Background/objective: Both polymethylmethacrylate (PMMA) and traditional calcium phosphate-based cements have some deficiencies as augmentation materials for pedicle screw fixation. Here, a novel calcium phosphate-based nanocomposite (CPN) for the augmentation of pedicle screw fixation was developed based on previous study, and the handling properties, biomechanical performance, and biodegradation behaviour of CPN were evaluated and compared with clinical PMMA by means of a cadaver study and animal tests.
Methods: Bone mineral density of the lumbar vertebrae was tested. Pedicle screws were placed into the lumbar vertebrae under the guidance of three dimensionally printed templates; each of which was designed based on computed tomography (CT) reconstruction of each vertebrae and augmented with either PMMA or CPN. X-ray and CT scan were used to evaluate the accuracy of screw placement and dispersion as well as interdigitation of bone cement. The axial pull-out strength and maximum torque were tested using a mechanical testing machine. Degradation behaviour of CPN was evaluated by in vitro immersion tests for 8 weeks and in vivo rabbit femur defect model for up to 6 months, respectively.
Results: Standard mechanical tests revealed that PMMA was much stronger than CPN after setting (compressive strength 95 vs. 49 MPa, respectively, p < 0.001). Results of the projection area and volume distribution of cement along the distal end of the screws revealed that CPN exhibited unique dispersing and interdigitation abilities compared with PMMA. Specifically, CPN dispersed uniformly and symmetrically along the screw, while PMMA was limited to the proximal part of the screw. Axial pull-out test results showed that the axial pull-out strengths of CPN- and PMMA-augmented pedicle screws were similar (1199 ± 225 N vs 1337 ± 483 N, respectively) and not significantly different (p = 0.47), although CPN was an intrinsically weaker material than PMMA. Similarly, CPN showed average torque values of 0.72 ± 0.31 N·m slightly lower than those of PMMA (0.96 ± 0.23 N·m), but statistically there was no significant difference between CPN and PMMA (p = 0.21). In a rabbit model of femoral bone defect, the implanted CPN maintained its clear boundary and there is no disintegration in the cement clump after 20 days and 24 weeks, and there was moderate bioabsorption of CPN and clearly new bone ingrowth at the absorbed sites after 24 weeks.
Conclusion: A new nanocomposite cement CPN, designed for replacing the nondegradable PMMA cement and overcoming the mechanical inferiority of calcium phosphate cement, was evaluated for its biomechanical and biodegradation behaviours in cement-injectable cannulated pedicle screws (CICPS) application. Although CPN is a mechanically weaker material than PMMA, CPN demonstrates similar biomechanical properties to PMMA in the application of augmentation for CICPS fixation in cadaveric vertebrae. This improvement in biomechanical property is attributed to a better dispersion and interdigitation mode of CPN. In addition, the animal study results suggest the in vivo absorption of CPN is slow enough and matches the bone ingrowth.
The translational potential of this article: This work reports a cadaveric and biomechanical study of novel CPN for the application in the augmentation of CICPS. The results suggest that CPN has equivalent or better biomechanical and interdigitation performance compared with PMMA. Together with the biodegradability and ossointegration capability, CPN reveals high translational potential as a new bone cements for load-bearing bone fixation and repair.
Keywords: Calcium phosphate–based cement; Injectable; Osteoporosis; PMMA; Pedicle screw.
© 2019 The Authors.