Innovative sacropelvic fixation using iliac screws and triangular titanium implants.

PURPOSE: Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for the correction of severe spinal deformities. The purpose of this study was to explore the effects of the triangular titanium implants on the iliac screw fixation. Our hypothesis was that the use of triangular titanium implants can increase the stability of the iliac screw fixation. METHODS: Three T10-pelvis instrumented models were created: pedicle screws and rods in T10-S1, and bilateral iliac screws (IL); posterior fixation and bilateral iliac screws and triangular implants inserted bilaterally in a sacro-alar-iliac trajectory (IL-Tri-SAI); posterior fixation and bilateral iliac screws and two bilateral triangular titanium implants inserted in a lateral trajectory (IL-Tri-Lat). Outputs of these models, such as hardware stresses, were compared against a model with pedicle screws and rods in T10-S1 (PED). RESULTS: Sacropelvic fixation decreased the L5-S1 motion by 75-90%. The motion of the SIJ was reduced by 55-80% after iliac fixation; the addition of triangular titanium implants further reduced it. IL, IL-Tri-SAI and IL-Tri-Lat demonstrated lower S1 pedicle stresses with respect to PED. Triangular implants had a protective effect on the iliac screw stresses. CONCLUSION: Sacropelvic fixation decreased L5-S1 range of motion suggesting increased stability of the joint. The combination of triangular titanium implants and iliac screws reduced the residual flexibility of the sacroiliac joint, and resulted in a protective effect on the S1 pedicle screws and iliac screws themselves. Clinical studies may be performed to demonstrate applicability of these FEA results to patient outcomes.

Biomechanics of sacropelvic fixation: a comprehensive finite element comparison of three techniques.

PURPOSE: Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for complex deformity correction and is commonly associated with pseudoarthrosis, implant failure and loosening. This study compared pedicle screw fixation (PED) with three different sacropelvic fixation techniques, namely iliac screws (IL), S2 alar-iliac screws (S2AI) and laterally placed triangular titanium implants (SI), all in combination with lumbosacral instrumentation, accounting for implant micromotion. METHODS: Existing finite element models of pelvis-L5 of three patients including lumbopelvic instrumentation were utilized. Moments of 7.5 Nm in the three directions combined with a 500 N compressive load were simulated. Measured metrics included flexibility, instrumentation stresses and bone-implant interface loads. RESULTS: Fixation effectively reduced the sacroiliac flexibility. Compared to PED, IL and S2AI induced a reduction in peak stresses in the S1 pedicle screws. Rod stresses were mostly unaffected by S2AI and SI, but IL demonstrated a stress increase. In comparison with a previous work depicting full osteointegration, SI was found to have similar instrumentation stresses as those due to PED. CONCLUSIONS: Fixation with triangular implants did not result in stress increase on the lumbosacral instrumentation, likely due to the lack of connection with the posterior rods. IL and S2AI had a mild protective effect on S1 pedicle screws in terms of stresses and bone-implant loads. IL resulted in an increase in the rod stresses. A comparison between this study and previous work incorporating full osteointegration demonstrates how these results may be applied clinically to better understand the effects of different treatments on patient outcomes. These slides can be retrieved under Electronic Supplementary Material.

Biomechanical in vitro comparison between anterior column realignment and pedicle subtraction osteotomy for severe sagittal imbalance correction.

PURPOSE: To investigate the biomechanical effects of anterior column realignment (ACR) and pedicle subtraction osteotomy (PSO) on local lordosis correction, primary stability and rod strains. METHODS: Seven cadaveric spine segments (T12-S1) underwent ACR at L1-L2. A stand-alone hyperlordotic cage was initially tested and then supplemented with posterior bilateral fixation. The same specimens already underwent a PSO at L4 stabilized by two rods, a supplemental central rod (three rods) and accessory rods (four rods) with and without adjacent interbody cages (La Barbera in Eur Spine J 27(9):2357-2366, 2018). In vitro flexibility tests were performed under pure moments in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) to determine the range of motion (RoM), while measuring the rod strains with strain gauge rosettes. RESULTS: Local lordosis correction with ACR (24.7° ± 3.7°) and PSO (25.1° ± 3.9°) was similar. Bilateral fixation significantly reduced the RoM (FE: 31%, LB: 2%, AR: 18%), providing a stability consistent with PSO constructs (p > 0.05); however, it demonstrates significantly higher rod strains compared to PSO constructs with lateral accessory rods and interbody cages in FE and AR (p < 0.05), while being comparable in FE or slightly higher in AR compared to PSO constructs with two and three rods. CONCLUSION: Bilateral posterior fixation is highly recommended following ACR to provide adequate primary stability. However, primary rod strains in ACR were found comparable or higher than weak PSO construct associated with frequent rod failure; therefore, caution is recommended. These slides can be retrieved under Electronic Supplementary Material.

In vivo kinematics of fixed-bearing total ankle arthroplasty.

BACKGROUND: A good recovery of the physiological mobility of the ankle is an indication of patients' satisfaction after total ankle arthroplasty, which does not generally match that of other consolidated procedures such as hip and knee replacement. The aim of this study was to investigate the kinematics of the Zimmer Total Metal Total Ankle (ZTMTA) during the different exercises. METHODS: Fifteen patients with ZTMTA were enrolled in this study. The patients performed non-weightbearing flexion-extension, stair climbing and descending, and fluoroscopic images were taken to capture the ankle movements. A combined images/three-dimensional models method was used to perform a kinematic analysis. RESULTS: Plantar-dorsiflexion resulted the main plane of movement, with the largest range of motion (ROM) of 23.3 ± 9.0° during flexion-extension. Inversion-eversion and adduction-abduction resulted lower than 10° in any trials. CONCLUSIONS: In the investigated population, the ZTMTA allowed a good recovery of the mobility, with ROMs comparable to the healthy subjects.

Computational and Experimental Fatigue Analysis of Contoured Spinal Rods.

Posterior fixation with contoured rods is an established methodology for the treatment of spinal deformities. Both uniform industrial preforming and intraoperative contouring introduce tensile and compressive plastic deformations, respectively, at the concave and at the convex sides of the rod. The purpose of this study is to develop a validated numerical framework capable of predicting how the fatigue behavior of contoured spinal rods is affected by residual stresses when loaded in lordotic and kyphotic configurations. Established finite element models (FEM) describing static contouring were implemented as a preliminary simulation step and were followed by subsequent cyclical loading steps. The equivalent Sines stress distribution predicted in each configuration was compared to that in straight rods (SR) and related to the corresponding experimental number of cycles to failure. In the straight configuration, the maximum equivalent stress (441 MPa) exceeds the limit curve, as confirmed by experimental rod breakage after around 1.9 × 105 loading cycles. The stresses further increased in the lordotic configuration, where failure was reached within 2.4 × 104 cycles. The maximum equivalent stress was below the limit curve for the kyphotic configuration (640 MPa), for which a run-out of 106 cycles was reached. Microscopy inspection confirmed agreement between numerical predictions and experimental fatigue crack location. The contouring technique (uniform contouring (UC) or French bender (FB)) was not related to any statistically significant difference. Our study demonstrates the key role of residual stresses in altering the mean stress component, superposing to the tensile cyclic load, potentially explaining the higher failure rate of lordotic rods compared to kyphotic ones.

Residual Stresses in Titanium Spinal Rods: Effects of Two Contouring Methods and Material Plastic Properties.

Posterior spinal fixation based on long spinal rods is the clinical gold standard for the treatment of severe deformities. Rods need to be contoured prior to implantation to fit the natural curvature of the spine. The contouring processes is known to introduce residual stresses and strains which affect the static and fatigue mechanical response of the implant, as determined through time- and cost-consuming experimental tests. Finite element (FE) models promise to provide an immediate understanding on residual stresses and strains within a contoured spinal rods and a further insight on their complex distribution. This study aims at investigating two rod contouring strategies, French bender (FB) contouring (clinical gold standard), and uniform contouring, through validated FE models. A careful characterization of the elastoplastic material response of commercial implants is led. Compared to uniform contouring, FB induces highly localized plasticizations in compression under the contouring pin with extensive lateral sections undergoing tensile residual stresses. The sensitivity analysis highlighted that the assumed postyielding properties significantly affect the numerical predictions; therefore, an accurate material characterization is recommended.

Biomechanical advantages of supplemental accessory and satellite rods with and without interbody cages implantation for the stabilization of pedicle subtraction osteotomy.

PURPOSE: To investigate the effect of anterior interbody cages, accessory and satellite rods usage on primary stability and rod strains for PSO stabilization. METHODS: Seven human cadaveric spine segments (T12-S1) underwent PSO at L4 with posterior fixation from L2 to S1. In vitro flexibility tests were performed under pure moments in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) to determine the range of motion, while measuring the strains on the primary rods with strain gauge rosettes. Six constructs with 2, 3 and 4 rods, with and without interbody cages implantation adjacent to the PSO site, were compared. RESULTS: All constructs had comparable effects in reducing spine kinematics compared to the intact condition (- 94% in FE and LB; - 80% in AR). Supplementation of 2 rods with lateral accessory rods (4 rods) was the most effective strategy in minimizing primary rod strains, particularly when coupled to cages (p ≤ 0.005; - 50% in FE, - 42% in AR and - 11% in LB); even without cages, the strains were significantly reduced (p ≤ 0.009; - 26%, - 37%, - 9%). The addition of a central satellite rod with laminar hooks (3 rods) effectively reduced rod strains in FE (p ≤ 0.005; - 30%) only in combination with cages. CONCLUSIONS: The study supports the current clinical practice providing a strong biomechanical rationale to recommend 4-rod constructs based on accessory rods combined with cages adjacent to PSO site. Although weaker, the usage of accessory rods without cages and of a central satellite rod with hooks in combination with interbody spacers may also be justified. These slides can be retrieved under Electronic Supplementary Material.

Preclinical evaluation of posterior spine stabilization devices: can we compare in vitro and in vivo loads on the instrumentation?

PURPOSE: To discuss whether the standard test method for preclinical evaluation of posterior spine stabilization devices with an anterior support correctly describes the effect of two short-segment posterior stabilization techniques frequently used in clinical practice for the treatment of traumatic, degenerative and iatrogenic instabilities. METHODS: A finite element study compared a validated instrumented L2-L4 segment undergoing standing, upper body flexion and extension to ISO 12189 standards model under a compressive load. A bridge instrumentation, with screws only at cranial and caudal levels, and a full stabilization, using screws at every level, are considered for both conditions. The internal loads on the spinal rod and the stress values on the implant are analysed in detail. RESULTS: Using ISO model and a bridge stabilization construct allow to overstress the pedicle screw more than a full stabilization with respect to the corresponding L2-L4 segment undergoing upper body flexion, while the stress on the spinal rod is comparable. Choosing softer/stiffer springs would involve higher/lower loads on every component. CONCLUSIONS: ISO model predicts the effects of using both a full and a bridge posterior instrumentation. The study justifies the use of both conditions during in vitro reliability tests to achieve meaningful results easy to compare to clinically relevant loading modes and known in vivo failure modes.

Anterior support reduces the stresses on the posterior instrumentation after pedicle subtraction osteotomy: a finite-element study.

STUDY DESIGN: The investigation was based on finite-element simulations. OBJECTIVE: Pedicle subtraction osteotomy (PSO) is an effective but technical demanding surgical technique, associated with a high risk of rod failure. The present study aims at investigating the role of the anterior support in combination with PSO, with a numerical comparative analysis. METHODS: An osteotomy was simulated at the L3 level of a lumbosacral spine. An implantation of various combinations of devices for the anterior (1 or 2 cages of different material) and posterior stabilization (1 or 2 rods) was then performed. ROM, loads, and stresses acting on the rods were calculated. RESULTS: A 4-8% reduction of the ROM was obtained introducing one or two cages in the instrumented model. However, the anterior support had only a minor influence on the ROM. The load on the posterior instrumentation decreased up to 8% using one cage and about 15% with two anterior devices. A 20-30% reduction of the stresses on the rods was calculated inserting one cage and up to 50% using two cages. Following the introduction of the anterior support, the greatest stress reduction was observed in the model having two cages and spinal fixators with two rods. CONCLUSIONS: The use of cages is crucial to ensure anterior support and decrease loads and stresses on the posterior instrumentation.

Instrumentation failure following pedicle subtraction osteotomy: the role of rod material, diameter, and multi-rod constructs.

PURPOSE: Pedicle subtraction osteotomy (PSO) has a complication rate noticeably higher than other corrective surgical techniques used for the treatment of spinal sagittal imbalance. In particular, rod breakage and pseudoarthrosis remain burning issues of this technique. Goal of this study was to investigate the biomechanical performance of several hardware constructs. METHODS: The study was performed using two validated finite element models of the lumbosacral spine (L1-S1) incorporating a PSO on L3 and L4, respectively. Both models were instrumented two levels above and below the osteotomy site. Different combinations of materials (Ti6Al4V and Cr-Co) and device configurations (bilateral single vs. double rod, rod diameters of 5 and 6 mm) were investigated. The loading was represented considering a force of 500 N (imposed along the spinal curvature and connecting the vertebral bodies) and pure moments of 7.5 Nm in flexion-extension, lateral bending and axial rotation. The results were evaluated in terms of range of motion (ROM), load, and stresses acting on the instrumentation. RESULTS: A comparable ROM was found for all the models. The simulations showed a different behavior of the devices: increasing the stiffness an 8-19% increase of the load was calculated on the rod. However, the stress on the instrumentation resulted higher on Cr-Co devices and on smaller rods. The highest stress reduction (up to 50%) was ensured using double rod constructs. CONCLUSIONS: The bilateral double parallel rods configuration resulted the best to reduce the stresses on the spinal fixators at the osteotomy site. However, the high loads acting on the rods with respect to the physiologic condition could slow down the bone healing at the osteotomy site.

Contact stresses, pressure and area in a fixed-bearing total ankle replacement: a finite element analysis.

BACKGROUND: Mobile-bearing ankle implants with good clinical results continued to increase the popularity of total ankle arthroplasty to address endstage ankle osteoarthritis preserving joint movement. Alternative solutions used fixed-bearing designs, which increase stability and reduce the risk of bearing dislocation, but with a theoretical increase of contact stresses leading to a higher polyethylene wear. The purpose of this study was to investigate the contact stresses, pressure and area in the polyethylene component of a new total ankle replacement with a fixed-bearing design, using 3D finite element analysis. METHODS: A three-dimensional finite element model of the Zimmer Trabecular Metal Total Ankle was developed and assembled based on computed tomography images. Three different sizes of the polyethylene insert were modeled, and a finite element analysis was conducted to investigate the contact pressure, the von Mises stresses and the contact area of the polyethylene component during the stance phase of the gait cycle. RESULTS: The peak value of pressure was found in the anterior region of the articulating surface, where it reached 19.8 MPa at 40% of the gait cycle. The average contact pressure during the stance phase was 6.9 MPa. The maximum von Mises stress of 14.1 MPa was reached at 40% of the gait cycle in the anterior section. In the central section, the maximum von Mises stress of 10.8 MPa was reached at 37% of the gait cycle, whereas in the posterior section the maximum stress of 5.4 MPa was reached at the end of the stance phase. DISCUSSION: The new fixed-bearing total ankle replacement showed a safe mechanical behavior and many clinical advantages. However, advanced models to quantitatively estimate the wear are need. CONCLUSION: To the light of the clinical advantages, we conclude that the presented prosthesis is a good alternative to the other products present in the market.

Preclinical evaluation of posterior spine stabilization devices: can the current standards represent basic everyday life activities?

PURPOSE: To discuss whether the available standard methods for preclinical evaluation of posterior spine stabilization devices can represent basic everyday life activities and how to compare the results obtained with different procedures. METHODS: A comparative finite element study compared ASTM F1717 and ISO 12189 standards to validated instrumented L2-L4 segments undergoing standing, upper body flexion and extension. The internal loads on the spinal rod and the maximum stress on the implant are analysed. RESULTS: ISO recommended anterior support stiffness and force allow for reproducing bending moments measured in vivo on an instrumented physiological segment during upper body flexion. Despite the significance of ASTM model from an engineering point of view, the overly conservative vertebrectomy model represents an unrealistic worst case scenario. A method is proposed to determine the load to apply on assemblies with different anterior support stiffnesses to guarantee a comparable bending moment and reproduce specific everyday life activities. CONCLUSIONS: The study increases our awareness on the use of the current standards to achieve meaningful results easy to compare and interpret.

Effect of intermediate ZrO2-CaO coatings deposited by cold thermal spraying on the titanium-porcelain bond in dental restorations.

STATEMENT OF PROBLEM: Metal ceramic systems are used for the majority of dental crowns and fixed dental prostheses. However, problems with porcelain bonding are encountered when titanium is used as the substrate. PURPOSE: The purpose of this study was to evaluate the effect of intermediate calcium oxide-stabilized zirconia (ZrO2-CaO) coatings deposited by cold thermal spraying on the titanium-porcelain bonding in dental restorations. MATERIAL AND METHODS: Two different types of ZrO2-CaO coatings obtained by oxyacetylene cold thermal spraying deposition were applied on commercially pure titanium bars before adding the porcelain layer. Type 1 was obtained by directly spraying the ZrO2-CaO powder on the titanium substrate. Type 2 was obtained by spraying a bond coat of nickel-aluminum-molybdenum alloy before spraying the ZrO2-CaO powder. Three-point bend tests according to International Organization of Standardization 9693-1:2012 were carried out to evaluate the debonding strength for the ZrO2-CaO-coated specimens (types 1 and 2) in comparison with a noncoated group (control), which received a traditional bonder-based adhesive technique. The results were compared with ANOVA, followed by the Student-Newman-Keuls test for pairwise comparisons. Scanning electron microscopy and energy dispersion spectroscopy were used to examine the interfacial properties and the failure mode of each group. RESULTS: Mean (±standard deviation) debonding strength values for type 1 coating (25.97 ±2.53 MPa) and control (23.51 ±2.94 MPa) were near the acceptable lower limit of 25 MPa indicated by the International Organization of Standardization 9693-1:2012 and were not significantly different (Student-Newman-Keuls test, P>.05). Type 2 coating produced an improved titanium-porcelain bonding (debonding strength=39.47 ±4.12 MPa), significantly higher than both type 1 (Student-Newman-Keuls test, P<.05) and control (Student-Newman-Keuls test, P<.05). Scanning electron microscopy-energy dispersion spectroscopy analysis confirmed these findings, which revealed a predominant cohesive failure mode for type 2. CONCLUSIONS: An intermediate coating layer of ZrO2-CaO plus a substrate of bonding nickel-aluminum-molybdenum alloy applied by oxyacetylene cold thermal spraying deposition provided an improved titanium-porcelain bond.

The role of collagen and crystallinity in the physicochemical properties of naturally derived bone grafts.

Xenografts are commonly used for bone regeneration in dental and orthopaedic domains to repair bone voids and other defects. The first-generation xenografts were made through sintering, which deproteinizes them and alters their crystallinity, while later xenografts are produced using cold-temperature chemical treatments to maintain the structural collagen phase. However, the impact of collagen and the crystalline phase on physicochemical properties have not been elucidated. We hypothesized that understanding these factors could explain why the latter provides improved bone regeneration clinically. In this study, we compared two types of xenografts, one prepared through a low-temperature chemical process (Treated) and another subsequently sintered at 1100°C (Sintered) using advanced microscopy, spectroscopy, X-ray analysis and compressive testing. Our investigation showed that the Treated bone graft was free of residual blood, lipids or cell debris, mitigating the risk of pathogen transmission. Meanwhile, the sintering process removed collagen and the carbonate phase of the Sintered graft, leaving only calcium phosphate and increased mineral crystallinity. Microcomputed tomography revealed that the Treated graft exhibited an increased high porosity (81%) and pore size compared to untreated bone, whereas the Sintered graft exhibited shrinkage, which reduced the porosity (72%), pore size and strut size. Additionally, scanning electron microscopy displayed crack formation around the pores of the Sintered graft. The Treated graft displayed median mechanical properties comparable to native cancellous bone and clinically available solutions, with an apparent modulus of 166 MPa, yield stress of 5.5 MPa and yield strain of 4.9%. In contrast, the Sintered graft exhibited a lower median apparent modulus of 57 MPa. It failed in a brittle manner at a median stress of 1.7 MPa and strain level of 2.9%, demonstrating the structural importance of the collagen phase. This indicates why bone grafts prepared through cold-temperature processes are clinically favourable.

Does soccer cleat design influence the rotational interaction with the playing surface?

Non-contact injuries in soccer players may be related to the interplay between cleat type and playing surface, and bladed shoes were often blamed for non-contact injuries with no research support. The aim of this study was to compare the rotational resistance (stiffness and peak sustainable torque) among three types of soccer cleats (metal studs, molded rubber studs, and bladed) in a controlled laboratory environment. The shoes were tested on both natural and artificial turfs under a compressive preload of 1000 N and with internal and external rotations. The three shoe models showed comparable performances with a good repeatability for each individual test on both playing surfaces. A less stiff behavior was observed for the natural turf. A tendency toward highest peak torque was observed in the studded model on natural surface. The bladed cleats provided peak torque and rotational stiffness comparable to the other models. Studded and bladed cleats did not significantly differ in their interaction with the playing surface. Therefore, soccer shoes with bladed cleats should not be banned in the context of presumed higher risk for non-contact injuries.

Stability and Instrumentation Stresses Among Sacropelvic Fixation Techniques With Novel Porous Fusion/Fixation Implants: A Finite Element Study.

BACKGROUND: Sacropelvic fixation is frequently combined with thoracolumbar instrumentation for correcting spinal deformities. This study aimed to characterize sacropelvic fixation techniques using novel porous fusion/fixation implants (PFFI). METHODS: Three T10-pelvis finite element models were created: (1) pedicle screws and rods in T10-S1, PFFI bilaterally in S2 alar-iliac (S2AI) trajectory; (2) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, triangular implants bilaterally above the PFFI in a sacro-alar-iliac trajectory (PFFI-IFSAI); and (3) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, PFFI in sacro-alar-iliac trajectory stacked cephalad to those in S2AI position (2-PFFI). Models were loaded with pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation. Outputs were compared against 2 baseline models: (1) pedicle screws and rods in T10-S1 (PED), and (2) pedicle screws and rods in T10-S1, and S2AI screws. RESULTS: PFFI and S2AI resulted in similar L5-S1 motion; adding another PFFI per side (2-PFFI) further reduced this motion. Sacroiliac joint (SIJ) motion was also similar between PFFI and S2AI; PFFI-IFSAI and 2-PFFI demonstrated a further reduction in SIJ motion. Additionally, PFFI reduced max stresses on S1 pedicle screws and on implants in the S2AI position. CONCLUSION: The study shows that supplementing a long construct with PFFI increases the stability of the L5-S1 and SIJ and reduces stresses on the S1 pedicle screws and implants in the S2AI position. CLINICAL RELEVANCE: The findings suggest a reduced risk of pseudarthrosis at L5-S1 and screw breakage. Clinical studies may be performed to demonstrate applicability to patient outcomes. LEVEL OF EVIDENCE: Not applicable (basic science study).

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.

Effect of Sacropelvic Hardware on Axis and Center of Rotation of the Sacroiliac Joint: A Finite Element Study.

BACKGROUND: The sacroiliac joint (SIJ) transfers the load of the upper body to the lower extremities while allowing a variable physiological movement among individuals. The axis of rotation (AoR) and center of rotation (CoR) of the SIJ can be evaluated to analyze the stability of the SIJ, including when the sacrum is fixed. The purpose of this study was to determine how load intensity affects the SIJ for the intact model and to characterize how sacropelvic fixation performed with different techniques affects this joint. METHODS: Five T10-pelvis models were used: (1) intact model; (2) pedicle screws and rods in T10-S1; (3)pedicle screws and rods in T10-S1, and bilateral S2 alar-iliac screws (S2AI); (4) pedicle screws and rods in T10-S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory ; and (5) pedicle screws and rods in T10-S1, bilateral S2AI screws, and 2 bilateral triangular implants inserted in a lateral trajectory. Outputs of these models under flexion-extension were compared: AoR and CoR of the SIJ at incremental steps from 0 to 7.5 Nm for the intact model and AoR and CoR of the SIJ for the instrumented models at 7.5 Nm. RESULTS: The intact model was validated against an in vivo study by comparing range of motion and displacement of the sacrum. Increasing the load intensity for the intact model led to an increase of the rotation of the sacrum but did not change the CoR. Comparison among the instrumented models showed that sacropelvic fixation techniques reduced the rotation of the sacrum and stabilized the SIJ, in particular with triangular implants. CONCLUSION: The study outcomes suggest that increasing load intensity increases the rotation of the sacrum but does not influence the CoR, and use of sacropelvic fixation increases the stability of the SIJ, especially when triangular implants are employed. CLINICAL RELEVANCE: The choice of the instrumentation strategy for sacropelvic fixation affects the stability of the construct in terms of both range of motion and axes of rotation, with direct consequences on the risk of failure and mobilization. Clinical studies should be performed to confirm these biomechanical findings.

Global stiffness and residual stresses in spinal fixator systems: A validated finite element study on the interconnection mechanism.

Posterior spinal fixation systems are the gold standard to treat different column disorders using rods and screws. The proper connection between them is guaranteed by the Interconnection Mechanism (IM), consisting of different metallic subcomponents held together through the application of tightening torque. The response of the fixation system is defined by its overall stiffness, which in turn is governed by the local residual stress field arising during tightening. Although literature computational models for studying spinal fixation are becoming increasingly anatomically complex, most studies disregard completely the realistic modeling of the IM, namely choosing elastic-plastic material models and proper contact interactions. In this frame, the present study aims at increasing awareness in the field of spinal fixation modeling by investigating the mechanical response of the IM in terms of overall stiffness and local residual stresses. Once validated through dedicated experiments, the results of the proposed model have been compared with the current literature, highlighting the key role of the IM in the reliable modeling of spinal fixation.

High-Demand Spinal Deformity With Multi-Rod Constructs and Porous Fusion/Fixation Implants: A Finite Element Study.

STUDY DESIGN: Basic science (finite element analysis). OBJECTIVES: Pedicle subtraction osteotomy (PSO) at L5 is an effective treatment for sagittal imbalance, especially in select cases of patients showing kyphosis with the apex at L4-L5 but has been scarcely investigated. The aim of this study was to simulate various "high-demand" instrumentation approaches, including varying numbers of rods and sacropelvic implants, for the stabilization of a PSO at L5. METHODS: A finite element model of T10-pelvis was modified to simulate posterior fixation with pedicle screws and rods from T10 to S1, alone or in combination with an L5 PSO. Five additional configurations were then created by employing rods and novel porous fusion/fixation implants across the sacroiliac joints, in varying numbers. All models were loaded using pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation. RESULTS: The osteotomy resulted in a general increase in motion and stresses in posterior rods and S1 pedicle screws. When the number of rods was varied, three- and four-rod configurations were effective in limiting the maximal rod stresses; values approached those of posterior fixation with no osteotomy. Maximum stresses in the accessory rods were similar to or less than those observed in the primary rods. Multiple sacropelvic implants were effective in reducing range of motion, particularly of the SIJ. CONCLUSIONS: Multi-rod constructs and sacropelvic fixation generally reduced maximal implant stresses and motion in comparison with standard posterior fixation, suggesting a reduced risk of rod breakage and increased joint stability, respectively, when a high-demand construct is utilized for the correction of sagittal imbalance.