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

Biomechanical comparison of different rod-to-rod connectors to a conventional titanium- and cobalt chromium posterior spinal fixation system

Introduction

Several types of rod-to-rod connectors are available for the extension of spinal fixation systems. However, scientific literature regarding the mechanical performance of different rod-to-rod connector systems is lacking.

Research question

The goal of this study was to evaluate the mechanical characteristics of axial and lateral rod connectors in comparison to a conventional pedicle screw rod (titanium and cobalt chromium) construct.

Material and method

Six types of instrumentations were investigated in a standardized test model to quantify the mechanical differences: 1: titanium rod; 2: titanium rod with axial connector; 3: titanium rod with lateral connector; 4: cobalt chromium rod; 5: cobalt chromium rod with axial connector; 6: cobalt chromium rod with lateral connector. All groups were tested in static compression, static torsion and dynamic compression and statistically compared regarding failure load and stiffness.

Results

In static compression loading, the use of connectors increased the construct stiffness, but unaffected the yield load. The use of a cobalt chromium rod significantly increased by approximately 40% the yield load and stiffness in comparison to the titanium rod configurations. Under dynamic compression, a similar or higher fatigue strength for all tested groups in comparison to the titanium rod configuration was evaluated, with the exception of titanium rod with axial connector.

Conclusion

Biomechanically, using rod connectors is a secure way for the extension of a construct and is mechanically equal to a conventional screw rod construct. However, in clinical use, attention should be paid regarding placement of the connectors at high loaded areas.

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.

Continuous Rod Load Monitoring to Assess Spinal Fusion Status-Pilot In Vivo Data in Sheep

Background and Objectives: Spinal fusion is an effective and widely accepted intervention. However, complications such as non-unions and hardware failures are frequently observed. Radiologic imaging and physical examination are still the gold standards in the assessment of spinal fusion, despite multiple limitations including radiation exposure and subjective image interpretation. Furthermore, current diagnostic methods only allow fusion assessment at certain time points and require the patient's presence at the hospital or medical practice. A recently introduced implantable sensor system for continuous and wireless implant load monitoring in trauma applications carries the potential to overcome these drawbacks, but transferability of the principle to the spine has not been demonstrated yet. Materials and Methods: The existing trauma sensor was modified for attachment to a standard pedicle-screw-rod system. Two lumbar segments (L2 to L4) of one Swiss white alpine sheep were asymmetrically instrumented. After facetectomy, three sensors were attached to the rods between each screw pair and activated for measurement. The sheep was euthanized 16 weeks postoperatively. After radiological assessment the spine was explanted and loaded in flexion-extension to determine the range of motion of the spinal segments. Sensor data were compared with mechanical test results and radiologic findings. Results: The sensors measured physiological rod loading autonomously over the observation period and delivered the data daily to bonded smartphones. At euthanasia the relative rod load dropped to 67% of the respective maximum value for the L23 segment and to 30% for the L34 segment. In agreement, the total range of motion of both operated segments was lower compared to an intact reference segment (L23: 0.57°; L34: 0.49°; intact L45: 4.17°). Radiologic assessment revealed fusion mass in the facet joint gaps and bilateral bridging bone around the joints at both operated segments. Conclusions: Observations of this single-case study confirm the basic ability of continuous rod load measurement to resolve the spinal fusion process as indicated by a declining rod load with progressing bone fusion. A strong clinical potential of such technology is eminent, but further data must be collected for final proof of principle.

Partial Threading of Pedicle Screws in a Standard Construct Increases Fatigue Life: A Biomechanical Analysis

This study proposed a pedicle screw design where the proximal 1/3 of the screw is unthreaded to improve fixation in posterior spinal surgery. This design was also expected to reduce the incidence of mechanical failure often observed when an unsupported screw length is exposed outside the vertebra in deformed or degenerated segments. The aim of this study was to evaluate the fatigue life of the novel pedicle screw design using finite element analysis and mechanical testing in a synthetic spinal construct in accordance with American Society for Testing and Materials (ASTM) F1717. The following setups were evaluated: (i) pedicle screw fully inserted into the test block (EXP-FT-01 and EXP-PU-01; full thread (FT), proximal unthread (PU)) and (ii) pedicle screw inserted but leaving an exposed shaft length of 7.6 mm (EXP-FT-02 and EXP-PU-02). Corresponding finite element models FEM-FT-01, FEM-FT-02, FEM-PU-01, and FEM-PU-02 were also constructed and subjected to the same loading conditions as the experimental groups. The results showed that under a 220 N axial load, the EXP-PU-01 group survived the full 5 million cycles, the EXP-PU-02 group failed at 4.4 million cycles on average, and both EXP-FT-01 and EXP-FT-02 groups failed after less than 1.0 million cycles on average, while the fatigue strength of the EXP-FT-02 group was the lowest at 170 N. The EXP-FT-01 and EXP-FT-02 constructs failed through fracture of the pedicle screw, but a rod fractured in the EXP-PU-02 group. In comparison to the FEM-FT-01 model, the maximum von Mises stress on the pedicle screw in the FEM-PU-01 and FEM-PU-02 models decreased by −43% and −27%, respectively. In conclusion, this study showed that having the proximal 1/3 of the pedicle screw unthreaded can reduce the risk of screw fatigue failure when used in deformed or degenerated segments.

Biomechanical characterization of vertebral body replacement in situ: Effects of different fixation strategies

BACKGROUND AND OBJECTIVE

Artificial vertebral implant with a lateral or posterior screw-rod fixation system are usually employed in lumbar reconstruction surgery to rebuild the lumbar spine after partial resection due to a tumor or trauma. However, few studies have investigated the effect of the various fixation systems on the biomechanics of the reconstructed lumbar system. This study aims to evaluate the influence of different surgical fixation strategies on the biomechanical performance of a reconstructed lumbar spine system in terms of the strength and long-term stability.

METHODS

Two typical lumbar spine reconstruction case models that correspond to lateral or posterior fixation systems were built based on the clinical data. Finite element analyses were performed, and comparisons were made between the two models based on the predicted stress distribution of the reconstructed lumbar spine model, bone-growth area of the endplate, and the range of motion under various normal daily activities.

RESULTS

The load from the upper vertebral body was found to be effectively transmitted onto the lower vertebral body by a vertebral implant with the lateral fixation system; this was favorable for bone growth after surgery. However, significantly high stresses were concentrated around the interaction region between the screws and bone, owing to the uneven lateral fixation structure; this may increase the risk of bone fractures and screw loosening in the long term. For the posterior fixation case, stably posterior fixation structure was favorable to maintain stability for the reconstructed lumbar spine. However, the load was mainly transmitted via the fixation rod rather than the vertebral implant, owing to the stress shielding effect. Therefore, the predicted strain on the endplate were insufficient for bone ingrowth under most of the spinal activates, which could cause bone loss and prosthesis loosening.

CONCLUSIONS

In this study, the comparisons of the reconstructed lumbar spine system with lateral and posterior fixation strategies were conducted. The Pros and Cons of these two fixation strategies was deeply discussed and the associated clinical issues were provided. The results of this study will have a clear impact in understanding the biomechanics of the lumbar spine with different fixation strategies and providing necessary instructions to the design and application of the lumbar spinal fixation system.

The 'armed concrete' approach: stent-screw-assisted internal fixation (SAIF) reconstructs and internally fixates the most severe osteoporotic vertebral fractures

BACKGROUND

The treatment of severe osteoporotic vertebral compression fractures (VCFs) with middle-column (MC) involvement, high fragmentation, large cleft and/or pedicular fracture is challenging. Minimally invasive 'stent-screw-assisted internal fixation' (SAIF) can reduce the fracture, reconstruct the vertebral body (VB) and fix it to the posterior elements.

OBJECTIVE

To assess feasibility, safety, technical and clinical outcome of the SAIF technique in patients with severe osteoporotic VCFs.

METHODS

80 treated vertebrae were analyzed retrospectively. Severe VCFs were characterized by advanced collapse (Genant grade 3), a high degree of osseous fragmentation (McCormack grade 2 and 3), burst morphology with MC injury, pediculo-somatic junction fracture, and/or large osteonecrotic cleft. VB reconstruction was evaluated on postprocedure radiographs and CT scans by two independent raters. Clinical and radiological follow-ups were performed at 1 and 6 months.

RESULTS

SAIF was performed at 28 thoracic and 52 lumbar levels in 73 patients. One transient neurological complication occurred. VB reconstruction was satisfactory in 98.8% of levels (inter-rater reliability 96%, κ=1). Follow-up at 1 month was available for 78/80 levels and at 6 months or later (range 6-24, mean 7.9 months) for 73/80 levels. Significant improvement in the Visual Analog Scale score was noted at 1 and 6 months after treatment (p<0.05). Patients reported global clinical benefit during follow-up (Patient's Global Impression of Change Scale 5.6±0.9 at 1 month and 6.1±0.9 at 6 months). Fourteen new painful VCFs occurred at different levels in 11 patients during follow-up, treated with vertebral augmentation or SAIF. Target-level stability was maintained in all cases.

CONCLUSIONS

SAIF is a minimally invasive, safe, and effective treatment for patients with severe osteoporotic VCFs with MC involvement.

Clinically relevant finite element technique based protocol to evaluate growing rods for early onset scoliosis correction

OBJECTIVE

The emergence of distraction-based growing rods has provided the means to reduce the progression of spinal deformity in early onset scoliosis (EOS). The current protocols for evaluating spinal implants (ie, ASTM-F1717 and ISO-12189) were developed for fusion/dynamic devices. These protocols do not feature long unsupported rod lengths subjected to distraction. Due to the unsuitability of the existing guidelines for the evaluation of growing rods, a new distraction-based finite element protocol is presented herein for the first time.

METHOD

A vertebrectomy (VO) model from current protocols was modified to accommodate multi-spinal segments (ie, MS model) in which springs with appropriate stiffness were simulated in between the plastic blocks. To assess the efficacy of the protocol, two different computational studies were conducted: (a) compression-bending (MS-CB) with no distraction, and (b) distraction followed by compression-bending (MS-D + CB). In each study, the model with no axial connector (rods-only) was modified to include a) 80-mm long tandem (LT) connectors, and b) side-by-side (SBS) connectors. Stiffness and yield loads were calculated as per ASTM-F1717 guidelines and compared with the corresponding VO models with no distraction. In the MS-D + CB models, distraction was applied at the top block, stretching the spring-block construct in a simulation of scoliosis surgery prior to locking the construct at the connector-rods' interface.

RESULTS

MS-CB models predicted higher stiffness and yield loads, compared to the VO models. The locking mechanism produced pre-existing stresses on the rod-connector interface, which caused a shift in the location of high-stress regions to the distraction site. Distraction led to a decrease in the construct's stiffness and yield load.

DISCUSSION

The proposed protocol enables the simulation of clinical parameters that are not feasible in the F1717 models and predicted stress patterns in the hardware consistent with observed clinical failures.

A novel dynamic fixation system with biodegradable components on lumbar fusion between articular processes in a canine model

The objective of this study was to design a novel dynamic fixation system with biodegradable components, apply it for lumbar fusion between articular processes and compare the fusion results and biomechanical changes to those of conventional rigid fixation. The novel dynamic fixation system was designed using a finite element model, stress distributions were compared and 24 mongrel dogs were randomly assigned to two groups and subjected to either posterior lumbar fusion surgery with a novel dynamic fixation system or titanium rods at the L5-L6 segments. Lumbar spines were assessed in both groups to detect radiographic, manual palpation and biomechanical changes. Histological examination was performed on organs and surrounding tissues. In the novel dynamic fixation system, stress was mainly distributed on the meshing teeth of the magnesium alloy spacer. The magnesium alloy components maintained their initial shape 8 weeks after the operation, but the meshing teeth were almost completely degraded at 16 weeks. The novel dynamic fixation system revealed an increased lateral bending range of motion at 8 weeks; however, both groups showed similar radiographic grades, fusion stiffness, manual palpation and histological results. The novel dynamic fixation system design is suitable, and its degradation in vivo is safe. The novel dynamic fixation system can be applied for posterior lumbar fusion between articular processes and complete the fusion like titanium rods.

Incomplete insertion of pedicle screws in a standard construct reduces the fatigue life: A biomechanical analysis

Pedicle screws are commonly used for posterior stabilization of the spine. When used in deformed or degenerated segments, the pedicle screws are often not fully inserted into the bone, but instead the threaded portion is exposed by 1 or 2 threads to accommodate rod placement and ensure alignment between the tulip of the screw and the rod. However, broken pedicle screws have been reported with the use of this method. The aim of this study was to determine how the fatigue life of the screw is affected by not fully inserting the screw into the bone. Spinal constructs were evaluated in accordance with ASTM F1717. The following three screw positions were subjected to compression bending fatigue loading; (i) pedicle screw fully inserted in the test block with no threads exposed (EXP-T0), (ii) pedicle screw inserted with one thread exposed outside the test block (EXP-T1), (iii) pedicle screw inserted with two threads exposed outside the test block (EXP-T2). Corresponding finite element models FEM-T0, FEM-T1 and FEM-T2 were also constructed and subjected to the same axial loading as the experimental groups to analyze the stress distribution in the pedicle screws and rods. The results showed that under a 190 N axial load, the EXP-T0 group survived the full 5 million cycles, the EXP-T1 group failed at 3.7 million cycles on average and the EXP-T2 groups failed at 1.0 million cycles on average, while the fatigue strength of both the EXP-T1 and EXP-T2 groups was 170 N. The constructs failed through fracture of the pedicle screw. In comparison to the FEM-T0 model, the maximum von Mises stress on the pedicle screw in the FEM-T1 and FEM-T2 models increased by 39% and 58%, respectively. In conclusion, this study demonstrated a drastic decrease in the fatigue life of pedicle screws when they are not full inserted into the plastic block.

Stent-Screw Assisted Internal Fixation of Osteoporotic Vertebrae: A Comparative Finite Element Analysis on SAIF Technique

Vertebral compression fractures are one of the most relevant clinical consequences caused by osteoporosis: one of the most common treatment for such fractures is vertebral augmentation through minimally invasive approaches (vertebroplasty or balloon-kyphoplasty). Unfortunately, these techniques still present drawbacks, such as re-fractures of the treated vertebral body with subsidence of the non-augmented portions or re-fracture of the non-augmented middle column at the junction with the augmented anterior column. A novel minimally-invasive augmentation technique, called Stent-Screw Assisted Internal Fixation, has been recently proposed for the treatment of severe osteoporotic and neoplastic fractures: this technique uses two vertebral body stents and percutaneous cannulated and fenestrated pedicular screws, through which cement is injected inside the expanded stents to achieve optimal stents' and vertebral body's filling. The role of the pedicle screws is to anchor the stents-cement complex to the posterior column, acting as a bridge across the middle column and preserving its integrity from possible collapse. In order to evaluate the potential of the new technique in restoring the load bearing capacity of the anterior and middle spinal columns and in reducing bone strains, a Finite Element model of an osteoporotic lumbar spine has been developed. Both standard vertebroplasty and Stent-Screw Assisted Internal Fixation have been simulated: simulations have been run taking into account everyday activities (standing and flexion) and comparison between the two techniques, in terms of strain distribution on vertebral endplates and posterior and anterior wall, was performed. Results show that Stent-Screw Assisted Internal Fixation significantly decrease the strain distribution on the superior EP and the cortical wall compared to vertebroplasty, possibly reducing the re-fracture risk of the middle-column at the treated level.

Biomechanical analysis of spinal implants with different rod diameters under static and fatigue loads: an experimental study

Spinal implants are commonly used in the treatment of spinal disorders or injuries. However, the biomechanical analyses of them are rarely investigated in terms of both biomechanical and clinical perspectives. Therefore, the main purpose of this study is to investigate the effects of rod diameter on the biomechanical behavior of spinal implants and to make a comparison among them. For this purpose, three spinal implants composed of pedicle screws, setscrews and rods, which were manufactured from Ti6Al4V, with diameters of 5.5 mm, 6 mm and 6.35 mm were used and a bilateral vertebrectomy model was applied to spinal systems. Then, the obtained spinal systems were tested under static tension-compression and fatigue (dynamic compression) conditions. Also, failure analyses were performed to investigate the fatigue behavior of spinal implants. After static tension-compression and fatigue tests, it was found that the yield loads, stiffness values, load carrying capacities and fatigue performances of spinal implants enhanced with increasing spinal rod diameter. In comparison to spinal implants with 5.5 mm rods, the fatigue limits of implants showed 13% and 33% improvements in spinal implants having 6 mm and 6.35 mm rods, respectively. The highest static and fatigue test results were obtained from spinal implants having 6.35 mm rods among the tested implants. Also, it was observed that the increasing yield load and stiffness values caused an increase in the fatigue limits of spinal implants.

Evaluation of iliac screw, S2 alar-iliac screw and laterally placed triangular titanium implants for sacropelvic fixation in combination with posterior lumbar instrumentation: a finite element study

PURPOSE

This study aimed to implement laterally placed triangular titanium implants as a technique of sacropelvic fixation in long posterior lumbar instrumentation and to characterize the effects of iliac screws, S2 alar-iliac screws and of triangular implants on rod and S1 pedicle screw stresses.

METHODS

Four female models of the lumbopelvic spine were created. For each of them, five finite element models replicating the following configurations were generated: intact, posterior fixation with pedicle screws to S1 (PED), with PED and iliac screws (IL), with PED and S2 alar-iliac (S2AI) screws, and with PED and bilateral triangular titanium implants (SI). Simulations were conducted in compression, flexion-extension, lateral bending and axial rotation. Rod stresses in the L5-S1 segment as well as in the S1 pedicle screws were compared.

RESULTS

One anatomical model was not simulated due to dysmorphia of the sacroiliac joints. PED resulted in the highest implant stresses. Values up to 337 MPa in lateral bending were noted, which were more than double than the other configurations. When compared with IL, S2AI and SI resulted in lower stresses in both screws and rods (on average 33% and 41% for S2AI and 17% and 50% for SI).

CONCLUSIONS

Implant stresses after S2AI and SI fixations were lower than those attributable to IL. Therefore, pedicle screws and rods may have a lower risk of mechanical failure when coupled with sacropelvic fixation via S2AI or triangular titanium implants, although the risk of clinical loosening remains an area of further investigation. These slides can be retrieved under Electronic Supplementary Material.

Toward the definition of a new worst-case paradigm for the preclinical evaluation of posterior spine stabilization devices

Mechanical reliability tests on posterior spine stabilization devices are based on standard F1717 by the American Society for Testing and Materials, which describes how to assemble the implant with vertebrae-like test blocks in a corpectomy model. A recent study proposed to revise the standard to describe the anatomical worst-case scenario, instead of the average one currently implemented, and introduce the unsupported screw length as a mechanical parameter. This article investigates the implications of such revisions on the endurance properties of an implant already on the market. Experimental fatigue tests demonstrate that the revision of F1717 standard leads to a reduction of 3.2 million cycles in the fatigue strength of the tested implant: this amount is comparable to the run-out number of cycles (5 million cycles) currently recommended. The numerical analysis, validated with static tests and strain gauges, supports the experimental findings and demonstrates that the stress on the implant may increase upon revision up to a 50% on the screw (most recurrent failure mode), with the unsupported screw length contributing alone up to 40%. The revision of ASTM F1717 standard would guarantee higher safety for the implant to test, potentially covering for a wider population of patients.