Biomechanical Comparison of the Load-Sharing Capacity of High and Low Implant Density Constructs With Three Types of Pedicle Screws for the Instrumentation of Adolescent Idiopathic Scoliosis

Apical stress redistribution during anterior vertebral body tethering for thoracic adolescent idiopathic scoliosis: a finite element analysis of a novel surgical technique

PURPOSE

Apical stress redistribution (ASR) is proposed to mitigate failure risks after anterior vertebral body tethering for adolescent idiopathic scoliosis. It consists in releasing set-screws at peri-apical levels following curve tensioning to redistribute stresses within the construct. This study determines the biomechanical impact and curve correction obtained with ASR.

METHODS

Finite element models of anterior vertebral body tethering were constructed for three typical scoliotic patients with Lenke 1 curves. ASR was simulated by releasing tension on the cable at the level of the three apical set screws (i.e. untightening three consecutive periapical set screws), followed by retightening of the set screws without further tensioning. Cable tension, implant forces and spine geometry were compared before and after performing ASR.

RESULTS

Periapical cable tension decreased post-ASR, and ASR also reduced the maximum tensions proximally and distally. Postoperative disc height was similar between conventional and ASR approaches. Apical intervertebral disc stresses were shifted from concave to convex compression intra and postoperatively, with a similar pattern between the conventional and ASR techniques. The ASR technique achieved scoliotic curve corrections of 54%, 68%, and 79%, while the conventional technique resulted in corresponding corrections (54%, 68%, and 80%) for subjects 1, 2, and 3. The periapical coronal curves exhibited similar patterns.

CONCLUSION

ASR demonstrated promising apical cable and implant forces re-equilibrium compared to the conventional approach. This novel technique did not impair immediate and postoperative curve correction, while maintaining similar apical intervertebral stress distribution. ASR shows potential to modulate growth while reducing maximum cable tension infra- and supra-apical.

Biomechanical modeling and assessment of patient positioning to facilitate spinal deformity instrumentation

Finite element models (FEM) were built based on clinical documentation of five AIS surgical cases to simulate patient positioning and spinal instrumentation. Various patient positioning and instrumentation configurations were simulated, and the associated corrections and screw pull-out forces were analyzed. Patient prone-positioning resulted in Cobb angle reduction of over 5°. Vertical, caudal, and cephalad displacement of thoracic cushions had significant impact on thoracic kyphosis. Pelvic rotation through lower-limb extension/flexion had significant effect on lumbar lordosis. The validated FEM enabled simulations of patient positioning and spinal instrumentation. Patient positioning configurations had significant effects on deformity correction and screw pull-out forces.

Biomechanical Comparison of Corticopedicular Spine Fixation versus Pedicle Screw Fixation in a Lumbar Degenerative Spondylolisthesis Finite Element Analysis Model

OBJECTIVE

To compare the stability of a corticopedicular posterior fixation (CPPF) device with traditional pedicle screws for decompression and fusion in adult degenerative lumbar spondylolisthesis.

METHODS

Finite element analysis (FEA) was used in a validated model of grade 1 L4-L5 spondylolisthesis to compare segmental stability after laminectomy alone, laminectomy with pedicle screw fixation, or laminectomy with CPPF device fixation. A 500-N follower load was applied to the model and different functional movements were simulated by applying a 7.5-Nm force in different directions. Outcomes included degrees of motion, tensile forces experienced in the CPPF device, and stresses in surrounding cortical bone.

RESULTS

At maximum loading, laminectomy alone demonstrated a 1° increase in flexion range of motion, from 6.35° to 7.39°. Laminectomy with pedicle screw fixation and CPPF device fixation both reduced spinal segmental motion to ≤1° at maximum loading in all ranges of motion, including flexion (0.94° and 1.09°), extension (-0.85° and -1.08°), lateral bending (-0.56° and -0.96°), and torsion (0.63° and 0.91°), respectively. There was no significant difference in segmental stability between pedicle screw fixation and CPPF device fixation during maximum loading, with a difference of ≤0.4° in any range of motion. Tensile forces in the CPPF device remained ≤51% the ultimate load to failure (487 N) and stress in surrounding cortical bone remained ≤84% the ultimate stress of cortical bone (125.4 MPa) during maximum loading.

CONCLUSIONS

CPFF fixation demonstrated similar segmental stability to traditional pedicle screw fixation whereas tensile forces and stress in surrounding cortical bone remained below the load to failure.

Effectiveness of Halo-Pelvic Traction and Thoracoplasty for Pulmonary Artery Pressure and Cardiopulmonary Function in Patients With Severe Spinal Deformity

STUDY DESIGN

Retrospective review.

OBJECTIVE

To evaluate the effectiveness of halo-pelvic traction and thoracoplasty for pulmonary artery pressure (PAP) and cardiopulmonary function in patients with severe spinal deformity.

SUMMARY OF BACKGROUND DATA

The effect of severe spinal deformity on pulmonary arterial hypertension, cardiac structure, and function has received little attention before.

PATIENTS AND METHODS

A total of 21 patients with severe spinal deformity were included in our study; all patients were examined by echocardiography and pulmonary function test before and after treatment. The correlations between PAP and pulmonary function were examined using Pearson correlation analysis.

RESULTS

The PAP decreased from 58.67 ± 20.24 to 39.00 ± 12.51 mm Hg, and the PAP of 42.86% of the patients returned to normal after treatment. Right cardiac enlargement, left ventricular diastolic function, and pulmonary function were improved at the same time. The ratio of left ventricular to right ventricular diameter returned to normal. Moderate correlations (correlation coefficient: -0.513 to -0.559) between PAP and forced vital capacity and forced expiratory volume in the first second were identified.

CONCLUSIONS

Pulmonary arterial hypertension, ventricular diastolic function, and pulmonary function were improved after halo-pelvic traction and thoracoplasty. A moderate negative correlation was identified between PAP and pulmonary function: the more pulmonary function improved, the more PAP decreased.

Finite element analysis comparing a PEEK posterior fixation device versus pedicle screws for lumbar fusion

BACKGROUND

Pedicle screw loosening and breakage are common causes of revision surgery after lumbar fusion. Thus, there remains a continued need for supplemental fixation options that offer immediate stability without the associated failure modes. This finite element analysis compared the biomechanical properties of a novel cortico-pedicular posterior fixation (CPPF) device with those of a conventional pedicle screw system (PSS).

METHODS

The CPPF device is a polyetheretherketone strap providing circumferential cortical fixation for lumbar fusion procedures via an arcuate tunnel. Using a validated finite element model, we compared the stability and load transfer characteristics of CPPF to intact conditions under a 415 N follower load and PSS conditions under a 222 N preload. Depending on the instrumented levels, two different interbody devices were used: a lateral lumbar interbody device at L4-5 or an anterior lumbar interbody device at L5-S1. Primary outcomes included range of motion of the functional spinal units and anterior load transfer, defined as the total load through the disk and interbody device after functional motion and follower load application.

RESULTS

Across all combinations of interbody devices and lumbar levels evaluated, CPPF consistently demonstrated significant reductions in flexion (ranging from 90 to 98%), extension (ranging from 88 to 94%), lateral bending (ranging from 75 to 80%), and torsion (ranging from 77 to 86%) compared to the intact spine. Stability provided by the CPPF device was comparable to PSS in all simulations (range of motion within 0.5 degrees for flexion-extension, 0.6 degrees for lateral bending, and 0.5 degrees for torsion). The total anterior load transfer was higher with CPPF versus PSS, with differences across all tested conditions ranging from 128 to 258 N during flexion, 89-323 N during extension, 135-377 N during lateral bending, 95-258 N during torsion, and 82-250 N during standing.

CONCLUSION

Under the modeled conditions, cortico-pedicular fixation for supplementing anterior or lateral interbody devices between L4 and S1 resulted in comparable stability based on range of motion measures and less anterior column stress shielding based on total anterior load transfer measures compared to PSS. Clinical studies are needed to confirm these finite element analysis findings.

Corrective Mechanism Aftermath Surgical Treatment of Spine Deformity due to Scoliosis: A Systematic Review of Finite Element Studies

This paper presents a systematic study in reviewing the application of finite element method for the analysis of correction mechanism of spine deformity due to scoliosis. The study is aimed at systematically (1) reviewing the use of finite element analysis in spine deformity case, (2) reviewing the modelling of pedicle screw and rod system in scoliosis surgery, and (3) analysing and discussing gap between the studies. Using the restricted key phrases, the review gathered studies from 2001 to 2021 from various electronic databases (Scopus, ScienceDirect, PubMed, Medline, and WorldCAT). Studies were included if they reported a finite element study on spine deformity. Studies that did not fully disclose their methodology and results had significant discrepancies, not published in English or not yet published were all disqualified. Regardless of inconsistencies in the methodological design of the studies, the quality of all papers was above the acceptable level. A total of fifteen manuscripts were considered for inclusion and were given a comprehensive review. This study indicates that analysing the forces acting on the spine, as well as the interrelationship between the force, stress, and degree of correction (which measured as the Cobb angle), could help to improve the corrective mechanism procedure of spine deformity. Pedicle screws and its placement strategies are also important as it influence the corrective forces for scoliosis treatment. Hence, the findings of this study could potentially be used as a guidance to develop a reliable finite element analysis that can predict the biomechanics responses during the corrective spine deformity treatment.

Coordination Between Trunk Muscles, Thoracolumbar Fascia, and Intra-Abdominal Pressure Toward Static Spine Stability

STUDY DESIGN

Numerical in-silico human spine stability finite element analysis.

OBJECTIVE

The purpose of this study was to investigate the contribution of major torso tissues toward static spine stability, mainly the thoracolumbar fascia (TLF), abdominal wall with its intra-abdominal pressure (IAP), and spinal muscles inclusive of their intramuscular pressure.

SUMMARY OF BACKGROUND DATA

Given the numerous redundancies involved in the spine, current methodologies for assessing static spinal stability are limited to specific tissues and could lead to inconclusive results. A three-dimensional finite element model of the spine, with structured analysis of major torso tissues, allows for objective investigation of static spine stability.

METHODS

A novel previously fully validated spine model was employed. Major torso tissues, mainly the muscles, TLF, and IAP were individually, and in combinations, activated under a 350N external spine perturbation. The stability contribution exerted by these tissues, or their ability to restore the spine to the unperturbed position, was assessed in different case-scenarios.

RESULTS

Individual activations recorded significantly different stability contributions, with the highest being the TLF at 75%. Combined or synergistic activations showed an increase of up to 93% stability contribution when all tissues were simultaneously activated with a corresponding decrease in the tensile load exerted by the tissues themselves.

CONCLUSION

This investigation demonstrated torso tissues exhibiting different roles toward static spine stability. The TLF appeared able to dissipate and absorb excessive loads, the muscles acted as antagonistic to external perturbations, and the IAP played a role limiting movement. Furthermore, the different combinations explored suggested an optimized engagement and coordination between different tissues to achieve a specific task, while minimizing individual work.Level of Evidence: N/A.

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.

Implant-Related Complications Using Uniaxial Implants In Pediatric Spinal Deformity Surgery

INTRODUCTION

 The successful surgical treatment of paediatric spinal deformity relies on robust anchors to achieve correction. Uniaxial pedicle screws are designed with articulation between the screw head and screw shaft, thus reducing the risk of anchor failure whilst permitting corrective manoeuvres. The purpose of this study was to describe the incidence, nature, and chronology of implant-related complications in pediatric spinal deformity treated with uniaxial pedicle screws.

METHODS

A retrospective radiographic analysis was carried out on paediatric patients treated for spinal deformity with more than two years of follow-up. Each was treated with posterior instrumented spinal fusion (PISF) using a uniaxial pedicle screw system by a single surgeon at a single institution. Surgical records, post-operative radiographs, and follow-up documentation were scrutinised for details of the implants used, implant failure, and revision procedures.

RESULTS

Three hundred and eighty-nine eligible patients with a mean follow-up of 3.3 years were identified. The mean anchor density was 1.7. Seven implant complications were observed. Early complications (<12 months) occurred in four cases and late (>12 months) in three cases. None of the early complications were associated with non-union. Two early and two late complications required revision surgery to manage implant failure and non-union. Patients who underwent fusion across the lumbosacral junction showed a higher than expected rate of implant-related complication (P=0.02).

CONCLUSION

 This study shows that there is a rate of implant-related failure of 1.8% after PISF with uniaxial implants in pediatric spinal deformities. There is a distinction between early and late implant-related complications, with early failure being due to loss of construct integrity, whereas late failure is due to pseudarthrosis and construct fatigue.

The importance of curve severity, type and instrumentation strategy in the surgical correction of adolescent idiopathic scoliosis: an in silico clinical trial on 64 cases

Adolescent idiopathic scoliosis is a three-dimensional deformity of the spine which is frequently corrected with the implantation of instrumentation with generally good or excellent clinical results; mechanical post-operative complications such as implant loosening and breakage are however relatively frequent. The rate of complications is associated with a lack of consensus about the surgical decision-making process; choices about the instrumentation length, the anchoring implants and the degree of correction are indeed mostly based on personal views and previous experience of the surgeon. In this work, we performed an in silico clinical trial on a large number of subjects in order to clarify which factors have the highest importance in determining the risk of complications by quantitatively analysing the mechanical stresses and loads in the instrumentation after the correction maneuvers. The results of the simulations highlighted the fundamental role of the curve severity, also in its three-dimensional aspect, and of the instrumentation strategy, whereas the length of the fixation had a lower importance.

Sex-Specific Intubation Biomechanics: Intubation Forces Are Greater in Male Than in Female Patients, Independent of Body Weight

Background

Studies of head, neck, and cervical spine morphology and tissue material properties indicate that cervical spine biomechanics differ between adult males and females. These differences result in sex-specific cervical spine kinematics and injury patterns in response to standardized loading conditions. Because direct laryngoscopy and endotracheal intubation require the application of a load to the cervical spine, intubation biomechanics should be sex-specific. The aim of this study was to determine if intubation forces during direct laryngoscopy differ between male and female patients and, if so, is the difference independent of body weight.

Methods

We pooled original data from three previously published adult clinical intubation studies that used methodologically reliable intubation force measurements (measured total laryngoscope force applied to the tongue, and force values were insensitive to or accounted for other laryngoscope blade forces). All patients had undergone direct laryngoscopy and orotracheal intubation with a Macintosh 3 blade under general anesthesia. Patient data included sex, age, height, weight, and maximum intubation force. Least squares multivariable linear regression was performed between the dependent variable (maximum intubation force) and two independent variables (patient sex and patient weight). A third term was added for the interaction between patient sex and weight.

Results

Among all patients (males n=42, females n=59), the median intubation force was 42.2 N (25^th to 75^th percentiles: 31.5 to 57.4 N). While controlling for patient body weight, intubation force differed between the sexes; P=0.011, with greater intubation force in male patients. While controlling for patient sex, there was a positive association between patient body weight and intubation force; P=0.009. In addition, there was a significant interaction between patient sex and weight; P=0.002, with intubation force in male patients having greater dependence on body weight. The difference in intubation force between male and female patients who had the same body weight exceeded 5 N when body weight exceeded 75 kg, and intubation force differences between male and female patients increased as patient body weight increased. Additional analyses using robust regression and using body mass index instead of weight provided comparable results.

Conclusion

In adult patients, the biomechanics of direct laryngoscopy and intubation are sex-specific. Our findings support the need to control for patient sex and weight in future clinical and laboratory studies of the human cervical spine and head and neck biomechanics.

Recent Trends, Technical Concepts and Components of Computer-Assisted Orthopedic Surgery Systems: A Comprehensive Review

Computer-assisted orthopedic surgery (CAOS) systems have become one of the most important and challenging types of system in clinical orthopedics, as they enable precise treatment of musculoskeletal diseases, employing modern clinical navigation systems and surgical tools. This paper brings a comprehensive review of recent trends and possibilities of CAOS systems. There are three types of the surgical planning systems, including: systems based on the volumetric images (computer tomography (CT), magnetic resonance imaging (MRI) or ultrasound images), further systems utilize either 2D or 3D fluoroscopic images, and the last one utilizes the kinetic information about the joints and morphological information about the target bones. This complex review is focused on three fundamental aspects of CAOS systems: their essential components, types of CAOS systems, and mechanical tools used in CAOS systems. In this review, we also outline the possibilities for using ultrasound computer-assisted orthopedic surgery (UCAOS) systems as an alternative to conventionally used CAOS systems.