Bone regeneration capacity of newly developed spherical magnesium phosphate cement granules

Abstract

Objectives: Magnesium phosphate-based cements begin to catch more attention as bone substitute materials and especially as alternatives for the more commonly used calcium phosphates. In bone substitutes for augmentation purposes, atraumatic materials with good biocompatibility and resorbability are favorable. In the current study, we describe the in vivo testing of novel bone augmentation materials in form of spherical granules based on a calcium-doped magnesium phosphate (CaMgP) cement.

Materials and methods: Granules with diameters between 500 and 710 μm were fabricated via the emulsification of CaMgP cement pastes in a lipophilic liquid. As basic material, two different CaMgP formulations were used. The obtained granules were implanted into drill hole defects at the distal femoral condyle of 27 New Zealand white rabbits for 6 and 12 weeks. After explantation, the femora were examined via X-ray diffraction analysis, histological staining, radiological examination, and EDX measurement.

Results: Both granule types display excellent biocompatibility without any signs of inflammation and allow for proper bone healing without the interposition of connective tissue. CaMgP granules show a fast and continuous degradation and enable fully adequate bone regeneration.

Conclusions: Due to their biocompatibility, their degradation behavior, and their completely spherical morphology, these CaMgP granules present a promising bone substitute material for bone augmentation procedures, especially in sensitive areas.

Clinical relevance: The mostly insufficient local bone supply after tooth extractions complicates prosthetic dental restoration or makes it even impossible. Therefore, bone augmentation procedures are oftentimes inevitable. Spherical CaMgP granules may represent a valuable bone replacement material in many situations.

Keywords: Bone replacement material; Calcium-magnesium phosphate cement; Cement pastes; Implantation; Prefabricated granules.

Fig. 1

A Schematic of the drill hole placement. B Overview of the timeline and groups (l: left femur, r: right femur)

Fig. 2

Size distribution of Ca_0.25Mg_2.75(PO₄)₂ granules (I-A) and Ca_0.75Mg_2.25(PO₄)₂ granules (I-B) fabricated via the emulsion process. II-A to II-C show pictures of the granule morphology which is shown in more detail in II-D to II-F

Fig. 3

X-ray diffraction patterns of the hardened cement granules; a: β-tricalcium phosphate, HA: hydroxyapatite, f: farringtonite, str: struvite, s: stanfieldite, n: newberyite

Fig. 4

Histological sections of Ca_0.75Mg_2.25(PO₄)₂ granules (A, D), Ca_0.25Mg_2.75(PO₄)₂ granules (B, E), and CPC granules (C, F) in the distal femoral epicondyle after 6 and 12 weeks post-implantation. Masson-Goldner-Trichrome staining; Dark green/black: granules (some examples are marked with small asterisks), red: keratin, muscle tissue, blue/turquoise: mineralized bone, orange: non-mineralized bone

Fig. 5

Higher magnification images of the implanted granules. Upper row after 6 weeks, lower row after 12 weeks. Bone replacement material is marked by asterisks. The formed bone around granules is marked by arrows and osteoid by arrowheads. There is no sign of inflammation

Fig. 6

Radiographs of the implanted granules in the distal femoral epicondyle after explantation

Fig. 7

Empty defect after 12 weeks. A Radiograph of the defect area. B Histology of the defect area. The boundaries of the drill hole can still be detected

Fig. 8

Mean circumference (A, B), number of granules per slice (C), and added circumferences of all granules per sample (D) calculated from the histological sections after 6 and 12 weeks of implantation. A total of 12 slices was analyzed per sample

Fig. 9

Osteoid-implant contact assessed from the histological sections for CPC and CaMgP granules after 6 and 12 weeks of implantation. There is no significant difference between osteoid proportion after 6 weeks and after 12 weeks

Fig. 10

Bone-implant contact assessed from the histological sections for CPC and CaMgP granules after 6 and 12 weeks of implantation. The difference of the CPC granules to Mg-containing samples is significant (p < 0.05), whereas there is no significant difference between the Mg-containing samples

Fig. 11

Elemental composition of exemplary implant sites after 6 weeks post-implantation. A–A”: Ca_0.75Mg_2.25(PO₄)₂- granules, Mg is only detectable in the granules (A), Ca (A’) and P (A”) can be detected in the granules as well as the surrounding bone (lower right). B–B”: Ca_0.25Mg_2.75(PO₄)₂- granules, Mg is only detectable in the granules (A), Ca (A’) and P (A”) can be detected in the granules as well as the surrounding bone (lower left). C–C”: CPC control granules, only Ca (C’) and P⁻ (C”) was detectable in the granules (and surrounding bone/tissue)

Fig. 12

Elemental composition of exemplary implant sites after 12 weeks post-implantation. A–A”: Ca_0.75Mg_2.25(PO₄)₂- granules, nearly no Mg was detected in the granules (A), only weak signals of Ca (A’) and P (A”) were detected. B–B”: Ca_0.25Mg_2.75(PO₄)₂- granules, no Mg was detected in the granules (B), only weak signals of Ca (B’) and P (B”) were detected. C–C”: CPC control granules, still no Mg was detectable (C), Ca (C’) and P (C”) were detectable in the granules as well as in surrounding bone