Resorbierbare Kalziumphosphatzemente

In Vivo Investigation of 3D-Printed Calcium Magnesium Phosphate Wedges in Partial Load Defects

Bone substitutes are ideally biocompatible, osteoconductive, degradable and defect-specific and provide mechanical stability. Magnesium phosphate cements (MPCs) offer high initial stability and faster degradation compared to the well-researched calcium phosphate cements (CPCs). Calcium magnesium phosphate cements (CMPCs) should combine the properties of both and have so far shown promising results. The present study aimed to investigate and compare the degradation and osseointegration behavior of 3D powder-printed wedges of CMPC and MPC in vivo. The wedges were post-treated with phosphoric acid (CMPC) and diammonium hydrogen phosphate (MPC) and implanted in a partially loaded defect model in the proximal rabbit tibia. The evaluation included clinical, in vivo µ-CT and X-ray examinations, histology, energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) for up to 30 weeks. SEM analysis revealed a zone of unreacted material in the MPC, indicating the need to optimize the manufacturing and post-treatment process. However, all materials showed excellent biocompatibility and mechanical stability. After 24 weeks, they were almost completely degraded. The slower degradation rate of the CMPC corresponded more favorably to the bone growth rate compared to the MPC. Due to the promising results of the CMPC in this study, it should be further investigated, for example in defect models with higher load.

Highly flexible and degradable dual setting systems based on PEG-hydrogels and brushite cement

With respect to the composition of natural bone, we established a degradable dual setting system of different poly(ethylene glycol) (PEG)-based hydrogels combined with a brushite cement. The idea was to reinforce the inorganic calcium phosphate mineral phase with an organic, polymeric phase to alter the cement's properties towards ductility and elasticity. Extremely flexible samples were produced via this dual setting approach with a fully reversible elasticity of the samples containing high molecular weight PEG-based hydrogel precursors. Using the decalcifying agent EDTA, the whole inorganic phase was dissolved due to Ca²⁺-complexation and dimensionally stable hydrogels were obtained, indicating a homogenous polymeric phase within the composites. This was also confirmed by SEM-analysis, where no discontinuities or agglomerations of the phase were observed. Additional XRD-measurements proved a significant influence of the coherent polymeric matrix on the conversion from β-TCP/MCPA to brushite with a decrease in signal intensity. The results confirmed a parallelly running process of setting reaction and gelation without an inhibition of the conversion to brushite and the formation of interpenetrating networks of hydrogel and cement. The strengths of this newly developed dual setting system are based on the material degradability as well as flexibility, which can be a promising tool for bone regeneration applications in non-load bearing craniomaxillofacial defects.

STATEMENT OF SIGNIFICANCE

Brushite based calcium phosphate cements (CPCs) are known as bone replacement materials, which degrade in vivo and are replaced by native bone. However, the pure inorganic material shows a brittle fracture behavior. Here, the addition of a polymeric phase can influence the mechanical properties to create more ductile and flexible materials. This polymeric phase should ideally form during cement setting by a polymerization reaction to achieve high polymer loads without altering cement viscosity and it should be degradable in vivo similar to the cement itself. Therefore, we developed a dual setting system based on simultaneous cement setting of brushite and lactide modified poly(ethylene glycol) dimethacrylate (PEG-PLLA-DMA)-based hydrogel. It was evident that the gels form a continuous phase within the cement after radical polymerization with a strong reduction of cement brittleness.