Adjacent vertebral failure after vertebroplasty: a biomechanical study of low-modulus PMMA cement

Abstract

PMMA is the most common bone substitute used for vertebroplasty. An increased fracture rate of the adjacent vertebrae has been observed after vertebroplasty. Decreased failure strength has been noted in a laboratory study of augmented functional spine units (FSUs), where the adjacent, non-augmented vertebral body always failed. This may provide evidence that rigid cement augmentation may facilitate the subsequent collapse of the adjacent vertebrae. The purpose of this study was to evaluate whether the decrease in failure strength of augmented FSUs can be avoided using low-modulus PMMA bone cement. In cadaveric FSUs, overall stiffness, failure strength and stiffness of the two vertebral bodies were determined under compression for both the treated and untreated specimens. Augmentation was performed on the caudal vertebrae with either regular or low-modulus PMMA. Endplate and wedge-shaped fractures occurred in the cranial and caudal vertebrae in the ratios endplate:wedge (cranial:caudal): 3:8 (5:6), 4:7 (7:4) and 10:1 (10:1) for control, low-modulus and regular cement group, respectively. The mean failure strength was 3.3 ± 1 MPa with low-modulus cement, 2.9 ± 1.2 MPa with regular cement and 3.6 ± 1.3 MPa for the control group. Differences between the groups were not significant (p = 0.754 and p = 0.375, respectively, for low-modulus cement vs. control and regular cement vs. control). Overall FSU stiffness was not significantly affected by augmentation. Significant differences were observed for the stiffness differences of the cranial to the caudal vertebral body for the regular PMMA group to the other groups (p \ 0.003). The individual vertebral stiffness values clearly showed the stiffening effect of the regular cement and the lesser alteration of the stiffness of the augmented vertebrae using the low-modulus PMMA compared to the control group (p = 0.999). In vitro biomechanical study and biomechanical evaluation of the hypothesis state that the failure strength of augmented functional spine units could be better preserved using low-modulus PMMA in comparison to regular PMMA cement.

The effect of standard and low-modulus cement augmentation on the stiffness, strength, and endplate pressure distribution in vertebroplasty

“…Of concern is the increased risk of adjacent vertebral body fractures observed in clinical [4,5] and experimental studies [6][7][8]. Although a recent randomized controlled trial comparing vertebroplasty to conservative treatment [9] found no significant difference in adjacent-level fracture incidence, clinically used poly-methyl-methacrylate (PMMA) cements caused marked changes in load transfer.…”

“…Such adapted cement materials better preserved the failure strength of augmented functional spine units compared to standard PMMA [7], because failure mostly occurred in the adjacent level. Chevalier et al [15] showed numerically that a decreased elastic modulus of the cement reduced the stresses above/below the cement region while still strengthening the vertebral body.…”

Biomechanical comparison of vertebral augmentation with silicone and PMMA cement and two filling grades

“…These filling grades resulted in an average augmentation volume of 5.4 ml for the 16 % filling groups and 11.8 ml for the 35 % augmentation groups. According to the literature, 16 % filling is required to restore the biomechanical properties and 35 % filling is equivalent to endplate-to-endplate filling [32,37,38]. An experienced senior spine surgeon performed a bipedicular VP in each vertebra under fluoroscopic guidance.…”

“…It is still under debate whether new vertebral fractures appear more often in patients who received cement augmentation than in conservatively treated patients; some studies support this hypothesis [1,6,7,[27][28][29], while others question it [2,3,26,30]. Reducing the augmentation volume [31], using low-modulus PMMA [32] and prophylactic augmentation of adjacent vertebrae [13,25,33] have been proposed as ways of minimizing the risk of subsequent fractures. In particular, a need for materials with less stiffness has been identified to prevent this adverse effect of VP [34].…”

“…VK100 was engineered to be less stiff than PMMA and to have a stiffness in the same range as cancellous bone. It is hypothesized that an augmentation material with mechanical properties similar to those of cancellous bone may reduce the rates of adjacent fractures and vertebral sintering [32].…”