Using inverse finite element analysis to identify spinal tissue behaviour in situ.

In computational modelling of musculoskeletal applications, one of the critical aspects is ensuring that a model can capture intrinsic population variability and not only representative of a "mean" individual. Developing and calibrating models with this aspect in mind is key for the credibility of a modelling methodology. This often requires calibration of complex models with respect to 3D experiments and measurements on a range of specimens or patients. Most Finite Element (FE) software's do not have such a capacity embedded in their core tools. This paper presents a versatile interface between Finite Element (FE) software and optimisation tools, enabling calibration of a group of FE models on a range of experimental data. It is provided as a Python toolbox which has been fully tested and verified on Windows platforms. The toolbox is tested in three case studies involving in vitro testing of spinal tissues.

The correlation between skeletal muscle index of the L3 vertebral body and malnutrition in patients with advanced lung cancer.

BACKGROUND: Studies have shown that the skeletal muscle index at the third lumbar vertebra (L3 SMI) had reasonable specificity and sensitivity in nutritional assessment and prognostic prediction in digestive system cancers, but its performance in lung cancer needs further investigation. METHODS: A retrospective study was performed on 110 patients with advanced lung cancer. The L3 SMI, the Patient-Generated Subjective Global Assessment score (PG-SGA score), body mass index (BMI), and serological indicators were analyzed. According to PG-SGA scores, patients were divided into severe malnutrition (≥9 points), mild to moderate malnutrition (≥3 points and ≤ 8 points), and no malnutrition (≤2 points) groups. Pearson correlation and logistic regression analysis were adopted to find factors related to malnutrition, and a forest plot was drawn. The receiver operating characteristic (ROC) curve was performed to compare the diagnostic values of malnutrition among factors, which were expressed by the area under curve (AUC). RESULTS: 1. The age of patients in the severe malnutrition group, the mild to moderate malnutrition group, and the no malnutrition group significantly differed, with mean ages of 63.46 ± 10.01 years, 60.42 ± 8.76 years, and 55.03 ± 10.40 years, respectively (OR = 1.062, 95%CI: 1.008 ~ 1.118, P = 0.024; OR = 1.100, 95%CI: 1.034 ~ 1.170, P = 0.002). Furthermore, the neutrophil to lymphocyte ratio (NLR) of the severe malnutrition group was significantly higher than that of the no malnutrition group, with statistical significance. The difference between the mild to moderate malnutrition group and the no malnutrition group were not statistically significant, with NLR of 4.07 ± 3.34 and 2.47 ± 0.92, respectively (OR = 1.657,95%CI: 1.036 ~ 2.649, P = 0.035). The L3 SMI of patients in the severe malnutrition and mild to moderate malnutrition groups were significantly lower than that of the patients in the no malnutrition group, with statistical significance. The L3 SMI of patients in the severe malnutrition group, mild to moderate malnutrition group, and no malnutrition group were 27.40 ± 4.25 cm2/m2, 38.19 ± 6.17 cm2/m2, and 47.96 ± 5.02 cm2/m2, respectively (OR = 0.600, 95%CI: 0.462 ~ 0.777, P < 0.001; OR = 0.431, 95%CI: 0.320 ~ 0.581, P < 0.001). 2. The Pearson correlation analysis showed that the PG-SGA score positively correlated with age (r = 0.296, P < 0.05) but negatively correlated with L3 SMI (r = - 0.857, P < 0.05). The L3 SMI was also negatively correlated with age (r = - 0.240, P < 0.05). 3. The multivariate analysis showed that the L3 SMI was an independent risk factor for malnutrition (OR = 0.446, 95%CI: 0.258 ~ 0.773, P = 0.004; OR = 0.289, 95%CI: 0.159 ~ 0.524, P < 0.001). CONCLUSION: 1. The differences in the L3 SMI was statistically significant among advanced lung cancer patients with different nutritional statuses. 2. In the nutritional assessment of patients with lung cancer, the L3 SMI was consistent with the PG-SGA. 3. The L3 SMI is an independent predictor of malnutrition in patients with advanced lung cancer.

Trabecular Architecture and Mechanical Heterogeneity Effects on Vertebral Body Strength.

PURPOSE OF REVIEW: We aimed to synthesize the recent work on the intra-vertebral heterogeneity in density, trabecular architecture and mechanical properties, its implications for fracture risk, its association with degeneration of the intervertebral discs, and its implications for implant design. RECENT FINDINGS: As compared to the peripheral regions of the centrum, the central region of the vertebral body exhibits lower density and more sparse microstructure. As compared to the anterior region, the posterior region shows higher density. These variations are more pronounced in vertebrae from older persons and in those adjacent to degenerated discs. Mixed results have been reported in regard to variation along the superior-inferior axis and to relationships between the heterogeneity in density and vertebral strength and fracture risk. These discrepancies highlight that, first, despite the large amount of study of the intra-vertebral heterogeneity in microstructure, direct study of that in mechanical properties has lagged, and second, more measurements of vertebral loading are needed to understand how the heterogeneity affects distributions of stress and strain in the vertebra. These future areas of study are relevant not only to the question of spine fractures but also to the design and selection of implants for spine fusion and disc replacement. The intra-vertebral heterogeneity in microstructure and mechanical properties may be a product of mechanical adaptation as well as a key determinant of the ability of the vertebral body to withstand a given type of loading.

A novel approach to evaluate the effects of artificial bone focal lesion on the three-dimensional strain distributions within the vertebral body.

The spine is the first site for incidence of bone metastasis. Thus, the vertebrae have a high potential risk of being weakened by metastatic tissues. The evaluation of strength of the bone affected by the presence of metastases is fundamental to assess the fracture risk. This work proposes a robust method to evaluate the variations of strain distributions due to artificial lesions within the vertebral body, based on in situ mechanical testing and digital volume correlation. Five porcine vertebrae were tested in compression up to 6500N inside a micro computed tomography scanner. For each specimen, images were acquired before and after the application of the load, before and after the introduction of the artificial lesions. Principal strains were computed within the bone by means of digital volume correlation (DVC). All intact specimens showed a consistent strain distribution, with peak minimum principal strain in the range -1.8% to -0.7% in the middle of the vertebra, demonstrating the robustness of the method. Similar distributions of strains were found for the intact vertebrae in the different regions. The artificial lesion generally doubled the strain in the middle portion of the specimen, probably due to stress concentrations close to the defect. In conclusion, a robust method to evaluate the redistribution of the strain due to artificial lesions within the vertebral body was developed and will be used in the future to improve current clinical assessment of fracture risk in metastatic spines.

Three-dimensional distribution of CT attenuation in the lumbar spine pedicle wall.

This study investigated in vivo the three-dimensional distribution of CT attenuation in the lumbar spine pedicle wall measured in Hounsfield Unit (HU). Seventy-five volunteers underwent clinical lumbar spine CT scans. Data was analyzed with custom-written software to determine the regional variation in pedicle wall attenuation values. A cylindrical coordinate system oriented along the pedicle's long axis was used to calculate the pedicular wall attenuation distribution three-dimensionally and the highest attenuation value was identified. The pedicular cross-section was divided into four quadrants: lateral, medial, cranial, and caudal. The mean HU value for each quadrant was calculated for all lumbar spine levels (L1-5). The pedicle wall attenuation was analyzed by gender, age, spinal levels and anatomical quadrant. The mean HU values of the pedicle wall at L1 and L5 were significantly lower than the values between L2-4 in both genders and in both age groups. Furthermore, the medial quadrant showed higher HU values than the lateral quadrant at all levels and the caudal quadrant showed higher HU values at L1-3 and lower HU values at L4-5 than the cranial quadrant. These findings may explain why there is a higher incidence of pedicle screw breach in the pedicle lateral wall.

Sensitivity analysis of biomechanical effect in vertebral body of two different augmenters.

BACKGROUND: Transvertebral Bone Graft and Augmentation (TBGA) has achieved good clinical effects in the treatment of osteoporotic vertebral compression fractures (OVCFs). This study aimed to investigate the postoperatively biomechanical effects of TBGA and compare the biomechanical sensitivity of two different augmenters: a cylindrical enhancement device (CED) and bone cement. METHODS: Finite element models of the spine segment T11-L3 were created, including one model based on normal segment and the other three with L1 augmentation for pathological conditions. Three treatments were simulated including CED implant treatment A, CED implant treatment B, and bone cement treatment. The stress distribution and maximum displacement of the four models under different treatments were analyzed. A method of linear fitting of dummy variables was used to analyze the sensitivity of biomechanical parameters to the degree of osteoporosis (DO) and load. FINDINGS: The reduction of stress with increasing DO in augmented and adjacent vertebral bodies under bone cement augmentation was less than that under CED augmentation. The stress of augmented vertebral body and the adjacent vertebral body was most sensitive to extension and rotation loading conditions. As DO increasing, the bone cement augmentation significantly increased the stress level on the upper and lower endplates. INTERPRETATION: When the degree of osteoporosis increased, CED outperforms bone cement in terms of the stress reduction in augmented vertebral and adjacent vertebral, which could be beneficial for avoiding re-fracture. Using TBGA to treat OVCFs, especially with Plan B method, the condition of the pathological spine is closer to the original status in terms of the sensitivity to stress and the spinal range of motion. The TBGA treatment is sensitive to lateral bending and torsion, therefore patients should be advised to avoid high-risk motions like lateral bending and rotation.

The effect of vertebral body stapling on spine biomechanics and structure using a bovine model.

BACKGROUND: Adolescent idiopathic scoliosis is a common condition affecting 2.5% of the general population. Vertebral body stapling was introduced as a method of fusionless growth modulation for the correction of moderate idiopathic scoliosis (Cobb angles of 20-40°), and was claimed to be more effective than bracing and less invasive than fusion. The aim of this study was to assess the effect of vertebral body stapling on the stiffness of a thoracic motion segment unit under moment controlled load, and to assess the vertebral structural damage caused by the staples. METHODS: Thoracic spine motion segments from 6 to 8 week old calves (n=14) were tested in flexion/extension, lateral bending, and axial rotation. The segments were tested un-instrumented, then a left anterolateral intervertebral Shape Memory Alloy (SMA) staple was inserted and the test was repeated. Data were collected from the tenth load cycle of each sequence and stiffness was calculated. The staples were carefully removed and the segments were studied with micro-computed tomography to assess physical damage to the bony structure. Visual assessment of the vertebral bone structure on micro-CT was performed. FINDINGS: There was no change in motion segment stiffness in flexion/extension nor in axial rotation. There was a reduction in stiffness in lateral bending with 30% reduction bending away from the staple and 12% reduction bending towards the staple. Micro-CT showed physeal damage in all the specimens. INTERPRETATION: Intervertebral stapling using SMA staples cause a reduction in spine stiffness in lateral bending. They also cause damage to the endplate epiphyses.

Análisis morfométrico de pedículos torácicos en escoliosis idiopática del adolescente / Morphometric analysis of thoracic pedicles in adolescent idiopathic scoliosis

Existen pocos estudios relacionados con la morfometría del pedículo en la población latinoamericana con escoliosis idiopática del adolescente (EIA). El objetivo del trabajo es destacar las características morfométricas del pedículo en pacientes venezolanos con EIA; por lo que se realizó un estudio prospectivo. Los pacientes fueron sometidos a rastreo tomográfico computarizado (TC). El pedículo de la concavidad es ligeramente de mayor diámetro y con mayor angulación medial que el lado convexo, especialmente en la región apical de la curva escoliótica; la profundidad de inclusión al cuerpo vertebral era menor del lado convexo en comparación con el cóncavo. La anatomía del pedículo en pacientes con escoliosis muestra altas variaciones individuales y un estudio cuidadoso de la TC preoperatoria es esencial para la planificación de instrumentación transpedicular. Tornillos ligeramente más largos pueden ser introducidos en el lado de la concavidad en comparación con el lado convexo(AU)

There are few studies related to pedicle´s morphometry in Latin American population and adolescent idiopathic scoliosis (AIS). Objective was to highlight Venezuelan´s AIS morphometric characteristics in this prospective study. Computer Tomographic (CT) Scan was performed to patients. The pedicle of the concavity has a slightly larger diameter and greater angle than the convexity, particularly in the apical region of the scoliosis curve; vertebral body inclusion depth was lower on the convex side as compared with the concave side. Pedicle´s anatomy in patients with scoliosis shows high individual variations and a careful study of preoperative CT planning is essential for transpedicular instrumentation. Slightly longer screws can be introduced into the side of the concavity compared with the convex side, despite the difference in the depth of inclusion in the vertebral body is not statistically significant in most of the levels(AU)