A comparative study on the effects of biodegradable high-purity magnesium screw and polymer screw for fixation in epiphyseal trabecular bone

Abstract

With mechanical strength close to cortical bone, biodegradable and osteopromotive properties, magnesium (Mg)-based implants are promising biomaterials for orthopedic applications. However, during the degradation of such implants, there are still concerns on the potential adverse effects such as formation of cavities, osteolytic phenomena and chronic inflammation. Therefore, to transform Mg-based implants into clinical practice, the present study evaluated the local effects of high-purity Mg screws (HP-Mg, 99.99 wt%) by comparing with clinically approved polylactic acid (PLA) screws in epiphyseal trabecular bone of rabbits. After implantation of screws at the rabbit distal femur, bone microstructural, histomorphometric and biomechanical properties were measured at various time points (weeks 4, 8 and 16) using micro-CT, histology and histomorphometry, micro-indentation and scanning electron microscope. HP-Mg screws promoted peri-implant bone ingrowth with higher bone mass (BV/TV at week 4: 0.189 ± 0.022 in PLA group versus 0.313 ± 0.053 in Mg group), higher biomechanical properties (hardness at week 4: 35.045 ± 1.000 HV in PLA group versus 51.975 ± 2.565 HV in Mg group), more mature osteocyte LCN architecture, accelerated bone remodeling process and alleviated immunoreactive score (IRS of Ram11 at week 4: 5.8 ± 0.712 in PLA group versus 3.75 ± 0.866 in Mg group) as compared to PLA screws. Furthermore, we conducted finite element analysis to validate the superiority of HP-Mg screws as orthopedic implants by demonstrating reduced stress concentration and uniform stress distribution around the bone tunnel, which led to lower risks of trabecular microfractures. In conclusion, HP-Mg screws demonstrated greater osteogenic bioactivity and limited inflammatory response compared to PLA screws in the epiphyseal trabecular bone of rabbits. Our findings have paved a promising way for the clinical application of Mg-based implants.

Keywords: epiphyseal trabecular bone; high-purity magnesium screw; macrophage; peri-implant bone quality; polylactic acid screw.

Graphical abstract

Figure 1.

High-purity Mg screw improved peri-implant bone microstructural characteristics. (A) Representative ROI of peri-implant trabecular bone tissues at the sagittal plane (left panel) and corresponding 3D reconstruction (right three panels). The large circle indicated the region where the implant was located. The small circle represented the individual ROI for following morphometric analyses. Scale bar in the 2D images = 5 mm; Scale bar in the 3D images = 1 mm. (B–G) Microstructural properties of peri-implant bone tissues (n = 4 for weeks 4 and 8; n = 6 for week 16). BV/TV, relative bone volume (%); BMD, bone mineral density (mg HA/ccm); Tb. N, trabecular number (1/mm); Tb. Sp, trabecular separation (mm); Tb. Th, trabecular thickness (mm); conn. D, connectivity density (1/mm³). *P < 0.05, **P < 0.01.

Figure 2.

Gross and microscopic morphology of peri-implant bone tissues. (A) Gross observation of peri-implant bone tissues. Arrows indicated cavities; scale bar = 1 mm. (B) Microscopic observation of peri-implant bone tissues by SEM. Peri-implant bone tissues at 4- and 16-week post-implantation were shown in the leftmost panel. Peri-implant bone tissues in panel a were magnified into panels b and c by SE and BE, respectively. Osteocytes within the dotted box in panels b and c were further magnified into panels b1 and c1, where the arrows indicated lacuno-canalicular network architecture formed by osteocytes located at peri-implant bone tissues; scale bar in panel A = 1 mm; scale bar in panel B = 10 μm. SE, secondary electrons imaging; BSE, back-scattered electrons imaging.

Figure 3.

High-purity Mg screw reinforced peri-implant bone remodeling at week 8 post-implantation. (A) Representative images of peri-implant tissues stained with villanueva bone stain at week 8 post-implantation. ROI was delineated by a dotted rectangle; scale bar in panel A = 1 mm. (B and C) Representative images of ROI stained with villanueva osteochrome bone stain and calcein green/xylenol orange fluorochromes stain at week 8 post-implantation. Regions within the dotted box were further magnified; scale bar = 100 μm.

Figure 4.

High-purity Mg screw improved biomechanical properties of peri-implant trabecular bones at matrix level. (A) Representative images of peri-implant trabecular bone tissues under the microscope of microhardness tester pre- and post-indentation. Dashed boxes showed the indentation area with diagonal lines; scale bar = 100 μm. (B) Vickers–Hardness of peri-implant newly regenerated bone matrix (NRBM) and surrounding host bones (SHB) at week 4 and 16 post-implantation (n = 4). (C) Elastic modulus of peri-implant trabecular bones at week 4 and 16 post-implantation (n = 4). (D) Bone maturation index (BMI) of peri-implant trabecular bones at week 4 and 16 post-implantation (n = 4). *P < 0.05.

Figure 5.

High-purity Mg screw improved the ingrowth of peri-implant trabecular bones into implant pitches. (A) H&E stain of peri-implant trabecular bone tissues. The arrows indicated bone-implant contact; Mg screw pitch that was filled with trabecular bones; scale bar in panel A = 500 μm. (B) ROI for calculating peri-implant bone area was enclosed by lines. TB, trabecular bone; BM, bone marrow; O, osteocyte; OB, osteoblast; I, implant; scale bar in panel B = 100 μm. (C) Comparisons of bone-to-implant contact ratio (%) at week 4, 8 and 16 post-implantation (n = 4 for weeks 4 and 8; n = 6 for week 16). (D) Comparisons of peri-implant bone area (%) at week 4, 8 and 16 post-implantation (n = 4 for weeks 4 and 8; n = 6 for week 16). *P < 0.05.

Figure 6.

High-purity Mg screw alleviated macrophage infiltration at early stages post-implantation. (A) Representative images of peri-implant trabecular bones by IHC staining targeting at macrophage-specific RAM11; regions within the dashed boxes were recognized as ROI and further magnified. (B) Representative images of IHC negative control. NC, negative control. (C) The IRS-based expression intensity of RAM11 at peri-implant bone regions (n = 4 for weeks 4 and 8; n = 6 for week 16). (D) Representative images of peri-implant trabecular bones by IHC staining targeting at TGF-β1; regions within red-dashed boxes were recognized as ROI and further magnified. (E) Representative images of IHC negative control. NC, negative control. (F) The IRS-based expression intensity of TGF-β1 at peri-implant bone regions (n = 4 for weeks 4 and 8; n = 6 for week 16). IRS, immunoreactive score. Scale bar = 100 μm. *P < 0.05.

Figure 7.

Three numerical simulation models with or without the insertion of the PLA or Mg screw exhibiting the stress distribution around the peri-implant bone tunnels.