefficients were used to assess relationships between WB parameters and ACL strain and cartilage biomechanics. Moreover, receiver operating characteristic curve analyses and multivariate binary logistic regressions were used to find the WB parameters that could discriminate high from low ACL strain trials. Results:Correlations showed that more lumbar rotation away from the stance limb at peak ACL strain had the strongest overall association (ρ = 0.877) with peak ACL strain. Higher knee anterior shear force (ρ = 0.895) and lower gluteus maximus muscle force (ρ = 0.89) at peak ACL strain demonstrated the strongest associations with peak articular cartilage stress or contact force in ≥1 of the analyzed tasks. The regression model that used muscle forces to predict high ACL strain trials during the dominant limb SLD yielded the highest accuracy (93.5%), sensitivity (0.881), and specificity (0.952) among all regression models. Conclusion:WB parameters that were most consistently associated with and predictive of high ACL strain and poor articular cartilage biomechanics during the SLCD and SLD tasks included greater knee abduction angle at initial contact and higher anterior shear force at peak ACL strain, as well as lower gracilis, gluteus maximus, and medial gastrocnemius muscle forces. Clinical relevance:Knowledge of which landing postures create a high risk for ACL or cartilage injury may help reduce injuries in athletes by avoiding those postures and practicing the tasks with reduced high-risk motions, as well as by strengthening the muscles that protect the knee during single-leg landings. 

Abstract PurposeTo assess biomechanics of a lumbar PSO stabilized with different multi-rod constructs (4-, 5-, 6-rods) using satellite and accessory rods. MethodsA validated spinopelvic finite element model with a L3 PSO was used to evaluate the following constructs: 2 primary rods T10-pelvis (“Control”), two satellite rods (4-rod), two satellite rods + one accessory rod (5-rod), or two satellite rods + two accessory rods (6-rod). Data recorded included: ROM T10-S1 and L2-L4, von Mises stresses on primary, satellite, and accessory rods, factor of safety yield stress, and force across the PSO surfaces. Percent differences relative to Control were calculated. ResultsCompared to Control, 4-rods increased PSO flexion and extension. Lower PSO ROMs were observed for 5- and 6-rods compared to 4-rods. However, 4-rod (348.6 N) and 5-rod (343.2 N) showed higher PSO forces than 2-rods (336 N) and 6-rods had lower PSO forces (324.2 N). 5- and 6-rods led to the lowest rod von Mises stresses across the PSO. 6-rod had the maximum factor of safety on the primary rods. ConclusionsIn this finite element analysis, 4-rods reduced stresses on primary rods across a lumbar PSO. Although increased rigidity afforded by 5- and 6-rods decreased rod stresses, it resulted in less load transfer to the anterior vertebral column (particularly for 6-rod), which may not be favorable for the healing of the anterior column. A balance between the construct’s rigidity and anterior load sharing is essential. 

Abstract PurposeMulti-rod constructs are used commonly to stabilize pedicle subtraction osteotomies (PSO). This study aimed to evaluate biomechanical properties of different satellite rod configurations and effects of screw-type spanning a PSO. MethodsA validated 3D spinopelvic finite element model with a L3 PSO (30°) was used to evaluate 5 models: (1) Control (T10–pelvis + 2 rods); (2) lateral satellite rods connected via offsets to monoaxial screws (LatSat-Mono) or (3) polyaxial screws (LatSat-Poly); (4) in-line satellite rods connected to monoaxial screws (InSat-Mono) or (4) polyaxial screws (InSat-Poly). Global and PSO range of motions (ROM) were recorded. Rods’ von Mises stresses and PSO forces were recorded and the percent differences from Control were calculated. ResultsAll satellite rods (save InSat-Mono) increased PSO ROM and decreased primary rods’ von Mises stresses at the PSO. Lateral rods increased PSO forces (LatSat-Mono:347.1 N; LatSat-Poly:348.6 N; Control:336 N) and had relatively lower stresses, while in-line rods decreased PSO forces (InSat-Mono:280.1 N; InSat-Poly:330.7 N) and had relatively higher stresses. Relative to polyaxial screws, monoaxial screws further decreased PSO ROM, increased satellite rods’ stresses, and decreased PSO forces for in-line rods, but did not change PSO forces for lateral rods. ConclusionMulti-rod constructs using in-line and lateral satellite rods across a PSO reduced primary rods' stresses. Subtle differences in biomechanics suggest lateral satellite rods, irrespective of screw type, increase PSO forces and lower rod stresses compared to in-line satellite rods, which had a high degree of posterior instrumentation stress shielding and lower PSO forces. Clinical studies are warranted to determine if these findings influence clinical outcomes. 

Background:The use of total hip arthroplasty (THA) femoral stems that transmit force in a favourable manner to the femur may minimise periprosthetic fractures. Finite element analysis (FEA) is a computerised method that analyses the effect of forces applied to a structure with complex shape. Our aim was to apply FEA to compare primary THA cementless stem designs and their association with periprosthetic fracture risk. Methods:3-dimensional (3D) models of a Dorr Type A femur and 5 commonly used primary THA cementless stem designs (short single wedge, standard-length single wedge, modular, double-wedge metaphyseal filling, and cylindrical fully coated) were developed using computed tomography (CT) imaging. Implant insertion, single-leg stance, and twisting with a planted foot were simulated. FEA was performed, and maximum femoral strain along the implant-bone interface recorded. Results:Femoral strain was highest with short single-wedge stem design (0.3850) and lowest with standard-length single-wedge design (0.0520). Location of maximum femoral strain varied by stem design, but not with implant insertion, single-leg stance, or twisting with a planted foot. Strain was as high during implant insertion as with single-leg stance or twisting with a planted foot. Conclusions:Our results suggest the risk of intraoperative and postoperative periprosthetic fracture with THA in a Dorr A femur is highest with short single-wedge stems and lowest with standard-length single-wedge stems. Consideration may be given to minimising the use of short single-wedge stems in THA. Implant-specific sites of highest strain should be carefully inspected for fracture. 

Study Design.Finite element analysis (FEA). Objective.The aim of this study was to determine biomechanical differences between traditional growing rod (TGR) and spring distraction system (SDS) treatment of early-onset scoliosis. Summary of Background Data.Many “growth-friendly” implants like the TGR show high rates of implant failure, spinal stiffening, and intervertebral disc (IVD) height loss. We developed the SDS, which employs continuous, dynamic forces to mitigate these limitations. The present FEA compares TGR and SDS implantation, followed by an 18-month growth period. Methods.Two representative, ligamentous, scoliotic FEA models were created for this study; one representing TGR and one representing SDS. initial implantation, and up to 18 months of physeal spinal growth were simulated. The SDS model was continuously distracted over this period; the TGR model included two additional distractions following index surgery. Outcomes included differences in rod stress, spinal morphology and iVD stress-shielding. Results.Maximum postoperative von Mises stress was 249MPa for SDS, and 205MPa for TGR. During the 6-month TGR distraction, TGR rod stress increased over two-fold to a maximum stress of 417MPa, compared to a maximum of 262 MPa in the SDS model at 6-month follow-up. During subsequent follow-up periods, TGR rod stress remained consistently higher than stresses in the SDS model. Additional lengthenings in the TGR model led to a smaller residual curve (16.08) and higher T1-S1 growth (359 mm) at 18-month follow-up compared to the SDS model (26.98, 348 mm). During follow-up, there was less stress-shielding of the IVDs in the SDS model, compared to the TGR model. At 18-month follow-up, upper and lower IVD surfaces of the SDS model were loaded more in compression than their TGR counterparts (mean upper: +112 ± 19N; mean lower: +100 ± 17N). Conclusion.In the present FEA, TGR treatment resulted in slightly larger curve correction compared to SDS, at the expense of increased IVD stress-shielding and a higher risk of rod fractures. Level of Evidence.N/A 

Study Design.Porcine intervertebral discs (IVDs) were excised and then drilled to simulate degeneration before being electrically stimulated for 21 days while undergoing mechanical loading. The discs were then analyzed for gene expression and morphology to assess regeneration. Objective.The purpose of this study was to investigate the effectiveness of the electrical stimulation of IVD treatment as an early intervention method in halting the progression of degenerative disc disease using an ex-vivo porcine model. Summary of Background Data.Treatments for degenerative disc disease are limited in their efficacy and tend to treat the symptoms of the disease rather than repairing the degenerated disc itself. There is a dire need for an early intervention treatment that not only halts the progression of the disease but contributes to reviving the degenerated disc. Methods.Lumbar IVDs were extracted from a mature pig within 1 hour of death and were drilled with a 1.5 mm bit to simulate degenerative disc disease. Four IVDs at a time were then cultured in a dynamic bioreactor system under mechanical loading for 21 days, two with and two without the electrical stimulation treatment. The IVDs were assessed using histological analysis, magnetic resonance imaging, and quantitative reverse transcriptase polymerase chain reaction to quantify the effectiveness of the treatment on the degenerated discs. Results.IVDs with electrical stimulation treatment exhibited extensive annular regeneration and prevented herniation of the nucleus pulposus (NP). In contrast, the untreated group of IVDs were unable to maintain tissue integrity and exhibited NP herniation through multiple layers of the annulus fibrosus. Gene expression showed an increase of extracellular matrix markers and antiinflammatory cytokine interleukin-4 (IL-4), while decreasing in pro-inflammatory markers and pain markers in electrically stimulated IVDs when compared to the untreated group. Conclusion.The direct electrical stimulation application in NP of damaged IVDs can be a viable option to regenerate damaged NP and annulus fibrosus tissues.