A novel injectable calcium phosphate-based nanocomposite for the augmentation of cannulated pedicle-screw fixation

Abstract

Polymethyl methacrylate (PMMA)-augmented cannulated pedicle-screw fixation has been routinely performed for the surgical treatment of lumbar degenerative diseases. Despite its satisfactory clinical outcomes and prevalence, problems and complications associated with high-strength, stiff, and nondegradable PMMA have largely hindered the long-term efficacy and safety of pedicle-screw fixation in osteoporotic patients. To meet the unmet need for better bone cement for cannulated pedicle-screw fixation, a new injectable and biodegradable nanocomposite that was the first of its kind was designed and developed in the present study. The calcium phosphate-based nanocomposite (CPN) exhibited better anti-pullout ability and similar fluidity and dispersing ability compared to clinically used PMMA, and outperformed conventional calcium phosphate cement (CPC) in all types of mechanical properties, injectability, and biodegradability. In term of axial pullout strength, the CPN-augmented cannulated screw reached the highest force of ~120 N, which was higher than that of PMMA (~100 N) and CPC (~95 N). The compressive strength of the CPN (50 MPa) was three times that of CPC, and the injectability of the CPN reached 95%. In vivo tests on rat femur revealed explicit biodegradation of the CPN and subsequent bone ingrowth after 8 weeks. The promising results for the CPN clearly suggest its potential for replacing PMMA in the application of cannulated pedicle-screw fixation and its worth of further study and development for clinical uses.

Keywords: biodegradable; calcium phosphate nanocomposite; injectable; lumbar degenerative disease; osteoporosis; pedicle screw.

Figure 1

(A) TEM image of nanoscale networks of gelatinized starch; SEM image of microscopic morphology of fractured surfaces of (B) CPC and (C) CPN. Abbreviations: TEM, transmission electron microscopy; SEM, scanning electron microscopy; CPC, calcium phosphate cement; CPN, calcium phosphate nanocomposite.

Figure 2

Representative compressive stress–strain curves of PMMA, CPC, CPC-BS, and CPN. Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite.

Figure 3

(A) FTIR spectra and (B) XRD patterns of different cements. Notes: Original CPC was the mixture of powders before solid–liquid reaction and other samples were after setting for 3 days. Standard XRD spectra of HA and BaSO₄ are also shown in (B). Abbreviations: CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite; HA, hydroxyapatite; FTIR, Fourier-transform infrared spectroscopy; XRD, X-ray diffraction.

Figure 4

Fluidity and dispersion ability of bone cements tested in Sawbones blocks. Notes: (A) Photographs of PMMA, CPC, and CPN lumps with different L:S ratios after dispersal and setting in Sawbones blocks. (B) Average projection areas and (C) estimated volumes of the bone-cement lumps. Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; CPN, calcium phosphate nanocomposite; L:S, liquid:solid.

Figure 5

X-ray radiographs and photos of the augmentation of cannulated and solid screws in Sawbones blocks by different bone cements. Note: Rightmost photos show the screw–cement complexes after axial pullout tests from Sawbones blocks. Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; CPN, calcium phosphate nanocomposite.

Figure 6

Axial pullout tests on cement-augmented pedicle screws. Notes: (A) Photo showing the setup of axial pullout test and representative load-displacement curves for (B) cannulated and (C) solid pedicle screws augmented without cement (Blank) and with PMMA, CPC, CPC-BS, and CPN. Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite.

Figure 7

Axial pullout strengths of cement-augmented pedicle screws. Notes: (A) Cannulated and (B) solid pedicle screws augmented without cement (Blank) and with PMMA, CPC, CPC-BS (indicated by the line) and CPN. The pullout strength was expressed by the maximum load reached during the pullout test. Data are mean ± standard deviation (n=5). Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite.

Figure 8

Torsion tests on cement-augmented pedicle screws. Notes: (A) Photo of the setup of torsion tests. Representative torque-angle curves for (B) cannulated and (C) solid pedicle screws augmented without cement (Blank) and with PMMA, CPC, CPC-BS, and CPN. Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite.

Figure 9

Maximum torque measured on cement augmented pedicle screws. Notes: (A) Cannulated and (B) solid pedicle screws augmented without cement (Blank) and with PMMA, CPC, CPC-BS, and CPN. Data are mean ± standard deviation (n=5). Abbreviations: PMMA, polymethyl methacrylate; CPC, calcium phosphate cement; BS, BaSO₄; CPN, calcium phosphate nanocomposite.

Figure 10

Histological analysis of the CPN implanted in rat-femur defect after 8 weeks, H&E-stained. Note: (A) The smooth outer surface of cement degraded, and (B) bone ingrowth was observed. Abbreviations: CPN, calcium phosphate nanocomposite; H&E, hematoxylin and eosin.