Experimental testing and biomechanical CT analysis of Chinese cadaveric vertebrae with different modeling approaches

Craniocaudal toggling increases the risk of screw loosening in osteoporotic vertebrae

BACKGROUND AND OBJECTIVE

Screw loosening remains a prominent problem for osteoporotic patients undergoing pedicle screw fixation surgeries but its underlying mechanisms are not fully understood. This study sought to examine the interactive effect of craniocaudal or axial cyclic loading (toggling) and osteoporosis on screw fixation.

METHODS

QCT-based finite element models of normal (n = 7; vBMD = 156 ± 13 mg/cm³) and osteoporotic vertebrae (n = 7; vBMD = 72 ± 6 mg/cm³) were inserted with pedicle screws and loaded with or without craniocaudal toggling. Among them, a representative normal vertebra (age: 55; BMD: 140 mg/cm³) and an osteoporotic vertebra (age: 64; BMD: 79 mg/cm³) were also loaded with or without axial toggling. The individual and interactive effects of craniocaudal toggling and osteoporosis on screw fixation strength (the force when the pull-up displacement of the screw head reached 1 mm) and bone tissue failure (characterized by equivalent plastic strain) were examined by repeated measure ANOVA.

RESULTS

A significant interactive effect between craniocaudal toggling and osteoporosis on screw fixation strength was detected (p = 0.008). Specifically, craniocaudal toggling led to a marked decrease in the fixation strength (68%, p < 0.05) and stiffness (83%, p < 0.05) only in the osteoporotic vertebrae but had no effect on screw fixation strength and stiffness of the normal vertebrae (p > 0.05). Likewise, most of the bone tissues around the screw in the osteoporotic vertebrae yielded following craniocaudal toggling whereas this result was not seen in the normal vertebrae. The axial toggling had no significant effect on bone tissue failure as well as pedicle screw fixation in normal or osteoporotic vertebrae.

CONCLUSIONS

Craniocaudal toggling substantially reduces the screw fixation strength of the osteoporotic vertebrae by progressively increasing tissue failure around the screw, and therefore may contribute to the higher rates of screw loosening in osteoporotic compared to normal patients, whereas axial toggling is not a risk factor for pedicle screw loosening in normal or osteoporotic patients.

Biomechanical MRI detects reduced bone strength in subjects with vertebral fractures

Vertebral fracture is one of the most serious consequences of osteoporosis. Estimation of vertebral strength from magnetic resonance imaging (MRI) scans may provide a new approach for the prediction of vertebral fractures. To that end, we sought to establish a biomechanical MRI (BMRI) method to compute vertebral strength and test its ability to distinguish fracture from non-fracture subjects. This case-control study included 30 subjects without vertebral fractures and 15 subjects with vertebral fractures. All subjects underwent MRI with a mDIXON-Quant sequence and quantitative computed tomography (QCT), from which proton fat fraction-based bone marrow adipose tissue (BMAT) content and volumetric bone mineral density (vBMD) were measured, respectively. Nonlinear finite element analysis was applied to MRI and QCT scans of L2 vertebrae to compute vertebral strength (BMRI- and BCT-strength). The differences in BMAT content, vBMD, BMRI-strength and BCT-strength between the two groups were examined by t-tests. Receiver operating characteristic (ROC) analysis was performed to assess the ability of each measured parameter to distinguish fracture from non-fracture subjects. Results showed that the fracture group had 23 % lower BMRI-strength (P < .001) and 19 % higher BMAT content (P < .001) than the non-fracture group, whereas no significant difference in vBMD was detected between the two groups. A poor correlation was found between vBMD and BMRI-strength (R² = 0.33). Compared to vBMD and BMAT content, BMRI- and BCT-strength had the larger area under the curve (0.82 and 0.84, respectively) and provided better sensitivity and specificity in separating fracture from non-fracture subjects. In conclusion, BMRI is capable of detecting reduced bone strength in patients with vertebral fracture, and may serve as a new approach for risk assessment of vertebral fracture.

Biomechanical CT-computed bone strength predicts the risk of subsequent vertebral fracture

Following primary fractures and percutaneous kyphoplasty (PKP), patients have a high risk of incurring a subsequent vertebral fracture (SVF). Given that SVF is a consequence of mechanical deterioration of the vertebra, we sought to examine whether vertebral strength derived from QCT-based finite element analysis (i.e., BCT) can predict the risk of SVF. Sixty-six patients who underwent PKP were categorized into two groups: control or non-SVF group (age: 70 ± 7 years; n = 40) and SVF group (age: 69 ± 8 years; n = 26). BCT was performed on L4 or L3 vertebrae to noninvasively measure vertebral strength. Vertebral strength was also estimated based upon the geometry and material properties of the vertebra. Additionally, trabecular volumetric bone mineral density (vBMD) and L1 Hounsfield unit (HU) were measured. t-Test, χ² test or Mann Whitney U test were used to compare differences in these parameters between the two groups. The predictive abilities of BCT strength and other measured parameters were evaluated using the receiver operating characteristic (ROC) analysis. Results showed no significant difference in either vBMD or L1 HU between the control and SVF groups (p > 0.05), whereas BCT-computed and estimated vertebral strength values were significantly reduced by 33 % and 24 % for the SVF group relative to the non-SVF group, respectively. ROC curve indicated that BCT strength had the largest area under the curve, compared to other parameters. These results suggest that BCT-computed vertebral strength may serve as a surrogate for assessing risk of SVF.

A Novel Screw Modeling Approach to Study the Effects of Screw Parameters on Pullout Strength

Screw loosening remains a prominent problem for osteoporotic patients undergoing pedicle screw fixation surgeries and is affected by screw parameters (e.g., diameter, pitch and thread angle). However, the individual and interactive effects of these parameters on screw fixation are not fully understood. Furthermore, current finite element modeling of an threaded screw is less computationally efficient. To address these issues, we (1) explored a novel "simulated threaded screw" approach (virtual threads assigned to the contact elements of a simplified screw) and compared its performance with threaded and simplified screws, and (2) examined with this approach the individual and interactive effects of altering screw diameter (5.5-6.5 mm), pitch (1-2 mm) and half-thread angle (20-30°) on pullout strength of normal vertebrae. Results demonstrated that the "simulated threaded screw" approach equivalently predicted pullout strength compared to the "threaded screw" approach (R2 = 0.99, slope = 1). We further found that the pullout strength was most sensitive to the change in screw diameter, followed by thread angle, pitch and interactions of diameter*pitch or diameter*angle. In conclusion, the "simulated threaded screw" approach can achieve the same predictive capability compared to threaded modeling of the screw. The current findings may serve as useful references for planning of screw parameters, so as to improve the complication of screw loosening.

Biomechanical analysis of vertebral wedge deformity in elderly women with quantitative CT-based finite element analysis

BACKGROUND

To explore the vertebral deformity angle (VD angle) of 1^st lumbar vertebral body (L1) in elderly women, investigate the influence of VD on vertebral stiffness (VS) by biomechanical analysis using quantitative computed tomography-based finite element analysis (QCT-FEA).

METHODS

Two hundred seventy eight participants were recruited, and underwent QCT scan. Measured VD angles of L1, and constructed QCT-FEA models of L1 with the minimum (0.59°), median (5.79°) and maximum (11.15°) VD angles, respectively. Loads in two directions were applied on the upper edge of L1 with a force of 700 N, and vertebral stiffness (VS) was defined as the ratio of 700 N and displacement at the superior reference point: (1) perpendicular to the upper edge of L1 (defined as VS-U); (2) perpendicular to the lower edge of L1(defined as VS-L).

RESULTS

Age was very weak positively correlated with VD angle, moderate negatively correlated with vBMD, and moderate negatively correlated with VS (P < 0.05). VS-U was significantly different among three VD angles, so was VS-L (P < 0.001). VS-U was higher than VS-L in 5.79° and 11.15° VD angles (P < 0.05), however no difference in 0.59° VD angles (P > 0.10).

CONCLUSIONS

VD angle of L1 was slightly increased with age and not correlated with vBMD, and VS was moderate negatively correlated with age, showing that the vertebral body was more likely to fracture with aging. VS-U and VS-L were gradually decreased with the increase of VD angle, and VS-L was lower than VS-U with the increase of VD angle, which showed that vertebral body was more prone to fracture when the load was perpendicular to the lower edge of the vertebral body as the VD angle increasing.